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Keywords:

  • diagnosis;
  • immunohistochemistry;
  • mesothelioma;
  • pathology

Abstract

  1. Top of page
  2. Abstract
  3. Introduction
  4. Reactive mesothelial hyperplasia versus epithelioid mesothelioma
  5. Reactive pleural fibrosis versus sarcomatoid or desmoplastic mesothelioma
  6. Mesothelioma versus metastatic adenocarcinoma
  7. Rare and unusual variants of mesothelioma
  8. Other tumours mimicking epithelioid mesothelioma
  9. Sarcomatoid mesothelioma versus other connective tissue tumours
  10. Conclusion
  11. References

Many centres are now seeing increasing numbers of patients with malignant mesothelioma. This presents pathologists involved in making the diagnosis with a number of problems, which can be divided into those encountered in making the distinction between mesothelioma and benign changes and those experienced in separating mesotheliomas from other types of epithelial and connective tissue tumours. Immunohistochemistry plays a major role in helping to make the diagnosis, but it should be interpreted with due regard to the clinical setting and radiological features, and with a knowledge of the wide morphological variations seen in mesothelioma. This review identifies some of these problems and addresses the uses and limitations of immunohistochemistry in different situations. It includes a discussion of some of the less common variants of mesothelioma and other pleural-based tumours that enter into the differential diagnosis.


Abbreviations:
CEA

carcinoembryonic antigen

CK

cytokeratin

D-PAS

diastase periodic acid–Schiff

EMA

epithelial membrane antigen

H&E

haematoxylin and eosin

WT-1

Wilms tumour product-1

Introduction

  1. Top of page
  2. Abstract
  3. Introduction
  4. Reactive mesothelial hyperplasia versus epithelioid mesothelioma
  5. Reactive pleural fibrosis versus sarcomatoid or desmoplastic mesothelioma
  6. Mesothelioma versus metastatic adenocarcinoma
  7. Rare and unusual variants of mesothelioma
  8. Other tumours mimicking epithelioid mesothelioma
  9. Sarcomatoid mesothelioma versus other connective tissue tumours
  10. Conclusion
  11. References

Once considered to be rare tumours, malignant mesotheliomas are now seen in increasing numbers. In hospitals serving industrial areas or areas where dockyards, shipbuilding and allied trades were major employers, biopsy specimens from patients with suspected mesothelioma are seen on an increasingly regular basis to the point where its incidence has been described as an epidemic. Mesothelioma death rates in the UK are some of the highest in the world, at around 30 cases per million per year, similar to Australia and Belgium.1 The highest rates in men between 1981 and 2000 were in those areas associated in the past with shipbuilding, such as West Dumbartonshire, Barrow-in-Furness, Plymouth and Portsmouth.2 The occupations with the highest risk in men include metal plate workers, vehicle body builders, plumbers and gas fitters, carpenters and electricians.2,3

The major aetiological factor for mesothelioma is exposure to amphibole asbestos, particularly blue asbestos or crocidolite.4,5 Although the use of asbestos declined sharply after about 1980, it is estimated that deaths from mesothelioma will continue to rise. This reflects the long latent period between asbestos exposure and the development of mesothelioma.1 In 1968 only 153 were recorded. In 2001 this rose to 1848, and in 2005 there were 2037 deaths.1,6 It is predicted that they will increase until they peak between 2011 and 2015 at up to 2450 per year. For men born in the 1940s this is a predicted lifetime risk of 0.6%.7

Although the treatment of mesothelioma is largely ineffective in prolonging life significantly,8,9 multimodality therapies are being evaluated that may improve the outcome of malignant mesothelioma in the future.10 Despite this dismal prognosis, accurate clinical assessment and biopsy at the earliest possible stage enables patients and their families to come to terms with the diagnosis and to begin litigation proceedings, when appropriate. It is important, therefore, to make a positive diagnosis at an early stage in the disease. This entails making a clear distinction from benign pleural disease and excluding other types of tumour, which carry different prognoses and may be more treatable.

As with any tumour, reliable diagnosis of mesothelioma depends on obtaining adequate, representative tissue samples. Closed punch biopsies generally produce only very small amounts of tissue, often consisting mainly of chest wall tissues. Pleural-based masses may be of sufficient thickness to allow image-guided, percutaneous, transthoracic needle core biopsies, but it is often necessary to resort to more invasive techniques, including video-assisted thoracoscopic or open biopsy.11,12 Occasionally, decortication of the lung may be justified on clinical grounds. Whatever technique is used, close correlation of the histological and radiological findings is essential: a confident radiological diagnosis of mesothelioma may sometimes help to determine the management pathway if the pathological appearances are equivocal.

A great deal of attention has been paid to immunohistochemistry in the diagnosis of mesothelioma and this has been the subject of a number of recent reviews. No one would deny its importance, but immunohistochemistry alone does not always provide the solution to diagnostic problems. It has to be interpreted in the context of the morphological findings and with an awareness of its limitations. The neoplastic mesothelial cell is capable of many guises. It is protean in both its morphological and immunohistochemical characteristics, which range from purely epithelioid, when it may mimic carcinoma, to mesenchymal, when it may be indistinguishable from other forms of sarcoma. Apart from the traditional separation into epithelioid, biphasic and sarcomatoid types, a variety of subtypes are described, some of which will be discussed in more detail. This review does not aim to produce a comprehensive guide to mesothelioma, particularly with regard to its very varied morphology and patterns, but is intended to address some of the frequently encountered problems in its diagnosis. While we recognize the importance of cytopathology and concede that in expert hands it can reliably suggest the diagnosis of mesothelioma, it is often equivocal and requires biopsy confirmation. In particular, cytology does not provide evidence of invasive malignancy, which is usually critical to the diagnosis.

