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Spiral CT of Colon Cancer: Imaging Features and Role in Management1 Karen M. Horton, MD • Ross A. Abrams, MD • Elliot K. Fishman, MD Colorectal cancer is a common malignancy that results in significant morbidity and mortality. Abdominal computed tomography (CT) is valuable in planning surgery for colon cancer because it can demonstrate regional extension of tumor as well as adenopathy and distant metastases. At CT, colorectal cancer typically appears as a discrete soft-tissue mass that narrows the colonic lumen. Colorectal cancer can also manifest as focal colonic wall thickening and luminal narrowing. Complications of primary colonic malignancies such as obstruction, perforation, and fistula can be readily visualized with CT. At CT, local extension of tumor appears as an extracolic mass or simply as thickening and infiltration of pericolic fat. Extracolic spread is also suggested by loss of fat planes between the colon and adjacent organs. The liver is the predominant organ to be involved with metastases from colorectal cancer. At CT, hepatic metastases usually appear as hypoattenuating masses, which are best visualized during the portal venous phase of liver enhancement. Other common sites of metastases from colon cancer include the lungs, adrenal glands, and bones. Use of CT is critical for identifying recurrences, evaluating anatomic relationships, documenting “normal” postoperative anatomy, and confirming the absence of new lesions during and after therapy.

Abbreviation: 3D = three-dimensional Index terms: Colon, CT, 75.12115 • Colon, neoplasms, 75.32 • Intestinal neoplasms, CT, 75.12115, 75.32 • Intestinal neoplasms, diagnosis, 75.32 • Intestinal neoplasms, staging, 75.32 RadioGraphics 2000; 20:419–430 1From the Department of Radiology, Johns Hopkins Medical Institutions, 601 N Caroline St, Baltimore, MD 21287 (K.M.H., E.K.F.); and the Department of Radiation Oncology, Johns Hopkins Hospital, Baltimore (R.A.A.). Presented as a scientific exhibit at the 1998 RSN A scientific assembly. Received March 3, 1999; revision requested April 21 and received May 14; accepted May 17. Address reprint requests to E.K.F. (email: [email protected]). ©RSNA,

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See also the article by Horton et al (pp 399–418) in this issue.

420 March-April 2000

Introduction Colorectal cancer is the second most common cause of cancer death in developed countries. In 1998, there were 131,000 new cases of colorectal cancer and 56,000 deaths in the United States (1). The initial diagnosis is usually made with colonoscopy or air-barium enema examination; however, with the increased use of computed tomography (CT) as the initial imaging modality in patients with a variety of gastrointestinal symptoms, the radiologist may be the first to suggest the diagnosis of colon cancer on the basis of CT findings. Nevertheless, at this time, CT is not routinely performed for detection of colon cancer, although continued advancements in scanner and computer technology may allow CT to play a future role in detection of polyps and early-stage colon cancer. The current role of CT in patients with known colon cancer is controversial. Accuracy rates for preoperative staging of colon cancer with CT have been disappointing, ranging between 48% and 77% (2–6). Limitations of CT staging include an inability to definitively identify nodes that contain tumor or to determine the exact depth of tumor invasion through the wall. Despite these limitations, CT is valuable in the management of colon cancer. Preoperative CT is useful for planning surgery or radiation therapy, particularly when local extension of tumor into adjacent organs or distant metastases are detected. In addition, preoperative CT provides baseline findings for comparison during the postoperative period and is the modality of choice for detection of local recurrence after surgical resection. Given the prevalence of colon cancer in the United States and the role of CT in preoperative staging, treatment planning, and postoperative follow-up, the radiologist should be familiar with the CT appearance of colon cancer. This article discusses the technique of colon CT, staging of colon cancer, primary tumors, local spread, metastases, tumor recurrence, and therapeutic considerations.

