Unusual presentation of advanced prostate cancer masquerading as metastatic and obstructing rectosigmoid cancer

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An 80‐year‐old man presented with a 1 week history of constipation, abdominal pain and distension and overflow faecal incontinence on a background of 4 weeks of anorexia. His past history included a total hip replacement, pharyngoesophageal diverticulum and an incarcerated right femoral hernia requiring resection of 5 cm of ileum (necrotic bowel). He had no past or family history of malignancy.
On examination, he appeared cachectic with a distended and tender abdomen. Digital rectal examination (DRE) revealed a rectal stricture 2 cm from the anal verge, which was too tight to pass.
Computed tomography (CT) scan of his abdomen and pelvis demonstrated extensive ascites, a 9 cm segment of mural thickening affecting the rectosigmoid colon (Fig. 1) with proximal dilatation of distal small intestine and large bowel consistent with developing large intestine obstruction (LBO). There was no evidence of perforation, ischaemia or metastatic disease. Gastrografin enema confirmed a high grade fixed stenosis involving the proximal rectum and rectosigmoid junction (Fig. 2). A presumptive diagnosis of rectosigmoid carcinoma was made.
The patient underwent emergency laparotomy with biopsies taken via flexible sigmoidoscopy at the same time. The caecum was noted to be 15–16 cm in diameter, with areas of ischaemia and perforation. There was extensive ascites, peritoneal metastases and omental caking present. He underwent caecal decompression followed by a caecotomy and caecostomy.
Histopathological examination of the caecotomy specimen showed ulceration, inflammation and multiple small foci of poorly differentiated adenocarcinoma in the serosa and subserosa (Fig. 3). Similar histology was seen in the biopsy specimen obtained from the rectum. The tumour cells were stained strongly for cytokeratin (CK) AE1/AE3 and moderately for prostate specific antigen (PSA). This immunohistochemical staining profile is most consistent with prostatic adenocarcinoma with a Gleason pattern 5 + 5 = 10. The tumour was negative with CK20, CK7, S100 and transcription factor 1. CK20 negativity suggests that it is not colorectal adenocarcinoma.
Post‐operatively, the patient's PSA was 154 ng/mL. There was no preoperative measurement to compare. Bone scan post‐operatively showed no definite evidence to suggest osteoblastic metastatic disease.
Bicalutamide was commenced. The patient's post‐operative course was complicated by myocardial infarction, atrial fibrillation, feeding issues and aspiration pneumonia. The patient died 22 days post‐operatively from cardiac and malignancy‐related causes.
This case represents a very rare presentation of prostate cancer masquerading as both obstructing and metastatic rectal cancer. Locally advanced prostate cancer with rectal infiltration is uncommon. Historical series have reported between 1 and 9% rectal involvement, but these were from the pre‐PSA era (1935–1978).1 Similarly, an autopsy series (1967–1995) suggesting peritoneal metastasis occurring in 7% of men with prostate cancer likely overestimates contemporary occurrence. Our patient presented acutely with LBO requiring emergent laparotomy and was subsequently diagnosed with both locally invasive prostate cancer and extensive peritoneal carcinomatosis. Due to delayed presentation, the need for emergency laparotomy and pre‐existing co‐morbidities, our patient died 22 days after surgery from cardiorespiratory and cancer‐related causes.
Diagnosis of prostate cancer in this man was not considered because of the unusual clinical presentation. The abnormal DRE may have prompted PSA testing; however, the clinical picture was more in keeping with LBO secondary to a stenosing colorectal lesion. Prostate cancer metastasizes distantly via the lymphatic and haematogenous routes, most commonly seeding to lymph nodes and bone.1 Peritoneal carcinomatosis rarely occurs in the absence of bone metastasis.2 In men with metastatic prostate cancer, visceral metastasis, poor performance status and PSA ≥ 65 are poor prognostic factors.3
There are no series describing outcomes of men with both locally obstructing prostate cancer and peritoneal carcinomatosis.

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