Pedro Andreo, Dosimetry and Medical Radiation Physics Section, IAEA David T Burns, Bureau International des Poids et Measures (BIPM) Klaus Hohlfeld, Physikalisch-Technische Bundesanstalt (PTB), Braunschweig, Germany M Saiful Huq, Thomas Jefferson University, Philadelphia, USA Tatsuaki Kanai, National Institute of Radiological Sciences (NIRS), Chiba, Japan Fedele Laitano, Ente per le Nuove Tecnologie L’Energia e L’Ambiente (ENEA), Rome, Italy Vere Smyth, National Radiation Laboratory (NRL), Christchurch, New Zealand Stefaan Vynckier, Catholic University of Louvain (UCL), Brussels, Belgium

/pdf/absorbed-dose-determination-in-external-beam-radiotherapy-an-53v80q702z.pdf

Absorbed Dose Determination in External Beam Radiotherapy: An International Code of Practice for Dosimetry based on Standards of Absorbed Dose to Water

An addendum to the AAPM's TG-51 protocol for the determination of absorbed dose to water in megavoltage photon beams is presented. This addendum continues the procedure laid out in TG-51 but new kQ data for photon beams, based on Monte Carlo simulations, are presented and recommendations are given to improve the accuracy and consistency of the protocol's implementation. The components of the uncertainty budget in determining absorbed dose to water at the reference point are introduced and the magnitude of each component discussed. Finally, the consistency of experimental determination of ND,w coefficients is discussed. It is expected that the implementation of this addendum will be straightforward, assuming that the user is already familiar with TG-51. The changes introduced by this report are generally minor, although new recommendations could result in procedural changes for individual users. It is expected that the effort on the medical physicist's part to implement this addendum will not be significant and could be done as part of the annual linac calibration.

https://www.aapm.org/pubs/reports/RPT_67_Addendum.pdf

Addendum to the AAPM's TG-51 protocol for clinical reference dosimetry of high-energy photon beams.

Ultrahigh dose-rate radiotherapy (RT), or 'FLASH' therapy, has gained significant momentum following various in vivo studies published since 2014 which have demonstrated a reduction in normal tissue toxicity and similar tumor control for FLASH-RT when compared with conventional dose-rate RT. Subsequent studies have sought to investigate the potential for FLASH normal tissue protection and the literature has been since been inundated with publications on FLASH therapies. Today, FLASH-RT is considered by some as having the potential to 'revolutionize radiotherapy'. FLASH-RT is considered by some as having the potential to 'revolutionize radiotherapy'. The goal of this review article is to present the current state of this intriguing RT technique and to review existing publications on FLASH-RT in terms of its physical and biological aspects. In the physics section, the current landscape of ultrahigh dose-rate radiation delivery and dosimetry is presented. Specifically, electron, photon and proton radiation sources capable of delivering ultrahigh dose-rates along with their beam delivery parameters are thoroughly discussed. Additionally, the benefits and drawbacks of radiation detectors suitable for dosimetry in FLASH-RT are presented. The biology section comprises a summary of pioneering in vitro ultrahigh dose-rate studies performed in the 1960s and early 1970s and continues with a summary of the recent literature investigating normal and tumor tissue responses in electron, photon and proton beams. The section is concluded with possible mechanistic explanations of the FLASH normal-tissue protection effect (FLASH effect). Finally, challenges associated with clinical translation of FLASH-RT and its future prospects are critically discussed; specifically, proposed treatment machines and publications on treatment planning for FLASH-RT are reviewed.

https://iopscience.iop.org/article/10.1088/1361-6560/abaa28/pdf

Physics and biology of ultrahigh dose-rate (FLASH) radiotherapy: a topical review

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,

The current Code of Practice for high-energy photon dosimetry (HPA 1983) is based on the use of an NE 2561 ionisation chamber calibrated in air by comparison with the primary standard of exposure or air kerma for 2 MV x-radiation at the National Physical Laboratory (NPL). To derive absorbed dose to water from a measurement in water, the instrument reading is multiplied by the NPL calibration factor and by an energy-dependent conversion factor, C,,. The phantoms and methods recommended for the transfer of a calibration from a secondary standard to other instruments and for the measurement of machine output were described by NPL (1974). With the aim of improving the accuracy of photon dosimetry, NPL has launched a calibration service for high-energy photon beams in terms of absorbed dose to water, based on a graphite calorimeter as the primary standard (Burns et a1 1988). It is expected that the 2 MV, C,, derivation of absorbed dose to water and the absorbed dose calibration service will co-exist as alternative approaches to high-energy photon dosimetry for the foreseeable future and that NPL will continue to maintain the 2 MV primary standard. The purpose of this Code of Practice is to recommend procedures for calibrating field instruments using a secondary standard which has been calibrated by the NPL absorbed dose service, and procedures for measuring radiation output from a cobalt-60 unit or a high-energy accelerator. The procedures assume that the secondary standard and the field instrument each consist of an unsealed ionisation chamber connected to an electrometer by a cable. The recommended secondary standard dosemeter for high-energy photon dosimetry in the UK continues to be the NPL Secondary Standard X-ray Exposure Meter manufactured by NE Technology Ltd (chamber NE 2561 and electrometer NE 2560 or Dosemaster NE 2590). The previous Code of Practice (HPA 1983) remains valid and should continue to be used where a secondary standard has been calibrated in air only, and not by the NPL absorbed dose service. Secondary Standard Centres will need to consider whether to change to the absorbed dose calibration service and, if so, when. Advice for UK centres will be issued by the Institute of Physical Sciences in Medicine.

https://iopscience.iop.org/article/10.1088/0031-9155/35/10/301/pdf

Code of Practice for high-energy photon therapy dosimetry based on the NPL absorbed dose calibration service

The recombination correction for a small cylindrical ionization chamber exposed to pulsed radiation is reexamined in the light of new experimental evidence. A simple two-voltage testing technique is described and demonstrated. In general the existing theory for an air-filled chamber (Boag 1950) is found to be accurate over the range of charge densities examined (0.013 p/pulse to 3.8 p/pulse) where p=3.33 X 10(-4)C. m-3 (1 esu cm-3).

Current collection and ionic recombination in small cylindrical ionization chambers exposed to pulsed radiation

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Current collection and ionic recombination in small cylindrical ionization chambers exposed to pulsed radiation