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Localized pleural tumors are relatively uncommon and tend to produce diagnostic confusion, in part because of nomenclature, and in part because of lack of a set of definitive rules for separating these various entities. All of these lesions are circumscribed (except for the occasional malignant tumor that has grown into surrounding structures), more or less nodular (sometimes ovoid, discoid) masses attached to the pleural surface. Localized pleural tumors can be divided into 4 groups:
Solitary fibrous tumor
Malignant solitary fibrous tumor
Localized malignant mesothelioma
Localized sarcomas
Solitary fibrous tumor (SFT). These are by far the most common tumor of this whole group, with two large series (1,2) reporting a total of almost 600 examples. Nomenclature is a source of considerable confusion in this area, since SFT has been called "localized mesothelioma", "fibrous mesothelioma", "benign mesothelioma", and "submesothelial fibroma". The evidence for a mesothelial origin has always been dubious, and recent reports of identical tumors in the retroperitoneum and orbit make it clear that these are not mesothelial proliferations (4).Solitary fibrous tumors are circumscribed lesions that occur at any age. Most are asymptomatic. SFT are not associated with asbestos exposure. The majority are found to be attached to the visceral pleura; in half the cases the pleural attachment is by a thin stalk and about half the cases are sessile. Microscopically they show a variety of patterns including the so-called "patternless" pattern, hemangiopericytoma-like pattern, and cellular pattern. Rare cases have epithelial appearing areas, although immunochemical staining makes it clear that there is no true epithelial differentiation in these lesions. By electron microscopy, SFT show nondescript spindled cells. The characteristic immunochemical profile is CD34 positivity and vimentin positivity with keratin negativity (3,4).The vast majority of typical appearing SFT are benign (see below for criteria for malignancy).
Malignant solitary fibrous tumor (MSFT). A number of features suggest that a solitary fibrous tumor is malignant (1,2). Clinically these include presentation with chest pain, shortness of breath, and pleural effusion. Pathologically, large (>10cm) tumors; tumors attached to the visceral pleura, mediastinum, or inverted into the lung; tumors with hemorrhage and necrosis; and tumors with > 4 mitoses per 10 hpf frequently behave in a malignant fashion. SFT that recur are usually malignant. Microscopically MSFT tend to be cellular tumors composed of mitotically active spindled cells, but areas of more ordinary SFT may be seen. The immunochemical staining pattern of MSFT is said to be identical to benign SFT, but this conclusion appears to be based on very few cases (3,4).Malignant SFT often recur locally and may metastasize, but do not spread over the pleural surface. The proportion of SFT that are malignant was 37% in the series of England et al (1) and 12% in the series of Briselli et al (2), although referral patterns and problems of definition may bias these numbers. England et al (1) concluded that about half the tumors that they called malignant ultimately lead to the death of the patient.
Localized malignant mesothelioma. Localized malignant mesotheliomas (LMM) are a newly recognized entity (5), although it is clear from the literature that occasional cases have been reported previously in series of SFT. LMM are discrete circumscribed masses that may be attached to the pleura by a pedicle or may be sessile. The M:F ratio appears to be equal. Although some of the reported cases have had asbestos exposure, probably most have not, and the high proportion of tumors in women also argues against an asbestos etiology. Microscopically LMM are identical to ordinary diffuse malignant mesotheliomas (DMM) and occur in epithelial, sarcomatous, and mixed forms. The ultrastructure of the epithelial component is identical to that reported for DMM, and the immunohistochemical findings are also identical; of note is that these tumors are keratin positive and negative for the usual carcinoma markers (CEA, B72.3, etc.).The prognosis of LMM and criteria for malignancy are uncertain. Probably half the reported cases appear to have been cured by surgical excision. LMM may recur locally and may metastasize widely, but do not spread over the pleura in the fashion of DMM.
Localized sarcomas. This is somewhat of an exclusionary group, composed of nodular, pleural based, malignant appearing spindle cell tumors that are keratin negative (otherwise they would be classified as sarcomatous variants of l mm), and cd34 negative (otherwise they would be classified as MSFT). Few series exist, but some specifically differentiated localized pleural sarcomas have been reported. For example, Moran et al (6) recently reported 4 tumors that immunochemically marked as smooth muscle tumors (smooth muscle actin and desmin positive), but one was also keratin positive, raising the question of whether this was really LMM. Of interest, despite microscopic evidence of malignancy, none of the 3 tumors that could be resected has recurred at time periods up to 8 months.
Conclusions. A variety of nodular tumors can occur in the pleura. The vast majority are solitary fibrous tumors, and most of these are benign. Epithelial forms of localized malignant mesothelioma are microscopically distinctive, but the distinction between malignant solitary fibrous tumor, sarcomatous forms of localized mesothelioma, and other localized sarcomas appears to depend on immunohistochemical classifications that may or may not be reliable. From a clinical point of view, many of the pathologically malignant tumors nonetheless appear to behave in a benign fashion if they can be resected, but, other than resectability, there are no clear criteria to predict which of these malignant forms will recur or metastasize.
