Metastasis of Unknown Origin in Head and Neck Cancer

A patient presenting with a neck swelling is commonly evaluated for infection, thyroid disease, lymphoma, or a primary head and neck tumour.

Occasionally, however, examination reveals metastatic cancer in one or more cervical lymph nodes, while the original tumour cannot be identified.

This clinical situation is commonly described as head and neck carcinoma of unknown primary, cervical metastasis of unknown origin, or cancer of unknown primary presenting in the neck.

It is important to understand that “unknown primary” does not mean that the cancer has no origin. It means that the primary tumour remains undetected despite a detailed clinical, radiological, pathological, and endoscopic assessment.

Some primary tumours are extremely small, hidden within the tonsillar crypts or base of tongue, or may have regressed after releasing metastatic cells. Others may be below the limits of currently available diagnostic methods.

Metastasis to cervical lymph nodes from an occult primary accounts for an important but relatively small proportion of head and neck cancers. Published reviews describe it as approximately 5–10% of patients with carcinoma of unknown primary, although the exact frequency depends on the definition and population studied.

The condition is clinically challenging because treatment must control the involved neck, reduce the risk of a later-discovered primary tumour, and avoid unnecessary toxicity.

Metastasis of unknown origin in the head and neck usually refers to malignant tissue found in cervical lymph nodes without an identifiable primary tumour after an appropriate evaluation.

In most cases, the lymph-node deposit is a squamous cell carcinoma, reflecting a tumour arising from the mucosal lining of the upper aerodigestive tract. However, other histologies—including thyroid carcinoma, salivary gland carcinoma, melanoma, lymphoma, and metastatic cancers from below the clavicle—can also present as a neck mass.

The term should be used carefully. A patient should not be labelled as having a true unknown primary after only a basic clinical examination and one imaging test.

Comprehensive Diagnostic Work-Up

A comprehensive work-up generally includes a detailed history, examination of the upper aerodigestive tract, ultrasound-guided needle sampling, cross-sectional imaging, metabolic imaging where appropriate, endoscopic evaluation under anaesthesia, and directed biopsies.

Undiagnosed Primary vs True Occult Primary

There is also a distinction between an undiagnosed primary and a true occult primary. In the first situation, the primary tumour may subsequently be discovered with additional investigations or during follow-up. In the second, the primary remains unidentified even after a thorough diagnostic process.

Several biological and anatomical factors may explain why the primary lesion is not visible.

The oropharynx particularly the palatine tonsils and base of tongue is a frequent source of cervical nodal metastasis. These areas contain irregular lymphoid tissue and deep mucosal crypts in which a small tumour may remain concealed. A lesion may be too small to see during routine examination or may be missed on imaging.

HPV-Associated Oropharyngeal Cancer

Human papillomavirus-associated oropharyngeal cancer has further changed the presentation of unknown-primary disease. HPV-related tumours may arise in the tonsillar tissue or lingual tonsil, where the primary tumour can be tiny but the nodal metastasis may be clinically prominent.

In some cases, the primary tumour undergoes spontaneous regression, leaving metastatic disease in the neck.

Other Possible Hidden Primary Sites

Other hidden sites include the nasopharynx, hypopharynx, larynx, oral cavity, salivary glands, and thyroid.

The pattern of lymph-node involvement, tumour histology, HPV or EBV status, and radiological findings can help narrow the likely site of origin.

The most common presentation is a gradually enlarging, painless neck swelling. Patients may notice the mass while shaving, bathing, or turning the neck.

Some present with multiple enlarged lymph nodes or a firm, fixed mass. Pain is not essential and may occur only when the disease involves nerves, muscles, skin, or adjacent structures.

Common Symptoms

Symptoms include:

• A neck lump lasting more than two to three weeks.
• Difficulty or pain while swallowing.
• Persistent sore throat or pain referred to the ear.
• Change in voice.
• Nasal blockage or bleeding.
• Unexplained weight loss.
• Coughing or blood-stained saliva.
• Trismus or difficulty opening the mouth.
• Persistent unilateral ear pain without an ear abnormality.

Many patients have no symptoms related to the primary tumour.

This absence of symptoms should not provide false reassurance. Any persistent or enlarging adult neck mass requires timely assessment because metastatic lymphadenopathy may be the first sign of a head and neck malignancy.

Evaluation begins with a careful history.

Medical History

Clinicians should ask about the duration and progression of the swelling, pain, fever, weight loss, swallowing difficulty, voice change, ear symptoms, nasal symptoms, dental problems, tobacco exposure, alcohol use, prior cancers, radiation exposure, and family history.

Sexual history may be relevant in a sensitive and non-judgmental manner because HPV-associated oropharyngeal cancer is linked to prior exposure to HPV. The aim is not to assign blame but to understand tumour biology and guide testing and treatment.

Physical Examination

The physical examination should include inspection and palpation of the entire neck, assessment of cranial nerves, oral cavity examination, dental assessment, and flexible nasopharyngolaryngoscopy.

