The argument is that while detecting cancer earlier is usually a good thing, you have to be careful how you measure the effects. Looking at survival X years from detection can give you false positives about what is best.
-
I'll lay out a particular scenario where this happens:
Currently we detect a cancer moderately far along, and aggressively treat it. The five year survival rate is 50%.
We figure out how to detect it two years earlier. We use the same aggressive treatments, and the five year survival rate is now 70%. Hooray! But looking closer, if we wait another two years to correct for the early detection, the survival rate is only 45%. Only some of those tumors would have continued growing. Of those, attacking early is only marginally helpful. In others, the tumor wouldn't have killed the patient, but the radio/chemotherapy killed a quarter of them.
In this case, we magically know we detected everything two years earlier, so we could look at the seven year survival rate instead of five year. But the real world is not so clean. It's very hard to figure out a timescale and normalize everyone to it.
-
Catching cancer earlier screws up your ability to measure survival. A naive analysis will see higher survival rates when smaller/earlier tumors are detected. A sophisticated analysis that corrects for this is actually hard to do.
Even when you do save lives by treating earlier, it's very hard to figure out how many lives are saved, and how much of a confounding factor your detection method is.
-
I'll lay out a particular scenario where this happens:
Currently we detect a cancer moderately far along, and aggressively treat it. The five year survival rate is 50%.
We figure out how to detect it two years earlier. We use the same aggressive treatments, and the five year survival rate is now 70%. Hooray! But looking closer, if we wait another two years to correct for the early detection, the survival rate is only 45%. Only some of those tumors would have continued growing. Of those, attacking early is only marginally helpful. In others, the tumor wouldn't have killed the patient, but the radio/chemotherapy killed a quarter of them.
In this case, we magically know we detected everything two years earlier, so we could look at the seven year survival rate instead of five year. But the real world is not so clean. It's very hard to figure out a timescale and normalize everyone to it.
-
Catching cancer earlier screws up your ability to measure survival. A naive analysis will see higher survival rates when smaller/earlier tumors are detected. A sophisticated analysis that corrects for this is actually hard to do.
Even when you do save lives by treating earlier, it's very hard to figure out how many lives are saved, and how much of a confounding factor your detection method is.