IPEM code of practice for high-energy photon therapy dosimetry based on the NPL absorbed dose calibration service

Abstract

The 1990 code of practice (COP), produced by the IPSM (now the Institute of Physics and Engineering in Medicine, IPEM) and the UK National Physical Laboratory (NPL), gave instructions for determining absorbed dose to water for megavoltage photon (MV) radiotherapy beams (Lillicrap et al. Phys. Med. Biol. 1990 35 1355. The simplicity and clarity of the 1990 COP led to widespread uptake and high levels of consistency in external dosimetry audits. An addendum was published in 2014 to include the non-conventional conditions in Tomotherapy units. However, the 1990 COP lacked detailed recommendations for calibration conditions, and the corresponding nomenclature, to account for modern treatment units with different reference fields, including small fields as described in IAEA TRS483 (International Atomic Energy Agency, Vienna 2017). This updated COP recommends the irradiation geometries, the choice of ionisation chambers, appropriate correction factors and the derivation of absorbed dose to water calibration coefficients, for carrying out reference dosimetry measurements on MV external beam radiotherapy machines. It also includes worked examples of application to different conditions.The strengths of the 1990 COP are retained: recommending the NPL2611 chamber type as secondary standard; the use of tissue phantom ratio (TPR) as the beam quality specifier; and NPL-provided direct calibration coefficients for the user's chamber in a range of beam qualities similar to those in clinical use. In addition, the formalism is now extended to units that cannot achieve the standard reference field size of 10 cm × 10 cm, and recommendations are given for measuring dose in non-reference conditions. This COP is designed around the service that NPL provides and thus it does not require the range of different options presented in TRS483, such as generic correction factors for beam quality. This approach results in a significantly simpler, more concise and easier to follow protocol.

Evaluation of a micro ionization chamber for dosimetric measurements in image-guided preclinical irradiation platforms

“…Dose traceability for the small fields delivered by preclinical devices would benefit from a similar strategy as the one recommended by the IAEA TRS-483 CoP (IAEA 2017) and recently adopted by the update of the IPEM code of practice on reference dosimetry for megavoltage radiotherapy devices (Eaton et al 2020). TRS-483 formalism is based on the introduction of an intermediate calibration field in the form of a static machine specific reference (msr) or a field nominated as a plan class specific reference (pcsr) which, by definition, are closer to specific reference and clinical fields delivered by the radiotherapy devices.…”

Report dose-to-medium in clinical trials where available; a consensus from the Global Harmonisation Group to maximize consistency

“…One inconsistency in radiation oncology is the medium in which radiation transport and dose deposition are calculated. While radiotherapy equipment is calibrated in terms of dose-towater according to protocols such as TRS-398 [3], TG-51 [4,5], or the UK protocol [6,7], the corresponding dose calculation in the treatment planning system (TPS) is not consistent in terms of which medium is used and can be described either as dose to water or dose to medium.…”

Evaluation of a complementary metal oxide semiconductor detector as a tool for stereotactic body radiotherapy plan quality assurance

“…These measurements were replicated using a farmer chamber, and a ratio of CMOS reading/chamber reading was used to define the detector’s DPP sensitivity. Measurements for ion recombination effects at each DPP were performed according to the two voltage method [22] . DPP values were calculated by scaling a literature value for a Elekta versa 6MV FFF [23] by ion chamber ratios measured under reference and our conditions.…”