SU‐GG‐T‐336: Effect of Treatment and Beam Parameters on Surface Dose in Proton Beam Therapy

Abstract

Purpose: The advantage of lower skin dose in proton beam therapy may result in less radiation‐related side effects which are typically seen in hypo‐fractionated conventional external beam therapies. In this study, we evaluate the surface dose (SD) in proton therapy as a function of various beam parameters. Materials & Methods: SD is defined as the ratio of absorbed dose on CAX at surface to that at the middle of the Spread‐Out Bragg Peak (SOBP). SD in proton therapy is affected by several parameters: energy (E), SOBP, source‐to‐surface distance (SSD), air gap (g), field radius (r), material thickness upstream of surface besides air (t), atomic number of medium (Z), beam angle relative to surface (q), and nozzle type (N). The parameters, t, Z and q are not included in this study.. Results: Giving a proton range in water of 27 cm, SD rises from 30 to 90% with SOBP increasing from 0 to17cm. At high energy, SD is 5% higher in the uniform scanning (US) nozzle than in double scattering (DS) nozzle. This can be explained by the large difference in source‐to‐axis distances in US and DS nozzles (250 cm vs 320 cm). SSD and g have minimal impact on SD for r > 2.5cm. For small fields (r < 2.5cm), SD increases significantly with field size decreases. Within 15mm of the surface there is a small but pronounced 2% buildup in proton beam at high energies, which may be partially due to secondary electrons, secondary protons and heavy charged particles. Conclusions: In a clinical setting, SD is most significantly affected by SOBP extent, followed by field size, SAD, and beam energy. In general, SD ranges from 50 to 95%. It is important to clearly understand and minimize SD in Proton therapy, especially in hypo‐fractioned treatments.