Check the Calcium first in any Bone Lesion: Brown Tumour of the Talus Misinterpreted as Giant Cell Tumour.

Introduction: Although the vast majority of bone tumours are benign, they are a heterogeneous group of lesions with a wide spectrum of biological properties. Osteolytic, osteoblastic, or mixed-pattern changes may be observed, reflecting the heterogeneity of the underlying pathophysiological processes. Osteolytic lesions, most commonly arising from metastatic disease, multiple myeloma, or hyperparathyroidism, are the most frequently encountered presentations in clinical practice. Case presentation: Here, we report the case of a 59-year-old woman with a 3-month history of right ankle pain, in whom radiological evaluation revealed an osteolytic talar lesion initially interpreted as a giant cell tumour (GCT) and surgically excised. During postoperative oncological evaluation, a parathyroid adenoma was detected, and the patient was referred to our endocrinology clinic. Laboratory results were consistent with primary hyperparathyroidism. Re-evaluation of the bone pathology, in light of the biochemical and radiological findings and the absence of H3F3A immunostaining, supported the diagnosis of a brown tumour (BT). Following parathyroidectomy, laboratory results normalized. Discussion: Since hypercalcemia is not typically observed in GCTs, its presence should raise suspicion for metastatic or granulomatous diseases, as well as BTs associated with hyperparathyroidism. Upon detection of hypercalcemia, parathyroid hormone (PTH) levels should be evaluated as part of the differential diagnosis. Subsequent histopathological and immunohistochemical analyses can help distinguish these entities, with H3F3A immunostaining serving as an important marker for differentiating GCTs from BTs. Conclusion: This case highlights the importance of assessing serum calcium and parathyroid hormone levels, together with relevant clinical, radiological, and histopathological findings, to establish an accurate diagnosis in patients presenting with bone lesions. LEARNING POINTS: Serum calcium and parathyroid hormone measurements are essential in the evaluation of bone lesions. Hypercalcemia may indicate primary hyperparathyroidism but can also occur in myeloma, metastases, or granulomatous diseases.Brown tumours (BTs) and giant cell tumours (GCTs) of bone may overlap; BTs are associated with hypercalcemia and H3F3A negativity, while GCTs are characterized by normal calcium levels and H3F3A positivity.A multidisciplinary approach combining clinical, biochemical, and pathological data improves diagnostic accuracy in bone lesions by distinguishing benign, malignant and metabolic causes, ensuring appropriate management.