Response assessment criteria
RECIST vs PERCIST: — Size vs Metabolic Activity
Your scan report uses one of these scoring systems — or both — to describe how your cancer is responding to treatment. They measure different things, which is why two reports for the same patient can look very different from each other.
Medically reviewed by Dr. C. Raghavendra Reddy, Medical Oncologist, MBBS (Gold Medal) · DNB · DM (Medical Oncology, Gold Medal) · Last reviewed September 2026
- RECIST measures size — It uses CT or MRI measurements to track whether tumours have grown, shrunk, or stayed the same.
- PERCIST measures metabolic activity — It uses PET to track whether tumour tissue is consuming glucose — a marker of how biologically active it is.
- They can point in different directions — A tumour stable in size can become metabolically inactive, or vice versa. That is not a contradiction — it is two different measurements.
- Your oncologist reads both together — Neither score alone decides your treatment. Your team weighs scan results alongside your symptoms and blood tests.
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RECIST measures how much a tumour has changed in size. PERCIST measures whether the tumour is metabolically active on PET. These two criteria use different scan types to answer different questions, which is why one report can say 'stable' while the other says 'responding'. Your oncologist decides which criteria apply to your treatment plan.
What do these terms in my scan report actually mean?
- RECIST
- Response Evaluation Criteria in Solid Tumours. A standard system that uses the longest diameter of selected tumours on CT or MRI to track whether treatment is working.
- PERCIST
- PET Response Criteria in Solid Tumours. A system that uses metabolic activity on PET scan to score treatment response — independently of whether the tumour has changed in size.
- Target lesion
- A specific, measurable tumour chosen at the start of imaging follow-up. RECIST tracks its diameter over time. PERCIST tracks its metabolic activity.
- SUV (standardised uptake value)
- The number describing how much radioactive tracer a tissue absorbs on PET. PERCIST compares your follow-up SUV to your baseline SUV to calculate a response score.
- Complete response (CR) / Complete metabolic response (CMR)
- CR on RECIST: all measurable target lesions have disappeared on imaging. CMR on PERCIST: no detectable metabolic activity above background on PET. These terms are not interchangeable — each belongs to its own scoring system.
- Partial response (PR) / Partial metabolic response (PMR)
- PR on RECIST: tumours have shrunk by a meaningful amount on imaging. PMR on PERCIST: metabolic activity has decreased meaningfully. Each term belongs only to its own criteria.
- Stable disease (SD) / Stable metabolic disease (SMD)
- The cancer has not changed enough to count as a response or as progression under that scoring system.
- Progressive disease (PD) / Progressive metabolic disease (PMD)
- PD on RECIST: tumours have grown or new lesions have appeared on imaging. PMD on PERCIST: metabolic activity has increased significantly.
What should I ask my oncologist when I get a scan report?
- Which scoring system does this report use — RECIST or PERCIST?
- What were my results under the same system at my last scan?
- If I had both CT and PET scans, do the results point in the same direction?
- Which specific lesions are being tracked as target lesions?
- Does this result affect my current treatment plan?
- When is my next scan, and will it use the same criteria?
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Why do RECIST and PERCIST sometimes give different results?
A tumour can shrink in size but remain metabolically active on PET. In that case, RECIST may record a response while PERCIST does not — both are correct, because they are measuring different things.
The reverse also happens. Dead tumour tissue left behind after effective treatment can persist as a visible mass on CT for months. PERCIST will not flag that inactive tissue as a response failure. RECIST still measures the mass.
Neither result is wrong. Your treating team is aware that these two systems can diverge and reads them alongside your full clinical picture.
Which scan score matters more for my treatment?
Which scan does each system actually need?
RECIST uses CT or MRI — standard cross-sectional imaging that shows the physical shape and size of tumours. PERCIST uses PET, which tracks how much radioactive glucose tracer tissue absorbs. If your assessment does not include a PET scan, PERCIST cannot be applied to your results. Which scan is ordered at each time point depends on your cancer type, the treatment you are on, and your centre's protocol.
Can both scoring systems apply to the same patient at the same time?
Yes, and some protocols use both — particularly in lymphoma, where PET is standard, and in some clinical trials. When both are used, each report carries its own set of results. The two sets are read alongside each other, not combined into a single score. Each set of results belongs to its own criteria and is not directly comparable to the other. Your oncologist will clarify which results are guiding your treatment decisions.
What is pseudoprogression, and how does it affect the score?
Pseudoprogression is seen in some patients on immunotherapy. The tumour appears to enlarge on CT — producing a RECIST score of progressive disease — even though the immune system is attacking it. The apparent growth is caused by immune cells flooding into the tumour, not the cancer spreading. PET-based scoring can sometimes help distinguish this from true progression, though it is not always straightforward. Whether pseudoprogression is relevant to your results is a question for your treating oncologist.
What does 'not evaluable' mean on a report?
Not evaluable (NE) means a lesion or result could not be scored under the criteria — commonly because the image quality was insufficient, a lesion was not measurable at baseline, or it fell below the minimum size threshold. It is not the same as normal or unchanged. Your radiologist's report will usually state why. If a lesion your team was tracking closely is listed as not evaluable, ask whether a repeat scan would give a clearer answer.
