PET vs CT response
Tumour Size Unchanged but — SUV Is Falling
Your PET and CT scans measure completely different things, so they can point in different directions. A falling SUV when the tumour has not yet shrunk is a recognised pattern — and understanding the difference helps you ask better questions at your next appointment.
Medically reviewed by Dr. Bharati Devi Gorantla, Medical Oncologist, MBBS · MD · DM (Adyar, Chennai) · ECMO · MRCP SCE (UK) · Last reviewed September 2026
- Two different measurements — PET measures how metabolically active cancer cells are; CT measures how physically large the tumour mass is.
- They do not have to move together — Metabolic activity can fall weeks before any visible shrinkage appears on CT — because dead cells still take up physical space.
- A falling SUV is meaningful — Reduced SUV is generally read as a sign that treatment is affecting the tumour, even when the mass looks unchanged on CT.
- Your oncologist interprets the result — What a metabolic response means for your plan depends on the full scan, your cancer type, and the criteria your team is using.
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Yes, a tumour's metabolic activity can fall significantly while its physical size stays the same. PET detects this metabolic response earlier than CT shows any shrinkage. A lower SUV alongside an unchanged tumour size is generally a sign that treatment is affecting the tumour. Your oncologist will tell you what it means for your specific plan.
Can a tumour's metabolic activity fall while its size stays the same?
Yes. PET and CT measure completely different things, and they do not have to move in step.
CT shows physical dimensions — the size and shape of the mass. PET shows metabolic activity — how actively cells inside that mass are consuming glucose. When treatment kills cancer cells, those cells stop consuming glucose almost immediately. But the mass of dead and dying cells, together with any scar tissue forming around them, continues to occupy space on CT for weeks or months.
This gap between metabolic and physical change is one of the main reasons PET is used alongside CT for certain cancers. Metabolic response can appear on a PET scan before any size reduction is visible.
How do PET and CT measure treatment response differently?
| PET scan | CT scan | |
|---|---|---|
| What it measures | Metabolic activity — how actively cells consume glucose | Physical dimensions — tumour size and shape |
| What response looks like | A fall in SUV (standardised uptake value) | A reduction in the longest tumour diameter |
| How quickly changes appear | Within days to weeks of effective treatment | Weeks to months after treatment begins |
| Response criteria used | PERCIST; Deauville score for lymphoma | RECIST 1.1 |
| Main limitation | Cannot replace CT for precise anatomical mapping | Cannot detect changes in cellular metabolic activity |
What do the terms on your PET report mean?
- SUV (Standardised Uptake Value)
- A number that reflects how much radiotracer a tissue has absorbed. Higher values indicate more metabolic activity. A lower value between scans indicates less activity in that area.
- SUVmax
- The single highest uptake value recorded within the tumour on that scan. Most reports track this figure when comparing scans over time.
- Metabolic response
- A meaningful fall in SUV between two scans, indicating that cancer cells in that area have become less metabolically active.
- Morphological response
- A reduction in tumour size measured on CT or MRI, reflecting physical shrinkage of the mass rather than changes in cell activity.
- PERCIST
- A set of standardised criteria used to define and grade metabolic response on PET scans, analogous to how RECIST defines size response on CT.
- RECIST 1.1
- The standard system for measuring tumour size changes on CT or MRI. Used widely across cancer types to define stable disease, partial response, complete response, and progression.
- CMR (Complete Metabolic Response)
- No detectable abnormal FDG uptake in the area that was previously involved. The tumour site looks metabolically normal on PET.
- PMR (Partial Metabolic Response)
- A significant reduction in SUV that does not reach the threshold for complete metabolic response — an improvement, but not full clearance on PET.
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What should I ask at my scan review appointment?
- Ask which scan — PET or CT — is the primary measure of response for your cancer type.
- Ask how your SUVmax has changed compared with your previous scan.
- Ask whether the response criteria being used are PERCIST, Deauville, or RECIST — so you know what the threshold is.
- Ask what the next decision point is: which scan will determine whether treatment continues.
- Bring both the PET and CT reports to the appointment, not only the summary letter.
- If the words 'stable' or 'no change' appear, ask whether that refers to size, metabolic activity, or both.
Does a falling SUV mean the treatment is working?
A falling SUV is generally read as a sign that cancer cells in that area are becoming less metabolically active — which is what treatment aims to achieve.
Whether it counts as a meaningful response, and what it means for your specific plan, depends on how much the SUV has fallen, the criteria your team is using, your cancer type, and what the rest of the scan shows.
A metabolic response in one area alongside new activity elsewhere is a different clinical situation from a consistent metabolic response throughout. Your oncologist will interpret your specific result in that full context.
Questions families ask about metabolic and size response
Why is the tumour not shrinking if the treatment is working?
