A good number of health care professionals, though perhaps not enough, are obsessed with screening to catch various diseases early - "when we can do more about it." Cancer screening, in particular, is an area of great interest, one that the public has really latched on to over the last 20 years or so. As the row over breast cancer screening illustrates, there is a huge debate on when to screen people. Should we screen at, say, age 40 for breast cancer, or start yearly mammograms at age 50?
It turns out that the evidence to motivate one set of screening guidelines over another isn't all that great. That is, while we know that certain kinds of screening (breast and colon, for example) work, randomized clinical trials of screening tests have too few people in each age group to definitively assess the best age cutoff for these modalities.
Enter this neat paper by Srikanth Kadiyala and Erin Strumpf. They utilize our existing screening guidelines as "natural experiment" to study the effectiveness of screening at the population level. Specifically, while the person aged 39 years old and the person aged 41 years old may be similar in terms of their cancer risk, our national guidelines lead to one person being screened and the other not. Is the 41 year old better off as a result?
U.S. cancer screening guidelines recommend that cancer screening begin for breast cancer at age 40 and for colorectal cancer and prostate cancers at age 50. What are the marginal returns to physician and individual compliance with these cancer screening guidelines? We estimate the marginal benefits by comparing cancer test and cancer detection rates on either side of recommended initiation ages (age 40 for breast cancer, age 50 for colorectal and prostate cancers). Using a regression discontinuity design and self-reported test data from national health surveys, we find test rates for breast, colorectal, and prostate cancer increase at the guideline age thresholds by 78%, 65% and 4%, respectively. Data from cancer registries in twelve U.S. states indicate that cancer detection rates increase at the same thresholds by 25%, 27% and 17%, respectively. We estimate statistically significant effects of screening on breast cancer detection (1.3 cases/1000 screened) at age 40 and colorectal cancer detection (1.8 cases/1000 individuals screened) at age 50. We do not find a statistically significant effect of prostate cancer screening on prostate cancer detection. Fifty and 65 percent of the increases in breast and colorectal case detection, respectively, occur among middle-stage cancers (localized and regional) with the remainder among early-stage (in-situ). Our analysis suggests that the cost of detecting an asymptomatic case of breast cancer at age 40 is approximately $100,000-125,000 and that the cost of detecting an asymptomatic case of colorectal cancer at age 50 is approximately $306,000-313,000. We also find suggestive evidence of mortality benefits due to the increase in U.S. breast cancer screening at age 40 and colorectal cancer screening at age 50.
This is a neat, well-crafted study. The methodology the authors use, called regression discontinuity, utilizes sharp cutoffs in decision rules/policies/etc in the context of otherwise inconsequential changes in the variable used to determine this cutoff (i.e., in the immediate neighborhood of the cutoff). It is a useful way to get quasi-experimental evidence where there is no other way to get it. Indeed, regression discontinuity is now considered second below the gold standard randomized clinical trial in the pantheon of statistical approaches.
Of course, the one problem with this study is that, while we know our existing cutoffs are useful, we don't know if there is some other cutoff that would be better (one of the motivating questions of the paper). Here is a weakness of their research design: unless there is a policy change to a different age cutoff, regression discontinuity will only allow us to evaluate our current guidelines. Either we'll have to turn to evidence from other countries, evidence from different eras - both of which have problems insofar as that the epidemiology and treatment of cancer likely varies across time and space - or turn to larger randomized controlled trials where we can be sure to have large numbers of individuals around the cutoff ages we want to test. Alternatively, we could perhaps exploit current confusion over breast cancer screening guidelines, taking advantage of the fact that some providers may choose age 40 and others 50 to commence yearly mammograms, to assess whether one or the other is better.
No comments:
Post a Comment