Oral Cancer Imaging and Staging
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Direct answer
The eighth edition of the AJCC staging rewrote oral cavity T categories around depth of invasion: a 2 cm tumour invading 5 mm still ranks T2, but the same surface tumour burrowing past 10 mm becomes T3 — because depth, not diameter, predicts prognosis. T1 is now 2 cm or less with depth up to 5 mm; T2 covers larger surface extent or depth 5-10 mm; T3 exceeds 4 cm or 10 mm; T4a means moderately advanced disease (outer cortex of bone, skin of the face, maxillary sinus or deep tongue muscle involvement) and T4b very advanced (masticator space, pterygoid plates, skull base, carotid sheath). Imaging assigns these categories: MRI for primary tumour extent, depth and perineural spread; CT for cortical bone invasion and nodal assessment; and PET-CT for unknown primaries, distant disease and recurrence. Nodal staging adds extranodal extension, N3b, to the eighth edition's vocabulary.
What you must remember
- T categories with depth (AJCC 8th): T1 — 2 cm or less and depth 5 mm or less; T2 — up to 4 cm with depth 5-10 mm; T3 — over 4 cm or depth over 10 mm; T4a moderately advanced (cortical bone, skin, sinus, deep musculature of tongue); T4b (masticator space, skull base, carotid sheath).
- What changed in the 8th edition: depth of invasion added to T; extrinsic tongue muscle invasion removed as an automatic T4; extranodal extension added to N (N3a over 6 cm; N3b any node with ENE).
- Depth of invasion arithmetic: measured from the level of the adjacent normal mucosal basement membrane — 5 mm and 10 mm are the two thresholds every MCQ quotes.
- Nodal categories: N1 single ipsilateral node 3 cm or less; N2a single ipsilateral 3-6 cm; N2b multiple ipsilateral; N2c contralateral or bilateral; N3 over 6 cm or any ENE.
- Imaging by task: MRI — primary extent, tongue depth, marrow and perineural spread; CT — cortical mandibular invasion, nodal necrosis and rim enhancement; ultrasound-guided cytology for neck nodes; PET-CT — unknown primary, distant metastasis, recurrence.
- Nodal imaging clues: short-axis enlargement, rounded shape, loss of the fatty hilum, central necrosis, rim enhancement — level I and II drain the oral cavity first.
- Bone invasion assessment: CT for cortex (sensitivity high), MRI for medullary involvement; the consequent T4a upstaging and mandibular management decisions ride on this distinction.
Staging one tongue cancer from clinic to chart
A 55-year-old man presents with an indurated left lateral tongue ulcer. Step one: measurement — the ulcer measures 2.6 cm; depth of invasion on biopsy and MRI measures 7 mm. Step two: T category — 2-4 cm with depth 5-10 mm makes this T2 (an identical lesion 12 mm deep would be T3). Step three: neck assessment — contrast CT shows a 1.8 cm level II node with central necrosis: N1, but necrosis suggests ENE-risk disease; suspicious nodes are sampled by ultrasound-guided cytology if needed. Step four: local invasion — MRI shows the tumour abutting but not invading the mandibular cortex, so bone CT confirms no cortical breach and T4a is withheld. Step five: distant screening per protocol, with PET-CT where suspicion arises. The chart closes at T2 N1 M0, stage III — MRI for the primary, CT for bone and nodes, functional imaging reserved.
Where students slip
The edition trap dominates: candidates still answering with seventh-edition rules — diameter-only T categories, extrinsic muscle as T4 — lose the question outright, and "what did the 8th edition change" is itself a recurring stem. The second slip is threshold arithmetic: 5 mm and 10 mm of depth are the numbers that move T1 to T2 and into T3, and they are measured from the adjacent basement membrane, not the ulcer base. The third is modality matching: MRI for the tongue primary and perineural spread, CT for cortex and nodes — reversing them answers the wrong question. A fourth slip: calling every enlarged node N2 — the exam's node is usually single, ipsilateral and under 3 cm (N1).
Frequently asked questions
How does the 8th edition define T1 and T2 oral cavity cancer?
T1: 2 cm or less in greatest dimension with depth of invasion 5 mm or less; T2: 2-4 cm or depth of invasion between 5 and 10 mm.
What did AJCC 8th edition change in oral cavity staging?
Depth of invasion was added to T categories, extrinsic tongue muscle invasion was removed as automatic T4, and extranodal extension was incorporated into N staging.
Which imaging best assesses the primary tumour and mandibular invasion?
MRI for primary extent, depth and medullary involvement; CT for cortical bone erosion and nodal anatomy — together assigning T4a when bone is invaded.
What nodal features suggest metastasis on CT?
Rounded shape, short-axis enlargement, loss of fatty hilum, central necrosis and rim enhancement — earliest in levels I and II for oral cancer.
What is N3b in the eighth edition?
Any regional node with clinically evident extranodal extension, regardless of size — reflecting the poor prognosis of ENE.