Diagnostic problems can be categorized into several groups. Determining whether a biopsy specimen is benign or malignant presents problems in two main areas: the distinction between reactive mesothelial hyperplasia and epithelioid mesothelioma, and the distinction between reactive pleural fibrosis and sarcomatoid or desmoplastic mesothelioma. Having decided that malignancy is present, the distinction must be made between epithelioid mesothelioma and metastatic carcinoma, particularly in patients who have a previous history of malignancy or atypical radiology, and between sarcomatoid mesothelioma and other types of malignant connective tumour that may occasionally involve the pleura primarily. Finally, there are recognized but rare forms of mesothelioma that may resemble other tumours, and a knowledge of these is useful if pitfalls are to be avoided.

Reactive mesothelial hyperplasia versus epithelioid mesothelioma

  1. Top of page
  2. Abstract
  3. Introduction
  4. Reactive mesothelial hyperplasia versus epithelioid mesothelioma
  5. Reactive pleural fibrosis versus sarcomatoid or desmoplastic mesothelioma
  6. Mesothelioma versus metastatic adenocarcinoma
  7. Rare and unusual variants of mesothelioma
  8. Other tumours mimicking epithelioid mesothelioma
  9. Sarcomatoid mesothelioma versus other connective tissue tumours
  10. Conclusion
  11. References

Mesothelial cells respond to the presence of air, fluid, blood, infection or other irritants in the pleural cavity by undergoing proliferation. This is recognized in pleural biopsy specimens by the presence of multiple layers and loose surface papillary aggregates of mesothelial cells. In contrast to mesotheliomas, the papillae in benign proliferations tend to lack a vascular connective tissue core. Mitoses are often present, and some degree of atypia is not unusual. When this is more marked, it may be very difficult to make a confident distinction from mesothelioma. In larger specimens, the presence of invasion, with groups of cells infiltrating through the pleura, provides the best clue to malignancy,13,14 but this raises the question of what constitutes invasion. In the normal pleura there is no clearly identifiable basement membrane between the mesothelial cells and the underlying loose fibrocollagenous tissue with its irregular elastic layers. At an early stage in benign conditions, hyperplastic mesothelial cells may form layers within the connective tissue that are parallel to the surface. In biopsy specimens, cross cutting and folding of the pleura results in the false impression of deeper cell clusters, which may be seen to be continuous with the surface in deeper sections. In longstanding effusions, where there is organization of inflammatory exudate, the mesothelial cell inclusions trapped within the layers tend to form small lumens, usually in a recognizable plane. Where this pattern is disturbed and the cells become more numerous and more disorganized, particularly with infiltration perpendicular to the surface, there should be at least a suspicion of malignancy. If invasion of fat can be clearly identified, this is regarded as a more conclusive indicator, although tangential cutting or cross-cutting through the tissue may make this difficult to interpret. Neoplastic cells invading fat can usually be readily identified as they induce some formation of stroma, but more subtle infiltration between the fat cells may easily be missed.

In their early stages, many mesotheliomas appear as multiple small superficial nodules on the pleura. The adjacent mesothelium may be monolayered or folded with cellular atypia and could be regarded as in situ malignancy.15 In practice, the concept of in situ mesothelioma is not a particularly helpful one: in the absence of clear features of invasion, or the presence of definite tumour elsewhere, either in the same or subsequent biopsy specimens, it is usually impossible or unwise to make this diagnosis.16 A more pragmatic approach in such borderline cases is to use the term ‘atypical mesothelial proliferation’, in the expectation that further correlation with clinical and radiological findings may clarify the issue or that a further specimen will be diagnostic (Figure 1).

image

Figure 1.  Some degree of cellular atypia may be seen in benign mesothelial proliferations. Where this is more marked but without invasion, the term atypical mesothelial proliferation is appropriate. In this case the diagnosis of mesothelioma was proven subsequently.

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In this situation, immunohistochemistry has a limited role. It is useful in identifying the cells as mesothelial rather than epithelial, a distinction that is discussed in more detail below, but there are few markers that help in determining malignancy. Epithelial membrane antigen (EMA) has been demonstrated to show strong membranous immunoreactivity in malignant mesothelioma, but only weak focal reactivity in reactive mesothelial hyperplasia,16–18 particularly using the clone E29.19 Expression of p53 is also found more often in malignant mesothelioma than reactive hyperplasia.17,18 Conversely, desmin expression is seen more frequently in reactive mesothelial hyperplasia than malignant mesothelioma. The sensitivities and specificities of these antibodies are given in Table 1, taken from King and colleagues’ review of 15 papers evaluating the ability of immunohistochemistry to distinguish between benign and malignant pleural disease.20 The combination of EMA and p53 positivity, together with lack of desmin expression, may give additional weight to a benign or malignant diagnosis, but, in our opinion and that of other authors,21 should not override the morphological features.

