Target definition in salvage postoperative radiotherapy for prostate cancer: 18F-fluorocholine PET/CT assessment of local recurrence

<p><b>Purpose:</b> 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 <sup>18</sup>F-Fluorocholine (18F-FCH) positron emission tomography/computed tomography (PET/CT) and to compare different CTV-delineation guidelines in a cohort of postoperative patients.</p> <p><b>Material and methods:</b> 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<sub>RTOG</sub>, CTV<sub>FROGG</sub> and CTV<sub>EORTC</sub> 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.</p> <p><b>Results:</b> 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<sub>RTOG</sub> was not significantly different from that included in the CTV<sub>FROGG</sub> (84% versus 83%, <i>p</i> = .5). Significantly more recurrences were included in the CTV<sub>RTOG</sub> volume compared to the CTV<sub>EORTC</sub> (84% versus 68%, <i>p</i>=.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.</p> <p><b>Conclusions:</b> In our study, the CTV<sub>RTOG</sub> and CTV<sub>FROGG</sub> 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.</p>