Management of patients presenting with cervical lymph node metastasis from an occult primary tumour remains a significant clinical challenge. Initial clinical evaluation, supported by imaging, often establishes the presence of metastatic disease in the neck, which is subsequently confirmed through histopathological assessment. However, despite a comprehensive diagnostic workup, the primary tumour remains unidentified in a subset of patients. These cases are commonly referred to as Carcinoma of Unknown Primary (CUP) or Metastasis of Unknown Origin (MUO).

The majority of such patients harbour metastatic squamous cell carcinoma, most likely arising from occult primaries within the upper aerodigestive tract. Nevertheless, this group is heterogeneous, encompassing a wide spectrum of pathological subtypes and clinical presentations. Although multiple guidelines and an extensive body of literature address the evaluation and management of these patients, significant diagnostic and therapeutic dilemmas persist in routine clinical practice. This review aims to provide a comprehensive overview of current evidence on the management of Neck Metastasis of Unknown Origin (NMUO), with particular emphasis on diagnostic strategies, treatment approaches, and emerging concepts. 1, 2

This review was conducted to synthesise current evidence on the evaluation and management of Neck Metastasis of Unknown Origin (NMUO). A comprehensive literature search was performed across multiple electronic databases, including PubMed, Scopus, and Google Scholar, prioritising the recent publications from the last 10 years [ January 2006 to March 2017]. The search strategy incorporated combinations of keywords and Medical Subject Headings (MeSH) terms such as “cervical lymph node metastasis,” “unknown primary,” “occult primary tumour,” “head and neck cancer,” “CUP,” “MUO,” “diagnostic workup,” “PET-CT”, “HPV and treatment outcomes”, “Neck nodal staging”, “Radiation Therapy for Occult Primary” and “Neck dissection” In addition to database searches, relevant guidelines and consensus statements from organisations such as the National Comprehensive Cancer Network and the European Head and Neck Society were reviewed to ensure inclusion of current standard-of-care recommendations. Reference lists of selected articles were also manually screened to identify additional pertinent studies. Studies eligible for inclusion comprised randomized controlled trials, prospective and retrospective cohort studies, systematic reviews, meta-analyses, and key guideline papers focusing on adult patients with cervical lymph node metastasis from an unknown primary. Articles not published in English, case reports with limited generalizability, and studies lacking clear diagnostic or therapeutic relevance were excluded. Data from selected studies were qualitatively synthesized, with emphasis on diagnostic strategies (including imaging modalities, endoscopic evaluation, and molecular profiling), treatment approaches (surgery, radiotherapy, systemic therapy), and emerging concepts such as HPV-associated disease and minimally invasive surgical techniques. Given the heterogeneity of included studies, a formal meta-analysis was not performed.

Chronic suppurative otitis media (CSOM) is a chronic inflammation of the middle ear with perforation of the “Unknown primary”

The incidence of neck metastasis of unknown origin (NMUO) accounts for approximately 1–7% of newly diagnosed head and neck cancers. This proportion has shown a declining trend over time, largely attributable to advances in diagnostic modalities and more systematic evaluation protocols. The likelihood of identifying the primary tumor increases with meticulous clinical examination, including endoscopic assessment under general anaesthesia, supplemented by advanced imaging techniques such as positron emission tomography–computed tomography (PET-CT). 3-5

The anatomical distribution of cervical lymph node metastases often provides important clues regarding the potential site of the primary tumor. The American Academy of Otolaryngology–Head and Neck Surgery classification system standardizes cervical lymph node levels (I–VII), thereby aiding in clinical localization and management strategies. 6

Metastases involving lymph node levels I to V are most commonly associated with squamous cell carcinoma, typically arising from occult primaries within the upper aerodigestive tract. Cutaneous malignancies of the head and neck region may also metastasize to cervical lymph nodes, frequently involving the parotid basin and upper deep jugular chain.

In contrast, involvement of the central compartment (levels VI and VII) is more suggestive of a thyroid gland primary. Lateral cervical lymphadenopathy may also be seen in a variety of other malignancies, including salivary gland tumors, lymphomas, and non-squamous skin malignancies such as melanoma and Merkel cell carcinoma. The presence of metastasis in the supraclavicular lymph nodes, particularly the classical “Virchow’s node,” should prompt evaluation for infraclavicular primaries, including malignancies of the gastrointestinal tract, genitourinary system, esophagus, hepatobiliary system, pancreas, lung, breast, and uterus. 7, 8

In recent years, the increasing detection of human papillomavirus (HPV)-associated disease in cervical lymph nodes has significantly influenced the diagnostic approach. HPV positivity strongly suggests an oropharyngeal origin, even in the absence of an identifiable primary lesion on initial evaluation.9

Although less common, rare entities such as soft tissue sarcomas and cervical metastases from central nervous system tumors may also present as neck masses, further contributing to the heterogeneity of NMUO. 10

EVALUATION of NMUO

Early recognition and timely referral are critical in the management of neck metastasis of unknown origin (NMUO). General physicians encountering patients with persistent or suspicious cervical lymphadenopathy should maintain a low threshold for referral to a head and neck specialist, as delayed evaluation may adversely impact outcomes. 11-13

The evaluation of NMUO can be broadly categorised into three key components:

1. Clinical evaluation, including endoscopic assessment
2. Imaging (radiological evaluation)
3. Pathological evaluation, including molecular studies

Importantly, the diagnostic approach should not be restricted to the loco-regional region alone but must include a comprehensive systemic evaluation to exclude infraclavicular primaries.

