___________________________________________________________________________ Colorectal Cancer
Imaging Modality After colorectal cancer is diagnosed, additional imaging is required for disease staging. Liver and chest imaging, preferably using CT, is necessary to detect metastases. Rectal cancers should be staged using endorectal ultrasonography or MRI. Positron emission tomography (PET) imaging is increasingly used in colorectal cancer to detect extrahepatic metastases in patients considered for hepatic resection of presumed liver- only metastatic disease. PET is also used to localize disease in patients thought to have a recurrence, as reflected by emergent symptoms or rising CEA [171; 198; 199]. Practice guideline recommendations for imaging to stage colorectal cancer have been published by the American Society of Colon and Rectal Surgeons (ASCRS) and by Cancer Care Ontario [188; 200]. They recommend contrast-enhanced CT of the chest, abdomen, and pelvis should be performed in all patients with colon cancer (unless contraindicated) to estimate disease stage and identify metastases. If local excision is considered for low rectal cancer (0–5 cm from the anal verge), transrectal ultrasonography is preferred over MRI to improve discrimination between T1 and T2 lesions. For upper rectal cancers (10–15 cm above the anal verge), whereby the mesorectal fascia is not threatened, MRI is not considered superior to pelvic CT. MRI can stage the local rectum but is not adequate to assess regional disease at the level of the inferior mesenteric artery or distant disease. CT of the abdomen and pelvis should be used to assess for distant metastases and regional disease, including lymph node involvement along the inferior mesenteric artery. Pelvic CT and/or transrectal ultrasonography are recommended with contraindications to MRI. All patients with rectal cancer should have preoperative radiologic staging with contrast-enhanced CT to assess for metastatic disease [188; 200]. Histologic Assessment Histologic confirmation of colon cancer is ideal, and for rectal cancer, it is essential [171]. Research has demonstrated an association between the number of lymph nodes examined in colon and rectal cancer surgery and oncologic outcomes [201]. In patients with colon or rectal cancer, the American Joint Committee on Cancer (AJCC) and National Cancer Institute jointly recommend examination of a minimum of 12 lymph nodes to rule out regional lymphatic node involvement [202].
The TNM Classification System The AJCC has developed the TNM classification system, and this approach is the universal standard in clinical cancer care [202]. The AJCC TNM classification system is identical for colon and rectal cancer. The 2018 update to the AJCC system uses the pathologic stage (also called the surgical stage), as this is likely to be more accurate than the clinical stage, which takes into account the results of the physical exam, biopsies, and imaging tests done prior to surgery ( Table 6 ) [202; 203; 204]. The system was initially developed as a prognostic tool. While numerous studies have evaluated other clinical, pathologic, and molecular parameters for validity in outcome prediction, none have been validated in multi-institutional prospective trials, and the TNM system remains the only prognostic tool validated in multi-institutional prospective studies. With TNM [203]: • T describes the extent of primary tumor growth into the intestinal wall or adjacent areas. This grade reflects the extent of tumor spread in the colon and rectum wall, from the inner to the outermost layers. • N describes the extent of primary tumor spread to nearby (regional) lymph nodes. • M indicates whether the tumor has metastasized to other organs (most commonly, the liver or lungs). When the T, N, and M categories have been determined (usually after surgery), the information is combined for stage grouping, with stage I the least advanced and stage IV the most advanced ( Table 7 ) [127; 187; 188]. In rectal cancer, AJCC staging does not apply to the following malignant histologies [205]: • Sarcoma • Lymphoma • Carcinoid tumors • Melanoma
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