Investigation of MTAP and BAP1 staining loss and P16/CDKN2A deletion in pleural cytology specimens and its role in the diagnosis of mesothelioma

Background: Mesothelioma is a malignant neoplasm with a poor survival rate. We aimed to investigate the importance of BAP1, MTAP (IHC), and p16/CDKN2A homozygous deletion (FISH) in cytologic material obtained from pleural effusion sampling, which is a less invasive procedure in the diagnosis of mesothelioma.

Methods: Our study discussed pleural cytology samples of cases with histopathologically proven mesothelioma diagnoses between 2017 and 2022. As the control group, materials that had pleural effusion sampling for other reasons and reactive mesothelial hyperplasia were included in the study. Cell blocks prepared from these materials were subjected to fluorescent in situ hybridization for p16/CDKN2A homozygous deletion and immunohistochemistry for BAP1 and MTAP.

Results: The specificity of the P16/CDKN2A homozygous deletion in diagnosing mesothelioma is 100%. Its sensitivity is 68.75%. The specificity of BAP1 immunohistochemical nuclear expression loss is 95%, while the sensitivity is 60%. Loss of nuclear expression of MTAP alone has the lowest specificity and sensitivity, with a specificity of 86% and a sensitivity of 43%. The highest sensitivity is reached when BAP1 loss and p16/CDKN2A homozygous deletion are evaluated together, increasing to 81%. The specificity is 95%.

Conclusion: It has been determined that any marker alone cannot be used for a definitive mesothelioma diagnosis in pleural effusion cytological specimens; however, sensitivity increases in some combinations. The combination of BAP1 immunohistochemistry and p16/CDKN2A homozygous deletion detected by FISH, which has a higher specificity and sensitivity, can be routinely used in the diagnosis of mesothelioma under the guidance of clinical and radiologic information.