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Ionization chambers used for reference dosimetry require a local secondary standard ionization chamber with a {sup 60}Co absorbed dose to water calibration coefficient N{sub D,w}{sup 6{sup 0Co}} traceable to a national primary standards dosimetry laboratory or an accredited secondary dosimetry calibration laboratory. Clinic based (in-house) transfer of this coefficient to tertiary reference ionization chambers has traditionally been accomplished with chamber cross calibration in water using a {sup 60}Co beam; however, access to {sup 60}Co teletherapy machines has become increasingly limited for clinic based physicists. In this work, the accuracy of alternative methods of transferring the N{sub D,w}{sup 6{sup 0Co}} calibration coefficient using 6 and 18 MV photon beams from a linear accelerator in lieu of {sup 60}Co has been investigated for five different setups and four commonly used chamber types.

The authors present a radiochromic film dosimetry protocol for a multicolor channel radiochromic film dosimetry system consisting of the external beam therapy (EBT) model GAFCHROMIC film and the Epson Expression 1680 flat-bed document scanner. Instead of extracting only the red color channel, the authors are using all three color channels in the absorption spectrum of the EBT film to extend the dynamic dose range of the radiochromic film dosimetry system. By optimizing the dose range for each color channel, they obtained a system that has both precision and accuracy below 1.5%, and the optimized ranges are 0-4 Gy for the red channel, 4-50 Gy for the green channel, and above 50 Gy for the blue channel.

Percentage depth dose (PDD) distributions and beam profiles of very small diameter ( 1.5 – 5 mm ) megavoltage radiosurgical beams calculated with Monte Carlo(MC) technique critically depend on the diameter of the circular focal spot used in the simulation: The smaller is the field diameter, the larger is the effect. Thus, in simulations of radiosurgical fields that have diameters of the order of the focal spot size, an accurate focal spot geometry should be used. We used a simplified moving slit technique in conjunction with a diode detector for evaluation of the focal spot size and shape of a megavoltage 6 MV linac as well as for determination of the equivalent focal spot diameter of the linac for use in MC simulations. The measured total diode signal contains three components: A direct focal spot signal, a background signal, and an extra-focal radiation signal. A single profile scan of the focal spot signal is Gaussian like in shape, and its full width at half maximum is used to define the focal spot dimension for this scan. The focal spot of our 6 MV linac is approximated with a Gaussian circle, and when the geometry of the effective focal spot circle is used in MC simulations, the agreement between MC-calculated and measured PDD distributions as well as beam profiles is good even for radiosurgical fields as small as 1.5 mm in diameter. Our results also confirm that matching the penumbral areas of accurately measured large-field beam profiles to the same areas of the MC-calculated beam profiles reliably leads to a realistic effective focal spot size for use in MC simulations of very small diameter beams.

Fractionated high dose rate endorectal brachytherapy (HDR-EBT) using CT-based treatment planning is an alternative method for preoperative down-sizing and down-staging of advanced rectal adeno-carcinomas. The authors present an image guidance procedure that was developed to ensure daily dose reproducibility for the four brachytherapy treatment fractions. Since the applicator might not be placed before each treatment fraction inside the rectal lumen in the same manner as it was placed during the 3D CT volume acquisition used for treatment planning, there is a shift along the catheter axis that may have to be performed. The required shift is determined by comparison of a daily radiograph with the treatment planning digitally-reconstructed radiograph (DRR). A procedure is developed for DRR reconstruction from the 3D data set used for the treatment planning, and two possible daily longitudinal shifts are illustrated: above and below the planning dose distribution. The authors also describe the procedure for rotational alignment illustrated on a clinical case. Reproduction of the treatment planned dose distribution on a daily basis is crucial for the success of fractionated 3D based brachytherapy treatments. Due to the cylindrical symmetry of the applicator used for preoperative HDR-EBT, two types of adjustments are necessary: applicator rotation and dwellmore » position shift along the applicator's longitudinal axis. The impact of the longitudinal applicator shift prior to treatment delivery for 62 patients treated in our institution is also assessed.« less

Since taken with megavoltage, forward-directed bremsstrahlung beams, the image quality of current portal images is inferior to that of diagnostic quality images produced by kilovoltage beams. In this paper, the beam quality of orthogonal bremsstrahlung beams defined as the 90° component of the bremsstrahlung distribution produced from megavoltage electron pencil beams striking various targets is presented, and the suitability of their use for improved radiotherapy imaging is evaluated. A 10 MeV electron beam emerging through the research port of a Varian Clinac-18 linac was made to strike targets of carbon,aluminum, and copper. PDD and attenuation measurements of both the forward and orthogonal beams were carried out, and the results were also used to estimate the effective and mean energy of the beams. The mean energy of a spectrum produced by a carbon target dropped by 83% from 1296 keV in the forward direction to 217 keV in the orthogonal direction, while for an aluminum target it dropped by 77% to 412 keV , and for a copper target by 65% to 793 keV . An in-depth Monte Carlo study of photon yield and electron contamination was also performed. Photon yield and effective energy are lower for orthogonal beams than for forward beams, and the differences are more pronounced for targets of lower atomic number. Using their relatively low effective energy, orthogonal bremsstrahlung beams produced by megavoltage electrons striking low atomic number targets yield images with a higher contrast in comparison with forward bremsstrahlung beams.

The introduction of radiochromic films has solved some of the problems associated with conventional 2D radiation detectors. Their high spatial resolution, low energy dependence, and near-tissue equivalence make them ideal for measurement of dose distributions in radiation fields with high dose gradients. Precise knowledge of the absorption spectra of these detectors can help to develop more suitable optical densitometers and potentially extend the use of these films to other areas such as the measurement of the radiation beam spectral information. The goal of this study is to present results of absorption spectra measurements for the new GAFCHROMIC ™ film, EBT type, exposed to 6 MV photon beam in the dose range from 0 to 6 Gy . Spectroscopic analysis reveals that in addition to the two main absorption peaks, centered at around 583 and 635 nm , the absorptionspectrum in the spectral range from 350 to 800 nm contains six more absorption bands. Comparison of the absorption spectra reveals that previous HD-810, MD-55, as well as HS GAFCHROMIC ™ film models, have nearly the same sensitive layer base material, whereas the new EBT model, GAFCHROMIC ™ film has a different composition of its sensitive layer. We have found that the two most prominent absorption bands in EBT model radiochromic film do not change their central wavelength position with change in a dose deposited to the film samples.

