Definition
Otherwise known as a giant osteoid osteoma, this lesion is characterized by the formation of woven bone or osteoid, and it is larger than 1.5 centimeters in diameter (arbitrarily decided by McLeod).
Epidemiology
3% of benign bone tumours.
Peak prevalence between 20 and 30, rare after 40.
M>F 3:1.
Site
Most commonly located in the spine, particularly posterior elements.
Affects cervical and lumbar regions more commonly than thoracic regions.
Histology
Gross: Haemorrhagic, granular, friable and calcified tissue
Micro: Characterized by woven bone and osteoid, with an intervening hypervascular stroma
There is no production of lamellar bone
Osteoblastic rimming is characteristic. There are large plump osteoblasts and the lesion can be difficult to distinguish from a low grade osteosarcoma.
No cartilaginous material is produced.
Less regular tissue pattern than in osteoid osteoma.
Changes characteristic of aneurysmal bone cyst can be present in some osteoblastomas
Clinical
Pain is main feature
Usually present for a long time
“Less easily characterized” than the pain of osteoid osteoma
If located in the spine there may be associated neurological lesions or scoliosis
Natural history is progressive enlargement
Rarely can act aggressively and cause local destruction and invasion of adjacent soft tissues. These lesions don’t metastasize.
Radiology
Medullary position
Fusiform expansion
Thin rim of periosteal new bone
Centre may be lucent or blastic
Can arise in either the metaphysis or diaphysis
Well circumscribed
MRI: T1 low signal, T2 high signal traversed by lines of low signal (bone).
Differential diagnosis
Osteoid osteoma
Giant cell tumour
Aneurysmal bone cyst
Osteosarcoma
Treatment
En bloc resection for expendable bones
Marginal or intralesional excision with curettage
If the spine is involved and neural structures are nearby then cryosurgery, radiation or chemotherapy may be required.
Recurrence rate is around 20%.