Benign Skull Tumors Workup

Laboratory studies are not helpful in making the diagnosis.

Plain skull radiography and head CT scanning ^[7]

• Most of the benign skull tumors appear as radiolucent lesions with the exception of osteomas, ossifying meningiomas, the sclerotic form of fibrous dysplasia, and the later stages of Paget disease.

  • Osteomas appear as round sclerotic lesions arising from the outer table (less frequently inner table) of the skull without involvement of diploë. The spongy osteoma may be radiolucent. Osteoid osteomas consist of a radiolucent nidus with a surrounding zone of dense sclerosis.

  • In ossifying fibromas, the initial lesion is radiolucent, but it progressively becomes radiopaque, with sharp margins and dilated vascular channels, as shown below.

    Composite CT scan, MRI, and angiogram of a symptomatic ossifying fibroma with extensive involvement of the skull base in a 12-year-old girl whose primary symptom was exophthalmos and loss of vision bilaterally.

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• Osteoblastomas appear as well-demarcated noncontrast-enhancing lytic lesions with smooth calcified margins.

• Chondromas are well circumscribed, with distinct areas of lucency.

• Chondroblastoma manifests as a well-demarcated osteolytic area with varying degrees of calcification.

• Chondromyxoid fibroma is radiolucent with tissue calcification.

• Desmoplastic fibromas are well-defined lytic and expansile lesions with a typical soap bubble appearance. They cause thinning of the overlying cortex without periosteal reaction.

• Giant cell granulomas are radiolucent, well demarcated, and multiloculated, with expansion and thinning of the bone cortex. CT scanning shows an isodense lesion, which may erode the overlying cortical bone.

• Nonossifying fibromas and xanthomas are radiolucent with sclerotic margins and bony trabeculae with a soap bubble appearance.

• Eosinophilic granuloma is a radiolucent, oval, well-demarcated lesion without sclerosis. It has the appearance of a punched-out defect or a doughnut-shaped lesion that involves both the inner and outer table. On CT scan, it appears as a soft tissue mass within an area of bony destruction. A central density may also be present.

• Hemangiomas are well-defined nonenhancing lytic lesions with a characteristic honeycomb appearance. One third show peripheral sclerosis. Intralesional calcifications are common. See the images below.

  Lateral skull radiograph of a 73-year-old patient with a slow-growing, nontender skull lesion. Note the typical honeycomb appearance.

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  Head CT scan of a 73-year-old patient with a slow-growing, nontender skull lesion shows a well-defined nonenhancing lytic lesion with calcification and honeycomb appearance.

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  Sagittal magnetic resonance imaging (MRI) section of the brain of a 73-year-old patient with a slow-growing, nontender skull lesion showing a nonenhancing soft tissue mass. This lesion proved to be a hemangioma.

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• Lymphangiomas manifest as cystic defects of the bone.

• Aneurysmal bone cysts are well-demarcated lesions that arise from the diploë and expand both the inner and outer tables of the skull. On CT scanning, they are multiloculated expansile bone lesions with characteristic fluid levels.

• Epidermoids and dermoids are round lytic lesions arising within the diploë and have dense sclerotic borders. CT scanning shows a hypodense nonenhancing lesion with irregular borders, as shown below.

  Lateral skull radiograph of a 17-year-old adolescent male with a painless slow-growing mass. The single round lytic lesion was found to be an epidermoid.

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• Intraosseous meningiomas appear as irregular bone deposition on the inner and outer tables, usually in the vicinity of the coronal suture.

• Fibrous dysplasia has 3 different forms: the cystic form, which involves mainly the outer table; the sclerotic form, which is characterized by bone thickening; and the mixed form, which usually manifests after the third decade. All these manifest as skull lucency with patches of increased density, as shown below. CT scanning shows a multilobulated intradiploic lesion.

  Fibrous dysplasia involving the sphenoid sinus and pterygoid plates as well as the sella. This is an asymptomatic lesion; observation was recommended.

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• Paget disease manifests as a lytic lesion that resembles cotton wool, with varying degrees of bone formation and no clear edges, as shown below.

  Head CT scan of a 78-year-old woman with Paget disease. Note the cotton wool appearance of the lesion, with varying degrees of bone formation and no clear edges. Observation was recommended.

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Magnetic resonance imaging ^[8]

• Most tumors are hypointense on T1-weighted images and hyperintense on T2-weighted images.

• Hemangiomas are isointense on T1-weighted images.

Bone scanning: Bone scanning with technetium Tc-99m shows an area of increased radioisotope uptake in osteomas, ossifying fibromas, and osteoblastomas.

Arteriography

• Arteriography shows high vascularity in tumors of vascular origin.

• It is not helpful in making the diagnosis of other benign tumors.

Biopsy of the lesion is of paramount importance for establishing the diagnosis and considering treatment options.

Osteomas are composed of mature lamellar bone. The typical appearance is a nidus of osteoid tissue in a background of osteoblastic connective tissue, which is enclosed completely by reactive bone. Ossifying fibroma consists of fibrous spindle cells with varying amounts of woven bone. The periphery of the tumor is composed of mature lamellar bone.

Osteoblastoma consists of a fibrous stroma with irregular osteoid deposition. Chondromas (enchondroma, juxtacortical chondroma, osteochondroma) are rare skull tumors consisting of mature hyaline cartilage. Chondroblastomas consist of immature cartilage cells.

Chondromyxoid fibroma is characterized by chondroid and myxoid differentiation with lobular growth. Desmoplastic fibroma is of fibrous connective tissue origin marked by the formation of collagen. Giant cell granuloma manifests with giant cells around hemorrhagic foci, numerous spindle-shaped fibroblastic cells, and new bone formation. The tumor cells are smaller than those of the giant cell tumor of the bone, whereas stromal cells and giant cells resemble each other.

Nonossifying fibroma and xanthoma consist of fibroblast proliferation with multinucleated giant cells and foamy xanthomatous cells. In eosinophilic granuloma, mononuclear histiocytes are mixed with eosinophils. Giant cells and areas of hemorrhage or necrosis may also be observed. The histiocytes stain positive for the protein S-100. On electron microscopy, the Birbeck granules that characterize the Langerhans or X cells are noted. Hemangiomas are visualized macroscopically as brownish red lesions under the skull periosteum. Microscopically, they consist of capillary, cavernous, or venous blood vessels. ^[9]

Lymphangiomas consist of lymph vessels. Aneurysmal bone cysts consist of large vascular spaces separated by trabeculae of connective tissue and bone. The vascular spaces lack endothelial lining. Epidermoids consist of an epithelial capsule filled by desquamated epithelial cells and keratin.

Dermoids usually contain hair follicles and sebaceous and sweat glands. Fibrous dysplasia is a developmental anomaly in which the normal bone formation is arrested at the woven stage; thus, lamellar bone is not formed. This results in an overgrowth of the fibrous tissue among woven bone, which is the typical histologic feature of this lesion. Paget disease is initially characterized by increased osteoclastic activity, which results in bone resorption, followed by increased osteoblastic activity and bone formation.