Atypical and malignant solitary fibrous tumors in extrathoracic locations - Evidence of their comparability to intra-thoracic tumors

Solitary fibrous tumor (SFT), first described as a pleural lesion, has been reported at numerous extrathoracic sites over the past 10 years. About 10% to 15% of intrathoracic SFTs are histologically or clinically malignant, b ut such cases have very rarely been described at other locations. Among 92 cases of extrathoracic SFT in our files, we identified 10 that either had r ecurred (2 cases) or had a least one atypical histologic feature (8 cases). The ten tumors occurred in five men and five women, 32 to 81 years old (me dian 56), measured 1.9 cm to 20 cm (median 11.5 cm), and were located in th e abdomen/pelvis (4 cases), retroperitoneum (3 cases), groin, trunk, and up per arm. Nuclear atypia (8 cases), markedly increased cellularity (6 cases) , areas of necrosis (4 cases), and greater than 4 mitoses/ 10 HPFs (3 cases ) were seen in addition to the typical histologic features of SFT. Six tumo rs had at least two of these atypical histologic features. Nine cases were positive for CD34, six were positive for O-13, and one was focally positive for smooth muscle actin. Eight were excised completely. Subsequent follow- up revealed tumor relapse in eight cases (follow up 6-180 months, median 24 ). Four patients had local recurrence at 12 to 168 months. Distant metastas is developed at 1 to 6 years in five cases with spread to lung (2 cases), l iver (4 cases), and bone. Metastasis or local recurrence developed within 2 years in five patients. To date, no patient has died of their tumor. These findings demonstrate that nuclear atypia, hypercellularity, greater than 4 mitoses/10 HPFs, and necrosis may be seen in up to 10% of extrathoracic SF Ts, and are associated with, but are not by themselves predictive of, aggre ssive clinical behavior. In addition, our findings confirm that the behavio r of extrathoracic SFTs is unpredictable, entirely comparable to that of th eir better known pleural counterparts, and confirm that patients with SFTs in all locations require careful, long-term follow up. It is probably unwis e to regard any such lesion as definitely benign.