Clinical Evaluation

A detailed clinical history remains the cornerstone of evaluation in patients presenting with cervical lymph node metastasis of unknown origin. Particular attention should be directed toward symptoms related to the neck mass, including its duration, rate of progression, and the presence or absence of pain. A slowly enlarging, painless, firm-to-hard cervical swelling is highly suggestive of a neoplastic process, either primary or metastatic in origin.

Assessment of risk factors is crucial and should include a history of tobacco and alcohol use, as well as high-risk sexual behaviour, which is associated with human papillomavirus (HPV)-related malignancies. Even in the absence of overt symptoms, targeted questioning may reveal subtle indicators of an underlying head and neck primary. These may include referred otalgia, persistent throat discomfort, dysphagia, odynophagia, intolerance to spicy foods, voice changes, nasal obstruction, or headache. Nonetheless, a significant proportion of patients with NMUO remain asymptomatic apart from the neck swelling.

A comprehensive physical examination should follow, including systematic palpation of all cervical lymph node levels as defined by the American Academy of Otolaryngology–Head and Neck Surgery classification. The presenting lymph node should be evaluated in detail with respect to its location, size, number, shape, consistency, mobility, and relationship to surrounding structures. It is also important to note whether the nodes are discrete or matted. Metastatic deposits from squamous cell carcinoma are typically firm to hard and non-tender in early stages, with progressive fixation to adjacent structures as the disease advances.

In contrast, lymph nodes that are firm to rubbery in consistency, especially when associated with “B symptoms” such as fever, weight loss, and night sweats, may suggest lymphoma. These clinical distinctions, while not definitive, provide important diagnostic clues. 14,15

A thorough evaluation of the upper aerodigestive tract is essential and is commonly performed using flexible fibreoptic nasopharyngoscopy and laryngoscopy. Particular attention should be paid to anatomical “blind spots,” including the base of tongue, tonsillar fossae, nasopharynx, hypopharynx, and supraglottic larynx, where occult primaries are frequently located.

Examination should also include careful inspection of the scalp and skin of the head and neck region to identify potential cutaneous primaries. Additionally, assessment of the salivary glands and thyroid gland is necessary. A focused cranial nerve examination should be performed to detect subtle neurological deficits, which may indicate perineural invasion or advanced locoregional disease.

IMAGING

Ultrsound

Ultrasonography plays a valuable role in the initial evaluation of cervical lymphadenopathy. It is particularly useful in differentiating solid from cystic lesions, assessing nodal size, number, and precise anatomical location, and providing detailed information on intranodal architecture.

Ultrasound is especially important in the evaluation of thyroid malignancies, where it serves as a primary imaging modality for both thyroid nodules and cervical lymph nodes. It can detect pathological features such as microcalcifications, cystic change, loss of fatty hilum, and abnormal vascularity, even in lymph nodes that are not clinically enlarged. 16 In addition, ultrasonography is an essential adjunct in guiding fine-needle aspiration (FNA). It enables accurate targeting of representative solid components within a lesion, particularly in partially cystic or necrotic nodes, thereby improving diagnostic yield and cytological accuracy.7

Cross-Sectional Imaging (CT / MRI)

Contrast-enhanced computed tomography (CT) is typically the first-line imaging modality in the evaluation of cervical lymph node metastasis of unknown origin. A scan extending from the skull base to the thoracic inlet is recommended for upper neck nodal disease, while inclusion of the thorax and, when indicated, the abdomen is essential in cases of supraclavicular lymphadenopathy to evaluate for potential infraclavicular primaries.

Magnetic resonance imaging (MRI), performed with and without gadolinium contrast, is particularly useful in scenarios where superior soft tissue delineation is required, such as assessment of the tongue base, nasopharynx, and parapharyngeal spaces. MRI may also be preferred in patients with contraindications to iodinated contrast, including those with contrast allergy or pregnancy. In patients with renal impairment, non-contrast CT or MRI may be considered as alternative imaging options.

Both CT and MRI provide excellent delineation of cervical nodal anatomy, including the size, extent, and characteristics of lymphadenopathy, as well as their relationship to adjacent vascular and visceral structures. Importantly, these modalities can detect additional non-palpable but suspicious lymph nodes in regions such as the parapharyngeal, retropharyngeal, paratracheal, and mediastinal compartments.

The pattern of nodal involvement may offer valuable clues regarding the location of the occult primary tumor. For instance, retropharyngeal lymphadenopathy may suggest a nasopharyngeal origin, whereas paratracheal nodal involvement raises suspicion for thyroid or esophageal primaries.

