Fundamentals of Rectal Cancer Surgery
Preoperative Staging
Overview
Preoperative evaluation of a patient with rectal cancer provides data that allows for appropriate treatment selection. Rectal cancer staging involves a complete colonoscopy with biopsy, measurement of carcinoembryonic antigen (CEA) levels, contrast-enhanced computer tomography (CT) of the chest and abdomen, and magnetic resonance imaging (MRI) of the pelvis using a rectal cancer MRI protocol. A Fluorodeoxyglucose positron emission tomography (FDG-PET)/CT is not indicated in the initial staging.[1] Table 1 lists routine staging tests for rectal cancer.
Pretreatment staging of rectal cancer differs from colon cancer due to the need for local T-stage and N-stage evaluation of the primary tumor, which determines whether the patient should undergo neoadjuvant therapy and/or whether the cancer is amenable to local excision. Thus, rectal cancers should have a pelvic MRI. One of the most common mistakes in the management of rectal cancer is the failure to recognize that the tumor is in the rectum, which may occur because flexible endoscopy frequently overestimates the distance of the cancer from the anal verge. To ensure accurate localization of the cancer, surgeons should perform a rigid proctoscopy for cancers described on colonoscopy as being "rectosigmoid" or "sigmoid."[2] MRI has largely replaced endorectal ultrasound (ERUS) for local staging of rectal cancers because it gives better anatomic guidance for surgery and is less dependent on the operator (Figure 1). Cancer Care Ontario created an MRI protocol and synoptic MRI report for rectal cancer, which improves the quality and consistency of rectal cancer MRI reporting.[3] This protocol also has been widely used in the United States. More recently, the American Society of Abdominal Radiology created a rectal cancer MRI synoptic report based on the Ontario report.[4]
The National Accreditation Program for Rectal Cancer (NAPRC) has adopted this protocol as the recommended synoptic MRI report, which can be accessed through the Society of Abdominal Radiology’s website.[5]
Management of rectal cancer is different than management of colon cancer because of confinement of the pelvic organs within a fixed small space and proximity of the rectum to the anal sphincter. This location results in a technically challenging surgical dissection, with a higher risk for anorectal and genitourinary dysfunction. Preservation of the anal sphincter while avoiding permanent colostomy is an important consideration for the patient and surgeon.[6]
The patient’s symptoms and physical examination can supplement imaging and other tests. For example, a rectal mass encroaching on the lumen may cause symptoms of decreased stool caliber, bleeding, tenesmus, and incomplete rectal evacuation. A mass invading the anal sphincter may cause significant pain. Severe symptoms and risk for complete obstruction may be an indication for a diverting colostomy prior to neoadjuvant therapy. Clinical assessment of pretreatment continence is also important. Patients with incontinence will usually have a poor functional outcome with a low colorectal or coloanal anastomosis.
Digital rectal examination (DRE) allows the surgeon to evaluate the tumor’s distance from the levator muscles and anal sphincter. The distance of the lesion from the upper sphincter is best assessed with DRE, but this distance should also be recorded after rigid proctoscopy, flexible endoscopy, and sagittal MRI measurement. The relationship of the distal edge of the lesion can also be assessed as above or below the peritoneal reflection. While some may use the middle rectal valve as an estimation, the peritoneal reflection can be significantly lower in overweight Individuals and women with a history of multiple vaginal deliveries.[7]
Depth of invasion and lymph node involvement help determine the recommendation of neoadjuvant therapy and are best assessed using MRI.[8] ERUS may be used in some centers to evaluate superficial lesions being considered for local excision or it may be used when the patient is not a candidate for MRI. CT is used to assess paraaortic lymph nodes and distant metastases that lie outside of the imaging field of the pelvic MRI.[9]
It is essential to assess the circumferential resection margin (CRM), which is the distance between the tumor and the mesorectal fascia. The current standard for assessing CRM is MRI.[8] Rectal cancer within 1 mm of the mesorectal fascia (or levator for low tumors) is considered an involved CRM.[1] MRI CRM assessment has less accuracy for lesions anteriorly located and at the anorectal junction where there is paucity of mesorectal fat.
At these locations, ERUS may complement MRI of CRM.[10]
Lesions not invading the anal sphincter can be considered for sphincter-preserving surgery when adequate preoperative anal sphincter function is present.
The diagnosis of invasive adenocarcinoma is confirmed by endoscopy and biopsy. At endoscopy, the lesion is assessed for distance from the anal verge, size, percent circumference of the lumen occupied by the lesion, and location on the wall of the rectum. A complete endoscopic examination of the large intestine should be performed to exclude synchronous colon neoplasms. Synchronous colon adenomatous polyps may occur in 20%–33% of patients, and synchronous colon cancers are found in 4%–8% of patients with rectal cancer.[11] Cancers can be tattooed to mark their location in the event of complete clinical response to neoadjuvant therapy. If a colonoscopy cannot be completed up to the cecum, a second attempt should be made after neoadjuvant therapy and prior to surgery to perform a preoperative and complete endoscopic examination.[12][13]
Histologic characteristics on biopsy associated with higher potential for lymph node metastasis and worse prognosis include poor differentiation and lymphovascular invasion.[14] These histologic characteristics are contraindications for local excision. Biopsies from high-risk individuals (e.g., family history of colorectal/endometrial cancer, multiple colorectal tumors, younger age) should be routinely assessed for microsatellite instability. If microsatellite instability is found the patient should not only be appropriately be referred to a genetic counselor to exclude the possibility of a hereditary disorder,[15] but they should also be considered for preoperative immunotherapy.
A patient’s serum CEA level should be measured preoperatively to allow trending following treatment.[16] Serial CEA measurement is used with serial medical imaging for surveillance.[17][18]
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