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A project from the EMPIR normative call has helped revise a guide for the safe calibration of radiotherapy equipment used in cancer treatment

The EMPIR project “kQ factors in modern external beam radiotherapy applications to update IAEA TRS-398” (16NRM03, RTNORM).

Around 3.4 million Europeans are diagnosed with cancer every year and about half of the resulting treatments involve radiation therapy with ionising radiation.  Accurate radiation beam delivery and dosimetry are critical for patient health as using too high a dose of radiation can cause damage to healthy tissues and too low means that the cancer may not be eradicated. 

Hospitals are required to perform measurements, in accordance with validated measurement codes of practice or protocols, ensuring that doses delivered to patients at European hospitals are traceable to the quantity ‘absorbed dose to water’ which relates closely to the biological effects of radiation.

These protocols are required to correct the dosimeter response for differences between the variations in beam quality at the calibration laboratory (Q0) and the beam qualities at the hospitals (kQ). These corrections are called ‘beam quality correction factors’ and are represented by  kQ,Q0 factors.

Before the project

Previously the ‘absorbed dose to water’ value used in calibrations was reached via an ‘air kerma’ route – a measurement of radiation quantity that is done in air and that required some extensive calculations involved several correction factors for conversion to reach the desired value of absorbed dose to water, a complicated process which can lead to errors in the dose delivered to patients.

To avoid this added complexity recommendations were provided by the International Atomic Energy Agency (IAEA) in a code of Practice (the TRS-398) published in 2000 which recommends the determination of the quantity ‘absorbed dose to water’ using measurements with ionisation chambers directly traceable to primary standards of absorbed dose to water, such as calorimeters.

However, in the 20 years since this guidance was introduced new types of chambers, modelling software and beam types – such as high energy photon beams (MV) and proton therapy, have been introduced – meaning for the guide to be as relevant and accurate as possible it requires updating.

Project achievements

This is now in progress with a large input from the now completed project RTNORM within EURAMET’s European Metrology Programme for Innovation and Research.
During its lifetime the project:

1. Updated kQ,Q0 factors for kV x-rays between 100 kV and 250 kV – during which three calorimeters for measuring ‘absorbed dose to water’ were improved in the project.
2. Updated kQ factors for high-energy (MV) photons beams between 4 MV and 20 MV, including flattening filter free beams (FFF) for 10 different types of ionisation chambers.
3. Updated kQ,Q0 factors for scanned proton beams – this included the first ever work using different Monte Carlo codes capable of transporting protons kQ-factor calculation in scanned proton beams.

Results have now fed into 3 of the 6 chapters being updated in the revised guide, expected to be released in 2022 or early 2023:  TRS-398 TG ‘kV X-rays’, TRS-398 TG ‘high energy photons’, and TRS-398 ‘protons and heavy ion beams’.

Massimo Pinto (ENEA-INMRI), who coordinated the project and presented an invited keynote lecture in Japan on some of the results said about the work 

"RTNORM offered the opportunity to bring together measurements and computation experts from European metrology institutes and universities. This allowed us to go beyond the conventional way we each, singularly, measure and compute such values, with more appropriate uncertainty estimates and a deeper confidence in the accuracy of our results.”

The RTNORM project, and the substantial work performed, will help ensure that the new update is as accurate and simple as possible, ensuring those who suffer from cancer can expect the best treatment available in Europe and worldwide.

This EMPIR project is co-funded by the European Union's Horizon 2020 research and innovation programme and the EMPIR Participating States.

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