Ill Defined Solid Orbital Lesions

The clinical differential diagnosis of the most common solid ill-defined orbital lesions in children includes capillary hemangioma, lymphangioma, plexi-form neurofibroma, idiopathic orbital inflammation, and metastasis.1-3,25 In adults there can be idiopathic orbital inflammation, metastasis, primary orbital tumor, and lymphoproliferative disorder.1-3,25

TABLE 10.2. MR Features of the Less Common Well-Circumscribed Orbital Lesions on Spin-Echo Sequences.

Lesion appearance and signal with respect to vitreous Tl-weighted image T2-Weighted image

Degree of lesion enhancement after Gd-DTPA

Lymphoproliferative disorders

Capillary hemangioma

Orbital varix

Thrombosed varix

Orbital metastasis (skin malignant melanoma carcinoid)

Homo

Iso/Hyper

Homo/Hetero

Iso/Hyper

Homo/Hetero

Iso/Hyper

Hetero

Iso/Hyper

Homo

Iso/Hypo

Homo/Hetero

Iso/Hypo

Homo/Hetero

Iso/Hypo

Hetero

Iso/Hypo

Homo

Homo/Hetero

Homo/Hetero

Hetero

Homo

+/+ + +

FIGURE 10.4. Orbital cavernous hemangioma. (A) Axial postcon-trast Tl-weighted image. This well-circumscribed lesion demonstrates heterogeneous enhancement shortly after contrast injection.

Solid ill-defined orbital lesions usually present on MR studies as a diffuse, infiltrating, nonencapsulated pattern that often involves the extraocular muscles, the lacrimal gland, and sometimes the bony orbit. Id-iopathic orbital inflammation has a homogeneous isointense to slightly hyperintense signal with respect to the vitreous and a hypointense signal with respect to the orbital fat on Tl-weighted images.19 On T2-weighted images, the lesion may appear isointense to hypointense with respect to the vitreous.19 The increased signal of an inflammatory process is related to its acute stage and its high concentration of free wa-ter.18 Other differential diagnoses that can conceivably have identical MR characteristics most often include capillary hemangioma, plexiform neurofibroma, metastasis, lymphoproliferative disorder, and rhabdo-myosarcoma (Table 10.3, Figure 10.5).1-4,27-29 Malignant processes and occasionally inflammatory lesions may produce bone changes as disruption of the regular signal void of the adjacent cortical bone or replacement of the high signal of the fat marrow by the hypointense lesions.1-4,27-29

After administration of Gd-DTPA, these solid, ill-defined orbital lesions demonstrate diffuse moderate

(B) Fat-suppressed coronal postcontrast T1-weighted image. Fifteen minutes later, the lesion shows marked homogeneous enhancement owing to the pooling of the contrast agent.

to marked enhancement on T1-weighted images. This enhancement as well as the extent of the lesion is best delineated on fat-suppressed scans. Minimal and heterogeneous enhancement is usually seen in the scle-rosing type of idiopathic orbital inflammation, and marked enhancement is actually seen in the acute type, making its radiologic differentiation easier (Table 10.3).19,20

MRI does not provide sufficient tissue specificity to allow reliable differentiation among benign reactive lymphoid hyperplasia, atypical lymphoid hyperplasia, and malignant non-Hodgkin or Hodgkin lymphoma and between lymphoproliferative disorders and idio-pathic orbital pseudotumor.19,28 The main role of MRI in evaluating an ill-defined orbital lesion is to delineate the extent of the lesion if surgery is contemplated.

There are no specific CT features of ill-defined orbital mass that can help the clinician or the radiologist to identify the histologic nature of the lesion.3 CT may give some information regarding the malignancy or chronic behavior of the lesion if bone changes are identified on bone window scans.3,23 Radiolucent areas within the tumor may suggest necrotic changes. The age of the patient, the clinical presentation, and

TABLE 10.3. MR Features of the Most Common Ill-Defined Orbital Lesions on Spin-Echo Sequences.

Lesion appearance and signal with respect to vitreous T1-Weighted image T2-Weighted image

Degree of lesion enhancement after Gd-DTPA

Metastasis

Homo/Hetero

Homo/Hetero

Homo/Hetero

Hyper

Hypo

+/+ + +

Primary orbital tumor

Homo/Hetero

Homo/Hetero

Homo/Hetero

Hyper

Hypo

+/+ + +

Lymphoid proliferative disorder

Homo

Homo

Homo

Hyper

Hypo

+++

Capillary hemangioma

Homo/Hetero

Homo/Hetero

Homo/Hetero

Iso/Hyper

Iso/hypo

+++

Acute idiopathic inflammation

Homo

Homo

Homo

Iso/Hyper

Iso/Hypo

+++

Chronic idiopathic inflammation

Homo

Homo

Homo/Hetero

(sclerosing type)

Iso/Hyper

Hypo

-/+

FIGURE 10.4. Orbital cavernous hemangioma. (A) Axial postcon-trast Tl-weighted image. This well-circumscribed lesion demonstrates heterogeneous enhancement shortly after contrast injection.

(B) Fat-suppressed coronal postcontrast T1-weighted image. Fifteen minutes later, the lesion shows marked homogeneous enhancement owing to the pooling of the contrast agent.

FIGURE 10.5. Ill-defined infiltrative orbital lesion with a radiological differential diagnosis including idiopathic orbital inflammation, metastasis, and malignant lymphoma. (A) Coronal precontrast and (B) postcontrast Tl-weighted images show an infiltrative lesion in-

the anatomic location of the lesion will help the clinician or radiologist in this differential diagnosis.

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