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False negatives & accuracy

Sampling Error: — Why the Needle Might Miss

A biopsy can come back benign when cancer is present. The reason is usually not a laboratory mistake — it is that the needle collected tissue from the wrong part of the lesion. Understanding this is the first step to knowing when a result should be questioned.

Medically reviewed by Dr. Bharati Devi Gorantla, Medical Oncologist, MBBS · MD · DM (Adyar, Chennai) · ECMO · MRCP SCE (UK) · Last reviewed September 2026

  • Not a lab error — The tissue reaches the laboratory correctly. The question is whether it came from the part of the lesion that matters.
  • More common with uneven lesions — A large or mixed tumour may contain cancer in one area and near-normal tissue in another.
  • Reduced but not eliminated — Multiple cores and imaging guidance lower the risk considerably, but no biopsy technique removes it entirely.
  • A mismatch is the signal — When the biopsy result does not match what imaging showed, that disagreement is worth raising with your team.
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Sampling error happens when a biopsy needle takes tissue from a part of a lesion that does not contain cancer cells, even though cancer is present elsewhere in the same growth. It is one of the main reasons a benign biopsy result can be wrong. Radiologists and pathologists use several strategies to reduce it.

What do these terms mean?

Sampling error
When the biopsy needle collects tissue from a part of the lesion that does not contain what is being looked for, even though it is present elsewhere in the same growth.
False negative
A result that says no cancer was found when cancer is in fact present. Sampling error is one of the main causes.
Heterogeneous tumour
A tumour where different areas look and behave differently. Some parts may contain cancer while other parts, sampled by the needle, may appear almost normal.
Representative sample
A sample that reflects the most concerning part of a lesion, not just tissue from any point within it.
Pathology-imaging discordance
When the pathologist's finding does not match what imaging predicted. This disagreement is a recognised flag for possible sampling error.

What is sampling error and why does it happen?

A biopsy needle collects only a narrow column of tissue from wherever it lands. If the cancer is concentrated in one part of a lesion and the needle samples a different part, the result comes back benign — not because the analysis was wrong, but because the wrong tissue was analysed.

This is more likely when a lesion is large, when it contains mixed areas of living and dead tissue, or when cancer occupies only a portion of the growth. Sampling error is not a sign of poor technique — it is an inherent limitation of tissue sampling that oncology teams account for when interpreting results.

How is sampling error reduced?

  1. Multiple cores are taken

    The radiologist takes several needle samples from different areas of the lesion, giving a broader picture than a single pass.

  2. Real-time imaging guides the needle

    Ultrasound, CT or MRI guidance lets the radiologist steer toward the most suspicious-looking area rather than sampling at random.

  3. The most abnormal zone is targeted

    Where a lesion shows different characteristics in different areas, the needle is directed toward the part most likely to hold diagnostic tissue.

  4. Pathology is compared against imaging

    If a lesion looked concerning on imaging but the biopsy returns only normal tissue, that discordance is flagged before a management decision is made.

  5. Re-biopsy is arranged when concern remains

    If the clinical picture does not fit a benign result, guidance from ASCO and other bodies supports re-biopsy rather than routine follow-up.

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Which lesions carry the most risk of sampling error?

Large lesions are harder to sample representatively than small ones — there is simply more tissue the needle might miss.

Lesions with mixed areas on imaging — partly solid, partly fluid-filled, or with visible dead tissue — carry more risk because the cancer may be confined to a small viable rim that the needle bypasses.

Deep lesions and those in sites where movement affects accuracy, such as lung biopsies taken through breathing tissue, are harder to target precisely than fixed, accessible lesions.

What should you do if your result does not feel right?

Ask your oncologist one direct question: do the imaging finding and the biopsy result agree with each other? If imaging was described as suspicious and the biopsy returned benign tissue, those two findings disagree — and that disagreement should have been assessed.

Requesting a second opinion on the slides or the procedure is a supported step. A benign finding may be entirely correct, or it may warrant re-biopsy. Only the treating team, with your full imaging and clinical history, can make that call.

Questions families often ask

Does sampling error mean the biopsy was done badly?

