CLASSIFICATION METHODS
In the last years a compartment-based classification has become popular.
This classification method is easier to manage and remember and creates less confusion between radiologists.
We will revise this classification,
emphasizing the most frequent tumour-types on each category and their main radiological features.
COMPARTMENT-BASED CLASSIFICATION
GLOBE TUMORS
Not included in this review.
ANTESEPTAL COMPARTMENT
Tumours related to skin and conjunctival tumours,
accessible to biopsy.
Imaging techniques are used to assess orbital invasion.
POSTSEPTAL COMPARTMENT
Intraconal space
Optic nerve-sheath complex tumours:
- Optic nerve glioma
- Optic nerve meningioma
Muscle-cone and retrobulbar fat lesions:
Intraconal invasion from globe tumours
Extraconal space
Lacrimal gland and lacrimal ducts tumours
- Benign mixed tumour or pleomorphic adenoma
- Adenoid cystic carcinoma (or cylindroma)
- Malignant mixed tumour
- Lymphoproliferative tumours
Mesenchymal and fibrohystocitic tumours
- Solitary fibrous tumour
- Lipoma
- Meningiomas: sphenoid wing meningioma,
neighbourhood’s meningioma
Osseous and cartilaginous tumours
- Osteoma
- Osteosarcoma or osteogenic sarcoma
- Fibro-osseous lesions: ossificant fibroma,
fibrous dysplasia,
aneurysmal bone cyst.
Cysts
Extraconal invasion from neighbourhood: cranial,
nasal cavity and paranasal sinuses tumours.
MULTICOMPARTMENTAL
May affect indistinctly any of the compartments:
- Lymphoproliferative tumours
- Schwannoma (or neurilemoma)
- Neurofibromas (plexiform neurofibroma).
- Veno-lymphatic malformation (lymphangioma or cystic hygroma)
- Rhabdomyosarcoma
Patient’s age helps to narrow the differential diagnosis.
In the table below,
common orbital tumours according to age of presentation are shown:
CHILDHOOD: <20 años
- Optic nerve glioma
- Fibrous dysplasia
- Rhabdomyosarcoma
- Lymphangioma
- Plexiform neurofibroma.
YOUNG ADULT: 20-40 años
- Cavernous haemangioma
- Lacrimal gland: adenoid cystic carcinoma and malignant mixed tumour
- Fibrous dysplasia
- Schwannoma
ADULT: >40 años
- Optic nerve meningioma
- Benign mixed tumour of the lacrimal gland
- Lymphoproliferative tumours
Crossing two lists (location and age),
we may provide an acceptable differential diagnosis.
Further analysis of the characteristics of the tumour may sometimes suggest a final diagnosis,
specially using advance MRI sequences.
RADIOLOGICAL FEATURES
INTRACONAL
Optic nerve-sheath complex tumours
Optic nerve meningioma (ONM) (Fig.
1)
Epidemiology: most common primary tumour of the optic nerve sheath.
5% of all primary orbital tumours.
2% of all meningiomas.
Sex and age of presentation: Women between 30-70 years.
Rare in children except for neurofibromatosis type 2.
Most frequently benign and slow growing.
CT features: tubular or fusiform thickening of the optic nerve sheath,
although they might also appear as eccentric globular masses.
Calcifications are very suggestive of meningiomas.
These calcifications usually adopt a plaque-shaped or a diffuse (psammomatous) appearance.
Remodelling or enlargement of the optic nerve channel and hyperostosis may also be seen.
MR features: MR is better to assess intracranial extension.
Meningiomas show signal intensity similar to the optic nerve,
both in T1 and T2.
After contrast,
the tumour enhances while the optic nerve reminds isointense,
giving the typical appearance of “tram track or rail road” on the axial plane and “target sign or bull's eye” on the coronal plane (fig.
2).
Although this findings are very suggestive,
they are not pathognomonic.
Because of its hypercellular nature,
restriction is seen on DWI,
with low ADC values.
Differential diagnosis: with optic nerve glioma.
Others: optic neuritis,
idiopathic orbital pseudotumor,
granulomatous optic neuropathy,
lymphoma and metastasis.
Optic nerve glioma (ONG) (Fig.
3)
Epidemiology: most frequent primary tumour in the optic nerve.
3-4% of all orbital tumours and 66% of optic nerve tumours.
