Introduction to RECIST 1.1 (4)

Navigating the "Nadir": A Guide to Progressive Disease (PD) in RECIST 1.1

In cancer clinical trials, determining when a treatment has stopped working is just as important as knowing when it is succeeding. Under the RECIST 1.1 guidelines, this transition is captured by the term Progressive Disease (PD).

To understand how PD is derived, you must first understand a concept called the Nadir.

What is the Nadir?

The nadir is the "all-time low" for a patient's tumor burden during the study.

  • Initial Reference: At the start of a trial, the Baseline Sum of Diameters is the first nadir.

  • Dynamic Updating: If the tumors shrink during treatment, the new, smaller sum of diameters becomes the updated nadir.

  • The "Smallest on Study" Rule: The nadir is always the smallest sum of diameters recorded at any assessment since the beginning of the trial.

The Two-Part Rule for PD

To declare Progressive Disease based on target lesions, the current sum of diameters must meet two specific criteria compared to the nadir:

1. The 20% Relative Increase

There must be at least a 20% increase in the sum of diameters compared to the smallest sum (nadir) recorded on the study.

  • Crucial Detail: Notice that progression is measured against the nadir, not the original baseline. If a tumor shrinks significantly and then starts growing again, it can be declared "Progressive" even if it is still smaller than it was on day one.

2. The 5 mm Absolute Increase

The sum must also demonstrate an absolute increase of at least 5 mm.

  • Why this exists: This rule acts as a safety net. It prevents "over-calling" progression when tumors are very small and a 20% change might be caused by tiny measurement errors rather than actual cancer growth.

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