Mesorectal nodal metastasis with seminal vesicle invasion in biochemically recurrent prostate cancer

Objectives To determine the prevalence and predictors of mesorectal lymph node (MLN) metastases on prostate‐specific membrane antigen (PSMA)‐based positron emission tomography/computed tomography (PET/CT) in patients with biochemically recurrent prostate cancer (PCa) following radical therapy. Materials and Methods This was a cross‐sectional analysis of all PCa patients with biochemical failure following radical prostatectomy or radiotherapy who underwent an 18 F‐DCFPyL‐PSMA‐PET/CT at the Princess Margaret Cancer Centre between December 2018 and February 2021. Lesions with PSMA scores ≥2 were considered positive for PCa involvement (PROMISE classification). Predictors of MLN metastasis were evaluated using univariable and multivariable logistic regression analyses. Results Our cohort consisted of 686 patients. The primary treatment method was radical prostatectomy and radiotherapy in 528 (77.0%) and 158 patients (23.0%), respectively. The median serum PSA level was 1.15 ng/mL. Overall, 384 patients (56.0%) had a positive scan. Seventy‐eight patients (11.3%) had MLN metastasis, with 48/78 (61.5%) having MLN involvement as the only site of metastasis. On multivariable analysis, presence of pT3b disease (odds ratio 4.31, 95% confidence interval 1.44–14.2; P = 0.011) was significantly associated with increased odds of MLN metastasis, whereas surgical factors (radical prostatectomy vs radiotherapy; performance/extent of pelvic nodal dissection), surgical margin positivity, and Gleason Grade were not. Conclusions In this study, 11.3% of PCa patients with biochemical failure had MLN metastasis on 18 F‐DCFPyL‐PET/CT. pT3b disease was associated with 4.31‐fold significantly increased odds of MLN metastasis. These findings suggest alternate drainage routes for PCa cells, either via alternate lymphatic drainage from the seminal vesicles themselves or secondary to direct extension from posteriorly located tumours invading the seminal vesicles.