Tumor volume in prostate cancer assessed by [11C]Choline PET/CT: Validation with surgical specimen

1268 Objectives [11C]Choline PET/CT is an increasingly important tool for imaging prostate cancer. The aim of this study was to evaluate [11C]Choline PET/CT for delineation of gross tumor volume (GTV) in prostate cancer radiation therapy and to validate results by the use of the macroscopic tumor volume derived from histopathology. Methods 22 patients underwent [11C]Choline PET/CT before total prostatectomy. The surgical specimens were scanned with CT ex-vivo to determine volume changes after surgery and fixation. The prostates were cut in 5mm whole prostate step sections and the carcinoma were marked. Step sections were digitalized and the carcinoma was segmented semi-automatically. The tumor volume was calculated taking into account volume changes after fixation. [11C]Choline positive lesions were compared to histopathology. For each tumor the optimal threshold for tumor delineation in terms of SUV value and percentage of maximum lesion uptake was estimated to delineate the GTV. Results Prostate volume changed in average (21±12)%. In 8 patients no focal [11C]Choline uptake corresponding to pathological findings was found. In 14 patients corresponding lesions were co-localized and the optimal SUV threshold was between 2.7 and 9.4 (mean 4.9, SD 1.9). The optimal percentage of maximum uptake varied between 50 and 97% (mean 80%, SD 13%). Conclusions In 14/22 patients (64%), choline PET localized the tumor correctly. The threshold for GTV delineation showed a high intersubject variability. Different characteristics of benign prostate tissue, e.g. benign prostate hyperplasia, may be accountable for the results