Skin dose assessment at diagnostic and therapeutic photon energies: A Monte Carlo study on TLDs

In radiotherapy but also as a part of quality assurance in diagnostic imaging, assessment of radiation dose to the skin of the patient is of particular importance. While skin dose is recommended to be measured at a reference depth of 70 μm, in practice in both diagnostic radiology as well as therapeutic applications, the dosimeter is placed on the skin surface, the measured dose depending in large part on dosimeter thickness. Thermoluminescent dosimeters (TLDs) are commonly applied as skin dosimeters, most of the commercial forms being thicker than 140 μm, also with a density greater than that of soft tissue, accordingly representing in effective point of measurement deeper than 70 μm. On the other hand, Monte Carlo (MC) simulations can be used to accurately calculate the dose at an intended depth. In this work we first validate our MC simulations, comparison being made with a previously published correction factor for LiF TLD of dimensions 3 × 3 × 0.1 mm3. Then correction factors to convert the dose measured by various commercial LiF-based TLDs (with thicknesses ranging from 0.15 to 1 mm) to skin dose at 70 μm depth are calculated for RQR reference diagnostic photon spectra and some common MV photon therapy spectra. These results are useful for correction of TLD responses in various clinical situations. •ICRP recommends the skin dose to be assessed at the depth of 70 μm in skin.•TLDs are common dosimeters for skin dose measurements.•Due to TLD thickness, its effective point of measurements is typically deeper than 70 μm.•In this work, various size commercial LiF TLDs are simulated using MCNPX code.•Correction factors to convert TLD dose to skin dose in various photon beams are calculated.