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The Impact of Hormone Replacement Therapy on the Incidence of Breast Cancer in Norway

Departments of Community Medicine and Pharmacy, University of Tromsø, Tromsø, Norway

Anna Elise Eggen

Department of Pharmacy, University of Tromsø, Tromsø, Norway

To the Editor:

In a letter to the Editor of the Journal of Clinical Oncology, Hemminki et al¹ analyzed the effects of nationwide screening activity on the age-incidence relationships of breast cancer.

Referring to Swedish data, they concluded that the large increase in the incidence of breast cancer is mainly due to mammographic screening. Hemminki et al also referred to an article by Zahl et al² that made the same assumption. Zahl et al assumed the increase in cancer incidence in Norwegian women aged 50 to 69 years to be closely associated with the introduction of screening in the counties. They considered it unlikely to be caused by other factors such as hormone replacement therapy (HRT). The authors did not, however, present individual data on HRT.

In the Norwegian Women and Cancer (NOWAC) study, we found the adjusted relative risk (RR) of breast cancer in current users compared with never-users of HRT to be 2.1 (95% CI, 1.7 to 2.7).³ In the multivariate analyses, we adjusted for time since menopause, age at menarche, ever use of oral contraceptives, body mass index, maternal history of breast cancer, parity, age at first childbirth, and living in a region with a screening program. The prevalence of current use was as high as 35% and the estimated attributable risk of HRT was 27%.

The present reanalysis provides individual data on women’s use of HRT as well as mammograms taken. In postmenopausal women aged 45 to 64 years who never used HRT, we found a 20% increase in the risk of having breast cancer diagnosed by administering a mammogram (RR, 1.2; 95% CI, 0.9 to 1.7). Current users of HRT doubled their relative risk of having breast cancer (RR, 2.2; 95% CI, 1.5 to 3.1), whereas users who had a mammogram had an RR of 2.4 (95% CI, 1.8 to 2.3; Fig 1). We conclude that in the study period (1996 to 2000), use of HRT in Norwegian women aged 45 to 64 years had greater impact on the increase in breast cancer incidence than did screening.

View larger version (68K): [in this window] [in a new window]   Fig 1. Relative risk of breast cancer according to use of hormone replacement therapy (HRT) and mammography

The authors indicated no potential conflicts of interest.

REFERENCES

1. Hemminki K, Rawal R, Bermejo J: Mammographic screening is dramatically changing age-incidence data for breast cancer. J Clin Oncol 22 : 4652 -4653, 2004[Free Full Text]

2. Zahl PH, Strand BH, Maehlen J: Incidence of breast cancer in Norway and Sweden during introduction of nationwide screening: Prospective cohort study. BMJ 328 : 921 -924, 2004[Abstract/Free Full Text]

3. Bakken K, Alsaker E, Eggen A, et al: Hormone replacement therapy and incidence of hormone dependent cancers in the Norwegian Women and Cancer (NOWAC) study. Int J Cancer 112 : 130 -134, 2004[CrossRef][Medline]

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Related Reply

• In Reply:
  Kari Hemminki and Justo Lorenzo Bermego
  JCO 2005 23: 3637-3638 [Full Text]

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