+ General Considerations

• Benign and malignant tumors occur with equal frequency in the rectum of dogs
• Benign tumors include adenomatous polyps which are described as sessile, raised, pedunculated, single, or multiple
• Malignant rectal tumors in cats and dogs include ADC and LSA
• Rectal ADC in dogs is described as infiltrative, ulcerative, or proliferative

+ Adenocarcinoma

• Mean age 8.5 years (range, 2-14 years)
• Sex predilection: 60%-70% male
• Sites: large intestine and mid-to-distal rectum

Gross appearance is variable:

• Nodular (single or multiple)
• Pedunculated (mid-to-distal rectum)
• Annular constriction or obstruction (colon to mid-rectum)
• Metastatic rate is highly variable depending on the study

+ Polyps

• Site: distal rectum
• 80% are single lesions and 20% are multiple
• 0%-41% local tumor recurrence rate after surgical resection
• 18% rectal polyps undergo malignant transformation
• Tenesmus, hematochezia, dyschezia, rectal bleeding unassociated with defecation, and polyp prolapse
• Other signs can include vomiting, diarrhea and weight loss

+ Physical Examination

• Cachexia is common
• Abdominal mass is frequently palpable via either abdomen and rectal palpation
• Other findings include dehydration and abdominal pain

+ Laboratory Tests

• Anemia and leukocytosis reported but occur less commonly than small intestinal tumors
• Paraneoplastic leukocytosis reported with adenomatous rectal polyp

+ Endoscopy

• Endoscopy is recommended prior to definitive treatment
• Biopsy samples should be interpreted histologically as cytologic misdiagnosis is common with intestinal ADC being misdiagnosed as either septic inflammation or LSA
• Biopsy samples are often small and superficial resulting in false-negative diagnosis if lesion is either submucosal or associated with surface ulceration and necrosis

+ Surgical Techniques

• Surgical approaches: rectal eversion, rectal pull-through, or resection and end-to-end anastomosis
• Fecal incontinence is uncommon if rectal resection < 4 cm
• However, fecal incontinence and incisional dehiscence is common if > 4 cm rectum is resected with rectal pull-through surgery
• Transanal endoscopic resection of benign rectal tumors has been described in 6 dogs
• Incontinent end-on colostomy has been described in the management of 1 dog with rectal ADC

+ Cryosurgery

• Cryosurgery has been used in 1 dog rectal leiomyosarcoma complications include rectal prolapse, perineal hernia, and stricture formation
• Cryosurgery of pedunculated stalk can be considered with rectal polyps

+ Chemotherapy

• Multiagent protocols recommended for LSA as either adjuvant therapy or sole treatment for diffuse disease
• Chemotherapy is also recommended for cats with intestinal plasmacytoma and MCT
• No effective chemotherapy for ADC, but combination of 5-fluorouracil and cisplatin may be effective
• Piroxicam (suppository or oral) for rectal tubulopapillary polyps with significant PR or CR in 88% (7/8)

+ Rectal Polyp

• 0%-41% local tumor recurrence rate after surgical resection or cryosurgery
• 18% rectal polyps undergo malignant transformation
• Survival time > 1 year with few deaths related to polyp
• Survival time for polyps diagnosed as carcinoma in situ 5-24 months

+ Benign Rectal Tumors

• 5 dogs alive between 5-20 months after surgery
• 3 local recurrence with recurrence as carcinoma in one dog

+ Rectal Carcinoma

• Mean survival time 22 months following surgery
• Radical surgery associated with high complication rate and poor survival (< 1 week)
• Local appearance may be predictive of survival:
• Mean survival time 32 months if pedunculated
• Mean survival time 12 months if nodular or cobblestone
• Mean survival time 1.6 months if annular
• Mean survival time 24 months following cryosurgery
• Survival times following electrocautery are reported between 18-29 months
• MST 7 months following intraoperative orthovoltage radiation therapy with 12-month survival rate 67%, local tumor recurrence in 54%, and median DFI 6 months