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Gastrointestinal Imaging

E2782. “Dirty Masses” in the Abdomen: Their CT Appearance and Significance

Herskovits P,  Kafer I,  Friedman B,  Hines J. Northwell Health, Manhasset, NY

Address correspondence to P. Herskovits (pnina.herskovits@gmail.com)

Background Information: Colonic perforation is a surgical emergency with high morbidity and mortality. Its etiologic factors vary, and include spontaneous perforation, diverticulitis, ulcers, iatrogenic injury, tumor, and volvulus. As the clinical presentation of patients with colonic perforation is often vague and nonspecific, radiologists play a key role in making this diagnosis. CT is usually the first-line modality used in suspected acute gastrointestinal pathology. The purpose of this study is to highlight a particularly crucial CT finding in the setting of colonic perforation, namely the “dirty mass.”

Educational Goals/Teaching Points: The dirty mass is a low attenuation soft tissue mass with small foci of gas on CT. It implies fecal spillage from the bowel, and is specific for colonic perforation. In a study performed by Saeki et al. (1998), all patients with a dirty mass on CT had peritoneal fecal material confirmed intraoperatively.

Key Anatomic/Physiologic Issues and Imaging Findings/Techniques: Recognition of a dirty mass is made all the more important because findings of colonic perforation are often subtle. Perforations are frequently localized and walled off, and the amount of extraluminal gas may be small and difficult to detect due to its proximity to gas-filled bowel loops. Other specific findings of bowel perforation, such as a discontinuous wall sign and oral contrast extravasation, are helpful when present but are insensitive, unfortunately. In our experience, the dirty mass is frequently missed on CT, as its mottled soft tissue and gas appearance can be mistaken for a loop of bowel. Associated imaging findings, such as ascites, bowel wall thickening, bowel wall dilatation, and the dirty fat sign should alert the radiologist to look closely for a bowel perforation and a possible dirty mass. Once found, the dirty mass not only signifies fecal spillage but its location can point to the actual site of perforation, as it usually forms just adjacent to the involved bowel. Localizing the perforation site can aid in management of the patient and offer valuable preoperative information to the surgeon.

Conclusion: In conclusion, the dirty mass is a specific finding for colonic perforation, and recognition of this finding on CT may have a tremendous impact on patient management. These masses can be subtle on CT and easily overlooked. In this exhibit, we will critically review findings of a dirty mass on CT, including their appearance, location, and associated findings. This will be done in a case-based fashion, and include clinical and surgical follow-up. Many of these cases were found during the quality improvement process of our department, as they were missed by the radiologist making the original interpretation. The ultimate objective of this exhibit is to make the reader aware of a dirty mass and its clinical implications, and to provide teaching points to assist in the detection of this finding.