Invasive carcinoma of prostate presenting as rectal carcinoma.

Prostate carcinoma occasionally can present with rectal obstructive symptoms and an annular constricting lesion of the rectum. Discriminating between primary rectal carcinoma and prostate carcinoma locally invasive to the rectum is of obvious importance because of the different treatments and prognoses. History and physical examination play only a marginal role in differentiating between these two lesions. The diagnosis of prostatic malignancy in patients in this circumstance can be supported by an elevated serum acid phosphatase as well as a bone scan that demonstrates a pelvic/vertebral distribution of bony metastases. The rectal mucosa is usually spared, and a barium enema often will demonstrate tapered margins as opposed to a tumor edge in primary rectal malignancy. Excretory urography often demonstrates hydronephrosis. Rectal biopsy with immunohistochemical staining for prostate specific antigen can direct the origin of a poorly differentiated adenocarcinoma to the prostate. Treatment involves hormonal manipulation with estrogen therapy or orchiectomy. Radiation therapy to the obstructed rectum has provided satisfactory palliation when hormonal manipulation fails.