ANATOMY

To undestand orbital pathology is important to have a basic knowledge of certain anatomic details that are shown below

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Fig. 5: The superior image represents intraconal space while the inferior image represents extraconal space
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Computed tomography (CT) is the first-line imaging modality for orbital evaluation in the acute setting (inflammatory and traumatic) and to asses bony structures. Magnetic resonance (MR) has an important role in the diagnostic work-up of other entities.

We describe radiologic features of the main pathologies illustrating with cases of our center.

Differential diagnosis of orbital pathology:

1. CONGENITAL

2. VASCULAR

3. INFECTIOUS/ INFLAMMATORY

4. NEOPLASTIC

4.1 BENING TUMORS

4.2 MALIGNANT TUMORS

1. CONGENITAL

1.1-Dermoid cyst:

Dermoid cyst constitutes the most common congenital lesion of the orbit. It represents 2-8 % of the overall orbital masses.

Normally it appears as a well circumscribed, non enhancing cystic mass, localized on the supero-external quadrant of the orbit or in the lacrimal fossa in 80% of cases.

It may have either liquid or solid contents. Lipid component is present in 40-50% of cases. Fig. 11 Fig. 12

1.2- Optical dysplasia:

Septo-optic dysplasia consist on optic nerve hypoplasia and septum pellucidum agenesia or hypoplasia. Hypothalamic and pituitary dysfunction is present in 66% of patients.

Associated malformations are: schizencephaly, Chiari II malformation, Corpus callosum agenesia, aqueductal stenosis and rhombencephalosynapsis.

The clinical presentation is variable. Visual impairment is most frequent when there is no schizencephaly associated.  Fig. 13

1.3- Coats disease:

Congenital vascular malformation characterized by retinal vessel telangiectasia and aneurysmal dilations that manifests as leukocoria.

It is unilateral in 90 % of cases and non-inherited related, affecting boys between 6 and 8 years of age.

Ophthalmoscopy is often sufficient to make the diagnosis of Coats’ disease, although in advanced cases with ultrasound may be helpful to rule out an underlying mass lesion and CT and MR may also have some utility in atypical cases to identify calcification or enhancement that may be indicative of retinoblastoma.

Imaging findings:

- US: posterior vitreum hyperechoic mass without acoustic shadow. Pre- and subretinal exudates and hemorrhage are commonly seen.

- CT: Increased attenuation of the ocular globe secondary to protein exudates without calcifications.

- MR: Exudates are lipid rich, thus, they are hyperintense on T1 and T2 weighted images. Hemorrhage and fibrosis may give a heterogeneous appearance on T2.

- Contrasted studies show a linear “V-shape” enhancement of the anterior margin corresponding to a thickened retina with vessel telangiectasia and aneurysmal dilations. Subretinal space doesn¥t enhance.  Fig. 14

1.4- Coloboma:

It is an ocular globe defect present from birth secondary to the failure in choroid fissure fusion. Symptoms may be mild or more severe, depending on the size and location of the gap. Coloboma may be related to CHARGE and COACH syndrome.

On CT and MR it appears as a posterior focal defect with vitreum herniation and retrobulbat cyst with liquid contents.  Fig. 15

1.5- Persistent hyperplastic primary vitreous (PHPV):

This is caused by a persistent and hyperplastic fibrovascular tissue that belongs to the primary vitreous and its arterial supply (hyaloid artery). It used to be unilateral (90-98%) being the second most common cause of leukocoria.

Imaging findings:

-  Microphthalmia

-  Retinal detachment

- Soft-tissue replacement of the vitreous body: high attenuation on CT secondary to fibrovascular replacement and recurrent hemorrhage

- Persistent canal that goes from the optic nerve to the posterior surface of the lens

-  Absence of abnormal calcification within the orbit

-  Contrast administration shows enhancement of the retrolental mass.

CT is superior than MR to exclude retinoblastoba, even though MR is better to differentiate PHPV from pseudoretinoblastomas (late stages of Coats disease, inflammatory conditions (toxocariasis), severe hemorrhage...) Fig. 16

2.VASCULAR:

2.1. Carotid cavernous fistula:

Abnormal communication between the carotid arterial system and the cavernous sinus.

They can be classified as:

− Direct fistula: direct communication between cavernous segment of the internal carotid artery and the cavernous sinus (type A).

