Exhibit Hall | Forum 3
Purpose: MR-integrated linear accelerators (MR-linac) are becoming more prevalent in the field of radiotherapy. The AAPM’s TG51 and its addendum currently form the reference dosimetry protocols for conventional high-energy photon beams. The reference conditions defined by these protocols cannot be met by commercial MR-linac devices and additional considerations are required to account for the presence of the magnetic field. Therefore, practical guidelines are required for reference dosimetry measurements in high-energy photon beams in the presence of a magnetic field.
Methods: The AAPM Task Group 351, approved by the AAPM in 2020, is comprised of international members. To date, the task group has produced a draft manuscript that specifies a dosimetry protocol for reference dosimetry in available commercial MR-linac devices, addresses technical issues with dosimetry equipment in the presence of the magnetic field, and recommends best practices for performing beam quality and reference dosimetry measurements while providing a sample uncertainty budget, in a similar fashion as the TG51 addendum.
Results: Building on the formalism introduced by TG51, the AAPM TG351 introduces a magnetic field dependent beam quality conversion factor k_Q,B that is a function of the magnetic field, the chamber model and orientation of the detector with respect to the radiation beam and the magnetic field. Another major deviation from the AAPM TG51 protocol is the use of TPR²⁰₁₀ as the beam quality specifier, rather than PDD(10)ₓ. The draft report disseminates data from literature to provide the user with k_Q,B factors as a function of beam quality for various chambers appropriate for use with MR-linacs.
Conclusion: Guidance on clinical implementation and the use of high-quality data for correction factors for clinical reference dosimetry in external MR-guided radiotherapy will enable accurate reference dosimetry measurements for these systems.