In addition to nodal assessment, cross-sectional imaging may directly identify subtle mucosal abnormalities within the upper aerodigestive tract. Detection of asymmetry, focal thickening, or abnormal enhancement can guide targeted clinical examination and biopsy, thereby improving the likelihood of identifying the primary tumor.

METABOLIC IMAGING:

Positron Emission Tomography- Computed Tomography (PET-CT)

Positron emission tomography combined with computed tomography (PET-CT) has emerged as a crucial imaging modality in the evaluation of neck metastasis of unknown origin (NMUO). By detecting areas of increased metabolic activity, PET-CT can help identify potential occult primary sites within the head and neck region, as well as reveal other locoregional or  distant metastases that may not be apparent on conventional imaging.

PET-CT is particularly valuable in guiding further diagnostic workup, including targeted biopsies and endoscopic evaluation. When performed prior to panendoscopy or direct examination under general anesthesia, it may improve the likelihood of detecting the primary tumor and assist in narrowing the field of examination.

However, PET-CT is not without limitations. Several studies have highlighted its propensity for false-positive findings, often due to physiological uptake or inflammatory processes, thereby reducing its overall specificity. Despite these limitations, a substantial body of literature supports its role as an adjunctive tool in the diagnostic algorithm for NMUO.17

A meta-analysis by Zhu et al. demonstrated that PET-CT performed prior to endoscopic evaluation identified the primary tumor in approximately 44% of cases, with a high sensitivity of 97% but a moderate specificity of 68%. These findings underscore the importance of cautious interpretation.18

Therefore, PET-CT findings should always be integrated with clinical examination, histopathological results, and other imaging modalities to ensure accurate diagnosis and optimal management planning.

TISSUE EVALUATION

Fine Needle Aspiration Cytology (FNAC)

When a suspicious primary site is identified on clinical examination, a directed biopsy is performed. In the absence of an identifiable primary, fine needle aspiration cytology (FNAC) of the cervical lymph node is the initial diagnostic modality of choice to confirm metastatic disease and establish a pathological diagnosis. Importantly, FNAC does not compromise subsequent surgical management and is therefore preferred as the first-line invasive investigation.

For deep-seated lymph nodes, lesions in proximity to vital structures, or nodes with cystic components, ultrasound-guided FNAC enhances both safety and diagnostic yield. This approach is particularly valuable in targeting viable solid areas within partially cystic or necrotic nodes.

In solid lymph nodes harbouring metastatic squamous cell carcinoma, FNAC demonstrates a high diagnostic yield, with positivity rates exceeding 80%. However, in cystic neck masses, the sensitivity is lower due to higher false-negative rates, often resulting from hypocellular aspirates. The use of ultrasound guidance has been shown to improve diagnostic accuracy in such cases.19

A comprehensive review by Tandon et al., which included 30 studies along with 2,702 aspirates from their own institution, reported sensitivity, specificity, positive predictive value (PPV), negative predictive value (NPV), and overall accuracy rates of 89.5%, 98.5%, 97.3%, 94.0%, and 95.1%, respectively. Specifically, for metastatic squamous cell carcinoma, FNAC has demonstrated a sensitivity of approximately 92%. 19

Despite its utility, FNAC has inherent limitations. As a cytological technique, it provides a dispersed cellular sample without preservation of tissue architecture. Therefore, a negative FNAC result in the setting of a clinically suspicious lymph node should not be interpreted as benign but rather as non-diagnostic. False-negative results may arise due to several factors, including low cellularity (especially in cystic metastases), sampling of necrotic areas, peritumoral inflammation, or excessive vascularity resulting in a bloody specimen, and other possible reasons.

To enhance diagnostic accuracy, ancillary techniques such as immunohistochemistry (IHC) and molecular testing are increasingly utilized on cytological specimens. These adjuncts are particularly useful in identifying tumor origin (e.g., HPV/p16 status, thyroid markers) and refining the diagnosis.

In cases where initial FNAC is non-diagnostic and clinical suspicion persists, repeat aspiration, preferably under ultrasound guidance, should be performed, with emphasis on sampling viable solid components. 20

Core Needle Biopsy

Core needle biopsy (CNB) is an important diagnostic adjunct in cases where fine needle aspiration cytology (FNAC) is non-diagnostic or inadequate despite strong clinical suspicion of malignancy. Unlike FNAC, core biopsy provides preserved tissue architecture, thereby allowing more accurate histopathological assessment, immunohistochemistry (IHC), and molecular analysis.

Although core biopsy is more invasive and traumatic compared to FNAC, several large series have demonstrated a low incidence of clinically significant complications such as hematoma, infection, or tumor seeding.21-31 The introduction of newer self-aspirating powered biopsy needles has further reduced the theoretical risk of tumor implantation along the biopsy tract.

Allison et al. recommended the addition of a single “small core” using a 20-gauge aspiration core device at the time of initial FNAC to improve diagnostic yield. Their study demonstrated excellent diagnostic accuracy, including nearly 100% concordance for HPV detection using immunohistochemistry and in-situ hybridization techniques. 32

Current guidelines from the National Comprehensive Cancer Network recognize core biopsy as a useful option for confirmation of malignancy prior to definitive treatment.

