Compare like with like — same scanner, same uptake time, same criteria — and know which scoring system the tumour type uses.
Orient first
- RESPONSE IS A COMPARISON, so the validity of the report depends on the comparability of the two studies. SUV is exquisitely sensitive to uptake time, blood glucose, scanner and reconstruction; comparing across scanners or uptake times produces changes that are technical, not biological.
- The criteria are TUMOUR-SPECIFIC. Lymphoma uses the 5-point Deauville scale against mediastinal blood pool and liver; solid tumours generally use size-based criteria with metabolic supplements. Applying the wrong system produces a confidently wrong category.
- On the Deauville scale, 1–3 is generally a complete metabolic response and 4–5 is residual disease — the reference points being MEDIASTINAL BLOOD POOL and LIVER, not an absolute number.
- TIMING drives false positives: post-chemotherapy inflammation, post-radiotherapy change, colony-stimulating factor effect on marrow, and post-surgical healing all take up avidly. Scanning too early manufactures disease.
- FLARE and PSEUDOPROGRESSION are recognised, particularly with immunotherapy, and have their own response criteria.
Acquire the study
- Confirm the patient was fasted, the blood glucose was within protocol, and the UPTAKE TIME matches the baseline study — record all three.
- Confirm the same scanner and reconstruction as the baseline where possible; state it when they differ, because the comparison is then qualified.
- Confirm the interval from the last treatment is adequate for the modality and the agent.
- Review the CT component in its own right — it carries findings the PET does not.
The manoeuvre
- State the indication, the treatment given, and the interval since the last cycle.
- Confirm and record uptake time and blood glucose, and compare them with the baseline study.
- Identify the reference regions: mediastinal blood pool and liver.
- Score each site of previously known disease against the appropriate system and its reference regions.
- Measure SUVmax at each site the same way as at baseline, and note the anatomical location so the same lesion is being compared.
- Look for NEW sites of uptake, which outrank any improvement elsewhere.
- Interpret marrow and splenic uptake against recent growth-factor use.
- Review the CT for non-avid disease, complications and incidental findings.
- Give an overall response category, naming the criteria and the version.
What confirms it
- A complete metabolic response is uptake at or below the defined reference region on a comparable study at an adequate interval.
What licenses you to exclude it
- ⚠️ PET IS NOT A BIOPSY. Residual uptake is not proof of viable tumour, and absent uptake is not proof of its absence — small-volume, mucinous and some low-grade tumours are poorly avid.
- A scan performed too soon after treatment cannot exclude residual disease and should be reported as too early rather than as a response.
- A non-comparable study (different uptake time, glucose out of range, different scanner) qualifies every conclusion — say so explicitly.
The classic misread
- Comparing SUV across different uptake times or scanners.
- Applying solid-tumour criteria to lymphoma or vice versa.
- Calling post-treatment inflammation residual disease.
- Reporting the PET and ignoring the CT component.