Photon absorbed dose standards

Abstract

In this review the current status of absorbed dose to water standards for high-energy photon beams (60Co—50 MV nominal accelerating potential) is discussed. The review is focused on calorimeter-based absorbed dose standards for photon radiation therapy calibrations with typical dose rates of a few gray per minute. In addition, two alternative types of absorbed dose standards are also discussed. The overall uncertainty on measured dose to water in static reference fields is nowadays on the order of 0.4% to 0.5%. The components contributing to the uncertainty budgets are discussed. The discussed absorbed dose to water standards are expected to continue to have their place not only in the dissemination of absorbed dose to water but also in the determination of beam quality conversion factors essential in reference dosimetry in high-energy photon beams.

Aerrow: A probe‐format graphite calorimeter for absolute dosimetry of high‐energy photon beams in the clinical environment

“…In addition to this type A uncertainty, there are systematic uncertainties associated with photon cross‐sections and mean excitation energies that have not been considered in this work. These have been assigned a standard uncertainty of 0.35% to be consistent with other graphite calorimeters . For variance reduction, a cross‐section enhancement factor of 256, and a Russian roulette survival factor ( N R ) of 512 with an associated ESAVE of 1 MeV were used.…”

“…Even among primary standards, calorimetry is considered the most direct and absolute method of measuring absorbed radiation dose since device calibration can be achieved in terms of quantities with traceable standards (i.e., electrical and temperature), entirely independent of radiation . This avoids the need to rely on dosimetric quantities such as (the average energy required to produce an ion pair in dry air) and () (the product of the molar extinction coefficient and the radiation chemical yield of ferric ions), the knowledge of which are relatively more uncertain than current electrical and temperature‐based standards . To date, calorimeter designs have primarily been driven by national metrology institutes, whose principal motivation is to achieve the lowest possible measurement uncertainty .…”

Electron beam water calorimetry measurements to obtain beam quality conversion factors

“…Calorimeters developed by National Measurement Institutes (NMIs) worldwide have focused on two low‐Z materials relevant to the dosimetry of radiation therapy beams, water and graphite. Each material offers certain advantages for the measurement of dose and DuSautoy, Seuntjens and Duane and McEwen provide comprehensive reviews of calorimetry development and operation of both water and graphite calorimeters. Much of the research to date has, perhaps not surprisingly, focused on calorimetry for 60 Co and megavoltage photon beams, although investigations have been carried out for 192 Ir HDR brachytherapy and kV x‐ray beams .…”

Absolute dosimetry of a 1.5 T MR‐guided accelerator‐based high‐energy photon beam in water and solid phantoms using Aerrow

“…In general, the MK‐V Aerrow prototype shows a better overall level of agreement with established reference dosimetry techniques than its predecessor, the MK‐IV (Renaud et al 2018), partly due to an improved understanding of the mass impurity effect, but also a higher resolution thermal control system. Better estimates of the mass impurity and heat transfer corrections for the MK‐V have also decreased the overall standard uncertainty on the determination of absorbed dose to water in large‐field MV photon beams from 0.9% with the MK‐IV to 0.65%, bringing it considerably closer to being on par with standards‐level graphite calorimeters (~0.4%) . There remains room to further improve the mass impurity modeling, particularly as a function of field size, beam quality, and in regions of higher dose gradients, as well as a need for thermal modeling of the isothermal mode and an investigation into the effects of thermal conduction and convection in the surrounding water phantom on the response of the detector.…”

“…() assumes that convection and radiative heat transfer processes are negligible relative to conductive transfer between the core and the surrounding jacket. It also assumes that temperature gradients within each graphite component are generally much smaller than the gradients between bodies . This permits the assignment of a single temperature to each graphite body, and the use of effective heat transfer coefficients, h , between components.…”