Response assessment
Tumour Flare and Pseudoprogression — on Your PET Scan
A PET scan after immunotherapy can show new brightness or apparent growth — and still be good news. Pseudoprogression is a recognised phenomenon where the scan appears to worsen before a real response to treatment becomes visible.
Medically reviewed by Dr. Bharati Devi Gorantla, Medical Oncologist, MBBS · MD · DM (Adyar, Chennai) · ECMO · MRCP SCE (UK) · Last reviewed September 2026
- Not always what it looks like — A more active-looking scan after immunotherapy does not automatically mean the cancer is growing.
- Immune cells light up too — Immune cells attacking the tumour absorb the PET tracer, making the area appear brighter or larger on the scan.
- A repeat scan usually clarifies — A confirmatory scan a few weeks later shows whether the change was real progression or a temporary flare.
- Your oncologist reads the whole picture — Symptoms, clinical markers and how you feel all contribute — the scan result alone does not decide the next step.
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Pseudoprogression, also called tumour flare, is when a PET scan shows increased tumour activity or apparent new growth after treatment — but the cancer is actually responding. It happens most often after immunotherapy. Oncologists distinguish it from real progression by repeating the scan after a few weeks and reviewing the full clinical picture.
What do these terms mean?
- Pseudoprogression
- An apparent increase in tumour size or metabolic activity on a scan, occurring in the early weeks of treatment, that turns out not to be real growth. The prefix pseudo means false — the scan looks like progression, but is not.
- Tumour flare (flare reaction)
- The same phenomenon from a different angle: the tumour site briefly appears more active or inflamed before the response to treatment becomes visible. Some teams use flare and pseudoprogression interchangeably; others use flare specifically for swelling or tenderness you can feel at the tumour site.
- SUV (Standardised Uptake Value)
- The number on a PET report measuring how much radioactive tracer a tissue absorbed. A higher SUV means more metabolic activity. After immunotherapy, a rising SUV can reflect immune-cell activity at the tumour site rather than tumour growth — which is why no single number tells the whole story.
- iRECIST criteria
- A response-assessment framework from the RECIST Working Group, designed specifically for immunotherapy. Unlike standard RECIST 1.1, it allows an apparent worsening to be called unconfirmed progression rather than definitive progression — keeping the door open for a confirmatory scan before any treatment decision is made.
- Confirmatory scan
- A repeat imaging study, usually scheduled several weeks after an ambiguous result, to determine whether the apparent change was pseudoprogression or true progression. Under iRECIST, confirmed progression requires two consecutive scans showing worsening — not one.
Why does a scan show growth when the cancer may be responding?
Immunotherapy activates your immune system to recognise and attack cancer cells. When this begins to work, immune cells — particularly T-cells — travel to the tumour site in large numbers.
Those immune cells are metabolically active. They absorb the radioactive glucose tracer used in a PET-CT, just as cancer cells do. A tumour site flooded with attacking immune cells can look brighter, larger, or more active on the scan — even as the cancer is beginning to respond.
On a repeat scan several weeks later, the immune activity typically settles and the underlying tumour response becomes visible. That repeat scan is what separates pseudoprogression from real progression.
Standard response criteria were not designed for this pattern, which is why the RECIST Working Group developed iRECIST specifically for immunotherapy assessment.
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Pseudoprogression or true progression — what is the difference?
| Feature | Pseudoprogression (flare) | True progression |
|---|---|---|
| Timing after immunotherapy | Typically first weeks to months | Can occur at any point |
| Trend on repeat scan | Improves or stabilises on follow-up | Continues to worsen on follow-up |
| New or worsening symptoms | Usually absent — feeling same or better | Often present alongside scan change |
| Clinical markers | Stable or improving | Worsening, consistent with scan |
| Under iRECIST, classified as | Unconfirmed progression — confirmatory scan needed | Confirmed progression after two worsening scans |
Does a worsening scan always mean the treatment has stopped working?
Does pseudoprogression mean my immunotherapy is working?
Pseudoprogression suggests the immune system has been activated and is reaching the tumour site, which is the mechanism immunotherapy depends on. But not every flare leads to a lasting response, and not every response comes with a visible flare — many patients respond well without any pseudoprogression on their scans. Your oncologist will follow the pattern across multiple scans and your full clinical picture before drawing any conclusion about whether the treatment is working for you.
