[en] To compare the accuracy of "1"8F-FACBC and "1"1C-choline PET/CT in patients radically treated for prostate cancer presenting with biochemical relapse. This prospective study enrolled 100 consecutive patients radically treated for prostate cancer and presenting with rising PSA. Of these 100 patients, 89 were included in the analysis. All had biochemical relapse after radical prostatectomy (at least 3 months previously), had "1"1C-choline and "1"8F-FACBC PET/CT performed within 1 week and were off hormonal therapy at the time of the scans. The two tracers were compared directly in terms of overall positivity/negativity on both a per-patient basis and a per-site basis. Sensitivity, specificity, positive predictive value (PPV), negative predictive value (NPV) and accuracy were calculated for both the tracers; follow-up at 1 year (including correlative imaging, PSA trend and pathology when available) was considered as the standard of reference. In 51 patients the results were negative and in 25 patients positive with both the tracers, in eight patients the results were positive with "1"8F-FACBC but negative with "1"1C-choline, and in five patients the results were positive with "1"1C-choline but negative with "1"8F-FACBC. Overall in 49 patients the results were false-negative (FN), in two true-negative, in 24 true-positive (TP) and in none false-positive (FP) with both tracers. In terms of discordances between the tracers: (1) in one patient, the result was FN with "1"1C-choline but FP with "1"8F-FACBC (lymph node), (2) in seven, FN with "1"1C-choline but TP with "1"8F-FACBC (lymph node in five, bone in one, local relapse in one), (3) in one, FP with "1"1C-choline (lymph node) but TP with "1"8F-FACBC (local relapse), (4) in two, FP with "1"1C-choline (lymph nodes in one, local relapse in one) but FN with "1"8F-FACBC, and (5) in three, TP with "1"1C-choline (lymph nodes in two, bone in one) but FN with "1"8F-FACBC. With "1"1C-choline and "1"8F-FACBC, sensitivities were 32 % and 37 %, specificities 40 % and 67 %, accuracies 32 % and 38 %, PPVs 90 % and 97 %, and NPVs 3 % and 4 %, respectively. Categorizing patients by PSA level (<1 ng/ml 28 patients, 1 - <2 ng/ml 28 patients, 2 - <3 ng/ml 11 patients, ≥3 ng/ml 22 patients), the number (percent) of patients with TP findings were generally higher with "1"8F-FACBC than with "1"1C-choline: six patients (21 %) and four patients (14 %), eight patients (29 %) and eight patients (29 %), five patients (45 %) and four patients (36 %), and 13 patients (59 %) and 11 patients (50 %), respectively. "1"8F-FACBC can be considered an alternative tracer superior to "1"1C-choline in the setting of patients with biochemical relapse after radical prostatectomy. (orig.)