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Purpose: It is generally believed that “hot” spots should be avoided in radiotherapy because they lead to complications. Dose homogeneity within the target volume is much more difficult to achieve during brachytherapy than during external beam irradiation, and implants are rarely geometrically perfect. To not underdose some parts of the target volume, therefore, it may be necessary to accept hot spots in other parts of the target volumes, but it is not at all clear from the literature how much dose heterogeneity should be considered excessive. We undertook this study in an effort to determine just how high a dose to a hot spot is associated with clinically significant complications. Methods and Materials: We studied 40 patients treated by high-dose rate brachytherapy with or without external irradiation. For each patient, we calculated the minimum dose to the “hottest” 1 cubic centimeter (cc) volume (Dmax1) and, for 18 patients, the minimum dose to the hottest 10 cc volume (Dmax10) as well. Results: Considerable dose heterogeneity existed within the target volume. The Dmax1 ranged from 150–2000% (median 320%) of the minimum target dose (MTD). The median MTD/fraction was 2.50 Gy (range 1.50–25.00), and the median Dmax1/fraction was 10.00 Gy (range 3.75–150.00). The median Dmax1 from the entire course of brachytherapy was 75.00 Gy (range 25.00–550.00). Adding the doses from planned external irradiation, plus any prior irradiation to the same area, the median total Dmax1 was 112.50 Gy (range 30.00–580.00), yet the incidence of complications, even among those in the highest quartile of this dose range, was not greater than the lowest quartile. The total median Dmax10 was 85.00 Gy (range 32.00–130.00), but the incidence of complications was, again, similar whether the dose was in the lower or the upper half of this range (32.00–85.00 Gy, or 86.00–130.00 Gy, respectively). Conclusions: We had expected to find that the patients with the highest Dmax1 and/or Dmax10 would be the ones most likely to suffer complications, but the results did not support this hypothesis. Thus, dose heterogeneity, within the scope of our study, turned out to be rather unimportant with regard to complications. This finding contradicts the conventional wisdom and suggests that concerns about hot spots need not preclude optimization to ensure adequate dosage to all parts of the target volume.

Purpose: Microscopically positive or close margins after surgical resection results in an approximately 21-26% local failure rate despite excellent postoperative external radiation therapy. We sought to demonstrate improved local control in head and neck cancer patients who had a resection with curative intent, and had unexpected, microscopically positive or close surgical margins. Methods and Materials: Twenty-nine patients with microscopically close or positive margins after curative surgery were given definitive, adjuvant external radiation therapy and 125 I brachytherapy. All 29 patients had squamous cell cancer and tonsil was the most common subsite within the head and neck region. After external radiation therapy and thorough discussions with the attending surgeon and pathologists, the slides, gross specimens, and appropriate radiographs were reviewed and a target volume was determined. The target volume was the region of the margin in question and varied in size based on the surgery and pathologic results. Once the target volume was identified the patient was taken back to the operating room for insertion of 125 I seeds. Activity implanted (range 2.9-21.5 millicuries) was designed to administer a cumulative lifetime dose of 120-160 Gy. Results: Twenty-nine patients were followed for a median of 26 months (range 5-86 months). Two-year actuarial local control was 92%. Conclusion: 125 I, after external radiation therapy, is an excellent method to improve local control in the subset of patients with unexpectedly unsatisfactory margins.