− Indirect fistula: the communication occurs through meningeal branches of internal carotid artery (type B), external carotid artery (type C) or both (type D). They have been proposed to be secondary to sinus thrombosis with revascularization.

Typical symptoms are: pulsatile exophtalmus (75% of cases), chemosis and subconjuntival hemorrhage.

Imaging findings:

- Proptosis

- Dilated superior ophthalmic vein

- Diffuse thickening of extrinsic ocular muscles

- Cavernous sinus enlargement (convex surface)

- Orbital edema

- Subarachnoid/intraparenchimatous hemorrhage secondary to a cortical vein rupture.

Angiographic findings:

- Abnormal arterial phase filling of the cavernous sinus

- Dilatation of drainage veins and inversion of circulation from cavernous sinus to superior ophthalmic vein. 

Fig. 17 Fig. 19

2.2 Ophtalmic Varix:

An uncommon vascular malformation compound of dilated solitary or multiple venous structures that communicates directly with systemic veins.

It may appear secondary to other pathologies (intracranial A-V malformations, carotid-cavernous sinus fistula, dural A-V fistula...) or isolated (primary).

Classical symptoms are dyplopia or intermittent proptosis during increasing venous pressure maneuvers. Intraocular hemorrhage and thrombosis are possible complications.

Imaging findings:

- CT: phebolites may be seen. After contrast administration we will see a regular or an irregular structure with enlargement with Valsalva maneuver and enhancement during the venous phase, localized on the orbital ·pex. The enhancement may be absent if there is thrombosis. It is important to perform studies during Valsalva maneuvers.

- MR: T1 hypointense and T2 hyperintense structure with enhancement after gadolinium administration.  Fig. 20

2.3. Cavernous haemangioma:

Most common orbital mass in adulthood. It is a slow growing hamartomatous malformation, oval shaped and with fibrous pseudocapsule. Clinical symptoms are painless proptosis. Localization: 80% intracranial.

Imaging features: isointense on T1, hyperintense on T2 and progressive enhancement. It may have calcifications but they are rare.  Fig. 21  Fig. 22

2.4. Lymphangioma:

It appears in early infancy, appearing as a unilateral mass that involves either superficial or deep orbital structures, with frequent multicompartimental location: pre- and postseptal, intra- and extraconal.

It has an irregular shape with no capsule and causes a diffuse infiltration of adjacent structures, crossing anatomical barriers. Lymphangioma has liquid contents and occasional solid poles.

CT is useful to detect phebolytes. Mass attenuation depends on the presence of hemorrhage.  MR gives a better anatomical description of the lesion and its vascular components as well as detects different hemoglobin products. Blood-fluid levels are useful to establish the diagnosis.

It appears as a slightly hyperintense mass on the T1 and strongly hyperintense on T2. There is no fluid voids or enlarge afferent vessels, opposite to infantile capillary haemangioma. After contrast administration it may have a slight to moderate enhance either diffuse or irregular.  Fig. 23

3. INFLAMMATORY-INFECTIOUS:

3.1.Ocular toxocariasis:

It is a granulomatous reaction in the vitreous or uvea in response to infestation by the dead or dying larvae of Toxocara Canis or T cati after ingestion of eggs in fecally contaminated soil or sandboxes.

Most commonly affected organs are the liver, lungs, brain, and eye. Ocular involvement tends to occur in the absence of systemic infestation, in children 5–10 years of age.

Signs and symptoms are caused by hypersensitivity and granulomatous inflammatory reaction to the dead or dying larvae. The most common presenting symptom is unilateral visual impairment, that often progresses to blindness, as well as leukocoria.

Imaging Findings are nonspecific. The most important finding is the absence of calcification opposite to retinoblastoma.

US: echogenic mass in the vitreous. CT appearance: is identical to that of Coats' disease; high attenuation mass with no calcification. At MR imaging: central vitreous mass is usually visualized, although it may be isointense to vitreous on T1-weighted images. The mass may be hyperintense relative to the vitreous on T2-weighted images. Alternatively, the granuloma may be of low signal intensity owing to dense fibrosis.

The latter appearance is similar to retinoblastoma, but the origin of the mass in a central granulomatosis rather than posterior, as in retinoblastoma.  Fig. 24

3.2. Subperiostic abcess and cellulitis:  Fig. 27  Fig. 28  Fig. 29  Fig. 30

Orbital infections represent >50% of primary orbital disease processes.

