Robotic Radiosurgical Boost After Whole-Brain Radiotherapy for 12 Brain Metastases: En Bloc Consecutive Irradiation With Comprehensively Optimized Single Plan for Eight Lesions Totaling 118 cc.

General radiotherapeutic management for >10 brain metastases (BMs) totaling >100 cm3 , including multiple large lesions (>10-30 cm3 ) in close proximity, demonstrated limited efficacy and/or safety. We describe a case of 12 BMs, summating 122.2 cm3 , including a 39.6 cm3 maximum lesion and adjacent ones. The patient had an 8.1-year treatment history for recurrent/metastatic breast cancer refractory to endocrine and chemotherapy. BMs were treated with conventional whole-brain radiotherapy (WBRT) with 30 Gy/10 fractions (fr), followed by an immediate stereotactic radiosurgery (SRS) boost with 27 Gy/5 fr (52-64% isodoses) which covers the gross tumor boundaries of selected eight lesions (total 118.4 cm3 ). The SRS dose was defined to ensure the cumulative biologically effective dose (BED10 ) of just ≥80 Gy while minimizing the risk of radiation injury. The SRS was performed using a CyberKnife (CK) robotic system (Accuray Incorporated, Sunnyvale, California, United States) with a variable-sized collimator (10-40 mm), for which en bloc consecutive irradiation, using 215 beams based on a comprehensively optimized single plan (path), was adopted. The treatment time per fraction was ≤45 min (mean 5.6 min per lesion). Afterward, BMs demonstrated remarkable regression over six months, causing the total residual visible lesions of 12.6 cm3 (10.3%) at 11.4 months, despite the absence of obvious lesion shrinkage during the radiotherapy. WBRT, followed by an immediate 5-fr SRS boost with a total BED10  of 80 Gy to large and/or culprit lesions, can be an efficacious and safe treatment option for multiple BMs, totaling >120 cm3 . En bloc consecutive irradiation with a single path provides overwhelmingly more efficient delivery for treating multiple lesions using CK in terms of irradiation time and comprehensive reduction of normal brain dose compared to individual planning. Volumetric-modulated arc-based >10-fr SRS with simultaneously integrated reduced-dose WBRT may be an alternative to further enhance efficacy and safety.