Overview of Orbital Tumors
The frontal, zygomatic, maxillary, sphenoid, lacrimal, palatine, and ethmoid bones bones form the orbit. The bony orbit protects the globe and allows for passage of the optic nerve, the ophthalmic division of the trigeminal nerve, the ocular motor nerves (third, fourth, and sixth), and the vessels required for proper function of the eye. These bones may be involved in fibroosseous tumor growth, such as osteomas, ossifying fibromas, fibrous dysplasia, aneurysmal bone cysts, and osteogenic sarcoma.
Osteomas arise most commonly from anterior ethmoid cells, although they are known to originate from posterior ethmoidal cells and sphenoid cells. This tumor exhibits a slowly invading growth pattern, and it can be associated with the presence of a mucocele if normal sinus drainage is occluded. Ossifying fibromas are benign, progressive, recurrent tumors that cause painless facial swelling and displacement of the eye; because of their possible extension into the anterior and middle cranial fossae, they can be problematic. These lesions are readily identified on CT scans by their diffusely dense signal.
Fibrous dysplasia commonly involves the sphenoid or frontal bone. It produces a general expansion of the involved bones and does not form a definable soft-tissue mass. Pathologically, they contain fibrous stroma with patches of osteoid but no osteoblasts (Figure 1). This characteristic leads to a low-intensity signal on T1 - and T2 -weighted MR images. On administration of gadolinium, however, they show enhancement. The lack of osteoblasts prevents proper bone formation, and the lesion becomes clinically significant by either encroaching on the optic canal or by causing cosmetic disturbances.
Aneurysmal bone cysts commonly affect the frontal bone in young children. These cysts form a large cavernous space filled with blood. It is thought that the inflammatory reaction caused by the surgical procedure helps to induce resorption of the lesion. Clearly, it is important to differentiate benign lesions of this type from the cystic changes of more sinister lesions, such as hemangiomas of the bone, giant cell tumor of bone, or osteogenic sarcoma. These lesions are definitively treated surgically.
. A fibrous dysplastic lesion was obtained in the orbit of a 40-year-old woman. Photomicrograph showing spindle-cell proliferation and metaplastic bone formation (A). A high-magnification photomicrograph emphasizing the absence of osteoblastic reaction of the bony trabeculae (B). H & E, original magnification x 100 (A) and x 200 (B).
Osteogenic sarcoma is rarely found in the orbital region. It may occur more frequently in cases in which there are genetic defects and a history of radiation therapy usually in the setting of an irradiation-treated retinoblastoma. Even radical excision may not improve the poor prognosis in patients with these tumors.
The periosteal lining of the orbit, also known as the periorbita, is an important structure in the orbit because it helps serve as a barrier to the intracranial extension of neoplastic growth. The periorbita proper can also be involved in nonneoplastic lesions; in young children, this can be the site of an idiopathic inflammatory condition known as infantile cortical hyperostosis (Caffey disease).
The only cartilaginous structure of the orbit is the trochlea, a U-shaped structure that suspends the tendon of the superior oblique muscle. Tumors arising from the trochlea such as chondromas and chondrosarcomas are exceedingly rare.
Vascular lesions of the orbit include capillary hemangiomas, cavernous hemangiomas, lymphangiomas, and hemangiopericytomas, as well as AVMs of intraorbital vessels. Together, these lesions comprise 12 to 15% of orbital tumors. Capillary hemangiomas usually present by the age of 6 months. These lesions are benign, yet widely infiltrative, and they can be associated with cutaneous manifestations such as strawberry nevi. Spontaneous resolution usually occurs over a 3- to 5-year period, but in some cases there may be significant residual cosmetic disfigurement and amblyopia. They can be treated with argon laser therapy or corticosteroid therapy to promote involution.
