Distal rectal carcinoma

Gastrointestinal tract images Rectal Carcinoma: Rectal wall penetration and pelvic lymph node involvement are the major prognostic factors in predicting recurrence. Some lymph nodes <5mm in diameter already contain metastases, a limitation in the imaging prediction of tumor spread. Nevertheless, the sensitivity for detecting positive lymph nodes is greater for rectal tumors than for more proximal colonic tumors because benign perirectal adenopathy is uncommon. Barium Enema: The relative roles of barium enema, flexible sigmoidoscopy, and colonoscopy are not established. A number of studies have shown a superiority of colonoscopy over barium enema. The problem with most of these studies is that colonoscopy is used as a gold standard and the studies are performed by gastroenterologists, but barium enemas were performed by general radiologists and the results are often biased against barium enema. A barium enema, however, does detect most pedunculated, sessile and infiltrating colon and rectal carcinomas.
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radiology images of Colon adenocarcinoma. A: A double-contrast barium enema identifies a tight circumferential cancer. The entire colon could be studied in spite of the tight obstruction.B: This patient presented both with bleeding and obstruction.Barium enema reveals an ulcerated (arrows), circumferential sigmoid cancer.
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This is radiology images of A: Distal rectal carcinoma (arrows) presenting as a diffuse carpet-like infiltrate. (Courtesy of Arunas Gasparaitis, M.D., University of Chicago.) B: Polypoid, infiltrating rectal carcinoma (arrow).

Computed Tomography: Computed tomography of some large, fungating ascending colon carcinomas infiltrating to pericolic fat identifies segmental distal colonic wall thickening. The histopathology of resected specimens reveals submucosal and subserosal edema, chronic inflammation and fibrosis, or both. Not all focal colorectal tumors detected by CT or MR are neoplastic. An adjacent abscess can readily mimic a necrotic cancer and vice versa. Endometriosis is another example. Intravenous contrast enhancement aids polyp detection; both benign and malignant polyps enhance with contrast, while residual content does not. Enhancement significantly improves visualization of 6–9 mm polyps (75% postcontrast versus 58% precontrast). Contrast also aids detection of local tumor extension and any lymphadenopathy.
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This is radiology images of Pararectal abscess mimicking a rectal carcinoma. Constipation developed after prostatic resection 4 months previously for benign hyperplasia. A,B: Two pelvic CT images reveal a rectal tumor narrowing the lumen (arrows). C,D: T1– and T2–weighted images show rectal wall thickening and an adjacent fluid-filled structure (arrow), suggesting an abscess or necrotic tumor.

               Studies suggest that CT colonography is competitive with conventional colonoscopy in detecting both benign and malignant polyps >1cm. In patients with colonic tumors (confirmed at endoscopy or surgery), axial and multiplanar CT detected all malignancies; all missed benign tumors were <8mm in diameter. Computed tomographic colonoscopy performed the same day as conventional colonoscopy achieved a 58% sensitivity and 52% specificity in identifying polyps, with sensitivity for polyps ≥1cm being 86%.
               After bowel preparation and colon air insufflation, 300 patients underwent CT scanning in supine and prone positions using 3-mm collimation and single breath hold; transverse CT images, sagittal and coronal reformations, and 3D endoluminal images completed the CT colonography.Using conventional colonoscopy results as a gold standard, this study achieved a sensitivity of 90% for detecting polyps 10mm or larger, 80% for polyps 5.0 to 9.9mm, and 59% for polyps <5mm; of note is that CT colonography detected all carcinomas. Intravenous contrast provides colonic wall and tumor enhancement. Enhancement significantly improved visualization of 6- to 9-mm polyps (75% postcontrast versus 58% precontrast). One potential pitfall for CT colonography is the occasional carpet-like or flat cancer. Most studies suggest that multiplanar 3-D endoluminal images achieve better sensitivity and specificity than 2-D images; nevertheless, in any one patient a combination of images is often necessary for full evaluation.
              Computed tomographic colonoscopy is an alternative to barium enema and conventional colonoscopy, especially in frail, elderly patients. Detection of small polyps in these patients is not as relevant as in younger patients. Ultrasonography: Endorectal US detects rectal tumors. Attempts have been made to
detect a malignancy arising within a rectal villous adenoma, but results have been disappointing. In general, rectal villous adenomas are resected regardless of imaging or biopsy findings.
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