ESTRO 36 Abstract Book

S953 ESTRO 36 _______________________________________________________________________________________________

considered. Three beam widths (1, 2 and 3 mm FWHM) and 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

Table 1: Optimal and mandatory dose constraints Conclusion Two sequential planning excercises have demonstrated dose escalation in anal cancer patients is achievable without sacrifice of OAR sparing. This shows OAR sparing is achievable across multiple centres using a variety of planning techniques, giving expectation of consistent quality plans for trial patients. Over 30 sites will join the trial in the next phase and will complete the same RTQA process. References [1] A Computational Environment for Radiotherapy Research, CERR; Online: http://www.cerr.info/about.php EP-1733 Proton grid therapy (PGT): a parameter study T. Henry 1 , A. Valdman 2 , A. Siegbahn 1 1 Stockholm University, Department of Medical Physics, Stockholm, Sweden 2 Karolinska Institutet, Department of Oncology and Pathology, Stockholm, Sweden Purpose or Objective Proton grid therapy (PGT) with the use of crossfired and interlaced proton pencil beams has recently been proposed by our research group. A clear potential for clinical applications has been demonstrated. The beam sizes used in our proof-of-concept study were in the range 7-12 mm, full-width at half maximum (FWHM), representing the typical range of available proton pencil- beam widths at a modern proton therapy facility. However, to further take advantage of the dose-volume effect, on which the grid therapy approach is based, and thereby improve the overall outcome of such treatment, smaller beams are desirable. In this present study, Monte- Carlo (MC) simulations of a simple PGT treatment were performed with varying beam sizes and center-to-center (c-t-c) distances between the beams. The aim was to determine which combinations of those two parameters would produce the most therapeutically desirable dose distributions (high target dose and low valley dose outside of the target). Material and Methods MC calculations were performed using TOPAS version 2.0 in a 20x20x20 cm 3 water tank. The beam grids were aimed towards a 2x2x2 cm 3 cubic target at the tank center. Two opposing (or 2x2 opposing) grids were used. The target was cross-fired in an interlaced manner. Grids containing planar beams (1-D grids) or circular beams (2-D grids) were