ESTRO 2025 - Abstract Book

S2656

Physics - Detectors, dose measurement and phantoms

ESTRO 2025

3853

Poster Discussion Revision of IAEA TRS-398; a code or practice for absorbed dose determination in external beam radiotherapy.

Karen Christaki, Zakithi Msimang, Jamema Swamidas, Mauro Carrara Dosimetry and Medical Radiation Protection., IAEA, Vienna, Austria

Purpose/Objective: IAEA TRS-398 [1] was published in 2000, the basic concepts are still sound after 25 years but many things have changed. The revised version, TRS-398 (Rev. 1) [2], was published in February 2024 and endorsed by ESTRO. Material/Methods: Compared to 2000, the standards based on absorbed dose to water are now well established worldwide for most of the radiotherapy modalities. Also, advances include availability of published new key data for measurement standards for the dosimetry of ionizing radiation, the introduction of new detectors that are now commercially available and the introduction of new radiotherapy techniques. The dosimetry concepts for the most common modalities of radiotherapy used in the clinic namely photon and electron has changed little over the years. The greatest change was needed for the dosimetry of i) proton and light ion beams, with pencil beam scanning delivery systems now being widely adopted; ii) low and medium energy kilovoltage X ray beams, with very few primary standards laboratories having adopted absorbed dose to water standards in the last 25 years. Results: For photon dosimetry TRS-398 (Rev. 1) includes the dosimetry for flattening filter free beams. Also included in TRS 398 (Rev. 1) are new beam quality correction factors, k Q , based on the paper by Andreo et al, that are derived from measurements from the standards laboratories and Monte Carlo calculations adopting the key data of the ICRU Report 90. For proton and light ion beam dosimetry, not only scattered beams, but also pencil beam scanning delivery systems are now included. For low and medium energy kV X ray dosimetry absorbed dose to water standards are a challenge. Therefore TRS 398 (Rev. 1) gives the option of dosimetry based on an ionization chamber calibrated in terms of air kerma. In addition, it has been well understood that half value layer (HVL) is not the ideal beam quality index for these energies so TRS-398 (Rev. 1) makes use of a GUI that uses both HVL and kVp as the beam quality index based on work by Andreo.

Conclusion: It is intended that TRS-398 (Rev. 1) replaces TRS 398, maintaining its status as a code of practice adopted internationally for the determination of absorbed dose to water in external beam radiotherapy.

Acknowledgments The IAEA wishes to express its gratitude to the experts committee, Pedro Andreo, David Burns, Ralf-Peter Kapsch, Malcolm McEwen and Stan Vatnitsky for their major contribution to drafting TRS-398 (Rev.1). Keywords: Code of practice, dosimetry, TRS398 (Rev. 1) References: [1] INTERNATIONAL ATOMIC ENERGY AGENCY, Absorbed Dose Determination in External Beam Radiotherapy: An International Code of Practice for Dosimetry Based on Standards of Absorbed Dose to Water, Technical Reports Series No. 398, Vienna (2000). [2] INTERNATIONAL ATOMIC ENERGY AGENCY, Absorbed Dose Determination in External Beam Radiotherapy: An International Code of Practice for Dosimetry Based on Standards of Absorbed Dose to Water, Technical Reports Series No. 398 - Second Edition, Vienna (2024).