Analysis of early pleural fluid samples in patients with mesothelioma: A case series exploration of morphology, BAP1, and CDKN2A status with implications for the concept of mesothelioma in situ in cytology

Abstract

BACKGROUND The concept of mesothelioma in situ has been revisited and is a new World Health Organization diagnostic entity. The definition centers on ancillary techniques used in pleural mesothelioma (PM) assessment. At the authors' institution, most PM diagnoses are made on cytologic specimens. Effusion samples obtained before definitive PM diagnosis were interrogated using BRCA1‐associated protein 1 gene (BAP1), cyclin‐dependent kinase inhibitor 2A gene (CDKN2A) and cytologic evaluation to assess whether early or possible in situ disease could be characterized. METHODS All cases of PM diagnosed between January 2008 and December 2019 were identified at a tertiary referral center. Patients who had a pleural fluid sample collected 24 months before the diagnosis were selected, numbering 8 in total. The cytomorphology of each sample was reviewed; and, retrospectively, BAP1 immunohistochemistry (IHC) and CDKN2A fluorescence in situ hybridization (FISH) were performed on initial and diagnostic samples. RESULTS The initial samples were deemed benign in 5 cases and atypical mesothelial proliferations in 3 cases. A spectrum of apparently normal to atypical cytomorphologic changes was identified. BAP1 loss was present in 6 of 8 initial cases, whereas CDKN2A homozygous deletion was identified in 1 of 7 initial cases. Either abnormality was identified in 7 of 8 initial samples. CONCLUSIONS Detectable abnormalities of BAP1 IHC and CDKN2A FISH were present in pleural fluid specimens before the development of cytomorphologic features diagnostic of PM. This is the largest series to date describing cytology samples early in the course of PM development, thereby highlighting a possible cytological equivalent for mesothelioma in situ.

The cytological features of effusions with mesothelioma in situ: A report of 9 cases

“…Louw et al evaluated fluid samples retrieved up to 24 months earlier than the diagnosis of mesothelioma. 27 Of the eight cases, five were diagnosed as negative for malignancy (NFM) and three as atypical. BAP1 was lost in 5 (2 negative and 3 atypical), focally lost in one and retained in two NFM cases respectively.…”

“…Louw et al evaluated fluid samples retrieved up to 24 months earlier than the diagnosis of mesothelioma 27 . Of the eight cases, five were diagnosed as negative for malignancy (NFM) and three as atypical.…”

Comparison of immunohistochemical mesothelial biomarkers in paired biopsies and effusion cytology cell blocks from pleural mesothelioma

“…4,10,11 Recent studies have demonstrated that the use of BAP1 and MTAP in effusions can establish the mesothelioma diagnosis earlier. 12,13 It is known that differences in fixation and preparation of cytology compared to biopsies may affect the results of IHC. [14][15][16] Only a few studies have included a limited number of mesotheliomas with paired biopsies and effusion cytology specimens for comparison of protein expression by IHC.…”

“…Several immunohistochemical (IHC) markers are regularly used in the diagnosis of mesothelioma, including calretinin, cytokeratin 5 (CK5), podoplanin (often referred to by its clone name D2‐40), and Wilms tumour 1 (WT1) for detection of mesothelial lineage; and desmin, epithelial membrane antigen (EMA), BRCA1‐associated protein 1 (BAP1), and methylthioadenosine phosphorylase (MTAP) for distinguishing mesothelioma from reactive mesothelial cells 4,10,11 . Recent studies have demonstrated that the use of BAP1 and MTAP in effusions can establish the mesothelioma diagnosis earlier 12,13 …”