Tumors of the rectum are uncommon. Benign and malignant tumors occur with equal frequency in the rectum of dogs. Adenomatous polyp is the most common benign tumor. It occurs most commonly in the distal rectum or anorectal region. An adenomatous polyp consists of branching lamina propria covered by abnormal epithelium and is described as sessile, raised, or pedunculated. Polyps are single (80%) or multiple (20%). Malignant transformation of rectal polyps is described to occur in 18–50%. Chances increase with larger size (> 1 cm) and prolonged presence. Clinical signs consist of tenesmus, hematochezia, dyschezia, rectal bleeding unassociated with defecation, and polyp prolapse. Also vomiting, diarrhea and weight loss can occur. Leiomyoma is the second most common benign tumor. This intestinal smooth muscle tumor has a typical well-circumscribed intramural aspect. Its dominant clinical signs are related to partial or complete extraluminal rectal obstruction. Rare benign masses are fibroma, plasmacytoma, ganglioneuroma, inflammatory pseudopolyposis and idiopathic eosinophilic masses. Adenocarcinoma is the most common malignant rectal tumor. It occurs in the mid-to-distal rectum of elder dogs (mean age 8.5 years) with a male sex predisposition (60–70%). The gross appearance is variable: nodular (single or multiple), pedunculated (mid-to-distal rectum) or annular constriction or obstruction (colon to mid-rectum). Their biological behavior is characterized by invasion of the surrounding tissue layers but a slower metastatic rate when compared to a small intestinal adenocarcinoma. Clinical signs are attributed to a combination of luminal obstruction and hematochezia. Other malignant masses are lymphosarcoma, leiomyosarcomas, hemangiosarcoma, extramedullary plasmacytoma, mast cell tumor, melanoma and fibrosarcoma.

Digital rectal exam is an important diagnostic step to confirm the presence of a rectal tumor (63% of rectal tumors is palpable), the number of tumors present (solitary or multiple), the location in the wall (proliferating mucosa or deeper layers) and the presence of blood. Laboratory tests are performed to evaluate anesthetic risk. Anemia and leukocytosis are reported but occur less commonly than small intestinal tumors. Paraneoplastic leukocytosis is reported with adenomatous rectal polyps. Endoscopy (proctoscopy and colonoscopy) is recommended prior to definitive treatment to evaluate the cranial rectum. However, cytological misdiagnosis is common with intestinal adenocarcinoma (being misdiagnosed as either septic inflammation or lymphosarcoma) and endoscopic biopsy samples for histologic interpretation are often small and superficial resulting in false-negative diagnosis if the lesion is either submucosal or associated with surface ulceration and necrosis.

Surgery is frequently the preferred treatment, but can be challenging because the surgical approach to the distal portion of the rectum is complicated by the regional anatomy, which increases the potential for postoperative complications.

Transanal endoscopic resection involves the piecemeal removal of a rectal tumor, with a coagulating endoscopic loop and has been recommended for the treatment of benign rectal tumors. Because of the piecemeal nature of this technique, complete excision is difficult to achieve for both benign and malignant tumors. The procedure is often repeated several times to achieve a successful result. Furthermore, there is a high risk of rectal perforation because the depth of resection is difficult to determine intraoperative. In a case series of 13 dogs complete excision of benign rectal tumors was achieved in 3 dogs, 5 dogs had incomplete excision of the tumor with clinical improvement, 2 dogs died of rectal perforation and 3 dogs were euthanized due to poor response to treatment.

Rectal mucosal eversion involves the use of stay sutures to evert the rectal mucosa to a level just beyond the tumor. A submucosal resection technique, thoracoabdominal stapler, or rectal amputation and anastomosis is used to remove the rectal tumor. The primary advantage of this procedure is that minimal surgical dissection results in a low risk for postoperative complications. In a case series of 23 dogs with rectal masses 9 dogs developed hematochezia and mild tenesmus, which resolved within 7 days, and 1 dog had partial dehiscence, which healed by second intention and required no further intervention. A disadvantage is that the limited surgical exposure due to folding of the rectal mucosa makes achieving clean surgical margins sometimes difficult. Nevertheless complete tumor excision was achieved in 22 of 23 dogs and median disease free interval was 1122 days. Consequently, this approach is recommended for benign disease confined to the mucosa of the mid-to-distal portion of the rectum.