Dead and dying cancer cells still occupy physical space, and so does the scar tissue that forms around them. The mass does not disappear the moment cells stop functioning. In many cancers, meaningful size reduction on CT takes weeks to months after the metabolic response is already visible on PET. This is a recognised and expected pattern — it is one of the main reasons PET and CT are read together rather than in isolation.
Which number matters more — the SUV or the size?
It depends on the cancer type and the criteria your oncologist is using. For lymphoma, metabolic response assessed on the Deauville scale is typically the primary measure of how well treatment is working. For other cancers, RECIST size criteria may be more central. In many situations both are considered together, and neither alone tells the whole story. Ask your oncologist which measurement your team is using as the main decision point, and why.
The report says 'metabolic response' but the mass looks the same — is that bad news?
A metabolic response with an unchanged or only slightly smaller mass is a recognised and often favourable finding. It most commonly means the cells inside the mass are no longer active, while the structural remnant of dead cells and scar tissue has not yet resolved on CT. Your oncologist will interpret this in the context of your cancer type, the treatment you have received, and what the full scan shows — not from the size measurement alone.
What if the SUV stays high but the tumour shrinks in size?
A tumour that reduces in size on CT while retaining high metabolic activity on PET may contain a proportion of still-active cancer cells. This is one of the situations where reading both scans together matters: size alone can overstate the response, and residual metabolic activity after treatment has implications for what happens next in some cancers. Your oncologist will weigh both measurements in reaching a conclusion about your plan.
The report mentions a Deauville score. What does that mean?
The Deauville score is a five-point scale used mainly in lymphoma to compare the remaining FDG uptake in a tumour with the uptake in normal reference tissue. A lower score indicates less residual metabolic activity. The exact thresholds and what each score means for your treatment plan are interpreted by your oncologist alongside your full clinical picture — the number alone does not determine the next step.
Can the PET show response while the CT shows the disease progressing?
Yes, though less commonly than the reverse. It is one of the patterns that makes reading both scans together important — either measurement alone can give an incomplete or misleading picture. If your PET and CT reports appear to conflict, that is precisely the kind of finding your oncologist is trained to interpret. Do not draw a conclusion from one scan or one number without the full clinical context your team holds.
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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
- Thyroid Uptake on PET: Diffuse vs Focal
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
- Why Your Report Mentions the Liver and Blood Pool
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
What does it mean if my SUV has dropped but the tumour is the same size on CT?
It means the two scans are measuring different things and have moved at different rates, which is a recognised pattern rather than a contradiction. PET measures metabolic activity — how active the cancer cells are — and CT measures physical size. When treatment works, metabolic activity often falls before the mass physically shrinks, because dead cells still occupy space. Whether your specific result represents a good response is a clinical judgement your oncologist will make from the full scan and your overall situation.
Which is more reliable for showing treatment response — PET or CT?
Neither is universally more reliable — they answer different questions. PET shows what the cells are doing; CT shows how large the mass is. For some cancers and treatment types, metabolic response on PET is the better predictor of long-term outcome. For others, size response on CT is the standard measure. In many situations, both are used together. Ask your oncologist which measure is the primary response criterion for your specific cancer and treatment, because the answer varies significantly.
How much does the SUV need to fall to count as a response?
The threshold depends on which criteria are being applied, and those differ by cancer type. Under PERCIST, a commonly referenced system, a specified percentage fall in SUV in the most active lesion defines a partial metabolic response, and no detectable uptake defines a complete response. For lymphoma, the Deauville scale uses a different approach based on comparison to reference tissue rather than a percentage fall. NCCN, ASCO, and ESMO all reference these criteria in their guidance. Your team will tell you which apply to your situation.
Can I have a complete metabolic response on PET but still have cancer?
Yes. A complete metabolic response means no abnormal FDG uptake is detectable at the sensitivity of the scan — it does not mean every cancer cell has been eliminated. Some cancers can persist at levels below PET's detection threshold. A CMR is an encouraging finding, but what it means for your ongoing management — including how often you are scanned and whether further treatment is needed — is a clinical decision. We do not yet know that a CMR equals cure across all cancer types.
Why does my report use different words for response on PET versus CT?
Because the two scans use entirely separate response criteria developed for each modality. CT response is described in RECIST language — stable disease, partial response, complete response, progressive disease — based on measurements in millimetres. PET response is described in PERCIST or Deauville language based on changes in SUV or comparison to reference tissue. The same underlying situation can generate different labels from each system, which is why your oncologist reads both reports together rather than treating either in isolation.
Should we be worried if only one of the two scans shows a response?
Partial concordance between PET and CT is common and does not automatically mean something has gone wrong. Metabolic response on PET frequently precedes size response on CT, so a PET response without a CT response can simply mean one measurement is ahead of the other. The reverse — CT shrinkage with persistent high SUV — has different implications and is worth asking your oncologist about specifically. In both cases, the interpretation belongs with your treating team, who can weigh both findings against your full clinical picture.