The Myth of Early Cancer Detection: Part (too many to count)
It has become a mantra of the war on cancer: early detection saves lives. Mammograms, colonoscopies, Pap tests, prostate tests—you name it, and if you undergo the screening test often enough your risk of dying of cancer decreases.
If only there were scientific support for that claim. But studies concluding just the opposite keep arriving like a drumbeat of bad news. In the latest, researchers find that a shorter time between screenings for prostate cancer do not reduce how many aggressive tumors are found—even though more prostate cancers are detected among men screened every two years than men screened every four years. The study is published online today in the Journal of the National Cancer Institute.
There is nothing more intuitive than the idea that cancer screening leads to early cancer detection, and that earlier detection leads to longer survival. Surely the earlier a cancer is caught, the greater the chance that you’ll be alive in five years, the benchmark for "survivorship."
But the effect of cancer screening on reducing cancer mortality remains unproved. And the evidence keeps going in the other direction. To be sure, some tumors discovered through screening would, if left untreated, have killed patients; some of the improvement in breast-cancer survival rates reflect earlier detection. But many other tumors are so slow-growing that detecting them “early” makes no difference to survival; catch them now or in 10 years, and modern chemotherapy and radiation can still wipe them out before they metastasize (it is metastasis that kills some 90 percent of cancer patients, not the primary tumor). The incidence of prostate cancer has soared since the prostate specific antigen, or PSA, test was introduced in the 1980s, for instance, but mortality has hardly budged: the test is picking up tumors that would have posed little mortality risk even if they had been detected only after symptoms appeared. Men may think the test saved them, and give moving testimonials to that effect, but there’s no scientific proof of that. Just the opposite. As a 2006 study concluded, "results do not suggest that screening with PSA . . . is effective in reducing mortality" from prostate cancer.
In the latest study, scientists led by Monique Roobol of Erasmus Medical Centre in Rotterdam, The Netherlands, compared men who were screened bi-annually for prostate cancer (using the standard PSA test) to those who were screened every four years. In particular, the scientists examined so-called interval prostate cancers, those discovered between screenings when the men developed symptoms of prostate cancer before it was time for their next PSA test. Over ten years, there was no statistically significant difference in the total number of interval cancers, or in the number of aggressive interval cancers, in the two groups. That means more-frequent screening did not reduce the number of interval cancers. And larger, more worrisome cancers can be found equally well whether screening occurs every two years or every four.
The authors say it is “more realistic to believe that each screening will again lead to prostate cancer diagnoses among some men from a large pool harboring small and often clinically insignificant disease.” The phrase “often clinically insignificant” is the key: yes, more prostate cancers are found if you look for them more often, but all you do is make men worry that they have prostate cancer and jack up the rates of that disease (which has indeed been happening), but not affect mortality.
In an editorial accompanying the study, E. David Crawford of the University of Colorado Health Sciences Center writes, “Although many of us believe that early detection is saving lives, definitive evidence is lacking.”