Table 1.   Antibodies distinguishing between malignant mesothelioma and reactive mesothelial hyperplasia (15)
AntibodySensitivity (%)Specificity (%)
Epithelial membrane antigen7489
p535891
Desmin8383

Other techniques that have been suggested for assessing malignancy, but rarely used in everyday practice, include assessment of mean nuclear volume, which is significantly higher in malignant than benign mesothelial cells,22 and silver staining of the nucleolar organizer regions (the argyrophilic nucleolar organizer region technique), which shows a significantly higher number of nucleolar organizer regions in malignant than benign mesothelium.23 Assessment of the number of cells in cycle, using the antibody MCM2, has been proposed as a useful adjunct.24 Although not widely used, immunohistochemistry for telomerase reverse transcriptase can be performed on formalin-fixed tissue, and the presence of telomerase is a useful indicator of malignancy in mesothelial cells.25

Reactive pleural fibrosis versus sarcomatoid or desmoplastic mesothelioma

  1. Top of page
  2. Abstract
  3. Introduction
  4. Reactive mesothelial hyperplasia versus epithelioid mesothelioma
  5. Reactive pleural fibrosis versus sarcomatoid or desmoplastic mesothelioma
  6. Mesothelioma versus metastatic adenocarcinoma
  7. Rare and unusual variants of mesothelioma
  8. Other tumours mimicking epithelioid mesothelioma
  9. Sarcomatoid mesothelioma versus other connective tissue tumours
  10. Conclusion
  11. References

In response to inflammation, and in addition to the reactive changes seen in the mesothelial cells, fibrinous exudate is laid down on the surface of the pleura. This undergoes organization, with in-growth of capillaries and fibroblasts and subsequent collagenization. This is most characteristically seen in empyema, and at this stage it is not usually a problem to recognize the characteristic layering. Organizing surface fibrinous exudate merges with a layer of granulation tissue and this in turn merges with deeper layers of more densely collagenized fibrous tissue. A zone of entrapped mesothelial cells may be apparent where the two layers of the pleura become fused and these are recognized as benign by their lack of invasion beyond this layer. When the inflammatory process is more chronic, or when it subsides, the thickened pleura becomes more uniformly fibrotic in appearance. The distinction between these appearances and sarcomatoid or desmoplastic mesothelioma can be very difficult26 and depends largely on the recognition of cellular atypia. Very cellular sarcomatoid tumours with marked atypia present little difficulty, but when the cellularity is less marked and nuclear atypia less obvious, it would be useful to have more objective evidence of malignancy. As with epithelioid mesothelioma, extension of the process into fat is a useful pointer.27 The normally clear boundary between parietal pleura and the underlying adipose tissue is often preserved in reactive fibrosis, when it may be accompanied by a variable zone of lymphoid infiltration extending into the fat. Regular, pointed, tooth-like extensions of acellular fibrous tissue into the fat are not infrequently seen in benign conditions and should not necessarily be taken as evidence of malignancy. In sarcomatoid mesothelioma, the cellularity is usually increased, either throughout the full thickness of the pleura or more focally, and it is more variable without recognizable zonation. This increased cellularity extends into the fat as deeper and more irregular elongations than are seen in reactive fibrosis, and may be seen around vessels.

The nuclei in reactive fibrosis are typical of fibroblasts: they are pale-staining, irregular, and vesicular with a small nucleolus. In contrast, malignant mesothelial cells are more irregularly spaced, often with more cellular clusters. To a variable extent, they show the typical nuclear features of malignancy: greater nuclear pleomorphism, more marked nuclear irregularity and hyperchromasia and variable numbers of mitoses. Not infrequently, however, these features are too subtle to allow a confident diagnosis on morphological grounds alone (Figure 2A).

image

Figure 2. A, In sarcomatoid mesotheliomas the atypical mesothelial cells may be difficult to distinguish among the fibroblasts. B, Staining for cytokeratins highlights the mesothelial cells with a disorganized growth pattern.

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Unfortunately, this is the area where immunohistochemistry is least helpful. The antibodies that typically stain benign or malignant mesothelial cells, and those discussed in further detail below, are either negative or, at best, weakly positive in sarcomatoid mesothelioma, and cannot be relied on to provide the diagnosis. The only antibody that is generally considered to be of value is a broad-spectrum cytokeratin (CK), such as MNF116, which highlights the distribution of mesothelial cells, demonstrating any invasion into underlying tissues.21,27,28 Here a note of caution is needed. In benign, reactive pleural fibrosis, CK immunoreactivity will often demonstrate layers of thin, spindle-shaped cells in the superficial zone of the thickened pleura, becoming less numerous in the deeper, more densely fibrotic areas (Figure 3). These are inconspicuous on routine haematoxylin and eosin (H&E) sections and are presumed to represent CK expression in submesothelial fibroblasts as they become incorporated into the fibrosis. They should not be taken as evidence of malignancy. If the pattern is more irregular, with disruption of the layering and CK+ cells extending more deeply, mesothelioma should be suspected. In contrast to entrapped benign mesothelial cells, deep clusters of cells often show irregular orientation, some with their long axes perpendicular to the surface (Figure 2B).

image

Figure 3.  Staining for cytokeratins in reactive pleural fibrosis (CAM5.2) shows layers of inconspicuous cytokeratin-positive cells parallel to the surface and becoming less numerous in the deeper layers.

