FOLLICULAR DENDRITIC CELLS IN EXTRANODAL NON-HODGKIN LYMPHOMAS OF MALT AND NON-MALT TYPE

Extranodal lymphomas of the thyroid (n = 19), kidney (n = 15) and test is (n = 30) were investigated histologically and immunohistochemically for follicular dendritic cell pattern using the monoclonal antibody K i-FDClP. This recognizes follicular dendritic cells in paraffin sectio ns. Follicular dendritic cells were most predominant in lymphomas of t he thyroid. These thyroid lymphomas showed the morphological features of mucosa-associated lymphoid tissue (MALT) type lymphomas in 18 of 19 cases and were classified as high-grade malignant lymphoma of MALT ty pe with evidence of a low-grade malignant component (n = 18). Ten of t hese cases contained destroyed reactive follicles of follicular dendri tic cells. In 6 of these 10 cases follicular dendritic cells occurred in a pattern of tumour-associated abortive follicle type. The remainin g lymphoma of the thyroid was an immunoblastic lymphoma of B-cell type showing no detectable follicular dendritic cells. In extranodal lymph omas of non-MALT type follicular dendritic cells occurred in only two cases where immunocytoma involved the kidney. Malignant lymphomas of t he kidney (chronic lymphocytic leukaemia, n = 2; immunocytoma, n = 4; centroblastic lymphoma, n = 9) and of the testis (immunocytoma, n = 2; centroblastic lymphoma, n = 27; immunoblastic lymphoma of B-cell type , n = 1) revealed no characteristics of MALT type lymphoma, cytologica lly or with respect to follicular dendritic cells. Classical lymphoepi thelial lesions formed by centrocyte-like cells, a hallmark of MALT, o ccurred exclusively in thyroid lymphomas of MALT type. Although occurr ence of classical lymphoepithelial lesions formed by centrocyte-like c ells was limited to thyroid lymphomas of MALT type, a growth pattern o f lymphoid blasts, with formation of lesions mimicking lymphoepithelia l lesions superficially, was found in 6 of 27 testicular centroblastic lymphomas. Follicular dendritic cells in non-Hodgkin's lymphomas of M ALT type show distinct follicular patterns not found in other extranod al lymphomas such as those found in the kidney and testis.