Radiologic Findings

On T1-weighted (T1w) MRI, the imaged portions of the femur and tibia show scattered spots of relatively low signal intensity (Fig. 7.1 a, b), which appear bright in the water-sensitive sequence (Fig. 7.1 c). The spots enhance after intravenous (IV) contrast administration (Fig. 7.1 d). Similar changes were found on the contralateral side.

Location

All the findings are located in the medullary cavity of the diametaphysis while sparing the epiphyses.

Pathoanatomic Background of the Findings

The foci must have a moderately high proton density and must have a sufficient vascular supply based on their enhancement characteristics. They could be aggregates of normal but heterotopic bone marrow cells, or myeloproliferative lesions such as mastocytosis or malignant lymphoma.

Assignment to a Possible Basic Entity

Normal variant or malformation?

Possibly. Islands of bone marrow in medullary cavities containing mostly fatty marrow may occur in the long bones of healthy individuals, especially women. Some persons, currently healthy, may also have a prior history of one or more episodes of significant blood loss or heavy menstruation.

Inflammation?

No clinical manifestations.

Tumor or tumorlike lesion?

Yes, in principle, such as localized non-Hodgkin lymphoma, plasmacytoma, etc.

Synopsis and Discussion

Since the lesions were detected incidentally during knee MRI in a clinically healthy young woman, the spotty areas of altered signal intensity in the medullary cavity should be classified as an incidental finding or normal variant. This is even more likely when the same changes are found on the opposite side. The referring physician should be told, however, that a clinical examination (liver, spleen, and lymph nodes) and blood count are necessary to exclude an incipient hematologic disease. There is no need, however, to alarm the patient or referring physician.

Finally, it should be mentioned that islands of bone marrow in fat-containing diametaphyseal segments of long bones may not only form in the setting of increased hematopoiesis, for whatever reason, but may also occur physiologically during the conversion of red bone marrow to fatty marrow at about 20 years of age and then disappear completely by about 24 years of age. Apparently this process does not occur in some individuals, however, with the result that islands of bone marrow are still detectable for years or even for life.

Final Diagnosis

Islands of red marrow detected incidentally in the distal femoral metadiaphyses.

Radiologic Findings

Both initial MRI (Fig. 7.2 a) and the follow-up series (Fig. 7.2 c, d) demonstrate a predominantly medial area of edema-like signal in the middle third of the left femoral shaft. A linear hyperintensity is visible along the medial cortex.

Location

The above MRI findings are arranged eccentrically about the medial side of the femoral cortex, so the cortex may be their epicenter. This suggests that the process does not originate from the bone marrow or medullary cavity (compare with Case 130).

Pathoanatomic Background of the Findings

As noted under Location, a primary process arising from the medullary cavity (Ewing sarcoma, eosinophilic granuloma, osteomyelitis) appears unlikely. Edema-like signal changes with a cortical epicenter may have a traumatic etiology.