PET-CT vs Tumour Markers — in Follow-Up After Cancer

For cancers with reliable markers, the blood test often detects recurrence first. A tumour can release marker proteins before it is large enough for any scanner to see. PET-CT is more sensitive than conventional CT for many cancers, but it still depends on lesions reaching a detectable size. The honest answer is that it varies by cancer type — in some, the marker leads by months; in others, PET-CT finds disease the marker never showed. Your oncologist's choice of follow-up reflects which tool gives the earliest signal for your specific cancer.

A normal marker is reassuring, but it is not a guarantee. Some recurrences happen in cancers with low or no marker expression. Some tumours recur but stop producing the marker they originally made. And some metastatic sites do not shed marker proteins into the blood as reliably as the original tumour did. A normal marker alongside normal imaging is the most reassuring combination. A normal marker with new symptoms or clinical concern still warrants assessment — do not delay reporting symptoms because a blood test looks fine.

Not necessarily. A single raised result is less significant than a clear upward trend across several tests. Your oncologist will usually look at the speed of the rise, your cancer type, and how high the result is before deciding whether imaging is needed and which scan is most appropriate. PET-CT is one option, but the right imaging depends on your cancer — for some types, a standard CT or MRI gives more useful information. If you are worried, call your team and ask what the next step is, rather than assuming a scan is automatically required.

Yes. PET-CT using FDG — the standard tracer — works by detecting cells consuming glucose faster than normal. Some cancers are not reliably FDG-avid, meaning they do not show up well on this scan even when active. These include some prostate cancers (where PSMA-PET is often more useful), mucinous bowel cancers, low-grade lymphomas, some thyroid cancers, and certain slow-growing tumours. If PET-CT has been recommended for you, it means your team believes your cancer type is one where the scan is likely to be informative. If you are unsure, ask why that scan specifically was chosen.

Yes, and this happens in both directions — a rising marker with nothing on PET, and something on PET with a normal marker. Some deposits are metabolically active enough to show on a scan but do not shed marker proteins into the blood in detectable amounts. This is one reason markers and imaging are complementary rather than interchangeable. It also explains why clinical assessment — what you are actually experiencing as symptoms — remains part of the picture regardless of what the blood tests and scans show.

Your follow-up schedule is built from guidelines published by bodies including NCCN, ASCO, and ESMO, then adapted to your specific situation — your stage, the treatment you had, and your oncologist's assessment of your recurrence risk. Ask your team to explain the plan in writing: which tests, how often, and what result would change the approach. If you have been discharged to a GP for follow-up, you are entitled to know what to watch for and at what point to come back to the specialist.