Extended field radiotherapy measurements in a single shot using a BaFBr-based OSL-film

This work evaluated the use of a class solution specific calibration for an extra-large BaFBr-based optically stimulated luminescence film (OSL; 43  ×  35 cm²; Z _eff  =  4.55). The clinical need for such large dosimeters follows from the increased use of extended-field radiation therapy (EFRT). E.g. for prostate cancer EFRT is currently used in the first prospective trial investigating the benefit of adding elective irradiation of the para-aortic lymph nodes in pN1 prostate cancer. The full extent of these EFRT dose distributions is not covered by the well-established standard sized radiochromic film or 2D detector arrays. Here we investigate an OSL calibration methodology, that tackles BaFBr-based OSL's inherent energy dependence by a class solution specific calibration. 10 EFRT treatment plans used in the PART trial were investigated. One plan was used to build a class solution specific bilinear calibration model, that distinguishes between in-field and penumbra dose contributions. The effect of this calibration was evaluated with respect to a standard linear calibration, using standard IMRT patterns, the nine remaining patient plans, and to smaller prostate treatment plans. A single OSL-dosimeter could be reused for all measurements. The dosimeter captured the full extent of the dose distributions (maximum EFRT field size  =  33.5 cm). The bilinear correction reduced the residual dose differences from above 10% to an average of 0.7% (max 3.6%) in comparison with a Monte Carlo simulation. Consequently global gamma agreement scores (3%-3 mm) of 95.5%  ±  2.7% were reached. A more strict local evaluation resulted in an average gamma-agreement score of 93.3%  ±  3.2%. The BaFBr-based OSL film, with reduced Z _eff requires a class-solution specific correction. The current work shows that such a correction can be as simple as a bilinear residual dose correction driven by the measured signal. As far as we know this is the first 2D dosimeter combining reusability, a sub-mm resolution, and a size covering the typical EFRT treatment plans.