SU‐FF‐T‐268: Study of Field Size Factors for Passively Scattered Proton Therapy Beams

Purpose: To measure the field size factors (FSF) for passively scattered proton therapy beams and study its dependence on aperture size, proton energy, air gap or snout position, and spread‐out Bragg peak (SOBP) width. Method and Materials: A pinpoint ion chamber in plastic phantoms was used to measure the FSF for square apertures with sides of 2, 3, 5, 18 cm and circular apertures with 2 to 6 cm diameter relative to the 10 × 10 cm² apertures. The chamber location was kept fixed at the isocenter. The FSF were measured at the center of SOBPs for 120 to 250 MeV proton beams as a function of air gap between the aperture and phantom surface and SOBP widths. Results: The FSF dropped significantly when the aperture size became smaller than 3 × 3 cm². For a 6 cm SOBP and largest possible air gap, the FSF for a 2 × 2 cm² field was 0.95 for a 120 MeV beam and decreased to 0.84 as the energy increased to 250 MeV. Increase in SOBP width, decrease in the air gap, and the increase of aperture size beyond 3 × 3 cm² caused relatively smaller change in the FSF. Conclusion: The change in FSF for small fields may be attributed to two competing processes, the reduction of the primary fluence due to the decrease in aperture size and increase in aperture edge scattering. For low energy beams and small air gaps, the FSF is close to 1 with the increased edge scattering compensating the loss of fluence. With increasing energy and air gap, primary fluence reduction overtakes the aperture edge scattering and causes larger reductions in FSF. A possible model for predicting the FSF for small irregular fields based on this study is being explored and will be discussed.