Will my doctor stop immunotherapy because of one concerning scan?
Under iRECIST guidance, an apparent worsening on a single scan is not by itself sufficient reason to stop immunotherapy if you are clinically stable — meaning your symptoms are unchanged or improving. Your oncologist will typically arrange a confirmatory scan before making any treatment change. The decision to continue, pause or stop belongs entirely to your treating team based on the complete clinical picture. If your symptoms are clearly worsening at the same time as the scan, tell your team promptly — that changes the assessment.
What does iRECIST or unconfirmed progression mean on my report?
Seeing iRECIST on your scan report means the radiologist used the framework designed for immunotherapy response assessment. Unconfirmed progression is a holding category — the radiologist has noted an apparent change and flagged it for follow-up, not declared that your treatment has failed. It is a prompt for a confirmatory scan, not a final verdict. Your oncologist is the right person to explain what the specific finding means for your situation and what the next step will be.
Can pseudoprogression happen with chemotherapy or targeted therapy?
Pseudoprogression as a defined pattern is primarily associated with immunotherapy — particularly checkpoint inhibitors — because the mechanism depends on immune-cell infiltration at the tumour site. With chemotherapy and most targeted therapies, this specific pattern is not typically observed. Your oncologist will interpret your scan in the context of the treatment you are receiving, which determines how any apparent change is assessed.
How long does pseudoprogression last before it resolves?
There is no single timeframe that applies to all patients. Published literature on checkpoint inhibitors has observed pseudoprogression resolving over weeks to a few months, but the range varies considerably. We do not yet have reliable tools to predict in advance how long a flare will last for any particular patient. What matters practically is the interval your team sets for the confirmatory scan and what that follow-up scan shows.
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Explore 94 more Understanding Your PET-CT Report topics
Response Assessment Scores & Criteria
- Complete Metabolic Response: What It Means
- Deauville Score 1 to 5: The Lymphoma Scale Explained
- End-of-Treatment PET Scan: What It Decides
- How Doctors Judge Whether Treatment Is Working on PET-CT
- Interim PET Scan During Chemotherapy: Why It Is Done
- Metabolic Response vs Size Response: Why They Differ
- PERCIST Criteria: How Metabolic Response Is Measured
- Partial Metabolic Response: Is That Good Enough?
- Progressive Metabolic Disease: What Happens Next
- RECIST vs PERCIST: Size vs Activity
- Stable Metabolic Disease: Good News or Bad?
- Tumour Flare and Pseudoprogression on PET
- What Is a Deauville 3 and Why Is It Ambiguous?
- Why the Scan Says One Thing and You Feel Another
False Positives, False Negatives & Pitfalls
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- Bone Marrow Uptake After Growth Factor Injections
- Bowel Uptake: Normal, Inflammation or Something Else?
- Brown Fat Uptake: A Common Cause of Confusing Findings
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- How Often Is a PET-CT Scan Wrong?
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- Thymic and Rebound Uptake in Younger Patients
- Uptake After Radiotherapy: Inflammation or Disease?
- Uptake at a Biopsy or Surgery Site
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- What Causes a False Positive on a PET Scan?
- What Happens If Two Scans Disagree?
- Which Cancers Do Not Show Well on PET-CT?
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Incidental Findings
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- An Incidental Bone Finding on PET-CT
- An Incidental Breast Finding on PET-CT
- An Incidental Kidney or Adrenal Finding
- An Incidental Liver Lesion on PET-CT
- An Incidental Lung Nodule Found on PET-CT
- An Incidental Prostate Finding on PET-CT
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- Do All Incidental Findings Need Investigation?
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- Incidental Findings on a PET-CT Scan: What They Are
- Incidental Heart Findings on a Cancer PET Scan
- The Cost of Chasing an Incidental Finding
- What to Ask Your Doctor About an Incidental Finding
Infection, TB & Inflammation on PET
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- Fungal Infection and Abscess on PET-CT
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- If It Might Be Infection, What Test Comes Next?
