Small orangiophilic squamous-like cells: an underrecognized and useful morphological feature for the diagnosis of malignant mesothelioma in pleural effusion cytology.

BACKGROUND The cytological diagnosis of malignant mesothelioma (MM) on serous effusion is challenging due to significant morphologic overlap with reactive mesothelial cells and adenocarcinoma. One of the morphologic features of MM in effusion cytology, small orangiophilic squamous-like cells (SOSLC), has received little attention. To the best of the authors' knowledge, the current study is the first to assess the sensitivity and specificity of SOSLC in serous effusion specimens from histology-proven MM cases along with those of reactive mesothelial and adenocarcinoma cases. METHODS A total of 130 cases of pleural (86 cases) and peritoneal (44 cases) effusion cytology cases (30 with histology-proven MM, 41 with adenocarcinoma, and 59 with reactive mesothelial cells) were studied. The presence or absence of SOSLC was recorded in each case. RESULTS The cytological diagnoses of the 30 histology-proven MM cases included 1) atypical mesothelial cells, favor reactive (4 cases); 2) atypical mesothelial cells, suspicious for or cannot exclude mesothelioma (18 cases); and 3) positive for MM (8 cases). SOSLC were found in 10 of the 30 MM effusion cases (33.3%), 1 of the 41 adenocarcinoma cases (2.4%), and 5 of the 59 reactive mesothelial cell cases (8.5%). SOSLC were more likely to be present in MM effusions compared with either adenocarcinoma (P < .0001) or reactive mesothelial cell (P < .02) effusions. All 10 cases of MM with SOSLC were from pleural fluids. One case of peritoneal serous adenocarcinoma had SOSLC and 5 cases of reactive mesothelial cells in peritoneal fluid were found to have SOSLC. CONCLUSIONS Although not sensitive, the presence of SOSLC is quite specific for MM in pleural fluid cytology specimens. Finding this morphological feature in pleural fluid should alert the pathologist to a possible diagnosis of MM. Cancer (Cancer Cytopathol) 2014;122:70–75. © 2013 American Cancer Society.