Differential Diagnosis of Benign and Malignant Mesothelial Proliferations on Pleural Biopsies

During recent decades, many studies have investigated the differential diagnosis of diffuse malignant mesothelioma (DMM) versus other types of cancer, especially epithelial DMM versus adenocarcinomas metastatic to the pleura. However, the surgical pathologist not infrequently encounters a different problem with pleural biopsies, namely, whether a mesothelial proliferation represents a malignant neoplasm or a benign reactive hyperplasia.1–13 

Reactive pleural processes may be associated with pleural effusion and thickening of the pleura and in some cases raise the clinical possibility of malignancy. Reactive mesothelial cell hyperplasia and organizing pleuritis may demonstrate increased cellularity and reactive cytologic atypia of epithelial mesothelial cells and of spindle mesothelial cells, fibroblasts, and endothelial cells, respectively. It is not unusual for both epithelial and spindle proliferating cells to occur together in a reactive process. These benign reactive mesothelial proliferations may mimic malignancy because of high cellularity, cytologic atypia, and other histologic features, whereas some DMMs are cytologically bland and may be sampled in minimally invasive areas that disguise their malignant nature. Obviously, it is of considerable importance to patients to know that they have a benign pleural process rather than DMM or stage IV carcinoma.

Biopsies that include DMM in the differential diagnosis can be referred to the United States–Canadian Mesothelioma Reference Panel for the opinions of a panel of experts. In 22% of 217 cases circulated to the entire United States–Canadian Mesothelioma Reference Panel between 1994 and 1999, 1 or more panelists disagreed with the majority on whether the case represented a benign reactive proliferation or a malignancy.6 Only difficult cases are circulated to the entire panel, but this rate of disagreement among experts highlights the diagnostic dilemma that can be posed by the question of benign reactive proliferation versus malignancy on pleural biopsy.

As with biopsies from other sites, it is always best to interpret pleural biopsies within the clinical context, including clinical history and findings as well as radiologic findings. Pleural processes that may be associated with benign reactive mesothelial hyperplasia and/or organizing pleuritis on a pleural biopsy are listed in Table 1. A unilateral, bloody pleural effusion is characteristic of many pleural effusions caused by malignancy, but several benign processes listed in Table 1 may be unilateral and some (pulmonary infarct, pneumothorax, surgery, trauma) may produce bloody effusions. Occasionally, a patient has exuberant pleuritis overlying a subpleural lung cancer that may or may not be invasive into the pleura, and the radiology may be very helpful in understanding the full context of the biopsy.

A history of asbestos exposure should not be given any inordinate weight in making a diagnosis of DMM for at least 3 reasons: (1) asbestos exposure can cause benign pleural effusions, benign pleural plaques, and benign mesothelial reactions, and these outnumber malignant mesotheliomas; (2) asbestos exposure is fairly common and asbestos-exposed individuals are susceptible to the same nonasbestos causes of pleural effusions and reactions as everyone else; and (3) not all DMMs are caused by asbestos exposure.

The impression of the surgeon on thoracoscopic or thoracotomy examination is helpful information. Classically, malignancies involving the pleura appear as multiple tumor nodules or encasement of the pleura by tumor. However, gross appearances may occasionally be misleading and, if the biopsy is not conclusive for malignancy, it is recommended that the pathologist not yield to pressure to make a diagnosis of malignancy based solely on reported gross impressions. On the other hand, if the surgeon states that the process appears benign on thoracoscopic or thoracotomy examination, the pathologist should be circumspect about diagnosing malignancy.

Histologic features of benign reactive mesothelial hyperplasia that may potentially mimic histologic features of malignancy are listed in Table 2. In subsequent sections, we discuss the histologic features of mesothelial proliferations that can be encountered on a pleural biopsy. Diagnosis of malignancy on pleural biopsy may be problematic not only because of potential benign reactive mesothelial hyperplasia, but also because of sampling and other issues that may limit the usefulness of any biopsy. We advocate a very guarded approach to the diagnosis of malignancy on pleural biopsies when a reactive mesothelial proliferation is in the differential diagnosis. Given the consequences to the patient of a diagnosis of malignant mesothelioma, when there is any doubt about malignancy on a pleural biopsy, we recommend the term atypical mesothelial proliferation or atypical mesothelial hyperplasia be used for the diagnosis and additional tissue obtained if clinically indicated.

