Water equivalent thickness of immobilization devices in proton therapy planning – Modelling at treatment planning and validation by measurements with a multi-layer ionization chamber

Abstract Purpose To investigate the range errors made in treatment planning due to the presence of the immobilization devices along the proton beam path. Methods The measured water equivalent thickness (WET) of selected devices was measured by a high-energy spot and a multi-layer ionization chamber and compared with that predicted by treatment planning system (TPS). Two treatment couches, two thermoplastic masks (both un-stretched and stretched) and one headrest were selected. At TPS, every immobilization device was modelled as being part of the patient. The following parameters were assessed: CT acquisition protocol, dose-calculation grid-sizes (1.5 and 3.0 mm) and beam-entrance with respect to the devices (coplanar and non-coplanar). Finally, the potential errors produced by a wrong manual separation between treatment couch and the CT table (not present during treatment) were investigated. Results In the thermoplastic mask, there was a clear effect due to beam entrance, a moderate effect due to the CT protocols and almost no effect due to TPS grid-size, with 1 mm errors observed only when thick un-stretched portions were crossed by non-coplanar beams. In the treatment couches the WET errors were negligible ( 0.5 mm with a 3.0 mm grid-size. In the headrest, WET errors were negligible (0.2 mm). Conclusions With only one exception (un-stretched mask, non-coplanar beams), the WET of all the immobilization devices was properly modelled by the TPS.