Purpose: To assess the impact of utilizing avoidance structures versus avoidance sectors on dosimetric performance, planning efficiency and patient-specific quality assurance (PSQA) of multi-isocenter frameless stereotactic radiosurgery (SRS).
Methods: Five patients each with 4 to 6 lesions were included in this study. Three VMAT plans were generated for each patient, to treat one target and avoid the remaining lesions. While the treatment geometry was identical between the three plans, the first VMAT plan employed avoidance sectors (SRS-AvS) to minimize the dose to the untargeted lesions. The second and third plans designated the untargeted lesions as avoidance structures and prohibited "Entrance and Exit" of the beam through them for the second plan (SRS-NEE) and only "Entrance" for the third plan (SRS-NE). The three plans were evaluated using commonly employed dose-volumes and plan quality indices (Paddick Conformity Index [CIPaddik], Homogeneity Index [HI], normal brain [V12Gy], maximum dose to OARs and untargeted lesions) and total number of monitor units (MUs). PSQA was performed for the SRS-AvS and SRS-NEE plans and evaluated for gamma passing rate of 3%/3mm.
Results: All three plans achieved similar target coverage, CIPaddik, HI, V12Gy and sparing of OARs and untargeted lesions. On average, the SRS-AvS plans resulted in 44% and 17% reduction in the total MUs when compared to SRS-NEE and SRS-NE plans, respectively. The greater the overlap between the targeted lesion and avoidance structures, the larger the reduction in total MUs offered by SRS-AvS plans. Regarding PSQA, two of the SRS-NEE plans that resulted in highest increase in total MUs did not satisfy the clinical criteria of 95% passing rate.
Conclusion: Although the use of avoidance structures improves planning efficiency by eliminating the need of identifying avoidance sectors in the VMAT arcs it could potentially result in substantial increase in MUs and ultimately beam-on time.