Determination of absorbed dose at different methods of target irradiation with scanning proton beam by means of chemical dosimeter FBX

In highly conformal proton therapy treatments it is very important that doses calculated with the use of the treatment planning system (TPS) and virtual absorbed doses to a tumor and surrounding tissues should coincide. This requirement is necessary to be fulfilled, in order to protect surrounding healthy organs and tissues. The purpose of the study is to estimate possibility of using chemical dosimetry for measuring an absorbed dose and verification of the computed TPS doses when dose is delivered to a target volume at one or several different angles. The high sensitivity FBX-system (Ferrous (II) sulfate - Benzoic acid v Xylenol orange) served as a dosimeter. We performed irradiations at the proton accelerator “Prometheus” (manufacturer - PROTOM, Protvino, Russia) from one and three directions (0°, 90°, 180°) in three different ways: from one direction either with one dose fraction or with several fractions of 1 Gy; and from three directions with one dose fraction, the prescribed total dose being equal for all scenarios. The dose range was 1-5 Gy. The results obtained with the FBX-dosimeter show that there are no statistically significant differences between the dose delivery methods (one or three fields, one or several dose fractions). Comparison of the proton irradiation data with the calibration curve for 60Co g-radiation shows significant differences, which we explain by different values of linear energy transfer for protons (LET range ~5-20 keV/μm) and 60Со g-radiation (~0.25-0.3 keV/μm). The data obtained allow to conclude that in the clinical and experimental dosimetry of scanning proton beams, the FBX dosimetric system can be used effectively. It also points out the necessity to consider the dose average LET in the TPS to enhance the therapy efficiency and quality assurance. However, one must preliminary calibrate the FBX dosimeter under the same proton energy-LET conditions as in the tested TPS dose distributions for successful use in clinical and experimental applications.