An argument in favor of core biopsy prior to endoscopy, particularly if the FNA cytology is completely non-diagnostic, is to detect diagnoses other than squamous cell carcinoma before taking the patient to operating room, to avoid the rare, inadvertent misdiagnosis at frozen section leading to misguided treatment. 33,34 In patients where panendoscopy also fails to identify the primary tumor, a core biopsy may still be necessary to establish a histological diagnosis before initiating radiotherapy or chemotherapy based solely on cytological findings.

The ability of core biopsy specimens to support ancillary testing, including HPV/p16 analysis, Epstein–Barr virus (EBV) studies, and other molecular markers, has further increased its relevance in the modern diagnostic workup of NMUO.

Open Biopsy

The role and timing of open biopsy in the evaluation of neck metastasis of unknown origin (NMUO) remain important considerations, as they carry significant implications for subsequent management. Although core needle biopsy (CNB) has improved diagnostic accuracy, false-negative results may still occur, particularly in cystic lesions, where sampling errors and hypocellularity are common. Consequently, a subset of patients may ultimately require open biopsy to establish a definitive diagnosis. Frozen section analysis performed at the time of open biopsy has demonstrated excellent sensitivity for metastatic squamous cell carcinoma, approaching nearly 100% in some series, although its diagnostic accuracy is comparatively lower for other malignancies.

When malignancy has already been confirmed by a less invasive modality such as FNAC or CNB, premature excisional lymph node biopsy without planned completion neck dissection is generally considered inappropriate, as it offers no therapeutic benefit and may potentially complicate future surgical planes. Furthermore, in many patients, the overall radiotherapy dose and treatment strategy remain unchanged despite prior open biopsy. However, it is important to recognize that performance of an open lymph node biopsy does not necessarily mandate subsequent neck dissection if definitive chemoradiation remains the preferred treatment approach based on the eventual identification and staging of the primary tumor.35,36

In patients with suspected lymphoma, tissue architecture is essential for accurate histopathological classification and immunophenotyping. Therefore, either core needle biopsy or incisional/excisional biopsy may appropriately serve as the initial diagnostic procedure in such cases.

Advanced Methods for Diagnosis of the Primary Malignancy

Immunohistochemistry (IHC) and Molecular Techniques:

Immunohistochemistry (IHC) and molecular diagnostic techniques have become increasingly important in the evaluation of neck metastasis of unknown origin (NMUO). These modalities assist not only in establishing the histopathological diagnosis but also in improving diagnostic accuracy and identifying the likely site of the primary tumor. Their role is particularly significant in cases of poorly differentiated cancers, non-squamous malignancies or supraclavicular lymphadenopathy, where the possibility of an infraclavicular primary must be considered. Commonly utilized immunohistochemical markers include cytokeratin profiles, p53 mutations, thyroid transcription factor-1 (TTF-1), thyroglobulin, calcitonin, Paired-box gene 8 (PAX8), S100 protein, human melanoma black 45 (HMB 45) and other lineage-specific markers that help narrow the differential diagnosis and determine tumor origin. 37, 38

In cases where metastatic thyroid carcinoma is suspected, particularly in cystic cervical lymph nodes, measurement of thyroglobulin levels in aspirated cyst fluid can provide valuable diagnostic information.

For poorly differentiated lymphomas, flow cytometry in combination with IHC markers such as CD20 and PAX5 is highly useful in establishing B-cell lineage. Additionally, heavy-chain gene rearrangement studies using reverse transcriptase polymerase chain reaction (RT-PCR) can demonstrate clonality and further support the diagnosis. 39

Patients presenting with cervical metastases in the setting of cutaneous malignancies or a prior history of skin cancer may pose a diagnostic dilemma regarding the need for extensive upper aerodigestive tract evaluation. Certain cutaneous squamous cell carcinomas may demonstrate p16 positivity without any association with HPV-related oncogenesis or prognostic significance, thereby potentially confounding interpretation. Accurate identification of the primary site in such cases is clinically important, as it directly influences decisions regarding irradiation of the upper aerodigestive tract and contralateral neck.40

Nordemar et al. demonstrated that the presence of aneuploidy on DNA analysis predicted malignancy in 53% of aspirates obtained from cystic neck lesions that were ultimately proven malignant, whereas benign branchial cleft cysts lacked aneuploidy. Such molecular analyses may therefore aid in differentiating malignant cystic metastases from benign congenital cystic lesions.41

More recently, advanced molecular technologies such as next-generation sequencing (NGS), gene expression microarrays, microRNA profiling, and DNA methylation analysis have emerged as promising tools in the evaluation of carcinoma of unknown primary. These techniques can predict the likely site of origin with reported accuracies ranging from 80% to 95%, thereby offering significant potential for individualized diagnostic and therapeutic planning. 42 -44

In-Situ Hybridization (ISH):

In-situ hybridization (ISH) plays an important role in identifying viral-associated malignancies, particularly those related to human papillomavirus (HPV) and Epstein–Barr virus (EBV). Detection of HPV, especially high-risk subtypes, strongly suggests an oropharyngeal primary, whereas EBV positivity is classically associated with nasopharyngeal carcinoma. 45,46

ISH-based techniques, often used in conjunction with p16 immunohistochemistry, have become increasingly relevant in regions with a high prevalence of HPV- or EBV-associated malignancies. These methods not only facilitate identification of the occult primary site but may also provide prognostic and therapeutic implications in the modern management of NMUO.

