Immunoblastic sarcoma: morphologic criteria and the distinction of B and T cell types.

Immunologic concepts have facilitated the recognition within the histiocytic lymphoma group of the Rappaport classification of five diagnostic categories of lymphoma (follicular center cell lymphoma large cleaved and large non-cleaved types, immunoblastic sarcoma B cell type, immunoblastic sarcoma T cell type and 'true histiocytic lymphoma'). The first four of these 'large cell lymphomas' are closely related biologically and are difficult to distinguish histologically. This paper purports to examine the morphologic criteria employed in the differential diagnosis of these four large cell lymphomas in an objective manner, utilizing carefully prepared histologic sections from 24 cases of lymphoma that had been subjected to detailed B and T cell characterization by surface marker methods. 18 morphologic criteria were evaluated by four observers independently. The results show that reliable histologic distinctions can be made on histologic grounds in a proportion of cases only and that individual criteria show more overlap within diagnostic criteria than previously recognized. This is attributed in part to the close biologic relationship of these neoplasms. It is also recognized that the morphologic criteria are subtle and that the pathologist should have the advantage of auxiliary techniques--cytochemistry, immunohistology, surface marker techniques--to achieve reliable distinction of functional types within the diffuse histiocytic group of Rappaport.