SU-E-T-362: Small Field Dosimetry for Double Scattering Proton Beams

Abstract

Purpose: This study examines the accuracy of dose calculation in small square fields for double scattering proton beams using an empirical method based on dose per incident fluence. Methods: Fractional Depth doses (FDD) at SSD = 220 cm for open beam and small square fields (s = 1, 2, 3, 5, 10 and 15cm) are renormalized to surface. The resulting pristine peaks depth dose is normalized to the broad beam FDD to determine a fractional FDD, FDDf for selected proton energies (90 – 200 MeV). The head scatter factor (H), defined as the ratio of proton energy fluence at point of interest to 10×10 cm2 at otherwise the same geometry, is measured using a diode in air without buildup cap as a function of square field sizes for proton energies. The phantom scatter factor, PSF=BF/H, defined as the ratio of blocking factor in water at reference depth and is measured as a function of field size, energy, and SSD. Results: It is found that H is a single function of renormalized equivalent square, sN = s*230/(SSD+d) and is independent of SSD for a given proton energy. Normally, the s dependence of H is a function of SSD. The extrapolated value of PSF from percent depth dose (PDD) renormalized to the surface is remarkably close to the measured PSF data, to within 2%. Conclusions: By normalizing the Fractional depth dose (FDD) to the surface, most characteristics of small field dosimetry can be simplified and adequately described by H, a quantity describing the incident proton energy fluence.