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Osteoblastomas are rare bone-forming tumors that may be locally aggressive. Compared to their histological relative, the osteoid osteoma, they are larger (>2 cm) and more frequently affect the axial skeleton 1.
Osteoblastoma accounts for the 'O' in the popular mnemonic for lucent bone lesions FEGNOMASHIC.
Epithelioid osteoblastoma is considered an acceptable alternative term 1. The terms pseudomalignant osteoblastoma or aggressive osteoblastoma are no longer recommended 1.
Osteoblastomas account for ≤1% of all primary bone tumors 1-4. Patients typically present around the second to third decades of life 1. There is a recognized male predilection with men approximately two times more commonly affected than women 1.
The diagnosis of osteoblastoma is based on a combination of typical radiological and pathological features.
Diagnostic criteria according to the WHO classification of soft tissue and bone tumors (5thedition) 1:
Essential features include 1:
lytic bone tumor >2 cm in size on imaging
well-defined tumor borders
no evidence of permeation of the host bone
histological evidence of a bone-forming tumor consisting of trabeculae of remodeled woven bone framed by plump osteoblasts in a vascularized background
Osteoblastomas commonly present with an insidious onset of dull pain, worse at night, with minimal response to salicylates (only 7% of patients respond, unlike osteoid osteoma) 1. The area will characteristically be swollen and tender with a decreased range of motion.
Spinal lesions might present with painful scoliosis or neurological symptoms 1.
Osteoblastomas locally aggressive lesions that are histologically similar to osteoid osteomas, are bone and osteoid forming with a rim of osteoblasts, and have rich vascularity 1.
spinal column: ~40% (range 32-46% 2); often involves the posterior elements
cervical spine: 9-39% of all spinal osteoblastomas 5
sacrum: 17% of all spinal osteoblastomas 6
usually located in the metaphysis and distal diaphysis of the long bones
Macroscopically osteoblastomas are usually well-defined tumors. They might show osseous expansion with thinning of the cortex rimmed by sclerotic host bone 1,4. Due to their rich vascularity, osteoblastomas display a red-tannish appearance and might show blood-filled cystic spaces 1,4.
Microscopically osteoblastomas are similar to osteoid osteoma and are characterized by the following 1,4:
interconnecting trabeculae of woven bone rimmed by a single layer of osteoblasts
trabeculae with different degrees of mineralization (from osteoid to pagetoid appearance) connecting to the bony edge in the periphery
richly vascularized loose stroma
possible central sclerotic nidus
scattered osteoclastic giant cells
well-defined borders without destructive bone permeation and no soft tissue extension
no atypical mitotic figures
possibly aneurysmal bone cyst-like changes
aggressive (malignant) osteoblastoma
has a high of number epithelioid osteoblasts with nuclear atypia
controversial diagnosis, not recommended by the WHO 1
epithelioid osteoblastoma is the preferred term 1
FOS gene rearrangement is present in ~90% of cases (similar to osteoid osteoma) 1
Osteoblastomas can have a wide range of radiographic patterns. Lesions are typically larger than 1.5-2 cm in size although smaller lesions may occur 7,8.
lesions are predominantly lytic, with a rim of reactive sclerosis
tend to be expansile
may have a bubbly appearance 7
internal calcification may sometimes be present
an associated soft tissue mass may also be present
demonstrate a rapid increase in size with associated cortical expansion in the vast majority of patients, sometimes with cortical destruction
there may be surrounding sclerosis or periostitis in up to 50%
there may be a secondary aneurysmal bone cyst-like changes in 20%
similar to the radiograph, lesions are often demonstrated as predominantly lytic
internal matrix mineralization is better appreciated on CT
MRI features tend to be non-specific and often overestimate the lesion 9:
T1: typically hypo to isointense on T1 with areas of decreased intensity that correspond to foci of calcification
T2: typically isointense to hypointense on T2 with foci of decreased intensity corresponding to the foci of calcification
a high signal may be seen in surrounding bone marrow and soft tissues due to the edema "flare phenomenon" 10
T1 C+ (Gd): this is a highly vascular tumor and therefore typically avidly enhances, with associated enhancement of the surrounding soft tissues 9
Tc-99m MDP or HMDL: often shows intense uptake although this is non-specific and is typical in all lesions exhibiting increased bone turnover
The radiological report should include a description of the following 11:
location and size
tumor margins and transition zone
aneurysmal bone cyst-like changes
aggressive periosteal reaction
surrounding bone marrow edema
solid mass-like enhancement
soft tissue extension
If features are typical the lesion can be categorized as Bone-RADS 4 on CT or MRI 11.
Treatment and prognosis
Management is surgical with the selection of the surgical procedure depending on the location and aggressiveness 12. Intralesional curettage can be performed in most cases but should be extended to the normal bone and can be combined with cryotherapy, chemical cauterization with phenol as adjuncts and bone grafting 12. En bloc surgical excision is associated with fewer recurrence rates and can be done in locally aggressive and/or large tumors or in recurrent lesions 12. Pre-operative embolization can be carried out to reduce bleeding risk. However, surgery needs to be performed at a very short time interval in order to avoid reconstitution of collateral blood supply ref. Percutaneous ablation is an emerging modality for the treatment of these lesions (as well as osteoid osteoma) 12. Recurrence rates may be as high as ~23% 1.
Lesions are prone to extensive intraoperative bleeding due to intrinsic vascularity ref.
History and etymology
Henry Lewis Jaffe and Leo Mayer first described osteoblastoma as an ‘osteoblastic osteoid tissue forming tumor' in 1932 12,13. It was later described as ‘osteogenic fibroma of bone’ by the American bone pathologist Louis Lichtenstein in 1952 14,15 and as ‘giant osteoid osteoma’ by D C Dahlin and E W Johnson, Jr 16 before the name osteoblastoma was suggested in 1956 again by Louis Lichtenstein 17.
Differential diagnoses of osteoblastoma include 4: