The Role of Prostate-Specific Membrane Antigen (PSMA) PET-CT in Characterizing Suspected Synchronous Rectal Lesions in a Case of High-Grade Prostatic Adenocarcinoma

We report a case of a 65-year-old man who presented with urinary voiding problems and inadequate bowel evacuation. On digital rectal examination (DRE), he was found to have a hard mass in the prostate. Colonoscopy revealed a rectal proliferative growth, which was also suspicious for malignancy. Initially, a clinical diagnosis of prostate carcinoma with rectal involvement was suspected. The patient then underwent radiological evaluation in the form of computed tomography (CT) and magnetic resonance imaging (MRI) scans, which revealed two separate lesions in the prostate and rectum with maintained fat planes along with pelvic and abdominal lymphadenopathy and skeletal metastases. Hence, a radiological diagnosis of prostatic cancer with synchronous rectal primary was made. Further histopathological evaluation revealed high-grade prostatic adenocarcinoma. However, rectal biopsy revealed an inflammatory lesion in the rectum, even in repeated biopsies, in spite of high clinical and radiological suspicion of synchronous rectal primary. Later, gallium-68-labeled PSMA positron emission tomography-computed tomography (Ga-68 PSMA PET-CT) scan revealed the prostatic lesion with low to intermediate expression of PSMA, along with multiple metastatic pelvic and abdominal lymph nodes and extensive skeletal metastases with intermediate to high PSMA expression. However, the rectal lesion showed low PSMA expression. This case highlights the fact that PSMA expression in suspicious rectal lesions can help in labeling them as benign or malignant. In this patient, it aligned with repeated biopsy results, which showed benign proctitis, despite high clinical and radiological suspicion of malignancy.