Speakers in this session will present overview and details of a specific rotation or feature of their Medical Physics Residency Program that is particularly exceptional and noteworthy. The featured rotations include foundational topics executed with exceptional acumen and innovative educational rotations perhaps not commonly found in Medical Physics Residency Programs. A site-specific clinical rotation will be described, where the medical physics resident follows the physician and medical resident for two weeks into patient consultations, simulation sessions, target contouring sessions, planning meetings with dosimetry, patient follow up visits, and tumor boards, to gain insight into the thought processes of the radiation oncologist. An incident learning rotation will be described where the residents learns about and practices evaluating clinical errors and investigates process improvements for the clinic. The residency environment at a Canadian medical physics residency program will be described, where the training and interactions with radiation oncology residents is integrated. And the first month rotation will be described, where the medical physics resident rotates through the clinical areas including simulation, dosimetry, and treatment units, gaining an overview of the clinical flow and meeting all the clinical staff to begin the residency program. This session will be of particular interest to residency programs who are interested in adopting or adapting these curricular ideas into their programs and to residency candidates who want to learn about programs already employing innovative practices. Learning Objectives: 1.To learn about exceptional and innovative clinical rotations or program features within existing Medical Physics Residency Programs. 2.To understand how to adopt/adapt innovative curricular designs into your own Medical Physics Residency Program, if appropriate.

The purpose of this study is to establish protocols for simulation and external-beam radiotherapy of pituitary tumors and to examine the performance of a new 3-dimensional treatment planning system (3-D RTP). The asymmetric collimation feature of a dual-energy photon beam linear accelerator allows the user to set up half-beam treatment fields posterior to the orbits with no divergence to anterior structures. Treatment planning was performed with a relatively new Food and Drug Adminisration (FDA)-approved 3-D RTP system. Computerized tomographic (CT) scans used by this system to generate isodose distributions and dose-volume histograms were obtained directly from the scanner which is connected via ethernet cabling to the 3D treatment planning system. These were used for evaluating the dose distribution to the treatment volume, clinical target volume, gross tumor volume, and certain critical organs. Using 6 and 18 MV photon beams, different configurations of standard treatment techniques for pituitary tumors, i.e., a 190° flying wedge arc, 3-fields (an anterior plus a lateral wedged pair), and a pair of lateral fields, were studied and the resulting dose distributions were analyzed. Real-time dose measurements on patients using diode dosimetry were made and compared with computed dose values. With regard to minimizing radiation dose to surrounding structures, (i.e., lens, temporal lobes, spinal cord, etc.) the 190° flying wedge arc technique gave the best isodose distributions, followed by 3 fields (an anterior plus a lateral wedged pair). The opposing lateral fields gave the highest integral dose to the temporal lobes. The dose measured on the patient during the treatment agrees to within ±2% with the computed dose. The protocols presented in this work for simulation, immobilization, and treatment planning of patients with pituitary tumors provide the optimum dose distributions in the target volume with reduced irradiation of surrounding non-target tissues, and can be easily implemented in a radiation oncology department. The presence of a real-time dose-measuring system plays an important role in verifying the actual delivery of radiation dose. © 1995 Wiley-Liss, Inc.

Purpose : To analyze the effect that immobilization devices used in conjunction with beam modifiers may have on the dose to the skin and build-up region. Methods and Materials : Central axis depth dose measurements were made in a polystyrene phantom in the build-up regions using the 6 and 15 MV photon beams, at two different source-to-phantom distances, and various field sizes. The effects of acrylic blocking trays, lead wedges, and cerrobend blocks were assessed in conjunction with the enhancement of dose in the build-up region due to immobilizing devices using plaster and thermoplastic casting materials of different thicknesses. Results : For the 6 MV photons, solid (3 mm) thermoplastic casting material was found to have the greatest effect on surface dose: for a 12 × 12 cm field we measured 79% of maximum dose when treating through the material versus 22% of maximum dose when no beam modifiers or immobilization devices are used. Measurements were also made to evaluate the effect of the immobilization of patients receiving three-dimensional conformal treatments using a 15 MV photon beam. Conclusions : The relevance of these results to treatments in the pelvis, breast, and head and neck regions is discussed. For 6 MV beams, special consideration should be given if the need arises to treat through the immobilization device, as unacceptable skin reactions may result.

