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the vitreous. Hemorrhage may be difficult to distinguish from mass on the T1 precontrast sequence as both may be T1 hyperintense, but the hemorrhagic component does not enhance.
12.3.3 Uveal Metastases
Carcinoma from primary lesions, such as those in the breast or lung, may metastasize to the orbit. Lesions generally are hypointense on T1 images, are generally hyperintense on T2 images, and demonstrate enhancement.
12.4 Orbital Lesions
12.4.1 Lymphoma
Involvement of the orbit with lymphoma may be primary or secondary to systemic disease. Orbital lymphoma is generally a low-grade marginal zone B-cell lymphoma of the mucosa-associated lymphoid tissue, accounting for 40–70% of lesions [14], and is associated with systemic disease in about 30% of patients [15]. In patients with lymphoma of the orbit, the risk of systemic involvement increases with grade. The average age at presentation is 50–70 years, but this disease can occur in children and young adults. Ocular adnexal lymphomas may involve the conjunctiva, eyelid, and orbital soft tissue, including the lacrimal gland [16]. Lymphoma is often contained by facial planes and grows by molding around adjacent structures rather than presenting as a solid mass. Lymphoma almost never indents the globe [16].
The appearance varies from diffusely infiltrative in high-grade lesions to solid tumors with defined margins in less aggressive forms. Usually, there is less bone destruction than in other disease processes. On CT, lymphoma is similar in density to muscle and demonstrates enhancement. Lymphoma is isointense to muscle on T1 images and demonstrates moderate to marked enhancement (Fig. 12.3a). Since lymphoma is hypercellular, it tends to be isointense to hypointense to muscle on T2 sequences (Fig. 12.3b), which differentiates this lesion from rhabdomyosarcoma. Central nervous system lymphoma may also show decreased diffusion (Fig. 12.3c) [17]. On PET/CT, lymphoma is seen as an FDG-avid lesion (Fig. 12.3d), and systemic disease may also be demonstrated (Fig. 12.4).
12.4.2 Orbital Rhabdomyosarcoma
Rhabdomyosarcoma arises from primitive undifferentiated mesenchymal cells in the orbit. The tumor can occur in patients at any age, but the average age at presentation is 7 years. Rhabdomyosarcoma has a predilection for the upper outer quadrant [18] of the orbit, but it may occur in the lid or conjunctiva. Lesions range from moderately well defined to ill-defined and demonstrate mild to moderate enhancement.
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Fig. 12.3 Orbital lymphoma, right orbit. (a) Coronal T1 postcontrast image demonstrates an enhancing superior right orbital mass (arrow). (b) Axial T2 image shows the mass is isointense to brain parenchyma (arrow). (c) Axial diffusion-weighted sequence shows decreased diffusion within the mass (arrow). (d) PET/CT axial plane image shows an 18F-fluorodeoxyglucose avid right orbital lesion (arrow)
Invasion of paranasal sinuses and intracranial extension may occur [19, 20]. On CT, lesions are similar in density to soft tissue and heterogeneous in appearance, and bone destruction may be seen. On MRI, rhabdomyosarcoma is isointense to muscle on T1 images, hyperintense to muscle on T2 images, and demonstrates enhancement (Fig. 12.5). The globe is often displaced but rarely invaded [18].
12.4.3 Orbital Nerve Sheath Tumors
Neurofibroma and schwannoma most commonly arise from the peripheral nerves of the orbit [21] and therefore have a predilection for the extroconal, superior orbit [22]; they are well defined and oval or fusiform. Nerve sheath tumors present in
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Fig. 12.4 Orbital lymphoma with systemic disease. PET/CT image shows 18F-fluorodeoxyglucose avid adenopathy within the neck, axilla, and mediastinum
Fig. 12.5 Orbital rhabdomyosarcoma, left orbit. (a) Axial T1 postcontrast image demonstrates an enhancing preand postseptal left orbital mass (arrow). (b) Coronal T1 postcontrast image demonstrates an enhancing mass (arrow) with inferior displacement of the left globe. Figures courtesy of Dr. Bita Esmaeli
young to middle-aged adults are noninvasive and are associated with a painless proptosis [21]. On CT, these tumors are isodense to slightly hyperdense and demonstrate mild to marked enhancement. On MRI (Fig. 12.6) these tumors are isointense to slightly hyperintense to muscle on T1 images, hyperintense on T2 images, and also
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Fig. 12.6 Plexiform neurofibroma, right orbit. (a) Axial T2 image demonstrates the lesion within the right temporal soft tissue and superior right orbit (arrows). (b) Coronal T1 postcontrast image shows the enhancing characteristics of the lesions (arrows)
demonstrate moderate to marked enhancement. As lesions age, they become more hypocellular with a predominance of collagen and may not significantly enhance. Associated expansion of the orbit and bone erosion may be seen.
12.4.4 Mesenchymal Tumors of the Orbit
Mesenchymal tumors of the orbit include fibrous histiocytoma, fibroma, and fibrosarcoma. Fibrous histiocytoma probably arises from a fibroblast precursor and may be benign or malignant [23, 24]. Fibroma is usually found in young adults [25] and grows slowly. On CT, these lesions appear well circumscribed and demonstrate moderate to marked enhancement. The benign form can cause bony remodeling, and the malignant form causes bone erosion [26, 27]. On MRI, the lesion is hypointense to isointense on T1 images and demonstrates moderate enhancement. On T2 images, fibrous tumors are hypointense to hyperintense. The fibrous component is hypointense on both sequences.
Other mesenchymal tumors include chondrosarcoma, osteosarcoma, Ewing sarcoma, and synovial sarcoma. These lesions may occur within or around the orbit with secondary extension. CT demonstrates a soft tissue mass of muscle density with variable enhancement (Fig. 12.7). Associated bone destruction may occur with more aggressive lesions. On MRI, lesions are often nonspecific with T1 signal hypointensity, T2 signal isointensity to hyperintensity, and variable enhancement.