Overdetection of Breast Cancer

Abstract

Overdetection (often referred to as overdiagnosis) of cancer is the detection of disease, such as through a screening program, that would otherwise remain occult through an individual's life. In the context of screening, this could occur for cancers that were slow growing or indolent, or simply because an unscreened individual would have died from some other cause before the cancer had surfaced clinically. The main harm associated with overdetection is the subsequent overdiagnosis and overtreatment of disease. In this article, the phenomenon is reviewed, the methods of estimation of overdetection are discussed and reasons for variability in such estimates are given, with emphasis on an analysis using Canadian data. Microsimulation modeling is used to illustrate the expected time course of cancer detection that gives rise to overdetection. While overdetection exists, the actual amount is likely to be much lower than the estimate used by the Canadian Task Force on Preventive Health Care. Furthermore, the issue is of greater significance in older rather than younger women due to competing causes of death. The particular challenge associated with in situ breast cancer is considered and possible approaches to avoiding overtreatment are suggested.

Keywords: Canadian National Breast Screening Study; breast cancer; breast cancer screening; microsimulation; overdetection; overdiagnosis; overtreatment.

Figure 1

(a) Illustrates the initiation and growth of breast cancers in an unscreened population (e.g., 1000 women). Size of the lesion is represented by the diameter of the discs. X indicates cancers that will result in death. (b) The effect of screening. In (a,b), the lower rows indicate cancers that have been detected and treated, while upper rows show cancers in the cohort that have not yet been detected. (c) Difference (excess) in the cumulative number of cancers found in screened (black curve) versus unscreened (red curve) individuals, depicted in the graph as the dashed line, increases during the period of screening. In this example where there is no overdetection, cancers are found and treated earlier in the screened group; however, after screening ends at Sn, the number in the unscreened group will catch up over time, eliminating the excess.

Figure 2

Initiation and growth of breast cancers in the presence of cancers with limited malignant potential. (a) An unscreened population. Grey discs indicate cancers that are destined not to be lethal. (b) The effect of screening. Lower row indicates cancers that have been detected and treated, while upper row indicates cancers in the cohort that have not yet been detected. (c) Overdetection. After screening ends at Sn, the initial excess of cancers in the screened grouped will not be completely eliminated over time.

Figure 3

Excess invasive cancers in the study group compared to the controls in the two CNBSS trials.

Figure 4

Predicted cases of breast cancer (OncoSim-Breast model) for women who are unscreened and in the Mammography or Usual Care arms of the CNBSS1 trial.

Figure 5

Modeled cumulative excess breast cancer detection in CNBSS1 from OncoSim-Breast. Shaded area represents screening period.

Figure 6

Predicted incidence of breast cancer (OncoSim-Breast model) for women who are unscreened and in the Mammography or Physical Examination arms of the CNBSS2 trial.

Figure 7

Modeled cumulative excess breast cancer detection in CNBSS2 from OncoSim-Breast.