The impact of reproductive factors on breast cancer risk - The feasibility of using Swedish population-based registers to account for the effect of confounding in cohort studies

Reproductive Factors and Breast Cancer Risk in Lithuanian Women: A Population-Based Cohort Study

Abstract. Background.

Although the relationship between reproductive factors and breast cancer is internationally proved, reliable data on former USSR countries are scarce. This study examines the association of parity, age at the first childbirth, number of children, and breast cancer risk in Lithuanian women.

Methods.

The study that included women from 40 to 79 years old was based on a dataset that was made up linking all records from the 2001 census, all cancer incidence records from the Lithuanian Cancer Registry and all death records from Statistics Lithuania between 6th April 2001 and 31st December 2009. Cox’s proportional hazards regression models were used to estimate the hazard ratios (HRs) for parity, age at the first childbirth, and number of children.

Results.

If compared to nulliparous women, parous women had a lower risk of breast cancer (HR=0.84, 95% CI 0.78–0.89) and this risk further decreased with an increasing number of children. Women who gave birth after the age of 25 had a significantly higher risk of breast cancer. This disadvantage became statistically insignificant or decreased after controlling for total number of children.

Conclusions.

Parity and age at the first childbirth are strong predictors of breast cancer risk among Lithu-anian women.

Breast cancer risk and possible mechanisms of radiation-induced genomic instability in the Swedish hemangioma cohort after reanalyzed dosimetry

The cohort of 17,200 female Swedish hemangioma patients, who had been exposed to ionizing radiation because of skin hemangioma, was analyzed for breast cancer incidence with descriptive excess relative risk models and mechanistic models of carcinogenesis. The dosimetry system has recently been updated, leading to substantially reduced doses for the most highly exposed part of the Stockholm cohort. The follow-up includes persons until December 2009 with 877 breast cancer cases. All models agree on the risk estimates. The excess relative and excess absolute risk at the age of 50 years are 0.48 Gy(-1) (95% CI 0.28; 0.69) and 10.4 (10(4)PYR Gy)(-1) (95% CI 6.1; 14.4) (95% CI 6.1; 14.4), respectively. These risk estimates are about a factor of 2 higher than previous analyses of this cohort as a consequence of the re-evaluation of the dosimetry system. Explicit models incorporating effects of genomic instability were developed and applied to the hemangioma cohort. It was found that a radiation-induced transition towards genomic instability was highly significant. The models indicate that the main effect of radiation-induced genomic instability is to increase the rate of transition of non-initiated cells to initiated cells with a proliferative advantage. The magnitude of such an acceleration cannot be inferred from epidemiological data alone, but must be complemented by radiobiological measurements.

Breast cancer risk after radiation treatment at infancy: potential consequences of radiation-induced genomic instability

Swedish hemangioma patients were treated in infancy mainly by external application of radium-226 starting from 1920. This work analysed the radiation risk among 17,158 women with a total of 678 breast cancer incidence cases with models of carcinogenesis and empirical excess relative risk models. Models incorporating effects of genomic instability were developed and applied to the hemangioma cohort. The description of the radiation risk was significantly improved with a model of genomic instability at an early stage of carcinogenesis.

Risk factors for breast cancer: a reanalysis of two case-control studies from 1926 and 1931

BACKGROUND

The first major case-control study, published by Janet Lane-Claypon in 1926, provided the first epidemiologic evidence that low fertility increases breast cancer risk. This study in the United Kingdom was replicated in 1931 by JM Wainwright using a US sample. Neither study used modern statistical inference to interpret their data. We have evaluated and reanalyzed data from both studies to assess the validity of the original conclusions about the etiology of breast cancer.

METHODS

We abstracted data from the published contingency tables for age at menarche, age at menopause, parity, age at marriage (as a proxy for age at first birth), and duration of lactation for each childbirth (as a proxy for lifetime duration of lactation). Study-specific odds ratios and associated 95% confidence intervals were calculated.

RESULTS

Findings from the quantitative reanalysis were consistent with contemporary epidemiologic evidence for age at menopause, parity, age at first birth, and duration of lactation.

CONCLUSIONS

Lane-Claypon's scientific efforts, as manifested in the 1926 UK study, are an excellent example of how one investigator's work can help develop a field of scientific inquiry.

BRCA1 gene mutations may explain more than 80% of excess number of ovarian cancer cases after breast cancer - a population based study from the Western Sweden Health Care region

AIM

In a previous cohort study, we showed that there was a significant variation in the frequency of ovarian cancer after having breast cancer in Sweden, with the highest risk occuring in the Western region. The present study aimed to evaluate whether the high prevalence of the founder mutation BRCA1 3171ins5 may explain the excess number of ovarian cancer.

METHOD

Among more than 26 000 women with breast cancer in the Western Swedish Health Care Region, 159 cases were subsequently diagnosed with ovarian cancer, whereas the expected number was 96. Archived tissue material was analysed for six common Scandinavian BRCA1 and BRCA2 gene mutations.

RESULTS

The excess number of cases was 63 (95% CI 47-77), based on person-years at risk and national incidence rates of ovarian cancer. A BRCA1 gene mutation was detected in 33 cases corresponding to 52% of the excess number. The founder mutation, BRCA1 3171ins5, was detected in 44% of the excess number. The identified mutations decreased from 45% in women less than 50 years of age at follow-up to 14% at 60+ years at follow-up. There was no obvious decrease in mutation frequency by excess numbers with age. Age at follow-up and first-degree relatives with breast and/or ovarian cancer were the best predictors of a mutation in this material.

CONCLUSION

The founder mutation, BRCA1 3171ins5, explains the excess of ovarian cancer after breast cancer in the region. From the relative frequency of the studied mutations found at the cancer genetic counselling clinic, it is estimated that BRCA1 gene mutations are associated with about 80-85% of the excess cases. This means that a negative screening for these mutations in similar cases may have a predictive value and could strongly reduce the risk of ovarian cancer in relatives.

Breast cancer risk among Swedish hemangioma patients and possible consequences of radiation-induced genomic instability

Breast cancer incidence among 17,158 female Swedish hemangioma patients was analyzed with empirical excess relative risk models and with a biologically-based model of carcinogenesis. The patients were treated in infancy mainly by external application of radium-226. The mean and median absorbed doses to the breast were 0.29 and 0.04Gy, and a total of 678 breast cancer cases have been observed. Both models agree very well in the risk estimates with an excess relative risk and excess absolute risk at the age of 50 years, about the mean age of breast cancer incidence, of 0.25Gy(-1)(95% CI 0.14; 0.37) and 30.7 (10(5) BYR Gy)(-1) (95% CI 16.9; 42.8), respectively. Models incorporating effects of radiation-induced genomic instability were developed and applied to the hemangioma cohort. The biologically-based description of the radiation risk was significantly improved with a model of genomic instability at an early stage of carcinogenesis.