Progression
The cancer is growing or spreading despite treatment, or after it. Progression usually means the current drug has stopped working and it is time to switch to the next line.
RECIST defines progression as at least a 20% increase in the sum of target-lesion diameters (and at least 5 mm absolute), or the appearance of any new lesion; clinical progression (worsening symptoms) can also count. Progression triggers the move from first-line to second-line therapy and, increasingly, a repeat biopsy or liquid biopsy to see what resistance mechanism has emerged. Time to progression is the basis of progression-free survival, the commonest primary endpoint in trials, and 'pseudoprogression', an apparent enlargement caused by immune cells flooding a tumour, can mislead early scans during immunotherapy.