Locally Aggressive Desmoid Tumor With L4-L5 Vertebral Invasion and Progressive Neurological Deficit: A Case Report.

Desmoid-type fibromatosis is a rare, non-metastatic fibroblastic neoplasm that may cause major morbidity through locally infiltrative growth. Spinal involvement with vertebral destruction and neurological deficit is particularly uncommon and may mimic malignant spindle-cell or primary osseous tumors. We report a 59-year-old woman with severe low back pain, progressive right lower-limb weakness, sensory loss, swelling, and functional decline. Imaging showed a markedly enlarging right retroperitoneal-pelvic mass with invasion and partial destruction of the L4-L5 vertebral bodies. Although initially diagnosed as spindle cell sarcoma, histopathology, strong nuclear β-catenin positivity, and CTNNB1 mutation analysis confirmed desmoid-type fibromatosis. A pathogenic APC variant was also detected, raising the need to consider familial adenomatous polyposis evaluation. The patient underwent posterior decompression and long-segment stabilization for neural compromise and spinal instability. This case highlights the severe local morbidity possible despite non-metastatic histology.