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Estimates of overdiagnosis of invasive breast cancer associated with screening mammography
Cancer Causes and Control, 11/10/09

Morrell S et al. – Overdiagnosis of invasive breast cancer attributable to mammography screening appears to be substantial. Our estimates are similar to recent estimates from other screening programmes. Overdiagnosis merits greater attention in research and in clinical and public health policy making.

Methods

Calculated overdiagnosis as observed annual incidence of invasive breast cancer in NSW in 1999–2001 (a screened population) minus expected annual incidence in population at same time, as percentage of expected incidence
Estimated expected incidence without screening in 1999–2001 from incidence of invasive breast cancer in: (1) women in unscreened age groups (interpolation method); and (2) women in all age groups prior to implementation of screening (extrapolation method)
Adjusted these estimates for trends in major risk factors for breast cancer that may have coincided with introduction of mammography screening: increasing obesity, use of HRT and nulliparity
Adjusted for lead time to produce estimates of expected incidence in 1999–2001
Compared with the observed incidence in 1999–2001 to calculate overdiagnosis of breast cancer associated with screening

Results

Overdiagnosis of invasive breast cancer among 50–69 year NSW women estimated to be 42 and 30% using interpolation and extrapolation methods, respectively

Exclusive Author Commentary
Stephen Morrell, 11/11/09

There is no doubt that mammography screening reduces breast cancer mortality through early detection and treatment of breast cancer. However, we think the price to be paid for this is that some women with screen-detected cancer will be treated for a cancer that may never have manifested clinically in their lives. This is similar to PSA testing and prostate cancer. Accordingly, women do need to be informed of the possibility that if a breast cancer is detected at screening, then there is also a chance of it being treated unnecessarily. Consequently, all parties to mammography screening are in somewhat of a quandary, largely because there is no established method for triaging screen-detected breast cancers. Presently, once a cancer is found through screening, everyone concerned would be obliged to have it treated promptly as an elementary precaution. Accordingly, overdiagnosis of breast cancer poses starkly the urgency of developing and establishing a reliable, technologically based approach to ascertaining which screen-detected breast cancers of a given type and (small) size are more are likely to progress, remain indolent, or even regress. Otherwise a substantial proportion of women with screen-detected breast cancer will continue to be overtreated for it.