Navigating diagnostic challenges in low-grade spindle cell lesions of the breast: a retrospective review

Low-grade spindle cell lesions of the breast (LGSCL-B) are uncommon, with malignant counterparts being particularly rare. Management recommendations for desmoid-type fibromatosis (DF), in particular, have evolved.

A retrospective review of cases from 2000 to 2024 was conducted using LGSCL-B-related keyword searches. Clinical features were assessed, and when available, core needle biopsy (CNB) findings were correlated with excision specimens and imaging. Of 68 LGSCL-B samples, DF was the most frequent subtype (28 cases, 41%). A total of six resections were negative for β-catenin expression, including three resections from cases diagnosed as DF and three resections from cases categorized as undetermined on CNB and subsequently finalized as DF on resection. Myofibroblastoma occurred in 12 patients, more often in men (58%), with lipid-rich (n = 3) and myxoid (n = 2) variants. One lesion initially diagnosed as a solitary fibrous tumour was reclassified as a palisaded myofibroblastoma. Malignancy was identified in two biopsies: a spindle cell sarcoma and a fibromatosis-like metaplastic carcinoma; the latter was reclassified at resection as a malignant phyllodes tumour with heterologous bone formation. Thirteen CNB (19%) were indeterminate, including one described with ambiguous wording, a spindle cell sarcoma with a differential encompassing both benign and malignant entities.

Most LGSCL-B can be classified on CNB, but approximately one-fifth yield indeterminate results. Diagnostic challenges include overlapping morphological features, immunohistochemical profiles and potential reclassification at resection. Awareness of these pitfalls, along with selective use of adjunct molecular testing, may enhance diagnostic accuracy and reduce overtreatment.