Benign Skull Tumors 

Tumors of the skull are uncommon lesions that are not reported systematically in the medical literature. Therefore, assessing their true incidence and consequences to the health of the general population is difficult. Recent diagnostic advances have made such lesions easier to recognize, and new skull-base surgery techniques have provided access to previously inoperable skull tumors. Skull tumors are estimated to account for approximately 1% of bone tumors.

Treatment for most tumors is not controversial. However, the differentiation and identification of the tumor type is the greatest clinical challenge. The usual presentation is an enlarging skull mass, with or without pain, or cranial nerve deficits if the tumor involves the base of the skull.

Plain skull radiography with special projections is an important diagnostic tool. The initial classification of a lesion into radiolucent (osteolytic) or radiopaque (osteoblastic) is of considerable significance. The presence of sharply defined or irregular margins, presence or absence of sclerotic borders, and calcifications in the lesion are also important. Head CT scanning, with and without contrast, is useful in determining the extent of intracranial extension and other tumor characteristics.

Most skull tumors share certain MRI characteristics, such as hypointensity on T1-weighted images, hyperintensity on T2-weighted images, and some degree of contrast enhancement. The capability of imaging in multiple planes and enhanced soft tissue discrimination has made MRI an important diagnostic tool. The classification of benign and malignant brain tumors is based on that by Wilkins and Rengachary^[1] (which is a modified version of the system outlined by Huvos^[2] ).

The tumor type and behavior determine radiographic appearance (eg, radiolucent, radiopaque). Depending on the primary proliferating cell, benign skull tumors can be any of the following:

• Bone forming
• Cartilage forming
• Tumors of connective tissue
• Histiocytic tumors
• Tumors of blood or blood vessel origin
• Other types, including fibrous dysplasia, Paget disease, or epidermoid, dermoid, or aneurysmal bone cysts^{[3, 4, 5]}

Frequency

United States

One of the most comprehensive series of bone tumors with classification originated from the Mayo Clinic. Of the 7975 patients in the series, 4% had tumors involving the skull (excluding the mandible, maxilla, and nasal cavity). Of these tumors, 19% were benign and 81% were malignant. Because the Mayo Clinic is a tertiary referral center, this series probably reflects some degree of selection bias. Other studies estimate that skull tumors comprise 1% of bone tumors.

Bone-forming tumors: Osteomas are the most common primary brain tumors of the calvaria, affecting 0.4% of the general population. Osteoid osteomas and ossifying fibromas are rare. Osteoblastomas account for approximately 1% of bone tumors.

Cartilage-forming tumors: Chondromas and chondromyxoid fibromas are rare. Chondroblastoma, although rare in some studies, accounted for 10% of the benign skull tumors in the Mayo series.

Connective tissue tumors: Desmoplastic fibroma is very rare in the skull (in the literature, only case reports exist).

Histiocytic tumors: Giant cell granuloma, nonossifying fibroma, and xanthoma are very rare in the skull.

Tumors of blood or blood vessel origin: Eosinophilic granuloma commonly affects the skull. Hemangiomas account for 10% of benign skull tumors (70% in the Mayo series).

Lymphangiomas: These tumors are rare.

Miscellaneous conditions: Aneurysmal bone cysts, epidermoid and dermoid tumors, intraosseous meningiomas, and fibrous dysplasia are relatively rare conditions. The prevalence of Paget disease is believed to be 1-5% in those older than 40 years, with involvement of any bone in the body, but most individuals remain asymptomatic and the condition is undiagnosed.

Mortality/Morbidity

• Morbidity is due to recurrent sinusitis (tumors affecting sinuses), recurrence of tumor after excision, and cranial nerve compression at the skull base.
• Malignant transformation to osteosarcoma, fibrosarcoma, or chondrosarcoma is observed in 2% of patients with Paget disease and 0.5% of patients with fibrous dysplasia.

Sex

Most tumors demonstrate no sex predilection.

• Osteomas, ossifying fibromas, chondromas, and giant cell granulomas are observed more often in females than in males.
• Osteoid osteomas, osteoblastomas, eosinophilic granuloma, and Paget disease affect males more often than females.
• The female-to-male ratio of hemangiomas is 1:2.

Age

• Bone-forming tumors, connective tissue tumors, giant cell granulomas, and fibrous dysplasias usually manifest in young adults. Cartilage-forming tumors affect those aged 20-50 years.
• Eosinophilic granuloma, nonossifying fibroma, and xanthoma usually manifest in those younger than 20 years. Epidermoids, dermoids, and lymphangiomas are usually observed in children.
• The usual age of presentation for hemangiomas is in the fourth to sixth decade of life. Intraosseous meningioma and Paget disease affect those older than 50 years.