Retromolar Trigone Cancer: Symptoms & Treatment

A sore behind the last lower tooth is easy to dismiss as a wisdom tooth problem. Most of the time it is. But the small patch of lining there, the retromolar trigone (RMT), is a place where a cancer can reach the jawbone and the chewing muscles earlier than cancers in many other parts of the mouth. That is why a sore, ear pain or stiffness in this area deserves a proper look if it does not settle.

This guide explains where the retromolar trigone is, what symptoms to watch for, how the cancer is staged, and how it is usually treated, including jaw surgery, neck surgery, reconstruction and recovery.

Understanding the retromolar trigone

The retromolar trigone is a small triangle of lining behind the last lower molar, over the front edge of the ascending ramus of the mandible. It forms a junction between the cheek, the gum, the tonsil pillar region and the deep spaces around the jaw. The lining here is thin and firmly attached to the periosteum, the membrane covering bone. Just behind it lies the pterygomandibular raphe, a band connecting the cheek muscle (buccinator) and the throat muscle (superior constrictor).

Because the lining is thin and sits over bone, tumours here do not have much soft tissue to grow through before reaching the mandible. Even a fairly small cancer may touch or enter the jaw, and larger ones can spread to the chewing muscles, the upper jaw behind the last upper molar, or the deeper spaces behind it. Surgeons treating oral cavity cancer therefore pay special attention to this site.

Symptoms of cancer behind the last molar

This area is hard to see, and early changes are often mistaken for an impacted wisdom tooth, gum infection or jaw joint strain.

1. A sore or growth that does not heal

The commonest sign is an ulcer, red or white patch or a raised, wart-like growth behind the last lower molar. A mouth ulcer that does not heal in two to three weeks should be examined by a specialist. A red patch (erythroplakia), white patch (leukoplakia) or a mix of both can also be an early warning.

2. Pain and referred earache

Soreness behind the lower back teeth, worse on chewing or swallowing, is common. As the tumour grows or irritates nearby nerves, pain can be felt in the ear on the same side even though the ear is normal.

3. Difficulty opening the mouth (trismus)

The area lies close to the chewing muscles, especially the medial pterygoid, and tumour spread into them stiffens the jaw. If you can no longer fit three fingers, held sideways, between your upper and lower front teeth, get seen soon. Other conditions such as oral submucous fibrosis can also reduce mouth opening, so it needs examination.

4. Loose teeth or a denture that no longer fits

If a tumour affects the bone, nearby molars may loosen without gum disease. Denture wearers may feel sore at the back of the lower ridge.

5. Numbness of the lip or chin

Tingling or numbness of the lower lip and chin on one side can mean the inferior alveolar nerve inside the jaw is involved. This needs prompt assessment.

6. A painless neck lump

Sometimes the first sign is a firm, painless lump high in the neck (a level II lymph node). Reports suggest that a meaningful proportion of retromolar trigone cancers already have neck node spread at diagnosis. See our guide to neck lumps and lymph node spread.

Causes and risk factors

Squamous cell carcinoma accounts for nearly all cancers here. It develops after long exposure to carcinogens and chronic irritation.

• Smokeless tobacco, gutka, pan masala and khaini. In India these are leading causes of oral cancer. They are often held in the back of the cheek and near the last molars, where their chemicals can cause precancerous change (OPMD). Read more on gutka and pan masala risk.
• Cigarette and bidi smoking, whose heat and chemicals directly injure the lining.
• Heavy alcohol use, which makes the lining more permeable to tobacco chemicals. Smoking and alcohol together raise risk much more than either alone.
• Chronic irritation from sharp or broken molars or ill-fitting dentures. This is a lesser cause on its own but can add to the risk.
• HPV is mainly linked to cancers of the tonsil and base of tongue (oropharynx), and has a much smaller role in oral cavity cancers.

Diagnosis and staging

Treatment depends on accurate staging. Because the retromolar trigone lies over bone and deep spaces, both soft tissue spread and jaw involvement must be assessed.

Examination

The surgeon examines the area visually and by feel, checking the size of the tumour, whether it is fixed to the jaw, spread to the front tonsil pillar and involvement of the cheek (buccal mucosa). The neck is carefully examined for enlarged nodes.

Biopsy

A small tissue sample, usually taken under local anaesthesia, confirms the diagnosis and shows the grade of the tumour.

Imaging

• MRI with contrast helps assess tumour thickness, spread into the pterygoid muscles and along nerves.
• CT with bone windows helps show erosion of the mandible or upper jaw.
• PET-CT is mainly used in advanced disease to look for spread to the lungs, liver or bones, and later to assess possible recurrence. See our comparison of PET-CT, MRI and CT scans for oral cancer.

TNM staging

The AJCC 8th edition stages oral cavity cancers, including the retromolar trigone, by tumour size and depth of invasion (DOI), nodal disease and distant spread.

• Stage I (T1 N0): tumour 2 cm or smaller with DOI 5 mm or less, no nodes.
• Stage II (T2 N0): tumour 2 cm or smaller with DOI over 5 mm, or 2 to 4 cm with DOI up to 10 mm, no nodes.
• Stage III: T3 (over 4 cm, or DOI over 10 mm) without nodes, or T1 to T3 with a single same-side node of 3 cm or less without extranodal extension.
• Stage IVA: T4a (tumour through the cortical bone of the jaw, or into the skin of the face) or more extensive node disease such as N2.
• Stage IVB: T4b (tumour into the masticator space, pterygoid plates or skull base) or N3 disease, including extranodal extension in large nodes.
• Stage IVC: distant spread.

Superficial erosion of the tooth socket alone does not make a tumour T4a. Whether a tumour has truly entered bone is a judgement based on the examination, scans and sometimes the findings at surgery.