Technique When abdominal CT is performed to image a wide variety of colonic diseases, colonic opacification must be optimal. Oral contrast material (diatrizoate sodium meglumine [Hypaque 3%;

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Nycomed Amersham, Princeton, NJ] or barium sulfate [Barocat 2%; Lafayette Pharmaceuticals, Lafayette, Ind]) can be administered the night before the study as well as 30–90 minutes prior to the study to ensure that adequate contrast material reaches the colon. In urgent cases or in patients in whom limited rectosigmoid disease is suspected, positive contrast material can be gently administered via the rectum. A topogram can then be obtained to confirm filling of the entire colon before CT is performed. Neutral agents (water) (7,8) or negative agents (air) (9) can also be easily administered via a rectal tube and provide excellent contrast for colonic imaging. In a small series by Gazelle et al (5), CT performed after administration of a water enema was effective in staging colorectal cancers; this technique may improve the ability of CT to demonstrate the depth of tumor invasion of the wall and extension into the pericolic fat. Air or carbon dioxide can also be used to distend the colon and is particularly helpful for detection of polyps and small masses when used after bowel cleansing. In addition, water or air is preferred over positive agents (diatrizoate sodium meglumine or barium) when performing three-dimensional (3D) imaging of the abdomen for CT angiography because positive contrast agents interfere with data manipulation, thus necessitating extensive editing. Recently, CT (ie, virtual colonoscopy) is being investigated as a potential method for screening patients for polyps and early-stage colon cancers. However, significant advances in data acquisition and computer image processing techniques will be needed before the technology can become a reliable, cost-effective alternative to conventional colonoscopy. If CT is performed for polyp or tumor detection, bowel cleansing is essential to help avoid confusion between adherent stool and polyps or masses. Go-Lytely solution (polyethylene glycol; Braintree Scientific, Braintree, Mass) taken the evening before the study will reliably cleanse the colon but may leave residual fluid within the colon. Imaging in both the supine and prone positions will help shift fluid, which may obscure underlying lesions, and help distend collapsed segments that were dependent on the supine images. Also, imaging in different positions may help one definitively identify fecal matter by demonstrating its mobility. Glucagon can be administered in selected patients to help relieve spasm and cramping if necessary.

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Table 1 TNM Classification for Staging of Colorectal Cancer Primary tumor (T) TX = primary tumor cannot be assessed T0 = no evidence of primary tumor Tis = carcinoma in situ T1 = tumor invades the submucosa T2 = tumor invades the muscularis propria T3 = tumor invades through the muscularis propria into the subserosa or into nonperitonealized pericolic or perirectal tissues T4 = tumor directly invades other organs or structures or perforates the visceral peritoneum Regional lymph nodes (N) NX = regional lymph nodes cannot be assessed N0 = no regional lymph node metastasis N1 = metastasis in 1–3 pericolic or perirectal lymph nodes N2 = metastasis in ³4 pericolic or perirectal lymph nodes Distant metastasis (M) MX = distant metastasis cannot be assessed M0 = no distant metastasis M1 = distant metastasis

Administration of intravenous contrast material is essential for complete staging of known colorectal cancer and for evaluation of recurrent or metastatic disease. At our institution, we routinely administer 100–120 mL of iohexol (Omnipaque 350; Nycomed Amersham) intravenously at a rate of 2–3 mL/sec. When evaluating a patient with known or suspected colon cancer, the abdomen should be routinely imaged from the diaphragm to the symphysis pubis. When spiral CT is used, 5-mm collimation can be performed with a table speed of 8 mm/sec and reconstruction of the data at 5-mm intervals. Our standard spiral CT protocol is to acquire data during the portal venous phase of liver enhancement, 45–50 seconds after the start of contrast material injection, to maximize the detection of hepatic metastases. Scanning too late may result in streak artifact in the abdomen due to concentrated contrast material within the renal collecting system. The patient is usually scanned in the supine position. If necessary, repeat imaging can be performed with the patient prone or after administration of more air or contrast material to distend collapsed segments. Also, multiplanar reconstruction and 3D volume rendering techniques can be used in problem cases to better visualize colon anatomy and the location of the suspected mass or abnormality.

Staging Not all patients with colon cancer will require CT evaluation prior to initial surgical management. The diagnosis is usually made with colonoscopy and biopsy or after barium enema and colonoscopy. Overt metastatic disease at initial presentation is uncommon (