The study reviewed here suggests that some breast cancers may go away (regress) without treatment.
The researchers came to this conclusion after reviewing records of breast cancer diagnoses in Norway before and after the country started a national breast cancer screening program. The researchers compared how often breast cancer was diagnosed and treated over 6 years in two similar groups of women ages 50 to 64 with different breast cancer screening histories:
Because the two groups of women had similar characteristics that could affect breast cancer risk, the researchers thought the two groups would have similar breast cancer risk during the 6 years they were followed. But the researchers found that the women who had mammograms every 2 years were 22% more likely to have been diagnosed and treated for invasive breast cancer compared to the women who only had one mammogram during the study period. The researchers hypothesized that undetected breast cancers developed in the women who didn't get regular screenings, but went away without treatment.
It's hard to imagine how breast cancer could go away without treatment more than 20% of the time. Still, there are reasons why this could happen. In some cases, the genetic changes that caused the cancer cells to develop ultimately make it impossible for the cells to survive. In other cases, the immune system may be able to get rid of the cancer over time in some women.
This hypothesis is controversial and it's important to look at the results thoughtfully. The results DO NOT say that regular mammograms aren't valuable. Even if some of the undetected cancers may have gone away without treatment, most of the cancers didn't, and some were detected later, when they were potentially harder to treat.
These findings also DO NOT suggest that not treating breast cancer and hoping it will go away on its own is a good option.
Right now, annual screening mammograms starting at age 40 for all women are the best way for breast cancer to be diagnosed early, when it's most treatable. Depending on your unique circumstances, your doctor may recommend a more aggressive screening plan. Stick with the screening plan that you and your doctor have decided is the best for YOU.
HANOVER, N.H., Nov. 24 (MedPage Today) -- Screening mammography detects some breast cancers that would otherwise have regressed without notice, researchers said.
Even after controlling for prevalence in unscreened women, cumulative incidence of invasive breast cancer diagnosis remained significantly higher, by 22%, in women who underwent screening, H. Gilbert Welch, M.D., M.P.H., of the Dartmouth Institute for Health Policy and Clinical Practice here, and Norwegian colleagues reported in the Nov. 24 issue of the Archives of Internal Medicine.
Their population-based study of breast cancer rates around initiation of a public screening program in Norway "raises the possibility that the natural course of some screen-detected invasive breast cancers is to spontaneously regress," they said.
While provocative, this hypothesis is hard to rule out, commented Robert M. Kaplan, Ph.D., of the University of California Los Angeles, and Franz Porzsolt, M.D., Ph.D., of the University of Ulm in Ulm, Germany.
In their accompanying editorial, Drs. Kaplan and Porzsolt cautioned physicians not to dismiss the findings on the basis that spontaneous remission of breast cancer is considered extremely rare.
"It highlights how surprisingly little we know about what happens to untreated patients with breast cancer," they wrote.
The findings could explain observations that have troubled investigators for years, such as the lack of screening benefits in randomized trials, particularly among women younger than 50, they said.
Further study is warranted because if credible, major reevaluation of the approach to breast cancer research and treatment would be needed, the editorialists said.
"The hypothesis suggests that about 20% of the women in the multiple screen group received unnecessary treatment because their tumors would have disappeared on their own," they said.
Studying the natural history of breast cancer would be difficult since physician and patient preference ensures that most women with screen-detected cancers undergo treatment, they noted.
Because of this, Dr. Welch's group took advantage of "an exceptional natural experiment" with the rapid, comprehensive introduction of biennial screening in four Norwegian counties.
They compared six-year cumulative breast cancer incidence in two cohorts of women ages 50 to 64 years.
The screened group included all women invited for three rounds of mammography over a six-year period after initiation of the screening program in 1996.
As a control group, the researchers also observed incidence in all women who would have been eligible for screening in the four years prior to the screening program and were invited to undergo a one-time prevalence screen at the end of their observation period in 1997.
The number of women who responded to the screening invitation at the end of their respective study periods was similar at 78.3% in the screening group and 79.5% among controls.
Because reproductive history, education, income, and other key factors were similar between cohorts, cumulative incidence of breast cancer would have been expected to be equal if spontaneous regression did not occur, the researchers said.
But using national breast cancer registry data, they found this was not the case.
The screening group had a dramatic 72% increase in invasive breast cancer incidence with the first round of mammography relative to the age-matched controls, as expected (two-year cumulative incidence 660 versus 384 per 100,000 population, relative rate 1.72, 95% confidence interval 1.53 to 1.94).
The control group caught up somewhat over time as their cancers became clinically evident (four-year cumulative incidence 1,268 versus 810 controls per 100,000 population, RR 1.57, 95% CI 1.44 to 1.70).
However, even after the controls had prevalence screening at the end of the observation period, cumulative incidence remained significantly elevated in the screened group (six-year cumulative incidence 1,909 versus 1,564 controls per 100 000 population, RR 1.22, 95% CI 1.16 to 1.30).
The editorialists noted that the higher number of mammograms in the screening group could have accounted for the difference because between 20% and 30% of lesions may be overlooked on radiologic review.
Additional screening in the control group would be expected to narrow the difference between groups, they said.
However, a sensitivity analysis using eight years of observation comparing the groups with an additional round of biennial screening (four total for the screening group and two for the control group) did little to attenuate the excess incidence (RR 1.20, 95% CI 1.14 to 1.25).
Dr. Welch's group also noted that the case ascertainment in the registry did not change over time and the sensitivity of mammography appeared stable over the period of the study.
Increasing use of hormone therapy over time would account for only 3% of the difference, according to another sensitivity analysis.
The researchers urged further study to confirm the findings and explore why and how regression may occur and whether the regression is complete, but emphasized that the findings did not bear on whether screening reduces breast cancer mortality.
The study was supported in part by an award from the Department of Veterans Affairs.
The researchers reported no conflicts of interest.
Drs. Kaplan and Porzsolt reported no conflicts of interest.
Primary source: Archives of Internal Medicine Source reference: Zahl P-H, et al. "The Natural History of Invasive Breast Cancers Detected by Screening Mammography" Arch Intern Med 2008; 168: 2311-2316.Additional source: Archives of Internal MedicineSource reference: Kaplan RM, Porzsolt F "The Natural History of Breast Cancer" Arch Intern Med 2008; 168: 2302-2303.
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