Purpose: SRS of multiple brain metastases (m-bm) via single-isocenter/multi-target (SIMT) VMAT can improve treatment efficiency and patient comfort. Due to setup uncertainty, aligning m-bm on a single CBCT is nearly impossible and has shown clinically unacceptable loss of target coverage. Moreover, due to island-blocking, m-bm sharing the same MLC pair, low and intermediate dose spill causes an increase in normal tissue dose. This study proposes RESIST method to improve plan quality by reducing normal brain dose from island-blocking.
Methods: Six patients with 6-16 tumors were selected for this study with an average PTV size of 10.8±2.1cc (8.2-13.4cc). Patients were planned with SIMT-VMAT using 10MV-FFF beam with a 20Gy prescription to each lesion. Isocenter was placed at the geometric center of targets. Patients were replanned using RESIST, utilizing a single isocenter with lesions treated in two groups based on brain hemisphere location with corresponding non-coplanar partial-arc geometry. This technique uses DCA-based dose and aperture shape controller before VMAT optimization. It allows for two separate plans to be created for each group while sharing the same isocenter and all tumors to be treated in same session.
Results: Compared to SIMT-VMAT, RESIST plans provide slightly better coverage and conformity with less low and intermediate dose spread. RESIST showed reductions in normal brain V8Gy and mean brain dose by an average of 64cc and 1Gy with slight increase in V12Gy by 3cc.Overall monitor units decreased by 2278MU and decrease modulation factor by 1.1, on average. Slight decreases in beam on time were evident; however, treatment time would increase with an additional CBCT and couch-kicks.
Conclusion: For superior target coverage and reduced dose to normal brain, RESIST planning for m-bm is a simplified approach that provides patient comfort and compliance for those complex patients who may not tolerate traditional longer treatment times with multiple isocenters.