Why is it that the incidence of breast cancer has gone up so dramatically since screening mammography has been introduced?
H.Gilbert Welch, Per-Henrik Zahl and Jan Maehlen developed an ingenious way to address the question and reached the almost unfathomable conclusion that some breast cancers disappear without treatment.
The scientists knew Norway’s health system adopted a biannual screening program in 1996, so they compared the cumulative incidence of breast cancer in two age-matched groups of 100,000 Norwegian women.
The first group, followed from 1992-1997, received one mammogram at the end of the observation period. The second, followed from 1997-2002, had 3 mammograms over the same time duration.
Women in the frequently screened group had a 22% higher cumulative incidence of breast cancer.
The difference could not be attributed to differential use of hormone therapy or risk factor profiles. Nor was it caused by the use of more sensitive mammograms in the latter group. The possibility that multiple mammograms somehow increase screening yields, though plausible, explained almost none of the difference and no one believes the mammograms actually caused cancer.
In commenting on the study Barnett Kramer, director of the Office of Disease Prevention at the NIH, told the New York Times, “People who are familiar with the broad range of behaviors of a variety of cancers know spontaneous regression is possible. “But what is shocking is that it can occur so frequently.”
Even if true, the astounding possibility has no immediate implications since at the time of diagnosis there is no way to tell which cancers will regress and which ones will not.
So in the end came reassurances that mammograms save lives, warnings to continue all screening programs until further notice, and a lot of people asking for more. Like, can the study ever be replicated?
Yes, it turns out. Mexico is introducing mammography screening as we speak.