[77] Expanding the Spectrum of Dedifferentiation in Soft Tissue Tumors: Dedifferentiated Solitary Fibrous Tumor A Study of 8 Cases

JM Mosquera, CDM Fletcher. University Medical Group, Providence, RI; Brigham and Women's Hospital & Harvard Medical School, Boston, MA

Background: Dedifferentiation is a well-recognized form of tumor progression in certain types of low grade soft tissue and bone sarcoma, and confers a worse prognosis. To date, dedifferentiation has not been described in solitary fibrous tumor (SFT).
Design: Among 948 cases of intrathoracic and extrathoracic SFTs accessioned between 1988 and 2008, we identified 8 cases of primary dedifferentiated SFT. Immunohistochemistry was performed. Clinical and follow-up information was obtained from the referring pathologists and/or the treating physician.
Results: Dedifferentiated SFT occurred in three men and five women, 40 to 76 years old ( = 59 years), and measured 3.4 to 20.0 cm ( = 10.6 cm). Two cases were intrathoracic, two were in deep soft tissue of thigh, and single cases were located in omentum, scalp, retroperitoneum and abdominal wall. In addition to typical features of SFT there was abrupt transition to non-distinctive high-grade sarcoma in all cases. The latter included spindle and/or epithelioid components with increased mitotic activity, necrosis and cystic degeneration. All cases were CD34 positive in the usual SFT areas, while 5 showed loss of CD34 in the poorly differentiated component. Six of 7 cases stained for p53 and p16 showed either negative or scattered positive cells in well-differentiated SFT areas, in contrast to positive or stronger and more diffuse staining in the high-grade component. Follow-up information available in 7 patients ranged from 1 to 58 months ( = 22 months). Three patients with the largest tumors (20.0, 17.0, and 9.0 cm) died of disease after 1, 8, and 34 months of presentation. Three patients whose tumors measured less than 8.0 cm show no evidence of disease at last follow-up. One patient with an 11.5 cm intrathoracic dedifferentiated SFT is alive with disease at 58 months after recurrence and metastasis.
Conclusions: We describe, apparently for the first time, the phenomenon of dedifferentiation in SFT. Our results demonstrate that this phenomenon, as in other soft tissue tumors, poses a higher risk of tumor recurrence and/or metastasis, particularly in large and deep seated tumors. Similar to other dedifferentiated sarcomas, abrupt transition between low grade and high-grade areas is typically observed. Loss of CD34 positivity, and p53 and p16 overexpression in the high-grade component is common as in other dedifferentiated lesions, perhaps pertaining to the underlying mechanism.
Category: Bone & Soft Tissue

Monday, March 9, 2009 11:45 AM

Platform Session: Section E, Monday Morning