The transanal rectal pull-through procedure involves an incision around the anocutaneous junction or another area of the distal portion of the rectum and dissection of the external rectal attachments, which allows the isolated rectum to be pulled caudally. After opening the rectum dorsally to inspect the luminal side rectal amputation is performed, followed by sectional cutting and suturing of an end-to-end or end-to-skin anastomosis. A study on 74 dogs documented a 78% complication rate: permanent fecal incontinence (55%), diarrhea (43%), tenesmus (31%), stricture (22%) and rectal bleeding (11%) were the most common. Recurrence occurred in 14% and the median survival time was 1150 days. This approach is indicated for single or multiple benign or malignant rectal tumors located in the mid-to-distal rectum.

The dorsal approach involves a curved incision that begins at an ischiatic tuberosity, extends dorsal to the anus, and terminates at the contralateral ischiatic tuberosity. Soft tissue dissection involves the transection of the rectococcygeal and levator ani muscles to allow exposure of the caudal portion of the rectum. Advantages of this approach include good surgical exposure of the caudal and dorsal portions of the rectum and visibility of the pudendal nerve and pelvic plexus, which are essential for the maintenance of fecal continence. A disadvantage of the procedure is the inability to assess the abdomen for metastatic lesions. In an experimental study on 13 dogs that underwent rectal resection by means of a dorsal approach, postoperative complications included transient tenesmus (12 dogs), anastomotic dehiscence with rectocutaneous fecal leakage (4 dogs) and transient hematochezia (3 dogs). Fecal incontinence was observed in all dogs that had > 6 cm of rectum resected (4 dogs), and this had not resolved by the end of the 10-week follow-up period. This approach is indicated for solitary or multiple malignant tumors in the mid rectum. The ventral approach involves either a sagittal pubic osteotomy, an ischiopubic flap or bilateral pubic and ischial osteotomies. The major perceived drawback to this approach is that it involves an invasive technique to achieve adequate exposure of the diseased tissue. However, results of multiple studies indicate that dogs develop minimal complications and rapidly return to ambulation following osteotomy. A study involving 6 dogs and 1 cat with rectal tumors showed disturbed ambulation to be limited to 2 days (5 dogs, 1 cat) and 3 days (1 dog). No tumor recurrence was noted and median survival time was not reached after 7 and 8 months follow-up. This approach is indicated for solitary or multiple benign or malignant rectal tumors located in the proximal-to-mid rectum.

References

1.  Anderson GI, McKeown DB, Partlow GD, Percy DH. Rectal resection in the dog. A new surgical approach and the evaluation of its effect on fecal continence. Vet Surg. 1987;16(2):119–125.

2.  Danova NA, Robles-Emanuelli JC, Bjorling DE. Surgical excision of primary canine rectal tumors by an anal approach in twenty-three dogs. Vet Surg. 2006;35(4):337–340.

3.  Holt PE. Evaluation of transanal endoscopic treatment of benign canine rectal neoplasia. J Sm Anim Pract. 2007;48:17–25.

4.  Nucci DJ, Liptak JM, Selmic LE, Culp WT, Durant AM, Worley D, Maritato KC, Thomson M, Annoni M, Singh A, Matz B, Benson J, Buracco P. Complications and outcomes following rectal pull-through surgery in dogs with rectal masses: 74 cases (2000–2013). J Am Vet Med Assoc. 2014;245(6):684–695.

5.  Yoon HY, Mann FA. Bilateral pubic and ischial osteotomy for surgical management of caudal colonic and rectal masses in six dogs and a cat. J Am Vet Med Assoc. 2008;232(7):1016–1020.