FDG PET Limited in Evaluation of Mucinous Tumors

Abstract & Commentary

Synopsis: Although FDG PET can be useful for evaluation of primary or recurrent neoplasms, tumors containing abundant mucin may have false-negative results at PET.

Source: Berger KL, et al. FDG PET evaluation of mucinous neoplasms: Correlation of FDG uptake with histopathologic features. AJR Am J Roentgenol 2000;174:1005-1008.

Fluorodeoxyglucose positron emission tomography (FDG PET) is becoming an important technique in the evaluation of known or suspected neoplasms, whether primary or recurrent. As experience accrues, however, it is becoming clear that generalizations about the accuracy of PET for the diagnosis of malignancy can be dangerous. Specifically, some low-grade tumors grow slowly and metabolize glucose at relatively low rates, making them indistinguishable from benign processes at PET. Conversely, some inflammatory processes can consume glucose rapidly, and thus falsely simulate malignancy at PET.

In an attempt to better define the capabilities of PET, Berger and associates performed a retrospective review of FDG PET examinations in 22 patients being evaluated for primary (n = 12) or recurrent (n = 10) mucinous carcinoma. The results of PET were correlated with findings at histopathologic examination of core biopsy or surgical resection specimens. Interestingly, PET showed moderately or markedly increased FDG accumulation (relative to surrounding normal tissue or the corresponding normal contralateral structure) in the mucinous carcinoma in only 13 (59%) of the patients. The nine carcinomas showing no or minimal uptake of FDG measured 1-5 cm in diameter on other imaging studies or at pathologic examination; these tumors included recurrent colorectal cancer (n = 5), primary carcinoma of the esophagus/gastroesophageal junction (n = 2), primary lung cancer (n = 1), and metastatic breast cancer (n = 1). Carcinomas with low cellularity or containing abundant mucin were statistically significantly correlated with false-negative PET results. Berger et al conclude that evaluation of mucinous tumors with FDG PET is limited, especially in those tumors that contain abundant mucin and relatively few cells.

Comment by David M. Panicek, MD

This report provides an important cautionary reminder that the ability to demonstrate cancer at FDG PET examination also depends on the particular histologic composition of a given tumor; not all colon carcinomas, for example, will produce positive findings at PET, even if large. Such information is critical for properly determining the clinical significance of a negative result at PET, to avoid incorrectly concluding that a malignant lesion is benign.

The results of this study are particularly relevant because tumors that produce mucin are not rare, arising in various anatomic sites such as colorectum, stomach, pancreas, ovary, and lung. And, as Berger et al point out, one of the more common clinical applications of PET is for the detection of recurrent colorectal cancer; given that mucinous tumors comprise about 17% of colon carcinomas and that colon carcinoma is quite common, the potential magnitude of false-negative PET scans is substantial.

More clinical experience will be needed before the actual sensitivity of PET is known for various histologic types of various tumors. In the meantime, physicians should be cautious about interpreting a negative result at PET if other imaging techniques (such as CT, MRI, or nuclear scintigraphy) show findings suspicious for primary or recurrent malignancy; additional evaluation, such as with close interval follow-up radiologic examination or even biopsy, may be required. Particular caution is warranted in the subgroup of mucinous tumors.