Skin dosimetry in megavoltage radiotherapy using hydrogenated amorphous silicon on a tissue-equivalent flexible substrate

Acute skin toxicity after radiation treatments highlights the need for improved dose monitoring and measurement, as the treatment planning system (TPS) does not accurately estimate skin dose. Wearable radiation detectors may be the solution for most in vivo dosimetry needs, but they require innovative materials. The purpose of this work is to overcome the challenges that limit accurate skin dosimetry by exploring the use of hydrogenated amorphous silicon (a-Si:H) fabricated on a flexible polyimide substrate for direct radiation detection. The a-Si:H was fabricated on a 125 μm polyimide substrate with a thickness of 3.6 to 10 μm. The detector's response to the build-up region of the percentage depth dose (PDD) of the MV photon beam was compared to Geant4 radiation transport simulations and benchmarked to Attix ionization chamber (IC) measurements. Angular dependency and surface field factors at the phantom surface were compared to Attix IC. The PDD measurement is within 2% of Geant4's simulation and the Attix chamber's response from 150 μm to 25 mm in a plastic water phantom. All samples showed linear dose responses with 0.37% reproducibility. The a-Si:H device matches the Attix chamber for surface field factor measurements (6-10 MV photon beams, 5-25 cm field sizes). The angular dependence (-60o-60o) compared to the Attix IC confirms the sensor's WED of 150 ± 5 µm. This study demonstrates that a-Si:H sensors on flexible plastic substrates are suitable for accurate surface dose measurements and agree with reference detectors. This thin, flexible detector provides real time measurements and is stable under high radiation doses. The possibility to assemble with ease an array of a-Si:H pixels over large areas with different sizes and shapes makes this technology attractive for in vivo surface dose gradient monitoring for robotic and volumetric arc treatment modalities.