Author: Dylan Yamabe Breitkreutz, Lee Chin, Brige P. Chugh, Mark D'Souza, Brian M. Keller, Anthony Kim, Michelle K. Nielsen, Arjun Sahgal 👨🔬
Affiliation: Sunnybrook Health Sciences Centre 🌍
Purpose: The Elekta Unity MR-linac delivers highly conformal SBRT treatments using IMRT. These treatment plans require patient-specific quality assurance (PSQA) measurements to detect discrepancies between the TPS-calculated dose and the delivered dose, as well as specialized equipment, such as MR-compatible array detectors. We developed a PSQA system for MR-linac treatments, utilizing radiochromic film and solid water (SW), along with in-house developed software to calculate gamma pass rates (GPRs).
Methods: A streamlined absolute single-channel calibration protocol for EBT4 film was developed using five calibration points and an optimization function to align these points with a previously established calibration curve. A PSQA process was developed in which the patient treatment plan was first modeled on a 25 cm SW stack in our treatment planning system, Monaco. A clinically relevant slice was then selected as the dose plane. The film was then placed at the dose plane within a SW stack and irradiated with the patient plan. After scanning a Python program applied the calibration curve to the film, registered it to the Monaco dose plane, and performed gamma analysis using the open-source PyMedPhys module. Gamma analysis pass criteria were global 3%/2 mm with a 10% dose threshold and a 90% GPR.
Results: The film calibration protocol demonstrated agreement within 3% when compared to a conventionally generated calibration curve from the same batch at SBRT dose levels ~10 Gy. Among 51 patients, the average GPR was 96.61%, with 49 patients meeting the pass criteria. In the two failing cases, most failing gamma points occurred in the low-dose region (~200 cGy). These low-dose failures also occurred in otherwise passing patients.
Conclusion: The film-based PSQA system showed strong agreement with TPS dose distributions at SBRT levels, with robust GPRs above 90 %. Despite low-dose discrepancies, it remains a reliable and streamlined approach for MR-linac PSQA.