The diagnosis of malignant mesothelioma in effusion cytology: A reappraisal and results of a multi-institution survey

BACKGROUND: The diagnosis of malignant mesothelioma (MM) in effusion specimens is controversial. At the study institution (Northwestern University), a primary diagnosis of MM is made on fluid cytology specimens. In an effort to estimate the practice at other institutions, a survey was disseminated regarding cytologic diagnosis of MM. The authors also evaluated their own institution's experience with primary cytologic diagnosis of MM. METHODS: Patients with MM at the study institution were identified from 1992 through 2011. Fluid cytology specimens preceding histologic diagnoses were reviewed. A survey was sent to a number of cytology laboratories to assess practice patterns regarding the diagnosis of MM. RESULTS: At the study institution, 20 cases of MM had effusion specimens preceding the diagnostic histologic material. In 6 cases (30%), a definitive diagnosis of MM was rendered via cytology alone. There were no false-positive diagnoses of MM. Of 55 laboratories that responded to the survey, 36 reported making a definitive diagnosis of MM after cytologic analysis. Almost all laboratories (35) willing to diagnose MM in effusions reported performing immunohistochemistry to distinguish adenocarcinoma from MM. A smaller proportion (18) of these laboratories reported doing additional immunohistochemistry or fluorescence in situ hybridization for p16 to help distinguish benign from malignant mesothelial proliferations. Those who do not definitively diagnose MM in fluid specimens state inability to identify invasion and overlap with reactive mesothelial proliferation as factors supporting their practice. Most respondents (32) felt that the clinicians at their institution would manage a patient based on a cytologic diagnosis of MM. CONCLUSIONS: The majority of respondents reported making a definitive diagnosis of MM in effusion cytology specimens. The diagnosis of MM in effusions, although not sensitive, is extremely specific.