O. Ratib, M. Viallon, H. Zaidi, M. Becker, J-P. Vallee, M. Wissmeyer, J-P. Willi, P. Loubeyre, N. Ojha, P. Maniawski, and C. Becker Nuclear Medicine, Hopital Universitaire de Geneve, GENEVE, Switzerland, Radiology, Hopital Universitaire de Geneve, GENEVA, Switzerland, Radiology, Hopital Universitaire de Geneve, GENEVE, Switzerland, Radiology, Hopital Universitaire de Geneve, Breast Oncology, Hopital Universitaire de Geneve, GENEVE, Switzerland, Philips Healthcare, Cleveland, United States

To investigate the value of MRI and PET studies with either 18F-Fluorocholine (FCH) or 11C-acetate in patients with long term low and stable PSA levels after curative radiotherapy (RT) for prostate cancer. We aimed to test the ability of the above mentioned imaging modalities to differentiate benign radiation induced changes in the prostate from early local recurrences. Twenty patients in complete remission (PSA<0.5 ng/ml), >5 years (median 6.5) after RT for prostate cancer were included in this study. All patients underwent endorectal MRI including T2-weighted, diffusion, dynamic contrast-enhanced, and spectroscopic imaging. In addition, ten patients underwent FCH and ten 11C-acetate PET/CT studies. FCH PET images of prostate were performed at 0, 3 and 6 minutes after tracer injection followed by a late whole body run at 12 and at 40 minutes. Prostate centred PET frames with 11C-acetate were taken 5, 21 and 45 min. after the tracer injection. The evaluation of MR spectroscopy suggested positive or equivocal results in 15/20 (75%) patients. However, the combined interpretation of all MRI sequences (i.e., T2-weighted, diffusion, dynamic, and spectroscopic MR studies) dropped the rate of suspicious or equivocal studies to 40%. The evaluation of early PET frames showed suspicious or equivocal tumour images inside the prostate and/or the seminal vesicles in 16/20 (80%) patients. However, after a combined evaluation of all early and late PET frames only in 1 patient the study was judged equivocal among the ten FCH PET patients, while all 11C-acetate PET studies were reported to be negative. Multimodal endorectal MRI or early single PET (FCH or 11C-acetate) frames of the prostate after curative RT may not be accurate enough to differentiate remission from early recurrence. Multiple-phase evaluation of PET images, however, may reduce significantly the risk of reporting equivocal or false positive results.