Purpose : The specific aim of 3-dimensional conformal radiation therapy is to improve the target dose distribution while concomitantly reducing normal tissue dose. Such an approach should permit dose escalation until the limits of acceptable normal tissue toxicity are reached. To evaluate the feasibility of tumor dose escalation for nine patients with lung cancer, we determined the dose distribution to the target and normal tissues with 3-dimensional conformal radiation therapy and conventional planning. Methods and Materials : Plans were compared to assess adequacy of dose delivery to target volumes, dose-volume histograms for normal tissue, and normal tissue complication probabilities (NTCP) for nine patients with lung tumors. Results : The mean percentage of gross disease which received ≤ 70.2 Gy with 3-dimensional conformal radiation therapy (31)CRT) was 40% of the mean percentage of gross disease which received ≤ 70.2 Gy with conventional treatment planning (CTP). The mean NTCP for lung parenchyma with 3DCRT was 36% of the mean NTCP with CTP. The mean esophageal NTCP with 3DCRT was 88% of the mean NTCP with CTP. Conclusion : This preliminary analysis suggests that three dimensional conformal radiation therapy may provide superior delivery of high dose radiation with reduced risk to normal tissue, suggesting that this approach may have the potential to improve the therapeutic ratio of high dose radiation therapy for lung cancer.

Two-dimensional relative dose distributions have been measured around 125I brachytherapy seeds. The two seed models studied, models 6711 and 6712, were manufactured by the 3M Company. Silicon detectors immersed in water phantoms were used to measure the dose. A computerized data acquisition system that controlled the radial position of the diode and the angular rotation of the seed, as well as a manually controlled system were used to collect and store the data. Our results show that the two seed models have relative dose distributions which are quite similar; however, the absolute dose distributions are sufficiently different to warrant separate look-up tables for the two seed models. Additionally, our results are compared with dose distribution data previously obtained for the model 6711 seed.

This paper describes the beam characteristics and dosimetry measurements performed on the 6-MV photon beam of a new model of linear accelerator, three of which were recently introduced and installed in our institution. Percent depth dose and tissue maximum ratio tables for a variety of field sizes and depths, as well as other parameters used for treatment planning are presented. These accelerators are the first of their kind using both hardware and software tools to control interlocks. Checking procedures for these interlocks are available from the authors upon request. Comparison of characteristic parameters between these three new 6-MV linear accelerators and with the 6-MV beams of two other accelerators is also made.

Task Group 6 of the Diagnostic X-Ray Imaging Committee of the American Association of Physicists in Medicine (AAPM) was appointed to develop performance standards for diagnostic x-ray exposure meters. The recommendations as approved by the Diagnostic X-Ray Imaging Committee and the Science Council of the AAPM are delineated in this report and provide specifications on meter precision, calibration accuracy, calibration reference points, linearity, energy dependence, exposure rate dependence, leakage, amplification gain settings, directional dependence, the stem effect, constancy checks, and calibration intervals. The report summarizes recommendations for meters used in mammography, general purpose radiography including special procedures, computed tomography, and radiation safety surveys for x-ray radiography.

