Aneurysmal Bone Cyst and Telangiectatic Osteosarcoma: Diagnostic Boundaries Illustrated by a Clinical Case

29-year-old female patient presented with pain and progressive limitation of function in her left knee. Imaging data revealed an expansive cystic-destructive lesion with cortical thinning in the distal metaphysis of her left femur. Aneurysmal bone cyst and telangiectatic osteosarcoma were included in the differential diagnosis due to the similarity of the cystic-hemorrhagic structure and destructive nature of the lesion. Following a standard imaging examination, a biopsy was performed. Microscopic examination revealed fragments of a giant cell fibrous-osteogenic lesion with cystically dilated cavities containing blood and debris. Septa consisted of fibrous tissue with areas of reactive bone formation; osteoclast-like giant cells were unevenly distributed. No morphological features of telangiectatic osteosarcoma were detected: tumor osteoid, cytological atypia, pathological mitoses, and necrosis were absent. The diagnosis was an aneurysmal bone cyst based on morphological, clinical, and radiographic data. Molecular testing for USP6 rearrangement was not performed.