Electron beam reference dosimetry measurements obtained at multiple institutions using the Addendum to AAPM's TG-51 protocol

Background

The TG-51 protocol describes methods for obtaining reference dosimetry measurements for external photon and electron beams. Since the publication of TG-51 in 1999, research on reference dosimetry has allowed revisiting the procedures and data recommended in the protocol. An Addendum to TG-51 for electron beam reference dosimetry was published in 2024, which revises the formalism and procedures and provides updated $k_Q$ data.

Purpose

To compare clinical reference dosimetry measurements in electron beams obtained using the original American Association of Physicists in Medicine's (AAPM) TG-51 protocol and its associated Addendum (AAPM WGTG51 report 385).

Methods

Measurements were performed in electron beams using the data and methods prescribed by TG-51 and its Addendum. Nine participants (eight clinics and one primary standards laboratory) provided data and measurements. Results were obtained with 18 linacs using 87 total beam energies (4–6 energies per linac) between 4–22 MeV, representing the range of electron beam energies used clinically. Various cylindrical (6 types) and parallel-plate (4 types) ionization chamber types were employed, representing most of the chambers commonly used in modern radiation therapy clinics. An analysis was performed to determine if differences arise from the new data recommended for beam quality conversion factors or from changes to the procedure.

Results

Results for dose to water per monitor unit obtained using the Addendum are up to 2.3% higher in low-energy beams and 1.3% higher in high-energy beams compared to results obtained using the original TG-51 protocol. These differences are consistent with what was predicted by the Addendum. Differences arise from both the changes to procedure (up to 0.7% from not requiring the $P_{\mathrm{gr}}^Q$ correction for cylindrical chambers, 0.5% from the change in the shift of the point of measurement for parallel-plate chambers) as well as the recommended data (0.8% from differences in $k_{R_{50}}^{\prime }$, 0.5% from differences in $k_{\mathrm{ecal}}$).

Conclusion

This work elucidates where differences arise in results obtained using the original TG-51 protocol and its associated Addendum for electron beam reference dosimetry. The results presented here provide confidence in the new approach and data recommended by the Addendum. Clinical physicists can use these results to ensure that differences are as expected when implementing the Addendum.