Answer: Malignant lymphoma

Histology: Radiographs demonstrate a lytic and blastic lesion of the L3 vertabra. The vertebra shows partial collapse. Histologically, there are sheets of small round blue cells.

Discussion: Malignant lymphoma may occur as a primary osseous neoplasm. Generally lymphomas are large B-cell lymphomas, as in this case. Histologically, they show sheets of lymphoid cells which stain positively for CD20 (not shown).

The lesion is not metastatic carcinoma because of the absence of epithelial cells. However, metastatic carcinoma must always be considered in spinal lesions in a patient above the age of 55. Therefore, immunohistochemical stains are always necessary to differentiate a metastatic carcinoma from a primary bone tumor in this age group.

The diagnosis is not multiple myeloma because of the absence of plasma cells. Occasionally, plasma cells may be so poorly differentiated as to masquerade as lymphocytes. However, the plasma cell morphology is completely absent in this case. However, occasionally a CD138 stain is necessary to rule in or rule out multiple myeloma. In addition, since almost all patients with multiple myeloma have an elevated serum or urine M-protein, this test should always be performed on evaluating a round cell neoplasm in an older person.

The diagnosis is not Ewing’s sarcoma. Although, Ewing’s sarcoma may rarely occur in older patients, this tumor occurs most commonly in young adults or children. However, a CD99 immunostain for Ewing’s sarcoma should be performed when evaluating any small round blue cell neoplasm in any aged patient. In the present case, CD20 positivity identifies the neoplasm as a B cell lymphoma.