Tumour markers on immunotherapy — why they're less reliable than a scan

Not reliably, and not on their own. Tumour markers such as CEA, CA-125, CA 19-9 or PSA can rise, fall, or stay flat during immunotherapy for reasons that have little to do with whether the cancer itself is shrinking, so a single reading — up or down — should never be read as proof either way. Imaging using standard response criteria remains the primary tool your oncologist uses to judge response.

This page explains why markers behave this way in general — it does not, and cannot, interpret your own report. Whether your specific marker trend means anything is a judgement your treating oncologist makes alongside your scans, your symptoms, and often a repeat test. Nothing here is a substitute for that conversation.

The rest of this page sets out why markers can rise falsely on immunotherapy, how that's told apart from a marker genuinely reflecting the cancer, and what your care team actually leans on to decide whether treatment is working.

These are general tendencies for patient education, not diagnostic rules. Your oncologist weighs all of these together — including imaging — for your specific case; a single row here should never be read as an answer on its own.

Imaging is the primary tool. CT or PET-CT scans are read against standard response criteria — RECIST, or the immune-adapted iRECIST used specifically for checkpoint inhibitor treatment — comparing tumour size and number of lesions across scans taken a set number of weeks apart. Response-assessment PET-CT in this pathway is coordinated at partner imaging centres rather than performed in-house.

Tumour markers, where relevant to your cancer type, are tracked as a supporting trend over several tests rather than a single number, and circulating tumour DNA (ctDNA) is an emerging additional signal some teams now use alongside imaging. Markers are useful context for a conversation with your oncologist — they are not, on their own, what changes a treatment decision.

1. 1

   Correlate with imaging and symptoms

   Your oncologist checks the marker against your most recent scan and how you actually feel — a rising number alongside a stable scan and no new symptoms reads very differently from one alongside new growth.

2. 2

   The trend is checked, not the single value

   A repeat test some weeks later shows whether the rise continues, reverses, or was a one-off. One number, taken alone, is rarely acted on.

3. 3

   Non-cancer causes are ruled out where relevant

   Depending on the marker, your team may consider things like smoking (CEA), a benign ovarian cyst or menstrual cycle (CA-125), mild liver or bile-duct irritation (CA 19-9), or a urinary infection (PSA).

4. 4

   Treatment usually continues, if you're stable

   Unless imaging or your clinical picture also points to progression, the standard approach is to continue immunotherapy on schedule rather than stop on a marker number alone.

5. 5

   The plan is reviewed only when everything aligns

   A change in treatment is considered when the marker trend, imaging, and your clinical picture point the same direction together — not from any one of these alone.

A blood test is easy to get and easy to obsess over — a marker number is available within days, printed in black and white, while the next scan may be weeks away. It is completely understandable that patients on maintenance immunotherapy, feeling well and waiting between scans, fix on that one number as if it were the whole answer.

But a marker is a chemical signal produced, in variable amounts, by a variable proportion of tumours — influenced by inflammation, benign organs, lab variation, and now, on immunotherapy, immune activity itself. A scan directly measures the tumour. That is the practical reason imaging leads response assessment on this treatment, and why a single marker swing — in either direction — is not something to act on alone, however tempting it is to read too much into the one clear number you have between scans.

• How Often Will You Be Scanned on Immunotherapy? — the schedule your marker trend is checked against.
• Understanding Your Response Assessment Report Line by Line — for when your next scan report arrives and reads like a foreign language.
• Circulating Tumour DNA (ctDNA) for Monitoring Response — a newer blood-based signal some teams now use alongside imaging.
• Immunotherapy at CION Cancer Clinics — the full picture of how CION supports patients through response assessment and monitoring.

This page explains general terminology for patient education and does not interpret any individual marker report. It describes typical patterns across groups of patients, not a reading of your own results. Response-assessment PET-CT referenced on this page is coordinated at partner imaging centres.

Many patients have watched a marker number climb and worried

Wanting a clear answer instead of guessing is not overreacting — it's how patients on active treatment stay informed. Our team walks through your specific trend, honestly.