VMAT and IMRT plan-specific correction factors for linac-based ionization chamber dosimetry

Purpose: The determination of absorbed dose to water from external beam radiotherapy using radiation detectors is currently rooted in calibration protocols that do not account for modulations encountered in patient-specific deliveries. Detector response in composite clinical fields has not been extensively studied due to the time and effort required to determine these corrections on a case-by-case basis. To help bridge this gap in knowledge, corrections for the Exradin A1SL scanning chamber were determined in a large number of composite clinical fields using Monte Carlo methods. The chamber-specific perturbations that contribute the most to the overall correction factor were also determined. Methods: A total of 131 patient deliveries comprised of 834 beams from a Varian C-arm linear accelerator were converted to EGSnrc Monte Carlo inputs. A validated BEAMnrc 21EX linear accelerator model was used as a particle source throughout the EGSnrc simulations. Composite field dose distributions were compared against a commercial treatment planning system for validation. The simulation geometry consisted of a cylindrically symmetric water-equivalent phantom with the Exradin A1SL scanning chamber embedded inside. Various chamber perturbation factors were investigated in the egs_chamber user code of EGSnrc and were compared to reference field conditions to determine the plan-specific correction factor. Results: The simulation results indicated that the Exradin A1SL scanning chamber is suitable to use as an absolute dosimeter within a high-dose and low-gradient target region in most nonstandard composite fields; however, there are still individual cases that require larger delivery-specific corrections. The volume averaging and replacement perturbations showed the largest impact on the overall plan-specific correction factor for the Exradin A1SL scanning chamber, and both volumetric modulated arc therapy (VMAT) and step-and-shoot beams demonstrated similar correction factor magnitudes among the data investigated. Total correction magnitudes greater than 2% were required by 9.1% of step-and-shoot beams and 14.5% of VMAT beams. When examining full composite plan deliveries as opposed to individual beams, 0.0% of composite step-and-shoot plans and 2.6% of composite VMAT plans required correction magnitudes greater than 2%. Conclusions: The A1SL scanning chamber was found to be suitable to use for absolute dosimetry in high-dose and low-gradient dose regions of composite IMRT plans but even if a composite dose distribution is large compared to the detector used, a correction-free absorbed dose-to-water measurement is not guaranteed.