Conventional osteosarcoma of the ilium masquerading as a brown tumor in primary hyperparathyroidism: a rare case report and literature review.

The coexistence of primary osteosarcoma in the setting of primary hyperparathyroidism (PHPT) is exceedingly rare, and overlapping features between reactive brown tumors and malignancies pose a severe risk of misdiagnosis and diagnostic omission. We report the case of a 65-year-old female who presented with a left hip lesion. Although her clinical manifestations, imaging features, and initial pathology all favored a PHPT-induced brown tumor, a second pathology examination confirmed the coexistence of osteosarcoma within the pelvic lesion. In patients with PHPT, reactive giant cell hyperplasia and cystic degeneration can easily mask the malignant components of osteosarcoma. When encountering "red flags", such as a solitary, extensive bone destruction that is disproportionate to parathyroid hormone (PTH) levels, clinicians must remain vigilant for a concurrent malignant bone tumor and promptly perform expanded sampling to prevent missed diagnoses. Multimodal imaging evaluations and, when necessary, repeated or expanded biopsies are also critical to avoiding misdiagnosis.