Purpose: To evaluate 3D printed compensator-based preclinical IMRT plan quality as a function of number of angles for efficient hypoxia dose-painting in a murine tumor model.
Methods: Twenty mice with hind-limb tumors and previously acquired electron paramagnetic resonance oxygen images were randomly selected for this study. A preclinical inverse treatment planning system was used to generate IMRT plans targeting hypoxic tumor regions, ranging from one to seven per animal, using 2, 3, 5, 7, 9, and 15 angles. Each plan was normalized such that 95% of the hypoxic volume received 35.5 Gy. Plan quality was assessed in terms of conformity index (CI), difference between effective radii of the prescription and 50% isodose volumes (Δ50), homogeneity index (HI), and mean dose (Dmean) to surrounding non-hypoxic tissue. Quality metrics were evaluated for significance (p<0.05) using two-tailed paired t-tests versus plans with 3 angles.
Results: Preclinical IMRT dose distributions were significantly (p=0.01) less conformal with 2 angles than 3 angles, with mean CI values of 10.65 and 1.93, respectively. However, there was no significant difference (p≥0.06) for the 5 (CI=1.69) and 7-angle (CI=1.58) scenarios. CI was 1.56 with 9 (p=0.04) and 1.59 (p=0.05) with 15 angles. Dose fall-off (Δ50) measured 2.5, 3.1, 2.8, 2.7, 2.7, and 2.7 mm for 2, 3, 5, 7, 9, and 15 angles, respectively. From 3 to 15 angles, the mean HI varied from 120.0-118.8% versus 115.0% for the 2-beam case. Use of 3 or more angles significantly reduced non-target tissue Dmean by 6.2%, 8.2%, 8.6%, 9.1%, and 8.1% with increasing angle number relative to a mean dose of 22.1 Gy for the 2-angle plans.
Conclusion: The results indicate that using 3 angles for compensator-based preclinical IMRT does not compromise plan quality relative to treatments with 5 or more angles while providing improved experimental workflow and treatment delivery efficiency.
Funding Support, Disclosures, and Conflict of Interest: This work was supported by IRG-19-136-59 from the American Cancer Society and the University of Chicago Medicine Comprehensive Cancer Center. Daniela Olivera Velarde was supported by a 2020-2022 AAPM Fellowship for Graduate Study in Medical Physics.