Fibrous dysplasia: an unusual case of a very aggressive form with costo-vertebral joint destruction and invasion of the contralateral D7 vertebral body.

Fibrous dysplasia (FD) is a benign fibro-osseous disease of the bone that may be solitary or multicentric. It is important to distinguish this type of lesion from low-grade osteosarcomas (LGOS) and from secondary sarcomas, because malignant transformation has rarely been reported. It is classically described as having a ground-glass appearance, endosteal scalloping, and thinning of the cortex. Cortical disruption is considered evidence of malignancy, but it can also be present in benign FD with aggressive behavior. We present an unusual case of aggressive FD of the 7th left rib, already diagnosed more than 22 years ago, where cortical and costo-vertebral joint disruption and 7th thoracic vertebral body involvement were not evidence of malignant behavior. From a histological perspective, FD and LGOS are similar; even if histology is of fundamental importance, the diagnosis has to be made based on the clinical and radiological aspects as well, although at imaging, differentiation between FD and LGOS can be difficult. In the present case, even though the histological examination suggested a benign lesion, the radiological examination instead consistently suggests malignancy. It is for this reason that there should be a high index of suspicion during follow-up and a new biopsy should be scheduled in case any changes occur during follow-up.