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Desmoplastic mesotheliomas provide a particular diagnostic challenge. There is no strict definition of this tumour, and overlap exists between desmoplastic and sarcomatoid variants. Typically, mesothelioma is suspected radiologically and clinically, but even quite large biopsy specimens show only thickened, densely fibrotic pleura. This is usually acellular or paucicellular and shows few features that enable a diagnosis of malignancy to be made. Helpful features are said to include a storiform pattern, areas of bland necrosis with no inflammatory response and haphazard arrangement of the capillaries instead of being aligned perpendicular to the surface.13,27,29–31 Invasion of the chest wall tissues, with muscle fibres trapped in the fibrous tissue, is a very helpful feature, but it should be recognized that similar appearances may be seen in longstanding empyema with fibrosis. Malignancy may not be confirmed until autopsy, and even then it may be necessary to examine large numbers of tissue blocks to find areas with sufficient cellular atypia to make a confident diagnosis.

Mesothelioma versus metastatic adenocarcinoma

  1. Top of page
  2. Abstract
  3. Introduction
  4. Reactive mesothelial hyperplasia versus epithelioid mesothelioma
  5. Reactive pleural fibrosis versus sarcomatoid or desmoplastic mesothelioma
  6. Mesothelioma versus metastatic adenocarcinoma
  7. Rare and unusual variants of mesothelioma
  8. Other tumours mimicking epithelioid mesothelioma
  9. Sarcomatoid mesothelioma versus other connective tissue tumours
  10. Conclusion
  11. References

The distinction between epithelioid mesothelioma and metastatic carcinoma, particularly adenocarcinoma, is perhaps the most frequently encountered diagnostic dilemma. Even when the tumour cells appear characteristic of mesothelioma, metastatic carcinoma remains the main differential diagnosis, and this is the situation in which immunohistochemistry is most helpful. If the histological features are typical of mesothelioma, immunohistochemistry is usually required as supporting evidence. The best-differentiated examples of epithelioid mesothelioma consist of uniform cells with open nuclei, single central nucleoli and a moderate amount of pale-staining cytoplasm. They characteristically form complex tubulo-papillary aggregates. Even in tumours that are predominantly epithelioid, there is frequently some evidence of a biphasic pattern, with clusters of epithelioid cells merging with atypical stromal spindle cells, a pattern very characteristic of mesothelioma and unusual in metastatic carcinoma. In metastatic adenocarcinoma, the neoplastic glands tend to be more clearly distinct from the stroma, and the diagnosis may present no difficulty, particularly if the tumour is mucus-secreting, when all that may be required is a diastase periodic acid–Schiff (D-PAS) stain for epithelial mucin. Mesotheliomas rarely produce D-PAS+ mucin, although there are occasional reports of tumours secreting this type of mucin.32 Mesothelial cells do contain cytoplasmic glycogen, and the application of diastase to the PAS stain is essential to avoid confusion. They also produce stromal hyaluronic acid, and this may be abundant in some tumours, giving a myxoid or vacuolated appearance. Hyaluronic acid is alcianophilic and, in contrast to epithelial mucin, it is D-PAS−, although the mucin must be present within vacuoles in the cytoplasm of tumour cells rather than simply within the stroma and also must show sensitivity to pretreatment with hyaluronidase. In reality, positive Alcian blue staining of tumour cells is found only in about 20% of cases and has been superseded by immunohistochemistry. Some metastatic carcinomas, particularly those metastatic from primary tumours in the lung, show extreme pleomorphism with spindle cells and giant cell forms. Similar degrees of pleomorphism are occasionally seen in mesotheliomas and, as might be expected, in neither type of tumour do the cells retain their typical immunohistochemical features.33

A large number of antibodies have been used to help distinguish between epithelioid mesothelioma and metastatic adenocarcinoma. King and colleagues34 systematically reviewed 88 published papers comparing immunohistochemical markers used in the diagnosis of epithelioid malignant mesothelioma. Their data, extracted from the text of the paper, is presented in Tables 2 and 3.

Table 2.   Antibodies usually positive in pulmonary adenocarcinoma metastatic to the pleura
AntibodySensitivity (%)Specificity (%)
Carcinoembryonic antigen8395
Ber-EP48090
B72.38093
CD157293
MOC-319393
E-cadherin8682
Thyroid transcription factor-172100
Lewisy (BG8 clone)9393
Table 3.   Antibodies usually positive in epithelioid mesothelioma
AntibodySensitivity (%)Specificity (%)
CK5/68385
Calretinin8285
HBME 18543
Thrombomodulin6180
N-cadherin7884
Wilms tumour product-17796

Of the many other antibodies that have been studied, some are not widely available or data are very limited, and others have been demonstrated to be unhelpful. They include CD44S, IOB-3 (CD24), OV632, SM3, 44-3A6 and AUA-1.34–36 Both EMA and vimentin are readily available antibodies, but neither provides sufficient discrimination to be helpful.