Sex and age of presentation: more common in children,
especially low-grade gliomas.
In contrast,
high-grade gliomas are more frequent in adults.
There is a relationship between ONG and neurofibromatosis type 1.
In addition,
bilateral affectation is pathognomonic for neurofibromatosis type 1.
CT features: tubular or fusiform thickening and kinking of the optic nerve.
Calcifications are rare.
Cystic degeneration is more often than in meningiomas.
Enhancement is less intense than in meningiomas.
MR features: T1-hypointense and slightly T2-hyperintense (more than meningiomas).
Intracranial dissemination is frequent.
They might enhance,
although less intense than meningiomas.
Low cellularity leads to an increased diffusibility,
with high ADC values.
Differential diagnosis: mainly with Optic nerve meningioma
Muscle-cone and retrobulbar fat tumours
Cavernous haemangioma (Fig.
4)
Epidemiology: most frequent benign orbital tumour in adults. It is a low flow venous malformation.
Sex and age of presentation: Women between 20 - 40 years.
Usually benign and slow growing tumours.
Imagining features: well circumscribed mass with a peripheral fibrous pseudocapsule separating the tumour from the optic nerve and the ocular musculature.
They can exert mass effect on adjacent structures.
Variable MR signal can be seen in this tumours and behaviour on DWI does not defer significantly from other orbital tumours.
Therefore,
these features might not be helpful enough in the differential diagnosis.
Nevertheless,
there are two findings very suggestive of haemangiomas:
- Presence of calcified phleboliths.
- In dynamic study after contrast media injection,
these tumours show patchy enhancement in the arterial phase and progressive filling until homogenizing in the later venous phase.
Punctiform areas of T1 hyperintensity on non-contrast MR might be seen,
consistent with thrombi.
An important consideration is that not every calcification on CT is a phlebolith.
Calcifications should be considered together with other features.
If findings suggest haemangioma,
then,
calcifications may be consistent with phleboliths.
Otherwise,
calcifications are a common finding in other space-occupying lesions,
such as meningiomas and malignant tumours.
Differential diagnosis: Most important differentials should include venous varicose vein and schwannomas.
Varicose veins usually present with exophthalmos after Valsalva,
and thus,
they are clinically visible.
In differential with schwannomas post-contrast and DWI sequences are helpful.
In schwannomas enhancement in the early arterial phase begins in a wide area,
whereas in hemangiomas it is usually focal.
Mean ADC value of schwannomas (1,92) is greater than that of the haemangiomas (<1,4),
according to literature.
Others: neurinoma,
fibrous histiocytoma,
meningioma,
hemangiopericytoma.
EXTRACONAL
Lacrimal gland and lacrimal ducts tumours
Benign mixed tumour (BMT) or pleomorphic adenoma (Fig.
5)
Epidemiology: most frequent benign lesion of the lacrimal gland.
Sex and age of presentation: 40-50 years.
No sex predilection.
Usually slow growing tumours.
Imagining features: these tumours may present hyaloid,
myxoid or mucinous degeneration,
as well as areas of haemorrhage.
They also may vary the degree of cellularity.
All these factors will determine the radiological appearance,
most commonly seen as heterogeneous tumours.
- On CT,
BMT appear as well circumscribed,
usually encapsulated,
oval-shaped masses in the lacrimal fossa.
Bone remodelling (smooth concavity of the lacrimal fossa) without destruction or erosions is characteristic.
Calcifications might be seen.
- On MR, characteristically they are T2-hyperintense in relation to ocular muscles.
Because of the presence of myxoid and mucoid material areas of T1-hyperintensity may also be seen.
Features are similar to malignant tumours.
Differentiation between benign and malignant tumours using ADC values may be helpful.
ADC values <1 are highly suggestive of malignant tumour,
while BMT do not show significant restriction on DWI,
with overall ADC values > 1.3 (in any case >1).
Malignant epithelial tumours of the lacrimal gland
Includes adenoid cystic carcinoma (ACC) and malignant mixed tumour (MMT),
both with similar findings.
Epidemiology: malignant epithelial tumours constitute 29% of all epithelial tumours of the lacrimal gland and up to 50% of all malignant epithelial tumours of the lacrimal glands.
Sex and age of presentation: young people,
peak around 40 years.
