This earlier detection can also lead to an overestimation of the efficacy of treatment. That’s the grain of truth in the comment above. The reasons for this are two types of bias in treatment studies known as lead time bias and length bias. In the case of cancer, survival is measured from the time of diagnosis. Consequently, if the tumor is diagnosed at an earlier time in its course through the use of a new advanced screening detection test, the patient’s survival will appear to be longer, even if earlier detection has no real effect on the overall length of survival, as illustrated below:...
Unless the rate of progression from the point of a screen-detected abnormality to a clinically detected abnormality is known, it is very difficult to figure out whether a treatment of the screen-detected tumor is actually improving survival when compared to tumors detected later. To do so, the lead time needs to be known and subtracted from the group with the test-based diagnoses. The problem is that the use of the more sensitive detection tests usually precede such knowledge of the true lead time by several years. The adjustment for lead time assumes that the screening test-detected tumors will progress at the same rate as those detected later clinically. However, the lead time is usually stochastic. It will be different for different patients, with some progressing rapidly and some progressing slowly. This variability is responsible for a second type of bias, known as length bias.
Length bias refers to comparisons that are not adjusted for rate of progression of the disease. The probability of detecting a cancer before it becomes clinically detectable is directly proportional to the length of its preclinical phase, which is inversely proportional to its rate of progression. In other words, slower-progressing tumors have a longer preclinical phase and a better chance of being detected by a screening test before reaching clinical detectability, leading to the disproportionate identification of slowly progressing tumors by screening with newer, more sensitive tests.
https://sciencebasedmedicine.org/the-early-detection-of-canc...
Excerpt:
This earlier detection can also lead to an overestimation of the efficacy of treatment. That’s the grain of truth in the comment above. The reasons for this are two types of bias in treatment studies known as lead time bias and length bias. In the case of cancer, survival is measured from the time of diagnosis. Consequently, if the tumor is diagnosed at an earlier time in its course through the use of a new advanced screening detection test, the patient’s survival will appear to be longer, even if earlier detection has no real effect on the overall length of survival, as illustrated below:...
Unless the rate of progression from the point of a screen-detected abnormality to a clinically detected abnormality is known, it is very difficult to figure out whether a treatment of the screen-detected tumor is actually improving survival when compared to tumors detected later. To do so, the lead time needs to be known and subtracted from the group with the test-based diagnoses. The problem is that the use of the more sensitive detection tests usually precede such knowledge of the true lead time by several years. The adjustment for lead time assumes that the screening test-detected tumors will progress at the same rate as those detected later clinically. However, the lead time is usually stochastic. It will be different for different patients, with some progressing rapidly and some progressing slowly. This variability is responsible for a second type of bias, known as length bias.
Length bias refers to comparisons that are not adjusted for rate of progression of the disease. The probability of detecting a cancer before it becomes clinically detectable is directly proportional to the length of its preclinical phase, which is inversely proportional to its rate of progression. In other words, slower-progressing tumors have a longer preclinical phase and a better chance of being detected by a screening test before reaching clinical detectability, leading to the disproportionate identification of slowly progressing tumors by screening with newer, more sensitive tests.