If a pleural biopsy shows only mesothelial cells along the surface with no mesothelial cells observed in the underlying tissue, the proliferation may represent simple mesothelial cell hyperplasia or atypical mesothelial hyperplasia. It is likely that some instances of the latter represent mesothelioma in situ, but that diagnosis cannot be made with current histologic criteria unless invasive malignancy is also present.

The normal mesothelial lining of the pleura is composed of flat cells that are inconspicuous on casual histologic examination (Figure 1). Irritation of the pleural surface leads to simple hyperplasia of the mesothelium. The mesothelial cells in simple hyperplasia form a more conspicuous layer of relatively bland cuboidal cells regularly spaced along the pleural surface (Figure 2). Distinct nucleoli may sometimes be present. The hyperplastic cells usually show regular spacing along the membrane.

Atypical mesothelial hyperplasia is a more florid mesothelial proliferation along the pleural surface than simple hyperplasia. There is a spectrum of cytologic atypia and cellularity between cases of simple hyperplasia to cases of atypical mesothelial hyperplasia that are worrisome for noninvasive malignancy. Atypical mesothelial hyperplasia may consist of a single layer of cells with varying degrees of cytologic atypia along the surface (Figure 3) or may consist of piled-up accumulations of cells on the surface (Figure 4). The cells of atypical mesothelial hyperplasia are cuboidal to elongate, may have enlarged nuclei, and often have prominent nucleoli with significant cell-to-cell variation (Figures 5 and 6).

Simple mesothelial hyperplasia is usually not of diagnostic concern and clearly represents a benign reactive process. Some examples of atypical mesothelial hyperplasia represent the more cellular and more cytologically atypical spectrum of benign reactive processes, whereas other examples probably represent noninvasive malignancy, either at the margin of an invasive tumor or possibly a preinvasive stage of malignancy. Inflammatory changes and clinical history/findings of a benign process favor a benign reaction, and a lack of inflammatory changes or dense underlying fibrosis may increase concern for a malignancy, but neither situation is definitive. On the other hand, bulk tumor in nodules or masses on the pleural surface should be considered malignant (Figure 7).

Progressive morphologic steps from normal epithelium through varying degrees of dysplasia to carcinoma in situ to minimally invasive malignancy are established in the cervix, lung, and other organs. Therefore, it is probable that there are preneoplastic and in situ phases in the development of DMM.9 Some examples of atypical mesothelial hyperplasia could conceivably represent these preneoplastic and preinvasive phases of DMM. Whitaker et al3 and Henderson et al4 proposed potential criteria for a morphologic diagnosis of mesothelioma in situ, but noted that this diagnosis could only be confirmed when accompanied by adjacent or subsequent invasive tumor (Figure 8). Currently, there are no reliable morphologic criteria for the diagnosis of mesothelioma in situ in the absence of invasive tumor, and we suggest that the term mesothelioma in situ should not be used in diagnosis. Instead, a surface mesothelial proliferation of indeterminate malignancy should be diagnosed as atypical mesothelial hyperplasia or atypical mesothelial proliferation, with the understanding that some of these cases will eventually turn out to be benign and that others may turn out to be malignant. When in doubt, it is best to avoid a diagnosis that cannot be substantiated and instead ask for additional tissue if clinically indicated.

Pathologists recognize certain histologic features as being characteristic of malignancy, but are aware that some reactive processes may share degrees of these features. Mesothelial cells are particularly notorious for demonstrating greater degrees of cytologic atypia in benign reactive processes than they sometimes do when they are malignant. As is well known to cytologists, malignant mesothelial cells can be very cytologically bland. Indeed, although malignant mesothelial cells may occasionally be highly pleomorphic, the presence of highly pleomorphic cells is much more suggestive of a carcinoma than a DMM. Mitoses, which are often associated with malignancy, can be increased in a benign reactive mesothelial hyperplasia. Necrosis may occur in both DMM and benign reactive pleural processes. In DMM, tumor necrosis is typically bland, consisting of areas of ischemic necrosis without significant inflammation or cellular debris (Figure 9), whereas in inflammatory pleural conditions like tuberculosis, the necrosis is typically associated with inflammatory cells and cellular debris.

