Extrahepatic disease

CT technology has evolved rapidly from incremental to single-slice helical to 4-, 8-, 16-, 64-, and 256-slice multidetector scanners now commercially available. Images obtained from 16-slice (or greater) scanners can be near-isotropic (ie, nearly perfectly cubical voxels without distortion of anatomy). This allows for so-called ''volumetric imaging'' during a single breathhold in many instances covering the entire chest, abdomen, and pelvis. The acquisition of ultrathin slices (typically 0.5, 0.625, or 0.75 mm depending on the vendor)

allows reformation of images in any plane without loss of resolution (isotropic) [59]. This technologic advance, allowing multiplanar postprocessing, puts CT on par with or ahead of (because of better spatial resolution) MR imaging. Nonetheless, a remaining advantage of MR imaging is the lack of ionizing radiation.

Faster CT scanning requires faster injection rates, smaller volumes, the use of saline bolus chasers, more concentrated iodine contrast agents, and longer delays between injection time and scan time. For a complete discussion on this topic the reader is referred to the review by Brink [60]. Various CT angiographic techniques can be used for preopera-tive liver assessment (discussed later).

Although CT scan is widely used for preoperative staging of CRC, there is no consensus on its use in a A tumor nodule greater than 3 mm in diameter in perirectal or pericolic adipose tissue without histologic evidence of a residual lymph note in the nodule is classified as residual lymph node metastases; however, a tumor nodule up to 3 mm in diameter is classified in the T category as discontinuous extension (ie, T3)

b Cases not considered R0 (complete resection) if the following are evident: non-en-bloc resection; radial or bowel margin positive for disease; residual lymph node disease; or NX (incomplete resection)

From Nelson H, Petrelli N, Carlin A, et al. Guidelines 2000 for colon and rectal surgery. J Natl Cancer Inst 2001;93:8; with permission.

Table S: TNM staging system for colorectal cancer

Stage

Definition

Primary tumor (T)

TX

Primary tumor cannot be assessed

T0

No evidence of primary tumor

Tis

Carcinoma in situ: intraepithelial or invasion of lamina propria

T1

Tumor invades submucosa

T2

Tumor invades muscularis propria

T3

Tumor invades through muscularis propria into the subserosa or into nonperitonealized pericolic or perirectal tissues

T4

Tumor perforates visceral peritoneum or directly invades other organs or structures

Regional lymph nodes (N)a

NX

Regional lymph nodes could not be assessed

N0

No regional lymph node metastases

N1

Metastases in one to three regional lymph nodes

N2

Metastases in four or more regional lymph nodes

Distant metastases (M)

MX

Distant metastases could not be assessed

M0

No distant metastases

M1

Distant metastases

Extent of resection (R)b

RX

Presence of residual tumor cannot be assessed

R0

No residual tumor

R1

Microscopic residual tumor

R2

Macroscopic residual tumor

a A tumor nodule greater than 3 mm in diameter in perirectal or pericolic adipose tissue without histologic evidence of a residual lymph note in the nodule is classified as residual lymph node metastases; however, a tumor nodule up to 3 mm in diameter is classified in the T category as discontinuous extension (ie, T3)

b Cases not considered R0 (complete resection) if the following are evident: non-en-bloc resection; radial or bowel margin positive for disease; residual lymph node disease; or NX (incomplete resection)

From Nelson H, Petrelli N, Carlin A, et al. Guidelines 2000 for colon and rectal surgery. J Natl Cancer Inst 2001;93:8; with permission.

preoperative scanning of patients with intraperito-neal colon cancer. Although detection of primary colon lesions is usually made by colonoscopy or barium enema, the increased use of CT for a variety of gastrointestinal symptoms is such that the radiologist may be the first to detect CRC based on CT findings. CT is most useful for detecting metastatic disease and regional tumor extension. Complications, such as obstruction, perforation, and fistula,

I, T1,2,N0M0; IIA, T3 N0M0; IIB, T4N0M0; IIIA, T1,2N1M0; IIIB, T3,4N1M0; IIIC, any T, N2M0; IV, any T, any N, M1. Reprinted with the permission of the American Cancer Society, Inc. All rights reserved.

can be readily visualized. CT is also useful in identifying recurrences, evaluating anatomic relationships, documenting normal postoperative anatomy, and confirming the absence of new lesions during and after therapy. A recent study of CT in 130 Veterans Administration patients to determine clinical use and cost-effectiveness found disease previously unknown to the surgeon as follows: demonstration of local extension (9%); demonstration of metastases (15%); unsuspected vascular abnormalities (10%); second malignancies (4%); and other pathology (13%). The investigators determined that the preoperative scan directly aided operative planning in 43 (33%) cases; actually altered management (surgery canceled) in six cases (5%); and led to qualitatively different care in 16%. The sensitivity and specificity for all metastases were 75% and 99%, respectively. For liver metastases these were 90% and 99%, respectively [61]. Furthermore, a cost savings of $24,000 was realized over 5 years.