Human Papillomavirus (HPV)

Early determination of HPV status in patients with neck metastasis of unknown origin (NMUO) is of considerable clinical importance, both as a prognostic indicator and as a guide to the likely site of the occult primary tumor. It is now well established that human papillomavirus (HPV)-associated malignancies are strongly linked to oropharyngeal squamous cell carcinoma (OPSCC), typically representing a biologically distinct and less aggressive disease entity with improved treatment response and survival outcomes. 47

Histologically, HPV-related tumors often demonstrate basaloid morphology and an exophytic growth pattern. Immunohistochemical evaluation of p16 expression is widely used as a surrogate marker for HPV-associated carcinoma. However, p16 positivity alone may occasionally yield false-positive results, particularly in tumors lacking the characteristic basaloid histology or typical anatomical origin within the palatine or lingual tonsils. Therefore, confirmation of HPV status using more specific molecular techniques is recommended in atypical cases. The presence of HPV can be confirmed through detection of HPV deoxyribonucleic acid (DNA) by in-situ hybridization (ISH) or by real-time reverse transcription polymerase chain reaction (RT-PCR). In contrast, p16-negative tumors are more frequently infiltrative in nature and are commonly associated with p53 mutations. 48, 49

Identification of HPV positivity has significant implications for diagnostic evaluation. HPV-positive metastatic cervical nodes strongly direct attention toward an occult oropharyngeal primary, particularly within the tonsillar tissue and base of tongue. Conversely, p16-negative nodal metastases necessitate a broader evaluation of the entire upper aerodigestive tract.

Epstein–Barr Virus (EBV)

Testing for Epstein–Barr virus (EBV) in metastatic cervical lymph node specimens is particularly indicated when the histopathology suggests a nasopharyngeal-type carcinoma or when there is clinical suspicion of a nasopharyngeal primary, especially in cases where endoscopic evaluation fails to identify an obvious lesion.The significance of EBV testing varies geographically. In endemic regions such as Southern China, the Middle East, and North Africa, EBV-associated nasopharyngeal carcinoma (NPC) is highly prevalent, making EBV testing more clinically relevant than HPV testing in the diagnostic workup of NMUO. 50, 51

Certain patterns of nodal involvement may further raise suspicion for nasopharyngeal carcinoma. Enlarged posterior triangle lymph nodes and retropharyngeal adenopathy are particularly suggestive of a nasopharyngeal origin. However, metastasis to level II cervical lymph nodes is also common and has been reported in nearly 70% of NPC cases. 52

The standard method for EBV detection is in-situ hybridization for Epstein–Barr virus-encoded RNA (EBER), which is a well-validated and highly sensitive diagnostic technique. Identification of EBV positivity in metastatic nodes can significantly narrow the search for the occult primary and guide focused evaluation of the nasopharynx as well as subsequent treatment planning. 50-52

SURGICAL PROCEDURES IN EVALUATION OF NMUO:

Panendoscopy

“Panendoscopy” is the commonly used term for “triple endoscopy,” which traditionally includes direct laryngoscopy, rigid or flexible bronchoscopy, and rigid or flexible esophagoscopy performed in the evaluation of neck metastasis of unknown origin (NMUO). Historically, these procedures were undertaken to identify occult primary tumors within the upper aerodigestive tract. However, isolated cervical metastasis arising from occult malignancies of the esophagus, bronchus, or lung is exceedingly rare. Consequently, the routine use of bronchoscopy and esophagoscopy in all patients with NMUO remains controversial and has been abandoned in many centers unless clinically indicated by symptoms, imaging findings, smoking history, or specific patterns of nodal involvement.In contrast, meticulous examination of the nasopharynx, oropharynx, hypopharynx, and larynx through direct nasopharyngoscopy and laryngoscopy remains an essential component of evaluation. 53-57

Although flexible endoscopic assessment may be performed in the outpatient setting, examination under general anesthesia is considered superior for identification of occult primaries and for obtaining targeted biopsies.  Examination under anesthesia is particularly advantageous in children and anxious or poorly cooperative adults. It allows complete relaxation of the pharyngeal musculature, improved visualization, and better access to anatomically difficult “blind spots,” including the base of tongue, tonsillar fossae, nasopharynx, and pyriform sinuses. During endoscopic evaluation, any suspicious mucosal abnormality including ulceration, nodularity, mucosal irregularity, increased vascularity, asymmetry, or discoloration should undergo directed biopsy. 53, 58, 59

Identification of the primary tumor is of paramount importance, as it significantly influences treatment planning, radiation fields, morbidity, and overall survival. Nevertheless, beyond directed biopsy of suspicious lesions, the historical practice of extensive random “blind” biopsies from the upper aerodigestive tract has demonstrated a very low diagnostic yield and is now largely discouraged in contemporary practice. 60

Narrow Band Imaging (NBI)

Narrow band imaging (NBI) is a noninvasive adjunct to conventional fibreoptic endoscopic examination and has demonstrated considerable diagnostic value in the evaluation of neck metastasis of unknown origin (NMUO). By enhancing visualization of mucosal vascular patterns, NBI increases the likelihood of detecting subtle mucosal lesions responsible for regional metastatic spread to the cervical lymph nodes.

