Mesothelial Proliferations: Challenges and Ancillary Testing

Expires: August 3, 2023

Photo of Anja Roden, M.D.Presenter

Anja Roden, M.D.
Professor of Laboratory Medicine and Pathology
Division of Anatomic Pathology
Mayo Clinic, Rochester, Minnesota


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Transcript and References


I’m Anja Roden, M.D., one of the thoracic pathologists at Mayo Clinic in Rochester and I am also one of the medial directors of the immunohistochemistry labs at Mayo Clinic in Rochester.

Today we will be talking about mesothelial proliferations, their challenges, and which ancillary testing might help us to overcome some of the challenges.


I have nothing to disclose.


After a short introduction, I will show you the spectrum of mesothelial diseases, and then we will discuss the distinction of mesothelioma from carcinoma, and the distinction of reactive from malignant mesothelial proliferations, and which ancillary testing might help us in these distinctions, and I will leave you with a take home message.

Introduction – Malignant Mesothelioma1

As you know, malignant mesothelioma is an aggressive neoplasm of mesothelial differentiation. In the pleura, it can involve all sites including the parietal, visceral, mediastinal, or diaphragmatic pleura. It also can involve and originate from the pericardium and peritoneum. We distinguish between an epithelioid and a sarcomatoid subtype, or if we have both subtypes it will be a biphasic subtype. Specifically, malignant pleural mesothelioma has a very poor outcome with survivals ranging from 12 to 27 months for epithelioid malignant mesothelioma, and only 4 to 18 months for sarcomatoid malignant mesothelioma.

Introduction—Malignant Pleural Mesothelioma1-4

Malignant pleural mesothelioma is strongly associated with remote asbestos exposure, which could be over 40 years ago. It can be rarely occurring in young patients, usually after mantle radiation possibly for lymphoma, or in patients with a family history of breast cancer, or BAP1 germline mutations. It usually presents with unilateral pleural effusion and/or pleural thickening. Only occasionally there is a single pleural-based mass. On imaging studies we sometimes see pleural plaque, which is a sign of asbestos exposure, but does not mean malignant pleural mesothelioma.

Mesothelial Proliferation Behaviors

Here are the various mesothelial proliferations. There are the benign mesothelial proliferations, which include reactive mesothelial proliferation and adenomatoid tumor. This indeterminate behavior includes well differentiated papillary mesothelioma, which after resection usually has a favorable outcome, but occasionally can proceed to a malignant mesothelioma. And then the term atypical mesothelial proliferation, which we usually use in the biopsy setting where we cannot morphologically and immunophenotypically decide whether this is a reactive or a malignant proliferation.

Malignant behavior shows, of course, malignant mesothelioma, but also malignant mesothelioma in situ, which usually will progress to malignant mesothelioma after a few years and then, very rarely we encounter a diffuse intrapulmonary malignant mesothelioma.


Here is an autopsy case of a 92-year-old gentleman who was exposed to asbestos while he was in the Navy as a young man. Five months prior to his death, he was diagnosed with lung cancer, and was placed on hospice care. At autopsy, you can appreciate that the left lung appears unremarkable; however, the right lung has tumor nodules along the pleura and also including the fissure, and again here in the lower lobe there are tumor nodules along the pleura.

If we look at scanning power there are sheets of neoplastic cells, and although they appear to be well demarcated from the surrounding lung parenchyma, they are actually invading into the lung parenchyma. Also on this medium power you can appreciate that the cells are having an epithelioid morphology. On high power many of the cells are of epithelioid subtype. As you can appreciate here, the cells are having round to oval nuclei, prominent nucleoli, and ample cytoplasm. However, there are also areas of a sarcomatoid morphology as characterized by these elongated cells with dark nuclear chromatin.

Malignant Mesothelioma  Carcinoma

There is morphologic overlap between malignant mesothelioma and carcinoma, and if we look at the patterns of epithelioid mesothelioma, which can include solid, acinar or glandular, tubulopapillary, microcystic, clear cell, deciduoid, or small cell carincoma, you can imagine that they show quite a bit of morphologic overlap between mesothelioma and carcinoma. Very similarly, if we have a sarcomatoid subtype of mesothelioma there will be morphological overlap with sarcomatoid carcinoma. Occasionally, psammoma bodies or cytoplasmic mucin can occurm, and therefore these features will not help us either to distinguish mesothelioma from carcinoma.

Malignant Mesothelioma  Carcinoma5

In addition, keratin will be positive in both mesothelioma and carcinoma, and there is no immunohistochemical marker that might be specific for malignant mesothelioma. Therefore, the current recommendation is to start the workup with at least two markers for carcinoma and two markers for mesothelial differentiation. The selection of the markers should be based on our differential diagnosis.

