Purpose End‐to‐end testing with quality assurance (QA) phantoms for deformable dose accumulation and real‐time image‐guided radiotherapy (IGRT) has recently been recommended by American Association of Physicists in Medicine (AAPM) Task Groups 132 and 76. The goal of this work was to develop a deformable abdominal phantom containing a deformable three‐dimensional dosimeter that could provide robust testing of these systems. Methods The deformable abdominal phantom was fabricated from polyvinyl chloride plastisol and phantom motion was simulated with a programmable motion stage and plunger. A deformable normoxic polyacrylamide gel (nPAG) dosimeter was incorporated into the phantom apparatus to represent a liver tumor. Dosimeter data were acquired using magnetic resonance imaging (MRI). Static measurements were compared to planned dose distributions. Static and dynamic deformations were used to simulate inter‐ and intrafractional motion in the phantom and measurements were compared to baseline measurements. Results The statically irradiated dosimeters matched the planned dose distribution with an average γ pass rates of 97.0 ± 0.5% and 97.5 ± 0.2% for 3%/5 mm and 5%/5 mm criteria, respectively. Static deformations caused measured dose distribution shifts toward the phantom plunger. During the dynamic deformation experiment, the dosimeter that utilized beam gating showed an improvement in the γ pass rate compared to the dosimeter that did not. Conclusions A deformable abdominal phantom apparatus which incorporates a deformable nPAG dosimeter was developed to test real‐time IGRT systems and deformable dose accumulation algorithms. This apparatus was used to benchmark simple static irradiations in which it was found that measurements match well to the planned distributions. Deformable dose accumulation could be tested by directly measuring the shifts and blurring of the target dose due to interfractional organ deformation and motion. Dosimetric improvements were achieved from the motion management during intrafractional motion.

The linac latency associated with triggering the beam-on/off was determined to have a minor contribution to the total allowable system latency; thus, the majority of the total system latency can be attributed to linac-independent factors.

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PurposeLimits on mean lung dose (MLD) allow for individualization of radiation doses at safe levels for patients with lung tumors. However, MLD does not account for individual differences in the extent or spatial distribution of pulmonary dysfunction among patients, which leads to toxicity variability at the same MLD. We investigated dose rearrangement to minimize the radiation dose to the functional lung as assessed by perfusion single photon emission computed tomography (SPECT) and maximize the target coverage to maintain conventional normal tissue limits.Methods and materialsRetrospective plans were optimized for 15 patients with locally advanced non-small cell lung cancer who were enrolled in a prospective imaging trial. A staged, priority-based optimization system was used. The baseline priorities were to meet physical MLD and other dose constraints for organs at risk, and to maximize the target generalized equivalent uniform dose (gEUD). To determine the benefit of dose rearrangement with perfusion SPECT, plans were reoptimized to minimize the generalized equivalent uniform functional dose (gEUfD) to the lung as the subsequent priority.ResultsWhen only physical MLD is minimized, lung gEUfD was 12.6 ± 4.9 Gy (6.3-21.7 Gy). When the dose is rearranged to minimize gEUfD directly in the optimization objective function, 10 of 15 cases showed a decrease in lung gEUfD of >20% (lung gEUfD mean 9.9 ± 4.3 Gy, range 2.1-16.2 Gy) while maintaining equivalent planning target volume coverage. Although all dose-limiting constraints remained unviolated, the dose rearrangement resulted in slight gEUD increases to the cord (5.4 ± 3.9 Gy), esophagus (3.0 ± 3.7 Gy), and heart (2.3 ± 2.6 Gy).ConclusionsPriority-driven optimization in conjunction with perfusion SPECT permits image guided spatial dose redistribution within the lung and allows for a reduced dose to the functional lung without compromising target coverage or exceeding conventional limits for organs at risk.

Deformable 3D dosimetry represents a robust method of verifying the accuracy of clinical deformable dose accumulation algorithms used to monitor interfraction anatomical changes during radiotherapy treatments. For this study, a deformable abdominal phantom was developed incorporating a deformable nPAG gel dosimeter for the dose verification of Adaptivo™, a commercial software program with a deformable dose accumulation algorithm. A comparison was made for three single fraction irradiations of gel dosimeters, each with a different deformation state. Additionally, a comparison was made for the cumulative dose over a three-fraction treatment of a single gel dosimeter with individual fraction deformations matching those of the single fraction measurements. The single fraction irradiations resulted in target contour dose volume histograms (DVH) created by Adaptivo™ that were in close agreement with those determined by gel dosimeter measurements for doses similar to and higher than the planned target dose, with two of the three cases matching to within 5%. Discrepancies are attributed to a deformed contour compression during analysis in the cases where the phantom was deformed. The three-fraction treatment resulted in very close agreement between the DVHs determined through the gel dosimeter measurements and Adaptivo™ calculations across the full range of doses, with an average absolute discrepancy of 2.0% and a maximum absolute discrepancy of 6.3%.