Pseudoprogression on immunotherapy — when a bigger scan doesn't mean treatment failed

Pseudoprogression is an unusual pattern sometimes seen on scans during checkpoint inhibitor immunotherapy, where the tumour appears larger, or new small spots appear, in the weeks or months after starting treatment — yet the cancer is not actually growing. It is thought to happen because immune cells flood into the tumour to attack it, temporarily swelling the area a scan measures, before the tumour itself later shrinks.

This page explains the terminology in general — it does not, and cannot, interpret your own scan report. Whether a specific growth on a specific report is pseudoprogression or true progression is a judgement your treating oncologist makes using your full clinical picture, imaging, and often a follow-up scan. Nothing here is a substitute for that conversation.

The rest of this page sets out how common pseudoprogression actually is, how it's told apart from real progression, and — because families genuinely stop treatment over this — what a reasoned next step looks like if you see growth on your own report.

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

Published trial data cited in NCCN and ASCO patient-education material generally reports pseudoprogression in a small minority of patients on checkpoint inhibitor immunotherapy — roughly in the low single digits to around one in ten, depending on cancer type. It has been documented somewhat more often in melanoma than in cancers such as non-small cell lung cancer.

It is real and recognised, but it remains the less common explanation whenever a scan shows growth on immunotherapy; true progression is still the more frequent finding. That is exactly why the standard response to an ambiguous scan is a confirmation scan, not an assumption in either direction — and why families who stop treatment on the strength of one report alone are, more often than not, stopping on an unconfirmed result.

1. 1

   Clinical check, not scan-only

   Your oncologist checks how you actually feel — new pain, breathlessness, or weight loss point one way; feeling stable or better points another. The scan is never read in isolation.

2. 2

   Treatment usually continues, if you're stable

   Unless there is clear clinical deterioration, the standard approach is to continue immunotherapy on schedule rather than stop on an unconfirmed report.

3. 3

   A confirmation scan is booked, 4–8 weeks out

   Response-assessment PET-CT for this pathway is coordinated at partner imaging centres, not performed in-house. Your team schedules the exact timing for your case.

4. 4

   The confirmation scan settles it

   Shrinkage from the flagged scan supports pseudoprogression; further growth is treated as true progression, and the treatment plan is reviewed accordingly.

5. 5

   A biopsy, occasionally, if it's still unclear

   Where imaging alone doesn't settle it, a biopsy of the growing area can directly check for immune cell infiltration versus active tumour.

Not on the basis of the scan alone, and never as a decision made without your oncologist. This is, honestly, the single most consequential misunderstanding in this cluster: families see a report that says "increased size" or "new lesion" and read it as proof treatment has failed, then stop before the confirmation scan that might have shown the opposite.

If you are clinically stable — feeling reasonably well, without serious new symptoms — the usual approach is to continue treatment and repeat the scan in four to eight weeks. Stopping early on an unconfirmed report can mean abandoning a treatment that was actually starting to work. If a scan report has left you frightened, the right next step is a conversation with your treating oncologist about what it specifically shows for you, not a decision made from the report alone at home.

• When Is the First Scan on Immunotherapy, and Why the Wait? — if you haven't had your first response scan yet, this explains the timing logic behind it.
• 'Stable Disease' on Immunotherapy: Is That Good News? — another scan word that sounds worse than it usually is.
• Feeling Better but the Scan Looks Worse: Which Do You Trust? — the lived, day-to-day version of the mismatch this page explains clinically.
• 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 scan report. Incidence figures are approximate, drawn from published patient-education material, and describe groups of patients — they are not a reading of your own results. Response-assessment PET-CT referenced on this page is coordinated at partner imaging centres.

Many families have faced this exact scan result

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 report, honestly.