Understanding your response assessment report — line by line

A response assessment report from a CT, PET-CT or MRI scan generally follows five sections: patient and exam details, the imaging technique and comparison scan used, a target lesion measurements table, an assessment of non-target and any new lesions, and an overall impression naming the response category. Knowing this order helps you find the section that actually answers "is it working" — the impression — instead of getting lost in individual lesion measurements first.

• Patient & exam details — the scan type, date, and which prior scan it's being compared against.
• Technique & comparison — how the images were taken and which earlier report is the baseline or reference for this one.
• Target lesion measurements — a table re-measuring the same handful of tumour deposits chosen at your first scan.
• Non-target & new-lesion assessment — a note on other known disease and whether anything genuinely new has appeared.
• Overall impression — the line naming the response category: Complete, Partial, Stable, or Progressive Disease.

This page explains the general layout and terminology of a response assessment report — it does not interpret your specific report. Response-assessment PET-CT for CION patients is coordinated at partner imaging centres, not performed in-house.

Categories reflect the RECIST 1.1 criteria (Eisenhauer et al., 2009) and the iRECIST adaptation for immunotherapy (Seymour et al., RECIST working group, 2017), as commonly referenced in NCCN and ASCO patient-education materials, indicative as of August 2026. Your oncologist confirms which framework and comparison scan applies to your specific report; this table is general education, not an interpretation of any individual report.

1. 1

   Baseline selection

   At your very first scan, the radiologist picks up to five measurable lesions — no more than two per organ — to serve as targets for every future comparison.

2. 2

   Longest diameter recorded

   For each target lesion, the single longest diameter is measured (lymph nodes instead use their short axis) and logged against the date of that scan.

3. 3

   Sum of diameters calculated

   All target lesion measurements from that scan are added together into a single "sum of diameters" figure — the number that actually drives the response category.

4. 4

   Every follow-up compares to this sum

   The percentage change against your baseline sum — or against your smallest-ever sum since treatment started, called the "nadir" — is what determines Complete, Partial, Stable or Progressive Disease on the table above. Non-target and new-lesion findings are noted separately and can still affect the overall impression even when target measurements alone look stable.

Immunotherapy works by activating immune cells, and immune cells infiltrating a tumour can temporarily make it look larger — or make a new spot visible — on a scan, a pattern called pseudoprogression. Under iRECIST, this kind of finding is labelled "unconfirmed progressive disease" rather than true progression.

A repeat scan four to eight weeks later then confirms whether the growth continued (true progression) or reverses (pseudoprogression). It's an uncommon pattern, not the typical course, but it's genuinely described in the immunotherapy literature — which is exactly why an "unconfirmed" label exists in the first place, instead of every early increase being called progression outright. This is why a single scan is rarely read in isolation from your prior scans and how you're doing clinically.

• Which lesions were chosen as targets, and why — so you know exactly what's being tracked scan to scan.
• What the overall response category is — and whether it's compared against your very first baseline scan or your best-ever scan since (the nadir).
• Whether any new finding needs a repeat scan — before it's labelled true progression rather than an unconfirmed finding.
• Which framework was used — RECIST 1.1 or the immunotherapy-specific iRECIST — since the same finding can read differently under each.
• What non-target or incidental findings mean — anything noted that isn't a measured target lesion but is still worth understanding.

• Circulating Tumour DNA (ctDNA) for Monitoring Response — a blood-based way response is sometimes tracked alongside imaging.
• Immune-Related Findings on a Scan That Are Not Cancer — other scan changes on immunotherapy that can look concerning but aren't tumour-related.
• Why Two Radiologists Read the Same Scan Differently — a closer look at the interpretation variability behind a single report.
• Immunotherapy at CION Cancer Clinics — the full picture of how CION supports patients through diagnosis, treatment and monitoring.

This page explains general response-assessment report terminology and layout for education only, and does not interpret any individual patient's scan or report. Response-assessment PET-CT for CION patients is coordinated at partner imaging centres, not performed in-house. Bring your full report to a consultation for a doctor's explanation of what it means for you.

Thousands of families have made sense of a response scan report with us

A dense report full of measurements and unfamiliar labels can feel overwhelming on your own. Our oncologists take the time to explain, in plain language, what your specific report means.