No. Sampling error is an inherent property of tissue sampling, not a measure of the radiologist's skill. Even a technically correct biopsy can miss cancer present in another part of the lesion. What matters is whether protocols were followed — multiple cores, imaging guidance, comparison against what imaging showed — and whether the team acted on any discordance.

Can a second look at my imaging show whether the biopsy missed?

Imaging and pathology are assessed together, not in isolation. If imaging strongly suggested malignancy but the biopsy returned benign tissue, that combination — called pathology-imaging discordance — is itself a signal. Ask your team directly: do the imaging result and the biopsy result agree with each other? If discordance was assessed, they should be able to explain the reasoning.

Is there a biopsy method that avoids this problem?

No technique eliminates sampling error entirely, but some reduce it. Vacuum-assisted devices collect more tissue per pass than a standard core needle. Liquid biopsy analyses tumour DNA from the blood and is not affected by needle placement, but it has different limitations and is not a replacement for tissue biopsy in most clinical situations. Your team will choose the method best suited to your lesion and site.

How many core samples should have been taken?

The answer depends on the organ, lesion size and needle type. For breast biopsies, the American College of Radiology and European Society of Breast Imaging publish guidance on minimum core counts. Similar site-specific guidance exists for other organs. Ask your team how many samples were taken and whether that aligns with standard practice for your biopsy type.

I am still worried after a benign result. Can I ask for a repeat biopsy?

If imaging was described as suspicious, symptoms have continued, or your clinical picture has changed since the procedure, those are reasonable grounds to ask whether re-biopsy is indicated. Raise the specific concern at your next appointment and ask whether the pathology and imaging are concordant. A worry you cannot resolve through discussion is also a legitimate reason to seek a second opinion at another centre.

What does it mean when a report says 'concordant' or 'discordant'?

Concordant means the biopsy finding is consistent with what imaging predicted — the two agree. Discordant means they do not. Discordance does not automatically mean the biopsy was wrong, but it means the result needs careful review before any management decision is made. Ask your team which category your result falls into and what that means for your next steps.

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Common questions

Frequently asked questions

What is sampling error in a biopsy?

Sampling error is when the biopsy needle collects tissue from a part of the lesion that does not contain cancer, even though cancer is present elsewhere in the same growth. The laboratory analyses tissue that looks benign and the result comes back negative — not because the analysis failed, but because the wrong tissue was analysed. It is one of the main causes of a false negative biopsy result.

Which lesions are most affected by sampling error?

Large lesions, lesions with mixed areas on imaging, and deep lesions in sites where precise targeting is difficult carry the highest risk. Heterogeneous tumours — where cancer is concentrated in one part of the growth — are more vulnerable than uniform lesions. Small, solid, well-defined lesions are generally easier to sample representatively, though no biopsy is entirely free of this risk.

How do doctors reduce the risk of sampling error?

By taking multiple core samples, using real-time imaging guidance to steer the needle toward the most suspicious area, and comparing the result against what imaging showed. If the two do not agree — a situation called discordance — guidance from ASCO and other bodies supports re-biopsy rather than accepting the benign result without further review.

How do I know whether my benign result is reliable?

Ask your oncologist whether the imaging finding and the biopsy result agree with each other. If they do not, ask whether discordance was assessed and why the benign result is accepted. Your team should be able to explain this clearly. If you are not satisfied with the explanation, seeking a second opinion at another centre is a reasonable step.

Is liquid biopsy an alternative to a tissue biopsy?

Liquid biopsy analyses tumour DNA shed into the blood and is not affected by where a needle lands. However, it may not detect early or small tumours reliably and is not a standard replacement for tissue biopsy in most situations. In India, guidance from ICMR and CDSCO describes the contexts where it is validated. Ask your oncologist whether it is relevant to your case.

Can CION arrange a re-biopsy if I am concerned?

Yes. If your oncologist at CION determines that re-biopsy is indicated — because imaging and pathology are discordant, or because symptoms have not resolved — it can be arranged. Imaging-guided procedures are coordinated with partner centres. Raise the concern at your next appointment so the team can review your imaging and pathology report together before deciding on next steps.

Full index

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What Is a Biopsy?

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Types of Biopsy Compared

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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?

How Accurate Is a Biopsy? Errors and Second Opinions →

If Your Result Is Benign

Your Biopsy Is Benign: What It Means and What Comes Next →

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