Patients undergo scanning using multidetector scans after the ingestion of oral contrast and the intravenous injection of an equivalent dose of 45 g of

Table 4: Colon cancer 5-year survival rates

Stage

%

Stage I

9B

Stage IIA

85

Stage IIB

72

Stage IIIA

8B

Stage IIIB

64

Stage IIIC

44

Stage IV

8

I, T1,2,N0M0; IIA, T3 N0M0; IIB, T4N0M0; IIIA, T1,2N1M0; IIIB, T3,4N1M0; IIIC, any T, N2M0; IV, any T, any N, M1. Reprinted with the permission of the American Cancer Society, Inc. All rights reserved.

Barium Enema Polyp
Fig. 1. Double-contrast barium enema reveals a pedunculated left colon polyp with a long stalk.

iodine in the form of low or iso-osmolar contrast medium. Routine scans are currently acquired at 0.625-mm or 1.25-mm slice thickness, during a single breathhold, in the portal venous phase of liver enhancement and reconstructed axially for viewing at 5-mm or thinner slices. Oral contrast can be given the night before to opacify the colon, or can be given rectally, but this is not routinely indicated, because primary disease has usually already been confirmed. Reformatted images may also be used if determined helpful.

The primary lesion, unless sizable, may not be seen unless the colon has been previously cleansed, an unusual scenario. Even in the uncleansed colon on routine CT, polypoid or annular lesions can be well-appreciated because of their enhancement and more solid appearance compared with stool (Fig. 9). Associated findings, such as lymphadenop-athy, peritoneal implants (Fig. 10), tumor penetration through the bowel wall, and colonic obstruction, can be well-appreciated. Tumor appearances may vary from a discrete mass narrowing the lumen, to bowel wall thickening (see Fig. 10), to a necrotic mass appearing much like an abscess. In cases of associated inflammation and microperforation, the primary differential diagnosis is perforated diverticulitis. The presence of lymph nodes may help distinguish tumors from diverticulitis, whereas many other findings are shared by both [62]. Tumors may intussuscept and be easily recognized in longitudinal (Fig. 11) or axial plane. Mucinous tumors may be quite bulky. If mucinous colonic or appendiceal tumors perforate, patients may present with pseudomyxoma peritonei at CT (Fig. 12). Pericolic tumor extension can be suggested (T3 disease) on CT when the fat planes are blurred, but this appearance is not specific, nor is it sensitive. Inflammation or deep ulceration may cause blurred fat planes. Normal fat planes maybe seen with microscopic penetration of the muscularis propria.

Peritoneal surfaces may be involved with tumor in up to 10% to 15% of patients at the time of diagnosis of CRC, and 40% to 70% of patients with recurrent CRC [63]. Although CT may currently be the best modality to detect early disease, it remains limited, even with helical technology and thin slices. CRC peritoneal tumor nodules less than 1 cm were detected in 9% to 24% of 25 patients in a recent study in which non-picture archiving and communications system observation methods

Double Contrast Barium Enema
Fig. 2. (A) Double-contrast barium enema reveals a ring-like density etched-in-white in the transverse colon representing a polyp viewed en face (arrow). (B) More oblique view reveals a small pedunculated polyp (arrow).
Stage Iiib T3n1bm0 Cecum Crc
Fig. 3. A fecal filling defect (arrow) mimics a polyp in the cecum (confirmed at colonoscopy).

were used. Overall, for all sizes, two radiologists detected only 60% to 76% of peritoneal implants with a poor interobserver agreement level. Surprisingly, these poor results are little improved from the pre-helical CT era where up to one third of peritoneal metastases were missed in patients undergoing staging laparoscopy for gastric adenocarcinoma [64]. It is expected that with the use of multidetector scanners and picture archiving and communications system for reviewing images, greater accuracy can be achieved.

CT provides the best resolution of any known imaging modality and is the first choice for lung nodule detection. Despite its high sensitivity, specificity is quite poor. In a recent series of high-risk oncologic patients with nodules 3 cm or less (75%

Fig. 4. Large filling defect in sigmoid colon with a typical lacy, reticulated surface pattern, representing a villous adenoma.
Colon Carcinoma Barium Enem
Fig. 5. Double-contrast barium enema reveals an annular or apple-core-type appearance indicating primary colon cancer (arrow).

were equal or less than 1 cm), in only 60% of patients with a solitary nodule and in only 64% of patients with multiple nodules were the nodules malignant [65].

CRC may recur in between 37% and 44% of patients after curative resection, usually within 2 years. Local recurrence accounts for 19% to 48%, whereas distant metastases account for 25% to 44%. Multiple sites of recurrence are most common and local and distant recurrences are more common in rectal tumors. Local recurrences most often occur in the perianastomotic tissues or lymph nodes and may not be appreciated with luminal examinations like colonoscopy or barium enema. A recent study of recurrent CRC by CTC found that 46 of 51 local recurrences were in the extraluminal soft tissues [66]. It is critical to optimize the ability of CT to detect recurrence at anastomoses and perianastomotic tissues by ensuring a well-distended colon. CT is invaluable in assessing the response to chemotherapy through measurement of index lesions in the lung, liver, lymph nodes, or peritoneum.

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