Incidental detection of hematopoietic hyperplasia on routine knee MR imaging.

An unusual appearance of the marrow of the distal femur, characterized by confluent foci of diminished signal intensity replacing the normally expected bright signal of fatty marrow on all pulse sequences, was encountered in 10 asymptomatic patients undergoing routine knee MR imaging. This prompted initial concern about the possibility that this appearance was due to lymphoma or occult myeloproliferative malignancy. All patients were mild to moderately obese, and all but one were women. Peripheral blood analysis was obtained in nine patients and was normal in five. The other four were cigarette smokers with a peripheral leukocytosis. Bone-marrow biopsy of the distal femur in two patients and of the iliac crest in three, performed to exclude malignancy, showed hypercellular but otherwise normal-appearing hematopoietic (red) marrow. The patients have been followed for 4-15 months without evidence of malignancy. We conclude that extensive foci of hematopoietic marrow may be encountered incidentally in patients undergoing MR examination. Careful follow-up to date suggests that this most likely represents a benign process and that patients in whom this appearance is encountered can be managed conservatively.