ESTRO 36 Abstract Book

S443 ESTRO 36 _______________________________________________________________________________________________

Germany 2 German Cancer Consortium DKTK, Partner Site Freiburg, Freiburg, Germany 3 Medical Center University of Freiburg - Faculty of Medicine - University of Freiburg, Department of Radiation Oncology, Freiburg, Germany 4 Medical Center University of Freiburg - Faculty of Medicine - University of Freiburg, Department of Pathology, Freiburg, Germany 5 Medical Center University of Freiburg - Faculty of Medicine - University of Freiburg, Department of Nuclear Medicine, Freiburg, Germany 6 University of North Carolina, Department of Radiation Oncology, North Carolina, USA 7 Karolinska Institutet - Stockholm University, Department of Medical Radiation Physics, Stockholm, Sweden Purpose or Objective The goal of this work is to show the technical feasibility and to evaluate the normal tissue complication probability (NTCP) and the tumor control probability (TCP) of the intensity modulated radiation therapy (IMRT) dose painting technique using 68 Ga-HBED-CC PSMA-PET/CT in patients with primary prostate cancer (PCa). Material and Methods We studied 10 RT plans of PCa patients having PSMA- PET/CT scans prior to radical prostatectomy. One contour was semi automatically generated for each patient on the basis of the 30% of SUVmax within the prostate (GTV-PET). For each patient, two IMRT plans were generated: PLAN 77 , which consisted of whole-prostate radiation therapy to 77 Gy in 2.2 Gy per fraction; PLAN 95 , which consisted of whole-prostate RT to 77 Gy in 2.2 Gy per fraction, and a simultaneous integrated boost to the GTV-PET to 95 Gy in 2.71 Gy per fraction. The feasibility of these plans was judged by their ability to reach prescription doses while adhering to the FLAME trial protocol. Comparisons of TCPs based on co-registered histology after prostatectomy (TCP-histo) and normal tissue complication probabilities (NTCP) for rectum and bladder were carried out between the plans. Results Prescription doses were reached for all patients plans while adhering to dose constraints. The mean doses on GTV-histo for [Plan 77 and Plan 95 ] were 75.8±0.3 Gy and 96.9±1 Gy, respectively. In addition, TCP-histo values for Plan 77 and Plan 95 were 70±7 %, and 95.7±2 %, respectively. PLAN 95 had significantly higher TCP-histo (p<0.0001) values than PLAN 77 . There were no significant differences in rectal (p=0.563) and bladder (p=0.3) NTCPs between IMRT dose painting for primary PCa using 68 Ga-HBED-CC PSMA-PET/CT was technically feasible. A dose escalation on GTV-PET resulted in significantly higher TCPs without higher NTCPs. PO-0825 Multi-scenario sampling in robust proton therapy treatment planning E. Sterpin 1 , A. Barragan 2 , K. Souris 2 , J. Lee 2 1 KU Leuven, Department of Oncology, Leuven, Belgium 2 Université catholique de Louvain, Molecular imaging- radiotherapy and oncology, Brussels, Belgium Purpose or Objective Beam specific PTVs (BSPTV) or robust optimizers are superior to conventional PTVs for ensuring robustness of proton therapy treatments. In these planning strategies, realizations ('scenarios”) of a few types of uncertainties are simulated: errors in patient setup, CT HU conversion to stopping powers, and, more recently, breathing motion. However, baseline shifts of mobile targets should also be taken into account, which complicates the sampling of the space of possible scenarios. We compare the 2 plans. Conclusion

here several sampling strategies. We will also show that current robust optimizers sample scenarios in a statistically inconsistent way. Material and Methods Sampling must optimize the trade-off between clinical optimality and robustness. Both were assessed by computing the volume of the BSPTV and a confidence interval (CI), respectively. The latter is defined as the percentage of all possible ranges and beam positions that the BSPTV encompasses. The findings can then be applied later to robust optimizers. We have designed a simulation phantom to model uncertainties in lung tumors (Figure 1). Standard deviations of the Gaussian distributions for (systematic) setup errors, baseline shifts, and CT conversion errors were 5 mm, 5 mm, and 2%, respectively. The errors were sampled following three different methods: 1. M1 (conventional approach): sampling of setup errors and baseline shifts within conventional lateral PTV margin for systematic errors (encompassing 90% of possible beam positions). The distal and proximal margins encompass 98% of possible proton ranges scaled by a flat CT conversion error (±3.3% to include 90% of possible CT conversion errors). M2: same as M1 with random sampling of the CT conversion error. M3: all errors are simulated within an iso- likelihood hypersurface including 90% of all possible scenarios. A fixed breathing-induced motion amplitude of 1 cm has been considered for every scenario. 2. 3.

Results BSPTVs equaled 430, 420 and 564% of the CTV volume for the three methods, respectively (see figure 2 that illustrates the range margins). M1 does not ensure statistical consistency because of the flat CT conversion error, which overemphasizes unlikely scenarios (large geometrical AND large CT conversion errors) and makes non-trivial the computation of the CI. M3 guarantees at least 90% CI, but with a 34% increase of the irradiated volume. The latter is due to the non-prioritization of