Metastatic Prostate Adenocarcinoma in Rectal Polyp: A Rare Clinical Presentation

Abstract

Abstract Introduction/Objective Prostate cancer is known to metastasize to bone, lymph node, lungs, liver, and brain, with the GI tract being an uncommon site of metastasis. More commonly, prostate adenocarcinoma involves GI tract by direct colonic extension. Interestingly, few cases have been reported with metastasis presenting as a GI polyp. Methods We herein report a case of 69-year-old African American male who was diagnosed with metastasis of prostate adenocarcinoma presenting as rectal polyp. The patient originally presented with lower urinary tract symptoms. Further examination revealed PSA level of 220. The patient underwent TRUS-guided prostate biopsy which showed prostate adenocarcinoma with Gleason score of 9 (grade group 5). Follow-up CT and radioisotope scans showed direct extension of prostate lesion into bladder, seminal vesicle and rectum. Metastases were noted to right lesser trochanter, L5 vertebra and retroperitoneal lymph nodes. Colonoscopy was performed 1 month after prostate biopsy due to abnormal CT finding of rectal wall thickening. Endoscopy found a single 0.5 mm sessile polyp which was resected by cold snare polypectomy. Results Microscopically, a nodule of tubular glands arranged back to back containing pleomorphic cells with large nuclei, prominent nucleoli, and vacuolated cytoplasm was seen underlying normal colonic mucosa. The malignant cells expressed prostate specific antigen (PSA) and phosphate specific acid phosphatase (PSAP). It was negative for CK7 and CDX2, rendering a diagnosis of metastatic prostate adenocarcinoma. Patient is currently receiving androgen deprivation treatment and enzalutamide with future plans for external beam radiation. The germline alteration testing was positive for BRCA2 which is associated with increased risk of prostate cancer. Conclusion Albeit rare, it’s important for pathologists to be aware of this entity presenting uncommonly as a rectal polyp, particularly given the history of prostate adenocarcinoma which could be mistaken for colonic adenocarcinoma. In rare cases, tumor seeding following TRUS-guided prostate biopsy has also been implicated.