Evaluation of a modified formalism for precision electron beam dosimetry compared to TG-51 and TRS-398

This study aims to evaluate a modified formalism for electron beam reference dosimetry and to compare the results with the cases of previous protocols TG-51 and TRS-398.

Measurements were performed using three types of cylindrical chambers (TW30013, FC65-G, and A1SL) and plane-parallel chambers (PPC) (Roos, Advanced Markus, and NACP-02) with electron beams of energies 6, 12, and 18 MeV. Absorbed dose to water at the reference depths was determined following the modified formalism using different chambers and methods, including the direct application of ⁶⁰Co calibration or cross-calibration for PPCs in a high-energy electron beam. Data were also analyzed following TG-51 and TRS-398 formalisms for comparison.

Following the modified formalism, all the results obtained using cylindrical chambers, ⁶⁰Co calibrated well-guarded PPCs and cross-calibrated PPCs were in good agreement within the estimated uncertainty (0.7 - 0.9 %) at all the energies. However, one PPC exhibited strange behavior, producing 3 % higher doses when applying ⁶⁰Co calibration. This was confirmed due to chamber-to-chamber variation in P_wall. The modified formalism showed good agreement with TG-51 and TRS-398, but yielded consistently higher doses. The difference was reduced by applying the updated EPOM (effective point of measurement) shift to the previous protocols.

This study supports the reliability of the modified formalism for electron beam reference dosimetry with chambers calibrated under ⁶⁰Co, thereby reducing uncertainty. The modified formalism yielded consistent results within smaller uncertainty (0.7 - 0.9 %). However, it is recommended to check each PPC's chamber-to-chamber variation.