Some of the more promising recently studied antibodies include D2-40, podoplanin and h-caldesmon. D2-40 is a commercially available monoclonal antibody that stains lymphatic endothelial cells. It also shows strong membranous immunoreactivity in epithelioid malignant mesothelioma, and studies to date have demonstrated it to be both sensitive and specific.37–41 However, caution should be applied as D2-40 expression has been demonstrated in a high proportion of ovarian serous carcinomas,37 the epithelioid component of synovial sarcomas and angiosarcomas.39 Podoplanin is a membrane mucoprotein that is also expressed by lymphatic endothelium. It is positive in some vascular tumours, seminomas and haemangioblastomas.42 It has been shown to be expressed by malignant mesothelioma, and the small number of studies performed so far indicate good sensitivity and specificity.39,43 h-Caldesmon is a specific marker for smooth muscle tumours. Recently, it has been demonstrated to be a sensitive and specific marker for epithelioid malignant mesothelioma.44,45 Another potentially useful marker is MUC4, which is said to be expressed in about 90% of pulmonary adenocarcinomas, but not by mesotheliomas.46

It should be noted that many of the antibodies that were initially hailed as being very useful in the diagnosis of malignant mesothelioma have since fallen out of favour, as subsequent studies have demonstrated that they were not as specific or sensitive as originally thought. For example, mesothelin was initially believed to be highly specific for mesothelioma but has since been demonstrated to be positive in many adenocarcinomas, including lung, ovary, pancreas, endometrium, stomach, colon and cholangiocarcinoma.47,48 Further work is needed to test how robust the observations are on these newer diagnostic antibodies before they are included in routine practice.

As with most tumours, a panel of antibodies should be used. Different authors recommend different combinations,34,49,50 but we suggest the choice of antibodies should reflect the reproducibility of results achieved in your own laboratory. We prefer to use Ber-EP4, carcinoembryonic antigen (CEA), CD15, thyroid transcription factor-1, CK5/6, calretinin and Wilms tumour product-1 (WT-1). Unless the tumour is very undifferentiated, this combination usually provides an answer. In general, our experience is in accord with published figures: Ber-EP4 is least specific for adenocarcinoma and focal reactivity is frequently seen in mesothelioma; and both CEA and CD15 are more specific but less sensitive for adenocarcinoma. For the three antibodies that are most often positive in mesothelioma, immunoreactivity is most consistent and heaviest in epithelioid areas of the tumour and weakest in spindle cell areas: calretinin expression is lost most readily, CK5/6 is more likely to stain the spindle cell component (Figure 4) and WT-1 reactivity is often focally retained in these areas. Apart from adenocarcinoma, distinguishing mesothelioma from other types of lung carcinoma presents different problems. Squamous cell carcinoma usually expresses CK5/6,51,52 and small cell carcinoma has to be distinguished from small cell mesothelioma, as discussed below. In addition to spindle cell features, both mesotheliomas and carcinomas may show a considerable degree of pleomorphism with loss of normal antibody expression, and this presents particular problems with so-called pseudomesotheliomatous carcinomas, when the radiological appearances are similar.53 There is a suggestion that podoplanin is a useful marker in this situation, staining 72% of sarcomatoid mesotheliomas, with no staining of sarcomatoid carcinomas.43

image

Figure 4.  Characteristic immunohistochemical patterns are very variable in spindle cell/sarcomatoid areas. In this field cytokeratin 5/6 reactivity is retained in some of the spindle cells.

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Carcinomas metastatic from sites other than the lung may have their own morphological and immunohistochemical characteristics, and there is usually a history of malignancy elsewhere. Metastatic breast carcinoma is perhaps most commonly seen, and here the history is clearly important. Clues to the tumour type are the trabecular or single-file pattern of infiltration and the presence of intracytoplasmic lumina containing PAS+ material. Oestrogen, progesterone and herceptin receptor studies are required for therapeutic purposes and may help to confirm the diagnosis. Hormone receptors have also been evaluated in the distinction between peritoneal mesotheliomas and primary and secondary serous carcinomas of the peritoneum in women, and the findings suggest that the majority of carcinomas express oestrogen receptors but mesotheliomas are negative.45,54

A variable degree of clear cell change is not unusual in epithelioid mesothelioma and generally does not pose a problem, as other parts of the tumour are more typical. If the whole of the biopsy material has clear cell features (‘clear cell mesothelioma’), the differential diagnosis includes metastatic renal cell carcinoma and, in women, metastatic clear cell carcinoma of ovarian origin. Immunoreactivity for CD10 cannot be relied on to make the distinction between renal cell carcinoma and mesothelioma as about 50% of mesotheliomas show at least focal positivity. However, calretinin and CK5/6 are usually negative in renal cell carcinoma and CD15 is positive in about 70%.55–59

In individuals exposed to asbestos with a cigarette-smoking history, it is inevitable that there are occasional instances of synchronous malignant mesothelioma and lung carcinoma. Attanoos and colleagues60 identified six cases of synchronous lung carcinoma in a group of 500 cases of mesothelioma: three adenocarcinomas, two squamous cell carcinomas and one small cell carcinoma. The authors have encountered one case of synchronous small cell carcinoma and mesothelioma, with ‘collision’ of the two tumours. Awareness of this potential combination and careful interpretation of the immunohistochemical features is required in this situation.

Metastatic malignant melanoma

Metastatic malignant melanoma is occasionally pleural-based. If no history of a previous primary lesion is provided, this can produce confusing morphological and immunohistochemical features, particularly if a spindle cell pattern is dominant. Standard melanoma markers, S100, HMB45 and melan-A are often only variably positive, and the cells may express Bcl-2, CD56 and CD99. Apart from sarcomatoid mesothelioma, the differential diagnosis can, therefore, include synovial sarcoma and even primitive neuroectodermal tumour (Figure 5).

