A procedure for accurate dose measurement in radiotherapy research at millimetre-sized synchrotron beams

<i>Objective.</i>Synchrotron-based spatially fractionated radiotherapy and ultra-high dose rate (UHDR) radiotherapy have been shown to better spare healthy tissue function in comparison to conventional radiotherapy, while controlling the tumour with the same efficacy. In recent years, an increasing amount of research has been carried out in these fields with promising results. However, further experiments remain essential, since the underlying mechanisms of healthy tissue preservation are not yet fully understood. The characterisation of synchrotron beamlines at the Deutsches Elektronen-Synchrotron in Hamburg represents an opportunity to increase the number of sites where pre-clinical studies could be conducted in the future. However, the beams available at this synchrotron are only a few millimetres in size and measuring absorbed dose with established detectors and dosimetry protocols represents a challenge.<i>Approach.</i>We show a procedure to accurately determine the beam dose rate under such conditions by first characterising a monochromatic beamline. After validation, and with the support of Monte Carlo simulations, the procedure is adapted to investigate a white-beam beamline, at which photon flux and mean energy can be varied with Cu absorbers.<i>Main results.</i>With the developed procedure, it is possible to measure absorbed dose at these beamlines with relative uncertainties below 10%. In particular, at the white-beam beamline, the dose rate varies between about 20 Gy s^-1and about 1800 Gy s^-1, thus offering the opportunity to carry out much-needed systematic studies. Moreover, pilot experiments with a mouse phantom demonstrate that it is possible to treat small animals with such small-sized beams by using dose-painting techniques, with an agreement between prescribed and delivered dose within ±15%.<i>Significance</i>. This work represents a first step towards the implementation of reproducible pre-clinical studies at the PETRA III synchrotron, further contributing to a transition of spatially-fractionated and UHDR radiotherapy techniques into clinical practice.