[466] Microphthalmia-Associated Transcription Factor (MITF) Promiscuity: Staining Patterns in Fibrohistiocytic Lesions

Navid Farahani, David Frishberg, Bonnie Balzer, Barry Catherine, Julie M Wu. Cedars-Sinai Medical Center, Los Angeles, CA

Background: Microphthalmia-associated transcription factor (MITF) is used as a marker of melanocytic differentiation in challenging cases. MITF has also been noted to focally label histiocytes, which raises the concern of fibrohistiocytic lesions being confused with malignant melanoma due to MITF staining. Because the specificity of MITF in fibrohistiocytic lesions has not been rigorously studied, we explored this topic by investigating the presence of MITF labeling in a wide range of fibrohistiocytic lesions in the skin and superficial soft tissue.
Design: Ninety four surgical pathology cases of fibrohistiocytic lesions were reviewed and the diagnoses confirmed. The cases included benign fibrous histiocytoma (BFH, 29), angiofibroma (11), fibromatosis (14), keloid (10), atypical fibroxanthoma (AFX, 7), dermal scar (9), spindle cell carcinoma (2), and dermatofibrosarcoma protuberans (DFSP, 12). Immunohistochemical staining was performed on 4-µm tissue sections using monoclonal MITF antibody (clone 34CA5) from Novocastra (Yerevan, Armenia) at 1:60 dilution. Stain intensity (1-3+), and percentage of positive cells 0, 1 (1-10%), 2 (>10-50%), and 3 (>50%) were each evaluated. Focal positive staining was defined as having either 2+ or 3+ staining intensity and >10% positive cells. Strong diffuse staining was defined as having 3+ staining intensity and >50% positive cells.
Results: Benign fibrous histiocytomas and angiofibromas showed strong diffuse staining in the majority of cases (26/29, 90%; 7/11, 64% respectively). A subset of the AFX, keloid, and fibromatosis cases also showed strong diffuse labeling (2/7, 29%; 1/10, 10%; 1/14, 7% respectively). No cases of spindle cell carcinoma, DSFP, or dermal scar showed strong diffuse staining. Focal positive staining was seen in BFH (28/29, 97%), keloid (9/10, 90%), angiofibroma (9/11, 82%), AFX (5/7, 71%), fibromatosis (10/14, 71%), dermal scar (6/9, 67%), and DSFP (1/12, 8%). No cases of spindle cell carcinoma showed focal positive labeling.
Conclusions: MITF immunostain labels a wide variety of soft tissue lesions with BFH, and angiofibroma frequently displaying strong diffuse MITF staining. Even DFSP can occasionally have focal MITF positivity. Because of the promiscuity of MITF labeling, awareness of its pattern in fibrohistiocytic proliferation can avoid potential pitfalls in the diagnosis of spindle cell lesions.
Category: Dermatopathology

Tuesday, March 5, 2013 9:30 AM

Poster Session III # 46, Tuesday Morning


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