Equal censoring but still informative: When the reasons for censoring differ between treatment arms.

In randomized controlled trials, informative censoring has been described as a potential bias, mainly affecting time-to-event composite endpoints, like progression-free survival (PFS). It is usually suspected in the presence of unequal attrition rates between arms. Early censoring occurs for different reasons: patients may withdraw from a trial because of toxicity, or because of disappointment with their allocation arm. If censoring is more frequent in one arm due to increased toxicity, this removes the frailest individuals and introduces a bias favoring this arm. Conversely, patients who withdraw because of disappointment of their allocation arm may be more affluent and healthy patients, who will seek treatment options outside the protocol. In trials with one treatment arm presenting higher toxicity rates, and the other arm potentially leading to patient disappointment, censoring can occur for different reasons in each arm however with the same rates. We modeled this hypothesis in a randomized controlled trial where modifying only 15% of censored patients' fate in each arm at early time-points made the PFS gain fade. Equal censoring but for different reasons is a hitherto unexplored form of informative censoring with potentially large implications across the cancer clinical trials landscape.