The most common is rectal carcinoma. Less common types are plasmacytoma, adenomatous polyps, lymphoma, and mast cell tumor.
Rectal carcinomas usually have one of three structural types: circumferential, cobblestone, and polypoid. The circumferential tumors, also known as a napkin ring tumor extend around the circumference of the rectum. When a digital rectal examination is performed a marked narrowing of the rectum (stricture) is usually felt.
This type of tumor tends to be located in the mid rectum. The cobblestone type of rectal tumor is a wide-based tumor and literally looks like a patch of cobblestones (bumps). This type of tumor tends to be located in the mid rectum but can also be located close to the anus. The polypoid type of tumor usually is a carcinoma in situ, which is a type of cancer that grows in one place on the surface of the colon and is attached by a narrow stalk.
Signs and Diagnosis
The most common clinical signs of rectal tumors include straining to have bowel movements and blood in the stools. In some cases the bleeding can be fairly extensive. If the tumor is polypoid and located near the anus, it may intermittently protrude through the anal opening.
Weight loss is not a common finding but may be associated with spread of cancer to other regions of the body.
Your veterinarian will often be able to make the diagnosis of a rectal mass upon digital rectal palpation. For masses further into the colon, colonoscopy will be needed to identify the mass. A complete blood count, chemistry profile and urine testing are completed to evaluate your pet’s internal organ health in preparation for general anesthesia. Chest radiographs (X-rays) and abdominal ultrasound are used to identify visible spread (metastasis) of cancer to the lungs or other internal organs; however, neither of these tests can show microscopic spread of cancer from the primary tumor. Colonoscopy may be recommended in some cases prior to surgery to evaluate the extent of the tumor.
A surgical plan will be discussed with you based on the type of tumor that your pet has. Most polypoid tumors can be removed from within the rectum; therefore, your dog will not have any external incisions. The tumor is usually exposed at the time of surgery by pulling a section of the rectum through the anus to the outside. The tumor and a portion of normal bowel are removed and the incision is stitched together using dissolvable sutures.
Removal of a cobblestone adenocarcinoma frequently can be performed by prolapsing the affected bowel through the anus, thus not requiring an external incision. If the tumor is further up the rectum a horseshoe-shaped incision is made around the anus to assist removal of the tumor.
The circumferential (napkin ring) carcinoma needs to be removed via a more extensive approach. In some cases, an abdominal incision is required along with prolapsing of the tumor through the rectum. If the tumor is quite extensive, a pelviotomy may be required, which entails making four cuts in the pelvic bone.
The prognosis is dependent on the type of tumor present. The polypoid adenocarcinomas have low tendency to spread and usually can be cured with surgery alone. The cobblestone adenocarcinomas have a reasonably good prognosis with a median survival time of 12 months. The circumferential adenocarcinoma carries a guarded prognosis with a median survival time of seven weeks.