ESTRO 38 Abstract book

S354 ESTRO 38

any technique tested. For this subgroup of breast cancer patients the benefit from proton therapy is so large that it should be considered as an immediate indication for referral. OC-0673 LET variation as a function of different optimization approaches in proton beam therapy G. Martino 1 , N. Van Lobenstein 1,2 , A. Carlino 1 , A. Resch 3,4 , M. Stock 1 , G. Kragl 1 1 MedAustron Ion Therapy Center, Medical Department, Wiener Neustadt, Austria; 2 Delft University of Technology, Faculty of Mechanical- Maritime- and Materials Engineering, Delft, The Netherlands ; 3 Christian Doppler Laboratory for Medical Radiation Research for Radiation Oncology, Medical Radiation Research, Vienna, Austria; 4 Medical University of Vienna- Austria, Department of Radiotherapy, Vienna, Austria Purpose or Objective Compared to photons, protons allow for decreased integral dose and an enhanced biological effectiveness, typically set to a constant value of 1.1. Amongst other factors, RBE depends on the LET of the projectile, which is increasing with decreasing proton energy. This creates concerns that a constant RBE might not sufficiently describe clinical situations where it is unavoidable to use beams stopping in front of critical OARs. In mixed particle fields, a dose-averaged LET (LET d ) over the entire particle spectrum is a meaningful quantity to correlate to biological effect. In this work the computation of LET d was performed with an MC algorithm available in a research version of the TPS RayStation. The LET d computed in RS was benchmarked against Gate/Geant4. The aim of this work is to set up a validated tool to evaluate LET d distributions resulting from different optimization strategies for cases with critical beam incidences. Material and Methods For LET benchmarking two regular shaped fields (box of 5x5x5 cm 3 ) centered at a depth of 6 and 30 cm in water were optimized in RS and forward calculated in Gate/Geant4. All the plans were computed in a 1x1x1 mm 3 dose grid. We compared depth LET d profiles at the central axis of the two SOBPs and transverse LET d profiles at three different depths corresponding to the distal part, central and proximal part of each SOBP. Plans were generated for a pediatric skull base case in RS using different optimization strategies (Single Field Optimization (SFO) and Multiple Field Optimization (MFO)), different number of beams (1 and 2), different maximum spot weights and number of distal energy layers (1 to 3). Results The benchmarking of RS against Gate/Geant4 showed an agreement within ± 5% for all depth LET d profiles and transverse LET d profiles analyzed (Fig. 1). The evaluation of Dose-Volume and LET-Volume Histograms within and around the PTV in our pediatric case showed that different optimization strategies lead to different dose and LET d distributions in concentric rings around the PTV. Fig. 2 shows that the DVHs for a ring of 0.5 cm diameter around the PTV is similar for all optimization strategies. Here, the 2-fields SFO plan showed slightly higher integral doses. In all cases, the maximum value is the prescribed dose of 54 Gy(RBE). The corresponding LET d showed lower values in the single-field plan in 60% and higher values in 40% of the ring volume. Very similar LET d values were obtained in the SFO and MFO plans. The maximum LET d value of the single- field plan is almost double if compared to the SFO and MFO plans.

Conclusion LET d

calculated by RS is in good agreement with Gate/Geant4 and offers a reliable tool for LET distribution display. Preliminary results suggest that 2-fields plans offer a better balance between LET d and integral dose outside the target if compared to single-field plans. Altering the number of distal energy layers and the spot weight has an inferior impact (maximum 3% in the LET d ) compared to increasing the number of beams.

Symposium: Focus on the Pelvic Region

SP-0674 Status on adaptive strategies in the pelvic region – how far are we? Y. Seppenwoolde 1 1 Medizinische Universität Wien, Department of Radiotherapy, Vienna, Austria Abstract text Despite the increased accuracy of irradiation techniques like IMRT and VMAT, sparing all organs at risk (OAR) in the pelvic area is still challenging because large changes in shape and position can be present due to variations in filling. With the introduction of cone-beam CT scanners and other on-board imaging systems, it became possible to observe these changes of internal organ configurations during each treatment fraction. Theoretically, this enables re-adaptation of plans according to tumour shrinkage and changes in OAR morphology, resulting in reduction of toxicity and better target coverage. Full online plan adaptation requires that re-delineation, re- optimizing of dose distributions and repetition of all legally required quality assurance steps should be performed in less than a few minutes. These workload intensive procedures would require a high degree of automation and workflow-integration that is still largely absent in off-the-shelf products. Nonetheless, by finding a well-balanced compromise between full automation and degree of plan adaptation, it is possible to apply simplified schemes of adaptation that provides improved treatment. In clinical practice, different on- and offline methods are used. For cervix, bladder and rectum, volumetric imaging based library of plans, library of margins, ‘ITV’ with different steps of integration, (daily) reoptimisation or ‘CBCT-guided evolutive library ‘ are most appropriate,