Malignant phyllodes tumour with spinal metastasis presenting as cauda equina syndrome.

Malignant phyllodes tumours (MPTs) are rare fibroepithelial breast neoplasms. While local recurrence is relatively common, distant metastasis is uncommon. Spinal metastases are exceptionally rare, with only a handful of cases in the literature. This case describes a female in her early 50s with a prior diagnosis of borderline phyllodes tumour status post right mastectomy and adjuvant radiotherapy. Two years post treatment, she presented with progressive lower back and bilateral leg pain, later identified as severe spinal canal compression causing cauda equina syndrome secondary to an L4 vertebral lesion. A decompression with biopsy was performed, followed by a two-stage L4 vertebrectomy, cage reconstruction, and instrumented L2-S1 spinal fusion. Although histopathology was inconclusive, molecular profiling confirmed that the spinal lesion was metastatic MPT. This case underscores the aggressive potential of MPTs and the importance of considering spinal metastases in the differential diagnosis of patients with back pain and a history of phyllodes tumour.