Purpose: To evaluate the dosimetric effects of the Qfix SBRT Abdominal Compression Belt in proton radiotherapy and to validate its water equivalent thickness (WET) modelled by the Varian Eclipse treatment planning system (TPS).
Methods: Physical measurements of WET were performed using EBT3 film sandwiched between solid water slabs wrapped around by the compression belt in a way as it would be used clinically. A CT scan was acquired with the same setup for TPS WET calculation. The percent depth dose (PDD) was measured by delivering proton beam with range of 12cm on a Mevion S250 proton machine through a slit, so that the film was parallel to the beam direction and perpendicular to the long side of slit. Different parts of the belt were put in the beam path for each measurement. The relative range shift due to the presence of device was evaluated and compared to the WET modeled in the TPS.
Results: The relative range shift measured by film showed that the belt and velcro had almost 0mm WET, which is consistent with the reading of 0.1mm for belt only and 0.6mm for belt with velcro in TPS. The WETs of the paddle center and 5cm superior/inferior to the paddle center were estimated to be 4mm and 7.5mm on the film measurement, respectively, comparable to 5.3mm and 8mm, respectively, from TPS. The differences between TPS and measurements were within 1.5mm. The buckle of the belt showed high WET in TPS, with 10.5mm WET.
Conclusion: The TPS model of the compression system agrees well with measurement results, proving feasibility to use abdominal compression belt in proton therapy, provided that the system is included in dose calculation in TPS. The buckle should be avoided in the beam path due to its high WET and poor reproducibility in patient daily setup.