Fibroxanthomas are common lytic osseous lesions of childhood. Fibroxanthomas can be divided into two different entities: fibrous cortical defect (FCD) or non-ossifying fibroma (NOF). These lesions are histologically identical, consisting of whorled bundles of connective tissue. FCDs are typically smaller, and seen in younger children, whereas NOFs are slightly larger, and more common in adolescents and young adults. They are usually asymptomatic, but may occasionally cause pain. They will usually regress over time, and occasionlly will undergo ossification and become sclerotic during this regression. On Tc-99m MDP bone scan, active fibroxanthomas will demonstrate increased radiotracer uptake. However, involuting lesions will not take up radiotracer.
In this case, the sclerotic appearance and location of the lesion was suggestive of on ossified NOF. However, the lucent foci suggested that the lesion could be an incompletely ossified NOF verus osteoid osteoma with a lucent central nidus. Bone scan was recommeded to help differentiate an involuting NOF from an active osteoid osteoma.