癌症筛查通常被宣传为挽救生命的手段。随着对多癌种检测血液检测（即液体活检）日益增长的热情，是否癌症筛查确实能挽救生命的问题变得日益重要。在随机临床试验中，筛查可能减少所针对癌症的死亡人数，但不减少由各种原因导致的死亡。为探索与癌症特异性死亡和全因死亡相关研究的可行性，我们进行了一系列针对选定癌症（乳腺癌、结直肠癌、肝癌、胰腺癌和前列腺癌）及所有癌症的样本量计算的研究。筛查特定个体癌症的随机临床试验通常需要10万名或更多的参与者来测试其对特异性癌症死亡的影响。测试全因死亡需要超过一百万名参与者的试验。然而，当考虑到使用多癌种检测血液检测时，样本量要求发生了明显变化。在此情况下，是否癌症筛查能减少全因死亡的问题可以在不到10万名参与者的试验中合理解决。对于一个个体癌症筛查，测试全因死亡是不可行的。然而，对于多癌种筛查来测试全因死亡是可行的，因为癌症死亡是总体死亡的重要组成部分。关于癌症筛查效果的观察性数据是具有误导性的。多癌种筛查将涉及巨大的成本和潜在的重大伤害。出于这些原因，随机临床试验不仅是了解多癌种筛查是否挽救生命的必要手段，也是了解其造成伤害频率的必要手段。

Cancer screening is often promoted as a means to save lives. The question of whether cancer screening truly saves lives is becoming increasingly relevant given the growing enthusiasm for multicancer detection blood tests (ie, liquid biopsies). It is possible in randomized clinical trials for screening to reduce deaths due to the targeted cancer without reducing deaths due to all causes. To explore the feasibility of powering studies for cancer-specific vs all-cause mortality, a series of sample size calculations was performed for selected cancers (breast, colorectal, liver, pancreas, and prostate) and for all cancers combined.Randomized clinical trials of screening for an individual cancer typically require 100 000 or more participants to test its effect on cancer-specific mortality. Testing all-cause mortality requires trials of more than a million participants. However, the sample size requirements change markedly when considering a randomized clinical trial of screening for all cancers, as is envisioned when using multicancer detection blood tests. In this setting, the question of whether cancer screening reduces all-cause mortality can be reasonably addressed in a trial of fewer than 100 000 participants.It is not feasible to test all-cause mortality when screening for an individual cancer. However, it is feasible to test all-cause mortality for multicancer screening because cancer deaths are such a large component of deaths in general. Observational data on the effects of cancer screening are misleading. Multicancer screening would entail tremendous costs and potentially substantial harms. For these reasons, a randomized clinical trial is mandatory not only to learn if multicancer screening saves lives, but also to learn how frequently it causes harm.