Solitary fibrous tumor of the mediastinum.

A 62-year-old woman presented with an asymptomatic anterior mediastinal mass. Clinically considered to be a thymoma, the tumor was solid, firm, and composed of hypocellular dense collagen and cytologically bland, spindled fibroblastlike cells growing in a patternless pattern. There was no apparent connection to pleura or pericardium, yet the clinicopathologic features clearly fit with solitary fibrous tumor (SFT) of mediastinum. SFTs occur most commonly in pleura but have been reported in other locations, including the mediastinum, where aggressive behavior has been more common when these tumors are compared to those occurring in pleura. Although it is difficult to predict behavior for all cases of SFT occurring in the mediastinum by cytologic features alone, morphologic criteria for benign and malignant forms have been described. Roughly half the malignant forms will progress, yet the single most important indicator of clinical outcome is whether the tumor can be initially totally excised.