Fibromatosis of the Breast: Diagnostic Accuracy of Core Needle Biopsy.

Objectives: Fibromatosis of the breast is an uncommon neoplasm with potential for local recurrence. Treatment has traditionally been surgical excision with current trends toward conservative management. Given the option of observation after diagnosis by core needle biopsy (CNB), we sought to evaluate the accuracy of CNB for diagnosing fibromatosis.

Methods: We identified a total of 31 cases in which fibromatosis had been diagnosed or included in the differential diagnosis on a CNB, an excision, or both. Morphology and immunohistochemical results were reviewed.

Results: Aberrant nuclear immunoreactivity for β-catenin and absent staining for CD34 were the most useful studies to diagnose fibromatosis, and one or both were performed in 21 (68%) cases. High molecular weight cytokeratins and p63 were helpful to exclude spindle cell carcinoma. Of 26 cases confirmed as fibromatosis on excision, 22 (85%) were diagnosed as fibromatosis or fibromatosis was favored in the differential diagnosis on CNB. More frequent use of immunohistochemistry would likely have resulted in a greater number of definitive diagnoses. Fibromatosis was rarely mistaken for other nonmalignant stromal lesions, with no cases misdiagnosed as carcinoma.

Conclusions: CNB can be an accurate method of diagnosing fibromatosis, allowing observation for a select group of patients.