ESTRO 36 Abstract Book

S939 ESTRO 36 2017 _______________________________________________________________________________________________

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

Figure 1: Planning Study target doses Material and Methods

Eight clinical cases were identified as part of the initial planning study and independently re-planned per the RT trial protocol with and without dose escalation by two experienced IMRT planners. A single (female) case was then selected representative of a typical yet challenging case and used as the planning benchmark. Ten of the fifteen centres participating in the pilot phase of the trial completed the benchmark planning case, planning with dose escalation only. All pilot centre data was processed with CERR [1] software enabling dose distribution and dose volume histograms to be assessed. Results Dose escalated plans for the initial eight cases showed no statistically significant increase in dose to the OAR with dose escalation (p-value>0.1) whilst maintaining PTV coverage (D95%>95%). Ten centres participating in the pilot phase completed the pre-trial exercise. A range of plan beam configurations were used: 1x 2arc 6 flattening filter free (FFF) MV, 2x 3arc 6MV, 2x 7-field IMRT 6MV, 2x2arc 6MV, 2x Tomotherapy and 1x 4arc 6MV. All centres met all trial mandatory dose objectives for the benchmark planning case and the vast majority of optimal constraints, see table 1 .