Abstract Book

S45

ESTRO 37

OC-0087 Clinical validation of Monte Carlo dose calculation for pencil beam scanning proton therapy. L. Widesott 1 , S. Lorentini 1 , F. Fracchiolla 1 , P. Farace 1 , M. Schwarz 1 1 Ospedale Santa Chiara di Trento, Centro di protonterapia, Trento, Italy Purpose or Objective Validation of a commercial Monte Carlo (MC) algorithm (RayStation (version 6.0.024) ) for the treatment of head and neck (H&N) tumours with pencil beam scanning (PBS) proton therapy, comparing it via measurements and analytical calculations in clinically realistic scenarios. Material and Methods For the measurements a 2D ion chamber array detector (MatriXX PT, IBA Dosimetry GmbH) was placed underneath the following targets: 1) anthropomorphic head phantom (with two different thickness) and 2) a biological sample (i.e. half lamb’s head). In addition, we compared the MC dose engine vs. the RayStation pencil beam (PB) algorithm clinically implemented so far, in critical conditions such as superficial targets (i.e. in need of range shifter), different air gaps and different gantry angles to simulate both orthogonal and tangential beam arrangements. For every plan the PB and MC dose calculation were compared to measurements using a gamma analysis metrics with passing criteria of 3% of maximum dose, 3mm distance-to-agreement, global approach, and dose threshold of 5%. Results For both the configurations of the head phantom (i.e. one and two slabs) the gamma passing rate (GPR) was always >96% and on average > 99% for the MC algorithm; PB algorithm had a GPR ≤90% for all the delivery configurations with single slab (apart 95 % GPR from gantry 0° and small air gap) and in case of two slabs of the head phantom the GPR was >95% only in case of small air gaps for all the three (0°, 45°,and 70°) simulated beam gantry angles. Overall the PB algorithm tends to overestimate the dose to the target (up to 25%) and underestimate the dose to the organ at risk (up to 30%) (Figure 1). We found similar results (but a bit worse for PB algorithm) for the two targets of the lamb’s head where only two beam gantry angles were simulated (Table1).

Conclusion This study confirmed the advantages of DECT-based SPR estimation for organic tissue samples. We validated the SPR estimation methods on several homogenous tissue samples, including both soft tissues and bone, with reference SPR values obtained from proton pencil beam measurements. This study therefore showed that using clinical CT doses, DECT improved the SPR estimation accuracy for organic tissue samples compared SECT. These results show that DECT can be used to improve proton treatment planning and reduce range uncertainty margins.