The RECIST guidelines were first published in 2000, and updated to version 1.1 in 2009. Over time, researchers have noticed that the traditional RECIST guidelines don’t do a good job in detecting responses to immunotherapies.  The RECIST working group published iRECIST guidelines in 2017 to better guide clinicians in evaluating the responses to immunotherapies. These types of therapies can sometimes take longer to be effective and cause pseudoprogression. Pseudoprogression is when a tumor looks bigger on imaging before eventually shrinking as a result of effective treatments. iRECIST and irRECIST are two ways of analyzing the effects of immunotherapies, with the chief difference being whether new tumor measurements are included in the tumor burden.