Intensity-Modulated Radiosurgery: Improving Dose Gradients and Maximum Dose Using Post Inverse-Optimization Interactive Dose Shaping

Abstract

Intensity-modulated radiosurgery (IMRS) for brain metastases and arterio-venous malformations (AVM) using a serial tomotherapy system (Nomos Corp., Cranberry Township, PA) has been delivered in >150 cases over the last 5 years. A new software tool provided within the Corvus inverse planning software (ActiveRx) allows for post inverse planning re-optimization and individualization of the dose distribution. We analyzed this tool with respect to increasing the steepness of the dose gradient and in-target dose inhomogeneity while maintaining conformity. Fifteen clinically delivered IMRS plans for solitary brain metastases provided the basis for this analysis. The clinical IMRS plans were copied and the ActiveRx module was opened. The toolset in ActiveRx includes a hot spot eraser, a pencil tool to redefine isodose lines and a drag and drop tool, allowing reshaping of existing isodose lines. To assess changes in the steepness of the dose gradient and dose homogeneity, the 100%, 90%, 50% and 25% isodose volume, the volume of the target, maximum dose and mean dose to the target were recorded. We also recorded total monitor units and calculated treatment delivery times. Target volumes ranged from 0.6 to 14.1 cm3 (mean/median 3.9/1.8 cm3). Mean RTOG conformity index (CI) of plans clinically delivered was 1.23±0.31; mean homogeneity index (HI) was 115±5%. After using the ActiveRx tool-set, the mean CI was slightly improved to 1.14±0.1, with an associated increase in HI to 141±10%. The average, respective Ian Paddick CI for the 100%, 90% 50% and 25% isodose lines were 0.79 vs. 0.83, 0.44 vs. 0.59, 0.12 vs. 0.19, and 0.04 vs. 0.07, representing significant improvements after using ActiveRx post-optimization. Total MU were reduced by a mean of 12.3% using ActiveRx, shortening estimated treatment delivery times by 3.2 minutes on average. A post inverse planning optimization tool for IMRS plans allowed for statistically significant improvements in the steepness of the dose gradient, and increased maximum and mean target doses compared to clinically delivered plans that were already considered excellent. Gains were especially pronounced in the reduction of normal brain tissue included into the 90%, and 50% isodose lines. We have since made this process part of the clinical routine for all cranial IMRS procedures.