Is it a problem if the scoring criteria changes between my scans?
It depends on why it changed. If your protocol specifies different criteria at different assessment points, the change is planned. If CT is ordered one time and PET the next, different scoring systems will apply — and the results are not directly comparable to each other. Comparing a RECIST result from one assessment to a PERCIST result from another is not meaningful. Ask your team which criteria apply to each scan and what they are specifically tracking at each time point.
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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
- Bladder, Kidney and Ureter Activity on PET
- Bone Marrow Uptake After Growth Factor Injections
- Bowel Uptake: Normal, Inflammation or Something Else?
- Brown Fat Uptake: A Common Cause of Confusing Findings
- Does a Suspicious PET Finding Always Need a Biopsy?
- Getting a Second Read of Your PET-CT Images
- How Often Is a PET-CT Scan Wrong?
- Muscle Uptake After Exercise or Injections
- Thymic and Rebound Uptake in Younger Patients
- Uptake After Radiotherapy: Inflammation or Disease?
- Uptake at a Biopsy or Surgery Site
- What Causes a False Negative on a PET Scan?
- What Causes a False Positive on a PET Scan?
- What Happens If Two Scans Disagree?
- Which Cancers Do Not Show Well on PET-CT?
- Why the Brain Always Lights Up on PET
Incidental Findings
- A Second Unrelated Cancer Found on PET-CT
- 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
- An Incidental Thyroid Nodule on Your PET Scan
- Do All Incidental Findings Need Investigation?
- Focal Bowel Uptake: Could It Be a Polyp?
- 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
- Can Tuberculosis Look Like Cancer on a PET Scan?
- Dental Infection and Jaw Uptake on PET
- Fungal Infection and Abscess on PET-CT
- How Long Should You Wait After an Infection Before a PET Scan?
- If It Might Be Infection, What Test Comes Next?
- Inflammation After Injections, Vaccines and Cannulas
- Old Healed TB and Calcified Nodes on PET-CT
- PET-CT for Fever of Unknown Origin
- PET-CT for Suspected Infection in Implants and Prostheses
- PET-CT for Vasculitis and Inflammatory Conditions
- Pneumonia and Recent Chest Infection on PET
- Sarcoidosis on PET-CT: The Great Mimic
- TB or Cancer? How Doctors Tell Them Apart
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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?
- TNM Staging on a PET-CT Report
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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?
- Why Blood Sugar Changes Your SUV Result
- Why Did My SUV Go Down but Not to Zero?
- Why Did My SUV Go Up After Treatment?
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Common questions
Frequently asked questions
Does it matter which scoring system my oncologist uses?
Yes, because the two systems measure different things and use different scan types. RECIST answers whether the tumour has changed in size; PERCIST answers whether the tumour is metabolically active. Which system is used depends on your cancer type, the scans ordered for you, and your protocol. Comparing a RECIST result to a PERCIST result is not meaningful — the categories look similar but they measure different phenomena. Ask your oncologist which system applies to your assessment and what it is tracking.
What is a complete response, and does it mean the cancer is gone?
Complete response on RECIST means all measurable target lesions have disappeared on imaging. Complete metabolic response on PERCIST means there is no detectable metabolic activity above background on PET. Both are the best possible result under their respective systems. Neither is a definitive statement about whether cancer cells remain in the body — that is a separate clinical question, and what either result means for your ongoing care is a conversation with your treating oncologist.
Is PERCIST more accurate than RECIST?
Neither is more accurate — they measure different things. PERCIST can detect metabolic changes earlier than size changes appear on CT, which is relevant in cancers such as certain lymphomas. RECIST remains the global standard for most solid tumour trials because CT is more widely available and results are reproducible across centres. NCCN and ESMO guidance specifies which system to use for particular cancer types. Which is right for you depends on your cancer type and treatment protocol.
My report mentions target lesions — does that mean other tumours are not being watched?
Non-target lesions are still followed — just not measured with the same precision. RECIST separates lesions into target lesions, which are measured carefully at each scan, and non-target lesions, which are noted and assessed qualitatively. A new lesion appearing in a non-target area can still trigger a progression score even if target lesions have shrunk. Your radiologist's report will comment on non-target lesions separately, and the full picture includes both.
My report says stable disease — should I be worried?
Stable disease means the cancer has not changed enough to count as a response or as progression under the scoring criteria being used. Whether that is reassuring or concerning depends entirely on the goals of your treatment. In some settings, stable disease is the intended outcome and represents a good result. In others, it may prompt a discussion. The category tells you the measurement — only your oncologist can explain what it means for your individual plan.
What is SUV, and why does my PET report mention it?
SUV — standardised uptake value — describes how much radioactive tracer a tissue absorbs on PET. Active cancer cells tend to absorb more tracer, so a higher SUV generally suggests more metabolically active tissue. PERCIST compares your current SUV to your baseline scan rather than using a fixed threshold, so a single number is not meaningful without that comparison. Inflammation and normal tissue in some organs can also show elevated SUV, which is why your radiologist interprets it in the context of the whole scan.