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Figure 5.  This tumour shows a combination of small dark round cells and spindle cells in a loose stroma, and expresses CD99, Bcl-2 and S100. A history of malignant melanoma was subsequently elicited.

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Rare and unusual variants of mesothelioma

  1. Top of page
  2. Abstract
  3. Introduction
  4. Reactive mesothelial hyperplasia versus epithelioid mesothelioma
  5. Reactive pleural fibrosis versus sarcomatoid or desmoplastic mesothelioma
  6. Mesothelioma versus metastatic adenocarcinoma
  7. Rare and unusual variants of mesothelioma
  8. Other tumours mimicking epithelioid mesothelioma
  9. Sarcomatoid mesothelioma versus other connective tissue tumours
  10. Conclusion
  11. References

A number of different morphological variants of epithelioid mesothelioma are described, each of which carries a different differential diagnosis. Nevertheless, they do not differ significantly in their immunohistochemical profile from more typical cases.

Small cell mesothelioma

Small cells with less abundant cytoplasm, more uniformly and densely immunoreactive nuclear chromatin and less conspicuous nucleoli are occasionally seen in mesotheliomas and may predominate in small biopsy specimens (Figure 6). If more material is available, multiple blocks will show more areas more typical of mesothelioma, even if the small cell areas predominate. The resemblance to small cell carcinoma is superficial, and the other typical features of small cell carcinoma are absent: nuclei do not mould together, mitoses are far less frequent and the characteristic areas of necrosis with deposition of nuclear debris are lacking. Immunohistochemistry is characteristic of mesothelioma, possibly with enhanced reactivity for thrombomodulin,61 and lacks the neuroendocrine profile of small cell carcinoma: the cells are negative for CD56, synaptophysin and chromogranin.62

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Figure 6.  Small cell features in mesothelioma are usually only focal, but may predominate in small biopsy specimens.

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Other ‘small dark cell tumours’ occasionally require exclusion, particularly in younger patients. Peripheral primitive neuroectodermal tumours of the chest wall (Askin tumours) are part of the Ewing sarcoma group. They characteristically express a surface protein mic2 (CD99), associated with the t(11;22)(q24;q12) translocation, and lack mesothelial markers. However, CD99 immunoreactivity is not specific and may be found in up to 50% of mesotheliomas as well as in synovial sarcomas and malignant melanomas.

Deciduoid mesothelioma and rhabdoid mesothelioma

Deciduoid mesothelioma is a rare morphological variant of epithelioid mesothelioma in which the cells bear a striking morphological resemblance to decidual tissue. They form sheets of large rounded cells with well-defined borders and abundant eosinophilic cytoplasm, more densely immunoreactive around the nucleus and lighter peripherally. Nuclei are vesicular and may be multiple63 (Figure 7). The initial case reports were young women with peritoneal tumours, where the distinction from some form of decidual tissue is a real diagnostic problem.64 Subsequently, similar appearances have been described in pleural mesotheliomas.65,66 Deciduoid features may be confined to part of the tumour, other areas showing more typical features of epithelioid mesothelioma.65 The immunoprofile does not differ from conventional epithelioid mesothelioma: the cells typically express CKs, calretinin, CK5/6 and WT-1. A deciduoid morphology does not appear to carry prognostic significance.67

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Figure 7.  In deciduoid mesothelioma the cells have abundant cytoplasm that is more strongly eosinophilic in the perinuclear area.

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Rhabdoid features are even less common and appear to indicate more aggressive behaviour (Figure 8). The tumours may be epithelioid, biphasic or sarcomatoid, and the proportion of cells showing rhabdoid features varies. Provided that the existence of this variant is borne in mind, confusion with other forms of rhabdoid tumour should not occur. The tumour cells are negative for desmin and variably positive for the mesothelial markers.68

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Figure 8.  Rhabdoid mesothelioma is a rare variant in which the cells show hyaline cytoplasmic inclusions with eccentric nuclei.

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Lymphohistiocytoid mesothelioma

Lymphohistiocytoid mesothelioma is a rare variant that may simulate an inflammatory process or malignant lymphoma.69 The malignant mesothelial cells have features resembling histiocytes and are submerged in a mixed infiltrate that includes true benign histiocytes and small lymphocytes. The neoplastic cells may be difficult to identify in H&E sections as they are poorly cohesive (Figure 9A). Like histiocytes, their nuclei tend to be oval and irregular, but with greater hyperchromasia and more prominent nucleoli. In small biopsy specimens the diagnosis may be extremely difficult unless there is a strong suspicion of mesothelioma. The same rules apply as with more typical mesotheliomas, and infiltration into fat is a useful feature, provided the specimens are of sufficient size to appreciate this. Careful evaluation of the immunohistochemistry demonstrates a population of CD68+ histiocytes that are distinct from the mesothelial cells, which are positive for EMA, broad-spectrum CKs such as AE1/3 and MNF116, CK5/6 and calretinin70–72 (Figure 9B). The small lymphocytes are predominantly CD8+ T cells that are positive for the cytotoxic granule markers TIA-1, granzyme and perforin. In a large series, the prognosis of this variant has been shown to be similar to epithelioid mesothelioma.72

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Figure 9. A, In lymphohistiocytoid mesothelioma the neoplastic mesothelioma cells are dispersed in a background of histiocytes and small T lymphocytes. B, They become apparent with cytokeratin staining (MNF116).

