Postoperative thoracic paraspinal desmoid tumor: illustrative case.

backgroundDesmoid-type fibromatosis is a rare soft tissue tumor characterized by invasive growth within deep connective tissue without metastasis. The etiology is largely unknown, but several cases have been reported in the postoperative setting. The management is multidisciplinary and largely dependent on individual presentation. Here the authors report a postoperative case of a rapidly progressive and symptomatic paraspinal desmoid tumor (DT). OBSERVATIONS: A 51-year-old female with a history of familial adenomatous polyposis and a previous L3-5 decompressive laminectomy and fusion presented to the neurosurgery clinic with complaints of a painful left posterior flank mass. Imaging disclosed a left posterior thoracic soft tissue mass originating at the T8-9 level and extending through the T11-12 level, suggestive of sarcoma. Pathological analysis from an open biopsy specimen confirmed desmoid-type fibromatosis. The patient underwent total resection. At 5 months postoperatively, she reported a progressive resolution of her symptoms without recurrence. LESSONS: Paraspinal DTs are a rare occurrence that may be de novo in origin or arise following spinal surgery. These tumors have a predilection for middle-aged females and may present in any region of the spine. A biopsy is needed for definitive diagnosis. Management may be active surveillance, medical management, or surgical management. https://thejns.org/doi/10.3171/CASE25573.