Boneforming Lesions Osteoma

The most common tumor of the orbital bones is the osteoma, which is a benign lesion consisting primarily of mature cancellous bone (Figure 16.1).1,2 It has not been determined whether osteomas are true neoplasms or the result of an inflammatory reaction. Most of these lesions develop in the fourth to fifth decades of life; they are rare in children.1 There is evidence that osteoma is more commonly encountered in males.1,3

The standard way of labeling the cranial osteo-mas is to name them according to the bone of origin (e.g., zygomatic osteoma) whereas osteomas of the paranasal sinus are labeled according to which sinus they invade (e.g., frontal osteoma).3-5 Most os-teomas are asymptomatic, but if they grow to sufficient size, they may cause displacement and/or prop-tosis of the globe and signs of nasolacrimal duct obstruction, particularly by ethmoidal lesions.6,7 Osteomas usually develop anteriorly; therefore, they do not cause visual disturbances. The exception to this rule is the sphenoid sinus osteoma, which encroaches on the optic foramen and may produce optic nerve damage.8

Although these are solitary tumors, multiple lesions occasionally develop, particularly in patients with Gardner syndrome, a hereditary condition in which orbital osteomas can be associated with intestinal polyposis and carcinomatosis.9,10

Histopathologically, these tumors are composed of irregular bony trabeculae and fibrovascular tissue (Figure 16.1).1 Traditionally the osteomas are divided into three types based on histopathologic appearance: ebur-nated (ivory), fibrous, and mature. The eburnated os-teoma is primarily composed of thick bony trabecu-lae with little fibrous tissue. The fibrous type contains highly vascularized fibroadipose tissue between the bone elements, which may lead to a misdiagnosis of ossifying fibroma or osteogenic sarcoma.2 Typically, osteomas manifest with well-delineated homogeneous radiodensity in plain films and CTs. On plain skull films, osteomas are identified as well-delineated, dense lesions adjacent to a paranasal sinus.11

The differential diagnosis of these lesions should include endochondroma, osteogenesis imperfecta, bone infarction, fibrous dysplasia, low-grade osteogenic sarcoma, ossifying fibroma, and, in unusual cases, calcified meningioma.12-14

Only patients with symptomatic osteomas should be treated; when the osteomas are located anteriorly, simple excision will effect a cure, since recurrences are rare. However, when paranasal sinus osteomas invade the orbit posteriorly, particularly with optic canal involvement, the surgery is complex and should be performed by a craniofacial surgical team.

Osteoblastoma

Osteoblastomas are benign bone-forming tumors that account for approximately 10% of all tumors developing in skull bones. However, they are very rarely seen in the orbit.15

Osteogenic Sarcoma

Osteogenic sarcoma (osteosarcoma) is the most common primary malignant tumor of bone.16 It is also the most common malignant tumor of the craniofacial skeleton as well as of the orbit.17,18 The majority of patients seen are over the age of 40, and the disease more often affects men.19,20 The clinical presentation depends on the location and size of the tumor and its rate of growth.16

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BOX 16.1.

Fibro-osseous and

Cartilaginous Lesions of the Orbit

Bone-forming

Osteoma

lesions

Osteoblastoma

Osteogenic sarcoma

Parosteal osteogenic sarcoma

Cartilage-

Chondroma

forming

Chondrosarcoma

lesions

Mesenchymal chondrosarcoma

Multiple endochondromatoses

Fibro-osseus

Fibrous dysplasia

lesions

Ossifying fibroma

Fibrous

lesions

Giant-cell-rich

Giant cell reparative

lesions

granuloma

Osteitis fibrosa cystica

(brown tumor)

