Aetiological classification

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
  • 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).

Miscellaneous conditions:

  • Aneurysmal bone cysts
  • epidermoid and dermoid tumors
  • intraosseous meningiomas
  • fibrous dysplasia are relatively rare conditions.

WHO Classification

1)typical or benign (88-94%)

2)atypical (5-7%)

3)anaplastic or malignant (1-2%).

Specific syndromes involving skull tumors have been described.

Gardner syndrome is the triad of the following:

  1. Multiple osteomas of the skull, sinus, and mandible
  2. Soft tissue tumors of skin
  3. Colon polyps

McCune-Albright syndrome comprises the triad of the following:

  1. Polyostotic fibrous dysplasia
  2. Hyperpigmented skin macules (Coast of Maine)
  3. Precocious puberty

Hand-Schüller-Christian disease consists of the following:

  • disease of unknown cause in which lipids accumulate in the body and manifest as histiocytic granuloma in bone, particularly in the skull; the skin; and viscera, often with hepatosplenomegaly and lymphadenopathy.
  1. Diabetes insipidus
  2. Exophthalmos
  3. Bone lesions

Differentials for frontoorbital (sphenoidal) bony tumors causing proptosis

  1. Hyperostotic meningioma(older age groups)
  2. Intraosseous meningioma
  3. Pagets disease of bone
  4. Fibrous dysplasia - bone is enlarged but follows the normal position or contour.
  5. Osteoma - mature cortical bone is involved and typically the diploïc space and inner table are spared

Meningiomas

  • 2-10% of population have asymptomatic meningiomas
  • multiple in 5-40% of cases, particularly when they associated with neurofibromatosis-2
  • incidence increases with age.
  • arise from arachnoid cells, particularly those packing the arachnoid villi, which protrude as fingerlike projections into the walls of the dural veins and sinuses
  • 90% of meningiomas are located in the supratentorial compartment.
  • may cause symptoms by irritating the underlying cortex, compressing the brain or the cranial nerves, producing hyperostosis and/or invading the overlying soft tissues, or inducing vascular injuries to the brain.
  • 3 most common histologic subtypes of meningiomas are the meningothelial (syncytial), transitional, and fibroblastic meningiomas. Others - psammomatous, angioblastic, and malignant tissue.
  • may also cause hyperostosis of the inner table without bony invasion, or with extensive penetration into the skull, causing bone thickening and osteoblastic reaction. The exact nature of the cause of this hyperostosis is controversial (ie, reactive versus tumoral infiltration).
  • En plaque meningiomas are particularly prone to invading the bone.

Intraosseous meningiomas

  • 1-2% of all meningiomas
  • Located in intradiploic space with no connection to dura
  • Usually psammomatous type
  • Most commonly occurs in the region of the frontal bone, involving the orbit and presents with the symptom of proptosis
  • Theories

1)ectopic meningocytes or arachnoid cap cells trapped in the cranial sutures during moulding of the head at birth

2)entrapment of meningothelial cells into suture or fracture lines as a result of trauma

3)arachnoidal cells that accompany the vesselsor the exiting nerves

  • differential diagnosis includes plasmacytoma, chondroma, chondrosarcoma, haemangioma, myeloma, eosinophilic granuloma, aneurysmal bone cyst, metastatic cancer or fibrous dysplasia.