THE USE OF EPITHELIAL MEMBRANE ANTIGEN AND SILVER-STAINED NUCLEOLAR ORGANIZER REGIONS TESTING IN THE DIFFERENTIAL-DIAGNOSIS OF MESOTHELIOMAFROM BENIGN REACTIVE MESOTHELIOSES

BACKGROUND. The accurate diagnosis of pleural lesions obtained from sm all closed biopsy is difficult. As yet there is no single reliable tes t to distinguish between malignant and benign mesothelial tissue. METH ODS. Immunostaining of epithelial membrane antigen (EMA) and the quant itation of silver stained nucleolar organizer regions (AgNORs) each we re applied to benign and malignant histologic sections of pleural and peritoneal biopsies. The usefulness of these stains was tested both in dividually and in combination in the diagnosis of epithelial malignant mesothelioma. RESULTS. One hundred and three of the 141 malignant les ions (73%) were immunoreactive for EMA but only 3 of the 73 benign les ions (4%) reacted equivocally, and none positively. The average count of AgNORs/cell in malignant lesions (n = 80) was elevated compared wit h benign cases (n = 26), but a significant overlap was exhibited in th e AgNOR count and this form of analysis was considered to be of little value in distinguishing benign from malignant mesothelial processes. Much less overlap was observed when the average AgNOR area was measure d. By using the maximum benign AgNOR area of 0.6677 mu m(2) as the upp er threshold, 51 cases (63.8%) were identified as malignant; the test demonstrated 100% specificity and 63.8% sensitivity. By combining the EMA and AgNOR results, 76 of 80 of the malignant mesothelioma cases (9 5%) tested positive for at least 1 of the tests with no false-positive results identified. CONCLUSIONS. This study confirms the usefulness o f EMA in diagnosing malignant and benign mesothelial lesions, and demo nstrates the enhanced diagnostic value of combining EMA immunoreaction with the average area of AgNOR per cell, thereby increasing sensitivi ty in the diagnosis of epithelial malignant mesothelioma. (C) 1998 Ame rican Cancer Society.