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The wall of an ionization chamber is commonly assumed to have a negligible effect on chamber response in electron beams. For cylindrical chambers with thin walls this assumption is valid. However, parallel‐plate chambers commonly possess large mechanical supports which may affect chamber response in a manner not accounted for in current dosimetry protocols. This is due to changes with energy in the relative backscattered electron fluence between chamber support and phantom materials. To investigate this effect, electron backscatter from low atomic number materials has been measured with electrons from 6 to 20 MeV. The effect of the diameter and thickness of the backscatteringmaterial has also been studied. Based on these data, Lucite and polystyrene chambers in water phantoms are expected to underrespond by 1% and 2% at 6 MeV. The expected underresponse decreases to 0.8% and 0.4% for polystyrene and Lucite at 12 MeV and is insignificant above 16 MeV. Two commercially available parallel‐plate chambers were compared with a cylindrical chamber in electron beams from 6 to 20 MeV. Using the 20‐MeV intercomparison, the expected chamber responses at the lower energies were calculated and compared with measurements. Both parallel‐plate chambers underresponded by approximately 1% at 6 MeV and 0.5% at 9 MeV which is qualitatively consistent with the electron backscatter data. Recommendations for minimizing electron backscatter effects through chamber design are discussed.

A comparison of the AAPM ‘‘Protocol for the determination of absorbed dose from high‐energy photon and electron beams’’ (TG21) with currently used protocols for electron and photondosimetry is presented. These protocols are the International Commission on Radiation Units and Measurements Report 21, ‘‘Radiation Dosimetry: Electrons with Initial Energies Between 1 and 50 MeV’’ (ICRU21), and the AAPM ‘‘Protocol for the Dosimetry of X‐ and Gamma Ray Beams with Maximum Energies Between 0.6 and 50 MeV’’ (SCRAD). Assuming a given radiation exposure and chamber parameters, doses to water at d max for electron beams and at 5 g/cm2 for photon beams are calculated using the three protocols and then compared. The doses for photon beams calculated using the TG21 and SCRAD protocols are found to differ by 3% or less at energies below 10 MeV. The largest differences occur in photon doses at high energies where the dose calculated with the TG21 protocol is as much as 5.5% greater than that calculated with the SCRAD protocol for a typical thimble ionization chamber. For low electron beam energies, the doses calculated with the ICRU21 protocol are as much as 5% less than TG21 doses when using thimble chambers constructed of tissue‐equivalent materials in a water phantom. If dosimetry measurements are performed in polystyrene, the dose calculated using TG21 may be greater than the ICRU21 dose, depending on chamber size and composition. An explanation for some of the differences between the protocols is presented emphasizing the dependence on chamber geometry, chamber composition, and phantom composition.

A breast applicator has been designed to optimize 3 field breast treatments for 60 Co. The device has a six half value layer beam splitting block constructed in two sections. The larger permanently mounted section is sufficient for treating 90% of the patients. Slots are available for mounting cerroband blocks, and any of five brass half field wedges. A magnetically attached front and back-pointer assembly readily breaks away in the event of a collision between pointer and patient. With this design the breast applicator with wedges and blocks has in-field surface doses reduced to that of an open field without accessory devices. The 50–90% dose decrement of the radiation penumbra for the half field block is comparable to that for the field edge of a typical 6 MV X ray unit, although the 10%–50 decrement is larger. The average out-of-field dose at the surface is 8% and is 5% at the depth of dose maximum. The combination for this applicator of sharp penumbra and low out-of-field dose leads to reduced lung and opposite breast doses. The latter was confirmed with TLD measurements on 10 patients, and yielded an average opposite breast dose of 230 cGy for a 4600 cGy prescription. Thus, half-field blocking devices do not preclude, as has been stated in the literature, acceptable opposite breast doses. In addition, proper design of these devices can significantly improve the radiation characteristics for primary breast treatments.

The applicability of quantitative information contained in CT scans to diagnostic radiology and to radiation therapy treatment planning and the heterogeneity problem has been recognized by members of the radiological community and by manufacturers. Determination of the relationship between electron density and CT number is important for these applications. As CT technology has evolved, CT number generation has changed. CT number variation was limited in the early water bag systems. However, later generation "air" scanners may exhibit variation in CT numbers across a reconstructed image which are re-lated to positioning within the scan circle and scan field size. Results of experimental investigations using tissue-equivalent phantoms of different cross-sectional shapes and areas on the Technicare Delta 2020 are presented. Investigations also cover the effect of "shaped" and "flat" x-ray beam filters. A variation in CT number is demonstrated on this fan beam geometry scanner for phantoms of different sizes and for different scan circle diameters. An explanation of these effects is given. Differences of as much as 20% in determination of tissue electron density relative to water under different experimental conditions are obtained and reported. A family of curves (electron density vs. CT number) is presented for different patient cross-sectional areas and different scanner settings.