The clinician should evaluate the tonsils, base of tongue, vallecula, epiglottis, pyriform fossae, larynx, hypopharynx, and nasopharynx.

Cystic Neck Masses

A neck mass that is cystic on imaging should not automatically be assumed to be benign. In adults, a cystic cervical mass may represent a metastatic HPV-related oropharyngeal squamous cell carcinoma.

Delayed diagnosis can occur when such a lesion is repeatedly treated as a branchial cyst without adequate pathological evaluation.

The first essential investigation is usually an ultrasound-guided fine-needle aspiration or core-needle biopsy.

Needle sampling helps establish whether the lesion is malignant and may provide material for immunohistochemistry, HPV-related testing, flow cytometry, or molecular analysis.

Role of Needle Biopsy

Open biopsy of a cervical node is generally avoided as the initial procedure when needle sampling is feasible. An unplanned open biopsy may disrupt tissue planes, complicate subsequent neck dissection, and compromise radiation planning.

If needle sampling is non-diagnostic, a core biopsy or carefully planned excision may be considered after multidisciplinary discussion.

Pathology Assessment

The pathology report should describe:

• Histological type.
• Degree of differentiation.
• Presence of keratinisation.
• Lymphovascular or perineural features, when assessable.
• Nodal architecture and extranodal extension.
• HPV-related markers when appropriate.
• EBV-related markers in suspected nasopharyngeal disease.
• Immunohistochemical profile if the tumour is not clearly squamous.

 HPV and p16 Testing

For suspected oropharyngeal squamous cell carcinoma, p16 immunohistochemistry is commonly used as a surrogate marker for HPV-associated disease.

However, p16 positivity is not identical to direct proof of transcriptionally active HPV in every clinical context. HPV-specific testing may be needed when the clinical pattern is atypical, when the primary is not found, or when the result will affect classification or trial eligibility.

Contrast-enhanced CT or MRI of the head and neck helps define the number, size, and characteristics of involved nodes.

It also assesses possible mucosal primaries, deep spaces, bone involvement, vascular encasement, and extranodal spread.

CT and MRI

MRI may offer superior soft-tissue detail, particularly for the tongue base, oropharynx, nasopharynx, and skull base.

CT may be valuable for detecting nodal necrosis, calcification, lung lesions, and bony involvement.

The choice depends on the clinical question, available expertise, renal function, and local protocols.

FDG-PET/CT

FDG-PET/CT can identify metabolically active primary tumours and distant metastases that are not apparent on conventional imaging.

It may also reveal a second primary tumour or provide additional information about the extent of nodal disease.

Nevertheless, PET/CT is not infallible. Inflammation, infection, dental disease, and physiological uptake can mimic malignancy, while very small or low-metabolic tumours may remain undetected.

A systematic review of head and neck unknown-primary disease supports combining clinical examination, imaging, endoscopy, biopsies, and appropriate surgical evaluation rather than relying on a single test.

When imaging and office examination do not identify the primary, many patients undergo examination under anaesthesia.

This allows systematic inspection and palpation of areas that are difficult to assess in the clinic.

Procedures That May Be Performed

The procedure may include:

• Direct laryngoscopy.
• Oesophagoscopy where indicated.
• Careful examination of the oral cavity and oropharynx.
• Bilateral tonsillectomy or directed tonsil biopsy.
• Lingual tonsil evaluation and tongue-base mucosal biopsies.

Targeted Surgical Evaluation

The approach should be individualised. Blind biopsies from multiple normal-appearing sites may cause morbidity without guaranteeing a diagnosis.

Modern imaging, transoral robotic surgery, and transoral laser microsurgery can help identify small lesions in the tonsil or base of tongue in selected centres.

These techniques are particularly useful when a positive finding would change the radiation field, surgical plan, or staging.

A cervical mass is not synonymous with metastatic squamous cell carcinoma.

Conditions to Consider

The differential diagnosis includes:

• Reactive or tuberculous lymphadenopathy.
• Metastatic papillary thyroid carcinoma.
• Salivary gland malignancy.
• Nasopharyngeal
• Metastatic renal, lung, breast, gastrointestinal, or genitourinary cancer.
• Skin cancer of the scalp, face, or neck.
• Infection or congenital cyst.

Importance of Lymph-Node Location

The location of the nodes provides useful clues. Upper and middle jugular nodes are commonly associated with mucosal head and neck primaries, while lower cervical or supraclavicular nodes may raise concern for a primary below the clavicle.

However, lymphatic drainage patterns overlap, and exceptions occur. A lower-neck mass should therefore not be dismissed as a benign lesion or assumed to have a thoracic or abdominal origin without appropriate evaluation.

Staging depends on tumour histology, HPV or p16 status, nodal distribution, laterality, extranodal extension, and whether the primary is considered HPV-associated.

Patients with an unknown primary may be designated as having a T0 tumour, with the N category reflecting the burden and distribution of cervical nodal disease.