Polarity effects in ionization chambers are caused by a radiation induced current, also known as Compton current, which arises as a charge imbalance due to charge deposition in electrodes of ionization chambers. We used a phantom-embedded extrapolation chamber (PEEC) for measurements of Compton current in megavoltage photon and electron beams. Electron contamination of photonbeams and photon contamination of electron beams have a negligible effect on the measured Compton current. To allow for a theoretical understanding of the Compton current produced in the PEEC effect we carried out Monte Carlo calculations with a modified user code, the COMPTON/EGSnrc. The Monte Carlo calculated COMPTON currents agree well with measured data for both photon and electron beams; the calculated polarity correction factors, on the other hand, do not agree with measurement results. The conclusions reached for the PEEC can be extended to parallel-plate ionization chambers in general.

Megavoltage x-ray beams exhibit the well-known phenomena of dose buildup within the first few millimeters of the incident phantom surface, or the skin. Results of the surface dose measurements, however, depend vastly on the measurement technique employed. Our goal in this study was to determine a correction procedure in order to obtain an accurate skin dose estimate at the clinically relevant depth based on radiochromic film measurements. To illustrate this correction, we have used as a reference point a depth of 70 micron. We used the new GAFCHROMIC dosimetry films (HS, XR-T, and EBT) that have effective points of measurement at depths slightly larger than 70 micron. In addition to films, we also used an Attix parallel-plate chamber and a home-built extrapolation chamber to cover tissue-equivalent depths in the range from 4 micron to 1 mm of water-equivalent depth. Our measurements suggest that within the first millimeter of the skin region, the PDD for a 6 MV photon beam and field size of 10 x 10 cm2 increases from 14% to 43%. For the three GAFCHROMIC dosimetry film models, the 6 MV beam entrance skin dose measurement corrections due to their effective point of measurement are as follows: 15% for the EBT, 15% for the HS, and 16% for the XR-T model GAFCHROMIC films. The correction factors for the exit skin dose due to the build-down region are negligible. There is a small field size dependence for the entrance skin dose correction factor when using the EBT GAFCHROMIC film model. Finally, a procedure that uses EBT model GAFCHROMIC film for an accurate measurement of the skin dose in a parallel-opposed pair 6 MV photon beam arrangement is described.

A study of the induced activity in a medical linear accelerator (linac) room was carried out on several linac installations. Higher beam energy, higher dose rate, and larger field size generally result in higher activation levels at a given point of interest, while the use of multileaf collimators (MLC) can also increase the activation level at the isocenter. Both theoretical and experimental studies reveal that the activation level in the morning before any clinical work increases from Monday to Saturday and then decreases during the weekend. This weekly activation picture keeps stable from one week to another during standard clinical operation of the linac. An effective half-life for a given point in the treatment room can be determined from the measured or calculated activity decay curves. The effective half-life for points inside the treatment field is longer than that for points outside of the field in the patient plane, while a larger field and longer irradiation time can also make the effective half-life longer. The activation level reaches its practical saturation value after a 30 min continuous irradiation, corresponding to 12 000 MU at a "dose rate" of 400 MU/min. A "dose" of 300 MU was given 20 times in 15 min intervals to determine the trends in the activation level in a typical clinical mode. As well, a long-term (85 h over a long weekend) decay curve was measured to evaluate the long-term decay of room activation after a typical day of clinical linac use. A mathematical model for the activation level at the isocenter has been established and shown to be useful in explaining and predicting the induced activity levels for typical clinical and experimental conditions. The activation level for a 22 MeV electron beam was also measured and the result shows it is essentially negligible.

For reasons of phantom material reproducibility, the absorbed dose protocols of the American Association of Physicists in Medicine (AAPM) (TG-51) and the International Atomic Energy Agency (IAEA) (TRS-398) have made the use of liquid water as a phantom material for reference dosimetry mandatory. In this work we provide a formal framework for the measurement of absorbed dose to water using ionization chambers calibrated in terms of absorbed dose to water but irradiated in solid phantoms. Such a framework is useful when there is a desire to put dose measurements using solid phantoms on an absolute basis. Putting solid phantom measurements on an absolute basis has distinct advantages in verification measurements and quality assurance. We introduce a phantom dose conversion factor that converts a measurement made in a solid phantom and analyzed using an absorbed dose calibration protocol into absorbed dose to water under reference conditions. We provide techniques to measure and calculate the dose transfer from solid phantom to water. For an Exradin A12 ionization chamber, we measured and calculated the phantom dose conversion factor for six Solid Water phantoms and for a single Lucite phantom for photon energies between 60Co and 18 MV photons. For Solid Water of certified grade, the difference between measured and calculated factors varied between 0.0% and 0.7% with the average dose conversion factor being low by 0.4% compared with the calculation whereas for Lucite, the agreement was within 0.2% for the one phantom examined. The composition of commercial plastic phantoms and their homogeneity may not always be reproducible and consistent with assumed composition. By comparing measured and calculated phantom conversion factors, our work provides methods to verify the consistency of a given plastic for the purpose of clinical reference dosimetry.

In this study, a measurement protocol is presented that improves the precision of dose measurements using a flat-bed document scanner in conjunction with two new GafChromic® film models, HS and Prototype A EBT exposed to 6MV photon beams. We established two sources of uncertainties in dose measurements, governed by measurement and calibration curve fit parameters contributions. We have quantitatively assessed the influence of different steps in the protocol on the overall dose measurement uncertainty. Applying the protocol described in this paper on the Agfa Arcus II flat-bed document scanner, the overall one-sigma dose measurement uncertainty for an uniform field amounts to 2% or less for doses above around 0.4Gy in the case of the EBT (Prototype A), and for doses above 5Gy in the case of the HS model GafChromic® film using a region of interest 2×2mm2 in size.

The growth of a radioactive daughter in neutron activation is commonly described with the saturation model that ignores the consumption of parent nuclei during the radio-activation process. This approach is not valid when radioactive sources with high specific activities are produced or when the particle fluence rates used are very high. Assuming a constant neutron fluence rate throughout the activation target, a neutron-activation model that accounts for the depletion in parent nuclei is introduced. This depletion model is governed by relationships similar to those describing the parent-daughter-granddaughter decay series, and, in contrast to the saturation model, correctly predicts the practical limit of the daughter specific activity, irrespective of the particle fluence rate. Also introduced is a neutron-activation model that in addition to parent depletion accounts for the neutron activation of daughter nuclei in situations where the cross section for this effect is high. The model is referred to as the depletion-activation model and it provides the most realistic description for the daughter specific activity in neutron activation. Three specific neutron activation examples of interest to medical physics are presented: activation of molybdenum-98 into molybdenum-99 described by the saturation model; activation of cobalt-59 into cobalt-60 described by the depletion model; and activation of iridium-191 into iridium-192 described by the depletion-activation model.

