Osteoblastoma

Osteoblastoma is a rare benign bone forming tumour that is histologically like osteoid osteoma but larger (nidus over 1.5 to 2 cm), grows more, and does not respond predictably to NSAIDs. It is locally aggressive in some cases and can recur after incomplete removal.

It accounts for under 1% of primary bone tumours, and occurs mainly between 10 and 30 years of age, with a male to female ratio of about 2 to 1. About 30 to 40% arise in the spine and sacrum (posterior elements: pedicle, lamina, transverse and spinous processes), 30% in the long bones (femur, tibia, humerus), and the rest in the jaw, skull, hands and feet.

Variants include conventional osteoblastoma, aggressive (epithelioid) osteoblastoma, which has large epithelioid osteoblasts, more recurrence (up to 50%) and a risk of local invasion, and osteoblastoma like osteosarcoma, a malignant mimic. A secondary aneurysmal bone cyst component is present in 10 to 15%. Complications are painful scoliosis, nerve root or spinal cord compression, pathological fracture, recurrence and, rarely, malignant transformation or lung metastasis in the aggressive form.

Pathophysiology.

Like osteoid osteoma, most tumours carry rearrangements of FOS or FOSB (in about 75% of cases), which drive proliferation of osteoblasts producing woven bone and osteoid. The larger size is attributed to continued growth without the self-limiting behaviour of osteoid osteoma. Prostaglandin levels are lower, which explains why pain is less nocturnal and less responsive to NSAIDs.

A dull, persistent ache for weeks to months that is not typically worse at night and is only partly relieved by NSAIDs. Spinal lesions cause back pain, scoliosis, muscle spasm and, in a third, radiculopathy, myelopathy or cauda equina syndrome. Long bone lesions cause local pain, swelling and a limp. Jaw lesions cause swelling.

Examine the site for tenderness, swelling, range of motion, spinal alignment and a full neurological examination (motor, sensory, reflexes, sphincters).

• Radiographs show an expansile, well defined lytic lesion over 2 cm with variable mineralisation, a thin sclerotic rim, and sometimes cortical expansion. Aggressive lesions show cortical destruction and a soft tissue mass.
• CT shows the expansile lesion and its calcification in the posterior spinal elements, the thin shell, and canal involvement.
• MRI shows marked oedema in adjacent bone and soft tissue, which can suggest malignancy or infection, and shows nerve root and cord compression.
• Bone scan: intense uptake.
• Biopsy (CT-directed) before definitive surgery when imaging is not diagnostic: anastomosing trabeculae of osteoid and woven bone with a rim of osteoblasts, a vascular stroma, and few mitoses. Exclude osteosarcoma (permeative growth, atypia, atypical mitoses, lace like osteoid).
• Blood tests: full blood count, alkaline phosphatase and inflammatory markers (usually normal).

Differential diagnosis.

• Osteoid osteoma and osteosarcoma (osteoblastoma like).
• Aneurysmal bone cyst and giant cell tumour of bone.
• Osteomyelitis and chondroblastoma.
• Fibrous dysplasia and Langerhans cell histiocytosis.

Surgery is the main treatment. The aim is complete removal while preserving the spine, nerves and function.

Surgery.

• Extended intralesional curettage with a high speed burr (and adjuvant such as cryotherapy or phenol, where the site allows) with bone graft or cement is used for most long bone lesions, with recurrence in 10 to 20%.
• En bloc resection (marginal or wide) for aggressive, recurrent or expendable bone lesions; this lowers recurrence below 10%.
• Spine: resection by posterior approach, with instrumented fusion when stability is lost. Preoperative embolisation of a vascular lesion reduces blood loss. Plan the surgery with a spinal tumour team.
• Small, accessible lesions can be treated by CT-directed radiofrequency ablation, with a risk of incomplete treatment.

Radiotherapy. Reserved for unresectable spinal or skull base lesions, or recurrent disease that cannot be removed; it carries a small risk of malignant transformation, and its role is limited.

Systemic therapy. No chemotherapy is used for conventional disease. Denosumab has been used off label in recurrent or unresectable lesions with aneurysmal bone cyst components; dosing and calcium monitoring are set by the tumour service.

Medical pain control. NSAIDs are used before surgery: ibuprofen 400 mg orally every 8 hours with food (maximum 1200 mg daily; avoid with eGFR under 30 mL/min/1.73 m2, peptic ulcer, heart failure or anticoagulant use). Dexamethasone is not routinely used.

Operative antibiotic prophylaxis. Cefazolin 2 g intravenously (3 g when weight is 120 kg or over) within 60 minutes before incision as a single dose (repeat after 4 hours or blood loss over 1500 mL); with severe penicillin allergy, vancomycin 15 mg/kg intravenously starting 90 to 120 minutes before incision as a single dose, with levels and creatinine checked in prolonged cases.

Tranexamic acid (spinal and pelvic resections). Tranexamic acid 15 mg/kg (maximum 1 g) intravenously over 10 minutes before incision, repeated once after 3 hours if bleeding continues (maximum 2 g); avoid with a history of thromboembolism, active intravascular clotting, or seizures; reduce the dose at creatinine clearance under 50 mL/min.

Thromboprophylaxis. Enoxaparin 40 mg subcutaneously once daily, started 6 to 12 hours after surgery and continued for 10 to 14 days (28 to 35 days after major pelvic or reconstructive surgery) (20 mg once daily at creatinine clearance under 30 mL/min; adjust by anti-Xa level in obesity and renal impairment; check the platelet count on days 5 to 14 for heparin induced thrombocytopenia). Use mechanical prophylaxis until then; withhold if a spinal haematoma is a concern, and time with neuraxial procedures.

Analgesia.

• Paracetamol 1 g orally every 6 hours as needed for up to 5 to 7 days (maximum 4 g daily; 3 g daily when weight is under 50 kg, with chronic alcohol use or hepatic impairment).
• Oral morphine 5 to 10 mg every 4 hours as needed for severe pain for up to 3 days (start 2.5 to 5 mg with age over 75 or eGFR under 30 mL/min/1.73 m2; avoid exceeding 60 mg in 24 hours without senior review); monitor sedation and respiration and prescribe a laxative.

Monitoring. Clinical and imaging review (radiographs, CT or MRI) at 3 months, 6 months and 12 months, then yearly for 5 years, since most recurrences occur within 2 years. Neurological state and spinal alignment at each visit. Lung imaging if the tumour is aggressive.

Referral and escalation. Refer every suspected osteoblastoma to a bone tumour service before any surgery. Emergency referral for a new neurological deficit, cauda equina syndrome or an unstable spine.

Special groups.

• Children: care for the physis, and monitor for spinal deformity until skeletal maturity.
• Aggressive (epithelioid) variant: wide resection and long follow up.
• Pregnancy: avoid NSAIDs in the third trimester; defer non-urgent surgery.
• Kidney impairment: avoid NSAIDs, reduce morphine doses, and adjust tranexamic acid.