Medical Oncologist
Reading your scan results
Tumour Flare and Pseudoprogression: — When Growth Isn't Really Growth
A scan showing a larger tumour is frightening. But in some treatments, apparent growth is part of how the treatment works — not a sign it has failed. Understanding which situation you are in is what the follow-up plan is for.
Medically reviewed by Dr. T. Raghavender Reddy, Medical Oncologist, MBBS · DM (Medical Oncology) · MD (Radiation Oncology) · Last reviewed August 2026
- Not always real growth — Pseudoprogression and tumour flare both mimic progression on scans or in symptoms — neither means the cancer is winning.
- Treatment-specific — These patterns occur with immunotherapy and certain hormonal treatments, not with all cancer therapies.
- Your doctor expects this — Both are recognised phenomena with established criteria for managing them.
- A repeat scan decides — A follow-up scan at a defined interval is usually how apparent growth is confirmed or ruled out as real progression.
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Pseudoprogression and tumour flare both look like growth on a scan but are not real cancer progression. Pseudoprogression happens in immunotherapy when immune cells flood the tumour, making it appear larger before it shrinks. Tumour flare happens when certain hormonal treatments briefly stimulate the cancer before suppressing it. Both are usually temporary with continued treatment.
What is the difference between pseudoprogression and tumour flare?
| Feature | Pseudoprogression | Tumour flare |
|---|---|---|
| What is happening | Immune cells infiltrate and inflame the tumour, making it look larger on imaging — not new cancer growth | The treatment briefly activates hormone-sensitive cancer before suppression takes effect |
| Which treatments | Immunotherapy — checkpoint inhibitors; also seen after radiation to the brain | Hormonal therapies — LHRH agonists in prostate cancer; sometimes tamoxifen when there are bone metastases |
| When it appears | Typically within the first few months of starting immunotherapy | Usually within the first few weeks of starting the hormonal treatment |
| What the scan shows | Tumour appears larger or new spots appear; subsequent scans show shrinkage or stability | Lesions appear more active; bone pain or other symptoms may temporarily worsen |
| How you may feel | Often no worse than before — feeling well alongside apparent scan growth is a reassuring sign | You may notice more pain or worsening symptoms before they begin to settle |
| What happens next | A repeat scan at the interval your oncologist sets confirms whether the apparent growth was real | Symptoms and imaging usually settle as the treatment establishes hormonal suppression |
What do these terms on my scan report actually mean?
- Pseudoprogression
- When a tumour looks larger on imaging during immunotherapy — not because cancer cells are multiplying, but because immune cells have moved into the tumour and are attacking it. A follow-up scan typically shows the tumour stable or smaller.
- Tumour flare
- A temporary worsening in scans or symptoms caused by the treatment briefly stimulating the cancer before it establishes suppression. It resolves as treatment takes full effect.
- True progression
- When cancer cells are genuinely multiplying and the tumour is growing. Unlike pseudoprogression and tumour flare, true progression does not reverse on a repeat scan.
- iRECIST
- A revised set of imaging criteria developed specifically to account for atypical immunotherapy responses including pseudoprogression. Under iRECIST, apparent growth on one scan is called unconfirmed progression — a second scan is needed before any treatment change is considered.
- Unconfirmed versus confirmed progression
- An iRECIST term. Growth seen on one scan is unconfirmed — the follow-up scan settles the question. Only when a second scan at a defined interval still shows growth is progression confirmed and a treatment change considered.
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How does your doctor know it is not real progression?
The clearest early sign is how you feel. Pseudoprogression typically occurs while you still feel as well as before, or even better — real progression usually comes with symptoms that are getting worse.
Timing is also informative. Both pseudoprogression and tumour flare follow a recognisable pattern in relation to when treatment started. Growth that appears much later, after a period of confirmed response, is more likely to be true progression.
A repeat scan at a defined interval is usually how the question is settled. Under iRECIST — the criteria ESMO and ASCO recommend for immunotherapy response assessment — growth on one scan does not confirm progression. A second scan is needed before treatment is changed.
What happens to your treatment while the scan is being repeated?
In most cases, treatment continues unchanged while the follow-up scan is arranged. The iRECIST framework was designed specifically to prevent treatment being stopped too early on the basis of one discouraging image.
Ask your oncologist for the date of your next scan and what they are looking for on it. A clear timeline — not 'we will see how it goes' — makes the waiting much easier to carry.
Tell your team promptly if you develop new pain, breathlessness, or significant fatigue during this period. Worsening symptoms shift the clinical picture and may change what is decided, so your team needs to know straight away.
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Common questions
Frequently asked questions
Does pseudoprogression mean the immunotherapy is working?
Not necessarily, and it is important not to read too much into it. Pseudoprogression indicates that immune cells have reached and are active in the tumour — which is a biological response — but it does not guarantee that response will be durable or that the treatment will ultimately control the cancer. Some apparent cases of pseudoprogression turn out to be true progression when the follow-up scan is done. The follow-up scan is what settles the question, and your oncologist will tell you what they see on it.
What if the scan shows new spots as well as growth in existing tumours?
New spots on a scan during immunotherapy can be part of pseudoprogression, particularly in the first few months of treatment. Under iRECIST, new lesions alone do not confirm that treatment has failed — they are recorded as unconfirmed progression, and a follow-up scan at a set interval is the next step. Your oncologist looks at the pattern, your symptoms, and your overall clinical picture together before drawing any conclusion. A single scan with new spots is a data point, not a verdict.
How long does tumour flare last?
Tumour flare from hormonal therapy typically settles within a few weeks as suppression is established, though the timing varies between patients and between specific treatments. Your oncologist will tell you the window they expect for your treatment. If your symptoms are severe during this period, or are continuing beyond the timeframe your team described, tell them — that changes the clinical picture and may affect what is decided.
Should I keep taking my treatment if the scan looks worse?
In most cases, yes — if the apparent worsening fits the pattern of pseudoprogression or tumour flare and your symptoms are not getting worse. This is a decision your oncologist makes based on the full picture: the scan, how you feel, the timing, and any blood markers. Do not stop or pause treatment on your own without speaking to your team first. The iRECIST framework exists precisely because stopping at the first sign of apparent growth on one scan is often the wrong decision.
What is the difference between pseudoprogression and hyperprogression?
They are opposites. Pseudoprogression is apparent growth that is actually immune activity and resolves on the follow-up scan. Hyperprogression is genuine and rapid acceleration of tumour growth that occurs in a small number of patients receiving immunotherapy — the cancer grows faster than it was before treatment started. Hyperprogression is typically accompanied by a clear worsening in how the patient feels, which is the clinical signal that separates it from pseudoprogression. We do not yet fully understand which patients are at risk; it remains an area of active research.
How common is pseudoprogression?
It is seen in a minority of patients receiving checkpoint inhibitor immunotherapy, and published figures vary considerably by cancer type — ASCO and ESMO both note that rates differ across tumour types. Because pseudoprogression looks identical to true progression on a single scan, the challenge is less how often it occurs and more how reliably it is identified — which is exactly what the follow-up scan protocol addresses. If your oncologist has raised pseudoprogression as a possibility, they are already working within the framework designed to tell the two apart.