In the CT and MR imaging era, plain radiography has a limited role in the diagnostic workup of patients with suspected orbital tumor. Before the availability of high-resolution cross-sectional imaging, ultrasonography was very popular. Ultrasonography is, however, still useful in the evaluation of ocular masses and fluid-filled orbital tumors such as cysts and vascular lesions.
Computerized tomography scanning is the imaging modality of choice for bone lesions, because of the natural contrast afforded by the retrobulbar fat, bone, and air. Orbital imaging protocols must include coronal as well as axial slices. In general, 1- to 5-mm-thick slices are adequate for evaluation of orbital masses. Intravenous contrast administration helps in the characterization of individual lesions.
Magnetic resonance imaging superbly demonstrates the anatomy of the orbit is sensitive to the different soft-tissue characteristics of orbital lesions. Additionally, MR imaging has the advantage of being a nonradiation-based modality, and, as such, does not deliver radiation to the lens. Despite the availability of MR angiography, conventional angiography of orbital lesions remains the gold standard for detecting vascular lesions, such as AVMs and low- flow dural arteriovenous fistulas, and for defining the extent of the blood supply to orbital tumors, especially meningiomas. Cerebral angiography should be considered for any patient with pulsatile exophthalmos.
The variations in imaging characteristics of all different orbital lesions are diverse and exceed the scope of this review. Furthermore, it should be emphasized that diagnoses of neoplastic growth should not be established in the absence of histological evaluation of a biopsy specimen. In this review, we focus on characteristics that can be useful to distinguish the different classes of lesions and illustrate imaging features of common lesions. As a general rule, benign tumors cause displacement and molding of adjacent structures, whereas malignant lesions display an aggressive pattern of growth, destroying and obliterating the contours of surrounding structures. On imaging studies, these two patterns of growth are usually, albeit not always, distinguishable. In cases of primary orbital tumors the extensive bone destruction produced by metastatic lesions is rarely demonstrated.
The radiological findings in pseudotumor depend on the structures and the nature of the ongoing inflammatory process, but the following generalities can be made. Pseudotumors are enhancing lesions that are usually isodense with muscle on T1 -weighted images, whereas they are nearly isodense with fat on T2 -weighted images. Myositic pseudotumor may be seen as an enlargement of one or more muscle bellies in the orbit, but it may also involve the tendinous origin, unlike dysthyroid orbitopathy.
Dysthyroid orbitopathy, which results in extraocular muscle infiltration with lymphocytes, plasma cells, mast cells, and mucopolysaccharides may involve any muscle of the orbit but most commonly enlarges the inferior or medial rectus. This can be helpful in distinguishing Graves disease from neoplastic growth, whereas the sparing of the tendinous origin in Graves disease distinguishes it from the myositic variant of pseudotumor.
Noteworthy imaging characteristics include enlargement of the optic canal observed in lesions involving the optic nerve; destruction of bone can occur in cases of aggressive tumors such as rhabdomyosarcomas. A decrease in size of the canal can be observed in the presence of non-neoplastic lesions involving the bone such as fibrous dysplasia and ossifying fibroma.
Arteriovenous malformations are associated with the presence of phleboliths, (calcified deposits in a vein) which appear almost isodense with bone on CT scans. This type of lesion is also suggested by the presence of flow voids demonstrated on MR imaging. Calcium deposits in the mass of a lesion are also more suggestive of certain tumor types, most notably retinoblastoma and meningioma.
Neurosurg Focus. 2001;10(5) © 2001 American Association of Neurological Surgeons
Cite this: An Introductory Overview of Orbital Tumors - Medscape - May 01, 2001.