Room: AAPM ePoster Library
Purpose: Nowadays, vendors of multi-room proton pencil-beam scanning (PBS) facilities are capable of delivering “matched” rooms. As has long been the case for linac-based clinics, this similarly now offers proton centers the prospect of flexibility in patient scheduling because a single beam model may, in principle, be employed for all rooms. However, while matching is typically done at the primary spot sigma level, the residual unmatched, low-intensity spot “halo” can limit the degree of success of this endeavor. Here, we demonstrate a method to best handle this room-dependent halo when deriving a beam model unified across multiple rooms.
Methods: Since the variation in output with field size at the center of uniformly-irradiated PBS energy layers is sensitive to the underlying spot shapes, measurements of this variation were made in air for field-sizes of 40–250 mm in three different PBS rooms at our facility in order to characterize systematically each room’s spot halo as a function of energy (100–225 MeV) and 75 mm water-equivalent-thick range-shifter (absent/present). A room-averaged variation for each measurement condition was fitted with a parameterized, theoretical function, to extract the characteristics of the low intensity halo most appropriate for beam model input.
Results: For beams without the range-shifter, the resultant unified room-independent model of the spot halo was able to predict output in air to within ~2% for all field sizes and all rooms; this improved the higher the energy. Agreement to within ~1% was achieved, even in the worst case, when the range-shifter was included.
Conclusion: Measurements of output variation with field size, coupled with judicious determination of parameters characterizing the spot halo, can be used to generate a single, best, unified, inter-room beam model, maximizing the likelihood that clinically acceptable dose calculation accuracy can be achieved simultaneously in all rooms for any combination of beam parameters.