Letters

Risk factors for breast cancer

Smoking may be important

Authors' reply

Smoking may be important

EDITORThe review by McPherson et al on the epidemiology of and risk factors for breast cancer states that: "Smoking is of no importance in the aetiology of breast cancer."¹ The authors seem to be unaware of the mounting evidence that supports the relation between tobacco smoke and breast cancer. Seven of the eight studies that examined the relation between passive smoking and breast cancer suggest that there is an association between an increased risk of breast cancer and passive long term exposure to smoke among women who have never smoked; this is particularly true in cases of breast cancer occurring before menopause.^2-4

Furthermore, when passive smoking was controlled for in these studies, active smoking also seemed to be a risk factor for breast cancer. ^{2 3} Both long term passive smoking among women who have never smoked and active smoking almost double the risk of breast cancer. ^{2 3} Polycyclic aromatic hydrocarbons are found in tobacco smoke, cause mammary cancer in experimental models, and may contribute to the development of breast cancer in humans.⁵ Given the high prevalence of regular passive smoking and of active smoking among women in most industrialised nations, smoking may turn out to be of considerable importance in the aetiology of breast cancer.

Kenneth C Johnson, senior epidemiologist. 
Cancer Bureau, Centre for Chronic Disease Prevention and Control, Health Canada, LCDC Building, PL 0601C1, Tunney's Pasture, Ottawa, ON, Canada K1A 0L2

1.	McPherson K, Steel CM, Dixon JM. Breast cancerepidemiology, risk factors, and genetics. BMJ 2000; 321: 624-628[Free Full Text]. (9 September.)
2.	Wells AJ. Re: "Breast cancer, cigarette smoking, and passive smoking." Am J Epidemiol 1998; 147: 991-992[Free Full Text].
3.	Johnson KC, Hu J, Mao Y, the Canadian Cancer Registries Epidemiology Research Group. Passive and active smoking and breast cancer risk in Canada 1994-97. Cancer Causes Control 2000; 11: 211-221[CrossRef][Medline].
4.	Jee SH, Ohrr H, Kim IS. Effects of husbands' smoking on the incidence of lung cancer in Korean women. Int J Epidemiol 1999; 28: 824-828[Abstract/Free Full Text].
5.	Rundle A, Tang D, Hibshoosh H, Estabrook A, Schnabel F, Cao W, et al. The relationship between genetic damage from polycyclic aromatic hydrocarbons in breast tissue and breast cancer. Carcinogenesis 2000; 21: 1281-1289[Abstract/Free Full Text].

Authors' reply

EDITORRecent studies have shown that there is an association between smoking and breast cancer, as Johnson points out, but the association seems to be complex. Certain studies have found an association between smoking and breast cancer, but others have not. For instance, one study suggested that the risk of breast cancer in women younger than 45 is reduced among those who currently smoke and who began smoking at an early age or who have been long term smokers. This contrasts with the findings of other studies that have shown that it is exposure to cigarette smoke when young that is associated with the greatest odds of developing breast cancer. ^{1 2} Earlier studies found that smokers had a lower risk of breast cancer. This finding was confirmed by a recent study from Quebec which reported that smokers had a significantly lower risk of breast cancer, particularly those who smoked cigarettes without filters.³

Other studies have found no significant relation between smoking and breast cancer. The effect of smoking on breast cancer in carriers of the BRCA1 and BRCA2 genes is also interesting: a recent report found that participants with BRCA1 or BRCA2 mutations who had breast cancer were significantly more likely to be non-smokers than participants with the mutations who did not have breast cancer.⁴ In a multivariate analysis, participants with BRCA1 or BRCA2 mutations who smoked for more than four pack-years (number of packs per day multiplied by the number of years smoked) had a much lower risk of breast cancer (odds ratio 0.46, 95% confidence interval 0.27 to 0.80, P=0.006).

We have also investigated the effects of smoking on breast cancer. In a study of 415 symptomatic women there was a significantly higher rate of lymphatic or vascular invasion in patients who smoked. This is consistent with a recent report that found that patients who had a history of smoking had a lower rate of survival than non-smokers.⁵

We accept that there is mounting evidence to support the idea of a relation between tobacco smoke and breast cancer. It has been known for some time that smoking causes damage to the ducts underneath the nipple resulting in periductal mastitis, but the exact relation between smoking and breast cancer seems complex.

J M Dixon, senior lecturer, surgery. 
Edinburgh Breast Unit, Western General Hospital, Edinburgh EH4 2XU

C M Steel, professor of medical and biological sciences. 
School of Biological and Medical Sciences, But Medical Building, University of St Andrews, St Andrews, Fife KY16 9TS cms4{at}st-and.ac.uk