A review of studies examining the relationship between progression ...

effect on PFS and treatment effect on OS with three using individual patient data, whilst theremainder used aggregate data from multiple trials. A variety of statistical techniques were employedwithin these studies, with the most commonly used being rank correlation coefficients, linearregression and landmark analysis. The lack of a standardised approach made it difficult to establishwhether there is a consistent relationship between PFS/TTP and OS. The majority of the studiesfound a positive correlation between PFS/TTP and OS for individual patients, individual trial armsand the treatment effect between trial arms, although, the size of the correlation and its statisticalsignificance varied considerably across studies. This is not surprising given the variety of methodsemployed and the variation in studies characteristics such as differences in the tumour type and theline of therapy considered.ConclusionThis review suggests that the level of evidence available supporting a relationship between PFS/TTPand OS varies considerably by cancer type and is not always consistent even within one specificcancer type. Furthermore, even where strong consistent evidence supporting a correlation betweenthe treatment effects (i.e Level 1 evidence according to Elston and Taylor’s framework) is available,it is unclear how that should be converted into a quantified relationship between PFS and OStreatment effects within a cost-effectiveness model. Therefore, any cost-effectiveness analysis whichmakes a strong assumption regarding the relationship between PFS and OS should be treated withcaution. We would support Elston and Taylor in recommending that any cost-effectiveness analysisbased on a surrogate relationship between PFS and OS should be supported with a transparentexplanation of how the relationship is quantified in the model and should be accompanied bysensitivity analysis exploring the uncertainty associated with that relationship and a systematicreview of papers examining the relationship between PFS and OS in the relevant setting. This wouldallow decision makers to judge the appropriateness of the model in light of the evidence available inthat specific disease area.4