Mesenteric masses can be the first sign of malignancy or of tumor recurrence, as shown in 5 of our patients, and our experience unfortunately demonstrates that large mesenteric masses can be repeatedly missed at serial CTs, even when the examinations are interpreted by subspecialty trained abdominal imagers who are familiar with the assessment of oncological patients and who read CT scans on a daily basis. To us, this was a surprising and humbling realization, and our results emphasize the need to remain vigilant in searching for mesenteric lesions. A benefit of our preliminary findings is that we identify a potentially helpful finding that may assist in the search for mesenteric masses: all of the patients with missed mesenteric masses showed contact between the masses and the mesenteric vasculature at CT. This finding may be of particular value for evaluating the mesentery of the colon and the mid and distal small bowel where mesenteric vascular contact with bowel is rare. A search pattern that includes tracing the major mesenteric vessels on axial images to identify areas of mesenteric vascular contact may be of particular value in the evaluation of high-risk patients with known or suspected malignancy and may allow more prompt intervention.
It must be stressed that the finding of mesenteric vascular contact is not in itself diagnostic for tumor because normal bowel can occasionally abut a mesenteric vessel. Rather, when interpreting CT scans with a concern for possible malignancy, the search pattern should include the tracing of mesenteric vessels to locate areas of vascular contact. Of course, because bowel may occasionally contact the mesenteric vessels, the reader must further inspect these foci to determine whether the mesenteric vessels abut a true mass or a bowel segment. Nevertheless, tracing of the mesenteric vessels may be helpful to improve detection of otherwise difficult-to-find mesenteric masses. Ambiguous cases may require multiplanar reformation of the CT images or follow-up examination. In our experience, tracing of vessels, even when intravenous contrast is not present (), can be readily performed on a PACS workstation by scrolling the images.
The finding of mesenteric metastases is important for medical and surgical management. Enlargement and encasement of the mesenteric vessels by enlarged lymph nodes is referred to as the “sandwich sign” and is thought to be suggestive of lymphoma.5,6
Also, the finding of mesenteric lymphadenopathy in patients with focal colonic wall thickening favors the diagnosis of colon cancer rather than diverticulitis.7
Several studies have shown that the size of normal mesenteric lymph nodes at CT may range up to 5 mm in diameter in adults2
and 8 mm in children8
and that such nodes may reside on either side of mesenteric branch vessels.6
However, the frequency and locations of mesenteric vascular contact with mesenteric masses or bowel at CT have not been previously described. We found that missed mesenteric masses are frequently in contact with mesenteric vasculature at CT (noted in 17 of the 18 CT scans where the masses were missed and 6 of the 6 scans where the masses were eventually identified). This finding held true even for the 2 patients with ovarian tumors, which were expected to have peritoneal implants rather than tumor enveloped within the mesenteric fat. This association between mesenteric masses and macroscopic blood vessels may reflect local potential for angiogenesis: tumors that grow larger may be more capable of blood vessel recruitment, or conversely, prominent blood vessels adjacent to tumor foci may allow more dramatic tumor growth. Other causes for mesenteric vascular contact with masses or bowel, such as the patient’s body habitus or location of the mass on the peritoneum versus within the mesenteric fat or distance from the mesenteric root, are not answered by our initial data and will require further study.
Knowledge of findings that assist in mesenteric lesion detection at 5-mm slice thickness CT is of high clinical importance. Potentially, the use of PET9
or novel magnetic resonance contrast agents10,11
may allow for higher detection rates of mesenteric masses. However, some low-grade malignancies, such as carcinoid, may not show significant uptake of 18-FDG or may be poorly visualized at routine MRI because of mesenteric motion artifact, and neither modality has yet replaced routine CT scanning for oncological follow-up in most clinical practices. The need to recognize useful CT findings to detect mesenteric masses is further amplified by the increasing use of isoattenuation oral agents, such as oral water or sorbitol, rather than radiodense oral agents as a means to improve evaluation of the bowel wall at CT12,13
because this practice may reduce the conspicuity of mesenteric masses relative to similar-attenuation bowel. Some reports have shown that thin-section CT with multiplanar reformations may improve the detection of mesenteric lymph nodes,14,15
and the added value of tracing mesenteric vessels to identify mesenteric masses at multiplanar CT review will require further study. However, most imaging centers, including ours, have not been fortunate enough to upgrade all of their CT scanners to allow for thin-section multiplanar reformation, and 5-mm thick transverse section CT remains a widely used imaging method to assess for abdominal tumors in the United States.
It is important to note that our patient population is likely biased toward severe or clinically relevant disease because smaller or asymptomatic missed mesenteric masses were less likely to be discovered. Our patient sample is also likely biased toward patients in whom the mesenteric mass was the first sign of malignancy or tumor recurrence because the finding of a missed solitary mass is more likely to stir clinical consternation than would the finding of just another metastasis in the setting of widespread disease. Furthermore, because the patients with mesenteric masses were nonconsecutive, we were not able to determine the sensitivity of this sign for the detection of mesenteric masses. Nevertheless, our cases show that clinically relevant enlarging mesenteric masses may be associated with mesenteric vascular contact at CT and that assessment of the mesentery for this finding may be of value in the oncological patient population.
Our study has a number of limitations. First, all our patients with mesenteric masses had tumors, and none had inflammatory or infectious disease. Other mesenteric diseases, including retractile mesenteric panniculitis or tuberculosis16,17
might also result in mesenteric masses that contact mesenteric vessels. Further study in a wider scope of patients will be useful to determine the generalizability of finding contact with mesenteric vessels to identify mesenteric masses. Second, our sample size of patients with missed mesenteric masses was small and nonconsecutive. Unfortunately, it is intrinsically difficult to identify consecutive patients with missed mesenteric masses because the identification of “missed” masses requires that the CT report not mention the mass, hence many missed masses would not be identified by a systematic search. However, our patient sample is of particular clinical importance because each mass was both missed at initial CT and was eventually proven to be malignant either by growth on subsequent imaging or at surgery. Third, our evaluation of anatomy in the 129 consecutive nononcological patients included only CT scans obtained with 5-mm slice thickness evaluated in the axial plane. The value of tracing mesenteric vessels to assess for mesenteric masses using multiplanar reformations or thinner slice thicknesses will require further study in the future. Nevertheless, 5-mm slice thickness is a commonly used CT technique in routine abdominopelvic imaging, and therefore, the assessment of mesenteric vessels will likely be useful for the evaluation of oncological patients in a broad range of institutions.
Nonwithstanding these limitations, we found that at CT, mesenteric vessels greater than 1 mm diameter rarely contact bowel segments other than the duodenum and proximal jejunum; however, they often contact mesenteric masses. At CT scan review, inspection of the mesenteric vessels may facilitate mesenteric mass identification.