Novel dose-transfer based array calibration methods for an ionization chamber array on MR-linac

Purpose

This study aims to develop novel dose-transfer-based calibration methods (central field [CF] and daisy chain [DC]) to overcome the limitations of conventional wide-field (WF) calibration for IC PROFILER™ ionization chamber arrays in MR-linac environments, specifically resolving dose profile anomalies and enhancing measurement consistency with water tank reference data.

Methods

The central field (CF) and daisy chain (DC) calibration methods were evaluated with water tank measurements as a reference and compared to the wide-field (WF) method from Sun Nuclear Corporation (SNC). Profiles were measured at depths of 0.9, 2.9, and 4.9 cm for field sizes ranging from 10 × 10 cm² to 40 × 22 cm². Accuracy was evaluated using flattening filter-free (FFF) beam characterization and gamma passing rate (GPR) with a 1 %/1 mm criterion. Statistical analyses included the Kruskal-Wallis and Mann-Whitney U tests. Effect size of GPR calculated using Cohen's d.

Results

The CF and DC methods showed superior agreement with water tank profiles compared to the SNC method. In larger fields (40 × 22 cm² and 30 × 22 cm²), the CF method achieved mean GPRs above 95.9 % and median values over 99.0 %, outperforming the SNC method (mean and median GPR: 89.2 %; all p-values <0.05 and d-values >0.9). The DC method also performed well, achieving higher GPRs in the 40 × 22 cm² field and consistent accuracy across smaller fields. Additionally, beam characterization metrics—including peak position, symmetry, penumbra, unflatness, and slope—exhibited smaller deviations from water tank measurements when using CF and DC methods, as reflected in more concentrated difference distributions.

Conclusion

The SNC method is insufficient for MR-linac due to magnetic field-induced variations in detector response, leading to significant discrepancies in dose profiles. In contrast, the CF and DC methods demonstrate superior accuracy, improving the reliability of QA workflows. By minimizing the risk of beam miscalibration, these methods ensure the safety and effectiveness of MR-linac-based radiotherapy.