The skull bones are affected by lesions similar to those seen in the other bones, such as benign or malignant neoplasms or metastatic deposits, congenital dysplasias, metabolic disorders, and hemopoietic disorders. The vault of the skull is made up of membrane bones whereas the base is of cartilaginous origin.

It has been reported that the primary skull tumors account for 0.8% of all bone tumors. The vault is made up of membrane bones whereas the base is of cartilaginous origin. This influences the pathology. Lesions that are primarily intracranial may involve the skull secondarily, and similarly, skull tumors can spread intracranialy.

Clinical features:

Swelling and local pain are the usual presenting symptoms. Associated neurological deficit suggests intracranial extension.

They produce symptoms by expansion of the skull or compression of the adjacent brain or venous sinuses. 

Skull base lesions can invade or compress the cranial nerves. Headache and/or focal bony pain may result. When cerebral venous thrombosis occurs secondary to compression or invasion, the abrupt or insidious onset of headache is related to elevated intracranial pressure.

The site of skull base lesions can often be localized by the cranial nerve deficits. There are five clinical syndromes:

The orbital syndrome is characterized by progressive supraorbital pain over the affected eye and visual blurring, followed by diplopia. On examination, there is proptosis of the affected eye and external ophthalmoplegia; there is variable numbness in the first division of the trigeminal nerve and pre-orbital swelling. 

The parasellar or cavernous sinus metastasis syndromes are characterized by a severe, unilateral frontal headache and diplopia.  There is paresis of either or both of the oculomotor and abducens nerves, numbness in the first division of the trigeminal nerve, and possibly papilledema.  There is no proptosis. 

Presenting complaints in those with middle fossa (gasserian ganglion) syndrome are numbness, paresthesias, or pain in the area innervated by the second or third division of the trigeminal nerve.  Headache is uncommon. 

Involvement of the jugular foramen produces hoarseness or dysphagia and often retroauricular pain. Examination reveals evidence of glossopharyngeal, vagus, and accessory nerve dysfunction. 

In contrast, patients with the occipital condyle syndrome present with severe unilateral occipital pain, exacerbated by neck flexion and associated with neck stiffness, pain over the occipital area, and unilateral hypoglossal nerve paralysis. 

Diagnosis:

CT scan has mostly replaced plain x-rays. It provides accurate information regarding the involvement of the skull bones as well as the intracranial and extracranial soft tissues. Contrast studies assess the vascularity and the involvement of dural venous sinuses.

MRI scan provides better delineation of soft tissue involvement. As compact cortical bone lacks in unbound protons, the inner and outer tables appear as a signal void. The diploe has abundant fat and hence images well. It is specially useful in skull base lesions Contrast-enhanced MRI of the skull base that can detect soft tissue abnormalities and provide excellent visualization of the cavernous sinus and cranial nerves. 

Cerebral angiography has limited indications.

Management:

Surgery, radiotherapy, and chemotherapy are often used in combination as in other bone tumors.

Surgery depends on the suspected nature and the site as well as intracranial involvement. Needle biopsy is a simple out patient procedure and may provide a clue on pathology. Primary lesions, ideally, are excised completely whenever possible. Cranioplasty may be done at the same sitting or at a later stage. Skull base lesions may need special exposures and involvement of plastic and ENT surgeons.

Even when complete excision is not possible, surgical palliation may be considered to avert neurological or life-threatening events, especially in patients whose prognosis is uncertain.  Surgical debulking of a mass lesion may allow for more effective chemotherapy and radiation therapy. 

Radiation therapy, either curative or palliative, is the mainstay for malignant tumors. Multiple small fractionated doses per day spread out over a longer time, is more effective with less damage to normal structures. Generally, the required dose is 55 Gray or 5500 rads in 30 fractions over six weeks utilizing megavoltage photon radiation. There is 1% risk of delayed sarcoma in the irradiated bone. Higher energy photon or electron beam therapy is ideal and can be tailored to the tumor volume by computerized techniques.

Chemotherapy alone is useful in a few, such as, lymphomas. Currently, it is used as an adjuvant to other forms of therapy and as a palliative therapy.

Primary skull tumors:

a) Benign skull tumors:

Osteomas are the commonest (about 30%). They arise from membranous bone and proliferate into dense cortical or spongy cancellous bone. The frontal and mastoid air cells are common sites. These slow growing tumors form an outward excrescence which is hard and painless and are usually noted while combing the hair.  A compact osteoma may become hard like ivory.  The attachment to the skull may be narrow or broad.  Rarely, an osteoma may extend intracranially and cause seizures.   

