SU‐E‐T‐478: Prescription to 50–75% Isodose Line May Be Optimum for Linac Radiosurgery of Cranial Lesions

Purpose: The percentage‐isodose‐line to prescribe (RxIDL) in linac‐based SRS varies among institutions. An optimal RxIDL may lead to a treatment plan with the highest therapeutic ratio. Gradient index (GI), defined as the ratio of total volume receiving 50% of prescription dose and target volume receiving prescription dose, is an important parameter to characterize dose falloff. For radiosurgery prescribed at 16–20Gy, GI corresponds to the normal brain volume receiving 8–10Gy (V8–10Gy), which has been found to correlate to brain necrosis. Thus, GI can be considered as a clinically relevant criteria correlating to therapeutic ratio. This study aims to explore the optimal RxIDL using the GI criteria. Methods: 30 patients with brain tumors were retrospectively studied using iPlan system (BrainLAB, Feldkirchen, Germany). MLC‐based dynamic conformal arc technique was used for planning. For each patient, 6–8 plans with different RxIDLs were generated with different MLC margins. All plans were normalized so that 95% of target volume receives prescription dose. RxIDL was calculated as prescription dose divided by maximum dose. GI was calculated for each plan and the plan with minimum GI was considered optimal. Results: GI decreases (6.3 to 2.4) as target volumes increases (0.15cm³ to 50.09cm³), and the optimal RxIDL shifts to higher percentile. Median optimal RxIDL is 52.7±8.8% for targets <5cm³ and 71.8±3.5% for those >5cm³. Average actually treated RxIDL is 83.6±3.1%. The lower optimal RxIDL results in smaller V8–10Gy and higher mean dose to target volume. About 22cm³ reduction in V8–10Gy is observed in the case that has the largest tumor volume. Conclusion: The optimal RxIDL appears to be between 50% and 75% which is lower than the treated RxIDL. Larger targets tend to have higher optimal RxIDLs. By choosing an optimal RxIDL, we could reduce the volume of irradiated normal brain while delivering higher dose to target volume.