Oncologists often celebrate positive clinical trial results, particularly improvements in progression-free survival (PFS). However, a critical issue known as informative censoring bias can distort these results, potentially leading to the adoption of treatments that do not genuinely improve patient outcomes. This bias arises when treatment toxicity leads to discontinuation, thereby skewing PFS data.
Understanding Informative Censoring
Informative censoring occurs when patients discontinue a treatment due to its toxicity and are subsequently censored from the PFS analysis. According to John M. Burke, MD, a hematologist and medical oncologist at Rocky Mountain Cancer Centers, if treatment X is more toxic than treatment Y, more patients on treatment X will drop out due to toxicity. These patients are then censored before disease progression can be assessed. This skews the PFS analysis, making it appear that treatment X is superior, even if the perceived benefit is merely a result of its higher toxicity.
Identifying and Addressing the Bias
To identify informative censoring, it's crucial to examine the rates of treatment discontinuation due to toxicity across different treatment arms. A significant difference in these rates indicates a high likelihood of informative censoring bias. This is especially pertinent in adjuvant trials comparing a drug to a placebo, where the placebo arm inherently has no toxicity-related discontinuations.
Once identified, several strategies can help determine a treatment's true clinical benefit. One approach is to prioritize the overall survival (OS) endpoint, which is not susceptible to informative censoring bias. Another is to conduct a sensitivity analysis, assuming that all patients who discontinued treatment due to toxicity experienced a progression event. Burke notes that such sensitivity analyses are rare in trial reports. Alternatively, analyzing time-to-treatment failure (TTF), defined as death, progression, or treatment interruption for any reason, can provide a more accurate picture. If the PFS benefit is solely due to informative censoring, TTF curves should not show a significant advantage.
Implications for Clinical Practice
The key takeaway is that an improvement in PFS does not automatically translate to a clinical benefit for patients. As Burke emphasizes, the ultimate goals are to extend life (improve overall survival) and enhance quality of life. Therefore, when evaluating clinical trial results, it is essential to consider toxicity rates, overall survival, quality of life, and time-to-treatment failure, rather than relying solely on PFS data.
