Rising PSA after curative I125 brachytherapy in prostate cancer: patterns of 18F-Choline prostatic uptake on PET/CT.

1549 Objectives [18F]-choline (FCH) positron emission tomography/computed tomography (PET/CT) is an efficient technique to assess relapse in prostate cancer. In a context of PSA bounce after I125 brachytherapy, to distinguish relapse from inflammation in case of rising PSA remains difficult. To our knowledge patterns of prostatic (FCH) PET/CT has not been described in this setting yet.The aim of this study was to describe patterns of (FCH) PET/CT prostatic uptake during rising PSA after I125 brachytherapy. Methods Among 243 patients with rising PSA referred to our center for (FCH) PET/CT between 12/2012 and 12/2015, 27 patients were previously treated by I125 brachytherapy. A two-time protocol, 30 minutes after furosemide infusion, including dynamic pelvic acquisition immediately after 3 MBq/kg FCH and a later acquisition (> 10 minutes) - was retrospectively analyzed. Imaging prostatic characteristics recorded were: diffuse versus focal and peripheral versus central uptake, for visual analysis. SUVmax from dynamic and late step were also recorded. Delta SUV absolute value (ΔSUV) - defined by the difference between late and dynamic SUVmax values - was calculated. Positive gold standard was defined by histology on biopsy and/or prostatectomy. Negative gold standard was defined by negative biopsy and spontaneously - no hormone therapy - PSA drop below nadir + 2 ng/ml during follow-up. Patients were excluded if no gold standard for prostatic status was available. Results Fourteen patients raised gold standard criteria. Median follow-up was 14.3 months [2.5-36.5]. Ten patients had intra prostatic and/or seminal vesicle relapse histologically proven, among them 5 patients were treated with salvage prostatectomy. Uptake was focal and peripheral for all positive patients with mean SUVmax 2.1[1.0-2.8] at the dynamic acquisition and 3.1[2.3-3.9] at the late step, nearby Iodine seeds. A central, diffuse with soft to intense uptake - mean SUVmax 2.3[1.1-5.8] - was observed for the 10 positive patients. Increased uptake at the late step tended to by higher in prostatic zones corresponding to histologically proven recurrence - ΔSUV 1.0 vs 0.21 (p=0.31), respectively for peripheral and central area. Histological analyses after prostatectomy found no cancer cells in the central area (n=5). Moreover, uptake only in central area was found for 3 on 4 negative patients. For the last negative patient no prostatic uptake was observed. Conclusions Local recurrence of prostate cancer after I125 brachytherapy remains challenging but can still be assessed by (FCH) PET/CT. Imaging patterns during local recurrence was a focal and peripheral uptake - even if SUVmax values were modest - increasing at the late step. A frequent central, diffuse and homogeneous uptake, probably due to post brachytherapy inflammation, should not been confused with a local recurrence.