Progressive Pelvic and Gluteal Desmoid Tumor in a 16-Year-Old Female: A Five-Year Multimodal Imaging Follow-Up.

Desmoid tumors (desmoid-type fibromatosis) are rare, locally aggressive fibroblastic neoplasms that lack metastatic potential but grow infiltratively and follow an unpredictable course; pelvic tumors in adolescents are especially uncommon and difficult to manage. We report the case of a 16-year-old adolescent female who presented with right gluteal swelling in 2021; biopsy confirmed desmoid-type fibromatosis (Ki-67 index, 5%-10%; APC-negative). Serial MRI, fluorine-18 fluorodeoxyglucose PET/CT (18F-FDG PET/CT), and CT performed from 2024 to 2026 documented progressive enlargement of a right gluteal mass and a large presacral pelvic mass despite sequential treatment with imatinib, sorafenib, and off-label nirogacestat. Mass effect caused bilateral hydroureteronephrosis and hematocolpos, which were managed with percutaneous nephrostomies, image-guided drainage, and bladder catheterization; the most recent imaging showed stable disease on nirogacestat. This case illustrates the refractory behavior of adolescent pelvic desmoid tumors and the central role of longitudinal multimodal imaging in tracking progression and detecting complications amenable to interventional management. It adds to the limited experience with off-label nirogacestat in adolescents, for whom prospective pediatric data remain limited.