In plain X-rays, an osteoma is seen as a solid homogenous bony shadow.  There are no increased vascular channels.  Tangential projections reveal the base and the absence of involvement of the diploid and inner table.  When the inner table is involved, differentiation from a meningioma becomes necessary.  CT gives the precise diagnosis. Myltiple osteoma of the calvaria and mandible with soft tissue tumors of the skin and colonic polyposis form the triad of Gardner’s syndrome.  Microscopically, the tumor is a nucleus of osteoid tissue in a background of osteoblastic connective tissue and is completely enclosed by reactive bone.   Histological differentiation from fibrous dysplasia is difficult: but the presence of smooth, homogeneous and sharply defined sclerotic nodules is unusual 0in fibrous dysplasia.

Osteomas are surgically curable. The indications for removal are rapid growth, pain, obstruction to sinus out lets and noticeable deformity.  Small osteomas of the outer table may be resected easily without destruction of the inner table.  As these lesions are very hard, it is wiser to remove them by cutting around their base through the cancellous tissue.  A large lesion needs removal of the entire bone as a flap and the defect is closed by cranioplasty.  After excising the bone flap. The osteoma can be excised from the flap and the flap can be autclaved and used to close the defect primarily.

Hemangiomas constitute about seven per cent of all skull tumors.  About two thirds of haemangiomas of bone occur in the skull or the vertebral column.  They arise from the vascular elements of the diploe, mainly in the vault of the skull and to a lesser extent in the roof of orbit or petrous temporal bone.  They are slow growing and may reach a large size.  They are painless and the presence of a swelling is the chief complaint.  The swelling is hard, but may be soft in some places.  The skull is involved by erosion and the margins are imperceptible. Dilated veins may be present. In haemangiomas of the orbit, proptosis, blindless or extra ocular palsies may be seen.  Haemangioma of the petrous bone may present with deafness and cranial nerve palsies.

The plain X-rays show a swelling with a typical honeycombed or sunburst appearance.  The diploe is enlarged and both tables of the skull bulge, outer more than the inner.  Rarely, intracranial extension is seen.  The trabeculae are seen vertically oriented.  The edges are well defined and a thin margin of bony condensation may be evident.  CT images with “bone window” show the hypodense matrix with discrete, thickened, sclerotic and widely separated trabeculae.  Despite the vascular nature of the lesion, contrast enhancement is an exception rather than the rule.  Carotid angiography shows enlargement of the external carotid artery branches.  Rarely, there may be an internal carotid supply to these tumors.

Treatment is usually by enbloc excision or wide curettage.  The tumor appears as a blue domed hard mass under the pericranium.  Sometimes the dural surface may bleed profusely in which case circumferential incision of the dura and resuturing will help. Radiotherapy is advisable in situations where excision is not feasible. Doses up to 30 Gy(3000 rads), in three weeks, may be required. 

Giant cell tumors (osteoclastoma) arise from the cartilaginous bone in the sphenoid, mastoid or occipital areas.  They are extremely rare in the bones of the vault, as osteoclasts are not usually present in membrane bones.  Their pathogenesis is unknown, although trauma and hemorrhage may precede their occurrence. 

Osteoclastoma of the skull presents as a painless bony swelling and radiographs show evidence of rarefaction or destruction of bone.  Excision is the treatment of choice; but it is often incomplete and needs supplementary radiotherapy to ensure freedom from recurrence.  Occasionally, malignant changes have been reported after surgery and radiotherapy. 

Epidermoid and dermoid tumors of developmental origin are derived from epithelial cell rests ectopically included in the bone during development.  They are commonly seen in or near the midline in the vertex, the frontal or occipital regions or in the temporal bone.  However, they may occur anywhere in the clavarium.  They originate in the diploe and enlarge in the both directions expanding and thinning both the tables by continuous growth pressure.  The bone at the edge of the lesion gets sclerosed.  The lesion may break through the egg shell thin tables and expand under the scalp or extradurally.  The swelling under the scalp is firm, rubbery and non-tender. Sometimes a tract may extend through the inner table and dura to end in an intradural lesion. A lesion in the midline especially over the torcula may involve the venous sinuses.  Very large intracranial extensions may exist with surprisingly normal intracranial pressure and with no neurological deficits.  These are described as giant intradiploic epidermoids.  The larger lesions, especially epidermoids, tend to get infected and osteomyelitis may result.   

Plain radiographs show a clear cut area of radio-lucency in the skull sclerosed margins resembling an emissary foramen.  Tangential views show expansion of the tables.  CT accurately delineates the bony defect and the size, location and extension of the soft tissue mass outside and inside the skull and dura.  The lesion appears hypodense relative to the adjacent brain, due to the contained keratinized debris and cholesterol.  They do not enhance with contrast, but adjacent compressed brain shows marginal enhancement.  On MR these lesions appear hyperintense in T2 weighted images and most often hypointense in T1weighted images.

