Room: Exhibit Hall | Forum 4
Purpose: The purpose of this study is to retrospectively evaluate multiple dosimetric features for patients who have undergone intracranial single- and multi-fraction SIMT SRS to determine institutional dosimetric plan quality metrics.
Methods: We analyzed 253 patients and 282 plans (111 SRS and 171 SRT) who received treatment from 2015-2018. The total of 1,464 lesions were treated. The number and volume treated per plan ranged from 2â€“30 and 0.88ccâ€“165.35cc, respectively. We extracted different plan features and regression analysis was performed to determine possible correlations. All treatment plans were optimized using Varianâ€™s Photon Optimizeráµ€á´¹ (PO_V.13.6.23) and Progression Resolution Optimizeráµ€á´¹ (PRO V_13.6.23) with 6 MV photons and 6 MV photons flattening filter free(FFF) in Eclipseáµ€á´¹ and delivered with Varian TrueBeamÂ® Edge linear accelerators.
Results: Correlations were determined between the 20%(V20), 50%(V50), and 100%(V100) isodose volumes. The minimum R-squared measure for V50 versus V100 and V20 versus V100 was determined to be 0.934 and 0.601, respectively. Data was stratified by the number of lesions (1, 2, 3, 4, 5, 6, and >6). The minimum V20 and V100 correlation improved (RÂ²=0.885) when the >6 lesion data was excluded. The plans used an average of 5.0Â±1.0 arcs, arranged in a noncoplanar geometry. The conformity, Paddick Conformity Index, and selectivity achieved for the 282 plans in the model were 1.01Â±0.06, 0.92Â±0.04, and 0.96Â±0.04, respectively. Little correlation was seen between the gradient index of the 50%(GI50) and 20%(GI20) isodose lines versus the minimum distance between targets. A correlation, RÂ²=0.729, was found between the V12 of the brain and the V100 for single fraction patients.
Conclusion: This analysis allows us to quantify when current plans are at or greater than our previous experience. Different features showed strong correlations based on target volumes. Stratification based on PTV number allows for better prediction of V20.