In RECIST v1.1, what defines a partial response?

• A. At least a 50% decrease in sum of diameters.
• B. At least a 30% decrease in the sum of diameters of target lesions, taking baseline as reference.
• C. Any decrease in lesion size.
• D. Complete disappearance of all target lesions.

Answer: (B)

Partial response in RECIST v1.1 means a meaningful shrinkage of measurable tumor burden on imaging. Specifically, there must be at least a 30% decrease in the sum of the longest diameters of all target lesions, using the baseline sum as the reference. This reduction should be confirmed by a subsequent scan to ensure it’s not a temporary fluctuation. It’s distinct from complete response, which is when all target lesions disappear. A smaller decrease than 30% doesn’t meet partial response, and the appearance of new lesions or progression of non-target lesions would indicate progression rather than a partial response.