ESTRO 36 Abstract Book

S446 ESTRO 36 _______________________________________________________________________________________________

result in more conformal and homogenous irradiation, though robustness for anatomical and posture variations is possibly an issue. Compared to conventional plans, the beams are not fully opposing and fields cannot be opened manually outside the outer contour of the breast and the body. Therefore, in this study we evaluated the robustness of both an IMRT and a VMAT technique for daily variations in patient positioning in comparison to our conventional technique. Material and Methods 20 Patients treated with a dose of 16x2.66 Gy using a conventional technique to the breast and axillary lymph nodes levels I to IV (Figure 1a) were replanned with both an IMRT and a VMAT technique using Pinnacle autoplanning. The IMRT technique consisted of 6 beams with 20 o spacing, while the VMAT technique consisted of opposing pairs of 24 o arcs (Figure 1). The delivered dose was calculated using the cone beam CT (CBCT) (Elekta XVI) images for each fraction to quantify the influence of patient positioning, both for an online and offline correction protocol. Contours were transferred from planning CT to CBCT by deformable image registration using Mirada RTx. Density overrides were applied to account for imperfections in Hounsfield unit values on the CBCT. IMRT and VMAT techniques were compared to the conventional technique for the V95%, conformity index (CI), mean lung dose and mean heart dose. The CTV-PTV margin used is 7mm. Since the setup error is accounted for when evaluating dose on the CBCT, we used the CTV for the evaluation. Results Evaluation of the treatment plans for 20 patients showed that V95% coverage of IMRT and VMAT plans was comparable to conventional plans (Table 1). Conformity was significantly higher for IMRT and VMAT. Mean lung dose was approximately 0.7 Gy lower on average, while mean heart dose increased by approximately 0.7 Gy using IMRT or VMAT. Robustness evaluation of the dose on daily CBCT’s using an online positioning protocol showed that V95% coverage remained stable for conventional, IMRT an VMAT. Significant conformity improvement was obtained using both IMRT and VMAT, and small differences in mean heart dose (+0.7 G) and mean lung dose (-0.8 Gy) were found. Evaluation of an offline positioning protocol showed similar results. Conclusion Presented IMRT and VMAT techniques show a similar robustness for interfraction motion in locoregional breast irradiation compared to the conventional technique, while conformity of the target volume is increased significantly. An offline positioning protocol would be sufficient for clinically acceptable set-up accuracy.

PO-0830 Quantification of density and tissue changes in pencil beam scanning proton treatment. F. Van den Heuvel 1 , F. Fiorini 1 , B. George 1 1 University of Oxford, CRUK/MRC Oxford Institute for Radiation Oncology, Oxford, United Kingdom Purpose or Objective Proton pencil beam scanning (PBS) is becoming the methodology of choice to deliver proton therapy in many cases. Several authors have reported discrepancies between the dose distributions generated by commercial planning systems, using analytical models, compared to those using stochastic methods. The differences are greatest in areas with extensive tissue inhomogeneities. In analytically based commercial planning systems, inhomogeneities are taken into account using a water equivalent path length (WEPL) scaling. In this work we quantify and investigate the impact of different densities and tissue on the dose deposition characteristics of a A single pencil beam with nominal energy 226 MeV from an IBA-facility is modeled in homogenous cubic 40x40x40 cm3 phantom using FLUKA. The pencil beam’s dose deposition is uniquely characterised using a stable proton pencil beam. Material and Methods