Prostate Cancer Disguised as a Rectal Tumor

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Background In 2011, almost a quarter million men were diagnosed with prostate cancer in the U.S., of whom 33,720 died as a result. It is estimated that prostate cancer will represent nearly 10% of all newly diagnosed cancers among men. We demonstrate a case of abnormal extension of a prostate cancer into the rectum. Case Description A 65-year-old man with prostate adenocarcinoma (pT2bN0M0, Gleason 4+3, PSA 9.5 ng/mL) treated with radical retropubic prostatectomy developed local recurrence 1 year later diagnosed by transrectal ultrasound-guided (TRUS) biopsy. He was treated with radiation therapy and 2 years of androgen deprivation therapy (ADT). Two years later, a subsequent rise in PSA (peak 52.8) prompted re-initiation of ADT with appropriate biochemical response. Six years later, a colonoscopy performed for hematochezia, frequent bowel movements, and decreased stool caliber revealed a semicircumferential nodular and friable mass in the anterior rectum which extended from the anal verge to 15 cm proximally (Fig. 1). Histology confirmed poorly differentiated adenocarcinoma of prostatic origin (PSA +; CK7 -; CK20 -). Contrast-enhanced CT of the pelvis identified stable lymphadenopathy and a heterogeneous soft tissue mass in the old surgical bed with evidence of invasion into the rectum (Fig. 2). Serum PSA was 8.5. Two years later, the prostate cancer metastasized to lung and liver. Discussion Rectal involvement of prostate cancer is uncommon, occurring in only 4% of patients. Normally, the rectoprostatic fascia serves as a barricade between the rectum and prostate. Rectal involvement is possible via: direct invasion through the fascia; shared lymph nodes; or rarely, seeding through needle biopsy. In our case, the fascia was excised during the prostatectomy and TRUS biopsy was also performed, both occurred 10 years prior.Indeed, erroneous classification of prostate cancer as a primary colorectal cancer has been described. Among a series of 19 men with history of prostate cancer, twelve patients had rectosigmoid biopsies initially misinterpreted as primary colorectal cancers. Accurate histologic diagnosis may be difficult, particularly with small rectal biopsy specimens. Gastroenterologists and pathologists should be mindful that the differential diagnosis of adenocarcinoma from rectal specimens includes prostate cancer, especially in patients with known history of prostate cancer.1491_A Figure 1. Semi-circumferential anterior rectal mass1491_B Figure 2. Rectal invasion on CT

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