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Whole-Body PET/CT Colonography for Colon Cancer Staging

Although integrated PET/CT was somewhat more accurate than conventional CT for staging tumors, management was not altered for most patients.

Imaging units that combine positron emission tomography (PET) with computed tomography scanning provide improved anatomic correlation of PET and CT findings and more efficient patient processing. In this prospective study, German investigators evaluated the staging accuracy of a specific PET/CT colonography imaging protocol among 47 patients who underwent PET/CT colonography on the day after tumors were demonstrated by optical colonoscopy plus biopsy. Only clear liquids were allowed between colonoscopy and colonography. For PET/CT studies, the colon was distended with water, and patients were scanned with all modalities from the neck through the pelvis. Images were evaluated in three ways: combined PET/CT, CT followed by PET, and CT alone. The reference standard was histopathologic evaluation of surgically removed tumors.

Overall, PET/CT colonography provided correct TNM staging for 37 of 50 lesions, PET plus CT was correct in 32, and whole-body CT alone was correct in 26. Overall, about two thirds of incorrectly staged tumors were understaged. Compared with CT alone, PET/CT was more accurate in defining the TNM stage (74% vs. 52%; difference, 22%; 95% CI, 9%–36%; P=0.003), based primarily on a more accurate definition of T stage (86% vs. 66%). The accuracy of N staging (cutoff, 0.7-cm nodes) ranged from 86% with PET/CT to 76% for CT alone. All modalities were accurate for M staging, as judged during a mean clinical follow-up of 447 days.

Management was changed for four patients as a result of PET/CT colonography results. One of these patients had a liver tumor that PET/CT showed to be a hepatocellular carcinoma, rather than a colorectal metastasis. Another patient had a benign polyp that appeared to be residual tumor on CT but was shown by PET/CT to be scar tissue. In a third patient, PET/CT showed a 6-mm liver metastasis along with one other metastasis visualized on CT, resulting in more extensive hepatic surgery. The final patient had a goiter diagnosed by CT, which PET/CT showed to be a thyroid carcinoma.

Comment: Can definitive conclusions be drawn from these findings? Probably not. Two of the four patients in whom management was changed by PET/CT had tumors that were not colonic in origin. A third did not have cancer at all — just scar tissue at the site of a polypectomy, which could have been differentiated easily by repeat colonoscopy with inspection and biopsy of the site. Also, the clinical value of better T staging is shaky, except in the case of rectal cancer, because it is unlikely to affect the surgical approach, and requirements for adjuvant therapy will be determined by pathologic staging. However, gastroenterologists should be aware of so-called whole-body PET/CT colonography and its purported benefits.

— Douglas K. Rex, MD

Published in Journal Watch Gastroenterology March 23, 2007

Citation(s):

Veit-Haibach P et al. Diagnostic accuracy of colorectal cancer staging with whole-body PET/CT colonography. JAMA 2006 Dec 6; 296:2590-600.

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