Osteoblastoma is an uncommon benign bone-forming neoplasm which accounts for about 1% of all primary bone tumors. It commonly arises in the posterior elements of the spine and the sacrum. Accurate diagnosis of osteoblastoma is critical in determining the appropriate treatment modality and prognosis. In most cases, the basis for diagnosis is from clinical, radiological and mainly histopathological examination. Osteoblastomas have a variable radiologic appearance ranging from indolent to very aggressive. In general, osteoblastoma has a good prognosis, and patients are often cancer free after surgical treatments of intralesional curettage or marginal en bloc resection.
The exact etiology of osteoblastoma is still unknown, and so are the predisposing factors.
Osteoblastoma is a rare benign neoplasm, accounting for about 1% of all primary bone tumors. It affects mainly adolescents and young adults (mean age, 20 years) with a male predominance (2.5 to 1). Osteoblastoma can involve any bone. It commonly arises in the posterior elements of the spine and the sacrum (40 to 55%). In the appendicular sites, the proximal femur, distal femur, and proximal tibia are the most frequent. Osteoblastoma less commonly involves the tarsal bones (talus and calcaneum). The majority of the cases are intraosseous (medullary).
Cytogenetic studies of osteoblastoma are few with the characterization of only six cases. These analyses have demonstrated near-diploid complements with unrelated rearrangements.
Osteoblastoma has a copious vascular supply and therefore appears red or red-brown, often with a gritty or sandpaper consistency. The average size is 3 to 3.5 cm, with maximum diameter up to 15 cm. The tumor is usually round to oval with a thinned cortex and always with a thin periosteal reactive bone shell if there is cortex is destruction. The border between the tumor and medullary cavity is sharp, often with some reactive bone. The tumor has a ''pushing'' border than a permeative or infiltrative border against the endosteal cortical surface and trabecular bone of the marrow.
Osteoblastoma has identical histological features to osteoid osteoma. The tumor is composed of woven bone spicules or trabeculae. These spicules are haphazardly arranged and have a lining of a single layer of osteoblasts. The vascularity is rich, often with extravasated erythrocytes. Osteoblasts may have mitoses, but they are not atypical. Diffusely scattered osteoclast-type, multinucleated giant cells are often present. Osteoblastomas do not infiltrate or isolate pre-existing lamellar bone structures as do osteosarcomas, so special attention should be given to the border between pre-existing cortex or marrow trabeculae. The lesion can include areas of secondary aneurysmal cysts. The term epithelioid osteoblastoma, previously known as aggressive osteoblastoma, has been used for cases of osteoblastoma with large, plump osteoblasts with a prominent nucleus and nucleoli, sometimes with mitoses.
Dorfmann and Weiss classified osteoblastomas into four groups in 1984.
1- Low-grade osteosarcomas (Osteoblastoma like osteosarcoma).
2- Pseudomalignant osteoblastomas.
3- Osteoblastomas that have undergone malignant transformation to osteosarcoma. Some authors are of opinion that these should belong to osteoblastoma like osteosarcomas.
4- Aggressive osteoblastoma: Aggressive osteoblastomas have characteristics intermediate between those of osteoblastoma and low-grade osteosarcoma. Some authors have considered this group as malignant osteoblastoma.
To date, the WHO has not subclassified osteoblastoma. However, some intermediate-grade osteoblastomas lying on the borderline between the benign osteoblastoma and the osteosarcoma have been found, including aggressive osteoblastomas with locally aggressive behavior.
Beta-catenin can be used to differentiate between osteoblastoma and osteosarcoma. Osteoblastoma is characterized by nuclear beta-catenin staining whereas cytoplasmic or membranous staining of beta-catenin suggests osteosarcoma.
A recent study has recommended using Hypoxia-Related MicroRNA-210 ss a successful diagnostic Marker for Discriminating Osteoblastoma and Osteosarcoma.
Osteoblastomas generally grow slowly with minimal symptomatology, but aggressive lesions may cause severe symptoms such as soft tissue edema, joint stiffness, and contracture. Pain is the most common presenting symptom, and diagnosis may be delayed up to two years from the onset of symptoms. Osteoblastomas of the spine have similar symptoms to that of osteoid osteoma; namely back pain, scoliosis, and nerve root compression.[ The appendicular tumors also produce pain and/or swelling, but these symptoms may be mild enough to last for months before the patient will see a clinician. Toxic osteoblastoma is a very rare variant of osteoblastoma associated with systemic symptoms, including fever, anorexia, and weight loss.
Several imaging modalities are available for establishing the diagnosis of osteoblastoma. They include:
On imaging, osteoblastoma may be difficult to distinguish from osteoid osteoma. Imaging techniques generally show osteoblastomas as being osteolytic tumors; however, due to osteoid and bone components, osseous type mineralization may be seen. The lesions are usually well circumscribed, central to slightly eccentric in long bones, rounded or ovoid, and expansile.
On plain radiography, an osteoblastoma appears as a lucent, expansile lesion with a varying degree of sclerosis, usually in the neural arch.
Computed tomography scan (CT):
CT can also demonstrate cortical breakthrough if the osteoblastoma is aggressive. CT better demonstrates the lesion as well as potential matrix mineralization or trabecula
Magnetic resonance imaging (MRI):
On MRI, osteoblastomas have low to isointense T1 signal intensity and isointense to high T2 signal intensity, and they enhance.
Studies using technetium-99m scintigram have shown an increased uptake in the mass, and FDG-PET studies have revealed a high uptake in the tumor in spite of its pathologically benign features.
The treatment of patients with osteoblastoma is surgery. The lesion can be removed with intralesional curettage or wide resection, depending on the clinical situation, location within the bone and suspicion of malignancy. Recurrence rates vary with the surgical approach, and wide resection correlates with lower recurrence rates. There is no definite role for adjuvant chemotherapy or radiotherapy. Because of the risk of transformation into osteosarcoma, radiotherapy is increasingly not a treatment of choice.
The prognosis of osteoblastoma is excellent with a local recurrence rate of about 15% to 20% Therefore, long-term follow-up of patients with osteoblastoma is essential due to recurrence.[ Osteoblastomas located near the central neural axis has a worse prognosis due to the difficulty of complete excision. Progression of osteoblastoma to osteosarcoma is exceptional but has been reported
A number of factors can influence the diagnosis of aggressive osteoblastoma, including :
Osteoblastoma is ideally managed by a multidisciplinary team that consists of orthopedists, radiologists, and pathologists. Correlation between gross, radiographic, and microscopic features of the lesion is crucial to establish the definitive diagnosis of osteoblastoma. Postoperatively, patients require long term follow-up due to the possibility of tumor recurrence and malignant transformation.