Researchers in Norway found there was a 22 per cent higher rate of detected breast cancer among women who had mammograms every two years compared to women screened every six years raising the possibility that some breast cancers just regress naturally without treatment.
The study was the work of Dr Per-Henrik Zahl, from the Norwegian Institute of Public Health, Oslo, and colleagues and is published in the 24 November issue of Archives of Internal Medicine.
For the study, Zahl and colleagues compared breast cancer rates between two groups of women living in Norway: one screened three times over six years (the “screened group”) and one screened only once at the end of six years (the “control group”).
The “screened group” comprised 119,472 women aged 50 to 64 who had been invited to take part in the Norwegian Breast Cancer Screening Program where they underwent a total of three mammograms (two years between each) from 1996 to 2001. The “control group” comprised 109,784 women who were aged 50 to 64 in 1992 and did not have the biennial screening, they had one mammogram to assess final prevalence at the end of six years of being tracked for cancer via a national register.
The results showed that breast cancer rates were higher in the screened group than among the control group before they had their final prevalence screening. For the women screened every two years the breast cancer rate was 1,909 out of every 100,000 women. This compared with 1,564 out of 100,000 among the controls. Also, the screened women were more likely to have breast cancer at every age.
Equally surprising however, was that Zahl and colleagues found that:
“Even after prevalence screening in controls, however, the cumulative incidence of invasive breast cancer remained 22 percent higher in the screened group.”
These findings reflect a phenomenon that has emerged across Europe: the introduction of regular mammography screening is linked to higher rates of breast cancer, wrote the authors in their background information. What is puzzling is that if these newly detected cancers were all to progress and be clinically evident as women age, then should we not soon see a decrease in incidence among older women? But this has not happened, so Zahl and colleagues raised the question:
“What is the natural history of these additional screen-detected cancers?”
They offered a possible explanation:
“Because the cumulative incidence among controls never reached that of the screened group, it appears that some breast cancers detected by repeated mammographic screening would not persist to be detectable by a single mammogram at the end of six years.”
“This raises the possibility that the natural course of some screen-detected invasive breast cancers is to spontaneously regress,” they added.
The authors conceded that many doctors and cancer experts will be sceptical about this suggestion, but they ask that the possibility that some cancers may spontaneously regress be given careful consideration. They refer to a recent literature review that mentions 32 reported cases of spontaneous remission. Admittedly this is a small number for a common disease but just because there are few reports of it does not mean remission is rare.
“It may instead reflect the fact that these cancers are rarely allowed to follow their natural course,” wrote the authors.
However, Zahl and colleagues pointed out that these results do not answer the question of whether mammograms reduce breast cancer deaths. Instead they concluded that the findings simply highlight the possibility that mammograms may also do harm by causing women to have treatment for cancers that would naturally have regressed on their own.
In an accompanying editorial, Dr Robert M. Kaplan of the University of California in Los Angeles, USA, and Dr Franz Porzsolt of the Clincal Economics University of Ulm, Germany, wrote that:
“Despite the appeal of early detection of breast cancer, uncertainty about the value of mammography continues.”
Zahl and colleagues highlight how little we know about what happens to patients whose breast cancer goes untreated, they wrote.
“In addition to not knowing the natural history of breast cancer for younger women, we also know very little about the natural history for older women,” wrote Kaplan and Porzsolt. For instance there are autopsies that report women having died without knowing they had breast cancer, including ductal carcinoma in situ, a non-invasive form of breast cancer.
“The observation of a historical trend toward improved survival does not necessarily support the benefit of treatment,” wrote Kaplan and Porzsolt.
They conclulded that if the idea of spontaneous remission is a credible one then it should cause a major rethink in the approach to breast cancer research and treatment and is certainly worthy of further evaluation.
“The Natural History of Invasive Breast Cancers Detected by Screening Mammography.”
Per-Henrik Zahl; Jan Maehlen; H. Gilbert Welch.
Arch Intern Med. Vol. 168 No. 21, November 24, 2008, pp 2311-2316.
Source: JAMA and Archives Journals.
Written by: Catharine Paddock, PhD