Modeling and optimization of respiratory-gated partial breast irradiation with proton beams - A Monte Carlo study

The selection of a suitable duty factor (DF) remains a major challenge in respiratory-gated treatments. Therefore, this study aims at presenting a new methodology for fast optimizing the gating window width (duty factor (DF)) in respiratory-gated proton partial breast irradiation (PBI). To do so, GATE Monte Carlo simulations were performed for various target sizes and locations in supine and prone positions. Three different duty factors of 20, 25, and 33% were considered. Sparing factors (SF) for four organs-at-risk (OARs) were then assessed. The weighted-sum method was employed to search for an optimal DF. The results indicate that an SF higher than unity was obtained for all plans. The SF also depends on the target size/location and the patient positioning. By increasing the DF, SF monotonically decreases. Optimal DF was found to be 25% and 20% for shallow-/laterally- and medially-located targets, respectively. It can be concluded that for PBI using multiple passively scattered proton fields with large hinge angles, the respiratory-gated treatment addresses the intrafractional target motion and the extent of its impact remains patient specific.