Recent work has shown that there is significant uncertainty in measuring build-up doses in mega-voltage photon beams especially at high energies. In this present investigation we used a phantom-embedded extrapolation chamber (PEEC) made of Solid Water to validate Monte Carlo (MC)-calculated doses in the dose build-up region for 6 and 18 MV x-ray beams. The study showed that the percentage depth ionizations (PDIs) obtained from measurements are higher than the percentage depth doses (PDDs) obtained with Monte Carlo techniques. To validate the MC-calculated PDDs, the design of the PEEC was incorporated into the simulations. While the MC-calculated and measured PDIs in the dose build-up region agree with one another for the 6 MV beam, a non-negligible difference is observed for the 18 MV x-ray beam. A number of experiments and theoretical studies of various possible effects that could be the source of this discrepancy were performed. The contribution of contaminating neutrons and protons to the build-up dose region in the 18 MV x-ray beam is negligible. Moreover, the MC calculations using the XCOM photon cross-section database and the NIST bremsstrahlung differential cross section do not explain the discrepancy between the MC calculations and measurement in the dose build-up region for the 18 MV. A simple incorporation of triplet production events into the MC dose calculation increases the calculated doses in the build-up region but does not fully account for the discrepancy between measurement and calculations for the 18 MV x-ray beam.

Two recently introduced GafChromic film models, HS and XR-T, have been developed as more sensitive and uniform alternatives to GafChromic MD-55-2 film. The HS model has been specifically designed for measurement of absorbed dose in high-energy photon beams (above 1 MeV), while the XR-T model has been introduced for dose measurements of low energy (0.1 MeV) photons. The goal of this study is to compare the sensitometric curves and estimated dosimetric uncertainties associated with seven different GafChromic film dosimetry systems for the two new film models. The densitometers tested are: LKB Pharmacia UltroScan XL, Molecular Dynamics Personal Densitometer, Nuclear Associates Radiochromic Densitometer Model 37-443, Photoelectron Corporation CMR-604, Laser Pro 16, Vidar VXR-16, and AGFA Arcus II document scanner. Pieces of film were exposed to different doses in a dose range from 0.5 to 50 Gy using 6 MV photon beam. Functional forms for dose vs net optical density have been determined for each of the GafChromic film-dosimetry systems used in this comparison. Two sources of uncertainties in dose measurements, governed by the experimental measurement and calibration curve fit procedure, have been compared for the densitometers used. Among the densitometers tested, it is found that for the HS film type the uncertainty caused by the experimental measurement varies from 1% to 3% while the calibration fit uncertainty ranges from 2% to 4% for doses above 5 Gy. Corresponding uncertainties for XR-T film model are somewhat higher and range from 1% to 5% for experimental and from 2% to 7% for the fit uncertainty estimates. Notwithstanding the significant variations in sensitivity, the studied densitometers exhibit very similar precision for GafChromic film based dose measurements above 5 Gy.

A hybrid phantom-embedded extrapolation chamber (PEEC) made of Solid Water and bone-equivalent material was used for determining absorbed dose in a bone-equivalent phantom irradiated with clinical radiation beams (cobalt-60 gamma rays; 6 and 18 MV x rays; and 9 and 15 MeV electrons). The dose was determined with the Spencer-Attix cavity theory, using ionization gradient measurements and an indirect determination of the chamber air-mass through measurements of chamber capacitance. The collected charge was corrected for ionic recombination and diffusion in the chamber air volume following the standard two-voltage technique. Due to the hybrid chamber design, correction factors accounting for scatter deficit and electrode composition were determined and applied in the dose equation to obtain absorbed dose in bone for the equivalent homogeneous bone phantom. Correction factors for graphite electrodes were calculated with Monte Carlo techniques and the calculated results were verified through relative air cavity dose measurements for three different polarizing electrode materials: graphite, steel, and brass in conjunction with a graphite collecting electrode. Scatter deficit, due mainly to loss of lateral scatter in the hybrid chamber, reduces the dose to the air cavity in the hybrid PEEC in comparison with full bone PEEC by 0.7% to approximately 2% depending on beam quality and energy. In megavoltage photon and electron beams, graphite electrodes do not affect the dose measurement in the Solid Water PEEC but decrease the cavity dose by up to 5% in the bone-equivalent PEEC even for very thin graphite electrodes (0.0025 cm). In conjunction with appropriate correction factors determined with Monte Carlo techniques, the uncalibrated hybrid PEEC can be used for measuring absorbed dose in bone material to within 2% for high-energy photon and electron beams.

Relative to solid water, electron fluence correction factors at the depth of dose maximum in bone, lung, aluminum, and copper for nominal electron beam energies of 9 MeV and 15 MeV of the Clinac 18 accelerator have been determined experimentally and by Monte Carlo calculation. Thermoluminescent dosimeters were used to measure depth doses in these materials. The measured relative dose at dmax in the various materials versus that of solid water, when irradiated with the same number of monitor units, has been used to calculate the ratio of electron fluence for the various materials to that of solid water. The beams of the Clinac 18 were fully characterized using the EGS4/BEAM system. EGSnrc with the relativistic spin option turned on was used to optimize the primary electron energy at the exit window, and to calculate depth doses in the five phantom materials using the optimized phase-space data. Normalizing all depth doses to the dose maximum in solid water stopping power ratio corrected, measured depth doses and calculated depth doses differ by less than +/- 1% at the depth of dose maximum and by less than 4% elsewhere. Monte Carlo calculated ratios of doses in each material to dose in LiF were used to convert the TLD measurements at the dose maximum into dose at the center of the TLD in the phantom material. Fluence perturbation correction factors for a LiF TLD at the depth of dose maximum deduced from these calculations amount to less than 1% for 0.15 mm thick TLDs in low Z materials and are between 1% and 3% for TLDs in Al and Cu phantoms. Electron fluence ratios of the studied materials relative to solid water vary between 0.83+/-0.01 and 1.55+/-0.02 for materials varying in density from 0.27 g/cm3 (lung) to 8.96 g/cm3 (Cu). The difference in electron fluence ratios derived from measurements and calculations ranges from -1.6% to +0.2% at 9 MeV and from -1.9% to +0.2% at 15 MeV and is not significant at the 1sigma level. Excluding the data for Cu, electron fluence correction factors for open electron beams are approximately proportional to the electron density of the phantom material and only weakly dependent on electron beam energy.

