Disease Burden - Screening for Breast Cancer
By a sizable margin, breast cancer is the most common malignancy diagnosed in American women and the second leading cause of death from cancer. According to current incidence and mortality estimates, in a hypothetical cohort of women, approximately 1 in 8 will be diagnosed with breast cancer in her lifetime, and 1 in 29 will die from this disease.
Breast cancer is the second most common cause of person-years of life lost to cancer among men and women, accounting for an estimated 841,000 years of premature mortality. On average, a woman who dies of breast cancer has lost 19.3 years of life that she might have had if she had not died of this disease.
This count does not include the years of diminished quality of life and lower productivity from the time of diagnosis, which are difficult to factor into disease burden measures but nonetheless are important to acknowledge. As noted here, breast cancer is among the more serious threats to health that a woman experiences in her lifetime.
Sojourn Time and the Influence of Early Intervention
The detectable preclinical phase, also known as the sojourn time, is the estimated duration of time that an occult tumor can be detected before the onset of symptoms. The lead time is the amount of time actually gained by screening before the onset of symptoms. The breast cancer sojourn time, and therefore the lead time, varies in individuals, and analysis of trial data has shown that the mean sojourn time and mean lead time also vary by age and, in postmenopausal women, also by histology.
Tabar and colleagues have estimated that the mean sojourn time is 1.7 years in women aged 40 to 49, 3.3 years in women aged 50 to 59, and 3.8 years in women aged 60 to 69.
In a perfect world, the lead time would always nearly equal the sojourn time, meaning that there would be a coincidence between the occasion of a screening test and the onset of the detectable preclinical period.
This is rarely the case, but knowledge of sojourn time is important for determining screening intervals in a breast cancer screening program, because the sojourn time defines the upper limit of the lead time that might be gained. When a screening interval exceeds the mean sojourn time, there is increased potential for a higher rate of interval cancers and thus poorer prognosis in that subset of incident cases.
Early evidence of the influence of mean sojourn time on the interval cancer rate was seen in the Swedish Two-County study, which reported nearly twice the interval cancer rate in women aged 40 to 49 compared with women aged 50 and older when both groups were screened at intervals of 24 or more months.
Stage at diagnosis is an important factor in prognosis. When breast cancer is detected while still localized to the breast, 5-year survival is markedly improved compared with a diagnosis of regional disease or the presence of distant metastases. For cases diagnosed during the period 1986 to 1993, 5-year survival among women in the United States for localized diseased was 97.4%, compared with 77.4% for regional disease and 21.2% for women diagnosed with distant metastases.
Smart and colleagues recently reported adjusted survival rates after 20 years of follow-up cases diagnosed in the BCDDP; survival for women diagnosed with stage I disease was 86.8%, compared with 40.3% for women diagnosed with stage III disease. Data on long-term survival are remarkably consistent between surveillance programs, trials, and demonstration projects, with each showing an inverse association between tumor size and long-term survival. When grouped by stage at diagnosis, long-term survival is similar in all age groups of women.
Robert A. Smith and Carl J. D’Orsi
R. A. Smith: Cancer Screening, Department of Cancer Control, American Cancer Society, Atlanta, Georgia
C. J. D’Orsi: Diagnostic Radiology, University of Massachusetts Memorial Medical Center, Worchester, Massachusetts