Some early breast tumours spontaneously regress

Norwegian research published in the Archives of Internal Medicine suggests many breast tumours are treated when they would disappear naturally anyway, after studying the natural history of the disease.

The introduction of screening mammography has been associated with sustained increases in breast cancer incidence. The natural history of these screen-detected cancers is not well understood.

The researchers compared cumulative breast cancer incidence in age-matched cohorts of women residing in four Norwegian counties before and after the initiation of mammography every two years. The screened group included all women who were invited for all three rounds of screening during the period 1996 through 2001 (age range in 1996, 50-64 years). The control group included all women who would have been invited for screening had there been a screening program during the period 1992 through 1997 (age range in 1992, 50-64 years).

All women in the control group were invited to undergo a 1-time prevalence screen at the end of their observation period. Screening attendance was similar in both groups (screened, 78.3%, and controls, 79.5%). Counts of incident invasive breast cancers were obtained from the Norwegian Cancer Registry, excluding in situ cancers.
They found that before the ‘age-matched control group’ women were invited to be screened at the end of their observation period, the cumulative incidence of invasive breast cancer was significantly higher in the screened group than in the controls. Even after prevalence screening in controls, however, the cumulative incidence of invasive breast cancer remained 22% higher in the screened group. Higher incidence was observed in screened women at each year of age.

Because the cumulative incidence among the control groups never reached that of the screened group, they concluded that some breast cancers detected by repeated mammographic screening would not persist to be detectable by a single mammogram at the end of 6 years.

This raises the possibility that the natural course of some screen-detected invasive breast cancers is to disappear themselves.

The scientists added: “Although many clinicians may be sceptical of the idea, the excess incidence associated with repeated mammography demands that spontaneous regression be considered carefully. Spontaneous regression of invasive breast cancer has been reported, with a recent literature review identifying 32 reported cases. This is a relatively small number given such a common disease. However, as some observers have pointed out, the fact that documented observations are rare does not mean that regression rarely occurs. It may instead reflect the fact that these cancers are rarely allowed to follow their natural course.

Why and how such regression may occur and whether the regression is complete are questions of considerable biological and clinical interest. It is also important to emphasise that our findings have no bearing on the debate on whether screening mammography reduces breast cancer mortality. Our findings are equally consistent with the possibility that mammography either leads to a reduction in breast cancer mortality or has no effect at all. Instead, our findings simply provide new insight on what is arguably the major harm associated with mammographic screening, namely, the detection and treatment of cancers that would otherwise regress.”

Article ref: Arch Intern Med. 2008;168(21):2311-2316

"The Natural History of Invasive Breast Cancers Detected by Screening Mammography"