Lack of backscatter factor measurements in HDR applications with MOSkins

Measurements of backscatter correction factors for intra operative (IOBT) HDR brachytherapy applicators were made using Centre for Medical Radiation Physics (CMRP), MOSFET devices. In clinical use there is an absence of backscatter material above the IOBT applicator, leading to a lower dose than predicted by conventional TG-43 dose calculations. To estimate the uncertainty in the MOSFET measurements, the dosimetric characteristics, including reproducibility, stability, linearity, and angular and energy response were measured using a HDR Ir-192 source, kilovoltage treatment unit and a high energy linac. Measurements were compared with previously published Monte Carlo data. Variability of the response of the MOSFETs due to angular variation contributed the largest uncertainty in dose measurements. Using the IOBT applicator without adequate scatter material resulted in a reduction of delivered dose of on average 10%, but was dependent on the location on the applicator and the treatment field size. Theoretical calculations based on previously published study indicated an expected reduced dose of on average 4%. MOSFET devices provide an ideal measurement tool in the presence of high dose gradients, however, the dosimetric characteristics of the detector must be accounted for when estimating the uncertainty.