Purpose: To develop a clinically useful forward planning algorithm for determining the energy, shape, and weight of multiple abutted fields used in segmented‐field electron conformal therapy (SFECT) that conforms the 90% isodose to the PTV while minimizing the treatment of normal tissue. Method and Materials: An algorithm was developed that requires a beam's eye view (BEV) of depth of the distal PTV surface. First, the minimum energy for which R90 (10‐cm diameter field) exceeds the depth to the distal edge of the PTV is determined for each pixel of the BEV. Next, the minimum circular field size to treat R90 to the distal edge of the PTV is determined for each pixel and drawn around the pixel. Fields of the same energy are merged with higher energy fields taking precedence in areas of overlap, and the dose distribution is calculated. Finally, an iterative process is used to modify the fields to converge the 90% dose surface to the distal PTV surface. The algorithm was used on six hypothetical PTVs, and the dose distribution was compared to that of a single energy plan. A pencil‐beam algorithm calculated the dose distribution for both sets of plans. Results: The SFECT plans were able to deliver 90% of the dose to over 95% of the PTV, while irradiating an average of 16% less non‐PTV to 90% of the dose. Due to abutment dosimetry, SFECT plans increased dose range within the PTV by an average of 4.9%. Conclusion: The developed algorithm was useful for determining SFECT plans in a water phantom. SFECT plans irradiate significantly less non‐PTV volume than single energy therapy, but at the expense of increased dose range within the PTV (D90−10). Conflict of Interest: This work was funded in part by a research grant with Varian Medical Systems, Inc.

Purpose: To determine the impact of Cerenkov light on radiographic film response and to recommend a methodology for correcting absolute dose measurements when using bare film, but calibrated using prepackaged film. Method and Materials: The Gammex RMI Film Dosimetry Cassette Model 436‐AST (Solid Water) and two in‐house cassettes (white opaque, high‐impact polystyrene) were studied using Kodak XV or EDR2 film. Films were exposed perpendicular to 16‐MeV electron (15×15 cm2) or 6 MV x‐ray (20×20 cm2) beams. Films were oriented such that quadrant ♯1 had bare film; quadrant ♯2 had film covered by the prepackaged white paper; quadrant ♯3 had film in its prepackaged container; and quadrant ♯4 had film covered by the prepackaged carbon jacket. To account for beam asymmetry, dose response for each quadrant was normalized to that in the corresponding quadrant of a film irradiated in the carbon jacket, which blocked phantom‐produced Cerenkov light. A prepackaged film, irradiated using a multi‐exposure technique, provided the dose‐response calibration. Results: The “carbon jacket only” dose values averaged 96.1% of the “prepackaged” dose values, indicating that the prepackaged white paper produced Cerenkov light that increased film response by 4.0%. No significant difference due to radiation modality or film type was evident. The “white paper only” dose values ranged from 103.6–107.5% of the “prepackaged” dose values, indicating that Cerenkov light from the phantom material contributed to an increased film response. For white opaque, high‐impact polystyrene the “bare film” dose values ranged from 102.2–109.6% of the “prepackaged” dose values, depending on phantom and modality. For Solid Water the “bare film” dose value was 117.3% of the “prepackaged” dose values. Conclusion: When making absolute dose measurements using bare film and calibrating using prepackaged film, a correction for excess film response arising from Cerenkov light is required, and the reported quadrant method is recommended.

Purpose: To measure sample planes of two 3D dose distributions that can be used to evaluate the accuracy of planning and delivery of bolus electron conformal therapy (ECT). Method: The planning target volumes (PTV) from a parotid and a chest wall patient were modified to a CT image set of a cylindrical polystyrene phantom. A bolus ECT plan for each PTV was developed using .decimal p.d software to design a custom bolus and dose was calculated using Pinnacle. Once acceptable, .decimal fabricated the bolus from machinable wax. Approximately 45 cGy given dose was delivered for each bolus ECT plan, and planar dose measurements were taken in the mid‐sagittal and five transverse phantom planes (±5cm, ±2.5cm, 0cm) using Kodak XV radiographic film sealed within the in‐phantom cassettes. Films were developed and digitized to (0.5mm pixels) using a Vidar DosimetryPRO Advantage, and converted to dose using a film calibration curve created at the time of measurement. Four film measurements were taken in each plane, with the mean dose and standard error being determined at each pixel. The precision of the data was quantified in the low gradient regions (Dose > 90% and Dose <10%) by the mean percent standard error of all pixels and in the high gradient region (90% > Dose > 10%) by the mean distance to dose agreement. Results: The percent average standard errors in the low gradient, high dose region were less than 1.5% for parotid and chest wall measurements and less than 3% in the low gradient, low dose regions. Average distances to agreement in the high gradient region were less than 0.6 mm. Conclusions: For two PTVs bolus, ECT dose distributions in 6 planes were measured and their precision quantified for three dose regions. These data are well‐suited for comparison with calculated electron dose distributions. This work was supported in part by a research grant from .decimal, Inc.

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Purpose: To compare IMRT QA pass rates before and after an in‐house MLC leaf calibration procedure. Methods: The MLC leaves and backup jaws on four Elekta linear accelerators with MLCi2 heads were calibrated using the EPID‐based RIT Hancock Test as the means for evaluation. The MLCs were considered to be successfully calibrated when they could pass the Hancock Test with criteria of 1 mm jaw position tolerance, and 1 mm leaf position tolerance. IMRT QA results were collected pre‐ and postcalibration and analyzed using gamma analysis with 3%/3mm DTA criteria. AAPM TG‐119 test plans were also compared pre‐ and post‐calibration, at both 2%/2mm DTA and 3%/3mm DTA. Results: A weighted average was performed on the results for all four linear accelerators. The pre‐calibration IMRT QA pass rate was 98.3 ± 0.1%, compared with the post‐calibration pass rate of 98.5 ± 0.1%. The TG‐119 test plan results showed more of an improvement, particularly at the 2%/2mm criteria. The averaged results were 89.1% pre and 96.1% post for the C‐shape plan, 94.8% pre and 97.1% post for the multi‐target plan, 98.6% pre and 99.7% post for the prostate plan, 94.7% pre and 94.8% post for the head/neck plan. Conclusion: The patient QA results did not show statistically significant improvement at the 3%/3mm DTA criteria after the MLC calibration procedure. However, the TG‐119 test cases did show significant improvement at the 2%/2mm level.