In modern radiotherapy, three-dimensional conformal dose distributions are achieved through the delivery of beam ports having precalculated planar distributions of photon beam intensity. Although sophisticated means to calculate and deliver these spatially modulated beams have been developed, means to verify their actual delivery are relatively cumbersome, making equipment and treatment quality assurance difficult to enforce. An electronic portal imaging device of the scanning liquid ionization chamber type yields images which, once calibrated from a previously determined calibration curve, provide highly precise planar maps of the incident dose rate. For verification of an intensity-modulated beam delivered in the segmented approach with a multileaf collimator, a portal image is acquired for each subfield of the leaf sequence. Subsequent to their calibration, the images are multiplied by their respective associated monitor unit settings, and summed to produce a planar dose distribution at the measurement depth in phantom. The excellent agreement of our portal imager measurements with calculations of our treatment planning system and measurements with a one-dimensional beam profiler attests to the usefulness of this method for the planar verification of intensity-modulated fields produced in the segmented approach on a computerized linear accelerator equipped with a multileaf collimator.

Output of superficial and orthovoltage x-ray units may be measured with cylindrical or end-window parallel-plate ionization chambers. The air-kerma calibration factors for these chambers are usually determined free in air, and the x-ray machine output is stated as the air-kerma rate free in air, which, when multiplied with the appropriate backscatter factor, gives the air-kerma rate on the surface of a phantom or patient. For end-window chambers, especially when they are used for measurements of small fields or low x-ray energies, the air-kerma calibration factors may also be determined with the chamber embedded in a tissue-equivalent phantom. This results in field size dependent air-kerma in-air calibration factors but obviates the requirement for knowledge of back-scatter factors when determining the air-kerma rate on the surface of a phantom. Since there still is considerable uncertainty in tabulated backscatter factors as a function of field size and x-ray beam energy, the output measurement technique which determines the air-kerma rate on phantom surface with a phantom-embedded end-window ionization chamber offers a clear advantage over the in-air calibration method.

Objective:To assess the feasibility, toxicity, and local control of stereotactic radiosurgery followed by accelerated external beam radiotherapy (AEBR) for patients with glioblastoma multiforme.Materials and methods:Six males and eight females, with a median age of 67.5 years (range 45-78 years), entered the study. Karnofsky performance status was 90 for five, 80 for six, and 60 for three patients. Following surgery, the patients were left with a residual mass 4 cm. Radiosurgery was delivered with a single dose of 20 Gy to the 90% isodose surface corresponding to the contrast-enhancing edge of the tumour. A total AEBR dose of 60 Gy in 30 fractions was delivered using a concomitant boost technique over four weeks.Results:Median survival time was 40 weeks (range 17-80 weeks). Actuarial survivals at 12 and 18 months were 43% and 14%, respectively. The median time to progression was 25 weeks (range 2-77 weeks). One patient developed a seizure on the day of stereotactic radiosurgery. Two patients experienced somnolence at 47 and 67 days post-radiotherapy. Eight patients remained steroid-dependent. Radiological evidence of leukoencephalopathy was observed in one patient, and brain necrosis in two additional patients at 30 and 63 weeks. One of these two patients with brain necrosis developed complete loss of vision in one eye, and decreased vision in the contralateral eye at 63 weeks.Conclusion:Stereotactic radiosurgery followed by AEBR was feasible but was associated with late complications. The use of such radiosurgical boost for patients with glioblastoma multiforme should be reserved for those patients entering controlled clinical trials.

The characteristic angle-beta concept provides a simple semiempirical method for determination of dose distributions in electron arc therapy. Initially, the method required a set of measured radial depth dose distributions for each electron beam energy used for arc therapy. In this paper, we report an extension of the angle-beta concept that enables the determination of arc therapy depth doses for an arbitrary electron energy from the known set of depth dose data at a reference energy. Depth dose distributions of stationary and arc electron beams have been studied in the energy range from 9 to 18 MeV. The stationary electron beams used for electron arc therapy were collimated by photon collimators only, no secondary collimation was used in our experiments. For stationary electron beams and for arc electron beams with a given characteristic angle beta, the depths of dose maximum as well as the depths of a given percentage depth dose beyond the depth of dose maximum are linearly proportional to the mean incident electron energy. This simple geometrical and dosimetric relationship increases the potential usefulness of the angle-beta concept in clinical electron arc therapy.

Between December 1986 and June 1990, 112 patients (116 lesions), underwent treatment with dynamic stereotactic radiosurgery at McGill University. Of the treated lesions, 59 were arteriovenous malformations and 53 were a variety of other neoplastic or non-neoplastic conditions. In 86 lesions, the treatment was delivered in a single fraction and the treatment of the remaining 30 lesions was fractionated. Complications attributed to treatment developed in seven of the 112 patients (6.3%). No relationship was found between complications and prescribed dose, fractionation, collimator diameter, type and anatomical region of the lesion that was treated, or previous irradiation. Although extensive clinical experience will be necessary to determine optimal total doses, the potential role of fractionated treatment, and the tolerance of critical structures to radiosurgery, the relatively low incidence of complications in our series allows us to conclude that radiosurgery is well tolerated by the vast majority of patients.

A linac‐based radiosurgical technique is reported which produces cylindrical isodose distributions covering cylindrical targets of arbitrary orientations within the patient’s head. The technique uses rectangular collimators and 4 degrees of freedom: gantry and couch rotation, as defined by a previously known dynamic rotation technique, collimator rotation, and collimator length adjustment. The relationship between the four parameters is derived and because of its complexity, the cylindrical dynamic rotation technique is introduced as a pseudodynamic technique. For cylindrical targets, the cylindrical pseudodynamic technique in comparison to the standard spherical technique produces considerable dose savings to healthy tissue surrounding the target and gives a similar or better dose fall‐off outside the target.

The term radiosurgery has been used to describe a variety of radiotherapy techniques which deliver high doses of radiation to small, stereotactically defined intracranial targets in such a way that the dose fall-off outside the targeted volume is very sharp. Proton, charged particle, gamma unit, and linear accelerator-based techniques appear to be equivalent from the standpoint of accuracy, dose distributions, and clinical results. However, capital and operating costs associated with the use of linear accelerators in general clinical use are much lower. Radiosurgery has an established role in the treatment of arteriovenous malformations and acoustic neurinomas. Interest in these techniques is increasing in neurosurgical and radiation oncological communities, as radiosurgery is rapidly assuming a place in the management of several other conditions, including craniopharyngiomas, meningiomas, and selected malignant lesions.

