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Cancer of unknown primary

IHC Panel for — Cancer of Unknown Primary

When cancer is found but its starting organ is not, IHC is the first tool the pathologist reaches for. It reads the protein fingerprints in your tumour tissue to trace where the cancer most likely began.

Medically reviewed by Dr. T. Raghavender Reddy, Medical Oncologist, MBBS · DM (Medical Oncology) · MD (Radiation Oncology) · Last reviewed September 2026

  • Proteins remember their origin — Even after cancer cells spread, they often still make the proteins of the tissue they came from.
  • Panels, not single markers — Pathologists test several markers together — the combination, not any one result, points toward a site.
  • It succeeds in many cases — IHC identifies a likely primary site in a meaningful proportion of CUP cases, according to ESMO guidance.
  • A suggestion, not a verdict — IHC points toward an origin. Your oncologist interprets that alongside scans, clinical findings, and molecular testing.
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IHC staining tests the proteins your tumour makes and compares them to the fingerprints of known tissue types. For a cancer of unknown primary, pathologists build a panel of markers to trace the most likely origin. In a meaningful proportion of cases, the pattern points clearly enough for your oncologist to plan treatment.

What do the abbreviations on your IHC report mean?

CK7 / CK20
Two cytokeratin proteins present in different tissue types. Which one is positive and which is negative is one of the first clues pathologists use to narrow down the organ of origin.
TTF-1
A protein made by lung and thyroid tissue. A positive result points toward one of those two sites as the starting point.
CDX2
A protein made in the lining of the bowel. A positive result points strongly toward a colorectal or upper gastrointestinal origin.
PSA
Prostate-specific antigen. In a man, a positive result points very strongly toward the prostate as the starting site.
ER / PR
Oestrogen and progesterone receptors. Positive results are most often associated with breast or gynaecological cancers.
GATA3
A marker associated with breast cancer and also with bladder cancer. It helps distinguish between those two origins when the clinical picture is unclear.
PAX8
A protein present in kidney, thyroid, and gynaecological tissue. A positive result narrows the search to one of those three sites.
Chromogranin A / Synaptophysin
Markers that identify neuroendocrine tumours — a type of cancer with its own distinct behaviour and treatment approach, regardless of which organ it started in.

How does IHC piece together where the cancer came from?

Cancer cells carry molecular memories of where they grew from. Even after spreading to the liver or lymph nodes, a lung cancer cell still makes lung proteins. IHC applies antibodies that bind to those specific proteins, staining positive wherever they are present.

The pathologist runs a first panel, studies the pattern, then adds targeted markers to confirm or rule out the strongest candidates. Each result narrows the field — like a detective eliminating suspects. Sometimes the picture is clear by the second panel. When it is not, molecular profiling — which compares your tumour's DNA pattern against a library of known cancers — is the next step recommended by ESMO.

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Dr. Owais Mohammed

MBBS, MD (General Medicine), DrNB (Medical Oncology), ECMO, MRCP SCE (Medical Oncology) (UK)

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Dr. Paila Gowri Naidu

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Dr. Kirti Ranjan Mohanty

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What do common IHC marker patterns generally suggest?

Marker patternGeneral directionContext that matters
CK7 positive / CK20 negativeLung, breast, ovary, biliaryCommon pattern — needs further markers to narrow down between these sites
CK7 negative / CK20 positiveColorectalOne of the more specific combinations in CUP workup
CK7 positive / CK20 positiveUpper GI, pancreas, urothelialLess specific — clinical and imaging findings carry more weight here
TTF-1 positive / Napsin A positiveLung adenocarcinomaA strong suggestion when both are positive together
PSA positive (in a man)ProstateHighly specific; prostate is the strong working assumption
ER positive / GATA3 positiveBreastParticularly relevant for a woman presenting with adenocarcinoma of unknown origin
Chromogranin A or Synaptophysin positiveNeuroendocrine tumourIdentifies tumour type rather than organ; changes the treatment approach

What should you do if the IHC panel is not conclusive?

  • Ask your oncologist which markers were tested and why those specific ones were chosen for your case.
  • Ask whether additional markers could be added to the panel to narrow the picture further.
  • Ask whether molecular profiling or gene expression profiling is the recommended next step.
  • Bring the full IHC report — including the sample block number — to every specialist consultation.
  • Ask whether a result 'favours' an origin or is only 'consistent with' one — these carry different levels of confidence.
  • Ask whether your treatment plan changes if the primary site is confirmed later.

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Common questions

Frequently asked questions

Why does finding the primary site matter if the cancer has already spread?

Different cancers respond to different treatments, and those treatments are chosen based on where the cancer started — not just where it is now. A breast cancer cell behaves differently at a molecular level than a bowel cancer cell, even if both have spread to the same lymph nodes. Knowing the origin allows your oncologist to match treatment to the biology, which is why pathologists work hard to answer this question even when the primary is not visible on any scan.

How does the pathologist decide which markers to test first?

The starting panel is based on your clinical picture — where the cancer was found, what the cells look like under the microscope, and any clues from imaging. A carcinoma in a woman's axillary lymph node prompts a different first panel than the same finding in a man with a raised PSA. The pathologist and oncologist work together to choose the markers most likely to be informative for your specific case.

What do 'positive' and 'negative' mean on the report?

Positive means the antibody found its target protein in your tumour cells and produced a visible stain under the microscope. Negative means that protein was not detected at a meaningful level. Neither result alone tells you where the cancer started — it is the pattern across your whole panel, read alongside your clinical findings, that the pathologist and oncologist interpret together.

How often does IHC actually identify the origin in CUP?

IHC identifies a likely primary site in a meaningful proportion of CUP cases. ESMO guidance notes that success depends on how much tissue is available, how well the sample was preserved, and which markers were selected. When IHC alone is inconclusive, adding molecular profiling increases the proportion of cases where an origin can be suggested. Your oncologist can tell you what your own panel showed and whether further testing is being planned.

What happens if the IHC panel does not find the origin?

When IHC is inconclusive, molecular profiling is the next step ESMO recommends. Gene expression profiling compares your tumour's pattern against a large library of known cancers to suggest the most probable site. When that also does not give a clear answer, your oncologist will plan treatment using the best available evidence for CUP as a category, drawing on whatever partial information the panel provided. Not having a named primary does not mean there is no treatment path.

Can I get a second opinion on an IHC report?

Yes, and for CUP it is reasonable to ask. A specialist pathologist or a multidisciplinary tumour board can review the same slides and sometimes add markers the original panel did not include. Ask your oncologist to arrange a review, or ask what is needed to have the tissue block sent to another centre. Bring your existing report to any second opinion so the reviewing pathologist can see what has already been tested.

Full index

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Preparing for a Biopsy

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Recovery and Aftercare

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Biopsy by Body Part

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Understanding Your Report

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IHC and Molecular Markers

IHC and Molecular Markers on a Biopsy: What They Mean →

Grading and Scoring Systems

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How Accurate Is a Biopsy?

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