1255 - Optimizing Postoperative Radiotherapy Target Volumes in Prostate Cancer Based on PSMA PET Recurrence Mapping
Presenter(s)
H. Zhang1, X. Qi1, C. Chen2, X. H. Liao3, J. H. Zhang3, Y. P. Liu4, L. R. He5, N. N. Lu4, Y. L. Shen6, H. Wang7, Q. Z. Zhong8, Y. Li9, M. Zhang10, J. Y. Wu11, and H. Z. Li1; 1Department of Radiation Oncology, Peking University First Hospital, Beijing, China, 2Department of Urology, Peking University First Hospital, Beijing, China, 3Department of Nuclear Medicine, Peking University First Hospital, Beijing, China, 4Department of Radiation Oncology, Cancer Hospital Chinese Academy of Medical Science, Beijing, China, 5Department of Radiation Oncology, San Yat-sen University Cancer Center, Guang zhou, China, 6Department of Radiation Oncology, West China Hospital of Sichuan University, Chengdu, China, 7Peking University Third Hospital, Beijing, China, 8Department of Radiation Oncology, Peking Hospital, Beijing, China, 9Department of Radiation Oncology, Beijing Cancer Hospital, Beijing, China, 10Department of Radiation Oncology, Peking University Peolple's Hospital, Beijing, China, 11Department of Medical Imaging, Peking University First Hospital, Beijing, China
Purpose/Objective(s): Salvage radiotherapy (sRT) to the prostate bed (PB) can be curative for prostate cancer patients with biochemical recurrence after radical prostatectomy and the PB CTV delineation is a key factor to predict clinical success. In this study, we first analyze the clinical differences in PB CTV delineation across multiple centers and then determined patterns of local recurrences by the PSMA PET, providing imaging-based evidence to discuss optimal PB CTV delineations in comparison with current guidelines.
Materials/Methods: Interobserver variability in PB CTV delineation was assessed: 10 experienced radiation oncologists from 8 centers contoured a case. Differences were quantified using the Dice similarity coefficient (DSC) and 95% Hausdorff distance (HD95). A retrospective cohort with PB recurrence after radical prostatectomy was analyzed. PSMA PET-detected lesions were mapped to a six-zone model, and recurrence rates were calculated. Coverage by three guideline CTVs (ESTRO-ACROP 2023, GFRU 2020, and RTOG 2010) was evaluated.
Results: There was considerable interobserver variation in contouring. The mean DSC was 0.767 ± 0.046 and the mean HD95 was 8.244 ± 2.347 mm. A total of 97 PSMA PET-detected recurrences were identified in 63 patients. Mapping to the six-zone model revealed that recurrences were most common in zone 3 (bladder neck region; 28.9%) and zone 5 (posterior bladder region; 12.4% in zone 5a and 17.5% in zone 5b), followed by zone 2 (vesicourethral anastomosis area; 21.6%), zone 1 (below the superior edge of the penile bulb; 10.3%), zone 6 (seminal vesicle bed; 7.2%), and zone 4 (anterior/superior to the bladder base; 2.1%). Current guidelines did not uniformly cover these recurrences: zone 1 were not mentioned by all three guidelines (0% coverage), while zone 4 were only recommended by RTOG (100% coverage). Distance analysis showed that zone 1 lesions lay within 2.1 cm inferior to the penile bulb apex, zone 5a lesions lay within 0.5 cm anterior to the posterior bladder wall and within 1.3 cm anterior to the posterior bladder edge, and zone 6 lesions lay within 3.0 cm cranial to the superior pubic symphysis.
Conclusion: PSMA PET-detected recurrence mapping revealed rare recurrence in the superior-anterior region above the mid–pubic symphysis (zone 4), supporting avoidance of unnecessary cranial/anterior extension. Lesions in the anterolateral retrovesical space (zone 5a) clustered within 1.3 cm anterior to the posterior bladder edge, offering an objective boundary to optimize coverage while limiting bladder dose.