Neoplasms of the Prostate in Small Animals
The most common neoplasm of the prostate is carcinoma arising from ductal or urothelial tissue. Transitional cell carcinoma arising from the prostatic urethra occasionally invades the prostate. Not only does castration not protect against future development of prostatic neoplasia in dogs, but incidence of prostatic neoplasia is higher in castrated dogs.
The clinical signs of prostatic neoplasia may be similar to those of other prostatic diseases. Pain and fever may be present. If the neoplasm infiltrates the urethra, dysuria or urethral obstruction is likely. On rectal palpation, the prostate may be normal in size but feel asymmetrical and nodular. It may also be firmly adhered to the pelvic floor or adjacent structures. Ultrasonographic examination may show an irregularly shaped prostate gland with hyperechoic, heterogenous foci. Gross metastases are present at the time of diagnosis in >80% of dogs with prostatic carcinoma. The most common sites of metastases are the regional lymph nodes, lumbar vertebrae, and bony pelvis. Spread to distant sites (such as the lungs) is uncommon until late in the course of disease. Urethral obstruction caused by prostatic disease in dogs is highly suggestive of neoplasia, as is prostatomegaly in a previously castrated dog. Diagnosis is made by biopsy. Prostatic tumor markers used for human prostatic cancer, such as prostate-specific antigen or prostatic acid phosphatase, are not present in canine prostate glands.
There is no effective curative treatment for prostatic carcinoma in dogs. Because of the high incidence of metastases at the time of diagnosis, and the high incidence of urinary incontinence after prostatectomy in dogs, total prostatectomy is not recommended as a treatment. Radiation therapy for prostatic cancer often results in incontinence due to radiation-induced fibrosis of the urinary bladder. Alternative means of ablating prostatic tissue such as transrectal high-intensity focused ultrasound, transurethral intraprostatic absolute ethanol injections, transurethral laser vaporization, or transurethral electrocoagulation have been successful in experimental studies but have not been performed on dogs with prostatic carcinoma. A relatively simple treatment offered some efficacy for dogs with prostatic carcinoma in one study involving 32 dogs. Treatment with the cyclooxygenase inhibitors piroxicam (0.3 mg/kg/day, PO) or carprofen (2.2 mg/kg, PO, bid) significantly prolonged the median survival time of dogs with prostatic carcinoma compared with dogs receiving no treatment (6.9 vs 0.7 mo).