The location of infection is described with respect to the orbital septum, as either preseptal (periorbital) or postseptal (orbital). 

The orbital septum is a thin sheet of fibrous tissue that originates in the orbital periosteum and inserts in the palpebral tissues and separates the periorbital from the intraorbital space. The orbital septum provides a barrier against the spread of periorbital infections into the orbit.

The distinction between periorbital and orbital processes is clinically important because postseptal infections are treated more aggressively to prevent devastating complications such as cavernous sinus thrombosis and meningitis.

Periorbital cellulitis: is defined as preseptal process limited to the soft tissues anterior to the orbital septum. It most commonly arises from the contiguous spread of infection from adjacent structures (face, teeth, and ocular adnexa) or from local trauma.

Symptoms: swelling and erythema of the eyelids, chemosis, and, in severe cases, limitation of eye movement without proptosis.

Cross-sectional imaging: demonstrates diffuse soft-tissue thickening and areas of enhancement anterior to the orbital septum without abscess formation.

Periorbital cellulitis in adults typically is treated with antibiotics v.o. 

Orbital cellulitis: is a postseptal infectious (>50% under 4 years of age) process, most commonly caused by paranasal sinusitis (60-85%), which spreads to the orbit via a perivascular pathway (without bone destruction). Orbital cellulitis can be divided into intraconal (within the cone, which consists of the extraocular muscles and the fascia), extraconal (outside the muscular cone, between the orbital bone and intermuscular membrane), and subperiosteal locations.

Patients with orbital cellulitis also may present with proptosis. Visual acuity is usually maintained.

Imaging findings: hyperatenuation of intraconal fat, edema, extraocular muscles swelling, intraorbital and/or subperiosteal abscess.

Treatment of orbital cellulitis typically requires the intravenous administration of antibiotics.

Imaging is necessary to determine the infection spread (identify postseptal process and its complications), infectious origin and to rule out possible abscess formation.

In case of visual impairment, proptosis or ophtalmoplejia that predicts postseptal involvement as well as when there is no recovery 24-48 hours after adequate treatment.

Complications: Development of an orbital subperiosteal abscess is most commonly associated with ethmoid sinusitis. CT findings include sinus opacification, retrobulbar stranding, and extraconal fluid collections adjacent to the orbital wall, with mass effect upon the extraocular muscles. Of note, early abscesses do not necessarily demonstrate peripheral enhancement. Drainage of the abscess may be necessary to avoid a rapid elevation of intraorbital pressure and resultant visual impairment.

Since the advent of antibiotics, intraconal abscesses secondary to paranasal sinusitis have become rare. Intraconal abscesses usually require surgical drainage.

Additional complications of orbital cellulitis include:

- Thrombosis of the superior ophthalmic vein

- Thrombosis of the cavernous sinuses

- Bacterial meningitis

- Epidural and subdural abscess

- Parenchymal brain abscess

- Venous infarct.

Treatment in such cases includes the surgical evacuation of the intracranial abscess, as well as intravenous antibiotic therapy.

Frontal sinusitis may cause periorbital cellulitis or frontal bone osteomyelitis with a secondary extracranial abscess, known as a Pott puffy tumor.  Fig. 39

Dacryocystitis: is inflammation and dilatation of the lacrimal sac, which is located along the inner canthus. The diagnosis is based on clinical manifestations, imaging may be requested to exclude orbital cellulitis.  Imaging finding: is a well- circumscribed round lesion that is centered at the lacrimal fossa and that demonstrates peripheral enhancement. Treatment options include medical and surgical methods, with the method selected depending on the clinical signs and symptoms.  Fig. 31

Fig. 25

Fig. 26

3.3. Idiopathic orbital pseudotumor: 

It is also known as orbital pseudotumor, being the second most common cause of exophthalmos. The diagnosis is made by excluding other possible causes of exophthalmos since it is a nongranulomatous orbital inflammatory process with unknown cause.

Diagnosis is based on the medical history, clinical course, results of laboratory testing, and response to steroids.

The symptoms include unilateral painful proptosis and eyelid swelling with a sudden onset. It may be associated with diplopia and decreased vision.

The radiographic features of idiopathic orbital inflammatory syndrome vary widely and include orbital fat stranding, myositis (the superior and medial musculature are most commonly affected), poorly marginated enhancing mass with a generally infiltrative appearance. Lacrimal gland inflammation and enlargement, diffuse orbital involvement, and involvement of the optic nerve sheath complex, uvea, and sclera are possibly found.

