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Description
Introduction: Detectors that can provide accurate dosimetry for microbeam radiation therapy (MRT) must satisfy a number of challenging criteria including having intrinsic radiation hardness, a high dynamic range, energy and dose rate independence, and a spatial resolution able to resolve x-ray microbeam spacings on the order of hundreds of microns.
At the Centre for Medical Radiation Physics, several solutions using electronics grade silicon devices have been explored in the last decade to measure dose in MRT. The limitations of silicon-based devices includes a small dose rate dependence and energy dependence, but most significantly, a large radiation beam distortion caused by the silicon substrate. Even in thinned devices, this distortion has dimensions comparable with the primary radiation field, resulting in a measurable distortion of the dose profiles and in the under-estimation of the peak-to-valley dose ratios. In recent years, detector architectures based on amorphous materials as sensitive volumes have been explored due to their possibility to be deposited in the form of films on flexible substrates such as Polyimide (Kapton). Two examples of these technologies are hydrogenated amorphous silicon (a-Si:H) planar diodes and solution processable organic semiconductor bulk heterojunctions (OSC). The dosimetric performance of these detectors are reported for both broad beam and microbeam modalities, and over a range of beam filtrations with experiments performed at the Australian Synchrotron in Melbourne.
Results: The synchrotron x-rays were spatially fractionated into an array of 50 microbeams with a Full-Width-Half-Max of 50 µm and a peak-to-peak distance of 400 µm to explore the potential to use amorphous thin film detectors for MRT dosimetry. The sensitivity and energy dependence of the aSi:H detectors fabricated with a combination of N, P and intrinsic a-Si:H showed a high sensitivity and an energy dependence matching closely to the attenuation coefficient ratio of Silicon against Water, despite the substrate being only 0.8 µm thick. The radiation damage of a-Si:H detectors out to 40 kGy is limited and stabilises at approximately -17% of the response in pristine conditions. Percentage depth dose profiles from the a-Si:H detector matched those from a PTW microDiamond detector to within ± 5 % for all beam filtrations, except in 3T Al-Al due to the energy dependence of the material. The microbeam field profile was reconstructed with a high spatial resolution and returned microbeam widths and peak-to-peak distances of (51 ± 1) µm and (405 ± 5) µm, respectively. The peak-to-valley dose ratio was measured as a function of depth and agrees within error to the values obtained with the PTW microDiamond. Regarding organic (OSC) detectors, the highest sensitivity of the flexible 250 nm thick film under the broad beam was determined to be (1958 ± 31) pC Gy-1cm-2 under 0 V bias. The organic x-ray sensor measured a FWHM of (51.6 ± 1.9) µm averaged across 3 beam filter conditions. The radiation tolerance of the organic detector was explored by exposing the organic detector to continuous irradiation at a 4.5 kGy/s dose-rate. The direct response decreased by 35% after a total irradiation dose of 45 kGy. The dosimetric performance of the OSC sensor with Kapton packaging was compared to an identical sample with PET packaging. Broad beam measurements of the PET sample demonstrated an additional signal from the PET fluorescence incident on the OSC active layer generating an opposing negative charge that significantly reduced the sensitivity and reliability of the output for sensitivity measurements.
Conclusions: The a-Si:H detectors proved to be comparable to commercially available dosimeters employed for quality assurance in MRT. OSCs devices show a high radiation hardness, no energy dependence, and extreme spatial resolution. The results demonstrate the need for additional considerations that must be given to the device packaging when designing flexible and low-cost radiation detectors for real-world applications. This work proves that amorphous materials are interesting alternatives for dosimetry in synchrotron-based radiotherapy modalities.