The technique functions by restricting light to two narrow spectral wavelengths: blue light (400–430 nm) and green light (525–555 nm). The blue wavelength enhances superficial capillary networks, while the green wavelength highlights deeper submucosal vessels. This optical enhancement improves visualization of abnormal neoangiogenic patterns associated with dysplasia and early malignancy, thereby facilitating detection of occult mucosal primaries that may not be apparent under standard white-light examination.

NBI can be employed during office-based flexible nasopharyngoscopy as well as during panendoscopic examination under general anesthesia. It is particularly useful in evaluating anatomically challenging regions such as the nasopharynx, base of tongue, tonsillar region, hypopharynx, and supraglottic larynx.

A systematic review by Cosway B et al. reported pooled sensitivity and specificity rates of 74% and 86%, respectively, for detection of occult primary tumors in the NMUO setting. The high diagnostic accuracy was further supported by an area under the receiver operating characteristic curve exceeding 0.9, suggesting that NBI is an excellent adjunctive tool for localization of mucosal primary lesions in these patients.61

Tonsillectomy

Excisional biopsy of the palatine tonsils, even when they appear clinically normal, has become an established practice in many centers for the identification of occult primary tumors in neck metastasis of unknown origin (NMUO). Initially, ipsilateral tonsillectomy was routinely performed; however, subsequent studies demonstrated the possibility of contralateral occult disease, leading many surgeons to adopt bilateral palatine tonsillectomy as standard practice. Approximately 10% of occult tonsillar primaries have been reported in the contralateral tonsil. 62 Safely performed tonsillectomy significantly increases the rate of primary tumor detection without adding substantial long-term morbidity.63, 64

The same principle has been extended to excision of the lingual tonsils and adenoid tissue when clinically indicated. Unlike palatine tonsils, however, lingual tonsils are not commonly chronically infected, and lingual tonsillectomy is not routinely performed in general practice. Furthermore, lingual tonsils are non-encapsulated and not readily accessible through conventional transoral techniques. Their removal therefore requires specialized instrumentation, including FK retractors and angled telescopes, often with assistant support.

Transoral Laser Microsurgery (TLM): TLM, utilizing a combination of operating microscopes and telescopes, has been described by several authors as an effective approach for lingual tonsillectomy in NMUO. 65-67  Nagel et al. reported outcomes of 36 lingual tonsillectomies performed using laser microsurgery, with identification of the primary tumor in 86% of patients.68

Transoral Robotic Surgery (TORS): TORS has further revolutionized the management of occult oropharyngeal primaries. The robotic platform provides superior visualization, enhanced dexterity, and improved access to anatomically confined regions of the oropharynx through articulated robotic instruments.

The da Vinci Surgical System, manufactured by Intuitive Surgical, was initially introduced for transoral resection of oropharyngeal malignancies and has subsequently been applied in the evaluation of NMUO. 69

Mehta et al. first reported robotic-assisted bilateral lingual tonsillectomy in patients with NMUO in 2013. In their series of 10 patients who had previously undergone direct laryngoscopy, directed or random biopsies, and palatine tonsillectomy without identification of a primary, occult tumors were identified in lingual tonsil tissue in 9 out of 10 patients, with a mean tumor diameter of 0.9 cm.70  Multiple subsequent studies have confirmed the effectiveness of robotic lingual tonsillectomy in detecting occult primaries.71

Overall, palatine and lingual tonsillectomy together can identify occult primary tumors in nearly 70% of patients with NMUO who have negative office examination, cross-sectional imaging, and PET-CT findings, particularly in HPV-positive disease. 72

Regardless of the surgical technique employed, careful operative planning is essential. Excessively aggressive mucosal excision and circumferential trauma should be avoided to minimise the risks of postoperative haemorrhage, severe odynophagia, and long-term oropharyngeal stenosis.

The timing and sequencing of procedures are also important to maximise diagnostic yield while minimising patient morbidity:

1. First step: Panendoscopy with directed biopsy of any suspicious lesion
2. Second step: Palatine tonsillectomy with adenoidectomy when indicated, along with frozen section analysis
3. Third step: Ipsilateral lingual tonsillectomy with frozen section, followed by contralateral lingual tonsillectomy if necessary

The first two stages may be performed during the same operative sitting, whereas lingual tonsillectomy may be deferred to a separate procedure after several days to reduce morbidity and postoperative discomfort. It should also be recognised that frozen section analysis may be less sensitive than final histopathological examination, and occult primaries may occasionally only become evident on permanent sections.