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- Old Healed TB and Calcified Nodes on PET-CT
- PET-CT for Fever of Unknown Origin
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Reading Your Report: Core Terms
- 'Cannot Be Excluded' and Other Cautious Phrases
- 'Deauville Score' on a Lymphoma PET Report
- 'FDG Avid': What It Means on Your Report
- 'Hypermetabolic': What the Word Actually Means
- 'Hypometabolic' and 'Photopenic' Findings
- 'Impression' vs 'Findings': Which Section Should You Read?
- 'Interval Change' and 'Stable Since Previous Study'
- 'Krenning Score' on a DOTANOC or DOTATATE Report
- 'Lymphadenopathy' on Your PET Report
- 'Metabolically Active Disease': What It Tells You
- 'Metastasis' and 'Distant Spread' on Your Report
- 'Mildly FDG Avid' vs 'Intensely FDG Avid'
- 'No Abnormal FDG Uptake': Is That a Clear Scan?
- 'Physiological Uptake': Normal Areas That Always Light Up
- 'Residual Disease' vs 'Complete Metabolic Response'
- 'Sub-Centimetre Node' and Why Size Matters on PET
- 'Suspicious for Malignancy': How Certain Is That?
- How Long Does a PET-CT Report Take?
- How to Read Your PET-CT Report: A Section-by-Section Guide
- Should You Read Your Own PET-CT Report Before the Consult?
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SUV Values Explained
- Comparing SUV Between Two Scans: Is It Meaningful?
- Does a Higher SUV Mean a More Aggressive Cancer?
- Factors That Change Your SUV Reading
- High SUV but Not Cancer: When the Number Misleads
- Is My SUVmax of 4 High? Reading Your Specific Number
- Low SUV but Still Cancer: The PET-Negative Cancers
- SUV in Non-Cancer Conditions
- SUVmax, SUVmean and SUVpeak: What Is the Difference?
- What Is SUV on a PET Scan?
- What Is a Normal SUV Value?
- What SUV Value Indicates Cancer?
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Common questions
Frequently asked questions
What is the difference between pseudoprogression and true progression on a PET scan?
Pseudoprogression is a temporary increase in scan activity caused by immune cells at the tumour site — the cancer may be responding even as the scan looks worse. True progression means the cancer is genuinely growing. The difference becomes clear on a repeat scan: pseudoprogression improves or stabilises, while true progression continues to worsen. Your oncologist uses the pattern across multiple scans and your clinical picture, not a single result, to tell them apart.
Should I be asking for a repeat scan after a worrying result?
If your scan shows apparent worsening and you are on immunotherapy, a confirmatory repeat scan is already standard practice under iRECIST — your team will typically arrange it without you needing to ask. What is worth asking: when is it scheduled, what are they watching for in the interval, and which symptoms should prompt you to call before that appointment. If no follow-up has been mentioned after a concerning immunotherapy scan, raise it directly with your oncologist.
Can I feel pseudoprogression, or does it only show on the scan?
Sometimes you can feel it. A tumour site can become temporarily tender, swollen or sore as immune cells converge on it — particularly in lymph nodes close to the skin surface. In many cases pseudoprogression shows only on the scan, with no symptoms at all. If you notice new pain or swelling around a known tumour site after starting immunotherapy, tell your team. They will assess whether it is a flare, a different immune reaction, or something else.
Does every patient on immunotherapy go through a flare phase?
No. Pseudoprogression is a recognised phenomenon but does not happen to all patients, and it is not required for treatment to be working. Many patients who respond well have scans that improve steadily from the start with no flare phase. Frequency varies by cancer type and the specific treatment used. Your oncologist will know whether pseudoprogression has been observed in patients with your diagnosis and can put your own result in that context.
What if my symptoms are getting worse at the same time the scan looks worse?
Worsening symptoms alongside a worsening scan make pseudoprogression a less likely explanation and true progression a more urgent concern. In that situation, waiting for a confirmatory scan may not be appropriate, and your oncologist may need to act sooner. Do not wait for your next scheduled appointment if you feel notably worse after a concerning result. The scan and your clinical picture are always read together — contact your team promptly.
What should I ask my oncologist after getting a report like this?
Ask three things: whether your result is being assessed under iRECIST criteria, when the confirmatory scan is scheduled and what it will look for, and which symptoms should prompt you to call before that appointment. Also ask whether your blood tests or tumour markers are moving in a direction that helps clarify the picture. Write the answers down — these conversations are difficult to retain afterwards, and having them in writing helps the rest of your family understand too.