Rectal-cancer mimic of recurrent high-grade prostate adenocarcinoma.

High-grade prostate cancer is usually monitored with prostate-specific antigen (PSA) and prostate-specific membrane antigen (PSMA)-targeted positron emission tomography-CT (PET/CT). We describe an older man with a history of Gleason 9 prostate adenocarcinoma treated with androgen deprivation therapy, external beam radiotherapy and brachytherapy who later developed haematuria and functional decline. Despite a large invasive pelvic mass, his PSA remained undetectable, and PSMA PET/CT was negative, initially suggesting an alternative diagnosis such as rectal carcinoma. Colonoscopic biopsy was non-diagnostic, but prostate biopsy confirmed recurrent Gleason 9 adenocarcinoma. The patient deteriorated rapidly and died within 3 months. This case underscores the need for histological confirmation when PSA, PSMA PET/CT, and the clinical picture are discordant, even in previously well-controlled disease.