Objective response rate (often used interchangeably with overall response rate) is a clinical endpoint that describes the number of patients who experienced tumor regression while on treatment. But what defines a response can vary depending on the form of cancer. For example, in many trials, you have to see a 30% shrinkage in the tumor to be considered a response. Defining the “response” criteria (ie, how much the tumor must shrink, and how long it must stay shrunk) is a critical part of the methods for any trial. If the tumor shrinks less than 30% or does not grow, this is termed “stable disease.” “Partial response” would be tumor shrinkage above 30% and below 100%. And “complete response” would be recorded for 100% tumor shrinkage.”
So to measure tumors for response in this way, you need to have a tumor that you can see somehow, and you have to be able to measure it. To arrive at the ORR, clinical researchers will add up the percentage of patients who had a partial or a complete response (PR+CR). You may wonder, “isn’t stopping the tumor also good?” Yes, it can be! But the response rate will not include it. Papers often also report the “disease control rate” (see below), which takes stable disease rates into account.