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Malignant Fat-Forming Solitary Fibrous Tumor (so-called “Lipomatous Hemangiopericytoma”): Clinicopathologic Analysis of 14 Cases

Lee, Jen-Chieh MD*,†; Fletcher, Christopher D.M. MD, FRCPath

American Journal of Surgical Pathology: August 2011 - Volume 35 - Issue 8 - p 1177–1185
doi: 10.1097/PAS.0b013e318219cd0b
Original Articles

Fat-forming solitary fibrous tumor is a rare variant of solitary fibrous tumor (SFT). Generally regarded as benign, very few fat-forming SFTs with malignant histologic features have been reported. Here, we report 14 histologically malignant fat-forming SFTs to better characterize this subset. Seven patients were female and 7 were male, with ages ranging 20 to 93 years (median, 57 y). Five tumors were located in the lower limb, 3 in the trunk, 3 in abdominopelvic locations, 2 in the head and neck region, and 1 in the upper limb. The tumor size ranged from 3.4 to 20 cm (median, 8.6 cm). Histologically, all exhibited at least focal hypercellularity; 12 tumors had mitoses >4/10 high-power fields (range, 2 to 37; median, 8), 12 showed at least moderate atypia, and 8 showed necrosis. It should be noted that 7 tumors contained only mature adipose tissue, whereas 5 contained multivacuolated lipoblasts and 2 had areas resembling atypical lipomatous tumor (ALT). Immunohistochemically, CD34 and CD99 were positive in most cases (11 of 14 and 8 of 10, respectively); MDM2 and CDK4 were both negative in all 4 cases tested (including both tumors with ALT-like areas). Follow-up data from 10 cases (median duration, 47.5 mo; range, 5 to 76) showed 2 patients with multiple metastases (both to lung and bones, and 1 each to breast and to soft tissue), both of whom died of disease. In conclusion, fat-forming SFTs exhibiting malignant histologic features have potential for aggressive behavior. The presence of lipoblasts and/or ALT-like areas, although described in some “benign” examples of fat-forming SFT, seems much more common in the malignant subset and may prompt a careful search for morphologic evidence of malignancy in any case of fat-forming SFT.

*Department of Pathology, National Taiwan University Hospital, Medical College, National Taiwan University, Taipei, Taiwan

Department of Pathology, Brigham and Women's Hospital, Harvard Medical School, Boston, MA

Correspondence: Christopher D.M. Fletcher, MD, FRCPath, Department of Pathology, Brigham and Women's Hospital, 75 Francis Street, Boston, MA, 02115 (e-mail: cfletcher@partners.org).

© 2011 Lippincott Williams & Wilkins, Inc.