Primary histiocytic lymphoma of bone: a light and ultrastructural study of four cases.

The light and ultrastructural features of primary histiocytic lymphoma of bone are indistinguishable from similar studies of respective cell types of nodular and diffuse large cell nodular and extranodal lymphomas. These ultrastructural features add further support to their lymphocytic histogenesis. There was no relationship between increasing nuclear clefts and convolutions in individual cases and the ultrastructural presence of increasing "histiocytic" features. Primary histiocytic lymphoma of bone is not a completely uniform monomorphic tumor cell population, but may vary in percentage of large noncleaved, large cleaved, large convoluted lymphocytic, histiocytic, and small atypical lymohocytic cells. Detailed study of four cases enabled division into two primary subgroups: Type I, consisting of a predominance of large noncleaved cells; and Type II, consisting of a mixture of cleaved, noncleaved, and convoluted cells. Further cases are being studied to determine whether these two groups are distinct or merely represent part of a continuous spectrum of cellular pleomorphism. In contrast to nodal lymphomas, there is no evidence at this time that proportional variations in the tumor cell population bear any relationship to biologic behavior or survival. Glycogen may be present in histiocytic lymphomas but was very rare (less than 1% total cells). Therefore, this feature as an isolated observation will not differentiate histiocytic lymphoma from Ewing's sarcoma.