Neoplastic Diseases

Colorectal cancer is a common disease and is one of the leading causes of cancer-related death in many Western countries. Imaging plays an important role in the detection of colorectal cancer and of their precursor polyps, and in staging of rectal cancer.

The vast majority of colorectal cancers are adenocar-cinoma and there is a well-established adenoma-carcinoma sequence. Cancer can present as a mass that may cause lumen obstruction. Some cancers may have a carpet-like presentation. Polyps are mostly sessile, but may be pedunculated or flat. Currently, it is not fully established whether flat adenomas are more aggressive.

Other malignancies of the colon and rectum are rare, and include soft tissue cancers, gastrointestinal stromal tumors and extension of tumor from adjacent organs and peritoneal metastases. The differentiation between diver-ticulitis and cancer may be difficult, particularly in the sigmoid colon. The presence of fluid in the root of the sigmoid mesentery, engorgement of adjacent sigmoid mesenteric vasculature and a tethered or sawtooth lumi-nal configuration favors the diagnosis of diverticulitis. Conversely, the presence of enlarged pericolic lymph nodes, mural thickness greater than 1.5 cm, and an abrupt transition zone raises the possibility of colon cancer. Ischemic colitis may also lead to bowel wall thickening, but the clinical history (e.g. extensive atherosclerosis) and imaging findings (no enlarged lymph nodes, more gradual transition) help to differentiate. At the rectosig-moid, endometriosis may simulate colorectal cancer, but the location (often anterior, adjacent to the pouch of Douglas or rectovaginal septum) and morphology help to differentiate between these two conditions. At MRI endometriosis has typical findings: fibrosis with small hy-perintense foci on T1-weighted sequences representing blood, or cystic masses that are often hyperintense on T1-weighted sequences and intermediate signal intensity on T2-weighted sequences (so-called shading) due to the presence of blood in a cyst.

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