Treatment is by surgery. Pure extracranial lesions can be excised enbloc. A careful search must be made for any intradiploic or intracranial extension along a thin track which, if present, needs to be excised.  While dealing with midline lesions the surgeon must be prepared to do an extensive craniotomy and venous sinus repair if necessary.

Chondromas arise mainly in the cartilaginous bones of the base of the skull.  They commonly occur between the ages of 20 and 40 years.  The common sites are the paranasal sinuses and the spheno-ethmoidal and spheno-occipital extended into the sellar or parasellar region, producing visual and ocular nerve palsies or endocrine dysfunction. The posterior lesions may compress the brainstem and involve the lower cranial nerves.  Radiographically, a chondroma appears a lytic lesion at the base of the skull with fairly sharp margins. Areas of stippled calcification maybe seen in more than 60 per cent.  CT reveals well marginated bone destruction and an associated homogenous, isodense and lobulated soft tissue mass with interspersed calcification.  Contrast enhancement is infrequent and when present is minimal. 

Sarcomatous changes occur in one to two per cent of these tumors, more frequently in individuals with Mafucci’s syndrome (multiple enchondromas and multiple subcutaneous hemangiomas). Rapid growth indicates a malignant change.  Histologically, malignancy is deduced by the presence of atypical cartilage cell nuclei in the actively growing peripheral portions of the neoplasm.  The treatment of a chondroma is total removal wherever possible.  Most often only partial removal is possible, especially at the skull base.  Decompression of neural structures by such partial removal is often beneficial.

Aneurismal bone cyst is a multiloculated expanding cystic tumor with a rich vascular network in the walls.  The encircling inner and outer tables are eroded to form thin bony shells.  Some cysts show a central core of hemangioma and repeated hemorrhage may be the cause of the cystic expansion of the skull tables.  These lesions may either becomes symptomatic or enlarge during pregnancy.  On CT, the contents of the cyst may be of the fluid.  The superficial temporal and middle meningeal arteries supply the tumor and this is made out well on selective external carotid angiography. 

Ossifying fibromas (benign osteoblastoma) are a rare solitary, vascular tumor predominantly osteolytic in character with varying degree of calcification and new bone formation. Treatment is local excision.

Osteoblastomas are seen occasionally in the base of the skull, but rarely in the vault. Depending on the site of origin, they produce signs of pressure on the optic nerves, the pituitary, the hypothalamus, the brainstem and other cranial nerves. Radio logically, islands of erosion with normal bone in between are seen. Local excision and post operative radiotherapy is the usual practice. 

b) Malignant tumors: 

Chondrosarcomas are rare, usually in adults, as a malignant transformation in a benign chondroma. The common site is the base of skull, in and around the sella, CP angle, or the fronto-ethomoidal sinuses. They grow for a long time and produce pain, deformity, and cranial nerve palsies. CT reveals an irregular destructive process with a homogenous, hyperdense, and enhancing soft tissue mass. Radical resection and radiotherapy is the traditional treatment. They are locally invasive and tend to recur.

Osteogenic sarcomas are rare; it occurs usually in the young and in the vault. Occasionally, it occurs as a late complication following radiotherapy. They grow rapidly and metastasize to the lungs and other bones. CT shows an enhancing irregular, heterogenous extradural mass with prominent new bone formation. Radical excision and radiotherapy is recommended. Adjuvant chemotherapy helps. 

Fibrosarcomas arise from the periosteum of the skull or the dura. They are rapidly growing and painful. X-rays show thinning of the outer table with irregular margins and an overlying soft tissue mass. Radical excision and post–op radiotherapy is advised. Chemotherapy has no role.

Chordomas are slow growing, locally invasive and of embryonal origin from the remnants of the chorda dorsalis. The smaller ones are pedunculated, the larger ones are diffuse. The usual sites are at the skull-base, in the sella, the clivus, and the nasopharynx. The sixth nerve is commonly affected. Brainstem compression occurs later. CT and MRI show a soft tissue mass in the midline with bone destruction. Occasionally (10%) there may be a sclerotic response. Extended skull base approaches may be needed for satisfactory excision. Post–op radiotherapy is advised.

Other rarer tumors, such as, reticulum cell sarcoma, angiosarcoma, malignant fibrous histiocytoma can occur. The diagnosis is usually made on histology. Wide excision and radiotherapy is advised.

