BMJ  2005;331:350 (6 August), doi:10.1136/bmj.331.7512.350-a

Letter

Model of outcomes of screening mammography

Spontaneous regression of breast cancer may not be uncommon

EDITOR—The model of outcomes of screening mammography by Barratt et al proposes that more breast cancer is diagnosed among screened than unscreened women.¹ The model predicts about 40% higher cumulative incidence in women screened biennially during a 10 year observation period.

Two important underlying assumptions in mammography screening are that the sensitivity at the screening is comparatively high (> 75%) and that spontaneous tumour regression is uncommon. From these assumptions, it emerges that most of the difference in the cumulative incidences between screened and unscreened women should disappear if a prevalence screening of previously unscreened women had been done at the end of the observation period.

Credit: ANDREA MOTTA/WELLCOME PHOTO LIBRARY

 

In particular, the model predicts that if women were followed up from 40 to 69, the cumulative 30 year difference between screened and unscreened per 1000 is (17.6-13.2)+(28.1-19.8)+(32.5-23.9) = 21.3 invasive breast cancers.¹ To compensate for this difference, the detection rate at a prevalence screening of previously unscreened women at age 69 should be 21.3 plus the background incidence. However, less than 50% of this difference is compensated for by prevalence screening at age 69.^2-4 Thus, at least one of the two assumptions above should be modified.

Prevalence screening detects most of the slow growing cancers, and the detection rates at the following screenings are stable at a lower level.² This indicates that it is true that the sensitivity is comparatively high. We are therefore left to conclude that for small invasive breast cancers, spontaneous tumour regression is not uncommon.

Norwegian Institute of Public Health, PO Box 4404 Nydalen, N-0403, Oslo, Norway per-henrik.zahl{at}fhi.no

Department of Pathology, Ullevål University Hospital, N-0407 Oslo, Norway

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Competing interests: None declared.

References

1. Barratt A, Howard K, Irwing L, Salkeld G, Houssami N. Model of outcomes of screening mammography: information to support informed choices. BMJ 2005;330: 936. (23 April.)[Abstract/Free Full Text]
2. Tabár L, Fagerberg G, Duffy SW, Day NE, Gad A, Gröntoft O. Update of the Swedish two-county program of mammographic screening for breast cancer. Radiol Clin North Am 1992;30: 187-210.[ISI][Medline]
3. Smith-Bindman R, Chu PW, Miglioretti Dl, Sickles EA, Blanks R, et al. Comparison of screening mammography in the United States and the United Kingdom. JAMA 2003;290: 2129-38.[Abstract/Free Full Text]
4. Zahl PH, Andersen JM, Mæhlen, J. Spontaneous regression of cancerous tumours detected by mammography screening. JAMA 2004;292: 2579-80.[Free Full Text]

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Related Article

Model of outcomes of screening mammography: information to support informed choices

Alexandra Barratt, Kirsten Howard, Les Irwig, Glenn Salkeld, and Nehmat Houssami
BMJ 2005 330: 936. [Abstract] [Full Text] [PDF]