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Cancer of unknown primary describes metastatic cancer where the site of origin has not been identified after an appropriate workup. Immunohistochemistry, imaging and increasingly molecular profiling are used to identify the likely origin. Where a treatable primary can be identified, or where an actionable alteration is found, the discussion changes substantially.
Also called CUP, Occult primary cancer.
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Cancer of unknown primary is a working diagnosis rather than a disease, and the goal of the workup is to convert it into a known one, because a named primary usually opens better options than a generic approach does.
Cancer of unknown primary describes metastatic cancer where the site of origin has not been identified after an appropriate workup. Immunohistochemistry, imaging and increasingly molecular profiling are used to identify the likely origin. Where a treatable primary can be identified, or where an actionable alteration is found, the discussion changes substantially.
01Before any molecular question
Usually established first: the pattern of metastatic disease, the histological type from biopsy, and the results of an immunohistochemistry panel. A structured workup is important because certain favourable subsets are treatable in a specific way and are worth identifying deliberately rather than by chance.
Usually already established
02
The divisions below are not labels. Each one changes what the treatment discussion is actually about.
Favourable subsets
Specific presentations treated as if the primary were known: for example isolated axillary nodal adenocarcinoma in a woman, treated as breast cancer; midline poorly differentiated carcinoma in a young man, treated as a germ cell tumour; single-site disease amenable to local treatment.
Adenocarcinoma of unknown primary
The largest group. Immunohistochemistry and molecular profiling are used to suggest an origin.
Squamous cell carcinoma of unknown primary
Often nodal, and frequently traced to a head and neck or anogenital origin with directed workup.
Poorly differentiated carcinoma
Where the pathology itself is uninformative and molecular approaches contribute most.
Neuroendocrine carcinoma of unknown primary
Managed on the neuroendocrine pathway once recognised.
03What is discussed
Comprehensive genomic profiling serves two purposes here. First, the pattern of alterations can suggest a tissue of origin, since certain alterations are strongly associated with particular cancers. Second, and often more useful, profiling may identify an actionable alteration that is treatable regardless of origin, such as an NTRK fusion, mismatch repair deficiency, BRAF V600E or high tumour mutational burden. Molecular and methylation-based tissue-of-origin classifiers exist and are used variably. Testing for the markers that would matter if a specific origin were confirmed, such as HER2, hormone receptors or PD-L1, is also part of a thorough workup.
NTRK
Fusions, actionable regardless of tissue of origin.
MMR / MSI
Actionable regardless of origin.
TMB
Assessed as part of the immunotherapy discussion.
BRAF
V600E, actionable across several tumour types.
HER2, hormone receptors, PD-L1
Worth testing where a specific origin is suspected, since they would change treatment if it were confirmed.
Tissue-of-origin classifiers
Molecular or methylation-based tests suggesting a likely primary site.
This is not an exhaustive list, and which markers are relevant depends on the individual case. Not every marker listed is assessed in every patient.
04What is tested, and from what
The workup should be structured rather than open-ended: history, examination, imaging, a targeted immunohistochemistry panel, and then comprehensive profiling. Adequate tissue matters, and a biopsy that yields enough material for the full panel is worth more than several small ones. Profiling in this setting has a higher chance of changing management than in many named cancers, because there is no standard alternative to fall back on.
Where profiling suggests an origin associated with inherited risk, or reveals a DNA repair or mismatch repair alteration, the germline question follows in the usual way. A family history suggestive of a hereditary cancer syndrome is worth taking carefully, because it can also point towards the likely primary site.
Cancer genetics and hereditary riskPlasma profiling is used where tissue is limited, and can occasionally suggest a tissue of origin. It is more often used to find an actionable alteration when a repeat biopsy is not feasible.
More on ctDNA monitoring05The timing question
Testing that arrives after the decision it was meant to inform has already been made is a common and avoidable problem. This is the sequence in which molecular information tends to become relevant.
06Take these to your team
When a decision is being made
Questions worth raising with a treating team.
At progression, recurrence or resistance
The molecular picture at diagnosis is not always the molecular picture later.
07What shapes eligibility
Cancer of unknown primary is often eligible for tumour-agnostic trials, which enrol by molecular alteration rather than by cancer type. These are frequently more relevant here than in named cancers, because there is no standard treatment to compare against.
| Term | What it means |
|---|---|
| Immunohistochemistry | Staining tissue for specific proteins to work out what kind of cell a cancer came from. |
| Favourable subset | A specific presentation of unknown primary cancer that is treated as if the origin were known. |
| Tissue of origin | The organ a cancer started in, even when it is first found elsewhere. |
| Tumour-agnostic | A treatment or trial that selects patients by molecular alteration rather than by cancer type. |
These are the services most often relevant to this cancer. Which of them applies is a clinical question, not an automatic one.
This entry is educational. It describes what is commonly discussed in this cancer, not what should happen for any individual. Which of it applies depends on the specific diagnosis, stage, prior treatment and the person themselves. Nothing here is a recommendation, and no list of biomarkers here is exhaustive. Decisions belong with a treating team who know the whole case.
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