ESTRO 36 Abstract Book

S695 ESTRO 36 2017 _______________________________________________________________________________________________

The purpose of our study was to show the potential benefits of using 68 Ga-PSMA-PET/CT imaging for integrated boost treatment planning or boost only treatment planning of prostate cancer patients. The potential gain of such an approach is the improvement of tumour control and reduction of the dose to organs at risk at the same time. Material and Methods 21 prostate cancer patients (70yrs average) without previous local therapy received 68 Ga-PSMA-PET/CT imaging. Body contour and organs at risk were manually defined on the obtained datasets. A PTV70 and PTV5920 were defined as planning target volumes. A PET active volume GTV_PET was segmented with a 40% of the maximum activity uptake in the lesion as threshold. Five different treatment plan variations were calculated for each patient (Monaco, Version 5.11.00, Elekta, St. Louis, MO) – Table 1. Analysis of derived treatment plans was done according to QUANTEC with in-house developed software. TCP (Tumor Control Probability) and NTCP (Normal Tissue Complication Probability) was calculated for Prostate and ProstatePET (TCP) as well as Rectum and bladder (NTCP). Student’s t-test method was applied for statistical analysis (paired, two-sided). A p level of smaller 0.05 was considered to be statistically significant. Results The median TCP of the PET-positive volume was found to be (89.9 ± 2.7) % for conventional Prostate plans. Comparing the conventional plans to the plans with integrated boost and plans just treating the PET-positive tumor volume, we found that the TCP increased to (95.2 ± 0.5) % for an integrated boost with 75.6 Gy, (98.1 ± 0.3) % for an integrated boost with 80 Gy, (94.7 ± 0.8) % for treatment of the PET-positive volume with 75 Gy, and to (99.4 ± 0.1) % for treating the PET-positive volume with 95 Gy (all p < 0.0001). For the integrated boost with 80 Gy, a statistically significant, but moderate increase of the median NTCP of the rectum was found. Only patients with a tumour directly adjacent to the rectum wall were found to have a significantly higher NTCP rectum . At the same time of course, these patient’s median TCP of the PET-positive volume was found to be significantly improved as well, if compared to TCPs of conventional plans. For all other plan variations no statistical significant increase of the rectum or bladder NTCP was found. Conclusion Our study demonstrates that the use of 68 Ga-PSMA-PET/CT image information would allow for more individualized prostate treatment planning and better targeting of active tumour volumes. TCP values of identified active tumour volumes can be increased, while rectum and bladder NTCP values either remain the same or are even lower for most plans. Clinical studies should be performed to confirm the theoretical benefits of PET target optimized prostate cancer treatment planning.

Abbreviations, Pre-CT; pre-radiotherapy computed tomography; Pre-RT, baseline Magnetic Resonance Imaging (MRI); Mid-RT, MR image at mid-point of the radiotherapy course (about 38 Gy); Post-RT, MR image at completion of the radiotherapy

course. Results

There was no significant difference between the estimated sizes of prostate during RT in all phases. A retreatment plan was well implemented in all patients.

Conclusion In this study, no significant change in prostate size was observed during the course of IMRT. On revising the radiation plan according to change in prostate size, no significant difference with respect to clinical outcomes associated with the use of a revised plan was observed. The mean prostate volume in our study population was 37 cc. The relatively smaller prostate may have led to this result. EP-1310 68Ga-PSMA-PET/CT imaging of localized prostate cancer patients for IMRT with integrated boost L. Thomas 1 , S. Kantz 2 , A. Hung 1 , D. Monaco 1 , M. Essler 3 , H. Strunk 4 , C. Thomas 1 , W. Laub 1 , R. Bundschuh 3 1 Oregon Health & Science University, Department of Radiation Oncology, Portland, USA 2 LMU Klinikum der Universität München, Klinik und Poliklinik für Strahlentherapie und Radioonkologie, Munich, Germany 3 Universitaetsklinikum Bonn, Nuklearmedizinische Klinik und Poliklinik für Nuklearmedizin, Bonn, Germany 4 Universitaetsklinikum Bonn, Radiologische Klinik, Bonn, Germany

EP-1311 Beyond IMRT for Prostate Cancer: The Effect of Modern Technique on Treatment Quality and Outcome J. Kao 1 , A. Zucker 1 , J. Timmins 1 , A. Wong 1 , A. Woodall 2 , E. Loizides 2 1 Good Samaritan Hospital Medical Center, Radiation Oncology, West Islip, USA

Purpose or Objective