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Characterization of the Low-Dose Halo of Proton PBS Spots with and Without a Range-Shifter

C Teng

C Teng*, C Ainsley , University of Pennsylvania, Philadelphia, PA


SU-I-GPD-T-130 (Sunday, July 30, 2017) 3:00 PM - 6:00 PM Room: Exhibit Hall

Purpose: The modeling of the low-dose spot “halo” is an important factor in determining the accuracy of dose calculation for proton PBS. We characterize this halo as a function of air-gap and water depth for different proton beam energies, both with and without a range-shifter.

Methods: We reported previously on our observation that the variation in output with field size at the center of uniformly-irradiated energy layers is sensitive to the underlying spot shapes. Here, we make measurements of this variation for field-sizes covering 40–250 mm at several depths in water spanning from the surface to near the end-of-range, for three different air-gaps, with and without a range-shifter, and for incident beam energies of 100 and 225 MeV (the two clinically-deliverable extremes). Via a novel technique to be presented, we extract the weight and width of the halo contribution to double-Gaussian functional descriptions of the spot profile at each point.

Results: With no range-shifter present, the shape of the output-versus-field-size curve (thus, the halo) for the low energy beam shows very little depth-dependence, but varies by room; by contrast, the curve for the high energy beam (thus, the halo) has significant depth-dependence, but exhibits little inter-room variation. The effects of including a range-shifter and of variable air-gap are currently being studied.

Conclusion: Without a range-shifter, the halo is dominated by the in-air contribution at 100 MeV and by the in-water contribution at 225 MeV. The consequences of including a range-shifter will be determined.

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