Introduction

Osteoblastoma is a rare primary bone tumour accounting for approximately 0.9% of all bone tumours.¹ Importantly, it is usually benign and the long-term prognosis is often very good. Surgical removal is the primary treatment for most patients, and once the tumor is removed, they are often cured.² Newer less invasive treatments are emerging which may speed up recovery and reduce any side effects. Younger people are more frequently affected and twice as many males than females. The most common areas to develop osteoblastoma include bones in the hands, spine, feet, legs and jaw bone. Extremely rare osteoblastoma affects the skull which can result in a variety of clinical signs - or commonly be asymptomatic. Due to the proximity to the brain, surgery can be more challenging to remove the tumour however, prognosis is still good when a complete resection has been performed.³ 

Causes and risk factors 

Currently, the cause of osteoblastoma is unknown. This makes it impossible to know what factors influence its development and who is at a higher risk of developing osteoblastoma.² 

Pathophysiology

Osteoblastomas share many features with osteoid osteomas (also a benign bone tumour). Similarly, they are characterised by rapid new bone formation however, osteoblastoma lesions tend to be more vascularised. The central lesions in osteoblastoma are usually less organised osteoid and trabecular bone, containing fewer nerve fibres and decreased levels of prostaglandins when compared to osteoid osteomas. They both express transcription factors involved in osteoblastic differentiation; Runx2 and Osterix. If osteoblastoma affects the skull, the most commonly affected area is the temporal bone.²

Symptoms

Symptoms can be variable and non-specific making diagnosis challenging, especially as osteoblastoma affecting the skull is so rare. Frequently patients are asymptomatic and are unaware of any issues. Osteoblastoma is usually slow-growing and is sometimes only diagnosed as an incidental finding after imaging for other reasons. Depending on the part of the skull involved, symptoms may include:

• Headaches
• Visual disturbances
• Hearing disturbances
• Facial paralysis
• Painful swelling
• Bone pain which worsens at night
• Neurological deficits³ 

Diagnosis

After a full physical exam to look for masses or swellings in addition to any other signs, further tests will be performed to reach an accurate diagnosis. Osteoblastoma has a very good prognosis however, more common bone tumours (such as osteosarcomas) are malignant and generally have a poorer prognosis with more intensive treatment therefore it is vital to ensure the correct diagnosis is reached. A diagnosis is generally made after imaging (this may involve X-rays, CT, MRI or/and ultrasound) and a biopsy of the lesion. 

Histopathology

To accurately make a diagnosis the biopsy will be sent to the laboratory for analysis. This will include macroscopic evaluation, histopathological examination and immunohistochemistry. 

Macroscopic evaluation

Gross examination commonly reveals a red/brown-coloured mass approximately 3-3.5cm in diameter - usually bigger than osteoid osteomas. Osteoblastomas are often very vascular with a gritty surface.

Histopathological examination

This involves fixing the specimen to allow examination of the cellular makeup of the lesion. Osteoblastoma consists of irregular woven bone spicules or trabeculae with a single layer of osteoblasts lining the spicules. Atypical mitoses are not commonly seen however, the mitotic rate may be high. 

Immunohistochemistry

Immunohistochemistry is a technique used to determine which markers are present or absent in the cells. One way to distinguish between benign osteoblastoma and malignant osteosarcoma is by assessing the intracellular location of beta-catenin. If the staining is more intense in the nucleus this is more consistent with an osteoblastoma however, if the staining is either cytoplasmic or membranous this suggests an osteosarcoma. Other markers are also being studied including hypoxia-related microRNA-210 which may be a useful marker in the future. 

Treatment

The gold standard for treatment is often surgery however, this can be difficult depending on the size and location of the tumour. Other minimally invasive techniques may be used depending on the features of the individual's tumour. Chemotherapy and radiotherapy are generally not indicated unless the tumour is unresectable or it has recurred and is inoperable. 

• Surgery 

Surgery aims to remove all the cells involved in the tumour to decrease the likelihood of the tumour returning. This may involve marginal resection (removing the affected section of bone) or curettage and bone graft (scraping out the tumour and filling the space with new bone). When the skull is affected it is recommended to remove the whole tumour and use reconstructive surgery when needed as these masses usually affect young patients therefore, it is important to reduce the chances of recurrence or transformation into a more malignant tumour.⁴

• Thermoablation

This is a minimally invasive technique which may be used to treat small bone tumours. Thermoablation uses heat passed through the skin (percutaneously) to destroy the tumour cells. There are many different types of ablative techniques including radiofrequency ablation, microwave ablation, laser ablation and cryoablation. Each treatment has its own advantages and disadvantages and is not always suitable for the patient.^5,6 

• High-intensity focused ultrasound

Another minimally invasive technique uses high-frequency sound waves to destroy the tumour cells. It is a form of thermoablation as the sound waves heat up the cells to kill them. The benefit of this treatment is it is very well tolerated and easy to administer. More recently a similar technique called magnetic resonance-guided focused ultrasound surgery (MRgFUS) has emerged which removes the need for ionising radiation and does not even need to be in contact with the patient. This combines the guidance of magnetic resonance imaging with high-frequency ultrasound waves.⁷ 

Prognosis

Osteoblastoma patients generally have a good long-term prognosis especially if all their tumours have been removed. However, recurrence does happen in around 20% of patients. These patients tend to have had curettage to remove the tumour cells rather than en-bloc surgery. This is because there is a higher chance of leaving some tumour cells behind when using curettage. Unsurprisingly, this technique is commonly used in patients with spinal lesions due to the proximity to the spinal cord and the inability to remove much bone from the vertebral column.⁸ 

It has been reported that benign osteoblastoma can transform into a malignant bone tumour however, this remains controversial within the research field. It is therefore important that a correct diagnosis is made initially so the most appropriate treatment can be administered. Long term follow-up and monitoring is indicated due to the risk of recurrence.² 

Summary

Osteoblastoma is a rare primary bone cancer that is predominantly benign. It affects young people and is commonly found incidentally at the time of imaging for another issue. Very rarely it develops within the skull with the most common area being the temporal bone. Treatment predominantly consists of either surgery or a non-invasive ablative technique to destroy the tumour cells. If all the tumour cells are removed the prognosis is very good, however, recurrence can be common. Osteoblastoma looks very similar to osteoid osteoma however, there are differences which can be seen on a biopsy. The main difference is osteoblastoma lesions are often larger and more locally aggressive than osteoid osteoma. Due to the rarity of osteoblastoma in the skull, research is limited and treatments for tumours in this location are often adapted from other benign bone tumours.