The most frequent benign bone tumor, known as a giant cell tumor (GCT), typically develops in the second and third decades of life. GCTs of the bone that have already been diagnosed and have already undergone treatment with denosumab therapy, curettage or excision, or radiotherapy frequently develop malignant transformation. A very uncommon occurrence involves a GCT of the bone that has always been malignant. Here, we describe the case of a 25-year-old man with a large cell tumor of the distal femur discovered after six months of symptom onset. The MRI suggested an aneurysmal bone cyst or subsequent modifications of an aneurysmal bone cyst in a GCT. A biopsy was performed, and the results pointed to a benign GCT of the bone. There were no pleomorphic or hyperchromatic lesions, unusual mitoses, or cellular atypia. The patient was treated with tumor removal and internal fixation using plate osteosynthesis two weeks later. The samples were sent for histopathology. The report was suggestive of osteogenic sarcoma or malignant transformation of the giant cell. This could happen due to the possibility of a biopsy sample being taken from an area not representative of the tumor site, which is not uncommon since the osteosarcoma also contains areas of conventional GCT. Thus, osteosarcoma usually mimics conventional GCT of the bone.
Keywords: aneurysmal bone cyst; denosumab; giant cell tumour; malignant transformation; osteogenic sarcoma.
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