History
    A 34-year old man was admitted due to left chest pain for 3 days. The patient had a history of valve replacement surgery owing to mitral stenosis 8years ago.

Findings
    Chest radiograph shows huge lobulated mass in the left lower lobe, which is broadly based on left diaphragm. Left lower cardiac border is straight, suggesting left lower lobe collapse. Some pleural effusion is noted. CT scans show a lobulated round mass in the left lower lung zone, which compresses the left lower lobe medially. The mass is composed of two components. One is homogeneous low attenuating, solid area, which is highly enhanced after the contrast infusion. The other is cystic portion, which is heterogeneous high attenuation on pre-contrast scan and is low attenuation on contrast enhanced scan, suggestive of cyst with hemorrhage. Ultrasonography (sagittal scan of the left lower chest) demonstrates the two components of the mass more clearly, showing the echogenic solid portion and peripheral hypo or anechoic cystic portion. The mass floats within the pleural effusion and separates from the diaphragm.

Discussion
    Solitary fibrous tumors are rare neoplasms that most commonly involve the pleura, mediastinum, and lung. They are believed to be submesothelial in origin. Histologically, fibroblast-like cells and connective tissue in varying proportions characterize them. The “patternless pattern” and the hemangiopericytoma-like pattern are the most common arrangements. Localized primary pleural neoplasm have been known by a variety of names in the literature; localized fibrous tumor of the pleura, benign fibrous mesothelioma, benign fibrous tumor of the pleura. Clinically solitary fibrous tumor of the pleura occurs in both sexes and in all age groups but predominantly affects persons more than 50 years old. Approximately 50 % of patients are asymptomatic, the lesion being discovered incidentally on a routine chest radiograph. In the remaining patients, the most frequent manifestations are chest pain, cough, and dyspnea, which are present in 40 % of symptomatic patients. Extrathoracic manifestations are frequent and include hypertrophic pulmonary osteoarthropathy in 4-35% and hypoglycemia. Hypoglycemia is usually associated with very large tumors and is noted in less than 5% of patients.
    The usual appearance of solitary fibrous tumor of the pleura is that of a solitary mass along the lung margin in the lower or middle chest. Calcification is relatively uncommon, but can occur. Pleural effusion can be seen, usually with larger lesions. When the tumor is bulky, its pleural origin may not be obvious; in these cases, aortography demonstrating arterial supply from the inferior phrenic, intercostal, or internal mammary arteries can establish the extrapulmonary origin of the mass. Pedunculated tumors can change in appearance with inspiration-expiration or with changes in patient positioning; such mobility is virtually pathognomonic. Despite their pleural origin, solitary fibrous tumors of the pleura often from acute angles of interface with adjacent chest wall structures, possibly because the pedicle permits the mass to impinge on adjacent lung parenchyma.
    On computed tomography, solitary fibrous tumors of the pleura present as well delineated, smooth, lobulated, usually noncalcified masses abutting a pleural surface. Crural thickening has been reported. Chest wall invasion is infrequent, but can occur. The tumors are usually elongated and somewhat lenticular in shape. As with plain films, the angle of interface between the mass and the chest wall is often not reliable in predicting the pleural origin of the mass. Dedrick et al. found that a gradually tapering contour of the mass, and its displacement (as opposed to invasion) of adjacent lung were more reliable sign of its pleural origin. Small tumors are usually homogeneously enhanced. Tubular or round low attenuation areas can be seen in larger lesions and represent cystic degeneration. Solitary fibrous tumors of the pleura exhibit low or intermediate signal on both T1- and T2-weighted magnetic resonance imaging sequences. This is likely due to the high content of fibrous, collagenous tissue with relatively few mobile protons. Most other pleural tumors demonstrate high signal on T2-weighted sequences, in contrast to the low or intermediate signal seen with solitary fibrous tumors of the pleura. The differential diagnosis includes solitary pleural metastasis, pleural lipoma, pleural fibrosarcoma, intercostal nerve neurilemoma, organized inflammation, and most important, peripheral bronchogenic carcinoma.

References
1. Dresser TS, Stark P. Pictorial essay: solitary fibrous tumor of the pleura. Journal of thoracic imaging 1998; 13: 27-35.
2. Feretti GR, Chiles C, Choplin RH, Coulomb M. Pictorial essay: localized benign fibrous tumors of the pleura. AJR 1997; 169: 683-686.
3. Feretti GR, Chiles C, Cox JE, Choplin RH, Coulomb M. Localized benign fibrous tumors of the pleura: MR appearance. JCAT 1997; 21: 115-120.
4. Lee KS, Im J, Choe KO, Kim CJ, Lee BH. CT findings in benign fibrous mesothelioma of the pleura: pathologic correlation in nine patients. AJR 1992: 158: 983-986.