Cavernous hemangiomas, the most common benign orbital tumors, are found primarily in young and middle-aged adults. These are low-flow circumscribed lesions usually situated behind the globe, most commonly within the muscle cone. They do not usually interfere with visual acuity, except when the tumor lies in the orbital apex, where it may affect the optic nerve. Cavernous hemagiomas are readily amenable to surgical treatment and do not recur. They are seen as well-delineated enhancing masses on CT scans (Figure 2). On T1 -weighted MR images, these lesions have a low-intensity signal (Figure 3 upper), whereas they are hyperintense on T2 -weighted MR images (Figure 3 lower). In the event of intralesional thrombosis, a higher-intensity signal will be demonstrated on T1 -weighted MR images.
. Postcontrast axial CT scan obtained in a patient with a cavernous hemangioma of the left orbit.
. Upper: Coronal T1 -weighted MR image revealing a hypointense mass in the right orbit consistent with an orbital hemangioma. Lower: Axial T2 -weighted axial MR image obtained in the same patient.
Lymphangiomas are slow-growing lesions that may occur in the orbit even though the orbit does not have a lymphatic drainage system. These lesions most commonly occur in children and young adults and may cause a slowly progressive exophthalmos. Hemorrhage into the tumor may cause rapid expansion of the orbital contents and subsequent rapidly progressive, worrisome exophthalmos. These episodes may resolve spontaneously. Lymphangiomas are exceedingly difficult to manage surgically because they involve critical orbital structures, and laser therapies are currently being used as adjuvants when treatment is necessary. Because of their repeated episodes of hemorrhage, they demonstrate a heterogeneous signal intensity on T1 - and T2 -weighted MR imaging as a result of the presence of blood breakdown products. On MR imaging these lesions are not as enhancing as hemangiomas and may not enhance at all.
Hemangiopericytomas are malignant tumors thought to arise from undifferentiated mesenchymal cells with pericytic differentiation (Figure 4). They are most common in young and middle-aged adults. They are invasive tumors with a known propensity for metastasis, although this is rare from the orbit. Total excision is necessary to prevent recurrence.
. Photomicrographs. An orbital hemangiopericytoma obtained in a 56-year-old woman. A: The tumor is characterized by numerous vascular channels intermixed with plump, polygonal cells with mild nuclear atypia B: Mitotic figures may be prominent. H & E, original magnification x 100 (A) and x 200 (B).
Other tumors of mesenchymal origin worthy of mention include rhabdomyosarcoma and fibrous histiocytoma. The former are the most common malignant orbital tumors affecting children. They are rapidly progressive lesions that respond readily to irradiation and chemotherapy and therefore should not be missed. On CT scans, they appear as bulky, mildly enhancing masses that erode bone, and they can invade into the surrounding sinuses. On MR images, they are of intermediate signal intensity.
Fibrous histiocytomas are the most common orbital tumors in adults. They develop insidiously and can be locally infiltrating, and although they are benign lesions, their rate of recurrence is high. They should be resected, and the resection margins should be wide. Malignant fibrous histiocytomas may metastasize and lead to death.
Although the rectus, superior and inferior oblique, and levator palpebrae muscles may give rise to fibromas, fibrosarcomas, leiomyomas, and leiomyosarcomas, these primary muscle tumors are exceedingly rare. Lipomas, which can originate from the voluminous orbital fat, are sometimes difficult to differentiate clinically and pathologically from normal fat.
Nerve sheath tumors within the orbit can give rise to neurofibromas, schwannomas, and malignant peripheral nerve sheath tumors, comprising approximately 15% of orbital tumors. These lesions do not involve the optic nerves because the optic nerve lacks Schwann cells; rather, they develop from the peripheral motor nerves that supply extraocular muscles, the first and second division of the trigeminal nerve, or from sympathetic or parasympathetic fibers. Neurofibromas can be subdivided into three classes: solitary, diffuse, and plexiform. Solitary neurofibromas are single, encapsulated lesions that are not associated with NF. Because these lesions are well circumscribed, complete resection is possible. In contradistinction, diffuse neurofibromas, which can be associated with NF, are not solitary lesions, and they may involve orbital tissues in a more extensive manner. Plexiform neurofibromas are pathognomonic of NF1. These extensive lesions involve bundles of nerves, each with perineurium-encased tumor, making total resection impossible. Subtotal resection is performed when the mass of tumor tissue produces cosmetic deformity. Patients with NF may harbor plexiform neurofibromas as well as optic nerve gliomas and meningiomas.
