Inadequate clinical target volume (CTV) definition is likely to be a major contributing factor to local recurrence (LR) rate after radiotherapy. Our aims were to identify sites of prostate cancer LR in biochemical recurrence post-prostatectomy using F-Fluorocholine (18F-FCH) positron emission tomography/computed tomography (PET/CT) and to compare different CTV-delineation guidelines in a cohort of postoperative patients.

Thirty-six patients presenting with LR within the prostatic bed on 18F-FCH PET/CT between 10/2011 and 06/2016 were included in this retrospective study. Median PSA at the time of 18F-FCH PET/CT was 2.7 ng/mL (0.8-9.4) and median PSA doubling time was 11 months (3-28). For each patient, the CTV, CTV and CTV following the corresponding guidelines were outlined and compared. Forty-one LR were delineated using a gradient-based method and the percentage of FCH uptake included in each CTV was evaluated.

The anastomosis was the most common recurrence site (52.8%), followed by the retrovesical region (31.7%) and the bladder neck (7%). The median SUV max value was 4.8 (2.3-16.1). The percentage of LR entirely included in the CTV was not significantly different from that included in the CTV (84% versus 83%, p = .5). Significantly more recurrences were included in the CTV volume compared to the CTV (84% versus 68%, p=.006), due to a better coverage of the bladder neck and retrovesical regions. Six out of 10 relapses occurring in the posterior region of the anastomosis were not covered by any of the CTVs.

In our study, the CTV and CTV ensured the best coverage of LR seen on 18F-FCH PET/CT. When outlining the prostatic fossa, greater coverage of the posterior vesico-urethral region may allow better coverage of potential microscopic disease.