Cauda Equina Syndrome Caused by Metastatic Sarcomatoid Carcinoma of Unknown Primary.

Sarcomatoid cancer of unknown primary site is an aggressive and rare clinical entity associated with poor patient outcomes. We describe a case of a 69-year-old woman who presented with low back pain, right leg weakness, urinary retention, and weight loss. Spinal imaging revealed a soft tissue mass at the right sacral ala infiltrating the S2-S4 neural foramina, with multifocal marrow replacement of the lumbosacral spine and adjacent bilateral iliac bones. Metastatic workup revealed additional lesions of the cervicothoracic spine, left proximal tibia and distal femur, and bilateral pulmonary nodules. Needle biopsy of the sacral mass was consistent with spindle cell carcinoma, a subtype of sarcomatoid carcinoma. IHC staining was positive for GATA3, suggesting mammary or urothelial origin. However, random bladder biopsies obtained at cystoscopy were negative for malignancy and no other discernable primary tumor was identified on full body imaging. This case highlights a rare histologic type of cancer - sarcomatoid carcinoma of unknown primary site (SCUP) in a patient that presented with cauda equina syndrome. Our patient was offered palliative radiation for symptom relief but elected to pursue hospice after the first radiation session. She passed away at a hospice facility within a few months after discharge from the hospital.