Purpose: Electron beam radiotherapy is frequently administered based on clinical setups without formal treatment planning. We felt, therefore, that it was important to monitor electron beam treatments by in vivo dosimetry to prevent errors in treatment delivery. In this study, we present our clinical experience with patient dose verification using electron diodes and quantitatively assess the dose perturbations caused by the diodes during electron beam radiotherapy. Methods and Materials: A commercial diode dosimeter was used for the in vivo dose measurements. During patient dosimetry, the patients were set up as usual by the therapists. Before treatment, a diode was placed on the patient's skin surface and secured with hypoallergenic tape. The patient was then treated and the diode response registered and stored in the patient radiotherapy system database via our in-house software. A customized patient in vivo dosimetry report showing patient details, expected and measured dose, and percent difference was then generated and printed for analysis and record keeping. We studied the perturbation of electron beams by diodes using film dosimetry. Beam profiles at the 90% prescription isodose depths were obtained with and without the diode on the beam central axis, for 6–20 MeV electron beams and applicator/insert sizes ranging from a 3-cm diameter circular field to a 25 × 25 cm open field. Results: In vivo dose measurements on 360 patients resulted in the following ranges of deviations from the expected dose at the various anatomic sites: Breast (222 patients) −20.3 to +23.5% (median deviation 0%); Head and Neck (63 patients) −21.5 to +14.8% (median −0.7%); Other sites (75 patients) −17.6 to +18.8% (median +0.5%). Routine diode dosimetry during the first treatment on 360 patients (460 treatment sites) resulted in 11.5% of the measurements outside our acceptable ±6% dose deviation window. Only 3.7% of the total measurements were outside ±10% dose deviation. Detailed investigations revealed that the dose discrepancies, overwhelmingly, were due to inaccurate diode orientation and positioning, especially in the regions with rapidly changing contours and/or sloping surfaces. The presence of a diode in the treatment field was found, in some cases, to cause significant dose reduction, most noticeable with smaller fields and lower energy beams. The reduction in dose ranged from 16% (for a 6 MeV beam and a 3 cm diameter circular field) to 4% (for a 12 MeV beam and a 10× 10 cm field). Conclusions: Diode dosimetry was found to be convenient and valuable for verifying in real time the dose delivery accuracy of electron beam treatments, but with some caveats. When treating a small field by low energy electrons, frequent use of diodes is undesirable, because it might result in appreciable reduction of dose to the target volume.

To compare different treatment techniques for unilateral treatment of parotid gland tumors.The CT-scans of a representative parotid patient were used. The field size was 9 x 11 cm, the separation was 15.5 cm, and the prescription depth was 4.5 cm. Using 3D dose distributions, tissue inhomogeneity corrections, scatter integration (for photons) and pencil beam (for electrons) algorithms and dose-volume histogram (DVH), nine treatment techniques were compared. [1] unilateral 6 MV photons [2] unilateral 12 MeV electrons [3] unilateral 16 MeV electrons [4] an ipsilateral wedge pair technique using 6 MV photons [5] a 3-field AP (wedged), PA (wedged) and lateral portal technique using 6 MV photons [6] a mixed beam technique using 6 MV photons and 12 MeV electrons (1:4 weighting) [7] a mixed beam technique using 6 MV photons and 16 MeV electrons (1:4 weighting) [8] a mixed beam technique using 18 MV photons and 20 MeV electrons (2:3 weighting) [9] a mixed beam technique using 18 MV photons and 20 MeV electrons (1:1 weighting).Using dose-volume histograms to evaluate the dose to the contralateral parotid gland, the percentage of contralateral parotid volume receivingor = 30% of the prescribed dose was 100% for techniques [1], [8] and [9], and5% for techniques [2] through [7]. Evaluating the "hottest" 5 cc of the ipsilateral mandible and temporal lobes, the hot spots were: 152% and 150% for technique [2], 132% and 130% for technique [6]. Comparing the exit doses, techniques [1], [8] and [9] contributed toor = 50% of the prescribed dose to the contralateral mandible and the temporal lobes. Only techniques [2] and [6] kept the highest point doses to both the brain stem and the spinal cord below 50% of the prescribed dose.The single photon lateral field [1] and the mixed electron-photon beams [8] and [9] are not recommended treatment techniques for unilateral parotid irradiation because of high doses delivered to the contralateral parotid gland and high exit doses which are associated with Xerostomia. The en face electron beam technique [2] and the mixed electron-photon beam technique [6] are unacceptable due to the excessive dose heterogeneity to the contiguous normal structures. In spite of optimal dose fall-off achieved using the en face technique [3], most patients cannot tolerate the resulting high skin doses. We conclude that the ipsilateral wedge pair [4], the 3-field [5], and the mixed electron-photon beam [7] techniques are optimal techniques in providing relatively homogeneous dose distributions within the target area and for minimizing dose to the relevant normal structures.

