EXTRAMEDULLARY MYELOID CELL TUMORS - AN IMMUNOHISTOCHEMICAL AND MORPHOLOGIC STUDY OF 28 CASES

In an attempt to correlate the morphologic and immunophenotypic findin gs in extramedullary myeloid cell tumors (EMT), we studied 28 cases wi th a large panel of antibodies using paraffin section immunohistochemi stry. A previous or concurrent diagnosis of acute myelogenous leukemia or chronic myelogenous leukemia was made in 25 cases. Six EMT were mo rphologically classified as well differentiated (WD-EMT), 17 as poorly differentiated (PD-EMT), and five as blastic EMT. The WD-EMT were eas ily recognized morphologically and displayed a relatively mature myelo id phenotype, with elastase, CD15, and CD68 positivity in all cases. O n the other hand, the five blastic-EMT displayed no morphologic eviden ce of myeloid derivation, were completely negative for CD15, and were weakly positive for elastase in only one case. The PD-EMT, with a morp hologic appearance that resembles large cell non-Hodgkin's lymphoma, v ariably expressed CD15 and elastase. CD68 and lysozyme were present in the majority of PD-EMT, with some variability, but were negative in m ost blastic-EMT. CD45 (LCA) was detected in 75% of all EMT and CD34 wa s positive in 36%; neither antigen was significantly associated with a specific morphology. CD30 reactivity was not evident in any case, but slight positive staining was seen with CD20 (L26) in one WD-EMT. CD43 (Leu 22) was the only antibody that was positive in 100% of cases; st aining was always intense and widespread. Antimyeloperoxidase (MPO) wa s positive in all cases but two, both with a blastic morphology. We co nclude that (a) an immunohistochemical panel including CD20, CD43, CD6 8, and MPO can successfully identify the vast majority (96%) of EMT in paraffin sections, and (b) there is an association between morphology and phenotype in these lesions.