Giant cell tumor

Cystic lesions

Simple bone cyst

Aneurysmal bone cyst

Epidermal cyst

Radiologic studies show a poorly defined infiltrating lesion that can be sclerotic, lytic, or mixed sclerotic and lytic. Early changes discernible on computed tomography (CT) include increased bone marrow attenuation and calcification (Figure 16.2).21 The tumor is densely sclerotic in approximately 50% of cases. In the other 50% of cases, osteogenic sarcoma has an ossifying, destructive appearance, or it may present as a pure osteolytic lesion. The classic "sunburst" pattern produced as a result of periosteal new bone formation is not a helpful feature in skull tumors. Contrast-enhanced CT can determine the vascularity and the distribution of the blood vessels within the soft tissue component. Radionuclide bone scan is more sensitive for early changes than plain films and CT, whereas MRI is the most sensitive method of imaging.22 MRI defines the extent of the tumor much more accurately than CT. High or low signal intensity can occur on T2-weighted images owing to cellularity and os-teosclerosis, respectively. The relationship of the tumor to the compartments of the cranium is best assessed with magnetic resonance imaging (MRI). Magnetic resonance angiography (MRA) may also prove to be useful to demarcate tumor vessels.

Clinical manifestations depend on the location of the tumor, its size, and rate of growth. Most patients present with swelling of the orbit, proptosis, and pain, particularly at night.23,24 Osteosarcomas originating from ethmoid and frontal bones present as palpable and often visible masses, whereas tumors originating posteriorly, particularly from the sphenoid bone, produce proptosis without a palpable mass. Serum alkaline phosphatase levels are often elevated in os-teogenic sarcoma.

Osteogenic sarcomas are composed of pleomorphic spindle cells containing hyperchromatic nuclei and numerous mitotic figures intermixed with neoplastic bone formations. Depending on the matrix of the tumor, they are subgrouped as chondroblastic or fibro-matoid types.

It is known that cranio-orbital osteosarcomas are more often seen with Paget's disease, with fibrous dys-plasia, and after radiation treatment with or without retinoblastoma.25-27 The incidence of osteogenic sar

FIGURE 16.1. (A) Coronal CT image showing a densely sclerotic osteoma of the frontal bone. Note the thickening of the superior orbital rim (white arrow). (B) The intraoperative picture of another case of superior orbital rim osteoma. (C) Gross photograph of the same tumor with bosselated, well-circumscribed appearance. (D) Histopathology of the same osteoma reveals dense cortical-type bone with haversian systems (black arrows) of varying size and shapes.

FIGURE 16.1. (A) Coronal CT image showing a densely sclerotic osteoma of the frontal bone. Note the thickening of the superior orbital rim (white arrow). (B) The intraoperative picture of another case of superior orbital rim osteoma. (C) Gross photograph of the same tumor with bosselated, well-circumscribed appearance. (D) Histopathology of the same osteoma reveals dense cortical-type bone with haversian systems (black arrows) of varying size and shapes.

FIGURE 16.2. (A) Proptosis and inferior displacement of the left eye secondary to osteogenic sarcoma of the superior orbit. (B) Coronal CT image shows a poorly defined, infiltrating lesion with alternating areas of sclerosis and lysis pushing the globe inferiorly. (C) The histopathology consists of clusters of fusiform, atypical connective tissue cells surrounding deposits of neoplastic bone. (D) Atypical osteoblastic proliferation.

FIGURE 16.2. (A) Proptosis and inferior displacement of the left eye secondary to osteogenic sarcoma of the superior orbit. (B) Coronal CT image shows a poorly defined, infiltrating lesion with alternating areas of sclerosis and lysis pushing the globe inferiorly. (C) The histopathology consists of clusters of fusiform, atypical connective tissue cells surrounding deposits of neoplastic bone. (D) Atypical osteoblastic proliferation.

coma developing as a second primary tumor in survivors of retinoblastoma is high (see Chapter 5).16'17'28 Treatment of osteogenic sarcoma is surgical, but clear margins in the craniofacial cases may be difficult to obtain because frozen section of the bone cannot be done during excision. CT and MRI are the best means of determining the bone and soft tissue extent of the tumor.29 Preoperative chemotherapy has been reported to increase the survival.30

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