ESTRO 2021 Abstract Book

S1626

ESTRO 2021

Purpose or Objective Proton therapy (PT) remains a limited resource due to cost and gantry size. Recently, a new design for PT without a gantry has been suggested, which may enable combined proton-photon therapy (CPPT) in conventional bunkers and allow the widespread use of protons. In this work, we explore this concept for breast cancer. Materials and Methods The treatment room consists of a LINAC for IMRT, a horizontal or 45° inclined fixed proton beamline (FBL) with beam scanning and a motorized couch for treatments in lying positions with accurate patient setup. Thereby, proton and photon beams can be delivered in the same fraction. Treatment planning is performed by simultaneously optimizing IMRT and IMPT plans based on their cumulative physical dose. The concept is applied to three breast cancer cases in deep inspiration breath-hold: two left-sided with and without lymph node involvement (patients 1 and 2) and one right-sided with lymph node involvement (patient 3). Main objectives are to minimize mean dose in lung and heart while delivering 40.05 Gy in 15 fractions to the PTV with a SIB of 48 Gy to the boost volume. To emulate VMAT treatments, IMRT planning uses 13 beams, whereas IMPT planning with the horizontal or inclined FBL uses 3 or 4 fields. For all patients, similar beam settings and objective functions are used and hybrid robust planning is employed by applying a 5 mm PTV margin for setup uncertainty and stochastic optimization to mitigate range uncertainties. Range uncertainty is modeled via two components: 1) expansion or contraction of the patient surface by ±3 mm with overwrite of relative electron density for water or air to model anatomical changes such as breast swelling and 2) up- and downscaling of relative electron densities by ±3% to model uncertainty in the CT imaging/calibration process. Results CPPT decreases the mean dose to the lung, heart and remaining healthy tissues compared to IMRT without compromising target coverage (table 1). Thereby, protons deliver dose to the edges of the target and parts of the breast whereas photons improve target conformity by irradiating the interface of lung and breast tissue with tangential beams (figure 1a-c). When assuming smaller range errors than ±3mm/3%, the proton contribution increases, while accounting for larger range errors increases the photon contribution (figure 1d). In addition, CPPT improves the robustness against range uncertainties compared to IMPT by reducing mean lung dose under conditions of range overshoot, whilst maintaining target coverage for range undershoot (figure 1e-f). Both FBL angles, horizontal and 45°, suggest a similar benefit of CPPT over single modality

treatments.

Conclusion