IHC Markers

Here’s a summary of all the markers that we can choose from for malignant mesothelioma and for carcinoma. So on the left side are the markers which we are usually thinking of when we think about mesothelial differentiation, and those include WT1, calretinin, podoplanin, CK5 or CK5/6, and more recently we found that a diffuse expression of GATA3 in a sarcomatoid morphologically malignancy is also suggestive of sarcomatoid malignant mesothelioma.

On the other hand for carcinoma, we think of the markers polyclonal CEA, TTF-1 and napsin, MOC31 and BerEP4 (although these two markers commonly are at least focally expressed also in mesothelial proliferations), furthermore B72.3. P40 for squamous cell carcinoma, claudin 4, a more recent marker for adenocarcinoma, and PAX8, for carcinoma of the GYN tract, or renal cell carcinoma, and thyroid carcinomas.

You will also see that various keratins can be expressed in both mesothelioma and carcinoma.

So if our differential diagnosis is between mesothelioma and adenocarcinoma of lung origin we might want to choose a WT-1 and a CK5, and for carcinoma a TTF-1 and polyclonal CEA.

However, if our differential diagnosis is between mesothelioma and metastatic carcinoma from the GYN tract, we might want to use calretinin and CK5, as well as PAX8 and polyclonal CEA. We might not want to use WT-1 as this can be expressed in both mesothelial differentiation and carcinoma of the GYN tract.

Coming back to our autopsy case, the neoplastic cells were diffusely positive for keratin AE1/AE3 as well as calretinin, and at least focally positive for CK5 while being negative for WT-1, TTF-1, and polyclonal CEA. With that morphology and immunophenotype this was diagnosed as malignant mesothelioma, biphasic type.

Reactive ↔ Malignant Mesothelial Proliferation

Now, how do we differentiate between reactive and malignant mesothelial proliferation, as that has an important implication for treatment and prognosis? Reactive mesothelial proliferations can occur with infections, pneumonia, pneumothorax, trauma, and fluid overload, and require a different treatment than malignant mesothelioma. Actually, most of the patients with malignant mesothelioma do not get any specific treatment. Some will undergo chemotherapy and specifically patients with a good performance status, and an epithelioid subtype might undergo tri-modal modality treatment, which includes chemotherapy, resection, and radiation. And of course, there is always the potential for medical legal action with malignant mesothelioma.

Malignant (vs. Reactive) Mesothelial Proliferation6-8

On morphology, signs of malignancy include invasion, for instance in the adipose tissue, lung, and skeletal muscle, and the keratin really can be helpful to highlight the invasion of the neoplastic cells. However, as you know, many times in small biopsies we will not have adipose tissue, lung, or skeletal muscle to even be able to assess invasion. Other indications for malignancy include tumefactive growth or haphazard growth of neoplastic spindle cells, and again a keratin might be helpful for that assessment. However, if the pleura is not perfectly oriented these assessments are also difficult. More recently, loss of expression of BAP1 and/or mTAP or homozygous deletion of CDKN2A, which encodes p16 and/or NF2, were identified to be helpful in the distinction between malignant mesothelial proliferations and reactive proliferations. However, immunostains such as GLUT-1, p53, desmin, EMA, IMP3, and EZH2, in our opinion, have an insufficient specificity and therefore are not useful for the distinction between malignant and reactive mesothelial proliferations.


If we look at this case here, of a 77-year-old gentleman who presented with recurrent right-sided pleural effusion and underwent excision of the right parietal pleura, you can see that the pleura is very thick. However, it appears to be a hypocellular lesion as there is a lot of pink fibrotic material. There might be a bit of inflammation here, and we know we are in the parietal pleura because there is adipose tissue.

If we go to higher power, there is this atypical spindle cell proliferation. However, in other areas it is more hypercellular, and then we also can see possible invasion of the atypical spindle cells into the adjacent adipose tissue. Here on the high power view there are the spindle cells and they are invading into adipose tissue.

A keratin stain highlights even better how the spindle cells do invade into the adipose tissue. So, this is a sign of invasion and is consistent with malignancy.

In addition, in this case at least some of the spindle cells are positive for calretinin, WT-1, and they are profusely positive for GATA3. We also performed a BAP1 immunostain and you can appreciate that these atypical spindle cells are negative for BAP1, so they lost expression of BAP1. This is BAP1 staining of benign endothelial cells. An internal control is very important to make sure the stain worked. So we do have a loss of expression of BAP1.

