AAPM task group report 351: Protocol for clinical reference dosimetry in external beam MR-guided radiotherapy

Background: Magnetic Resonance-guided Radiation Therapy (MRgRT) systems are increasingly used in clinical practice, necessitating a dedicated dosimetry protocol to address the challenges posed by the presence of strong magnetic fields. Purpose: AAPM TG351 introduces a reference dosimetry protocol for external beam MRgRT systems, aligning with AAPM TG51 reports and incorporating elements from the IAEA TRS398 code of practice. Methods: The protocol offers practical guidance on beam quality and dosimetry setup in the presence of strong magnetic fields. It reviews relevant literature to identify reference-class chambers suitable for external beam MRgRT systems and determines their beam quality conversion and other relevant correction factors through a detailed analysis of available data and associated uncertainties. Results: AAPM TG351 offers a comprehensive dosimetry protocol for external beam MRgRT systems, addressing the unique challenges posed by the presence of magnetic fields. It includes detailed guidance for measurement setups, a list of reference-class chambers appropriate for reference dosimetry in MRgRT systems, and updated beam quality conversion and correction factors. Additionally, the protocol presents an in-depth analysis of the uncertainty budget, identifying key contributors and providing strategies to minimize uncertainties, thereby enhancing clinical accuracy and reliability. Conclusions: This protocol provides a practical, evidence-based reference for medical physicists conducting reference dosimetry in external beam MRgRT systems.