treatment planning is challenging because the accumulated experience of any particular individual or institution in managing this tumour is rather limited .
this problem is further compounded by the absence of a universally accepted tumour staging system , making treatment outcome comparison difficult .
it is now known that the single most important factor determining survival is the local tumour extent at presentation .
it is therefore crucial to establish the disease extent in order to optimize treatment options .
the patterns of tumour spread are well described in the surgical literature . computed tomography ( ct )
can demonstrate bony erosion with exquisite detail but magnetic resonance ( mr ) imaging is preferred in the delineation of tumour spread along vascular channels , neural pathways , intracranial extension and involvement of the extra - temporal soft tissues .
on temporal bone ct , early carcinoma of the eac commonly presents as a soft tissue mass within the eac . as the disease progresses , aggressive underlying bony destructive changes are demonstrated . as the tumour infiltrates and spreads deep into the surrounding tissue ,
it is best shown as a heterogeneously enhancing lesion on contrast - enhanced mr imaging .
carcinoma of the eac most commonly spreads through the floor of the eac into the soft tissue below the temporal bone .
this may be related to the presence of the fissures of santorini along the inferior aspect of the cartilaginous eac .
less frequently , the tumour erodes posteriorly into the mastoid bone or anteriorly through the tympanic portion of the eac to invade the temporomandibular joint ( fig .
1 ) . on the other hand , superior extension into the middle cranial fossa and medial extension into the middle ear are rare . 

( a ) axial t1-weighted mr image shows intermediate signal tumour in the left eac ( asterisk ) extending into the temporomandibular joint and displacing the mandibular condyle ( arrow ) anteriorly .
( a ) axial t1-weighted mr image shows intermediate signal tumour in the left eac ( asterisk ) extending into the temporomandibular joint and displacing the mandibular condyle ( arrow ) anteriorly .
medial extension into the middle ear cavity is of special interest because of the resultant poor prognosis .
this negative prognostic indicator is related to the subsequent disease spread beyond the middle ear making surgical excision increasingly difficult .
tumour in the middle ear can readily extend superiorly through the thin tegmen tympani into the middle cranial fossa ( fig .
however , the presence of temporal lobe oedema associated with tumour extension into the middle cranial fossa does not necessarily indicate cerebral parenchymal infiltration . 

( a ) coronal contrast - enhanced mr image shows tumour in the right middle ear ( arrow ) with extension through the tegmen tympani into the middle cranial fossa .
( b ) coronal contrast - enhanced mr image ( posterior to ( a ) ) shows tumour in the right eac ( asterisk ) with middle ear and intracranial extension .
note enhancement in the labyrinth ( black arrow ) and the internal auditory canal ( white arrow ) .
( a ) coronal contrast - enhanced mr image shows tumour in the right middle ear ( arrow ) with extension through the tegmen tympani into the middle cranial fossa .
( b ) coronal contrast - enhanced mr image ( posterior to ( a ) ) shows tumour in the right eac ( asterisk ) with middle ear and intracranial extension .
note enhancement in the labyrinth ( black arrow ) and the internal auditory canal ( white arrow ) . from the middle ear cavity
tumour may gain access into the labyrinth through the lateral semicircular canal , oval or round windows .
malignant infiltration often extends further proximally along the vestibular or cochlear nerves into the internal acoustic canal ( fig .
the resistant cortical bone of the inner ear , especially the otic capsule , often limits the amount of gross bone destruction but aggressive tumours will eventually involve the cochlea ( fig .
3 ) . 
 figure 3(a ) axial ct shows a right eac tumour eroding the otic capsule and destructing the cochlea ( asterisk ) .
4a ) shows inferior spread of the right eac tumour involving the internal carotid artery and jugular bulb ( asterisk ) .
note the normal left carotid foramen ( black arrow ) and jugular foramen ( white arrow ) for comparison . 

figure 4(a ) axial t1-weighted mr image shows an intermediate signal intensity tumour in the left middle ear cavity with extension posteriorly into the mastoid ( arrow ) .
( a ) axial ct shows a right eac tumour eroding the otic capsule and destructing the cochlea ( asterisk ) .
4a ) shows inferior spread of the right eac tumour involving the internal carotid artery and jugular bulb ( asterisk ) .
note the normal left carotid foramen ( black arrow ) and jugular foramen ( white arrow ) for comparison .
( a ) axial t1-weighted mr image shows an intermediate signal intensity tumour in the left middle ear cavity with extension posteriorly into the mastoid ( arrow ) .
in contrast to the difficulty of invading cortical bone , tumour can readily spread from the middle ear posteriorly through the aditus into the mastoid antrum ( fig .
further posterior spread leads to extension into the posterior cranial fossa and involvement of the sigmoid sinus ( fig .
figure 5a 55-year - old female with a left eac tumour extending into the posterior cranial fossa .
( a ) axial t1-weighted mr image shows intermediate signal intensity tumour in the left eac ( asterisk ) with extension into the posterior cranial fossa ( arrow ) .
( b ) axial contrast - enhanced mr image shows heterogeneous contrast enhancement in the tumour .
( c ) coronal t1-weighted mr image shows intermediate signal intensity tumour in the posterior aspect of the left mastoid bone ( arrow ) and along the dural venous sinus ( arrowheads ) .
( d ) coronal contrast - enhanced image shows enhancement of the tumour ( arrow ) .
a 55-year - old female with a left eac tumour extending into the posterior cranial fossa .
( a ) axial t1-weighted mr image shows intermediate signal intensity tumour in the left eac ( asterisk ) with extension into the posterior cranial fossa ( arrow ) .
( b ) axial contrast - enhanced mr image shows heterogeneous contrast enhancement in the tumour .
( c ) coronal t1-weighted mr image shows intermediate signal intensity tumour in the posterior aspect of the left mastoid bone ( arrow ) and along the dural venous sinus ( arrowheads ) .
( d ) coronal contrast - enhanced image shows enhancement of the tumour ( arrow ) .
tumour erosion through the floor of the middle ear anteriorly results in the involvement of the carotid artery while inferior extension through the floor posteriorly involves the jugular foramen ( fig .
cancers with extensive inferior spread involving the jugular bulb and internal carotid artery exclude the surgical options of lateral temporal bone resection and subtotal temporal bone resection , and are often unresectable ( figs . 6 and 7 ) . 

figure 6a 57-year - old male with carcinoma of the right eac showing intracranial infiltration and spread below the skull base .
( a ) coronal ct shows tumour in the right eac eroding the cochlea ( short arrow ) and the floor adjacent to the carotid canal ( long arrow ) .
( b ) coronal ct ( posterior to ( a ) ) shows permeative erosion of the roof of the right middle ear ( short arrow ) , and the floor adjacent to the jugular foramen ( long arrow ) .
( c ) coronal contrast - enhanced mr image shows enhancing tumour in the right eac spreading into the right middle cranial fossa causing gross dural thickening ( asterisk ) .
figure 7(a ) coronal contrast - enhanced mr image shows a left eac tumour ( asterisk ) spreading and involving the left internal carotid artery ( arrow ) .
( b ) axial contrast - enhanced mr image shows the tumour with heterogenous enhancement surrounding the left petrous internal carotid artery ( long arrow ) .
a 57-year - old male with carcinoma of the right eac showing intracranial infiltration and spread below the skull base .
( a ) coronal ct shows tumour in the right eac eroding the cochlea ( short arrow ) and the floor adjacent to the carotid canal ( long arrow ) .
( b ) coronal ct ( posterior to ( a ) ) shows permeative erosion of the roof of the right middle ear ( short arrow ) , and the floor adjacent to the jugular foramen ( long arrow ) .
( c ) coronal contrast - enhanced mr image shows enhancing tumour in the right eac spreading into the right middle cranial fossa causing gross dural thickening ( asterisk ) .
( a ) coronal contrast - enhanced mr image shows a left eac tumour ( asterisk ) spreading and involving the left internal carotid artery ( arrow ) .
( b ) axial contrast - enhanced mr image shows the tumour with heterogenous enhancement surrounding the left petrous internal carotid artery ( long arrow ) .
this manner of spread can be diagnosed by the identification of soft tissues anterior and parallel to the petrous segment of the internal carotid artery ( fig . 8) .
when this manner of spread occurs , tumour comes into close proximity with the foramen ovale and the mandibular nerve in the central skull base .
proximal perineural spread along the mandibular nerve leads to intracranial extension . at this stage ,
signal changes in the muscles of mastication may be seen , indicating the onset of denervation atrophy .
the parapharyngeal space is a well - recognized route for the spread of inflammatory and malignant disease .
tumour may therefore extend into the parapharyngeal space and spread into the neck with relative ease . 

figure 8a 67-year - old male with recurrent carcinoma of left eac with spread to the nasopharynx .
( a ) axial t1-weighted mr image shows an intermediate signal intensity tumour in the left eac and middle ear extending anteriomedially along the eustachian tube ( arrow ) .
( b ) axial contrast - enhanced image shows heterogeneous enhancement of the tumour with non - enhancing areas , probably due to necrosis .
( c ) axial t1-weighted mr image shows tumour in the left parapharyngeal space and nasopharynx ( arrowheads ) .
( e ) coronal t1-weighted mr image shows an intermediate signal intensity tumour in the left nasopharynx ( asterisk ) extending to the left foramen ovale ( arrow ) .
( f ) coronal contrast - enhanced mr image shows tumour enhancement and intracranial spread ( arrow ) .
note the oedematous left temporal lobe , which may be due to radiation therapy , intracranial tumour or both .
a 67-year - old male with recurrent carcinoma of left eac with spread to the nasopharynx .
( a ) axial t1-weighted mr image shows an intermediate signal intensity tumour in the left eac and middle ear extending anteriomedially along the eustachian tube ( arrow ) .
( b ) axial contrast - enhanced image shows heterogeneous enhancement of the tumour with non - enhancing areas , probably due to necrosis .
( c ) axial t1-weighted mr image shows tumour in the left parapharyngeal space and nasopharynx ( arrowheads ) .
( e ) coronal t1-weighted mr image shows an intermediate signal intensity tumour in the left nasopharynx ( asterisk ) extending to the left foramen ovale ( arrow ) .
( f ) coronal contrast - enhanced mr image shows tumour enhancement and intracranial spread ( arrow ) .
note the oedematous left temporal lobe , which may be due to radiation therapy , intracranial tumour or both .
the incidence of carcinoma of the eac is estimated as between one and six cases in every one million population .
several aetiological factors have been implicated , including exposure to ultraviolet radiation , chlorinated disinfectants and human papillovirus infection .
chronic otitis media has been implicated for a long time as an aetiological factor but no definite correlation has been proven .
histologically , most of these tumours are squamous cell carcinomas , others being mucoepidermoid and adenoid cystic carcinomas .
carcinoma of the eac is treated with surgical resection with or without post - operative radiation therapy .
tumours confined to the cartilaginous portion of the eac are removed by local wide excision .
tumours arising from or spreading to the bony eac but lateral to the tympanic membrane are treated by limited partial temporal bone resection or  sleeve resection with subsequent reconstruction by a local flap or a microvascular myocutaneous free flap , and refashioning of an auditory canal for preservation of hearing .
tumour involvement of the tympanic membrane , malleus , incus and tympanic bone may necessitate lateral temporal bone resection , which was first reported by conley and novack .
subtotal temporal bone resection , pioneered by parson and lewis , is a combined extracranial and intracranial procedure for treating advanced cancers involving the middle ear , the temporomandibular joint and the middle cranial fossa .
the facial nerve is almost invariably sacrificed , as opposed to lateral temporal bone resection . however , the inferior margins of resection are similar in both lateral temporal bone resection and subtotal temporal bone resection , being just lateral to the internal jugular vein and internal carotid artery . in patients with even more extensive disease ,
total temporal bone resection may improve patient survival as well as increase the palliative benefits , notably decreased pain and improved hygiene
. however , there is increased risk of morbidity , including damage to the cavernous sinus and notably , the internal carotid artery as well as the third , fourth , fifth and sixth nerves .
a viable alternative in advanced disease is piecemeal resection of all visible tumour beyond the margins of a subtotal temporal bone resection , followed by post - operative radiation therapy .
en bloc surgical resection with complete tumour excision remains the primary aim and the critical determinant of patient survival . in two retrospective studies ,
the 5-year survival rate was almost 80% for patients with negative resection margins at the time of surgery , significantly higher than those with positive margins ( 035% ) . in order to achieve clear margins
, most cases will invariably require a superficial or total parotidectomy and not uncommonly , a partial mandibulectomy and clearance of the infratemporal fossa .
surgical planning therefore , involves meticulous pre - operative radiological tumour mapping . a high - resolution ,
non - enhanced bone algorithm temporal bone ct and contrast - enhanced mr imaging are recommended for this purpose . for early disease confined to the eac
ct can also readily detect erosion of the bony external auditory canal , which is often the first sign of local spread of tumour .
however , ct is inadequate to demonstrate the extent of tumour spread beyond the eac .
post - contrast t1-weighted sequencing with fat saturation in the axial , coronal and sagittal planes allows the best evaluation of the extent of tumour spread into the surrounding extra - temporal anatomic landscape .
intracranial extension is much better seen on coronal sections of contrast - enhanced mr images .
the earliest sign seen on mr imaging is mild dural thickening and enhancement along the floor of the middle cranial fossa in the vicinity of the tegmen tympani .
although ct demonstrates erosion of the cortical bone of the inner ear very well , spread into the labyrinth is better demonstrated on mr imaging .
perineural involvement of the seventh and eighth cranial nerves can not be diagnosed with confidence without contrast - enhanced mr imaging .
similarly , mr imaging provides further information on the status of the skull base neurovascular structures when bony erosions are identified on ct .
necrotizing external otitis tends to show comparatively more extensive inflammatory changes with cellulitis and abscesses involving the adjacent deep cervical spaces , most commonly the parotid space .
osteoradionecrosis of the temporal bone is a delayed complication following radiotherapy for head and neck tumours .
while cortical erosion of the bony eac may simulate carcinoma , the prominent features in osteoradionecrosis involve the mastoid air cells with disruption of the air cell septation , usually on a background of underlying radiation - induced otomastoiditis .
diffuse permeative demineralization of the skull base beyond the temporal bone can be seen in patients with severe osteoradionecrosis .
nevertheless , the clinical presentation and imaging appearance of eac carcinoma may exactly mimic necrotizing external otitis or osteoradionecrosis .
as early diagnoses significantly affect the eventual outcome , any clinically suspicious eac lesion with bony erosion should be subjected to early biopsy .
as with other squamous cell carcinoma of the head and neck , more than one biopsy should be taken as there may be non - diagnostic changes at the periphery of the tumour and malignant cells may be missed with only a single biopsy .
in addition to ct , mr imaging is essential for precise delineation of the extent of tumour spread beyond the eac , which at presentation , is the single most important prognostic factor .
this accurate pre - operative tumour mapping is also critical in deciding surgical options , with a clear resection margin the primary objective .
 abstractcarcinoma of the external auditory canal presents a challenge in management , largely due to limited experience in treating this rare disease and the lack of a universally accepted staging system . 
 prognosis is most dependent on the extent of local disease at presentation , while resection margin status is also a strong determinant of survival in post - operative patients . 
 the intent of this pictorial essay is to review the pattern of tumour spread and highlight the value of imaging , particularly magnetic resonance imaging in pre - operative tumour mapping . 
Introduction
Radiological features
Discussion
Conclusion