Validation of virtual water phantom software for pre‐treatment verification of single‐isocenter multiple‐target stereotactic radiosurgery

The aim of this study was to benchmark the accuracy of the VIrtual Phantom Epid dose Reconstruction (VIPER) software for pre‐treatment dosimetric verification of multiple‐target stereotactic radiosurgery (SRS). VIPER is an EPID‐based method to reconstruct a 3D dose distribution in a virtual phantom from in‐air portal images. Validation of the VIPER dose calculation was assessed using several MLC‐defined fields for a 6 MV photon beam. Central axis percent depth doses (PDDs) and output factors were measured with an ionization chamber in a water tank, while dose planes at a depth of 10 cm in a solid flat phantom were acquired with radiochromic films. The accuracy of VIPER for multiple‐target SRS plan verification was benchmarked against Monte Carlo simulations. Eighteen multiple‐target SRS plans designed with the Eclipse treatment planning system were mapped to a cylindrical water phantom. For each plan, the 3D dose distribution reconstructed by VIPER within the phantom was compared with the Monte Carlo simulation, using a 3D gamma analysis. Dose differences (VIPER vs. measurements) generally within 2% were found for the MLC‐defined fields, while film dosimetry revealed gamma passing rates (GPRs) ≥95% for a 3%/1 mm criteria. For the 18 multiple‐target SRS plans, average 3D GPRs greater than 93% and 98% for the 3%/2 mm and 5%/2 mm criteria, respectively. Our results validate the use of VIPER as a dosimetric verification tool for pre‐treatment QA of single‐isocenter multiple‐target SRS plans. The method requires no setup time on the linac and results in an accurate 3D characterization of the delivered dose.