Abstract Book

S864

ESTRO 37

Purpose or Objective To increase precision of radiation treatment (RT) delivery in prostate cancer and to reduce toxicity, MRI-based RT as well as the use of fiducials like goldmarkers (GM) have each shown promising results. However, their combined use is currently under investigation in clinical trials. Therefore, we aim to evaluate MRI-related distortions or deviations including artifacts by fiducials to estimate a A gel-phantom with two inserted GM was scanned by CT and three different MR-scanners of 1.5 T and 3 T (2mm slice thickness, T2 TSE and T1 VIBE-Dixon). After image fusion, deviations of the matches for fiducials and gel contours were measured and visibility/artifacts were evaluated. Results Visibility of GMs was shown for all modalities. The outer gel contours were matched with <1mm deviation, contour volumes varied between 0-1%. The deviations of the GMs were less than 2mm in any direction of MRI/CTand no relevant differences were found between 1,5 T and 3T MRIs. Shifts of peripherally or centrally located GM were randomly distributed. Match precision of a clinical sample including a fiducial-based MRI-CT match and consecutive MRI-match were in the same range. Conclusion In this phantom study we were able to measure small deviations of fiducial-based registrations of a reference CT with voxel-isotropic T2-TSE sequences of three different MR-scanners (1.5T and 3T). Spatial deviations of CT- and MR-contoured fiducials were less than 2mm, i.e. below the slice thickness of the applied modalities. Comparable small deviations were detectable in the clinical workflow of an example case using a MR-adaptive protocol. In addition, we were able to develop a workflow for MR-CT-image fusion and MR-only workflow. EP-1605 Outcome and toxicity in a cohort of prostate cancer patients treated with image-guided radiotherapy P. Toncheva 1 , N. Volegova-Neher 1 , K. Henne 1 , A. Grosu 1 , S. Kirste 1 1 Uniklinik Freiburg, Radiation Oncology, Freiburg, Germany Purpose or Objective Radiation therapy is an important primary treatment option in patients with prostate cancer. Fiducial marker image-guided radiotherapy allows correction of setup errors and interfraction motion resulting in improved accuracy. We aim to report outcome and toxicities of primary image-guided radiation therapy using an adaptive off-line planning protocol. Material and Methods This retrospective study includes 137 patients who received primary radiation therapy in our center between 2010 and 2014. All patients were treated with IMRT technique with IGRT including cone beam CT and daily 2D2D imaging. Before treatment 4 gold fiducials were implanted in the prostate in all patients. Treatment planning was done by MRI and/or PET-CT imaging. Patients with a large prostate volume and/or high risk disease were prescribed neoadjuvant and concomitant hormonal therapy. The total dose in the prostate was 74- 76 Gy. In case of node positive cancer up to 54 Gy were given to pelvic lymph nodes. Biochemical recurrence was defined as PSA >= 2ng/ml (or +2ng/ml from PSA nadir). potentially related bias. Material and Methods

Kaplan-Meier analysis was performed for biochemical recurrence free survival (bRFS) and overall survival (OS). Results Median follow-up was 48 months (2 – 81). In total 20 out of 137 patients developed a recurrence (10 biochemical recurrence only, 6 lymph node recurrences outside the radiation field, 2 distant metastases, 1 lymph node recurrence outside the radiation field and distant metastases and 1 recurrence inside radiation field). The 4 year bRFS was 82 %. 4 year OS was 92 %. Acute grade 3 genitourinary toxicity was observed in two patients (1,5 %). No patient had late grade 3 genitourinary toxicity. Acute grade 3 gastrointestinal toxicity was observed in five patients (3,6 %), late grade 3 gastrointestinal toxicity was observed in 6 patients (4,4 %). Factors that influenced bRFS were lymph node status, irradiation dose (cont.), concomitant and adjuvant hormonal therapy in univariate analysis. The following cox regression analysis showed significant influence of lymph node status and concomitant hormonal therapy. Conclusion Primary image-guided radiation therapy leads to very good local tumor control rates with low acute and late toxicity. We observed only 1 local recurrence in the radiation area. EP-1606 Probabilistic Planning Concept instead of Target Volume Margins - Prospective evaluation D. Wegener 1 , B. Berger 2 , Z. Outaggarts 1 , D. Zips 1 , F. Paulsen 1 , M. Bleif 3 , D. Thorwarth 4 , M. Alber 5 , O. Dohm 6 , A.C. Müller 1 1 Eberhard Karls University Tübingen, Department of Radiation Oncology, Tübingen, Germany 2 Oberschwabenklinik Ravensburg- Academic Hospital of Eberhard Karls University Tübingen, Department of Radiation Oncology, Ravensburg, Germany 3 Alb Fils Kliniken Göppingen- Academic Hospital of Eberhard Karls University Tübingen, Department of Radiation Oncology, Göppingen, Germany 4 Eberhard Karls University Tübingen, Department of Radiation Oncology- Research Section of Biomedical Physics, Tübingen, Germany 5 Ruprecht Karls University Heidelberg, Department of Radiooncology and Radiotherapy, Heidelberg, Germany 6 Eberhard Karls University Tübingen, Department of Radiation Oncology- Section of Medical Physics, Tübingen, Germany Purpose or Objective Cure- and toxicity rates after IMRT of prostate cancer can both suffer from ill-chosen PTV margins. We studied the potential for Organ at Risk (OAR) sparing of the probabilistic dose prescription concept i.e. the coverage probability concept (CovP) for compensation of tumor movement. Furthermore, outcome parameters of patients were evaluated. Material and Methods Twenty-eight prostate cancer patients (median age 70) with localized disease (cT1c-2c, N0, M0) and intermediate risk features (PSA<20, Gleason score ≤7b) were treated in a prospective study with the CovP. CTs were acquired on three subsequent days to capture form changes and movement of prostate and OAR. The clinical target volume (CTV, prostate) and the OARs (bladder and rectum) were contoured in each CT. The union of CTV1-3 was encompassed by an isotropic margin of 7mm to define the PTV (planning target volume). The CovP concept employs spatially variable weight factors for the target- and OAR cost functions that allow the optimizer