Introduction: Our task is to present information on the status of the Medical Physics practice in the different countries in Latin America. Because of its fundamental value, we would like to focus on the status of education and professional medical physics issues in the different countries. Purpose: The IAEA is interested in promoting harmonization of Medical Physics practice worldwide. As a contribution, we must first learn about the diverse status of the medical physics practice, possibilities and needs in the different countries of Latin America. Methods and Materials: The LAASC maintains a close communication with all (or most) Latin American Countries through the valuable contributions of our Latin American Liaisons and Consultants. We also rely heavily on the information and cooperation with ALFIM (The umbrella association of all Medical Physics Associations of the different countries in Latin America). This presentation is based on their contributions. Results: There is a large diversity in the status of Medical Physics practice in the different countries of Latin America. As expected, this discrepancy is mostly due to the different economy status in Latin American countries. However, in many instances the economic deficiencies are somehow compensated by the great creativity of medical physicists in the region. A large credit must be given to the extensive work of the IAEA and IOMP in the education and practice of the profession of medical physics in different countries of Latin America. Conclusions: LAASC always places emphasis on divulging to the Latin America medical physicists the professional and educational opportunities as well as the medical physics literature available through the AAPM website. Through the realization of ISEP workshops it has also been possible to exchange scientific information on the state of the art of the practice of medical physics, and also to divulge AAPM programs available to countries in development, such as PIP, International Affiliate, access to Medical Physics Journal and AAPM protocols, to name a few. We believe that through cooperation and work together with ALFIM, the AAPM could contribute to make a sustainable impact to the medical physics profession in Latin American Countries. Learning Objectives: 1. Understand the fundamental problems associated with the different medical physics practice levels encountered in Latin America 2. To become familiar with on‐going work in developing countries in Latin America in relation to medical physics education and professional recognition. 3. Learn how the AAPM could make a sustainable impact.

The Symposium is jointly sponsored by IOMP and IAEA with the International Affairs Committee of AAPM. So much is happening in large part of the world particularly when it comes to medical use of radiation and assessment of radiation doses to patients in over 50 developing countries, promoting harmonization of Medical Physics practice worldwide, and comparing doses with international standards and managing doses. In recent years many papers have been published in peer reviewed journals. Several premier international organizations have produced or are preparing publications impacting on the practice of medical physics. The momentum generated creates opportunities for collaboration and cooperation between medical physicists from developing countries and with those in America and Europe. AAPM members can contribute effectively in this very stimulating area of work. You can be part of the exercise of making a difference in the world, in making patients in need of diagnostic and therapeutic procedures safer and in improving the knowledge of colleagues in developing countries. Learning Objectives: 1. To learn about the MP practice in west Europe, in Asia, and in Latin America. 2. To learn about the MP practice in developing countries. 3. To become familiar with on‐going work in over 50 developing countries in Africa, Asia, Eastern Europe and Latin America on radiation protection of patients. 4. To understand the mechanism of cooperation and collaboration. 5. To consider development of new projects.

The purpose of this work is to improve the quality of patient care in radiation therapy by implementing a comprehensive quality assurance (QA) program aiming to enhance patient in vivo dosimetry on a routine basis. The characteristics of two commercially available semi-conductor diode dosimetry systems were evaluated. The diodes were calibrated relative to an ionization chamber-electrometer system with calibrations traceable to the National Institute of Standards and Technology (NIST). Correction factors of clinical relevance were quantified to convert the diode readings into patient dose. The results of dose measurements on 6 patients undergoing external beam radiation therapy for carcinoma of the prostate on three different therapy units are presented. Field shaping during treatments was accomplished either by multileaf collimation or by cerrobend blocking. A deviation of less than ±4% between the measured and prescribed patient doses was observed. The results indicate that the diodes exhibit excellent linearity, dose reproducibility, minimal anisotropy, and can be used with confidence for patient dose verification. Furthermore, diodes render real time verification of dose delivered to patients.

