Caught on Second Look: A Rare Case of Cutaneous Leiomyosarcoma of the Breast at the Crossroads of Rarity and Mimicry

Abstract

Cutaneous leiomyosarcoma (CLMS) is a rare smooth muscle malignancy of the skin, constituting 2–3%
of cutaneous soft tissue sarcomas and representing an exceedingly small fraction, approximately 0.04%,
of all reported neoplasms. Among cutaneous sarcomas, CLMS ranks third in frequency, following
dermatofibrosarcoma protuberans and Kaposi's sarcoma. We present a rare and instructive case
involving a 54-year-old female who developed a slowly enlarging, painful lesion on the left breast skin,
just above the nipple-areola complex. Initial clinical evaluation and investigations, including
mammography, fine-needle aspiration cytology (FNAC), and excisional biopsy, misidentified the lesion
as a benign spindle cell neoplasm, likely myofibroblastoma. However, the lesion recurred within four
months, prompting a wide local excision. Histopathological analysis of the excised specimen revealed
an infiltrative tumor consisting of atypical spindle cells with pleomorphic nuclei and an increased mitotic
rate. Immunohistochemical staining confirmed the diagnosis of cutaneous leiomyosarcoma through
positivity for smooth muscle actin, desmin, and h-caldesmon. Ki-67 labelling index was 40%, indicating
a high proliferative index. However, the wide local excision also further required a revision surgery for
margin clearance. The case highlights the diagnostic challenges posed by CLMS, especially when
clinical and histopathological features mimic benign entities. Accurate diagnosis relies on
comprehensive histopathological and immunohistochemical analysis. Wide local excision with negative
margins remains the cornerstone of treatment, as radiotherapy and chemotherapy are generally
considered ineffective. Given the high recurrence rate and metastatic potential of CLMS, vigilant long-
term follow-up is essential.