Utility of the cytological criteria in the diagnosis of malignant pleural mesothelioma: Making the bridge between theory and practice.

INTRODUCTION: The diagnosis of malignant pleural mesothelioma (MPM) depends on microscopic examination performed on pleural biopsies taken under thoracoscopy. However, it has recently been established that cytology presents a significant diagnostic contribution enabling an earlier diagnosis with a minimally invasive procedure.

AIM: To assess the diagnostic value of consensual cytological features of MPM in the differentiation between adenocarcinoma, mesothelioma and reactive mesothelial cells in pleural liquid.

METHODS: All available retrospective records from the computerized pathology database system and pathology reports were searched for malignant pleural effusion cytology specimens, over a 5-year period from January 2015 to February 2020. The cytological criteria based on the international Guidelines for cytopathologic diagnosis of epithelioid and mixed type of MPM were assessed. Malignant mesothelial cells, MNML and RL were considered as the gold standard.

RESULTS: 189 pleural biopsies with their corresponding cytology specimens were available for review. Among the reviewed cytologies, the diagnoses of 21/189 pleural cytologies were modified. The highest sensitivities were attributed to cytoplasmic blebbing, hypercellularity and cell ball clusters. The most specific feature was the absence of extracellular granular hyaluronic acid cores in reactive cytology and the absence of intercellular openings in NMML cell clusters. Extracellular granular hyaluronic acid cores had the highest positive predictive value and the highest negative predictive value was attributed to the cytoplasmic blebbing in both reactive cytology and NMML.

CONCLUSION: These results highlight the fact that no sign is pathognomonic of the diagnosis of MPM pointing out the necessity of immunocytochemical techniques in equivocal cases.