Background: Extramedullary plasmacytoma (EMP) is a rare plasma cell disorder that can occasionally involve the gastrointestinal tract, though involvement of the small intestine and mesentery is very uncommon. Diagnosis is challenging as patient presentation is variable with pain, obstruction, or bleeding, and imaging cannot differentiate between other malignancies. Diagnosis is not achieved until histopathological analysis of the resected mass. Treatment and prognosis depend on finding of primary EMP or secondary EMP in the setting of multiple myeloma.

Methods: We report a case of a 71-year-old male presenting with iron-deficiency anemia and melena who was found to have a 15.5 cm EMP of the small bowel and mesentery. The mass with adjoining intestine and mesentery was surgically resected. His postoperative course was uneventful. Follow-up indicated primary EMP without metastasis or multiple myeloma.

Conclusions: EMP of the small intestine often presents with nonspecific symptoms as seen in this case with no findings on colonoscopy or EGD. Further workup may only reveal a mass of undetermined origin. If the mass is easily accessible, then surgery is a reasonable course of action and can be curative in cases of primary EMP. Close follow-up is recommended due to risk of progression to plasma cell myeloma. Conclusions: This case adds to the current literature of rare cases of plasmacytoma of the small bowel and mesentery. The mass was first detected with CT imaging, which highlights the importance of further investigation for patients who present with symptoms concerning for GI malignancy but have no findings on colonoscopy or EGD.