How Often Is a PET-CT — Scan Wrong?

A suspicious finding on PET means your team needs to investigate further. It does not mean you have cancer. False positives are well-documented, and a PET report — however alarming the wording — is not a diagnosis. The next step is usually a biopsy so the tissue can be examined in a laboratory. Only that result, reviewed by your oncologist alongside your full clinical picture, can tell you what the finding actually is. Do not draw a conclusion from the scan report alone.

There is no single figure that covers all scans, and any number you read should be treated carefully. False positive rates vary significantly by cancer type, the body region being imaged, and clinical context — particularly in India, where tuberculosis and fungal infections are endemic. NCCN and ESMO do not publish one accuracy figure because no single number is meaningful across all situations. What they consistently say instead is that suspicious findings require tissue confirmation before any major decision is made.

Yes, and this is one of the most important limitations of PET-CT in India. Active TB, healed TB granulomas, and some fungal infections produce FDG uptake that looks indistinguishable from cancer on imaging. It is a known pitfall in TB-endemic settings. This is why your oncologist will ask about your history of TB or any prior exposure, and it is a core reason why biopsy is required rather than treating on the basis of the scan alone.

A false negative means the scan does not show something that is actually there. It is less common than a false positive, but it matters. Small tumours, low-grade cancers, and certain cancer types are more likely to be missed. High blood sugar at the time of the scan can also reduce accuracy. If your symptoms persist or worsen after a scan that appears clear, tell your oncologist — clinical assessment and further testing sometimes find what imaging does not.

A clear PET scan is generally reassuring, but it is one piece of information, not a guarantee. Low-grade tumours and very small lesions can produce signals too faint to appear. Your oncologist weighs the scan result against your symptoms, blood tests, and clinical picture. If something about your situation does not fit a clear result, that is information your team needs. Symptoms that are new or worsening after a clear scan should always be reported rather than assumed to be unrelated.

Ask what was found and where; whether a biopsy is recommended and of which site; what non-cancer causes have been considered given your history; and what the timeline is from biopsy to result to next decision. Write the answers down — these conversations are hard to hold onto under stress. You are entitled to ask for the imaging report in writing and to seek a second opinion on the interpretation if you want one.