Primary Tumor Not Identified

Despite extensive clinical, radiological, endoscopic, and pathological evaluation, the primary tumor remains unidentified in a proportion of patients, who are subsequently managed as cases of neck metastasis of unknown origin (NMUO). In many such patients, occult mucosal primaries may nevertheless receive incidental treatment during management. Patients undergoing radiotherapy directed to both the neck and upper aerodigestive tract may receive adequate treatment to microscopic primary sites. Similarly, removal of lymphoid tissues such as the palatine and lingual tonsils during diagnostic evaluation may inadvertently excise the occult primary tumor.

Interestingly, studies have demonstrated that among patients treated with neck dissection alone, a clinically apparent primary tumor subsequently develops in only approximately 10–30% of cases. This observation has led to the hypothesis that, in a substantial proportion of patients, the primary lesion may undergo spontaneous regression.

Tumor regression is a recognized immunologically mediated phenomenon in certain malignancies, particularly cutaneous tumors such as melanoma and Merkel cell carcinoma. Similar spontaneous regression has also been reported, albeit less commonly, in squamous cell carcinomas. 73,74

In 1999, Califano et al. performed microsatellite analysis and surveillance biopsies from mucosal sites considered at risk and demonstrated that in 10 of 18 patients, histologically benign mucosal specimens harboured genetic alterations identical to those identified in the metastatic lymph nodes. These findings led to the concept of “genetically malignant but phenotypically benign” mucosal microscopic lesions foci possessing sufficient genetic abnormalities to metastasize to cervical lymph nodes without demonstrating overt local proliferation or invasion at the primary site. The authors further proposed that molecular profiling of mucosal biopsies could potentially guide targeted radiotherapy to genetically implicated mucosal regions, even in the absence of clinically identifiable disease.75 This evolving understanding highlights the biological complexity of NMUO and underscores the growing role of molecular diagnostics in refining both localization and treatment strategies in the future.

TREATMENT

Treatment of neck metastasis of unknown origin (NMUO) is primarily guided by the presumed or identified primary tumor site and the extent of cervical nodal disease (N stage), similar to the management principles applied in other head and neck malignancies. In patients where the primary tumor is successfully identified, treatment follows standard site-specific protocols. However, management becomes considerably more complex in the subset of patients in whom the primary remains occult despite exhaustive evaluation.

Surgical Management

Unilateral or bilateral neck dissection, depending on disease extent for resectable NMUO, is a strategy favoured in National Comprehensive Cancer Network recommendations as it reduces or avoids radiation exposure while providing improved pathological staging information. 7, 76 

Patients with favourable pathological features, such as a single involved lymph node smaller than 3 cm without extranodal extension and with adequate nodal yield, may be managed with close observation alone following surgery. Surveillance typically includes serial clinical examinations and office-based fibre-optic naso-pharyngo-laryngoscopy. 7 Several studies have shown a low incidence of emergence of primaries in these patients on long term follow-up.

Post operative adjuvant Radiation is adviced for patients with multiple nodes or nodes larger than 3 centimeters with doses ranging from 44 to 63 Gy, with higher doses for areas with high risk pathological features. Extranodal extension, in particular, leads to an indication for adding chemotherapy to post-operative radiotherapy. 2,7 For advanced disease in the neck, surgery followed by radiation or combination of concurrent chemotherapy and radiation are the major options.

Radiation Therapy

For patients with low-volume neck disease, radiation therapy alone remains an accepted alternative treatment option, particularly after histological confirmation through core biopsy. 2, 76

Modern radiotherapy is typically delivered using intensity-modulated radiation therapy (IMRT) or three-dimensional conformal radiotherapy techniques to minimize dose exposure to surrounding critical structures. Radiation doses generally range between 66–70 Gy to gross nodal disease, administered in daily fractions of 2–2.2 Gy over approximately 6–7 weeks. Lower-risk regions typically receive doses ranging from 44–63 Gy. Target volumes are individualized according to nodal burden, involved cervical levels, and HPV/EBV status. One commonly employed strategy is to treat the patient similarly to a T1 oropharyngeal squamous cell carcinoma, given that many occult primaries likely arise from the tonsil or base of tongue. In such cases, radiation fields may include the oropharynx, retropharyngeal nodes, and bilateral neck. 7

Despite advances in radiotherapy techniques, late toxicities remain important considerations and include xerostomia, dysphagia, soft tissue fibrosis, osteoradionecrosis, mucosal atrophy, impaired wound healing, and secondary malignancies. These complications are substantially reduced with modern IMRT-based protocols compared to historical extended-field irradiation.

Many radiation oncologists prefer to treat both sides of neck if they cannot rule out midline primaries, including oropharynx, retropharyngeal nodes and nasopharynx. But routine inclusion of the larynx and hypopharynx does not affect outcome but significantly increases morbidity and should be avoided if possible. 77 Selected patients with unilateral low-volume disease may undergo ipsilateral neck irradiation alone with selective treatment of highest-risk mucosal regions. This approach significantly reduces long-term morbidity while maintaining excellent disease control. 2, 78

Role of Nodal Distribution in Treatment Planning:  The anatomical distribution of metastatic lymphadenopathy along with regional epidemiological patterns provides guidance on the course of further management. The most common presentation in NMUO is in  Level II, which usually suggests an occult primary in the upper aerodigestive tract [oropharynx, hypopharynx, or supraglottic larynx].

