Table 1.

Imaging and intraoperative macroscopically findings of calvarial lesions after histological diagnosis. CFD, color flow doppler sonography; CNS, central nervous system; CSF, cerebrospinal fluid; CT, computed tomography; MRI, magnetic resonance imaging; PET, positron emission tomography

Tomography findings CT and MRI	Other imaging methods	Macroscopically intraoperative
Primary neoplasia
Arachnoidal cyst (n=1; 0.75%)
- Calvarial (Tabula interna and externa) thinning - Well-circumscribed cyst containing CSF (isodense, isointense)		- rachnoidal Bubbles - Stretched veins - No solid mass
Dermoid cyst (n=7; 5%)
- Cystic lesion containing fat density - Fat in CT hypodense and in MRI hyperintense - In 20% calcification of the wall	Sonography: - Well-defined lobulated mass - Echo-rich/nonhomogeneous contents - Lack of ossification	- Circumscribed mass - Dents in the bone - Fat tissue - Bone destruction
Epidermal cyst (n=2; 1.5%)
- Round, lobulated lesion - Heterogeneous signal - Possible restricted diffusion		- Lytic bone lesion filled with keratinous material - Liquid/gelatinous content
Epidermoid cyst (n=7; 5%)
- Round, lobulated lesion - CT: sharply demarcated lytic lesion - MRI: T1 often slightly hyperintense to CSF T2 often iso-/slightly hyperintense to CSF - DWI: increased signal distinguishes cystic lesion from arachnoid cyst	Sonography: - Partial destruction of the tabula externa - CFD: no perfusion - Completely intraosseous cyst X-ray: - Lytic bone lesion	- Destroyed bone - Liquid, whitish pearl-colored mass - Vascularized skin - Bone thinning
Fibroma (n=1; 0.75%)
- CT: sharply demarcated lytic lesion - MRI: no specific signal changes	Skeletal scintigraphy: - Tracer uptake	- Whitish, soft - Lytic bone lesion
Hyperostosis (n=9; 7%)
- CT: osseous protrusion of the calvaria, starting from the tabula externa - MRI: no specific signal changes	Skeletal scintigraphy: - Focal uptake	- Only tabula externa affected
Intraosseous hemangioma (n=9; 7%) Fig. 2e
- CT: lytic-cystic lesion confined to the bone with involvement of the tabula interna and externa. - Low density. - No dura infiltration - MRI: contrast enhancement. T2 possible flow voids in large vessels T2* possible susceptibility artifact due blood breakdown in the vessels		- Palpable swelling - Hypervascularized - Soft consistency - Bluish to purple-reddish color - Fibrotic capsule
Meningioma (n=20; 15%) Fig. 2a–d     WHO I Fig. 2a–b
- MRI: extra-axial mass with dural adhesion; homogeneous contrast enhancement; “dural tail” in 30–85% of cases, but unspecific; T2: flow voids, possibly “sunburst” phenomenon - CT: bone thickening; transosseous growth; irregular cortex; hyperostosis; often calcifications - Necrosis and cystic parts are common, hemorrhage is rarely	X-ray: - Callus formation	- Infiltrative growth - Exophytic - Bone surface changed and uneven - Tumor intraosseous hard, intracranially soft - Hypervascularized - Dura can not be clearly defined - Reddish color
WHO II
- Like WHO I - Correlation between increased tumor grade and imaging with blurred tumor-brain delimitation, capsular, and heterogeneous enhancement	Skeletal scintigraphy: - No evidence of malignancy PET: - Metabolically active, osteodestructive	- Avascular to hypervascularized - Swelling above bone level - Infiltrating growth - Tough, reddish
WHO III Fig. 2c–d
- Like WHO I - Correlation between increased tumor grade and imaging with blurred tumor-brain delimitation, capsular and heterogeneous enhancement		- Infiltrating growth - Hypervascularized - Hard capsule - Middle soft and yellowish
Osteoid osteoma (n=1; 0.75%)
- Focal lytic lesion (“nidus”) within surrounding osteosclerotic reaction - MRI: enhancement of the nidus		- Macroscopically no tumor visible
Osteoma (n=16; 12%) Fig. 2f
- Homogeneous bone-dense structure with partly cancellous partly hypersclerotic swelling - Exostosis of the tabula externa	Skeletal scintigraphy: - Focal uptake	- Solid bone swelling - Macroscopically intact bone structure - Soft borders around the bone
Secondary neoplasia
Metastasis (n=19; 14%) Fig. 3a–b
- Most common malignant bone lesion - In patients > 40 years, metastasis should always be considered in the case of a lytic lesion - Variable morphology: lytic, plastic, and mixed lesions - Variable contrast enhancement - Possible necrosis or hemorrhages - Dura infiltration	Skeletal scintigraphy: - Uptake PET: - Metabolically active	- Infiltrating growth - Lytic bone lesion - Tough and hard lesions - Bone thickening and very soft - Soft cancellous bone - Hypervascularized
