Nuclear β-catenin expression distinguishes deep fibromatosis from other benign and malignant fibroblastic and myofibroblastic lesions

Deep fibromatoses (desmoid tumors) are clonal myofibroblastic proliferations that are prone to aggressive local recurrences but that do not metastasize. They must be distinguished from a host of fibroblastic and myofibroblastic lesions as well as from smooth muscle neoplasms. Virtually all deep fibromatoses have somatic β-catenin or adenomatous polyposis coli (APC) gene mutations leading to intranuclear accumulation of β-catenin. Since low-grade sarcomas in general lack β-catenin and since reactive proliferations would not be expected to have it, we predicted that nuclear β-catenin expression would be detected in deep fibromatoses but absent in other entities in the differential diagnosis. We evaluated the role of β-catenin to help differentiate distinguish deep fibromatoses from congeners. Formalin-fixed, paraffin-embedded sections from 21 lesions from 20 patients with deep fibromatoses were stained with monoclonal β-catenin antibody (Transduction Laboratories) and compared with low-grade fibromyxoid sarcoma (n = 12), leiomyosarcoma (n = 10), various other fibrosarcoma variants (n = 13, including 3 myofibrosarcomas, 3 sclerosing epithelioid fibrosarcomas, 5 low-grade fibrosarcomas, 1 classic fibrosarcoma arising in dermatofibrosarcoma protuberans, 1 inflammatory myxohyaline tumor/myxoinflammatory fibroblastic sarcoma), myofibroma/myofibromatosis (n = 12), nodular fasciitis (n = 11), and scars (n = 9). Nuclear and cytoplasmic staining was assessed. All 21 examples of deep fibromatosis displayed nuclear β-catenin (focal nuclear staining in one case to 90% staining). All other lesions tested (n = 67) lacked nuclear labeling for β-catenin, showing only cytoplasmic accumulation. β-Catenin immunohistochemistry separates deep fibromatosis from entities in the differential diagnosis, a finding that can be exploited for diagnosis. Most fibromatoses have diffuse nuclear staining although occasional examples only focally label.