ESTRO 36 Abstract Book

S294 ESTRO 36 _______________________________________________________________________________________________

dose levels (24 Gy, 30 Gy and 36 Gy all in 3 fractions) using a time-to-event continuous reassessment method design. Pazopanib was continued post-radiotherapy as maintenance therapy until disease progression. Results Thirteen patients were enrolled, with a median follow-up of 19 months. Their median age was 66 years, with 54% male and 46% female patients. No dose-limiting toxicities were noted at dose levels 1 or 2. Of 7 patients at dose level 3, 1 patient experienced a dose-limiting toxicity consisting of grade 4 hypoglycemia. Grade 3 to 4 pazopanib-related adverse events (AEs) occurred in 38% of patients (8%, 0%, 32% for respectively dose level 1, 2 and 3).

needed to study the impact of the combination on overall survival and PFS. PV-0551 PSMA PET/CT vs MRI for GTV delineation in prostate cancer: a comparison with histology C. Zamboglou 1 , V. Drendel 2 , C.A. Jilg 3 , H.C. Rischke 1 , B. Teresa I. 4 , T. Krauss 5 , M. Werner 2 , M. Bock 6 , M. Langer 5 , P.T. Meyer 4 , A.L. Grosu 1 1 Medical Center - University of Freiburg, Department of Radiation Oncology, Freiburg, Germany 2 Medical Center - University of Freiburg, Department of Pathology, Freiburg, Germany 3 Medical Center - University of Freiburg, Department of Urology, Freiburg, Germany 4 Medical Center - University of Freiburg, Department of Nuclear Medicine, Freiburg, Germany 5 Medical Center - University of Freiburg, Department of Radiology, Freiburg, Germany 6 Medical Center - University of Freiburg, Department of Radiology- Medical Physics Division, Freiburg, Germany Purpose or Objective The exact delineation of the intraprostatic tumour burden is crucial for treatment planning in primary prostate cancer (PCa). We compared 68 Ga-HBED-CC-PSMA PET/CT with multiparametric MRI (mpMRI) for gross tumour delineation in patients with primary PCa based on slice by slice correlation with histopathological reference material. Material and Methods Patients with histopathologically proven primary PCa underwent 68 Ga-HBED-CC-PSMA PET/CT (n=10) and MRI (n=7) followed by radical prostatectomy. Resected prostates were scanned by ex-vivo CT using a special localizer and prepared for histopathology in a customized cutting device. Invasive PCa was delineated on a HE stained histologic tissue slide and matched to ex-vivo CT to obtain gross tumor volume (GTV-)histo. Ex-vivo CT including GTV-histo and MRI data were matched to in-vivo CT(PET). Consensus contours based on MRI (GTV-MRI), PSMA PET (GTV-PET) or the combination of both (GTV- union/-intersection) were delineated. In each in-vivo CT slice the prostate was separated into 4 equal segments (total 340 segements) and sensitivity and specificity for PSMA PET and mpMRI were assessed by comparison with histological reference material. Furthermore, the spatial overlap with GTV-histo was measured. In the case of multifocal PCa (4/7 patients), SUV values (PSMA PET) and b-values (diffusion weighted MRI) were obtained for each lesion. Results GTV-histo was detected in 225 of 340 segments (66%). Sensitivity and specificity for GTV-PET, GTV-MRI, GTV- union and GTV-intersection were 75% and 87%, 70% and 82%, 82% and 67%, 55% and 99%, respectively. GTV-histo had on average the highest overlap with GTV-union (57±22%), which was significantly higher than overlap with GTV-MRI (p=0.016) and GTV-PET (p=0.016), respectively. In every patient with multifocal PCa there was one lesion which had both the highest SUV and the highest b-value (mean and max), which was always the largest lesion in histology. Conclusion 68 Ga-HBED-CC-PSMA PET/CT and mpMRI showed high sensitivity and specificity in detection of primary PCa. The combination of both methods performed even better in terms of sensitivity (GTV-union) and specificity (GTV- intersection). A good spatial overlap with GTV-histo was observed for PSMA PET/CT and mpMRI alone which was significantly improved by GTV-union. Further studies are warranted to analyse the impact of these preliminary findings for therapeutic (focal therapy) strategies in primary PCa.

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Treatment-related

adverse

events

Local control was achieved in all irradiated lesions, we noted a complete local response in 1 irradiated lesion (8%), partial response in 6 irradiated lesions (46%), and stable disease in 6 irradiated lesions (46%) as best response. Mean duration of local control was 23 months (95% confidence interval 16 - 31). Assessment of responses outside the radiation field revealed that 5 of 13 patients (38 %) developed a partial response, 7 patients (54 %) had stable disease and 1 patient (8%) had progressive disease as best response. Median progression-free survival (PFS) was 6.7 months (95% confidence interval 3 - 10). Figure: Local control of irradiated lesions and distant response of non-irradiated lesions: best response

Conclusion SBRT in combination with pazopanib treatment is well- tolerated with long-term local control and favourable response rates outside the radiation field. Larger trials are