Impact of Plan Quality and Complexity Metrics on Gamma Passing Rate SIMT SRS Technique

Abstract

<h3>Purpose/Objective(s)</h3> Multiple studies have reported different outcomes regarding the effect of plan complexity and quality metrics on gamma passing rate. However, there is very little information in literature regarding a single isocenter multitarget (SIMT) SRS technique. This study aims to evaluate the effect of different plan complexity metrics and quality metrics on gamma passing rate (GPR). Moreover, we aim to analyze the impact of distance to isocenter (DIS) & target size on plan quality metrics for the SIMT SRS technique. <h3>Materials/Methods</h3> 40 previously treated patient scans with 4–11 targets, an average volume of 1.2 cc and DIS range of 1.45 to 8.75 cm were used to create SIMT SRS in a treatment planning system (TPS) for a Linac using 6MV-FFF beam energy. In-house programs were developed to compute complexity metrics such as Plan averaged beam Area (PA), Plan averaged beam Irregularity (PI), Total MU, Leaf Travel (LT), mean Dose Rate variation (DR), mean Gantry Speed (GS) variation. Also, target volume, DIS, Paddick Conformity (PCI), and RTOG Conformity (RTOG_CI) indices were calculated per target & per plan. Paddick Gradient index (PGI) only was computed per plan. Patient QAs were then performed on a 4-D detector phantom using SNC software for comparison between measurements and calculated dose with objectives of 3%2mm & 10% threshold as per AAPM TG-218 recommendation. Spearman correlation was used for correlation and T-test for determining whether there are any statistically significant differences between the means of calculation per target vs per plan for PCI and RTOG_CI. <h3>Results</h3> The following results were observed: no correlation between GPR and PGI/RTOG_CI/number of targets/ target size, a positive but weak correlation between PCI & GPR (0.322, p-value = 0.023) and a negative weak correlation between DIS & GPR (-0.38, p-value = 0.01). However, a strong correlation was observed between target size & PCI/RTOG_CI/PGI (0.793, 0.719 & 0.636 respectively (p-value<0.001)), but no correlation between plan quality metrics and DIS. There was a positive weak correlation between GPR & DR/PA and a negative weak correlation between PI & GPR (0.35, 0.302 & -0.33 respectively). A moderate correlation was observed between GS & GPR (0.46, p-value= 0.0025), while no correlation was found between GPR & LT/total MU. Moreover, there was significant difference in mean PCI, RTOG_CI between per target and per plan. <h3>Conclusion</h3> GPR of SRS/SRT using single-isocenter VMAT for multiple targets is not associated with DIS, LT, total MU, and plan quality metrics. Also, DIS does not have any impact on plan quality metrics. Instead, there is a strong correlation between tumor volume & plan quality metrics, indicating volume is an important predictor for plan quality metrics. Moreover, DIS has a moderate impact on GPR, showing that a larger distance between target location and isocenter can result in lower GPR. For plan quality evaluation, calculation of PCI & RTOG_CI should be done per tumor.