On a single isocenter volumetric modulated arc therapy SRS planning technique for multiple brain metastases

Volumetric modulated arc therapy (VMAT) is a new technique for efficient delivery of intensity modulated dose distributions. This study investigates a single isocenter VMAT technique to treat multiple brain metastases to 15 - 24 Gy. The Pinnacle³ SmartArc VMAT optimization plugin was used for all VMAT plans. A non-coplanar arc technique using five 100° arcs and one isocenter was compared with a conformal arc technique which used anywhere from 5 to 9 arcs with at least one isocenter per target. Comparison was done using the Conformality Number (CN), Prescription Isodose to Target Volume (PITV), Homogeneity Index (HI), Conformity-Gradient Index (CGI) as well as the 12 Gy isodose volume in the normal brain from which the risk of symptomatic necrosis (S-NEC) was calculated. The VMAT technique resulted in plans with a maximum delivery of 15 minutes, regardless of the number of targets. The VMAT technique provided superior conformity for large targets but for small targets the conformal arc technique resulted in superior conformity. For all targets, the conformal arc technique resulted in superior dose fall off outside of the target. The VMAT technique resulted in an increase in the 12 Gy volume over the conformal arc technique, with an accompanying increase in risk of S-NEC. While the 12 Gy volume was still within an acceptable clinical range, 4 out 20 patients showed a significant increase (15-20%) in absolute risk of S-NEC. Thus the VMAT technique resulted in clinically acceptable plans with vast reductions in treatment time.