Routine electron beamquality assurance requires an accurate, yet practical, method of energy characterization. Subtle shifts in beam energy may be produced by the linac bending magnet assembly, and the sensitivity of a commercially available electron beam energy-monitoring device for monitoring these small energy drifts has been evaluated. The device shows an 11% change in signal for a 2 mm change in the I 50 energy parameter for low energy electron beams (in the vicinity of 6 MeV) and a 2.5% change in signal for a 2 mm change in the I 50 energy parameter for high energy electron beams (in the vicinity of 22 MeV). Thus the device is capable of detecting small energy shifts resulting from bending magnet drift for all clinically relevant electron beams.

Radiotherapy with orthovoltage techniques is often the prime treatment for localized superficial malignancies. Surface doses and depth doses measured with cylindrical and end-window Farmer chambers are presented for various orthovoltage x-ray beams in the range from 80 to 300 kVp, both for open beams and beams collimated with commercial intracavitary leaded-glass cones. For radiation fields collimated by a diaphragm positioned at a distance from the patient surface (open beams) there is a small skin-sparing effect. On the other hand, the surface doses with commercial leaded-glass intracavitary cones can exhibit a fivefold increase compared to the open-beam dose maxima. Beyond a depth of approximately 0.2 mm in a tissue-equivalent phantom, the doses measured for open beams and beams collimated with intracavitary cones are essentially identical. The increase in the surface dose observed with intracavitary cones is attributed to photoelectrons and recoil electrons produced in the cones. The high surface doses are measured by thin-wall parallel-plate ionization chambers but cannot be measured with cylindrical Farmer chambers since these chambers have wall thicknesses too large for the transmission of electrons produced in the cone. Since cylindrical Farmer chambers are typically used for calibration of radiation output, the high surface doses produced by the intracavitary cones may be overlooked; they can, however, be reduced to open-beam values by simple modifications to the cones.

A PC‐based, three‐dimensional treatment planning system, which may be used for planning of radiosurgical treatments with the Gamma unit or with any of the radiosurgical techniques based on isocentric linear accelerators(linacs), is described and used to calculate isodose distributions for various linac‐based radiosurgical techniques ranging from the single plane rotation to a 4‐π geometry. The latter gives an isotropic dose falloff outside the target volume but cannot be used for practical radiosurgery, while the single plane rotation is simple to use but gives unacceptably shallow dose falloffs in the transverse plane. Dose falloffs for several other techniques of varying degrees of complexity are shown and discussed. Also discussed is the effect of beam energy and beam profiles on radiosurgical dose distributions.

Stereotactic radiosurgery is often used for treating functional disorders. For some of these disorders, the size of the target can be on the order of a millimeter and the radiation dose required for treatment on the order of 80 Gy. The very small radiation field and high prescribed dose present a difficult challenge in beam calibration, dose distribution calculation, and dose delivery. In this work the dose distribution for dynamic stereotactic radiosurgery, carried out with 1.5 and 3 mm circular fields, was studied. A 10 MV beam from a Clinac-18 linac (Varian, Palo Alto, CA) was used as the radiation source. The BEAM/EGS4 Monte Carlo code was used to model the treatment head of the machine along with the small-field collimators. The models were validated with the EGSnrc code, first through a calculation of percent depth doses (PDD) and dose profiles in a water phantom for the two small stationary circular beams and then through a comparison of the calculated with measured PDD and profile data. The three-dimensional (3-D) dose distributions for the dynamic rotation with the two small radiosurgical fields were calculated in a spherical water phantom using a modified version of the fast XVMC Monte Carlo code and the validated models of the machine. The dose distributions in a horizontal plane at the isocenter of the linac were measured with low-speed radiographic film. The maximum sizes of the Monte Carlo-calculated 50% isodose surfaces in this horizontal plane were 2.3 mm for the 1.5 mm diameter beam and 3.8 mm for the 3 mm diameter beam. The maximum discrepancies between the 50% isodose surface on the film and the 50% Monte Carlo-calculated isodose surfaces were 0.3 mm for both the 1.5 and 3 mm beams. In addition, the displacement of the delivered dose distributions with respect to the laser-defined isocenter of the machine was studied. The results showed that dynamic radiosurgery with very small beams has a potential for clinical use.

The annual linac workload is often required by regulatory agencies to assess compliance with license conditions. Summation of the monitor units produced by the machine is generally used for this purpose. Various methods of estimating this value have inherent inaccuracies. We have built an integrating Monitor Unit “odometer” that is able to automatically accumulate all MUs delivered by the linac and segregate the total by mode (photon or electron) and energy. The device has been used to record clinical linac MU workloads for 10 months, and was installed in a new dual-energy linac during the acceptance and commissioning process.

Saturation currents and collection efficiencies in ionization chambers exposed to pulsed megavoltage photon and electron beams are determined assuming a linear relationship between 1/I and 1/V in the extreme near-saturation region, with I and V the chamber current and polarizing voltage, respectively. Careful measurements of chamber current against polarizing voltage in the extreme near-saturation region reveal a current rising faster than that predicted by the linear relationship. This excess current combined with conventional "two-voltage" technique for determination of collection efficiency may result in an up to 0.7% overestimate of the saturation current for standard radiation field sizes of 10X10 cm2. The measured excess current is attributed to charge multiplication in the chamber air volume and to radiation-induced conductivity in the stem of the chamber (stem effect). These effects may be accounted for by an exponential term used in conjunction with Boag's equation for collection efficiency in pulsed beams. The semiempirical model follows the experimental data well and accounts for both the charge recombination as well as for the charge multiplication effects and the chamber stem effect.

To evaluate treatment outcome and morbidity of stereotactic external-beam irradiation (SEBI) in pediatric patients, we reviewed 14 children treated with SEBI, using a 10-MV isocentric linear accelerator at McGill University between 1988 and 1994. The median follow-up was 46 months (range 6–82 months). The median age was 14 years. There were 8 low-grade astrocytomas, 3 neuromas and 4 other histologies. Twelve patients received fractionated treatments. The median collimator diameter was 2.5 cm (range 1–5 cm). The median biological effective dose delivered to the entire tumor volume was 57 Gy for astrocytomas and 43 Gy for the other histologies. The overall actuarial survival rate and disease-free survival rate at 5 years were 83 and 62%, respectively. For the patients with low-grade astrocytomas, the 5-year survival and disease-free survival rates were 100 and 60%, respectively. Four children had recurrence at a median of 37 months. Four patients developed treatment-related complications: 1 had edema alone, 2 had necrosis and 1 had edema associated with necrosis. Neither the physical nor radiobiological parameters were predictive of the treatment outcome or the treatment complications. Stereotactic irradiation is a valid option for progressive nonresectable tumors in children.

