(Sunday, 7/12/2020) [Eastern Time (GMT-4)]
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.
Keywords
Protons, Commissioning, Modeling
Taxonomy
TH- External Beam- Particle/high LET therapy: Proton therapy – dose optimization
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