On occasion, benign reactive mesothelial hyperplasias may exhibit architectural features like papillary excrescences and tubular lumens that mimic the architectural patterns of DMM or other malignancies. There are some architectural clues that favor a benign proliferation over a malignancy. Papillary excrescences of benign hyperplastic mesothelial cells tend to be tufts of cells without true fibrovascular cores (Figure 10). Tubule-like structures in benign mesothelial proliferations tend to be small, simple, nonbranching structures (Figure 11), but it should be noted that some DMMs and metastatic carcinomas are also composed of small, simple, nonbranching tubules or glands. Complex branching tubules or glands favor malignancy (Figure 12).

Histologic features strongly favoring a diagnosis of DMM over a benign reactive mesothelial hyperplasia are listed in Table 3. Invasion of underlying tissues is the most reliable criterion for the diagnosis of DMM, whether epithelial or sarcomatous. When invasion is minimal or equivocal, keratin immunostaining can help highlight the keratin-immunopositive neoplastic mesothelial cells infiltrating into underlying fat, connective tissue, or lung tissue (Figure 13). However, the mere fact that proliferating mesothelial cells are immunopositive for keratin, calretinin, or other markers does not make a diagnosis of benign versus malignant; the immunostains are used to demonstrate invasion of the cells into underlying tissues.

In addition to invasion, certain other criteria favor malignancy, both epithelial and sarcomatous types. Obvious cellular nodules of neoplastic cells may expand into the stroma, or neoplastic cells may have such conspicuous malignant cytologic features (severe pleomorphism, abnormal mitoses) that a diagnosis of malignancy is warranted. Benign reactive mesothelial proliferations tend to demonstrate zonation; that is, the proliferating cells are found toward the pleural surface and the deeper tissues are more fibrotic (Figure 14). Finding mesothelial cells deep in the pleural connective tissue or through the full thickness of the pleura favors malignancy. Bland necrosis is characteristic of DMM, particularly the desmoplastic variety, which may otherwise be difficult to differentiate from fibrous pleuritis or pleural plaque. A “soft” criterion that favors a benign diagnosis is capillaries in a biopsy that are parallel to each other and that are perpendicular to the pleural surface (Figure 15). Capillaries in malignant mesotheliomas, particularly desmoplastic mesotheliomas, tend to be inconspicuous and arranged in a haphazard fashion.

Entrapment refers to benign reactive mesothelial cells that have remained on a previous pleural surface that is now covered by fibrinous, organizing, or fibrotic pleuritis, giving an impression of invasion. As layers of fibrin and/ or fibrous connective tissue spread over the pleural surface in an inflammatory process, the entrapped mesothelial cells tend to form linear arrays marking the site of the previous surface (Figures 16 and 17). In some cases, including when there are adhesions of the visceral and parietal surfaces, the reactive mesothelial cells may line elongated clefts that may sometimes resemble glands (Figure 18). Histologic features that favor entrapment of nonneoplastic mesothelial cells over invasion are listed in Table 4.

En face cuts of unoriented or poorly oriented pleural biopsies may also give a false impression of invasion. Sections cut parallel to the free serosal surface rather than at right angles to the surface may give the impression that benign surface reactive mesothelial cells are surrounded by connective tissue. This may be particularly difficult to recognize on small biopsies. As always, we urge caution when the possibility of an en face section cut is encountered. The presence of mesothelial cells on 2 surfaces is a clue that a specimen has been cut en face.

Immunohistochemical examination is generally of little value in differentiating a benign reactive mesothelial proliferation from a DMM. As previously mentioned, broad-spectrum keratin immunostains can be used to confirm invasion into underlying tissues such as adipose tissue when the hematoxylin-eosin stain is equivocal, but virtually all active benign mesothelial reactions are also keratin positive; what keratin stains really do is to show the distribution of mesothelial cells within the tissue. While there are a number of markers such as calretinin that often indicate a mesothelial origin, both reactive and malignant mesothelial proliferations stain positively. Although a number of markers such as p53 and epithelial membrane antigen have been proposed to provide secondary support of a diagnosis of malignancy on pleural biopsy, none of these markers is absolutely specific or sensitive and, therefore, cannot be relied on to make a diagnosis.14–19