Dosimetry of intensive synchrotron microbeams

Intensive synchrotron X-ray microbeams form an integral part of microbeam radiation therapy (MRT). MRT is a novel radiation medicine modality being developed for inoperable and otherwise untreatable brain tumours. The extremely high dose rate (∼20 kGy/s), laterally fractionated radiation field and steep dose gradients utilized in this therapy make real-time dosimetry a significant challenge. In order for this treatment to advance to the clinical trial stage of development real-time dosimetry systems must be developed. This paper demonstrates the capabilities of a new dosimetry system based on an epitaxial silicon detector. The system combines high spatial resolution and real-time readout and we have measured the lateral dose profile of the MRT radiation field which incorporates 59 X-ray microbeams. All microbeam peaks and valley regions between two microbeams are clearly resolved. The measured detector response at any point is reproducible to within 0.5% after scaling for the known synchrotron storage ring beam current lifetime. The variation of the lateral dose profile at different depths in a PMMA phantom has been measured with the results compared to those from Penelope Monte Carlo simulations. The trend in the measured response with depth agrees with the simulation data (within the experimental variation of the central five microbeams peaks and valleys measured). However the measured peak-to-valley ratio response is a factor of 4.5 ± 0.1 times lower than that expected. The disagreement was further investigated and shown to be contributed to by charge recombination effects at the low bias voltages used.