In idiopathic orbital inflammatory syndrome, unlike Graves ophthalmopathy, there is tendinous involvement of the extraocular muscles.

Steroid therapy classically results in rapid improvement.

Fig. 32 Fig. 33

3.4. Orbital sarcoidosis:

Ocular involvement occurs in up to 80% of patients. Although any part of the eye or orbit can be involved, bilateral uveitis is the most common condition.

Acute uveitis usually resolves spontaneously or responds to local corticosteroid therapy. Involvement of bilateral lacrimal glands can also occur and is common.

Contrast enhanced CT or MR imaging typically demonstrates enlarged, enhancing lacrimal glands.  Fig. 34

3.5. Graves Basedow ophtalmopathy: 

Graves ophthalmopathy is the most common cause of exophthalmos in adults. Usually occurs 5 years after the onset of Graves thyroid disease and is postulated to be an autoimmune condition unrelated to thyroid function.

Orbital findings include lid retraction, proptosis, ophthalmoplegia, conjunctivitis, and chemosis. In Graves ophthalmopathy, classically bilateral and symmetric enlargement of the extraocular muscles is observed, with sparing of the tendinous insertion. They also may be unilateral.

Additional imaging findings include increased orbital fat, lacrimal gland enlargement, eyelid edema, stretching of the optic nerve, and tenting of the posterior globe.

Treatment is primarily conservative, with surgery and radiation therapy reserved to diminish tension on the optic nerve.  Fig. 35

3.6. Optic neuritis:

Optic neuritis, which involves inflammation or demyelination of the optic nerve, often manifests with unilateral eye pain and visual loss.

At T2-weighted MR imaging, it is manifested as an enlarged, hyperintense, enhancing optic nerve, whereas chronic optic neuritis is classically characterized by T2 signal hyperintensity in an atrophic, non- enhancing optic nerve.

Optic neuritis could be secondary to multiple sclerosis or to neuromyelitis optica, but some occurrences have been described as idiopathic or as associated with other processes (including systemic lupus erythematosus, viral infection, and radiation therapy). In addition, optic neuritis may be a secondary result of infection or inflammation of adjacent structures such as the sinuses.

Perineuritis, which is defined as inflammation of the optic nerve sheath, may mimic optic neuritis clinically with orbital pain, decreased visual acuity, and optic disc edema. At imaging, perineuritis is characterized by thickening and enhancement of the optic nerve sheath.

Because similar imaging findings may be seen in patients with dissemination of tumor cells in the cerebro-spinal fluid along the optic nerve sheath, a careful clinical evaluation is essential for accurate diagnosis.  Fig. 37  Fig. 36  Fig. 38

4. NEOPLASTIC LESIONS: 

4.1 BENIGN TUMORS:

4.1.1- Infantile Hemangioma

This is the most common tumor of infancy affecting children on their first weeks of life.

It has a fast-growing phase during 4-8 first months after diagnosis, followed by variable period of stabilization with spontaneous involution at 4-7 years of age.

It may be associated with PHACES syndrome.

Typical localization is preseptal (eyelid 85%) and extraconal.

US:

• Proliferative phase: variable echogenicity, predominantly hyperechoic. Doppler imaging demonstrates marked intralesional flow, with low resistance arterial flow.

• Involutional phase: decreased number of vessels and its caliber.

MR:

A lobulated mass, either circumscribed or infiltrative, is found. It is isointense to muscles on T1-weighted images and slightly hyperintense on T2-weighted images. Flow voids may be seen peripherally or within the lesion. Lipid component increases in the involutional phase, acquiring a higher signal at t1 and T2- weighted images.

It enhances intensely and uniformly during the proliferative phase being heterogeneous in involutive phase. Enlarged feeding arteries can be shown at MR angiography.  Fig. 40

4.1.2- Optic nerve glioma:

1,5-3,5% of the overall orbital tumors. When it is bilateral is strongly associated with NF type-1.

Radiological appearances are different in patients with and without NF-1. In patients with NF-1, it appears tortuous, kinked or buckled, and diffusely enlarged. In patients without NF-1, gliomas tend to be fusiform. are a fusiform swelling of the optic nerve with possible extension to chiasma and other intracranial structures.

On CT it is isodense without calcification opposite to meningioma.

