The patient is a 79-year-old woman with a history of arterial hypertension and cardiac arrhythmia, who comes to our centre after noticing a non-painful tumour in the left breast, which she refers to as having developed approximately one month ago. On examination, a solid, hard, poorly demarcated, fixed tumour of approximately 4 cm in diameter adhering to deep planes was palpated in the inner-inner quadrant of the left breast.
A mammographic study of both breasts was performed without being able to clearly appreciate the mass as it was in a deep plane. The ultrasound study revealed a hypoechogenic and heterogeneous mass measuring approximately 28 x 30 mm with signs of malignancy, irregular borders and posterior acoustic shadow.
A clinical tru-cut microbiopsy of the lesion was performed and histology showed a malignant infiltrative neoformation of fusocellular character surrounding non-neoplastic acinar ductal structures.

Malignant spindle cell neoplasms of the breast raise the differential diagnosis mainly between metaplastic spindle cell carcinomas and sarcomas, so complete excision of the lesion is indicated and the definitive diagnosis is deferred to histological study of the lesion.
Given the patient's age and after assessing her wishes and the therapeutic possibilities, surgical treatment was decided, and an infero-medial quadrantectomy was performed without axillary lymphadenectomy.
The histological size of the neoplasm is 5 x 4.8 x 3.5 cm, with infiltration of the skin and skeletal muscle and microscopically shows an infiltrating neoplastic proliferation made up of spindle cells arranged in irregular fascicles with abundant areas of necrosis and variable cell density. The neoplastic cells show marked nuclear atypia, hyperchromasia and abundant mitoses. In none of the areas studied is neoplastic ductolobulillar epithelium visible or forming part of the lesion and the few ducts observed appear to correspond to non-neoplastic structures infiltrated by the tumour.

In the immunohistochemical study the neoplastic cells are strongly positive for keratins AE1-AE3, cytokeratin 22, vimentin, smooth muscle actin, Cerb-2 and CD10 and negative for keratin 903, EMA, cytokeratin 7, CD31, CD34, p63, total actin, desmin, myosin, CD68, protein S-100 and HMB45. Hormone receptors for oestrogen and progesterone are negative.

Based on all these data, a diagnosis of metaplastic spindle cell carcinoma was established.
The patient showed no signs of residual tumour and adjuvant radiotherapy treatment was decided at a dose of 60 Gy, at 2 Gy per fraction, in 5 fractions per week.
The patient is currently asymptomatic, two months after the end of treatment.