Conformality Indices and Brain Dose Objective for Multi-Lesion Brain Single Isocenter Linac-Based Stereotactic Radiosurgery

Abstract

The lack of recommendation on Linac-based stereotactic radiosurgery (SRS) and radiotherapy (SRT) plan quality for multi-lesion brain metastasis results in planning heterogeneity. The use of MLCs in the delivery is expected to add extra leakage when multi-lesions are treated simultaneously, and no recommendations availability in literature. We aimed to identify conformality indices and plan objectives that would guide multi-brain metastasis SRS & SRT MATERIALS/METHODS: We performed a retrospective analysis of clinically accepted brain metastasis radiotherapy plans treated using SRS and SRT at our department. Each plan was assessed for four metrics: Two conformality indices, including R50% referring to the ratio of the 50% dose cloud to the PTV, and %D1cm referring to the percentage of dose at 1cm away from PTV in any direction. One dose objective, V12 Gy referring to the brain volume receiving 12 Gy, and one delivery efficiency parameter, the modulation factor, defined as the ratio of the total MU to the prescribed dose. Each parameter was evaluated as a function of total PTV volume and number of lesions. A total of 107 plans were analyzed. The analyzed plans included pre- and post-operative treated brain metastasis with 86% with ≤ 4 lesions (median is 2, IQR is 3), 80% of the plans received single fraction and 43% of plans have a total PTV volumes of ≤ 1cc (median is 2.2cc and IQR is 7.5cc). We used a two tail T-test to check for correlation. The 50% relative dose gradient, R50%, was found to be dependent on PTV volume (P < 0.001), and a power law was used to fit the data. The brain volume, V12 Gy, was also found to be dependent on total PTV volume (P < 0.0001) and a linear curve was used to fit V12 Gy vs PTV. We did not find correlation between the number of lesions and R50% and or V12 Gy (p-value = 0.154 and 0.736, respectively). The dose gradient at 1cm away in any direction from PTV, %D1cm, has a median, average and a S.D. of 32%, 33% and 9%, respectively. 85% of all plans have %D1cm<42% which falls within 1 S.D. In addition, we found %D1cm to be dependent on both PTV volume and number of lesions (P < 0.001 for both). The modulation factor (MF) has a median, average and a S.D. of 2.9, 3.0 and 1.5, respectively. We found that 81% of all plans has MF < 4.5, which is within the 1 S.D. It has a positive correlation of 0.628 with the number of lesions. We found a clear correlation between the total PTV volumes and R50% and V12 Gy, and a fit line was used to relate each parameter to the total PTV volume. As for %D1cm and MF, we found that the average value within 1 S.D. is adequate to include > 80% of the plan. We propose these four parameters to fill the gap for plan quality for Linac-based SRS and SRT. Further evaluation and validation of our results on larger cohort is needed to allow generalizability and applicability of the results and correlation with clinical outcomes to guide clinical decision making.