The potential for radiosurgery with an isocentric teletherapy cobalt unit was evaluated in three areas: (1) the physical properties of radiosurgical beams, (2) the quality of radiosurgical dose distributions obtained with four to ten noncoplanar converging arcs, and (3) the accuracy with which the radiosurgical dose can be delivered. In each of these areas the cobalt unit provides a viable alternative to an isocentric linear accelerator (linac) as a radiation source for radiosurgery. A 10 MV x-ray beam from a linac used for radiosurgery served as a standard for comparison. The difference between the 80%-20% penumbras of stationary radiosurgical fields in the nominal diameter range from 10 to 40 mm of the cobalt-60 and 10 MV photon beams is remarkably small, with the cobalt-60 beam penumbras, on average, only about 0.7 mm larger than those of the linac beam. Differences between the cobalt-60 and 10 MV radiosurgical treatment plans in terms of dose homogeneity within the target volume, conformity of the prescribed isodose volume to the target volume, and dose falloffs outside the target volume are also minimal, and therefore of essentially no clinical significance. Moreover, measured isodose distributions for a radiosurgical procedure on our Theratron T-780 cobalt unit agreed with calculated distributions to within the +/- 1 mm spatial and +/- 5% numerical dose tolerances, which are generally specified for radiosurgery. The viability of isocentric cobalt units for radiosurgery will be of particular interest to centers in developing countries where cobalt units, because of their relatively low costs, provide the only megavoltage source of radiation for radiotherapy, and could easily and inexpensively be modified for radiosurgery. Of course, the quality assurance protocols and mechanical condition of a particular teletherapy cobalt unit must meet stringent requirements before the use of the unit for radiosurgery can be advocated.

The procedure recommended by radiation dosimetry protocols for determining the collection efficiency f of an ionization chamber assumes the predominance of general recombination and ignores other charge loss mechanisms such as initial recombination and ionic diffusion. For continuous radiation beams, general recombination theory predicts that f can be determined from a linear relationship between 1/Q and 1/V 2 in the near saturation region (f>0.7), where Q is the measured charge and V the applied chamber potential. Measurements with Farmer-type cylindrical ionization chambers exposed to cobalt-60 gamma rays reveal that the assumed linear relationship between 1/Q and 1/V 2 breaks down in the extreme near-saturation region (f>0.99) where Q increases with V at a rate exceeding the predictions of general recombination theory. A comprehensive model is developed to describe the saturation characteristics of ionization chambers. The model accounts for dosimetric charge loss (initial recombination, ionic diffusion, and general recombination) and nondosimetric charge multiplication in an ionization chamber, and suggests that charge multiplication plays a significant role under typical chamber operating conditions (300 V) used in radiation dosimetry. Through exclusion of charge multiplication from the measured chamber signal Q, the model predicts the breakdown of the 1/Q vs 1/V 2 relationship and shows that the final approach to saturation is governed by initial recombination and ionic diffusion which are characterized by a linear relationship between 1/Q and 1/V. Collection efficiencies calculated with this model differ by up to 0.4% from those determined through a rigorous application of general recombination theory alone.

A single phantom technique has been developed to verify the full CT simulation and treatment-beam delivery procedures. The phantom consists of a target delineated by thin copper strips, affixed to therapy verification film, and inserted securely between two slices of water-equivalent material. The target is defined with the aid of the copper strips, and the position of the isocenter and beam parameters such as field size, and gantry and collimator angle are determined by CT simulation. With these parameters, the phantom is subsequently irradiated by the linear accelerator in the treatment position. Correspondence between the planned and the irradiated region is determined by the position of the copper strips on the film. The technique is a simple and practical method for verifying the entire CT simulation and treatment-beam delivery processes, and provides a permanent record of the correspondence between the planning digitally reconstructed radiographs (DRR) and the actual beam delivered.

Purpose: During the past 25 years, 185 patients have been treated with a rotational total skin electron irradiation (RTSEI) technique in our center. To modernize the technique we recently transferred it from a linac (Varian Clinac‐18) with a custom modified beam line to a linac (Varian 21EX) with a commercially available electron mode intended for total skin electron irradiation. Methods and Materials: The new technique uses a “high dose rate” mode and a “high dose per Monitor Unit” mode in conjunction with a custom‐made flattening filter to produce a uniform beam at an extended SSD of 378 cm. The accessory tray holds the custom‐made flattening filter and automatically selects the beam energy (6 MeV) and high dose rate (888 MU/min) while moving the collimators to the maximum 40 × 40 cm2field size. Beam parameters are monitored using the record‐and‐verify (VARIS) system. Results: Reference dosimetry for the stationary and rotational electron fields was performed to allow delivery of the prescription dose using the linac's transmission ionization chamber. Patients are treated on a rotating platform with a high dose rate rotational electron beam having a z max at the skin surface, an R 50 at 15 mm and a bremsstrahlung contamination of the order of 3%. The nominal dose rate to water at z max(surface) for the rotational technique was determined to be 24.1 cGy/1000 MU, and beam delivery is monitored with a secondary Farmer‐type ionization chamber located near the patient in the treatment field. Conclusions:Treatment times with the rotational total skin electron irradiation technique at an SSD of 378 cm for a daily dose of 2.0 Gy are of the order of 9.5 minutes and to date we have treated 15 patients with this technique.

Purpose: Fractionated high‐dose rate endorectal brachytherapy has been developed in our institution as a pre‐operative down‐staging modality. Since the treatment is fractionated (26 Gy/4) it is essential to reproduce the treatment planningdose distribution on a daily basis. In this paper we present the Quality Assurance (QA) steps developed to ensure patient based daily dose reproducibility. Method and Materials: The applicator used has a cylindrical symmetry. In addition to the applicator auto‐radiographs used for the catheter physical length determination, there are two additional steps that have to be performed in order to reproduce a treatment planningdose distribution on a daily basis. Another correction has to be performed on a daily basis for rotation of the catheter channels. For this purpose, we use uniquely coded “dummy” source inserts that show themselves on a daily radiograph. The applicator is rotated and radiograph repeated until perfect alignment is achieved. Results: Since the applicator might not be placed to the same depth inside the rectal lumen, there is a shift along the catheter axis that has to be performed on a daily basis. The amount of shift is determined by comparison of a daily radiograph and treatment planningDRR.Conclusions: Reproduction of the treatment planningdose distribution on a daily basis is crucial for the success of the fractionated 3D based brachytherapy treatments. Due to the cylindrical symmetry, two types of adjustments are necessary: applicator rotation and dose distribution shift along the applicator axis.

