What a PET-CT Scan Can Detect — — and What It Cannot

Yes. Two situations account for most missed findings. The first is a cancer that does not absorb glucose at a high rate — it is not FDG-avid, so it does not stand out against normal background activity. The second is a deposit that is too small for the scanner to resolve clearly. Neither means the scan was done incorrectly. It means the scan has a known biological limit for that cancer type, and your doctor will tell you which other investigations apply.

A normal PET-CT means no area of abnormally high glucose uptake was detected above the scanner's threshold. It does not mean there are no cancer cells anywhere in the body. Microscopic spread — too small to see on any available imaging — is not visible on PET-CT. Your oncologist interprets a normal result in the context of your full clinical picture and may still arrange further investigations if clinical suspicion remains.

Yes. Below a certain size, even a metabolically active deposit may not register clearly on the scan. The exact threshold depends on how active the cancer is and where in the body it is located. Lymph nodes containing only a small number of cancer cells may appear normal even when disease is present. This is one reason PET-CT is used alongside other clinical staging tools rather than as a complete picture on its own.

Yes, and this is a recognised limitation. Active infection, recent surgery, recent radiotherapy, and some autoimmune conditions all increase glucose uptake in the affected area. On the PET image alone, that uptake can look identical to active cancer. The CT component helps — anatomy can distinguish a lymph node from a site of inflammation — and the result is always read with full knowledge of your recent treatment and clinical history.

Because PET-CT is useful in proportion to how much glucose a cancer consumes. For lymphomas, lung cancer, and head and neck cancers, it is a standard staging tool because FDG uptake is consistently high. For most prostate cancers, some thyroid cancers, and low-grade neuroendocrine tumours, a different scan or a different tracer is more informative. The choice is based on what the evidence says works for that specific cancer type, not on cost or availability.

MRI is preferred for brain tumours and for detailed soft-tissue assessment — for example, the local extent of a rectal cancer or a sarcoma. Bone scan is used in some prostate cancer assessments. PSMA PET is now preferred over FDG PET for prostate cancer staging and recurrence detection. CT alone is sometimes sufficient for monitoring between treatment cycles. Your oncologist chooses the scan most likely to answer the specific clinical question, not the most sensitive scan in general.