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

Abstract

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.

Verification of stereotactic radiosurgery plans for multiple brain metastases using a virtual phantom-based procedure

“…We have previously reported the accuracy of the VIPER software to be used for PSQA of SIMT SRS plans [24]. In the current study its application for verification of clinical plans designed with the Eclipse TPS has been described.…”

“…Plans were calculated with 1 mm grid size using the anisotropic algorithm (AAA) algorithm of the Eclipse TPS. In a previous work, we investigated the Eclipse dose calculation accuracy, as well as the targeting accuracy of our linac to treat multiple targets with a single-isocenter [ 28 , 29 ].…”

“…In the context of PTV margins used for treatment, Bossuyt et al described the use of the CTV-PTV margin as DTA for transit in-vivo PSQA based on EPID measurements [ 33 ]. The 2 mm-DTA used in the present study coincides with the CTV-PTV margin implemented in our SIMT SRS policy [ 29 ]. So, a successful GPR using 2 mm-DTA is compatible with an adequate dosimetric coverage of the lesions.…”

“…The validation and feasibility of VIPER for SRS plan verification were described by our group in a recent publication [24].…”

“…There are several commercially available systems offering 3D dose reconstruction from EPID-based measurements, but none of them is designed to work using a virtual phantom [23]. Recently, the accuracy of the VIPER software has been assessed by our group to be used for 3D SRS verification [24]. The aim of this study is to describe our experience using the VIPER software for PSQA of SIMT SRS plans.…”