Atypical Parathyroid Tumor Causing Primary Hyperparathyroidism With a Clinical Presentation of a Brown Tumor.

Brown tumors are rare osteolytic bone lesions linked to primary hyperparathyroidism (pHPT), caused by excessive parathyroid hormone (PTH) production. They feature microfractures, hemorrhage, and hemosiderin deposition. Atypical parathyroid tumors (APT) are uncommon parathyroid neoplasms with histologic features resembling carcinoma but without definitive parathyroid carcinoma (PC) criteria. APTs can rarely lead to brown tumors. We report a 53-year-old male with a history of nephrectomy for renal cell carcinoma and Hodgkin disease. Imaging revealed a growing osteolytic lesion in the left iliac bone. Laboratory findings indicated pHPT with elevated PTH levels. Sonography and 18F-fluorocholine positron emission tomography-computed tomography (PET-CT) identified a right inferior parathyroid tumor. The patient underwent focused parathyroidectomy, and histology confirmed APT with fibrous tracts and trabecular growth features. APT accounts for 0.5% to 4.4% of parathyroid surgeries and may rarely lead to brown tumors. Differentiating brown tumors from malignancies requires biochemical, imaging, and histopathological evaluation. Although APTs generally follow a benign course, long-term surveillance is essential, particularly in familial cases. Further research is needed to investigate the malignancy potential and recurrence risk of rare atypical parathyroid neoplasms. This case highlights the importance of a multidisciplinary approach in their diagnosis and management.