MR is the modality of choice to assess intracranial involvement. The lesions are isointense on T1-weighted images and isointense to hyperintense on T2-weighted images. Enhancement is variable, and cystic spaces may be seen. Calcifications are rare. On T1-contrast images the nerve itself cannot be distinguished from the tumor, a characteristic that is useful for differentiating glioma from meningioma.  Fig. 41

4.1.3- Optic Nerve Sheath Meningioma

It is a slow-growing benign neoplasm derived from the arachnoid sheath of the optic nerve.

They are the second most common optic nerve tumor, accounting for 5% of orbital masses, affecting patients on the 5th decade of life.

On CT there is a fusiform enlargement of the optic nerve with calcification on 20-50% of cases.

Contrast-enhanced MR imaging is the study of choice for evaluation, particularly for its value in determining the extent of disease. A “tram-track” configuration is often observed in axial contrast-enhanced CT or MR imaging, since enhancing tumor lies on either side of the nerve. This is a specific but non pathognomonic sign, which is also seen in sarcoidosis, lymphoma and pseudotumor. T1 and T2 signal characteristics are variable depending on the presence of calcification.

Perioptic cysts are specific features, representing an accumulation of CSF on the distal nerve sheath, between the tumor and the ocular globe.

The presence of calcification as well as the enhancing pattern are features that allow the differentiation between optic nerve gliomas and meningiomas of the optic nerve.  Fig. 42

4.1.4- Solitary fibrous tumor:

It is a rare neoplasm of mesenchymal fibroblast-like cell origin.

It often manifests in the 4th decade of life.

Possible localization is intraconal and extraconal orbital spaces, as well as lacrimal gland, lacrimal sac and the eyelid.

Imaging findings:

A circumscribed solitary mass is seen, isointense signal on T1 and T2-weighted images, due to the fibrous component. High signal areas on T2-weighted images within the tumor may be seen secondary to hemorrhage, cystic degeneration or recent fibrosis.

Although bone remodeling could be found in large tumors, the presence of aggressive bone destruction suggests malignancy.

There is a rapid and marked contrast enhance on early phase followed by a progressive washout of contrast material.  Fig. 43

4.1.5-Schwannoma

Schwannomas constitute the 1 – 6% of orbital tumors and tend to occur in middle-aged individuals.

They are encapsulated, slowly progressive, benign proliferations of Schwann cells. Normally unilateral and malignant degeneration is rare.

Within the orbit they may arise from peripheral branches of the oculomotor, trochlear, and abducens nerves; parasympathetic and sympathetic fibers; and the ciliary ganglia.

MR imaging is the modality of choice for the diagnosis and for preoperative planning by revealing the exact location and extent of disease. T2 signal characteristics and contrast enhancement patterns may provide histopathologic information .They are usually T2 hyperintense but heterogeneous, a pattern that differs from the relatively homogeneous pattern of cavernous malformations. After intravenous contrast material administration, heterogeneous uptake is typically observed. A progressive enhancement on delayed venous phase images favors a diagnosis of cavernous malformation over schwannoma. 

4.1.6- Pleomorphic Adenoma or benign mixed tumor of the lacrimal gland

It is the most common benign neoplasm of the lacrimal gland, counting up to 2% of the overall orbital masses.

Pleomorphic adenomas are slow-growing tumors that most often manifest in the 2th or 5th decade of life, with extraconal location on the superoexternal margin of the orbit.

Pain is rare and suggest a malignant type of lesion.

Imaging features: unilateral lacrimal mass with bone remodeling in the lacrimal fossa. Hypointense to isointense signal on T1-weighted images and hyperintense signal on T2-weighted images. On contrast studies, it shows moderate enhance.  Fig. 44

4.2 MALIGNANT TUMORS:

4.2.1- Retinoblastoma:

Retinoblastoma is the most common intraocular tumor in children and arise on the retina.

Mean age at clinical presentation is 2 years in unilateral forms (60% of cases) and 1 year in bilateral forms. It could be trilateral when there is an intracranial neuroblastic tumor. Pineal region is the most common location followed by suprasellar or parasellar regions. Bilateral neuroblastoma associated with pineal and suprasellar tumors are known as tetralateral retinoblastoma.

Retinoblastoma may spread either by hematogenous dissemination or by direct extension either through the bulbar wall into the orbit or via the optic nerve and its meningeal sheath. Therefore, risk factors for metastasis and local recurrence include invasion of the optic nerve posterior to the lamina cribrosa (in particular if there is tumor at the surgical resection margin), anterior eye segment (AES), or extensive invasion of the ocular coats (massive choroidal and scleral invasin).