Purpose : To study the attenuation and scattering effects of ocular implants, made from different materials, on the dose distributions of a 6 MV photon beam, and 6, 9, and 12 MeV electron beams used in orbital radiotherapy. Methods and Materials : Central axis depth-dose measurements were performed in a polystyrene phantom with embedded spherical ocular implants using film dosimetry of a 6 MV photon beam and electron beams of 6, 9, and 12 MeV energy. The isodose distributions were also calculated by a computerized treatment planning system using computerized tomography (CT) scans of a polystyrene phantom that had silicone, acrylic, and hydroxyapatite ocular implants placed into it. Results : Electron beam dose distributions display distortions both on the measured and calculated data. This effect is most accentuated for the hydroxyapatite implants, for which the transmissions through ocular implants are on the order of 93% for the 6 MV photon beam, and range from 60% for 6 MeV electrons to 90% for 12 MeV electrons. Conclusion : We studied the effect of ocular implants of various materials, embedded in a polystyrene phantom, on the dose distributions of a 6 MV photon beam, and 6, 9, and 12 MeV electron beams. Our investigations show that while 6 MV photons experience only a few percent attenuation, lower energy electron beam with 60% transmission is not a suitable choice of treating tumors behind the ocular implants.

The relative dose distribution around the 103Pd model 200 implant seed was measured with a computerized data acquisition system employing a p-n junction silicon diode immersed in a water phantom. Data are acquired in polar coordinates by computer control of (1) the diode distance from the seed center and (2) the rotation angle of seed about a transverse axis. Transverse axis data are compared with thermoluminescent dosimeter (TLD) measurements and a Monte Carlo calculation by others.

Knowledge of the relative intensity distribution at the patient’s surface is essential for pencil beam calculations of three‐dimensional dose distributions for arbitrarily shaped electron beams. To calculate the relative intensity distribution, the spatial spread resulting from angular spread is convolved with a two‐dimensional step function whose shape corresponds to the applicator aperture. Two different approaches to obtain angular spread or the equivalent spatial spread are investigated. In the first method, the pencil beam angular spread is assumed to be Gaussian in shape. The angular spread constants (σθ ) are then obtained from the slopes of measured intensity profiles. In the second method, the angular spread, in the form of an array of numerical values, is obtained by the deconvolution of measured intensity profiles. After obtaining the angular spread, the calculation for convolution is done in a number of parallel planes normal to the central axis at various distances from the electron collimator. Intensity at any arbitrary point in space is computed by interpolating between intensity distributions in adjacent planes on either side of the point. The effects of variations in angular spread as a function of field size for two treatment machines, one with a scanned electron beam and the other with a scattering foil, have been studied. The consequences of assuming angular spread to be of Gaussian shape are also examined. The electron intensity calculation techniques described in this paper apply primarily to methods of dose calculations that employ pencil beams generated using Monte Carlo simulations.

It is difficult to make an accurate calculation of dose distribution incorporating blocks using a ray model. One approach is to simulate the blocking in a treatment planning distribution by using negatively weighted beams. A second is to employ an external contour. The parameters of the negative beam or contour can be adjusted using empirical dosimetric data. This paper discusses the calculation of the dose distributions using negatively weighted beams and external contours, compares them with measurements in and around blocked areas for a range of field sizes, block sizes, and depths of interest in treatment planning applications, for 60Co, 6 MV, and 10 MV beams, and assesses their applicability.

Asymmetric fields defined by independent jaws can be used to split a beam or to match adjacent fields. We have extended a method originally developed for symmetric fields to calculate the dose for asymmetric fields. The dose to a point is computed as the product of the tissue maximum ratio (TMR), the off center ratio (OCR), and the inverse square factor. The TMR is computed from the measured central axis depth doses for symmetric fields. The OCR is obtained by multiplying the primary OCR (POCR) and the boundary factors (BF's) for the four jaws. The POCR's and BF's were derived from measured beam profiles, which include the effect of off-axis beam quality variations. Using this method, the beam profiles and isodose distributions for asymmetric fields of a 6-MV accelerator were calculated and compared with the measured data. The agreement is within experimental errors both in the penumbra region and along the central ray of the asymmetric field.