Author: Catherine Coolens, Robert K. Heaton, Lalageh Mirzakhanian, Tasnim Rahman, Jan P. Seuntjens 👨🔬
Affiliation: Princess Margaret Cancer Centre & University of Toronto, McGill University, Princess Margaret Cancer Centre, Princess Margaret Hospital, University of Toronto 🌍
Purpose: Commissioning of a Gamma Knife® (GK) (Elekta, Sweden) unit requires precise dosimetric reference measurements, particularly for small fields. The IAEA TRS-483/AAPM TG-155 code of practice (2017) provides guidance and correction factors for reference and relative dosimetry. However, advancements in detectors, phantom materials, and modeling necessitate the validation and expansion of correction factors for updated equipment. This study calculates the correction factor, Qmsr,Q0kfmsr,fref, which addresses differences in field-size, geometry, phantom material, and beam quality between conventional and machine-specific reference fields.
Methods: Monte Carlo simulations of a GK Perfexion with various phantom and ionization chamber combinations were performed using EGSnrc. Phantom materials included Solid Water, PMMA, polystyrene, ABS, and Lucy (Standard Imaging). Ionization chambers, with electrode materials such as aluminum, C-552, and steel, were sourced from Standard Imaging, IBA, and PTW. The conventional reference field was a previously tabulated 60Co beam (10×10 cm², 100 cm SSD). The machine-specific reference field used a 16 mm field size GK Perfexion phase space file. Simulations measured the mean absorbed dose to water and the dose to the air cavity in ionization chambers for both reference and msr fields. Phantom and chamber dimensions were derived from literature and manufacturer blueprints.
Results: Correction factors were determined for various phantom/chamber combinations in parallel and perpendicular orientations. Linear trends with phantom material electron density were consistent across all chambers. Perturbations due to chamber components revealed that electrode material differences were most pronounced when comparing chamber orientations.
Conclusion: This study provides updated correction factors for TRS-483 protocols, aiding GK commissioning and routine quality assurance checks, and ensuring accurate dose is measured. Findings underscore the importance of chamber orientation on correction factors used due to influence of ionization chamber electrode. We also confirm the linear scaling of photon fluence ratios on electron density. These insights contribute to enhancing radiotherapy accuracy.