Cytomorphologic findings of malignant mesothelioma in FNA biopsies and touch preps of core biopsies

Background Given the lack of recent literature regarding the aspiration cytology of immunohistochemically confirmed malignant mesothelioma (MM), we were interested in reviewing the experience of our institution and establishing useful morphologic criteria. Methods Seventeen aspiration and touch preparation specimens with a diagnosis of MM obtained between 2002‐2013 were reviewed along with 20 cases of adenocarcinoma and 16 cases of squamous cell carcinoma. The utility of a number of morphologic features was evaluated. Results In most cases of MM, a consistent pattern emerged. Aspirates and touch preps were cellular with irregularly shaped 2 and 3 dimensional clusters. The individual cells were predominantly angulated and had dense cytoplasm with eccentric nuclei. In every case, a minority of tumor cells contained prominent microvacuoles. The chromatin pattern tended to be fine with small nucleoli. While most cases were cytologically monotonous, five cases displayed striking pleomorphism and three cases contained occasional large atypical cells. Two cases contained metachromatic background material. Features which were most useful in discriminating MM from adenocarcinoma were angulated cell shape(P = 0.0002), dense cytoplasm(P = 0.0001), and cytoplasmic microvacuoles(P = 0.0001). In our material, cases of squamous cell carcinoma were often difficult to distinguish from MM. Useful discriminatory features present in squamous cell carcinoma included ink dot nuclei(P = 0.0003), a “dirty” cystic, necrotic background (P = 0.0027) and tumor balls with peripheral spindling(P = 0.0041). Conclusion Most cases of MM have a consistent appearance in core biopsy touch preps and FNAs. Distinguishing MM from adenocarcinoma and squamous cell carcinoma can be facilitated by evaluating a few key morphologic features. Diagn. Cytopathol. 2016;44:14–19. © 2015 Wiley Periodicals, Inc.