Pre‐irradiation dose requirements for reference‐class ionization chambers: Quantifying measurement stability

Background Pre‐irradiation of ionization chambers is widely recommended to stabilize output readings for reference dosimetry in radiation therapy. However, the specific pre‐irradiation requirements for different chamber models, particularly under strictly controlled conditions that isolate chamber performance, remain unclear. Purpose This study aimed to quantitatively evaluate the pre‐irradiation dose required to stabilize the output of various reference‐class ionization chambers, while minimizing the influence of environmental and system‐level uncertainties. Methods Twelve reference‐class ionization chambers (Exradin A12, A1SL; IBA CC13, FC65G; PTW 30013, 31021) were evaluated under strictly controlled conditions (room temperature: 25 ± 1°C, water temperature: 25.00 ± 0.01°C, relative humidity: approximately 50%). Output stability was assessed using a 10 MV photon beam from a TrueBeam linac. Chamber readings were measured using an RT521R2 electrometer and normalized to an adjacent external monitor chamber. Readings are defined as stable if the reading variation is <0.1% and does not exhibit any trending. The optimal number of pre‐irradiation exposures and corresponding doses were determined for each chamber. Results For the large‐volume chambers (Exradin A12, IBA FC65G, PTW 30013), output variation was within ±0.05% from the first irradiation, suggesting that pre‐irradiation may be unnecessary. In contrast, the middle‐volume class ionization chambers (Exradin A1SL, IBA CC13, PTW 31021) exhibited variations of up to ±0.25%, and required up to 100 MU × 25 (approximately 21.25 Gy) of pre‐irradiation to achieve stability under the controlled experimental conditions of this study. Conclusions This study demonstrated that the required pre‐irradiation dose varies depending on the type of ionization chamber, and that the output variation is significantly lower than the previously reported 1%. These results may help streamline clinical reference dosimetry by tailoring procedures to each chamber.