iRECIST — why immunotherapy scans get graded by a different rulebook

iRECIST is a modified version of the standard RECIST 1.1 rules used to grade whether a scan shows a tumour shrinking, staying the same, or growing. It was created because checkpoint inhibitor immunotherapy can cause a tumour to look temporarily bigger, or a new small spot to appear, before it actually shrinks — a pattern called pseudoprogression. Ordinary RECIST rules would call that "progression" immediately; iRECIST builds in a confirmation step first.

This page explains the terminology in general — it does not, and cannot, interpret your own scan report. Whether your specific result is reassuring or concerning depends on your cancer type, treatment history and confirmation scan, which only your treating oncologist can weigh together. Nothing here replaces that conversation.

The rest of this page sets out exactly how iRECIST's categories differ from RECIST's, what "unconfirmed progression" means when it lands on your report, and why the extra confirmation scan exists rather than an immediate decision.

This table describes how the categories are generally defined in patient-education material, not a reading of your own scan. Your oncologist applies these definitions to your specific images and history.

1. 1

   Clinical check, not scan-only

   Your oncologist checks how you actually feel — new pain, breathlessness or weight loss point one way; feeling stable or better points another. The scan alone never decides.

2. 2

   Treatment usually continues, if you're stable

   Unless there is clear clinical deterioration, the standard approach is to continue immunotherapy on schedule through the iUPD label rather than stop on an unconfirmed finding.

3. 3

   A confirmation scan is booked, 4–8 weeks out

   Response-assessment PET-CT for this pathway is coordinated at partner imaging centres, not performed in-house. Your team schedules the exact timing for your case.

4. 4

   The confirmation scan reclassifies the finding

   Shrinkage on the repeat scan reclassifies it away from progression; further growth reclassifies it as iCPD, and the treatment plan is reviewed accordingly.

5. 5

   Occasionally, a biopsy if it's still unclear

   Where imaging alone doesn't settle it, a biopsy of the growing area can directly check for immune cell activity versus active tumour growth.

A single scan showing growth cannot reliably tell true progression apart from pseudoprogression, so the iRECIST framework builds the confirmation scan in as a deliberate safeguard rather than an afterthought. The four-to-eight-week gap gives a genuinely shrinking tumour enough time to show it on the second scan, rather than forcing a decision on the first, ambiguous one.

This is, honestly, the exact reason a report can sound contradictory: it can say "growth seen" and "treatment continuing" in the same sentence, and both statements are correct under iRECIST. Acting on the first scan alone — stopping treatment, or panicking before the confirmation scan — risks abandoning a treatment that was actually starting to work. If your own report has left you unsure which way it points, that is a conversation for your treating oncologist, not something to settle from a general guide.

• Partial Response and Complete Response: What the Words Mean — a plainer look at the shrinkage categories iRECIST keeps unchanged from RECIST.
• A Confirmation Scan Was Ordered: What Does That Mean? — the practical, day-to-day version of the four-to-eight-week wait explained on this page.
• SUV Values on PET After Immunotherapy: Why They Can Rise — another PET-scan number that can look worse than it is during immune activation.
• Immunotherapy at CION Cancer Clinics — the full picture of how CION supports patients through response assessment and monitoring.

This page explains general iRECIST terminology for patient education and does not interpret any individual scan report. Response-assessment PET-CT referenced on this page is coordinated at partner imaging centres, not performed in-house at CION.

Many families have faced this exact wording on a report

Wanting a clear answer instead of guessing is not overreacting — it's how patients on active treatment stay informed. Our team walks through your specific report, honestly.