Carcinomas can assume a variety of shapes. Smaller cancers may be identical to large polyps and flat lesions, while semi-annular and annular, and scirrhous cancers have a unique CT appearance. Semi-annular and annular cancers are seen as a focal, segmental regions of luminal narrowing, accompanied by focal wall thickening, usually with proximal and distal shouldering (Fig. 14.26). The intraluminal margins of the mass are irregular. Extension of soft tissue into the pericolonic fat signals invasion, as does regional lymphadenopathy or hepatic metastases. Annular cancers are best seen at 2D and 3D MPR images using soft tissue window settings.
Scirrhous cancers can constrict the lumen and are the most common type of cancer in our experience that are missed by radiologists learning colonography (Fidler et al. 2004). Scirrhous cancers are annular lesions that constrict the colonic lumen, and may be confused with luminal collapse, usually because adjacent collapse may obscure the shouldering of the carcinoma (Fig. 14.27). Focal wall thickening of soft tissue attenuation and irregular intraluminal margins clearly separate these lesions from collapse, however. Segmental regions of lumi-nal narrowing seen in one position or seen in both positions should be considered as potential scir-rhous cancers, until repositioning and reinflation can disprove their presence.
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