HPV-Positive and HPV-Negative Disease

HPV-associated and HPV-negative cancers have different biological behaviour and staging frameworks.

HPV-positive oropharyngeal cancers often have a more favourable prognosis, although individual outcomes vary according to smoking history, nodal burden, extranodal extension, comorbidities, and treatment response.

Multidisciplinary Staging

Staging should be reviewed by a multidisciplinary team.

The team commonly includes a head and neck surgeon, radiation oncologist, medical oncologist, radiologist, pathologist, nuclear medicine specialist, speech and swallowing therapist, dietitian, dentist, and specialist nurse.

Treatment is not identical for every patient.

It depends on the number and size of lymph nodes, side of the neck, extranodal extension, HPV status, histology, performance status, previous treatment, and likelihood of identifying the primary.

Surgery and Radiotherapy

For limited nodal disease, treatment may involve surgery alone or definitive radiotherapy.

A neck dissection provides pathological staging and removes involved lymph nodes. In selected patients, directed mucosal surgery may identify or remove the occult primary.

Patients with more extensive nodal disease, multiple involved levels, extranodal extension, or other high-risk features may require combined-modality treatment.

Options include neck dissection followed by postoperative radiotherapy, with concurrent chemotherapy in selected high-risk cases, or definitive chemoradiotherapy.

Reviews have described surgery followed by appropriate adjuvant treatment or primary chemoradiation as options for advanced disease.

Radiotherapy Planning

Radiotherapy planning is a major area of clinical judgment.

Traditional treatment may include comprehensive irradiation of likely mucosal sites and the involved neck.

More selective approaches may limit treatment to the oropharynx and involved neck when the clinical and pathological findings strongly suggest an oropharyngeal origin.

The aim is to preserve tumour control while reducing xerostomia, swallowing dysfunction, dental complications, hypothyroidism, and other late effects.

Treatment Side Effects

The choice between surgery and radiotherapy should be discussed with the patient.

Both approaches can be effective in appropriate settings, but their side-effect profiles differ.

Surgery may cause shoulder dysfunction, neck stiffness, numbness, nerve injury, and cosmetic changes.

Radiotherapy may cause mucositis, dry mouth, taste alteration, swallowing difficulty, dental problems, fibrosis, hypothyroidism, and long-term tissue changes.

Concurrent chemotherapy is generally considered when the risk of recurrence is high or when disease is unsuitable for surgery.

Cisplatin-based chemoradiotherapy remains an important approach for fit patients, but renal impairment, hearing loss, neuropathy, frailty, and other comorbidities may limit its use.

Treatment for Recurrent or Metastatic Disease

For recurrent or metastatic disease, systemic treatment is guided by tumour histology, HPV status, PD-L1 expression, previous treatment, symptoms, and patient preference.

Immunotherapy and targeted agents may be considered in appropriate clinical settings.

Molecular profiling is increasingly relevant, particularly for non-squamous tumours, unusual presentations, or disease that recurs after standard treatment.

Treatment should not be selected solely on the basis of a molecular result. Biomarkers must be interpreted alongside the clinical picture, pathology, imaging, treatment intent, access to therapy, and evidence from clinical trials.

Follow-up has two major goals: detecting recurrence and identifying a primary tumour that may become clinically apparent later.

Follow-up visits typically include a detailed history, head and neck examination, flexible endoscopy when indicated, assessment of swallowing and speech, and review of treatment-related toxicities.

Imaging During Follow-Up

Imaging is tailored to the stage and treatment received.

A post-treatment PET/CT may be useful in selected patients, particularly after definitive chemoradiotherapy, but timing is important because inflammation can produce false-positive uptake.

Additional imaging is guided by symptoms, examination findings, and the initial disease pattern.

Symptoms to Report

Patients should be advised to report new or persistent symptoms such as increasing neck swelling, pain, bleeding, progressive dysphagia, voice change, unexplained weight loss, or breathing difficulty.

Surveillance should also address hypothyroidism, dental health, nutritional status, shoulder function, speech, swallowing, fatigue, anxiety, and return to work.

The diagnosis can be psychologically difficult because patients may wonder how cancer could spread when the original tumour cannot be found.

Some may feel that the treatment is excessive or uncertain.

Clear communication is essential: the known disease is the metastatic lymph-node cancer, and treatment is directed at both the visible disease and the areas where the hidden primary is most likely to be located.

Rehabilitation and Supportive Care

Rehabilitation should begin early.

Speech and swallowing therapy can reduce functional deterioration and support safe nutrition.

Dental review before radiotherapy helps reduce the risk of osteoradionecrosis and dental complications.

Dietary counselling, exercise, smoking cessation, alcohol reduction, and psychosocial support are important components of survivorship care.

 Psychosocial Well-Being

The psychosocial impact of thyroid and head and neck cancers may include anxiety, depression, fear of recurrence, body-image concerns, and difficulty returning to normal activities.

These concerns should be actively assessed rather than considered secondary to tumour control.