[58] Malignant Fat-Forming Solitary Fibrous Tumor: Clinicopathologic Analysis of 14 Cases.

Jen-Chieh Lee, Christopher DM Fletcher. National Taiwan University Hospital, Taipei, Taiwan & Brigham and Women's Hospital, Harvard Medical School, Boston, MA

Background: Fat-forming solitary fibrous tumor (FF-SFT) is a rare variant of solitary fibrous tumor (SFT) containing variable amounts of usually mature fat. Generally deemed benign, FF-SFTs were very rarely reported to have malignant histologic features, at least one case of which recurred and proved fatal. Here we report 14 histologically malignant FF-SFTs to better characterize this subset.
Design: Of 81 FF-SFTs in our consultation files, 14 exhibited malignant features (mitoses ≥ 4/10HPF, hypercellularity, atypia, necrosis, and/or infiltrative margins, according to WHO criteria for malignant SFT) and were retrieved for analysis. The histology and immunostains were reviewed, and the clinical parameters and follow-up data were obtained from referring physicians.
Results: Seven patients were female and 7 male, with ages ranging from 20 to 93 years (median 57 years.) Most presented with a slowly enlarging mass, some with recent increase in growth or local discomfort. Five (36%) were located in lower limbs, 3 (21%) in trunk, 3 (21%) in abdominopelvic locations, 2 (14%) in the head or neck, and 1 (7%) in the upper limb. Grossly, the tumor size ranged from 3.4 to 20 cm (median 8.6 cm), all were well-demarcated, and most were yellow-white to pink / tan and rubbery. Histologically, all exhibited at least focal hypercellularity; 12 tumors had more than 4 mitoses /10HPF (range 2-37, median 7.5); 11 showed at least moderate atypia (focal or diffuse); 9 showed necrosis; 1 had focal infiltrative margin. All had variable amount of mature adipose tissue. Of note, 5 contained multivacuolated lipoblasts and 2 had adipocytes of varying size with atypical stromal cells, resembling atypical lipomatous tumor (ALT), which were not observed in the other 67 FF-SFT in our files. By immunohistochemistry, CD34 was positive in 11 of 14 cases tested, CD99 positive in 8 of 9, and MDM2 and CDK4 were both negative in all 3 tested (including both with ALT-like areas.) Follow-up data thus far (9 cases, median duration 55 months, range 2.5-76) showed no local recurrence and 1 patient with metastasis (lung, spine, and bilateral breasts) who died of disease at 57 months.
Conclusions: FF-SFT exhibiting malignant histologic features shows potential for aggressive behavior comparable to conventional malignant SFT. Presence of lipoblasts and/or ALT-like areas seems to be much more common in the malignant subset and may imply concurrent malignant change of both spindled and adipocytic components.
Category: Bone & Soft Tissue

Monday, February 28, 2011 1:00 PM

Poster Session II # 8, Monday Afternoon

 

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