Answer: Solitary fibrous tumor (pleural fibroma)

Histology: Histologic features of localized fibrous pleural tumor are somewhat variable but in most instances areas of low cellularity alternate with more cellular areas. The hypocellular areas consist of loosely arranged spindle or oval cells scattered among strands of dense collagen. Often there is a wire-like pattern of spindle cells, which often form interdigitating fascicles. Cleft-like spaces lined by low cuboidal (entrapped respiratory) epithelium are often seen around the border of the neoplasm. Alternating cellular and hypocellular areas may produce a pattern somewhat suggestive of schwannoma. This particular lesion was large and had areas of necrosis, but only rare mitotic figures and no pleomorphism.

Discussion: The lesion we now know as localized fibrous tumor of the pleura has been described in the earlier literature as localized or benign fibrous mesothelioma. It is now, however, clear that these lesions are not of mesothelial origin. They appear to represent pleural-based neoplasms that may arise from some underlying stromal cell in the pleura. In contrast to mesothelioma, the constituent tumor cells of localized fibrous tumor are negative for cytokeratin and are strongly positive with antibodies raised against CD34. There is no convincing epidemiological evidence of an association between localized fibrous tumor of the pleura with asbestos exposure, and the use of the term “mesothelioma” is inappropriate and should thus be discouraged. These neoplasms can arise in either the visceral or parietal pleura and range in size from 1 to over 30 cm in diameter (average 6 cm). Many of the tumors are incidentally discovered, but patients may present with symptoms of chest pain, cough and/or dyspnea. Pulmonary osteoarthropathy, digital clubbing, and fever have also been reported as clinical manifestations. Large tumors may give rise to pleural effusions or hypoglycemia. There is a spectrum of clinical behavior associated with localized fibrous tumors. Some are clinically malignant. Histologic features associated with malignancy include excess mitotic activity (> than 4 mitoses per10 hpf), cellular pleomorphism, hemorrhage and necrosis. In one series, however, 45 percent of tumors deemed to be malignant histologically were cured by simple excision, suggesting perhaps that resectability was the most important indicator of clinical outcome. Other factors associated with malignant behavior include origin of the tumor from the parietal and mediastinal pleura as well as large size. The immunohistochemical differences between fibrous pleural tumor and desmoplastic or sarcomatoid mesothelioma include the growth pattern and the immunohistochemical differences noted above. Fibrous pleural tumors are typically positive for CD34 and negative for cytokeratins, in contrast to mesothelioma. Similarly, the biphasic pattern may mimic schwannoma; however, the absence of S-100 protein staining in the tumor cells and the presence of CD34 positivity may assist in diagnosis. Malignant fibrous pleural tumor (FPT) can frequently be distinguished from other sarcomas by extensive examination, which usually reveals the more recognizable hypocellular areas common to benign FPTs.