Dosimetric comparison of gantry and horizontal fixed-beam proton therapy treatment plans for base of skull chordoma

IntroductionAustralia's first proton beam therapy (PBT) centre will house a fixed-beam room and two gantry rooms. As the only PBT facility in Australia for at least the short term, there is a need to efficiently allocate treatment appointments between the gantry and fixed-beam rooms. This planning study assesses the dosimetric differences between fixed-beam and gantry-based treatment plans for base of skull chordoma, one of the core indications likely to be referred for PBT in Australia.MethodsRetrospective gantry-based and fixed-beam treatment plans were generated for five patients with base of skull chordoma. Fixed-beam plans were generated with a conventional horizontal patient positioning system. Robust intensity modulated proton therapy (IMPT) optimisation and evaluation techniques were used for both delivery systems. Plans were designed to maximise target coverage while adhering to maximum dose constraints to neighbouring critical organs at risk.ResultsRobust target coverage and integral dose were found to be approximately equivalent for the gantry-based and fixed-beam plans. Doses to specific organs at risk could be reduced with the gantry-based geometry; however, the gantry-based plans did not exhibit a general decrease in doses to organs at risk.ConclusionA fixed-beam treatment plan was found to be non-inferior to a gantry-based treatment plan for all base of skull patients included in the current study. Fixed-beam systems significantly reduce the upfront cost of proton beam therapy. A dosimetric comparison of fixed-beam and gantry IMPT delivery systems is performed for five bases of skull chordoma patients. The fixed-beam dosimetry was found to be non-inferior to the gantry dosimetry for the patients studied.image