The most common symptom is leukocoria followed by strabism.

Almost all cases classic intratumoral calcifications can be detected by US providing high confidence rate regarding diagnosis.

High-resolution MR is important for pretreatment assessment: diagnostic confirmation, detection of local tumor extent, detection of associated developmental malformation of the brain and detection of associated intracranial primitive neuro- ectodermal tumor (trilateral retinoblastoma). It is slightly hyperintense on T1- weighted images and hypointense on T2- weighted images when comparing to the vitreum. After contrast adminstration there is a moderate to strong enhancement.

On CT it appears as a high density mass compared with the vitreous body, usually calcified and moderately enhancing after iodinated contrast medium administration. CT is better to detect calcifications but delineation of intraocular soft-tissue detail is limited.  Fig. 45

4.2.2 Primary uveal malignant melanoma or choroidal melanoma:

It is the most common primary tumour of the adult eye and arises from the ciliary body, iris or the choroid (90%).

The mean age at diagnosis is 56 years.

MRI is the modality of choice for the assessment of malignant uveal melanomas. We will observe a mass with high signal on T1 and low signal on T2-weighted images.

It is useful to evaluate tumoral size, extraocular extension and ciliary body infiltration (worse prognosis) as well as retinal detachment.

They have a moderate diffuse enhancement.

20% of them are amelanotic, therefore they dont posses their typical T1 and T2 signal, being indistinguishable from uveal metastases.  Fig. 46

4.2.3 – Lacrimal gland malignancies:

Malignant tumors of the lacrimal gland could be either epithelial or non- epithelial.

Epithelial subtypes:

- adenoid cystic carcinoma: most common malignant lacrimal gland tumor

accounts for 50% of malignant tumors of lacrimal gland and 25% of all lacrimal gland tumours

- adenocarcinoma

- acinic cell carcinoma

- squamous cell carcinoma

- mucoepidermoid carcinoma(rare)

Imaging findings: superoexternal orbital quadrant mass with infiltration of adjacent structures and possible intracranial extensin, either through direct invasion or by perineural dissemination.

Bone destruction is present in 70% of cases, suggesting a malignant origin.

On MR it is hypointense on T1 and hyperintense on T2-weighted images compared with muscles.

A moderate to strong enhancement is seen. Fig. 47

4.2.4- Metastases:

Metastases from other cancers constitute 1%–13% of orbital tumors.

The most common types in adults are: breast cancer (48%), prostate (12%), melanoma (12%) lung (8%) and kidney (7%). In children retinoblastoma and neuroblastoma are the most common types.

Metastases to the globe most frequently involve the choroid with unilateral involvement.

There are multiple forms of presentation with infiltration of fat, muscles or bone.

However, different malignancies have tendencies toward localizing to certain tissues. Breast cancer has a tendency to localize to orbital fat and muscle, prostate cancer has a tendency to disseminate to bone, and melanoma has a strong preference for muscle. Knowledge of these general trends can be useful for the radiologist in constructing a differential diagnosis and may also guide work-up in cases in which a primary tumor is not yet established.  Fig. 48

4.2.5- Lymphoproliferative lesions:

Lymphoproliferative lesions are the most common primary orbital tumor in older adults (≥60 years of age).

These lesions represent a spectrum of disorders from lymphoid hyperplasia to malignant lymphoma (60-90% of orbital lymphoproliferative tumors).

Lesions may be a manifestation of systemic lymphoma or arise primarily from the orbit. Non-Hodgkin lymphoma (MALT subtype) is the most common primary orbital lymphoma.

It is often unilateral (76% of cases) and extraconal.

Lacrimal gland is involved in nearly 40% of cases.

At CT imaging, half of the lesions are diffuse and ill defined and the other half appears a smooth, circumscribed mass. Uniform enhancement is characteristic.

Lymphoma has a tendency to mold to orbital structures, such as the globe, optic nerve, and orbital Wall. Bone remodeling may be present, however bone destruction is rare (aggressive types).

At MR imaging, most lesions are isointense relative to muscle with T1-weighted sequences and hyperintense relative to orbital fat with T2-weighted sequences (high cellularity). It shows moderate to strong enhancement.  Fig. 49  Fig. 18