A variable air-volume, parallel-plate extrapolation chamber forming an integral part of a Solid Water phantom was built to determine the absorbed dose in Solid Water directly. The sensitive air-volume of the extrapolation chamber is controlled through the movement of the chamber piston by means of a micrometer mounted to the phantom body. The relative displacement of the piston is monitored by a calibrated mechanical distance travel indicator with a precision on the order of 0.002 mm. Irradiations were carried out with cobalt-60 gamma rays, x-ray beams ranging from 4 to 18 MV, and electron beams between 6 and 22 MeV. The absorbed dose at a given depth in Solid Water is proportional to the ionization gradient measured in the Bragg-Gray cavity region with an extrapolation chamber embedded in the Solid Water phantom. The discrepancies between the doses determined in Solid Water with our uncalibrated extrapolation chamber and doses obtained with a calibrated standard thimble ionization chamber are at most 1% for photon and electron beams at all megavoltage clinical energies. Uncalibrated extrapolation chamber thus offer a simple and practical alternative to other techniques used in output measurements of megavoltage photon and electron machines.

The response of thermoluminescent dosimeters to electron irradiations depends on the radiation dose, mean electron energy at the position of the dosimeter in phantom, and the size of the dosimeter. In this paper the semi‐empirical expression proposed by Holt et al. [Phys. Med. Biol. 20, 559–570 (1975)] is combined with the calculated electron dose fraction to determine the thermoluminescent dosimetry (TLD) response as a function of the mean electron energy and the dosimeter size. The electron and photon dose fractions, defined as the relative contributions of electrons and bremsstrahlung photons to the total dose for a clinical electron beam, are calculated with Monte Carlo techniques using EGS4. Agreement between the calculated and measured TLD response is very good. We show that the considerable reduction in TLD response per unit dose at low electron energies, i.e., at large depths in phantom, is offset by an ever‐increasing relative contribution of bremsstrahlung photons to the total dose of clinical electron beams. This renders the TLD sufficiently reliable for dose measurements over the entire electron depth dose distribution despite the dependence of the TLD response on electron beam energy.

In electron beams, the dose in phantom under the central shielding depends on electron‐beam energy, depth in phantom, and shield area and thickness. In our experiments, all shield thicknesses were larger than the range of electrons in the shield material. At a given depth, the dose under the shield never exceeds the open field value; however, it can attain quite a large proportion of the open field value despite a shield thickness which exceeds the range of electrons in the shield material. The effects of shield area on the dose under the central shields were studied in detail and dose distributions are given as a function of shield lateral dimensions and electron‐beam energy. It is shown that in clinical use of central shielding, the best approach to dose estimation under the shield is direct measurement in phantom under conditions of the actual clinical setup.

Endocavitary rectal irradiation is a well established curative treatment for patients with selected early cases of adenocarcinoma of the rectum. At McGill we use a long source-skin distance (SSD) rectal irradiation technique as an alternative to the standard short SSD rectal irradiation technique originally described by Papillon. Eight patients, seven women and one man, were treated between April 1986 and November 1988. Median age was 72.5 years (range 46-86 years). Seven patients were treated with curative intent, and one for palliation. The mean follow-up period for all patients was 43 months, with a median of 54 months (range 3-65 months). Actuarial survival and disease-free survival were 86% and 72% at 65 months for the seven patients treated with curative intent. Treatments were well tolerated by all patients and only one patient developed localized proctitis that required steroid enemas, which resulted in complete resolution of symptoms. Our results are comparable with those in other reports in the literature. The complications were similar in type and frequency to other published series.

Stereotactic radiosurgery has become established as an effective treatment modality for certain non-malignant brain diseases such as arteriovenous malformations. This paper describes an extension of our linear accelerator-based radiosurgical technique to fractionated treatment of intracranial disease. The fractionated stereotactic radiotherapy technique expands the use of the modality by sparing normal cells within the treatment volume thus improving the therapeutic ratio. The first treatment is given using a stereotactic frame both for target localization and patient immobilization. The frame is then removed and subsequent treatments use a standard neurosurgical halo-ring for patient immobilization. The halo-ring is left in place on the skull for the duration of the course of treatment. Thus the physical requirements for fractionation pertain firstly to the patient immobilization and target localization using the halo-ring and secondly to the stringent quality assurance procedures required to maintain spatial accuracy under these new conditions. We describe a sensitive and effective technique for checking the rotational beam parameters and collimator alignment which we use immediately prior to treatment to ensure adequate accuracy of dose delivery to the target volume.

The depths of dose maxima of radiosurgical x-ray beams increase with the field diameter in the range from 10-30 mm. Furthermore, the degree of the increase is proportional to the photon beam energy. This behavior is in contrast to that of large radiotherapeutic fields, where the depths of dose maxima decrease with increasing field size. The surface doses, depths of dose maxima, and collimator scatter factors for radiosurgical beams with energies between 6 and 18 MV are presented. It is shown that the change in dmax for radiosurgical beams is a result of photon scatter in the phantom rather than in the collimator. Monte Carlo simulations of the radiosurgical beams are used to separate the total dose into two components: the primary dose and the scattered dose. Calculations indicate that buildup region characteristics are governed by the primary dose deposition in the medium. The scattered dose has a minimal effect on the total dose deposited in media by radiosurgical beams.

Between November 1986 and June 1990, 24 patients were treated with electron pseudoarc therapy at McGill University. There were 21 females and three males aged 27 to 81 years (median 62 years). 17 patients, nine of whom had received previous conventional locoregional irradiation, were treated palliatively for locally extensive breast carcinoma. Eight of these 17 patients achieved a complete, and six a partial, response to treatment; nine subsequently developed evidence of progressive disease within the treatment field after intervals of one to 27 months (median 5 months) following therapy. Eight patients developed moist desquamation of the treated chest wall, which was extensive in four; one of these patients developed chronic ulceration of the skin. The latter and one additional patient developed radiation pneumonitis. Seven patients were treated with radical intent, two following mastectomy for breast carcinoma, and one each for chest wall lymphoma, chest wall sarcoma, scalp angiosarcoma, scalp lymphoma and posterior cervical soft tissue sarcoma. Local control was achieved in six of these seven patients with minimal toxicity. Electron pseudoarc therapy is a treatment option for selected breast carcinoma patients for palliation of extensive chest wall disease, although morbidity may be considerable. The technique may, however, play a more useful role in other situations where the superficial portion of large curved surfaces is to be treated with curative intent.

