Operations to remove the rectum are most often performed for the treatment of rectal cancer. Rectal adenocarcinoma has the same histology and staging as colon cancer, however there are important differences in the treatment of this cancer due to the anatomical limitations imposed by the pelvic sidewalls and the anal sphincter complex.

Rectal cancer is loosely defined as an adenocarcinoma that is located within 15 cm of the dentate line. Once an endoscopic biopsy has confirmed rectal cancer, the first step is to perform a staging evaluation. A CT scan of the chest and abdomen is obtained to look for metastases, which most often occur in the lungs or the liver. Pulmonary metastases are more common in rectal cancer than in colon cancer due to the fact that the rectum drains directly into the systemic venous circulation as well as the portal venous circulation. Therefore, it is possible for tumor cells to spread hematogenously via the systemic circulation directly to the lungs, bypassing the liver.

Once metastatic disease has been ruled out, patients with rectal cancer require additional staging procedures to assess the local extent of the disease. Either an endoscopic ultrasound (EUS) or pelvic MRI can be used to evaluate tumor depth (T stage) and nodal involvement (N stage). Patients who are found to have early stage tumors proceed directly to resection, similar to colon cancer. However, patients with locally advanced tumors (T ≥ 3 or N ≥ 1) should receive chemotherapy and radiation prior to surgery. This neoadjuvant chemoradiation, which is not used in colon cancer, has been shown to decrease the risk of local recurrence and increase the anal sphincter preservation rate. Surgery to resect the rectum is then performed 4–6 weeks following the completion of neoadjuvant therapy.

Following resection, patients whose pathology demonstrates node-positive disease will go on to receive adjuvant systemic chemotherapy, in order to decrease the chance of recurrence.

Those patients found to have metastatic disease already present at presentation usually begin treatment with systemic chemotherapy, but may be candidates for metastasectomy depending on their clinical situation. Resection of limited pulmonary or hepatic metastases has been shown to prolong survival in well-selected patients.

Two features dictate the type of operation used for resection of a rectal cancer: the size/depth of the tumor and its distance from the anal sphincters. In very early stage rectal cancers, it is possible to perform a transanal excision of the lesion. This procedure is reserved for early cancers since a local excision does not include any of the lymph nodes draining the region. Therefore, the only candidates for transanal excision are those patients in whom lymphatic metastases are thought to be extremely unlikely. General criteria for local excision are patients with small, superficial T1 lesions, who have no radiographic evidence of metastatic disease to regional nodes. Finally, the tumor must be located in the lower segment of the rectum to be accessible via transanal access. These criteria may be extended somewhat in patients with multiple co-morbidities who would be at high risk for major intra-abdominal surgery. Serial postoperative imaging and endoscopic surveillance is necessary in patients undergoing transanal excision in order to detect any signs of recurrence.

The majority of patients with rectal cancer will not be candidates for local excision and instead require formal resection of the rectum. In most cases it is possible to resect the rectum and connect the proximal colon to the rectal stump, an operation known as a low anterior resection. This procedure maintains the continuity of the GI tract. However, for tumors located in the very lowest portion of the rectum, resection of the rectum en bloc with the surrounding anal sphincters is necessary in order to achieve tumor clearance (Fig. 16.1). This operation—known as an abdominoperineal resection (APR)—leaves the patient with a permanent colostomy, and is only employed in cases where sphincter preservation is not possible. This determination is heavily dependent on an individual patient’s size, body habitus, gender, and pelvic anatomy. If the distance from the tumor to the top of the anal sphincter is less than 5 cm, it is usually necessary to perform an APR.

Whereas rectal cancer arises from the glandular mucosa of the rectum, anal cancer is a squamous cell carcinoma arising from the transitional or squamous mucosa of the anal canal. Rather than a sharp transition, these cell types overlap significantly over the length of the anal canal. Therefore, it is the histologic type that determines the classification and treatment of a tumor in the anal canal.

The vast majority of anal cancers are associated with Human Papillomavirus (HPV) infection. While some affected individuals develop anal condylomata, others may be asymptomatic and unaware of an existing HPV infection. Women with a history of HPV-related cervical cancer are also at higher risk for developing anal cancer. Of note, a history of hemorrhoids, fistulae, or fissures is not associated with an increased risk of anal cancer.