ESTRO 36 Abstract Book

S940 ESTRO 36 2017 _______________________________________________________________________________________________

a wide range of c-t-c distances (3-12 mm) were studied. Peak and valley doses outside the target and the minimum, maximum and mean doses inside the target were scored. The objective of the planning was to obtain a nearly homogeneous target dose in combination with low peak doses in normal tissue as well as high peak-to-valley dose ratios (PVDRs) close to the target. Results The most appropriate c-t-c distances, according to our planning objectives, for 1, 2 and 3 mm beam-element widths, were 7, 8 and 10 mm, respectively. With these c- t-c distances, a very high entrance PVDR was obtained for the 3 beam sizes (>10000). At 1 cm distance from the target, the PVDR was 9, 10 and 14, for the three beam widths studied. Inside the target, a high dose homogeneity could be obtained for these cases (σ= ±4%). When decreasing the c-t-c distance further, the PVDR decreased dramatically outside of the target. With increasing c-t-c distances, the PVDRs also increased as expected, but the overall target dose homogeneity decreased due to the appearances of cold spots. Conclusion In this work we studied the possibility to use beam- element widths in the mm range for PGT combined with crossfiring. For each proton beam-element size studied, an optimal c-t-c distance was determined according to the selected planning objectives. With the optimal parameter setting, a high target dose homogeneity could be obtained together with high PVDRs outside of the target. EP-1734 AAPM TG-119 benchmarking of a novel jawless dual level MLC collimation system D. Mihailidis 1 , R. Schuermann 1 , C. Kennedy 1 , J. Metz 1 1 University of Pennsylvania, Radiation Oncology, Philadelphia, USA Purpose or Objective To study delivery accuracy for fixed beam and volumetric intensity modulated RT (IMRT & VMAT) of a new jawless MLC collimation system mounted on a straight through linac. The AAPM TG-119 1 recommended IMRT commissioning process was used to benchmark the new MLC system and compare it with the TrueBeam Millennium (120-MLC). This new MLC has faster moving leaves that may be more optimum for faster intensity modulated deliveries. Material and Methods A prototype jawless MLC system with 28 pairs of 1cm leaves provides a 28x28cm 2 field size at 100 cm. The leaves have maximum over-travel, i.e. over 28 cm, and 100% inter-digitization. After acquiring beam data and deducing the dosimetric leaf gaps (DLG) for modeling the MLC in the planning system, we applied the test plans in TG-119 IMRT for fixed IMRT and VMAT delivery. The same test plans, using 6X-FFF (filter-free), were planned and delivered, in an identical way, on a solid water phantom with a cc-13 ion chamber (IC), a MapCheck2 (for IMRT), and an ArcCHECK (for VMAT). Results obtained with the millennium and the new MLC system were compared based on γ-criteria of 3%/3mm-G (global normalization), and a more stringent 2%/2mm-L (local normalization). Results The TB DLG values (1.3mm) were adjusted to balance the confidence intervals for the IC measurements between IMRT and VMAT. For the new MLC system, the DLG values (0.1mm) were not adjusted. The TG-119 required IC measurements resulted for prototype MLC: 1.19% (mean), 1.28% (SD), 3.71% (CL) and 0.19% (mean), 0.47% (SD), 1.11% (CL) for high dose and low dose regions, respectively. For the TB MLC: 1.93% (mean), 0.5% (SD), 2.91% (CL) and 1.32% (mean), 1.17% (SD), 3.62% (CL) for high dose and low dose regions, respectively. The comparison of planned to delivered plans for all TG-119 targets for IMRT and VMAT deliveries are shown in Figures 1 and 2 below, and for the two MLC systems. The

prototype MLC system produced higher passing rates for both IMRT and VMAT than the TB MLC system for the various test plans. In addition, the prototype MLC system performs equally well for IMRT and VMAT, whereas the TB MLC is less optimum for VMAT delivery compared to IMRT (Fig. 1 and Fig. 2).

Conclusion The TG-119 test plans were performed on a prototype MLC system in comparison to the well-understood TB Millennium MLC. Our investigation showed, in the context of TG-119, this prototype MLC performs well for both IMRT and VMAT plans. 1 Ezzel G., et al., 'IMRT commissioning: Multiple institution planning and dosimetry comparisons, a report from AAPM Task Group 119.” Med. Phys. 36 :5359-5373 (2009). EP-1735 Total skin irradiation with helical Tomotherapy: Planning and dosimetry feasibility aspects A. Haraldsson 1 , P. Engström 1 1 Skåne University Hospital, Radiation physics, Lund, Sweden Purpose or Objective Mycosis fungoides (MF) is a lymhpatic disease that attacks the skin. The primary treatment for treating MF is total skin electron therapy (TSET). The procedure is technically challenging both in terms of dosimetry and treatment delivery. Helical TomoTherapy (HT) is due to its design especially advantageous when irradiating very long and complex targets. In this study we have explored the possibility of employing HT in the treatment of total skin irradiation (TSI). Material and Methods We used an anthropomorphic whole body phantom (PBU- 60 Kyoto Kaguka). The phantom was immobilized with whole body vacuum bag, a five-point open head net mask fixated to the couch and an individual neck rest. The phantom was covered with a 7 mm thick wet suit made of Neoprene (AquaLung) and CT scanned in two sets; from vertex to thigh and from toes to hip. The CTV was defined as skin with 5mm depth, with PTV as a 7 mm expansion.