Transitional Mesothelioma

Transitional mesothelioma is a histologic pattern of malignant mesothelioma in which the tumor cells show features that bridge the epithelioid and sarcomatoid forms. Under the microscope, the cells often appear plump, sheet-like, and elongated, with a look that doesn't fit neatly into either traditional category. The 2021 World Health Organization classification formally recognized it as a distinct subtype because its behavior and prognosis tend to resemble sarcomatoid mesothelioma more than epithelioid.

Origin

The pattern was described in the pathology literature for years as a difficult-to-classify variant, but it was formally named and accepted as a distinct subtype in the 2021 WHO Classification of Tumours of the Pleura and Pericardium, following studies (notably Galateau-Salle and colleagues, 2020) showing it had a survival profile closer to sarcomatoid disease.

Significance in Asbestos Context

For patients, the transitional designation matters because it usually signals a more aggressive course and a worse prognosis than pure epithelioid mesothelioma, and it can change treatment recommendations. For example, eligibility for aggressive surgery is often more limited. It can also affect immunotherapy decisions, since non-epithelioid subtypes (including transitional) have shown stronger responses to certain checkpoint inhibitor combinations in clinical trials. Because pathologists do not always agree on whether a tumor is transitional, epithelioid with atypical features, or sarcomatoid, a second pathology opinion from a mesothelioma-experienced center is often valuable. In a legal claim, the subtype listed on the pathology report can influence damages calculations related to life expectancy and treatment, so the exact wording matters.

Example Sentence

After her initial biopsy was read as epithelioid, a second review at a mesothelioma specialty center reclassified the tumor as transitional mesothelioma, which changed her oncologist's recommendation away from surgery and toward systemic therapy.

Related Terms

Epithelioid Mesothelioma, Sarcomatoid Mesothelioma, Biphasic Mesothelioma, WHO Classification, Immunohistochemistry, BAP1

Notes

• Ask whether your pathology report uses the 2021 WHO classification and whether the term "transitional" appears. Older reports may not use this category at all.
• Request a second pathology opinion at a mesothelioma center of excellence (such as a National Cancer Institute–designated center); subtype classification is one of the areas where expert review most often changes a diagnosis.
• Ask your oncologist specifically how the transitional designation affects your eligibility for surgery, clinical trials, and immunotherapy regimens approved for non-epithelioid mesothelioma.
• Preserve all pathology slides and blocks. These are the physical evidence behind your diagnosis and may be needed for re-review or for your legal claim.
• Make sure your legal team has the most current pathology report, including any reclassification, because subtype affects prognosis estimates used in damages.