Skull metastases: Hematogenous metastases are usually from the breast, the lungs, the prostate, the thyroid, and the kidney. Though metastases can affect any part of the skull, vault is the common site. They are usually osteolytic, except when the primary is from the prostate, when it is osteoblastic. Radiologically they appear as multiple, poorly marginated areas of slightly increased density. Occasionally in some cases of breast cancer, the skull metastases may appear as mixed lucent and sclerotic areas. Plain xrays are positive in 60% of cases and CT in 85%. Extradural and scalp extension may show as areas of contrast enhancement. Isotope scans are more sensitive. Biopsy may be needed to confirm the lesion. Treatment is correlated with that of the primary. Lymphomas of bone are a common feature in disseminated lymphomas and occur in 10-15% in Hodgkin’s and 7-25% in non Hodgkin’s. The more aggressive lymphosarcoma and reticulum sarcoma rarely involve the skull. Treatment is local radiotherapy and chemotherapy. Leukaemias appear as ill-defined areas of rarefaction with peripheral new bone formation similar to neuroblastoma. Low dose radiotherapy and chemotherapy of the systemic disease is recommended. Multiple myeloma affects the males more commonly between 40-60 years of age. Skull x-rays show multiple punched out areas of bone destruction. Biopsy or bone marrow biopsy may be needed to confirm. They are treated with chemotherapy. Occasional solitary plasmocytomas are treated with radiotherapy. Neuroblastoma is a common tumor of childhood. Skull metastases often precede the detection of the primary adrenal tumor. Diffuse nodular lucencies are seen radiologically. They are highly vascular and lift up the periosteum, producing radial bone speculation extending into the soft tissues. The dura often resist the spread; however, occasionally, an intratumoral hemorrhage may rupture into the brain parenchyma. Spontaneous resolution into a more benign form has been reported; skull metastasis suggest a poor prognosis. Local radiotherapy and systemic chemotherapy is the treatment of choice

Skull metastases from Ca. breast Multiple myeloma   Olfactory Neuroblastoma-MRI

Ewing’s sarcoma are often multicentric in origin involving the tibia, ribs, and vertebrae and rarely seen as a primary skull lesion. Local radiotherapy and chemotherapy is recommended.

Skull involvement by direct extension: Meningiomas may produce varying degrees of incidental bony changes in the overlying skull and may also invade the skull bone itself and may break through the outer table to present as a subcutaneous swelling. The involved bone is considerably vascular with tortuous arterial and large venous channels. The bony changes may resemble those of fibrous dysplasia. Sometimes, they remain entirely intra-diploic, expanding the two tables resulting in a doughnut like lesion. Occasionally, there may be bone destruction similar to that of a metastasis. Benign Nasopharyngeal tumors usually become symptomatic long before the skull involvement. Angiofibroma and squamous cell pappilloma are the common ones. Marked intracranial extension may require combined approach with ENT surgeons. The malignant tumors of the nasopharynx and the paranasal sinuses usually present with intracranial involvement and are treated with radiotherapy following biopsy. Glomus jugulare is discussed elsewhere. Conditions simulating skull tumors: Osteomyelitic changes appear radiologically long after the onset of clinical signs and symptoms. Multiple nodular lucent areas appear in the outer table or diploe. Later they condense into a large defect with scalp edema (Pott’s puffy tumor). Poorly defined sclerosis occurs at the edges of the bone with practically no subperiosteaL new bone or sequestrum formation. Leptomeningeal cyst as complication of growing fracture is children may mimic a tumor radiologically as a lucent area with scalloped marigins and a soft tissue shadow outside the skull. Occasionally, it may be in between the tables producing intraosseous cyst.

Meningioma with skull infiltration-MRI Intradiploic meningioma-MRI Nasopharyngeal Angiofibroma-MRI Nasopharyngeal tumor with parasellar extension-MRI Ethmoidal tumor  with intracranial extension-MRI Glomus jugulare tumor.-MRI

Cephalhematoma in the new born, following forceps delivery, may produce a soft tissue shadow radiologically. It is commonly seen in the parietal bone limited by sutures and mimics a tumor. Calcific edges gradulally project into the soft tissues of the scalp, resulting in a shell like calcification. Rarely, a deformity persists.

Paget’s disease (osteitis deformans) is multicentric involving pelvis, the femora, the vertebrae, and more commonly the skull. Men are more frequently involved. It starts as a diffuse mottled thickening in the frontal or occipital area as irregular patches or lysis which give the appearance of a geographical skull radiologically. Later, patchy sclerosis develops. Deafness and blindness due to foraminal involvement may occur. Except for neural decompression, there is no specific treatment. Sarcomatous change is infrequent.  

Sarcoidosis rarely involves the skull. It is seen as multiple punched out areas of rarefaction.