They present as painful solid masses.
Imagining features: they may be indistinguishable from pleomorphic adenoma (low grade) or show aggressive tumour characteristics (high grade).
- On CT, irregular or ill-defined masses in the lacrimal gland,
with bony destruction (one key to differentiate it from benign epithelial tumours!) and calcifications (Remember! Not every calcification is a phlebolith!).
- On MR,
they are T1-hypointense and T2-variable.
Intense enhancement and restriction on DWI (mean ADC value of 0,8) may be suggestive.
Perineural spread is relatively common.
Lymphoproliferative tumours
See multicompartmental.
Osseous and cartilaginous tumours
Fibrous dysplasia (Fig.
6)
Epidemiology: more frequently affecting frontal,
ethmoidal and sphenoid bones.
Sex and age of presentation: Children and mid-age (<30 years).
Predilection for female.
Monoostotic most frequently.
CT features: enlargement of the medullary cavity with ground glass attenuation.
Bone-cortex is spared.
MR features:
- T1 hypointensity (because of sclerosis and ground-glass opacity).
- Heterogeneous enhancement
- Liquid-liquid levels
- Absent of restricted diffusion and elevated ADC values,
suggesting benign lesion.
Differential diagnosis: meningioma (which shows cortical involvement and enhances vividly)
Cysts
Dermoid cyst (Fig.
7)
Epidemiology and clinical presentation: most frequent congenital orbital lesion.
Slow growing tumour,
usually asymptomatic.
They are formed from epithelial inclusions in fronto-cygomatic suture (most frequently) and in the fronto-ethmoidal suture.
Therefore,
they will be typically located in the superolateral portion of the orbit.
In addition,
when located in the fronto-cygomatic suture a characteristic morphology (“hourglass appearance”) might be seen due to the orbital and temporal components.
CT features: encapsulated tumours with cystic-low density appearance,
containing fat or fat-liquid levels.
Enhancement of the wall may appear.
MR features: same behaviour as fatty tissue in all MR pulse sequences.
Cancel their signal with fat suppression techniques.
Diffusion is variable usually non-diagnostic.
Only the capsule enhances.
Differential diagnosis: cephalocele (usually communicates with the anterior cerebral fossa and contains CSF,
not fat).
Others: cholesterol granuloma,
mucocele,
lipoma and congenital teratoma of the orbit
MULTICOMPARTMENTAL
Lymphoproliferative tumours
Epidemiology: most frequent malignant tumour of the orbit.
55% of malignant tumours of the orbit in adults and 10-15% of the orbital masses.
Most frequent type is LNH-B MALT.
It can be primary or secondary to systemic disease.
The lacrimal gland is the most frequent location.
Age of presentation: 60 - 70 years,
slow or fulminant growth depending on histopathologic subtype.
Some important aspects:
- They are characteristically hypercellular.
- They mold and accommodate to orbital structures,
encompassing them but not exerting mass effect.
- When bilateral involvement and metachronous foci it is very suggestive.
- Presence of lymphadenopathy in other levels usually accompanies orbital affectation.
CT features: Because they are hypercellular,
they commonly show high attenuation in non-contrast CT and intense and homogeneous enhancement.
Enhancement decreases in late phases (unlike inflammatory lesions).
No bony erosion.
Fig.
8,
9.
MR features: Masses showing intermediate T1 and T2 signal and intense and homogeneous enhancement.
Very low ADC values (0.44-0.92,
with a mean of 0.6) are highly associated.
This marked restriction in DWI is commonly named as "black-hole sign".
Fig 9.
Differential diagnosis:
- Benign lymphoproliferative disorders (atypical or reactive lymphoid hyperplasia,
also known as pseudolymphoma),
inflammatory pseudotumor and metastasis.
All of them can easily be differentiated due to ADC values.
- Granulomatous diseases (sarcoidosis).
Presence of hiliar lymphadenopathy,
pulmonary lesions and elevated ACE values will suggest de diagnosis.
- Others: pleomorphic adenoma.
Rhabdomyosarcoma (Fig.
10)
Epidemiology: most common malignant mesenchymal tumour of the orbit in children.
The orbit is the most frequent location within head and neck (40%).
Most frequent histological subtype is embryonic (very aggressive).
Sex and age of presentation: Childhood (peak 5-10 years) and slight predominance in males.