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Other tumours mimicking epithelioid mesothelioma

  1. Top of page
  2. Abstract
  3. Introduction
  4. Reactive mesothelial hyperplasia versus epithelioid mesothelioma
  5. Reactive pleural fibrosis versus sarcomatoid or desmoplastic mesothelioma
  6. Mesothelioma versus metastatic adenocarcinoma
  7. Rare and unusual variants of mesothelioma
  8. Other tumours mimicking epithelioid mesothelioma
  9. Sarcomatoid mesothelioma versus other connective tissue tumours
  10. Conclusion
  11. References

Adenomatoid tumours

Adenomatoid tumours most commonly arise in the genital tract, but occasionally occur in the pleura.73 They are small and solitary and usually discovered as incidental findings.74 Histologically, they are composed of vacuolated epithelioid mesothelial cells forming tubular spaces. Some examples of malignant mesothelioma have a very similar adenomatoid growth pattern75 and, as adenomatoid tumours have a mesothelial origin, their immunohistochemical profile is identical. Since some mesotheliomas are similarly localized, at least in their early stages,73,76 considerable caution is needed in making the diagnosis. The solitary nature of adenomatoid tumours, their sharp circumscription and bland cytological features are regarded as helpful features.77

Thymic tumours

Rarely, primary thymic epithelial tumours may mimic mesothelioma by diffusely infiltrating the pleura to form thickened, plaque-like areas. In the series of Attanoos et al.,78 the thymic tumours differed in their World Health Organization type, but were classified as thymomas rather than thymic carcinomas. The thymic epithelial component variably expresses CK5/6, calretinin, thrombomodulin and CD20, but the main distinction from mesothelioma is made on the pattern of the tumour and the lymphoid component of immature T cells, expressing CD3, CD5, terminal deoxynucleotidyl transferase and CD99, with a high proliferation fraction.

Sarcomatoid mesothelioma versus other connective tissue tumours

  1. Top of page
  2. Abstract
  3. Introduction
  4. Reactive mesothelial hyperplasia versus epithelioid mesothelioma
  5. Reactive pleural fibrosis versus sarcomatoid or desmoplastic mesothelioma
  6. Mesothelioma versus metastatic adenocarcinoma
  7. Rare and unusual variants of mesothelioma
  8. Other tumours mimicking epithelioid mesothelioma
  9. Sarcomatoid mesothelioma versus other connective tissue tumours
  10. Conclusion
  11. References

Osteosarcoma

An extreme degree of divergent differentiation and mesenchymal ‘metaplasia’ in sarcomatoid mesothelioma can lead to the production of osseous or cartilaginous matrix by cells indistinguishable from malignant chondrocytes or osteoblasts79 (Figure 10). Occasionally even rhabdomyoblastic differentiation can be seen.80 In these cases, the cells have completely lost their mesothelial characteristics and immunohistochemistry is unhelpful. If the biopsy specimens are sufficiently large, there may be areas more characteristic of mesothelioma but even at autopsy these may not be identifiable. In these patients the distinction from primary or metastatic sarcoma can be difficult or even impossible. There are sporadic case reports of primary pleural osteosarcoma,81,82 but it is uncertain how mesothelioma with osseous differentiation can be excluded, particularly if there is a clear history of asbestos exposure and the radiological or autopsy appearances show a tumour encasing the lung. We prefer to include these tumours in the spectrum of mesothelioma. Metastatic osteosarcoma, when there is a known primary elsewhere, usually presents as intrapulmonary lesions, but occasional pleural-based metastatic disease may occur.83

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Figure 10.  Malignant osteoid formation in a sarcomatoid mesothelioma.

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Synovial sarcoma

Primary pleural synovial sarcomas are very rare, more commonly representing a metastasis from a soft tissue primary site.73 They can present with chest pain, dyspnoea, cough84 or pleural effusion,73 all typical symptoms of malignant mesothelioma. However, they typically occur in a younger age group, the average age at diagnosis being 33 years,85 and they most commonly grow as a localized mass.84–86 Histologically, they may have a monophasic or biphasic growth pattern, the latter being more common in the pleura.73 Synovial sarcoma tends to be more densely cellular than sarcomatoid mesothelioma, with long fascicles of spindle cells usually displaying only a mild degree of pleomorphism, whereas mesothelioma typically shows short fascicles of less crowded spindle cells with more significant pleomorphism.86 The epithelial component, if present, can be subtle and become evident only when highlighted by immunohistochemistry, or it may be more obvious, with gland formation.73

Immunohistochemistry is useful in distinguishing synovial sarcoma from malignant mesothelioma. CK immunoreactivity is generally diffusely positive in malignant mesothelioma but only focally positive in synovial sarcoma.86 Synovial sarcoma is typically positive for Bcl-2, whereas malignant mesothelioma is only occasionally positive. It should be noted that synovial sarcoma can be positive for calretinin73,84 and that CD99, often used in the diagnosis of synovial sarcoma, is positive in approximately half of all malignant mesotheliomas.87 The diagnosis of synovial sarcoma can be confirmed by demonstration of the chromosomal translocation t(X;18).88

