They usually grow :( . Anyway, this is only a phase 1 trial, that is designed to test if the new proposed drug is safe and check the safe dose range. In later trials there is a control group that is important to check how often there are weird cases where the tumors mysteriously shrink. (For example, bad measurement methods, diet change, leftover cure from a previous treatment, a lucky mutation, divine intervention, whatever, a double blind randomized controlled group include all those cases.)