What is mesothelial hyperplasia?

Mesothelial hyperplasia is a noncancerous condition in which the cells that line the membranes around the lungs, abdomen, heart, or testis—called mesothelial cells—begin to grow or multiply more than usual. These membranes are known as the pleura (around the lungs), peritoneum (around the abdominal organs), pericardium (around the heart), and tunica vaginalis (around the testis).

The term “hyperplasia” means an increase in the number of cells. In mesothelial hyperplasia, the cells appear more crowded or thicker than normal. However, they are still benign (noncancerous) and do not show the destructive growth seen in cancers such as mesothelioma.

What causes mesothelial hyperplasia?

Mesothelial hyperplasia develops when the mesothelial lining is irritated, inflamed, or injured. The increased cell growth is a reactive process, meaning it happens in response to another underlying condition rather than being a disease on its own.

Common causes of mesothelial hyperplasia include:

• Inflammation or infection: Conditions such as pneumonia, peritonitis, or viral infections can irritate the mesothelial lining.

• Fluid buildup (effusion): Persistent or bloody fluid in the chest or abdomen can stimulate mesothelial cell growth.

• Trauma or surgery: Healing after a surgical procedure or injury can cause hyperplasia.

• Tumors or cysts nearby: A neighboring tumor or cyst may irritate the mesothelial surface.

• Heart or liver disease: Conditions that cause long-term fluid accumulation can also lead to reactive mesothelial changes.

Mesothelial hyperplasia often disappears once the underlying irritation or fluid buildup resolves.

What does mesothelial hyperplasia look like under the microscope?

Under the microscope, the mesothelial lining appears thicker and may show multiple layers of cells instead of the usual single layer. The cells themselves look normal, with smooth, round nuclei and clear boundaries. Sometimes they may form small clusters or papillary (finger-like) projections, but they do not invade the surrounding tissue.

Pathologists may describe the appearance as “reactive mesothelial hyperplasia” to emphasize that it is a benign response to irritation. This finding can look similar to early mesothelioma, so special tests are sometimes performed to confirm that the changes are reactive rather than malignant.

How do pathologists distinguish mesothelial hyperplasia from mesothelioma?

Because both conditions involve the same type of cell, distinguishing benign mesothelial hyperplasia from malignant mesothelioma can be challenging, especially when only a small biopsy is available.

To help make the distinction, pathologists may use immunohistochemistry or molecular tests, which detect specific proteins or genetic changes inside the cells.

Tests that support a diagnosis of mesothelial hyperplasia include:

• BAP1: Retained (normal) expression of the BAP1 protein supports a reactive process, while loss of BAP1 supports mesothelioma.

• MTAP and p16 (CDKN2A): Normal results suggest hyperplasia; loss or deletion supports mesothelioma.

• Calretinin, WT1, and cytokeratin 5/6: These markers confirm that the cells are mesothelial in origin but do not indicate malignancy on their own.

The most important feature that distinguishes mesothelial hyperplasia from mesothelioma is the invasion of cells into the underlying tissue. In mesothelial hyperplasia, invasion is not present.

Is mesothelial hyperplasia dangerous?

No. Mesothelial hyperplasia is a benign and reversible condition. The extra cells are part of the body’s normal healing or repair response. Once the cause of irritation or inflammation is treated, the mesothelial lining usually returns to normal.

However, because it can look similar to early mesothelioma, your doctor may recommend follow-up imaging or, in rare cases, a repeat biopsy to ensure that the process is reactive and not malignant.

How is mesothelial hyperplasia reported?

If your pathology report mentions mesothelial hyperplasia or reactive mesothelial hyperplasia, it means that:

• The cells lining the membrane have increased in number.

• The cells appear benign, with no evidence of invasion or cancer.

• The changes are likely due to an underlying cause such as inflammation, infection, or fluid accumulation.

The report may also describe the results of immunohistochemical tests that support a benign, reactive process.

What is the prognosis for mesothelial hyperplasia?

The prognosis is excellent. Once the underlying cause is identified and treated, the mesothelial lining usually returns to normal. Mesothelial hyperplasia does not progress to mesothelioma or other cancers.

Questions to ask your doctor

• What might have caused mesothelial hyperplasia in my case?

• Were any special tests, such as BAP1 or p16, performed on my sample?

• Do I need any follow-up imaging or repeat biopsy?

• How will my underlying condition (such as fluid buildup or inflammation) be treated?