Excessive Colon Activity from Antidiarrheal Medication Causing False-Positive In-111 Octreotide Studies

Antidiarrheal medications are a pitfall for physicians interpreting In-111 pentetreotide scans. In-111 octreotide scanning is sensitive for detecting neuroendocrine tumors located within the abdomen. However, lack of familiarity with the normal patterns of activity, such as may occur in the large bowel, may lead to false-positive interpretations. For patients taking such medications, colon activity is often very prominent, and the intensity of uptake cannot be used to distinguish tumor from physiologic large-bowel activity. Usually, additional views are obtained 24 hours later or a repeated examination will show a shifting pattern of large-bowel activity. Unfortunately, when the patient is constipated or is taking antidiarrheal medications, intense large-bowel activity may persist for days. The authors describe a patient in whom prominent right colon activity on two sequential examinations was erroneously interpreted as tumor uptake. Only a third In-111 octreotide examination, obtained after antidiarrheal medications were discontinued, yielded the correct diagnosis.