Three-dimensional dosimetry of dose degradations around gold markers in spot-scanning proton therapy

Fiducial markers are necessary for some tumor sites to ensure safe and accurate delivery of radiotherapy including proton therapy (PT). However, the high metal content of the markers may lead to shadowing of the proton beam. The aim of this study was to investigate the dose degradations around three fiducial marker types implanted in different configurations into a 3D dosimeter. The dosimeters were irradiated with a clinically relevant prostate cancer PT plan. Gamma comparisons (2%-2mm) between the control (no marker) and marker dosimeters resulted in a pass rate of 97%, and no significant differences were observed in the isodoses, indicating that markers did not affect dose coverage to the target. However, due to optical artifacts, the 3D dosimeter was unable to resolve the regions within a few millimeters from the markers.