Isolated level I [submental and submandibular] adenopathy is less common and more suggestive of oral cavity or cutaneous primaries. Ozer et al. demonstrated that level I metastases were uncommon in NMUO and were more frequently associated with oral cavity malignancies.79 Similarly, involvement of level V nodes raises suspicion for nasopharyngeal carcinoma or posterior scalp cutaneous malignancies. 52, 80

Residual disease: Salvage neck dissection is performed when feasible for patients with persistent or metabolically active residual nodal disease identified on post-treatment imaging, usually with PET-CT performed 3–6 months after completion of therapy.

HPV-related disease: In patients where there is evidence of HPV, definitive chemoradiotherapy is an established preferred modality as there is excellent locoregional control rates.

Since the landmark two-part review published by Strojan et al. in  2013, one of the most important developments in clinical practice has been the routine assessment of oropharyngeal squamous cell carcinomas for p16 expression and, in selected cases, HPV DNA or mRNA using in-situ hybridization or reverse transcriptase polymerase chain reaction techniques. This evaluation has implications on both prognostic stratification and therapeutic decision-making. The increasing recognition of favorable outcomes in HPV-associated disease has led to growing interest in treatment de-escalation strategies aimed at minimizing long-term toxicity without compromising cure rates.2

Emergence of Primary Tumor During Follow-Up

Several studies have shown that the subsequent appearance of a clinically evident primary tumor following treatment of NMUO is relatively uncommon. Patients treated with neck dissection alone may demonstrate a somewhat higher incidence of delayed primary emergence compared with those receiving mucosal irradiation; however, reported rates vary widely across studies due to differences in diagnostic intensity and regional tumor epidemiology. 81, 82

More recent reports, particularly in HPV-negative populations treated with unilateral neck irradiation without comprehensive mucosal radiation, have demonstrated remarkably low rates of primary emergence. Takes et al. reported excellent survival outcomes and absence of primary emergence following unilateral neck irradiation in selected low-volume unilateral NMUO patients. Interestingly, the incidence of delayed mucosal primary emergence approximates the expected rate of second primary tumors seen in patients previously treated for known head and neck cancers. This observation raises the possibility that some subsequently detected lesions may represent metachronous second primaries rather than initially occult tumors. 83

Modern management of neck metastasis of unknown origin (NMUO) requires careful recognition of its characteristic clinical presentation and avoidance of diagnostic pitfalls that may lead to inappropriate or unnecessarily morbid interventions. A structured, systematic, and multidisciplinary approach remains essential for optimal patient outcomes.

Comprehensive evaluation should include cross-sectional imaging with contrast-enhanced CT or MRI, supplemented by PET-CT for improved localization of occult primary tumors and detection of distant disease. Thorough assessment of the upper aerodigestive tract mucosa is mandatory and is best performed using endoscopic examination under anesthesia, preferably enhanced with adjunctive techniques such as narrow band imaging (NBI).

Ultrasound-guided fine needle aspiration cytology remains the preferred initial diagnostic modality for tissue confirmation. However, histological tissue acquisition may ultimately be required, particularly in cases with non-diagnostic cytology or when advanced pathological and molecular analyses are necessary. Whenever feasible, tissue diagnosis should ideally be obtained from the occult primary site itself.

The increasing use of immunohistochemistry, HPV and EBV testing, and advanced molecular technologies such as next-generation sequencing has substantially improved the ability to predict the likely site of origin and refine treatment strategies. These techniques are becoming central to the modern diagnostic workup of NMUO.

Palatine tonsillectomy and lingual tonsillectomy have significantly increased rates of primary tumor identification, particularly in HPV-associated disease, and now represent important components of contemporary evaluation algorithms.

Treatment decisions are individualized and depend upon identification of the primary tumor, extent of nodal disease, pathological risk factors, and viral status of the tumor. Neck dissection continues to play an important role in selected patients by improving pathological staging and, in some cases, reducing the need for extensive radiation therapy.

Current surgical and endoscopic techniques may be approaching the upper limit of achievable diagnostic yield in identifying occult primary tumors. Future advances are likely to emerge from molecular diagnostics, including gene expression profiling, microarray analysis, and other genomic technologies applied to mucosal fields at risk.

Despite considerable progress, significant variability still exists among institutions regarding both diagnostic evaluation and therapeutic strategies for NMUO. Differences persist in the utilization of core biopsy, lingual tonsillectomy, surgical management, radiation field design, and chemoradiotherapy protocols, even within similar geographic regions.

Although the incidence of NMUO has increased in the HPV era, it remains a relatively uncommon clinical entity, making large prospective randomized trials difficult to conduct. Continued collaborative research and multicenter studies are therefore essential to achieve greater consensus regarding the optimal evaluation, subclassification, and treatment of these patients.

Conflict of Interest:

The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.