The neoplastic cells were negative for MOC31 and BerEP4, and with that morphology and immunophynotype this case was diagnosed as malignant mesothelioma, sarcomatoid type.


If in doubt, a FISH can also be performed for CDKN2A. And we do miss the two red dots for CDKN2A here; we only have two green dots. So this is homozygous deletion of CDKN2A on FISH.


Here is the last case. This is a 61-year-old gentleman who presented with right upper and lower lobe consolidations. He also had chronic obstructive pulmonary disease (COPD) and bullae and he had recently a left pneumothorax and underwent VATS biopsy of the pleura. On this scanning power you can appreciate a slightly thickened pleura, and there is ample adipose tissue but there is no tumefective growth pattern and no invasion into adipose tissue. The pleura is slightly thickened because of these bland appearing epitheliod cells, and there are a lot of eosinophils which we often see with pneumothorax.   

Many of the epithelioid cells have a mesothelial differentiation as evident here by the WT-1. However, BAP1 was expressed, as was mTAP, and with that morphology and the preserved expression of BAP1 and mTAP, this was diagnosed as reactive mesothelial proliferation.

Malignant Mesothelioma

So, as I have mentioned, loss of nuclear BAP1, loss of cytoplasmic mTAP, homozygous CDKN2A deletion, have a specificity for malignancy of 100%. So if we see loss of BAP1 or cytoplasmic mTAP or we have homozygous CDKN2A deletion, this is consistent with malignancy. However, none of these markers have 100% sensitivity. That means that if we see expression of BAP1, mTAP, or wildtype CDKN2A, this does not necessarily exclude the possibility of malignancy.

If we combine some of these tests for instance, BAP1 and mTAP, or BAP1 and homozygous CDKN2A deletion, we can increase the sensitivity. However we still do not have a sensitivity of 100%.

BAP1 & mTAP Expression, Homozygous CDKN2A Deletion

So, the loss of BAP1 and mTAP expression and homozygous CDKN2A deletion are helpful in the distinction between reactive and malignant mesothelial proliferation and are consistent with malignancy. They are applicable in cytology preparations, and here specifically in cell blocks, specifically formalin-based cell blocks. However, their preserved expression does not entirely exclude the possibility of malignancy. Also, loss of expression of BAP1 and mTAP or homozygous CDKN2A deletion is not specific to malignant mesothelioma, as it can also be seen in other malignancies. So we still need at least two carcinoma markers, and two markers of mesothelial differentiation, to identify mesothelial differentiation in these cases.

Take-Home Message

In summary, the distinction between malignant mesothelioma and carcinoma or sarcoma requires a panel of immunostains including at least two carcinoma markers and two markers of mesothelial proliferation. The distinction between reactive and malignant mesothelial proliferation is based on morphology, and if morphology does not show signs of malignancy then immunostains and/or FISH might also be helpful.


Here are some more recent references that might be helpful for you, and with that, I would like to thank you for your attention.

  1. Kim R, Sterman D, Haas A. Malignant mesothelioma: has anything changed? Semin Respir Crit Care Med. 2019;40(03):347-60.
  2. Fels Elliott D, Jones K. Diagnosis of mesothelioma. Surg Pathol Clin. 2020;13(1:73-89).
  3. Abratt R, Vorobiof D, White N. Asbestos and mesothelioma in South Africa. Lung Cancer. 2004;45 Suppl 1:S3-S6.
  4. Viero M, Bueno R, Chirieac L. Clinicopatholigic and genetic characteristics of young patients with pleural diffuse malignant mesothelioma. Mod Path. 2018;31:122-131.
  5. Husain A, Colby T, Ordóñez N, et al. Guidelines for pathologic diagnosis of malignant mesothelioma 2017 update of the consensus statement from the international mesothelioma interest group. Arch Pathol Lab Med. 2018 Jan;142(1):89-108.
  6. Cigognetti M, Lonardi S, Fisogni S, et al. BAP1 (BRCA1-associated protein 1) is a highly specific marker for differentiating mesothelioma from reactive mesothelial proliferations. Mod Pathol. 2015;28(8):1043‐1057.
  7. Sheffield B, Hwang H, Lee A, et al. BAP1 immunohistochemistry and p16 FISH to separate benign from malignant mesothelial proliferations. Am J Surg Pathol. 2015;39(7):977‐982.
  8. Berg K, Dacic S, Miller C, et al. Utility of methylthioadenosine phosphorylase compared with BAP1 immunohistochemistry, and CDKN2A and NF2 fluorescence in situ hybridization in separating reactive mesothelial proliferations from epithelioid malignant mesotheliomas. Arch Pathol Lab Med. 2018;142(12):1549‐1553.
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