Does this MRI help narrow the differential?
Yes. The mass being low on T1 and T2 sequences suggests fibrous tissue, and as such excludes a unicameral bone cyst. Chondromyxoid fibromas are usually high on T2 weighted sequences. Absence of fluid fluid levels is against the diagnosis of an ABC and absence of bone marrow oedema or adjacent enhancement is against an osteoblastoma. Non-ossifying fibroma is therefore most likely.
Where are non-ossifying fibromas / focal cortical defects typically located?
They are typically located in the metaphysis, just distal to the physis, and most frequently in the distal femur and proximal and distal tibia. As the patient ages, they regress and appear to migrate away from the growth plate. Eccentric juxtacortical location is one of the main diagostic features of this lesion.
MRI demonstrates an eccentric lesion composed of soft tissue with low T1 and T2 signal, which enhances following contrast administration. No fluid fluid levels, no surrounding oedema or enhancement and no extra-osseous abnormality.