Schwannomas, also known as neurilemmomas, are tumors consisting exclusively of Schwann cells. These lesions grow slowly, allowing the eye to accommodate a severe degree of exophthalmos (Figure 5). The lesion is solid, and it is encapsulated by the perineurium of the nerve from which it arose; because of these features, it is amenable to total resection. Schwannomas are unlikely to undergo malignant degeneration. Malignant peripheral nerve sheath tumors (previously termed neurofibrosarcomas or malignant schwannomas) are very rare lesions, but they are known to be associated with NF1. These malignant lesions must be distinguished histopathologically from mesenchymal tissue tumors.
. Postcontrast axial CT scan obtained in a patient with a schwannoma involving the right orbit.
Paragangliomas (chemodectomas) can arise from the ciliary ganglion, although they are more typically seen around the carotid arteries and in the jugular fossa (growing from the ganglia of the vagus nerve). In general, nerve sheath tumors exhibit a low-intensity signal on T1 -weighted and high-intensity signal on T2 -weighted MR images. Imaging of plexiform neurofibromas may demonstrate involvement and enlargement of the eyelids, fat, extraocular muscles, and lacrimal gland, as well as some facial deformity. Solitary neurofibromas and schwannomas are indistinguishable radiologically, as both appear as solitary, ovoid masses resembling cavernous hemangiomas. The optic nerve is a white matter tract of the brain. Masses formed by abnormal proliferation of fibrillary astroglial cells in this area are termed optic nerve gliomas, which are the second most common orbital tumor in children, the fifth most common in adults, and the most common in patients with NF1. Most optic nerve gliomas are low-grade lesions and are most likely pilocytic astrocytomas; in approximately 50% of cases these lesions are diagnosed in patients before the age of 5 years. Patients with optic nerve gliomas typically present with exophthalmos, and examination shows a severely decreased visual acuity, with some patients being completely blind in the affected eye. Papilledema and pallor of the optic disc are also common findings. Optic gliomas are best visualized on MR imaging, which can demonstrate extraorbital extension, as well as a fusiform enlargement of the optic nerve. When these lesions are restricted to the orbit, they usually prove isointense with white matter on both T1 - and T2 -weighted MR images whereas lesions that involve the chiasm may have increased signal intensity on T2 -weighted MR images.
Meningiomas are thought to arise from arachnoid cap cells, and therefore, they generally follow the distribution of the meninges (Figure 6). Orbital meningiomas can be subdivided into primary and secondary types; the former arise in the orbit and the latter expand to include the orbit. They comprise approximately 10% of all intraorbital tumors, with a female/male preponderance of 3:1. Patients with orbital meningiomas present with a wide range of signs and symptoms, most commonly exophthalmos, decreased visual acuity, pallor of the optic disc, or visual field deficits. Unilateral boggy edema of the eyelid, resembling myxedema, is a rare, peculiar finding in cases of long-standing meningiomas. Orbital pain or headache is an uncommon symptom in patients with these lesions. Computerized tomography scanning may demonstrate calcification in patients with meningiomas (Figure 7). On T1 -weighted MR images, meningiomas are usually isointense with muscle, whereas on T2 -weighted MR images, they are variable. These lesions enhance markedly after administration of gadolinium. In general, angiography can determine the site of origin of these tumors by demonstrating their blood supply. Meningiomas originating from the optic nerve sheath are particularly slow growing. In the preimaging days they were often mistaken for "chronic optic neuritis."