It is a rapidly progressing and aggressive tumour,
which tends to infiltrate adjacent structures (sinus,
orbital fissures,
cavernous sinuses and intracranial invasion).
There may be lung metastasis.
Location: it is a tumour of the extraconal compartment; however,
due to its infiltrating nature,
extension to the intraconal space is often seen (multicompartmental).
Imagining features: these tumours will show radiologic features of aggressive tumours:
- Tumour exceeding anatomic limits.
- Adjacent bone destruction.
- Necrosis.
Rhabdomyosarcomas are isodense on CT and T1-isointense (T2 variable) on MRI.
Moderate to intense enhancement and low ADC values are characteristic.
Mean ADC values of 0,72 have been reported.
However,
lymphomas still show lower ADC.
Differential diagnosis:
- Orbital cellulitis: despite infectious,
it may also be infiltrating,
invading adjacent structures and causing bone destruction.
Keys to differentiate are clinical and radiological findings of infection: fever,
leucocytosis and presence of abscess or fat stranding.
- Capillary haemangiomas.
More common in younger children (12-18 months).
Cutaneous haemangiomas are associated in 1/3 of the cases.
“Flow gaps” and higher ADC values help in differentiation.
- Others: histiocytosis of Langerhans cells,
metastasis of neuroblastoma and lymphomas.
Veno-lymphatic malformation or lymphangioma
Epidemiology: young people (most frequent between 16-20 years).
5% of pediatric orbital tumours.
It grows with the patient,
especially at puberty.
Location: they are more frequently located in the extraconal compartment.
Nonetheless,
they can also be intraconal or multicompartmental.
Imagining features: multicystic masses with peripheral enhancement molding surrounding structures.
When large cysts,
blood-fluid levels might be seen.
On CT and MR, they show ill-defined and lobulated borders.
Haemorrhagic and proteinaceous content may appear hyperdense native CT.
Fluid-fluid levels are also suggestive.
On MR no “flow gaps” (unlike capillary hemangiomas) and no significant restriction are seen.
Schwannoma (or neurilemoma) (Fig.
11)
Epidemiology: represents about 1% of the orbital pathology and 1-6% of all orbital tumors.
It usually originates in the sensory branches of the ophthalmic branch (V1),
most frequently supraorbital and supratrochlear nerves.
Sex and age of presentation: young adult (women between 20-50 years).
There are divided into two subtypes based on Antoni´s classification: type A (cellular) and type B (myxoid).
Location: due to relationship with de V1,
its most frequent location is the orbital roof.
CT features: well-circumscribed mass (usually encapsulated),
rounded or oval.
It can have hypodense areas (cystic or necrotic component).
Heterogeneous uptake of contrast.
MR features: T1-isointense and moderately T2-hyperintense or heterogeneous.
Moderate enhancement.
Enhancement is more intense with myxoid component (Antoni B subtype).
It may have cystic-necrotic areas.
Differential diagnosis: cavernous hemangioma (when schwannoma is located in the intraconal compartment).
Others: orbital cyst,
adenoid cystic carcinoma (when the schwannoma has prominent areas of cystic-necrotic degeneration),
inflammatory lesions.
Neurofibromas
Most frequent tumor arising from peripheral nerves.
Incidence: 1-3% of all orbital lesions.
There are three subtypes.
We will focus on the plexiform as it is the most frequent in the orbit.
Plexiform neurofibroma (Fig.
12)
Epidemiology: 1-2% of all orbital tumors.
It is diagnostic of Type 1 Neurofibromatosis.
Sex and age of presentation: Childhood (first-second decade of life).
They are low grade tumors.
Malignant sarcomatous degeneration is seen in about 10%.
Diagnosis may be suspected when other signs of NF1 are presented.
Imagining features:
- On plain film classic sign of “harlequin eye appearance” due to alterations on the grater wing of the sphenoid.
- On CT and MR: infiltrative masses encompassing the globe.
On CT these masses show intermediate - soft tissue attenuation.
On MR the typical “target sign” may be presented on T2 (hyperintense on periphery with a centric hypointensity).
Variable enhancement.
Differential diagnosis: capillary or infantile hemangioma,
lymphangioma,
rhabdomyosarcoma.
Differentiation is made based on the absent of other stigmas of neurofibromatosis.