Malignant vascular tumours of the pleura

Primary malignant endothelial tumours of the pleura are rare. In other sites they are usually divided into two types, epithelioid haemangioendothelioma and angiosarcoma. Some overlap exists, as the latter may also have epithelioid features, and in the pleura it may be difficult to make a clear distinction between the two. Both may present with pleural effusion and diffuse pleural thickening and are capable of simulating mesothelioma clinically and radiologically, providing a further trap for the unwary histopathologist.89–92

In a typical example of epithelioid haemangioendothelioma, the cells form irregular clusters or cords in a hyaline, fibrous stroma. Nuclei are irregular and vesicular, usually with a small nucleolus, and the cytoplasm is eosinophilic. The similarity to mesothelioma may be even more pronounced as the epithelioid cells merge with more spindly cells, giving the tumour a biphasic appearance (Figure 11). The cytoplasm typically contains vacuoles, and careful examination will show occasional red cells within these. More pleomorphic variants, with more atypical nuclei and vascular spaces lined by malignant epithelioid endothelial cells, may be regarded as epithelioid angiosarcoma and account for sporadic case reports of this rare entity.92,93

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Figure 11.  Epithelioid haemangioendothelioma. The tumour is biphasic with both spindle cell areas (A) and epithelioid areas (B). It diffusely involves the visceral pleura with minimal infiltration into peripheral lung. Intracytoplasmic vascular channels, seen as vacuoles in (B), provide a clue to the correct diagnosis, supported by immunopositivity for vascular markers and immunonegativity for cytokeratins.

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Immunohistochemistry usually shows some expression of CKs, but this is weak and focal, an important clue that the diagnosis is unlikely to be mesothelioma. One or more of the endothelial markers—CD31, CD34 and Factor VIII-related antigen—will invariably be positive and provide the diagnosis.

As with osteosarcoma, it could be argued that some examples of primary pleural malignant endothelial tumours represent an extreme degree of divergent differentiation in mesotheliomas. Whereas some angiomatoid tumours of the thyroid gland do appear to show a mixed epithelial and endothelial immunophenotype, angiosarcoma-like neoplasms of other organs, including the lung, are essentially high-grade carcinomas and lack endothelial markers.94 In the absence of demonstrable mesothelial features, there is insufficient evidence that pleural angiosarcoma is related to mesothelial malignancy. Although some patients may give a history of asbestos exposure, this association could not be proven in cases investigated with asbestos fibre counts.90 The same authors also failed to find expression of endothelial markers in a series of mesotheliomas.90

Pyothorax-associated lymphoma

Although occasional examples have been described in Europe,95–97 most cases of pyothorax-associated lymphoma are reported from Japan, and the majority of patients have a history of artificial pneumothorax for tuberculosis with chronic empyema.98 The typical history provides a strong indication of the likely diagnosis, but the presence of atypical cells in thickened pleura makes mesothelioma an important differential diagnosis. The cells are dissociated and may appear plasmacytoid or anaplastic (Figure 12). They lack mesothelial markers and show variable expression of the B-cell markers, CD20 and CD79a. Their variable expression of other markers, including CD30 and T-cell markers, may cause confusion and some cases have been shown to have a mixed genotype.99 There is a strong association with latent Epstein–Barr virus infection, and the majority of cases are positive for LMP-1, ENBA-2 and EBER.96

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Figure 12.  Pyothorax-associated lymphoma may resemble sarcomatoid mesothelioma when the cells are pleomorphic and dispersed in a fibrous stroma. In this example, tumour cells stained for CD45, Cd79a, CD30 and Epstein–Barr virus encoded small nuclear RNAs by in situ hybridization, but were negative for CD20.

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Conclusion

  1. Top of page
  2. Abstract
  3. Introduction
  4. Reactive mesothelial hyperplasia versus epithelioid mesothelioma
  5. Reactive pleural fibrosis versus sarcomatoid or desmoplastic mesothelioma
  6. Mesothelioma versus metastatic adenocarcinoma
  7. Rare and unusual variants of mesothelioma
  8. Other tumours mimicking epithelioid mesothelioma
  9. Sarcomatoid mesothelioma versus other connective tissue tumours
  10. Conclusion
  11. References

Recent advances in immunohistochemistry have significantly improved the ability of the diagnostic pathologist to make an accurate pre-mortem diagnosis of mesothelioma. Previous figures of 20–30% diagnostic inaccuracy100,101 are now being improved to virtually 100% accuracy with epithelioid variants (the majority of cases) in an audit carried out in relation to a UK-based chemotherapy study.9 However, pathologists must be aware of the pitfalls of over-interpreting immunohistochemical data, particularly in those tumours that are so undifferentiated that they lack reliable immunohistochemical features. A multidisciplinary approach that encompasses careful morphological assessment as well as clinical and imaging data remains best practice.

References

  1. Top of page
  2. Abstract
  3. Introduction
  4. Reactive mesothelial hyperplasia versus epithelioid mesothelioma
  5. Reactive pleural fibrosis versus sarcomatoid or desmoplastic mesothelioma
  6. Mesothelioma versus metastatic adenocarcinoma
  7. Rare and unusual variants of mesothelioma
  8. Other tumours mimicking epithelioid mesothelioma
  9. Sarcomatoid mesothelioma versus other connective tissue tumours
  10. Conclusion
  11. References