. Photomicrographs. A transitional meningioma infiltrating the adipose periorbital tissues obtained in a 38-year-old woman (A). The typical meningothelial cells with bland nuclei and syncytial cellular arrangement (B). The meningioma diffusely infiltrated the orbital bony structures and the temporalis muscle. H & E, original magnification x 100 (A) and x 200 (B).
. Noncontrast axial CT scan obtained in a patient with right optic nerve sheath meningioma, revealing the presence of tumoral calcifications.
Dermoids and epidermoids are benign cystic lesions that arise from congenital rests. Most orbital dermoids are located anteriorly, becoming evident during childhood because of their superficial location (Figure 8 upper). The most frequent location is the frontozygomatic suture (Figure 8 lower). Growth usually results from the slow production and accumulation of dermal products within the mass. On CT scans, these lesions appear well delimited from the surrounding structures, have smooth margins, and can have a fluid level (Figure 9).
. Upper: Photograph showing upper-lid swelling in a patient with a frontal dermoid involving the orbit. Lower: Macroscopic view of a dermoid of the orbit during surgical removal. Note the typical location in the superotemporal orbit.
. Axial nonenhanced CT scan of the orbit obtained in a patient with a dermoid of the left orbit.
There is a variety of metastatic tumors (Figure 10). The patient's age at presentation helps differentiate among the possible tumor types. In children, the most common metastatic lesions include neuroblastoma, Ewing tumor, chloroma, and Langerhans cell histiocytosis. Neuroblastomas in pediatric patients are the most common metastatic tumor to spread to the orbit and have been reported in up to 40% of patients. These tumors arise from the adrenal medulla or parasympathetic or sympathetic structures, often spreading to both orbits, where they frequently cause an abrupt onset of exophthalmos and bilateral eyelid edema and ecchymosis (Figure 11). Ewing tumor is a malignant bone lesion that classically involves the limbs, ribs, or pelvis. When this tumor metastasizes to the orbit, it usually presents as an abrupt hemorrhagic exophthalmos. Chloromas are the extramedullary form of acute myelogenetic leukemia. This lesion is also heralded by the onsets of acute hemorrhagic exophthalmos. Langerhans cell histiocytosis is a term used to describe three similar but separate multisystem diseases: eosinophilic granulomatosis (or histiocytosis X), Hand-Schüller-Christian disease, and Letterer- Sewe disease. Only the first two subclasses cause orbital disease: eosinophilic granulomatosis with solitary granulomatous proliferation may occur in the orbit, causing painful eye swelling and exophthalmos, whereas Hand-Schüller-Christian disease has exophthalmos as part of its clinical triad, as well as diabetes insipidus and lesions of the bone.
. Photomicrographs. A male patient with a germinoma of the suprasellar region presented with visual difficulties. A: The optic chiasm and optic nerve were diffusely infiltrated by the germinoma. Large polygonal cells with large nuclei and prominent nuclei permeated the entire optic nerve. B: Lymphocytic inflammatory infiltrate accompanied the neoplastic cells. C: An immunohistochemical stain for placental alkaline phosphatase confirmed the germinal nature of the tumor. A and B: H & E, original magnifications x 100 (A), x 200 (B), and x 100 (C).
In adults, the most common primary tumors to metastasize to bone are, in decreasing order, breast, lung, prostate, melanoma, gastrointestinal tract, and kidney. Because the orbital tumor may be the presenting lesion in up to 25% of cases, it should be strongly considered in the differential diagnosis of adult patients with orbital lesions causing exophthalmos. Due to the associated abysmal overall median survival rates of 1 to 2 years postdiagnosis, these tumors are best treated with radiation therapy or chemotherapy. Obviously, treatment decisions must be modified according to the degree of systemic involvement and prognosis in each patient.
Neurosurg Focus. 2001;10(5) © 2001 American Association of Neurological Surgeons
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