EXPRESS: Suspected recurrent sarcoma resembling feline injection-site sarcoma with spinal cord invasion in a cat.

Case summary: A six-year-old neutered male domestic shorthair cat was referred for evaluation of a recurrent subcutaneous mass on the right dorsocaudolateral hip, four months after excision of a presumed feline injection site sarcoma (FISS). Advanced imaging revealed a bilobed soft tissue mass closely associated with the sciatic nerve and rectum. Surgical excision was limited by adjacent structures, and histopathology confirmed a sarcoma with incomplete margins. Definitive-intent radiation therapy was administered postoperatively. Follow-up imaging initially demonstrated no evidence of local recurrence. Approximately nine months after completion of radiation therapy, the cat developed progressive pelvic limb paraparesis, fecal incontinence, and urinary retention. Magnetic resonance imaging identified a large contrast-enhancing extradural spinal lesion extending from L5 to the sacrum, with suspected intramedullary invasion and marked thickening of the lumbosacral nerve roots. Surgical exploration with hemilaminectomy and spinal cord biopsy revealed a poorly differentiated sarcoma histologically similar to the original tumor. Progressive disease was confirmed on follow-up imaging, and palliative radiation therapy was administered. Despite treatment, the cat developed severe neurologic deterioration and was humanely euthanized. Relevance and novel information: This report describes the first documented case of spinal cord invasion secondary to a FISS-like fibrosarcoma without local recurrence at the primary site. Perineural invasion along the sciatic nerve is proposed as a biologically plausible mechanism of tumor extension. This case highlights an uncommon but severe pattern of disease progression and underscores the importance of considering perineural spread in incompletely excised FISS-like tumors associated with major peripheral nerves.