Older bunkers designed for dual energy linacs were often constructed with both a maze and high energy door. The door provides both radiation safety and access control, however, it may be desirable to limit use of the high energy door to reduce mechanical wear and improve workflow. We conducted a feasibility study to determine if our high energy linac (Varian 21EX with electrons and 6/18 MV photons) located in a bunker with a maze and high energy door could be operated with the door closed for 18 MV beams only. Dose estimation using NCRP-151, followed by measurement with TLD and survey meters was carried out. Workload and use factors from literature, and from realistic data derived from our ARIA database were used. NCRP-151 calculations underestimated the measured value, and this discrepancy became worse for points further into the maze. For our workload of 7 000 Gy m2/year an equal gantry angle distribution suggests 1.0 mSv extra per year, while a realistic workload and use factor derived from our database suggests 0.1 mSv extra per year for the 6 MV door-open scenario. A door occupancy factor of 0.125 further reduces these estimates. The worst case In-Any-One-Hour Time Averaged Dose-Equivalent Rate was determined to be 0.84 mSv/hr. A Last-Person-Out system linked to a light guard in parallel with a energy-select logic system was bench tested. Adapting existing high energy bunkers designed with maze and door for operation with the door closed for high energy photon beams only is a viable option.

Shielding design calculations used in megavoltage radiation therapy essentially reduce to determination of barrier thicknesses. A shielding barrier serves to attenuate the radiationdose from a high level produced inside a therapy room to a lower “design goal” level at a point‐of‐interest outside. The calculation must account for both radiation physics (barrier attenuation) and for the various clinic‐specific parameters that serve to modulate the design goal and the dose that impinges on the barrier. While the physical data relating to attenuation of megavoltage beams in concrete and other barrier materials is readily available (eg in NCRP Report 151 or via the NIST website), clinic‐specific parameters must be determined by the clinical physicist. Such parameters include the workload W of the treatment machine, the use factor U of the barrier in question and the occupancy T at the point‐of‐interest. In this submission we show how data archived in a clinic's record‐and‐verify system can be used to compile realistic workload and use factors.

Purpose: Concept of image guided radiotherapy(IGRT) is on the forefront of current efforts to improve tumorcontrol by increasing dose to target volume and decreasing the dose to the nearby critical structures. Most IGRT methods handle the inter‐fractional target position uncertainties. However, intra‐fractional motion is either incorporated into the remaining PTV margin, or subsequently incorporated into forthcoming treatments based on inter‐treatment Cone Beam CT acquired images.Brachytherapy, by its nature, does not suffer from the intra‐treatment target motion uncertainties. Adding the image guidance to improve the inter‐treatment target uncertainties makes the Image Guided Brachytherapy (IGBT) one of the most sophisticated image guided radiation therapy modalities. The incorporation of a Cone Beam Simulator CT (CBSCT) in the brachytherapy suite opens the door to a step forward, toward a 3D IGBT. Method and Materials: We describe a 3D image guidance method using a CBSCT for fractionated HDR brachytherapy, based on 3D CT based treatment planning.Results: Before every treatment a Cone Beam CT(CBCT)image set is acquired with the applicator and/or catheters in its place and patient in treatment position. Planning CTimages and CBCTimages are imported into appropriate treatment planning software and co‐registration of the two image sets is performed based on previously inserted radio‐opaque clips and bony and reliable soft‐tissue landmarks. Conclusion: The full three‐dimensional patient anatomy data with respect to brachytherapy catheters prior to treatment provides additional information to further decrease uncertainty in daily target position with respect to delivered dose distribution during HDR brachytherapy treatments. Cone Beam Simulator CT may provide the ability to move from 2D to 3D Image Guided Brachytherapy.

Purpose: To study the feasibility of developing a new standard for absorbed dose to water based on watercalorimetry for high dose rate iridium‐192 ( 192 Ir ) brachytherapysources.Method and Materials: The heat conduction pattern generated in water by the Nucleotron microSelectron‐HDR 192 Ir brachytherapysource was simulated using Comsol Multiphysics™ software.Source self‐heating due to self‐attenuation of photons was calculated with GEANT4. A smooth, well‐behaved three‐dimensional function was fit to the entire dose distribution data using TableCurve3D™. The heat‐loss correction Kc was calculated as the ratio of the temperature in the calorimeter under ideal conditions to realistic conditions. Results: The feasibility of a watercalorimeter based absorbed dose standard is determined by a balance between the requirements to obtain sufficient signal to perform a reproducible measurement, the effects of heat loss on the measured signal, and the positioning uncertainties. Due to self‐absorption, the source equilibrium temperature was found to be above the ambient temperature by a constant amount that depends only on setup conditions and source activity. For the 192 Ir source inside its nylon‐12 catheter inserted into water, the steady state excess temperature per unit source activity was found to be 0.5671 K/Ci. The source temperature reached 96% of its steady state temperature after 60 s. Conduction correction factors Kc were calculated for several exposure times and at various measurement points away from the 192 Ir source inside the calorimeter. A total exposure time between 140 s and 240 s at a distance that receives a minimum of 1 Gy of dose was found to allow reduction of Kc to below 0.1% of unity. Conclusions:Watercalorimetry for 192 Ir HDR brachytherapy is feasible and total uncertainties of significantly better than 5% on the dose can be achieved with current watercalorimetry techniques and instruments.

We have transferred and modernized our 25‐year‐old rotational total skin electron irradiation (RTSEI) technique from a linac (Varian Clinac‐18) with a custom modified beam line to a linac (Varian 21EX) with a commercially available electron mode intended for total skin electron irradiation. The new technique uses a “high dose rate” mode and a “high dose per Monitor Unit” mode in conjunction with a custom‐made flattening filter to produce a uniform beam at an extended SSD of 378 cm. Reference dosimetry for the stationary and rotational electron fields was performed to allow delivery of the prescription dose using the linac's transmission ionization chamber. The accessory tray holds the custom‐made flattening filter and automatically selects the beam energy (6 MeV) and high dose rate (888 MU/min) while moving the collimators to the maximum 40 × 40 cm2field size. Beam parameters are monitored using the record‐and‐verify (VARIS) system. Patients are treated on a rotating platform with a high dose rate electron beam having a z max at the skin surface, an R 50 at 15 mm and a bremsstrahlung contamination of the order of 3%. The nominal dose rate to water at z max(surface) for the rotational technique was determined to be 24.1 cGy/1000 MU, and beam delivery is monitored with a secondary Farmer‐type ionization chamber located near the patient in the treatment field. Treatment times for a daily dose of 2.0 Gy are of the order of 9.5 minutes and to date we have treated 15 patients with this technique.