Sore or stiffness behind your last molar that won't settle? Dr. Pradeep S. and Dr. Kalpa Pandya see patients at Apollo Main Hospital, Greams Road, Chennai. Book a consultation or call +91 96633 03747 for a careful examination and, if needed, a biopsy.

Surgical treatment

Surgery is the main treatment. The aim is complete removal of the tumour with about 1 cm of normal-looking tissue around it, so that the final microscopic margin is ideally 5 mm or more, while keeping speech, swallowing and appearance as far as possible. Our article on positive margins and frozen section explains how margins are checked.

1. Removing the tumour and part of the jaw

Because the lining lies on bone, the jaw is often part of the operation, even in small tumours. The choice depends on how deeply the tumour goes.

• Wide local excision of soft tissue: reserved for small, superficial tumours with no attachment to bone on examination or scans (see wide local excision).
• Marginal mandibulectomy: when the tumour lies against the jaw without clear invasion of the bone, a rim of the mandible is removed, often the front edge of the ramus, while the jaw stays continuous.
• Segmental mandibulectomy: when the bone is invaded, a full-thickness segment is removed to get clear margins. Read more about mandibulectomy.
• Composite resection: larger tumours are removed in one block with the involved jaw, soft tissue and, where needed, neck nodes (see composite resection).
• Upper jaw (maxillary) resection: because the area lies just below the upper wisdom tooth region, tumours extending upward may need part of the back of the upper jaw removed (see maxillectomy).

2. The neck

Retromolar trigone cancers carry a real chance of hidden spread to neck nodes, even when scans look normal. A neck dissection is therefore usually done at the same operation, either a selective dissection (commonly levels I to III) if the neck seems clear, or a more extensive one if nodes are involved. It removes hidden disease and gives accurate staging.

3. Reconstruction

Removing a retromolar tumour leaves a three-dimensional defect of lining, muscle and sometimes bone. Repairing it at the same operation helps to limit scarring, jaw deviation and trismus.

• Local and regional flaps: for smaller to moderate soft tissue defects, such as the buccal fat pad, a nasolabial flap or the PMMC flap.
• Free flaps: for larger defects, planned with a reconstructive colleague (see free flap reconstruction).
  • Radial forearm free flap: thin, pliable lining that suits soft-tissue-only defects and may help limit trismus.
  • Anterolateral thigh flap: more bulk, for deep defects extending into the cheek or throat wall.
  • Fibula free flap: after a segmental mandibulectomy, bone from the lower leg is shaped and joined with microsurgery to rebuild the jaw, which can allow dental implants later (see fibula free flap jaw reconstruction).

4. Treatment after surgery

After surgery, the full pathology report is reviewed at a tumour board. Radiation, planned and given by radiation oncologists, or chemoradiation with medical oncologists, may be advised when the report shows features such as:

• Positive or close margins
• Perineural or lymphovascular invasion
• Extranodal extension in neck nodes
• Advanced tumour stage (T3 or T4) or several involved nodes

Positive margins and extranodal extension usually lead to chemoradiation, and the other features usually to radiation alone. Our guide to radiation or chemo after oral cancer surgery explains what that involves.

Treatment approach by stage

This is a general outline. The actual plan depends on your scans, pathology and general health, and is agreed at a multidisciplinary tumour board. Some very advanced tumours are treated primarily with radiation or chemoradiation, or with systemic therapy.

Recovery and rehabilitation

Because the site is next to chewing and swallowing muscles, recovery focuses on three things.

Preventing trismus

Scar tightening at the back of the mouth can stiffen the jaw. Gentle jaw-opening exercises, for example with stacked tongue blades or a jaw-stretching device, usually begin when the team says the wound has healed enough, and continue through and after radiation.

Speech and swallowing

Surgery near the soft palate or throat wall can change speech and swallowing. Speech and swallowing therapists help to retrain these. See speech and swallowing therapy after oral surgery.

Teeth and chewing

Restoring chewing is a key goal of oral reconstruction. After a bone flap such as the fibula, dental implants can later support a fixed bridge or implant-retained denture. See dental implants after oral cancer.

Seeing the surgeons in Chennai

Dr. Pradeep S. (MDS, FHNS, FIBCSOMS, PGDCR) is an oral and maxillofacial surgeon with a head and neck surgical oncology fellowship from HCG Manavata Cancer Centre, Nashik. He has more than 15 years' experience in oral, head and neck cancer surgery, including three years in head and neck oncology at Apollo Proton Cancer Centre, and holds international board certification (IBCSOMS). He plans jaw and tumour resections, with free flap reconstruction done together with a reconstructive colleague.

Dr. Kalpa Pandya (BDS, MDS, FHNS) is an oral and maxillofacial surgeon with a head and neck oncology fellowship, a special interest in oral potentially malignant disorders, and training in dental implant rehabilitation. The two surgeons review cases together at Apollo Main Hospital, Greams Road, Chennai, and work with the hospital's imaging, radiation and medical oncology teams.

If you have a sore or stiffness behind the last molar, or a biopsy report that mentions the retromolar trigone, read about mandibulectomy to know what jaw surgery involves, then book an appointment or call +91 96633 03747.

References

1. Lydiatt, W. M., et al. "Head and Neck cancers: major changes in the American Joint Committee on Cancer eighth edition cancer staging manual." CA: A Cancer Journal for Clinicians, 2017.
2. National Comprehensive Cancer Network. NCCN Clinical Practice Guidelines in Oncology: Head and Neck Cancers (current version), nccn.org.
3. National Cancer Institute. Lip and Oral Cavity Cancer Treatment (PDQ), Health Professional Version.
4. Shah, J. P., Patel, S. G., & Singh, B. Jatin Shah's Head and Neck Surgery and Oncology, 5th ed. Elsevier, 2019.