Plasmacytoma (n=1; 0.75%) Fig. 3d
- Uni- (plasmacytoma) or multifocal (multiple myeloma) lytic lesion - Numerous, well-circumscribed lytic lesions (“raindrop skull”)	X-ray: - Numerous lytic bone lesions	- Grown through the bone - Fibrotic tissue
Spindle cell melanoma (n=1; 0.75%) Fig. 3c
- No specific imaging findings in case of bone involvement - The isolated case showed a tumorous mass of the scalp with transosseous and broad-based dural infiltration		- Reddened skin - Infiltrating growth
Squamous cell carcinoma (n=2; 1.5%)
- Soft tissue process with perifocal osseous erosions - Bone destruction from the outside in, starting from the tabula externa - MRI: no specific findings
Tumor-like lesions
Aneurysmal bone cyst (n=1; 0.75%)
- Very rarely in the calvaria - Typically age under 30 years - Well-circumscribed lytic lesion - MRI: multiple fluid levels with different signal behavior depending on the protein content		- Capsule strongly ingrown with the dura
Benign bone tissue with lytic lesions (n=1; 0.75%)
- No specific imaging findings - The isolated case showed a circumscribed lytic lesion in the diploe without erosion of the tabula interna and externa		- Grayish-colored cancellous bone
CNS-Aspergillosis (n=1; 0.75%)
- No specific imaging findings - Perifocal lytic bone lesion and infiltration of the adjacent tissue		- Altered, epidural, grayish tissue
Connective tissue (with foreign body granuloma) (n=3; 2%)
- No specific imaging findings - The individual cases showed a lobulated, well-circumscribed lytic lesion with hyperintense signal in T2 and mixed signal in T1.		- Tough capsule - Dermoid-typical content - Connective tissue in the area of the lytic lesion - Dura thinned - Fat tissue
Fibrous dysplasia (n=6; 4%) Fig. 4c–e
- Ground-glass opacities, homogeneously sclerotic - Thickened diploe - Mostly more than 1 bone affected (most often maxilla, orbit, frontal, ethmoid, and sphenoid bone) - MRI not indicated for primary diagnostic - Often not distinguishable from Paget’s disease. In fibrous dysplasia the tabula externa is more often affected	Skeletal scintigraphy: - Moderate focal uptake	- Hypovascularized - Grayish mass - Bone soft, hypertrophied, and reddish-glassy colored - No cancellous bone visible
Gorham-Stout disease (n=1; 0.75%)
- Extremely rare - Nonspecific lytic bone lesion		- Very soft bone - Almost no cancellous bone
Histiocytosis (n=20; 15%) Fig. 4a–b
- Well-circumscribed lytic bone lesion. Calvaria is most frequently affected (frontal > parietal > temporal > occipital bone), most rarely also mastoid, mandible, and orbit. Tabula interna more affected than tabula externa. - No periosteum reaction, possible perifocal soft tissue involvement - T1 iso-/hypointense; T2 hyperintense - Homogeneous contrast enhancement	Skeletal scintigraphy: - Uptake, moderately metabolically active	- Lytic bone lesion - Soft consistency - Grayish-glassy - Hypovascularized - Infiltrates the galea and periosteum
Paget’s disease (n=1; 0.75%)
- Either well-circumscribed bone defects or diffuse sclerotic, ground-glass opacities of the bone - Thickened diploe - The calvaria is most frequently affected, in isolated cases also the skull base - MRI not indicated for primary diagnostic		- Manifest thickened cancellous bone
Reactive bone remodeling zone (n=1; 0.75%)
- No specific imaging findings - The isolated case showed focal bone thinning with lobulated lytic lesion involving the diploe and the tabula interna and externa.		- Lytic bone lesion
Spindle cell tissue with abundant collagen connective tissue (n=1; 0.75%)
- No specific imaging findings - The isolated case showed a central lytic lesion with sclerosis around the border		- Tough yellowish tissue
Venous ectasia (n=1; 0.75%)
- Like hemangioma - Well-circumscribed lytic lesion - Thin peripheral sclerotic border - No destruction of tabula interna and externa - Expansion of the bone trabeculae in the diploe - MRI: in T2 heterogeneous hyperintense signal, in T1 with contrast agent diffuse enhancement with time-delayed filling	Sonography: - Liquid-filled cavity - Intracranial connection - CFD: minimal flow	- Bone thinned