Cancer of the prostate gland, also called prostatic neoplasia, is an uncommon but extremely serious disease that can affect both neutered and intact male dogs. Prostate tumors are aggressive, highly invasive, space-occupying masses that usually have spread to the spine, pelvis, lymph nodes, lungs and/or other remote locations by the time they are detected. Adenocarcinoma is the most common type of prostate cancer in dogs. Carcinoma, fibrosarcoma, leiomyosarcoma and squamous cell carcinoma can also affect the prostate, especially in neutered dogs. Symptoms of prostate cancer tend to develop gradually and include urination abnormalities, straining to defecate, constipation, scooting, bloody discharge from the penis, lameness, lethargy, appetite and weight loss, weakness and depression. Affected dogs can exhibit one, some, all or none of these symptoms. Owners who notice some of these symptoms should take their dog to a veterinarian as quickly as possible for diagnosis and treatment.