A technique is described for the use of lymphoscintigraphy in treatment planning of primary breast patients. During simulation of the treatment fields, the positions of the internal mammary nodes are projected back toward the source onto the patient skin surface and are marked by radio-opaque markers for visualization on films. Exact solutions for the coordinates of these surface projection points are derived. Approximate solutions are also given which are independent of the isocenter location and primarily dependent on the treatment field gantry angle. If a typical couch angle and field size are assumed, the projection points can be calculated for various gantry angles prior to simulation. Generally, a decision can then be made beforehand whether it would be better to use deep tangents or a separate field to treat the internal mammary nodes. During simulation, the surface projection points serve as visual and fluoroscopic guides to field design and optimization. A method is also presented for projecting the internal mammary node positions onto a single transverse patient contour for conventional 2-dimensional treatment planning. By accurately showing the projected location of the node with respect to the field edge, adequate treatment margin can be assured.

Radiographs of a phantom containing excised breast tissue and idealized test objects embedded in tissue equivalent plastic have been obtained using a variety of mammographic film-screen detectors. A CGR Senographe (molybdenum target, beryllium window) was modified to accept a series of x-ray filters: 0.03 mm molybdenum, 0.5 mm aluminum or 1.2 mm aluminum. Radiographs were obtained at kVp's ranging from 26 to 32 kVp with the mAs adjusted to achieve the same optical density on the films. Entrance skin exposure was measured for the range of techniques from which dependence of mean gland dose on kVp was derived. Indices of image quality were also determined from the radiographs. A discussion of the effects of kVp variation on mammographic image quality, especially on low contrast discrimination is included.© (1983) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.

registration. We now produce these masks through image segmentation making the registration completely automatic. We investigate how the accuracy of the segmented masks affects the registration. Materials/Methods: The program was developed using the Insight Toolkit. Prostate patient CTs were segmented via a number of algorithms. By comparing generated to manual contours, Neighborhood Connected was chosen as the preferred segmentation algorithm. Due to the difficulty in segmenting the bladder from prostate, a new mask combining the bladder and prostate volume, was also produced. Three registration methods will therefore be compared: using manual rectum and bladder masks (previous standard), segmented rectum and bladder masks, and segmented rectum and combined bladder prostate masks. Agreement of manual contours of the prostate (which is not masked individually), in the registered images was used to infer registration accuracy. Results: The DC between the segmented and manual masks was up to 96 and 93% for the bladder and rectum respectively. This allowed the segmented mask guided deformable registration algorithm to achieve the same agreement in prostate contours as when manual masks are used, (both average 90% DC). When the combined bladder prostate-segmented mask is used the DC of the prostate contour improves to 95%, (the rectum and bladder contour DC remains the same). Conclusions: A now fully automated hybrid deformable registration method was developed for the prostate region capable of recovering large initial organ variation. Good agreement of segmented masks to the organ they represent is required to achieve acceptable registration results. Using a combined bladder prostate mask is the preferred method as it simplifies the segmentation process, while allowing for improved prostate registration. This automatic registration is used to produce contours and delivered dose for adaptive therapy and cumulative dose for outcome analysis. Author Disclosure: A.R. Godley: None. M. Itomlenskis: None.

