SU‐F‐T‐52: Study of Energy Dependent Effect of Dosimetry Systems Used in Therapeutic Soft X‐Ray Energy Range

Abstract

Purpose:To investigate energy dependent effects of different dosimetry systems which can be used as in vivo dosimetry monitoring for intraoperative radiotherapy in therapeutic soft x‐ray energy range.Methods:Three dosimetry systems were evaluated in therapeutic soft x‐ray energy range: optically stimulated luminescent dosimeter (OSLD) nanoDots, radiochromic EBT2 and EBT3 films. The x‐ray photons were produced by a Zeiss Intrabeam 50 kV x‐ray radiotherapy system. Solid water and bolus slabs with different thicknesses were used in the process of irradiation. An aluminum filter set was used to measure HVLs of X‐rays. Calibration curves were made at different depth of boluses.Results:Half Value Layers at depths of 0, 3, 10, and 20 mm of solid water were measured to represent the energy change versus depth, yielding 0.306, 0.482, 0.865 and 0.901 respectively and indicating nearly unchanged HVL beyond 1 cm depth. The responses of each system at different depths were normalized to the response at 2 cm depth. In film dosimetry, the response is calculated as optical density (OD). The results show that there is nearly the same energy dependence for EBT2 and EBT3. At a HVL of 0.482 mm Al, the relative responses of nanoDots and EBT3 are 0.85 ± 0.04 and 0.89 ± 0.03 compared to those at 0.901 mm Al HVL, respectively, indicating no obvious difference between those two systems within the measurement uncertainty.Conclusion:It was observed that the studied dosimeter response increases about 13% from the x‐ray energy of 0.48 mm Al to 0.90 mm Al. Therefore, caution should be exercised in using an appropriate calibration curve, and x‐ray beam hardening effect has to be taken into account.