A Challenging Case of Locally Aggressive Polyostotic Fibrous Dysplasia

Fibrous dysplasia (FD) is a benign lesion observed mainly in children and young adolescents. Although benign, it can sometimes be challenging to discriminate these lesions from well-differentiated malignant tumours. Here, we describe a confusing case of asymptomatic locally aggressive polyostotic FD in a 49-year-old woman followed for a molar pregnancy. The case was particular due to the rare localization of FD in the ribs and vertebrae, and it posed a diagnostic dilemma with parosteal osteosarcoma. This case highlights the difficulty of distinguishing FD from a well-differentiated malignant tumour, especially when facing artefacts arising from the decalcification procedure.