Purpose: Hypofractionated prostate radiotherapy may benefit from both volumetric modulated arc therapy (VMAT) due to shortened treatment time and intrafraction real-time monitoring provided by implanted radiofrequency(RF) transponders. The authors investigate dosimetrically driven action thresholds (whether treatment needs to be interrupted and patient repositioned) in VMAT treatment with electromagnetic (EM) tracking. Methods: VMAT plans for five patients are generated for prescription doses of 32.5 and 42.5 Gy in five fractions. Planning target volume (PTV) encloses the clinical target volume (CTV) with a 3 mm margin at the prostate-rectal interface and 5 mm elsewhere. The VMAT delivery is modeled using 180 equi-spaced static beams. Intrafraction prostate motion is simulated in the plan by displacing the beam isocenter at each beam assuming rigid organ motion according to a previously recorded trajectory of the transponder centroid. The cumulative dose delivered in each fraction is summed over all beams. Two sets of 57 prostate motion trajectories were randomly selected to form a learning and a testing dataset. Dosimetric end points including CTV D95%, rectum wall D1cc, bladder wall D1cc, and urethra Dmax, are analyzed against motion characteristics including the maximum amplitude of the anterior-posterior (AP), superior-inferior (SI), and left-right components. Action thresholds are triggered whenmore » intrafraction motion causes any violations of dose constraints to target and organs at risk (OAR), so that treatment is interrupted and patient is repositioned. Results: Intrafraction motion has a little effect on CTV D95%, indicating PTV margins are adequate. Tight posterior and inferior action thresholds around 1 mm need to be set in a patient specific manner to spare organs at risk, especially when the prescription dose is 42.5 Gy. Advantages of setting patient specific action thresholds are to reduce false positive alarms by 25% when prescription dose is low, and increase the sensitivity of detecting dose limits violations by 30% when prescription dose is high, compared to a generic 2 mm action box. The sensitivity and specificity calculated from the testing dataset are consistent to the learning set, which indicates that the patient specific approach is reliable and reproducible within the scope of the prostate database. Conclusions: This work introduces a formalism for ensuring a VMAT delivery meets the most clinically important dose requirements by using patient specific and dosimetric-driven action thresholds to hold the beam and reposition the patient when necessary. Such methods can provide improved sensitivity and specificity compared to conventional methods, which assume directionally symmetric action thresholds.« less

registration. We now produce these masks through image segmentation making the registration completely automatic. We investigate how the accuracy of the segmented masks affects the registration. Materials/Methods: The program was developed using the Insight Toolkit. Prostate patient CTs were segmented via a number of algorithms. By comparing generated to manual contours, Neighborhood Connected was chosen as the preferred segmentation algorithm. Due to the difficulty in segmenting the bladder from prostate, a new mask combining the bladder and prostate volume, was also produced. Three registration methods will therefore be compared: using manual rectum and bladder masks (previous standard), segmented rectum and bladder masks, and segmented rectum and combined bladder prostate masks. Agreement of manual contours of the prostate (which is not masked individually), in the registered images was used to infer registration accuracy. Results: The DC between the segmented and manual masks was up to 96 and 93% for the bladder and rectum respectively. This allowed the segmented mask guided deformable registration algorithm to achieve the same agreement in prostate contours as when manual masks are used, (both average 90% DC). When the combined bladder prostate-segmented mask is used the DC of the prostate contour improves to 95%, (the rectum and bladder contour DC remains the same). Conclusions: A now fully automated hybrid deformable registration method was developed for the prostate region capable of recovering large initial organ variation. Good agreement of segmented masks to the organ they represent is required to achieve acceptable registration results. Using a combined bladder prostate mask is the preferred method as it simplifies the segmentation process, while allowing for improved prostate registration. This automatic registration is used to produce contours and delivered dose for adaptive therapy and cumulative dose for outcome analysis. Author Disclosure: A.R. Godley: None. M. Itomlenskis: None.

The purpose of this study was to compare two clinical immobilization systems for intracranial frameless stereotactic radiosurgery (fSRS) under the same clinical procedure using cone‐beam computed tomography (CBCT) for setup and video‐based optical surface imaging (OSI) for initial head alignment and intrafractional motion monitoring. A previously established fSRS procedure was applied using two intracranial immobilization systems: PinPoint system (head mold and mouthpiece) and Freedom system (head mold and open face mask). The CBCT was used for patient setup with four degrees of freedom (4DOF), while OSI was used for 6DOF alignment prior to CBCT, post‐CBCT setup verification at all treatment couch angles (zero and nonzero), and intrafractional motion monitoring. Quantitative comparison of the two systems includes residual head rotation, head restriction capacity, and patient setup time in 25 patients (29 lesions) using PinPoint and 8 patients (29 fractions) using Freedom. The maximum possible motion was assessed in nine volunteers with deliberate, forced movement in Freedom system. A consensus‐based comparison of patient comfort level and clinical ease of use is reported. Using OSI‐guided corrections, the maximum residual rotations in all directions were 1.1°±0.5° for PinPoint and 0.6°±0.3° for Freedom. The time spent performing rotation corrections was 5.0±4.1 min by moving the patient with PinPoint and 2.7±1.0 min by adjusting Freedom couch extension. After CBCT, the OSI–CBCT discrepancy due to different anatomic landmarks for alignment was 2.4±1.3 mm using PinPoint and 1.5±0.7 mm using Freedom. Similar results were obtained for setup verification at couch angles (