A Case Report of Osteoblastoma of Distal Femur: A Diagnostic Challenge

Key Points

Keywords: Osteoblastoma; Osteoid Osteoma; Benign Tumour; Bone Tumour

Introduction

Osteoblastoma is uncommon benign bone tumor, representing approximately 1% of all primary bone tumor and 3% of all benign bone tumor [1]. The nidus of osteoblastoma is simulating to that of osteoid osteoma on histopathology and imaging, but it is larger in size [2]. Osteoblastoma is usually diagnosed in the second decade of life and have a 2:1 male predominance [3, 4]. It occurs, in order of frequency, in the spine, femur, foot and ankle [4]. The basis for diagnosis in most of cases is from clinical, radiological, and mainly histopathological examination.  Osteoblastomas have a variable imaging appearance ranging from indolent to very aggressive, and many times difficult to differentiate from osteoid osteoma only based on imaging [5].

Discussion

Osteoblastoma is a rare benign bone-forming neoplasm, accounting for  ~1% of all primary  bone tumors and affects mainly  adolescents and young adults with a male predominance [1, 2, 3, 4]. It can involve any bone, commonly arises in the posterior elements of the spine and the sacrum followed by the proximal femur, distal femur, and proximal tibia [6]. It is also called giant osteoid osteoma, can occur in an intramedullary, cortical, or periosteal location [7]. It has no pathognomonic clinical or radiological features that could allow for a positive diagnosis of osteoblastoma [8]. Severe pain that is worse at night and relief from NSAIDS are not typically seen as with osteoid osteoma.

Osteoblastomas can have a wide range of radiographic patterns, however the lesions are typically larger than 1.5-2 cm in size although smaller lesions may occur [9, 10].  Radiographs usually show an expansile, well- circumscribed, lytic lesion with matrix ossification and mild surrounding sclerosis with cortical expansion in 75-94%, and cortical destruction in 20-22% [4, 7, 11]. Other imaging methods such as bone scintigraphy, CT and MRI may provide information about the extent of the lesion [11]. CT scan reveals similar findings as radiograph; however, the internal matrix mineralization is better appreciated on CT. Magnetic Resonance Imaging tend to be non-specific and often overestimate the lesion [12].

The radiological differential diagnosis may include osteoid osteoma, bone abscess, chondromyxoid fibroma, enchondroma and osteosarcoma [3, 4]. Chondroid like matrix calcifications may be present in the center of the nidus, at times resembling the dense osteoid matrix of osteosarcoma The lesion may mimic an osteoid osteoma when surrounded by a sclerotic rim and located intracortically or periosteally; however, osteoblastoma are typically larger than 2 cm at presentation and exhibit growth on serial examinations, while the osteoid osteomas tend towards regression [3, 7] because of absence of limited growth potential. Another important distinctive feature to differentiate it from osteoid osteoma is characteristic of pain, which is dull, localized and insidious onset in osteoblastoma with less severity than that of osteoid osteoma. It is not classically nocturnal and responds to NSAIDS only in 7% of the cases. While in osteoid osteoma, pain is classical and more severe at night but is dramatically relieved by NSAIDS, serves as an important clue to the diagnosis [13]. The lesion can be removed with intralesional curettage or marginal en block excision, depending on the clinical situation, and location within the bone [14].

Conclusion

A combination of thorough medical history, radiographic examination and clinical suspicion is at most important for differentiating osteoblastoma from osteoid osteoma. In our case, total resection of the lesion resulted in complete remission of the patient’s symptoms.