Osteomas are benign bone tumors that most commonly occur in the head. When occurring in the medullary cavity osteomas are called bone islands 5.
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Terminology
Osteoma is the preferred terminology with ivory exostosis, parosteal osteoma, and maxillary/mandibular torus not recommended per the WHO classification of soft tissue and bone tumors (5th edition) 5.
Epidemiology
Osteomas are common with an equal male/female distribution 5.
Diagnosis
Diagnostic criteria according to the WHO classification of soft tissue and bone tumors (5th edition) 5:
essential: bone tumor with compatible imaging; tumor arises on the bone source or within the medullary cavity; composed of lamellar/cortical-type bone
Clinical presentation
These lesions are benign, slow-growing, and usually asymptomatic. They may be incidentally identified as a mass in the skull or jaw, or as the underlying cause of sinusitis or paranasal sinus mucocele 5.
Pathology
Osteomas are benign osteogenic tumors composed of lamellar/cortical-type bone. The etiology is unknown 5.
Location
Osteomas most commonly occur in bones formed in membrane, almost exclusively occurring in the head 5, with the most common locations including:
Macroscopic appearance
These are well-circumscribed tumors broadly attached to the bone surface 5.
Microscopic appearance
Osteoma are primarily comprised of mature lamellar bone and divided into two histological subtypes based on their architecture: compact and cancellous or spongious 9,10.
The compact osteoma is comprised almost exclusively of mature lamellar bone with minimal if any bone marrow spaces and occasional Haversian canals 9.
Cancellous or spongious osteomas contain the bone marrow is filled with a well-vascularized and moderately cellular and fibrous stroma with inconspicuous osteoblasts and osteocytes and absent inflammatory infiltrate 5,9.
Associations
When osteomas are multiple, Gardner syndrome should be considered 5,10. These are due to mutations in the APC gene 5,10.
Genetics
In addition to cases occurring in the setting of Gardner syndrome, sporadic osteomas have also been shown to demonstrate frequent mutations in CTNNB1 gene (encoding β-catenin) 10.
Radiographic features
Plain radiograph / CT
Osteomas appearing ivory-like as round, very radiodense lesions, similar to the normal cortex 5. They usually arise on the skull outer surface (outer table) but can occur in the intramedullar or inner surface (inner table) of the skull 8. Some mature osteomas may demonstrate central marrow 8.
MRI
The MRI appearance will depend on the presence and quantity of bone marrow within the osteoma 5,7,8.
-
T1
usually low signal
-
T2
variable, depending on amounts of cortical and trabecular bone
-
T1 C+
no enhancement
Nuclear medicine
Mild uptake may be seen on bone scintigraphy 5.
Treatment and prognosis
Osteomas are benign and only require excision if they cause adjacent complications (e.g. mucocele formation) or mass-effect (functional or cosmetic impairment). They can slowly grow 5.
History and etymology
Osteomas have been documented as far back as 664-332 BCE in ancient Egypt 5.
Differential diagnosis
Considerations include many cranial and facial bone lesions 6: