1. Dept of Diagnostic Radiology Dept of Radiation Oncology
Control #: 1372 Retromolar Trigone Cancers: The overlooked ability of 16 section MDCT in evaluating bone erosion and assessing resectability.
eEdE
Dr. Supreeta Arya1,
Dr. Peter Paul1,Dr. J. P. Agarwal2
Dept of Diagnostic Radiology
Dept of Radiation Oncology
Tata Memorial Centre
Mumbai, INDIA

2. Disclosure of Commercial Interest
Neither we nor our immediate family members have a financial relationship with a commercial organization that may have a direct or indirect interest in the content.

3. Purpose
1. To demonstrate the normal anatomy of the retromolar trigone (RMT)
2. To depict the ability of multidetector CT (MDCT) to accurately evaluate bone erosion and soft tissue extent in RMT cancers.
using axial sections and multiplanar reformations (coronal and oblique) with puffed cheek technique on a 16 slice MDCT scanner

4. RETROMOLAR TRIGONE -Anatomy
“Retromolar trigone” is the triangular mucosal area behind the last molars on each side 1
Fig 1. Retromolar trigone- Graphical representation of frontal view.
Courtesy-

5. RMT -Anatomy RMT overlies the ascending ramus of the mandible 1
Figure : courtesy Arya S, Rane P, Deshmukh A.
Clin Radiol Sep;69(9):
RMT overlies the ascending ramus of the mandible 1
Behind the RMT is the Pterygomandibular raphe (superiorly attached to pterygoid hamulus and inferiorly to posterior end of mylohyoid line) 2
Fig 2. Sagittal line diagram of oral cavity shows the RMT (shaded region shown by arrowhead), Pterygomandibular raphe (long arrow) that is attached to the Pterygoid hamulus (*) and Mylohyoid line (dotted line)

6. SUBSITES OF ORAL CAVITY
The 7 subsites are—
1. Lips
2. Buccal mucosa ( )
3. Upper alveolus () & lower alveolus() with gingiva
4. Hard palate (^)
5. Tongue- anterior 2/3rds ( *)
6. Floor of mouth (│)
7. Retromolar trigone (not
shown—depicted in next slide) ^ *
Fig 3.Coronal reformation on 16 slice MDCT scanner using puffed cheek technique (requires patient to blow uniformly through pursed lips during quite breathing while separating tongue from hard palate)

7. Clinical examination versus Imaging
Clinical evaluation is difficult and incomplete because of the posterior location of RMT, dentition and trismus if present 1
Entire upper to lower limit of RMT can be visualized on the Oblique reformation on a 16 or higher section MDCT scanner using puffed cheek technique 1
Puffed cheek technique is possible even in those patients where trismus is present 1
Fig 4. Visualization of RMT mucosa and the contiguous raphe on an oblique reformation --requires use of puffed cheek technique that can separate the buccal and gingival mucosa with air that helps define the anterior margin of the RMT .
Also seen is the ascending ramus of mandible.

8. Retromolar trigone Anatomy on axial CT sectionsB
Fig 5. Axial CT sections, A. shows the upper limit of RMT behind the maxillary tuberosity (ellipse)
B. Shows lower limit behind the last mandibular molar ( arrow)

9. Relation of RMT with Pterygomandibular raphe & Pterygopalatine fossa*
Fig 6. 1 Sagittal bone algorithm reformat : shows the pterygoid hamulus () that gives attachment to the pterygomandibular raphe,
* shows the greater palatine foramen anterior to it leading to greater palatine canal ( ) and further to the pterygopalatine fossa () . Perineural spread along V2 occurs along this path.
Fig 7. Axial CT section at the level of base skull shows the pterygopalatine fossa () between posterior wall of maxillary sinus and base of pterygoid plates

10. RMT squamous cell cancers (SCC)
Common in South East Asia , risk factors being betel quid chewing, tobacco and alcohol 2
Relatively rare site of oral SCC in the West ( where it forms 7% of oral cancers3,4)
RMT cancers usually present late due to indolent growth3,4
The importance of RMT SCC lies in the complex spread patterns that need recognition by the radiologist to guide the clinician to optimize management

11. How RMT cancers spread Invades structures in close proximity 3-5
Tonsil and anterior tonsillar pillar (  )
Buccal mucosa is in continuity anteriorly ( )
Closely abuts ascending ramus of mandible (#)
Maxillary alveolus and inferiorly the body of mandible (  )
Medially related to base tongue and inferiorly to Floor of mouth(  )
Posteriorly related to Masticator space ( black arrow shows medial pteryoid muscle)
Perineural spread via V2 (shown in slide 9 & V3 shown in slide 18)
Fig- Courtesy Arya et al. Imaging in oral cancers. IJRI 20125 #
Fig 8. Pterygomandibular raphe shown by * is not seen on imaging -- superior spread along this extends into the pterygopalatine fossa ( shown earlier in Fig 6 & 7)

12. RMT SCC-Imaging Methods
MDCT with puffed cheek technique & reformations
Contrast enhanced MRI
Accurate for T staging and relations10
Perineural spread-- more accurate than CT
Problem- Can overestimate mandibular cortical invasion and inferior alveolar nerve invasion 7
Problem of motion and swallowing artifacts
High accuracy for bone invasion1,6-9
Adequate for soft-tissue extent
Perineural spread may be missed when early
Dental amalgam artifacts can be a problem

13. RMT SCCs- CT features
Contrast enhanced CT (CECT)-soft tissue algorithm : 
Mildly enhancing infiltrative mass in the region of the RMT.
Loss of adjacent fat planes may be a diagnostic clue.
Very small lesions may not be picked up due to opposition of mucosal surfaces. 'Puffed-cheek' technique helps to detect small obscure lesions 1,5.
Bone algorithm CT with multiplanar reformations
Features s/o osseous involvement are--
Cortical erosion --- seen as cortical discontinuity adjacent to the enhancing tumor (remember to look at cortex on bone window/algorithm image + soft tissue window to see the enhancing tumor)1
Marrow invasion ---seen as trabecular destruction with abnormal attenuation of bone marrow 1,7
Perineural spread (remember to see both soft tissue and bone windows)
Loss of normal fat density /excessive enhancement in various foraminae (foramen ovale, mandibular foramen, greater palatine foramen) in early cases 2
Foraminal widening or erosion in advanced cases 2

14. RMT SCC- MRI findings 2,7,10 T1W MRI:
Tumor is usually isointense to muscle. (Clue - Asymmetry of adjacent fat planes)
Replacement of normal hyperintense fat signal indicates marrow involvement .
T2W & STIR MRI: Tumor is usually hyperintense to muscle
T1W MRI +Contrast: Shows moderately enhancing infiltrative mass, adjacent soft tissue spread and can show perineural spread.
Perineural spread is seen as increased enhancement along the course of the nerve from mandibular foramen reaching foramen ovale ( V3) or along greater palatine canal reaching pterygopalatine fossa (V2) . Retrograde perineural spread can occur through the mandibular foramen along the inferior alveolar nerve .

15. Spread Patterns
Fig 9. Axial contrast enhanced CT sections show in A. a large RMT SCC ( t ) at maxillary level extending into masticator space posteriorly ( black arrow) , tonsil medially ( block arrow) & retro-antral buccal space anterolaterally (white arrow) .
B. shows superior extension into pterygopalatine fossa ( V2 invasion) resulting in soft tissue/ tumor density (shown by black arrow); compare with fat density on normal opposite side( white arrow).

16. Spread patterns
Only % RMT SCC are confined to the RMT ; rest spread to as many as 3 other subsites of oral cavity or oropharynx 11
80- 84% extend to anterior tonsillar pillar
( ATP) - part of oropharynx, resulting in labelling of these as RMT-ATP SCC 11 *
Fig 10. Axial CT shows RMT SCC ( * ) spreading to the ATP and tonsil (arrow)

17. Differences between RMT and ATP SCC11
It is important to distinguish between ATP and RMT SCCs as their growth patterns & prognosis differ
RMT SCC
ATP SCC
RMT -Subsite of oral cavity
ATP-Subsite of oropharynx
Higher incidence of 2nd primary in aerodigestive tract
Lower
Bone invasion is more frequent( 12-53% for mandible & 3-22% in maxilla)
Less frequent
Spread to pterygopalatine fossa and masticator space is more frequent in RMT SCC
Less common
Incidence of neck node metastasis is around 40%. RMT SCC preferentially spread to level I B nodes.
ATP SCC preferentially spreads to level II nodes.

18. Spread patterns- Perineural spread
Fig 11. A. Coronal CECT shows SCC ( * ) spreading to the foramen ovale along V3 (arrow). Examine region from mandibular foramen to foramen ovale B. Bone window shows foraminal widening (arrow)
Fig- Courtesy Arya et al. Imaging in oral cancers. IJRI 20125

19. Spread Patterns– tongue muscles and mandible erosion
Fig 12. Axial CT sections in same patient shows the tumor extension at its lower limit as in
C. Medial extension into tongue muscles ( white arrow) and across the gingivobuccal sulcus into buccal mucosa ( black arrow)
D. Bone algorithm image shows erosion of the anterior border of mandible (arrow)

20. Mandibular assessment with MDCT
16 slice MDCT scanner & above can provide very thin sections ( mm to 0.75mm)
Thin sections along with bone algorithm reconstructions make possible high resolution reformations
As the mandible is curved, sagittal reformation cannot display the entire body and the inferior alveolar canal
Oblique reformation best depicts the alveolar crest as well as almost entire inferior alveolar canal in RMT SCC 1
Coronal reformations in bone algorithm is best for measuring depth of erosion 1
Coronal, oblique and curved planar reformations with 16 or higher section CT obviate the need for Denta scan software 1

21. Oblique reformation— Bone window Vs Bone algorithm*
Figure 13. Oblique reformations on 16 slice MDCT with puffed cheek method. Shows eroded alveolar crest and invaded inferior alveolar canal
A. Bone window image on the soft tissue algorithm
B. Bone algorithm image shows a RMT SCC (*) well outlined by air in the buccal vestibule, invading cortex and marrow reaching upto inferior alveolar canal (arrows). Note the higher resolution of the cortex in B.
Figure : courtesy Arya et al , Clinical Radiology

22. Bone erosion – Role of Oblique reformationsD c
Figure 14 .Oblique reformations on 16 slice MDCT
A. Subtle erosion of alveolar crest (arrow)
B. Irregular pattern of erosion of alveolar crest (arrow) extending above to vertical ramus.
C. Lysis around root of third molar (short arrow) with erosion of alveolar crest (long arrow). This was false positive, cause of erosion being infective.
D. Smooth scalloping of cortex (arrow) without discontinuity (due to pressure of tumor). This was negative on HP.
Hence use of oblique reformation increases specificity for mandibular invasion.
Figure : courtesy Arya et al , Clinical Radiology A B C

23. Evidence for accuracy of MDCT In RMT SCC
Using ad hoc generated oblique reformations in addition to axial images + coronal reformations,
The reported sensitivity, specificity and accuracy of 16 slice MDCT for
Mandibular cortical erosion = 94%, 90% and 91.8% respectively.
Marrow invasion = 83%, 92% and 89% respectively.
Accuracy for inferior alveolar canal invasion = 100%.
Reference - Arya S, Rane P, Sable N, Juvekar S, Bal M, Chaukar D.
Retromolar trigone squamous cell cancers: A reappraisal of 16 section MDCT for assessing mandibular invasion. Clin Radiol Dec;68(12):e680-8.

24. How to generate oblique reformats ad hoc
Axial
Oblique
Figure 15 A-D.
Evaluation of the RMT on multiplanar reformations with triangulation on a GE volume viewer (AW suite). The spatial cursor is seen in the left RMT in all images.
The oblique reformat is generated by placing the spatial cursor in the diseased RMT on the axial image (Fig 15 B) and rotating the oblique image ( fig 15A) on the work station/ volume viewer.
Figure 15A shows the final plane of oblique reformation with the entire curved RMT region seen in one section . A B
Oblique
Axial C D
Sagittal
Coronal
Figure : courtesy Arya et al , Clinical Radiology

25. Table 1. Features to be studied on axial images & multiplanar reformations (bone and soft tissue algorithms) on16 slice MDCT in RMT SCC for evaluating bone invasion
Axial images
Coronal reformations
Oblique reformations
Sagittal reformations(for upper limit of RMT)
Anterior cortex of vertical ramus
Superior alveolar margin
Posterior cortex of maxillary alveolus
Buccal cortex
Inferior alveolar canal
Greater palatine canal
Lingual cortex
Depth of erosion
Pterygoid hamulus
Posterior margin of maxillary alveolus
Length of erosion
Pterygopalatine fossa
Mandibular foramen
Tooth sockets
Mental foramen
Greater palatine foramen
Reference –Arya et al , Clinical Radiology 2013

26. Management of RMT SCC– Principles 3-4,11-12
Goals of Management
Curative treatment of primary & nodes
Preservation of function & cosmesis  Reconstruction
Curative treatment
RMT SCC can be managed by one of following --
Only Surgery
Only Radiotherapy ( RT)
Surgery followed by RT +/- Chemotherapy

27. Management of RMT SCC 3-4,11-12
Early lesions --uni-modal/ combined modality ( increasing evidence that latter improves locoregional control)
Advanced lesions treated by combination modality
* When clinical/ imaging evidence of bone involvement seen, surgery is mainstay (RT is high risk due to osteonecrosis)
Teaching point--- * Imaging evidence of bone erosion impacts management . Surgery followed by adjuvant Rx is the optimal treatment when bone erosion present.

28. RMT SCC –treatment of Neck 3-4,11-12
If metastatic nodes present -- Modified radical neck dissection
If neck is clinically and imaging negative (N0) 
Selective neck dissection ( levels I-III) if primary treated with
surgery
Neck irradiation if primary treated with RT
Teaching Points–
Demonstration of metastatic nodes can influence extent of neck dissection
2) Bulky nodes at multiple levels necessitate postoperative radiation ± chemotherapy

29. Imaging in RMT cancers--- Why? 1,2,5
Usually not for detection except in cases of trismus where clinical examination is difficult
For accurate staging
T staging--- a ) Bone erosion & b) Soft tissue spread
N staging
Teaching points
Clinical evaluation of bone erosion unreliable & requires imaging
Deep posterior soft tissue extent is revealed only by imaging

30. Oral Cavity SCC- AJCC 7th edition TNM staging12
T staging
Tx Primary tumour cannot be assessed
T0 No evidence of primary tumour
Tis Carcinoma in situ
T1 Tumour 2cm or < in greatest diameter
T2 Tumour > 2cm , < 4 cm in greatest
diameter
T3 Tumour > 4cm in greatest diameter
T4a (oral cavity) - Tumor invades adjacent structures (eg, cortical bone, deep extrinsic muscle of tongue, maxillary sinus, skin of face).
T4b - Tumor invades masticator space, pterygoid plates, or skull base and/or encases internal carotid artery.
Neck Nodes
Nx Regional LN cannot be assessed
N0 No regional LN metastasis
N1 Ipsilateral Single node < 3cm
N2a Ipsilateral Single node 3-6cm
N2b Ipsilateral multiple nodes <6cm
N2c Bilateral/Contralateral nodes<6cm
N3 Lymph node > 6cm

31. Role of Imaging in staging and management 1- 2
Bone erosion
Presence or absence influences decision about mandibular resection ---Yes/ No
Extent of erosion influences extent of resection
(marginal, segmental or hemi-mandibulectomy)
Soft tissue spread
Influences resectability and extent of resection.

32. Bone erosion—planning resection 1 ,3-4,13
MARGINAL MANDIBULECTOMY (MM)
SEGMENTAL MANDIBULECTOMY (SM) / HEMI MANDIBULECTOMY ( HM)
When disease abuts mandible , but no erosion seen– for oncologically safe resection margins
When cortical erosion is extensive- SM
When cortical erosion is subtle and does not invade marrow or inferior alveolar canal
When inferior alveolar canal &/or mandibular foramen are involved-HM
Contraindicated in irradiated or edentulous mandibles
When soft tissue component is large with significant posterior spread to masticator space( even without erosion)– debated indication.

33. Soft tissue spread-- When to Resect? 14
T1-T3 & T4 a disease are resectable
A subset of T4 b with extension to masticator space below the mandibular sigmoid notch# is resectable (in South Asia where disease is prevalent)
T4 b disease above mandibular notch, and disease encasing carotid artery –unresectable
Fig 16. A. Axial CT at level of mandibular notch between coronoid process (thick arrow) & condyloid process ( thin arrow)
Fig 16 B. Oblique reformat on 16 slice MDCT scanner.
Horizontal black line through mandibular notch divides masticator space into a high (*) & low compartment *
# - Notch between coronoid and condyloid process ( black line in Figure 16 A & B) .

34. High and low masticator space as related to oral cancer spread 14
Figure 17 .shows high and low masticator space (MS).
Dashed yellow lines depict the entire MS. Horizontal yellow line is at approximate level of mandibular notch
The part of MS above this line (high MS) consists of the lateral pterygoid (Lp) and temporalis (T) muscles .
The part of MS below the notch ( low MS) has bulk of the masseter (m) and medial pterygoid muscles (mp) . m mp T Lp

35. T4 b disease extent Revealed only on imaging! 2
Unresectable
Resectable A * *
Fig 18 A. High masticator space disease extension (arrow) reaching skull base, invading lateral pterygoid muscle. Horizontal line shows the approximate level of mandibular notch. Bulk of medial pterygoid (*) muscle is below this level.
Fig 18 B. Low masticator space disease.
Arrow shows destruction of anterior border of vertical ramus of mandible with invasion only up to anterior margin of medial pterygoid muscle (* ).

36. Conclusion
Contrast enhanced MDCT is a good first line choice for bone evaluation and soft tissue extent in RMT SCC— “a one-stop shop” imaging method.
The speed of MDCT scanning is added advantage ( < 25 seconds).
MRI reserved for problem solving such as—
Those presenting with pain and sensory symptoms that suggest perineural spread
Extensive trismus with inability to perform adequate puffed cheek
Whenever CT is equivocal for precise soft issue extent

37. Imaging Check list for RMT SCC
Primary– Soft tissue
Tumor dimensions and thickness ( transverse extent)
Adjacent structures ( more important than T stage)
Buccal mucosa, gingiva
Base tongue, Floor of mouth
ATP & Tonsil
*Masticator space including low or high space extension
*Pterygopalatine fossa (PPF)
Perineural spread via V2 (greater palatine canal to PPF) and V3 (mandibular foramen to foramen ovale )
Primary– Bone
Bone erosion ( maxilla & mandible) ---present or absent
Mandible *– extent of erosion ( length & depth), invasion of marrow , inferior alveolar canal & mandibular foramen.
*Use– thin sections, soft tissue and bone algorithm, axial images, ad hoc coronal & oblique reformats with spatial cursor & puffed cheek technique!
*--High masticator space and invasion of pterygopalatine fossa are unresectable

38. Imaging Check list for RMT SCC
Nodes
*Level of abnormal nodes
Size & number
Presence of necrosis
Extracapsular spread
Invasion of IJV
Relation to common carotid artery and internal carotid artery
(circumferential contact , if < 180° –easily resectable,
if > 270° , unresectable)

39. Teaching points
Understanding anatomy of RMT and spread patterns of malignancies in RMT assumes significant relevance as the site has limited clinical accessibility.
Bone erosion, soft tissue extent and nodal status are important as prognostic markers as well as for management in RMT SCC

40. Teaching points
Contrast enhanced MDCT with puffed cheek technique is a good first line investigation for all the above. The thin sections with MDCT afford high quality reformations , together with bone algorithms and puffed cheek need to be exploited to a maximum.
MRI, even though useful in better delineation of soft tissue invasion and perineural spread, can be set aside as problem solving tool rather than primary imaging modality; with CT being a faster and cost-effective imaging modality.

41. References
Arya S, Rane P, Sable N, Juvekar S, Bal M, Chaukar . D.Retromolar trigone squamous cell cancers: A reappraisal of 16 section MDCT for assessing mandibular invasion. Clin Radiol Dec;68(12):e680-8.
Arya S, Rane P, Deshmukh A. Oral cavity squamous cell carcinoma: Role of pretreatment imaging and its influence on management. Clin Radiol. 2014;69 (9):
Genden EM, Ferlito A, Shaha AR, Rinaldo A. Management of cancer of the retromolar trigone. Oral Oncol Oct;39(7):633-7
Ayad T, Guertin L, Soulières D, et al. Controversies in the management of retromolar trigone carcinoma. Head Neck Mar;31(3):
Arya S, Chaukar D, Pai P. Imaging in Oral cancers. Indian J Radiol Imaging Jul;22(3):
Mukherji SK, Isaacs DL, et al. CT detection of mandibular invasion by squamous cell carcinoma of the oral cavity. AJR Am J Roentgenol , 2001 ;177:237–43.
Imaizumi, N. Yoshino, I. Yamada, et al.. A Potential Pitfall of MR Imaging for Assessing Mandibular Invasion of Squamous Cell Carcinoma in the Oral Cavity .AJNR Am J Neuroradiol 2006 ;27:

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