Effusion cytology diagnosis of malignant mesothelioma has always been controversial and is not accepted by many pathologists and clinicians. Although there are certainly cases of mesothelioma which require biopsy for diagnosis, in particular sarcomatoid and desmoplastic subtypes, the majority of cases can be diagnosed by effusion cytology. Failure to accept this diagnostic approach has led to unnecessary biopsies in many thousands of patients.
This talk will very briefly discuss how the cytological diagnosis is established and then address some of the criticisms surrounding cytological diagnosis.
- Establish malignancy by morphology +/- ancillary tests. Recent use of BAP1 immunohistochemistry and CDKN2A FISH for 9p21 homozygous deletion is a ‘game changer’.
- Establish phenotype of malignant population using an immunohistochemical panel.
Criticisms of cytological diagnosis:
- Low sensitivity – NOT a reason to abandon the test; diagnosis is accurate, with no false positives. Even a laboratory with a low sensitivity of 30 % is still saving 1 in 3 patients from a biopsy. BAP1 immunohistochemistry and CDKN2A FISH will increase sensitivity dramatically.
- Need expert pathologists- NO – this is a simple diagnosis; comprehensive cytological guidelines are available.
- Need a biopsy to demonstrate invasion- WHY? Cytology diagnoses malignancy, never invasion; why is pleura different? Correlate with imaging/clinical for extent of invasion. In situ disease now accepted, even by previously sceptical histopathologists.
- Need a biopsy for sub-typing - only if planning surgery; biopsy sub-typing not always accurate; outcome of MPM diagnosis in fluid is equivalent to outcome of epithelioid mesothelioma on biopsy.
- Need a biopsy for ancillary tests – NO - far more neoplastic cells in an effusion sample- suitable for all testing, including molecular.
- Need a biopsy to enrol in a trial – WHY? question the trial.
Pathologists should not be telling clinicians they need a biopsy in every case; claiming the need for “tissue biopsy confirmation” of a properly conducted cytological diagnosis of malignant mesothelioma should be a thing of the past.