3324 - A Novel Six-Zone Method to Guide Delineation of Clinical Target Volume (CTV) for Postoperative Radiotherapy of Prostate Cancer, Based on the Distribution of Prostate Bed Recurrence by PSMA PET

H. Zhang¹, X. Qi², H. Z. Li², X. S. Gao², X. H. Liao³, J. H. Zhang³, and S. Deng⁴; ¹Department of Radiation Oncology, Peking University First Hospital, Bejing, China, ²Department of Radiation Oncology, Peking University First Hospital, Beijing, China, ³Department of Nuclear Medicine, Peking University First Hospital, Beijing, China, ⁴Peking University First Hospital, Beijing, China

Purpose/Objective(s): Existing guidelines described the clinical target volume (CTV) of the prostate bed (PB) for postoperative radiotherapy are based on anatomical boundaries, which is not very convenient for guiding doctors to delineate the PB CTV. The aim of our study is to develop a novel six-zone method to guide the delineation of PB CTV for postoperative radiotherapy in prostate cancer patients, based on the distribution pattern of local recurrence observed through positron emission tomography/computed tomography using prostate specific membrane antigen (PSMA PET/CT). Materials/

Methods: A retrospective analysis was conducted on a cohort of 1391 patients with continuous prostate cancer patients who underwent PSMA PET/CT between September 1, 2021 and February 1, 2024. Sixty-three patients presenting PB recurrence were enrolled. The gross tumor volume (GTV) of the recurrence was delineated based on PSMA PET and transferred to a treatment planning system, which identified the central position of GTV. The recurrence rate of PB was analyzed according to predefined six-zone method: zone 1 - lower edge of penis bulb; zone 2 - anastomosis above upper edge of penis bulb; zone 3 - bladder neck below half pubic symphysis; zone 4 - anterior half of bladder above half pubic symphysis; zone 5 - posterior half of bladder above half pubic symphysis (subdivided as anterior [zone 5a] and posterior [zone 5b] based on the bladder posterior edge); and zone 6- seminal vesicle bed above upper edge of pubic symphysis. Recurrence lesions were recorded within and outside RTOG, GFRU and ESTRO consensus guidelines.
Results: The analysis included 63 patients with PSMA PET indicating local recurrence, encompassing a total of 97 sites of local recurrence. The median age and median PSA before PSMA PET were 68 years and 2.05 ng/ml, respectively. The recurrence rates in the six zones were as follows: zone 1 (10%), zone 2 (22%), zone 3 (29%), zone 4 (2%), zone 5a (12%), zone 5b (18%) and zone 6 (7%). Among these zones, the highest lesion in zone 6 was 3cm away from the superior border of the symphysis pubis. 2.1cm away from the superior border of symphysis pubis contained 85.7% of the lesions. The lowest lesion in zone 1 was 2.1cm away from the top layer of penis bulb, and 90% of the lesions were within 1.5 cm below the top layer of penis bulb. RTOG, GFRU and ESTRO guidelines agreed the CTV coverage in the 6 zones were as follows: zone 1 (0%, 0%, 0%), zone 2 (100%, 100%, 100%), zone 3 (96.43%, 100%, 100%), zone 4 (100%, 0%, 0%), zone 5a (100%, 91.67%, 83.33%) and zone 5b (82.35%, 94.12%, 100%), zone 6(100%, 85.71%, 100%).
Conclusion: The recurrence rate of zone 4 was low and could be excluded during PB CTV delineation to minimize bladder dose. Additionally, the recurrence rate of zone 1 reached as high as 10%, indicating the necessary for delineating the inferior aspect of the penis bulb, which is not covered by the existing guidelines.