Exposure to low-dose radiation and the risk of breast cancer among women with a familial or genetic predisposition: a meta-analysis

BACKGROUND

Women with familial or genetic aggregation of breast cancer are offered screening outside the population screening programme. However, the possible benefit of mammography screening could be reduced due to the risk of radiation-induced tumours. A systematic search was conducted addressing the question of how low-dose radiation exposure affects breast cancer risk among high-risk women.

METHODS

A systematic search was conducted for articles addressing breast cancer, mammography screening, radiation and high-risk women. Effects of low-dose radiation on breast cancer risk were presented in terms of pooled odds ratios (OR).

RESULTS

Of 127 articles found, 7 were selected for the meta-analysis. Pooled OR revealed an increased risk of breast cancer among high-risk women due to low-dose radiation exposure (OR = 1.3, 95% CI: 0.9- 1.8). Exposure before age 20 (OR = 2.0, 95% CI: 1.3-3.1) or a mean of ≥5 exposures (OR = 1.8, 95% CI: 1.1-3.0) was significantly associated with a higher radiation-induced breast cancer risk.

CONCLUSION

Low-dose radiation increases breast cancer risk among high-risk women. When using low-dose radiation among high-risk women, a careful approach is needed, by means of reducing repeated exposure, avoidance of exposure at a younger age and using non-ionising screening techniques.

Study design: evaluating gene-environment interactions in the etiology of breast cancer - the WECARE study

INTRODUCTION

Deficiencies in cellular responses to DNA damage can predispose to cancer. Ionizing radiation can cause cluster damage and double-strand breaks (DSBs) that pose problems for cellular repair processes. Three genes (ATM, BRCA1, and BRCA2) encode products that are essential for the normal cellular response to DSBs, but predispose to breast cancer when mutated.

DESIGN

To examine the joint roles of radiation exposure and genetic susceptibility in the etiology of breast cancer, we designed a case-control study nested within five population-based cancer registries. We hypothesized that a woman carrying a mutant allele in one of these genes is more susceptible to radiation-induced breast cancer than is a non-carrier. In our study, 700 women with asynchronous bilateral breast cancer were individually matched to 1400 controls with unilateral breast cancer on date and age at diagnosis of the first breast cancer, race, and registry region, and counter-matched on radiation therapy. Each triplet comprised two women who received radiation therapy and one woman who did not. Radiation absorbed dose to the contralateral breast after initial treatment was estimated with a comprehensive dose reconstruction approach that included experimental measurements in anthropomorphic and water phantoms applying patient treatment parameters. Blood samples were collected from all participants for genetic analyses.

CONCLUSIONS

Our study design improves the potential for detecting gene-environment interactions for diseases when both gene mutations and the environmental exposures of interest are rare in the general population. This is particularly applicable to the study of bilateral breast cancer because both radiation dose and genetic susceptibility have important etiologic roles, possibly by interactive mechanisms. By using counter-matching, we optimized the informativeness of the collected dosimetry data by increasing the variability of radiation dose within the case-control sets and enhanced our ability to detect radiation-genotype interactions.

Sarcoma as a second malignancy after treatment for breast cancer

BACKGROUND

Second malignant neoplasms may be a consequence of radiotherapy for the treatment of breast cancer. Prior studies evaluating sarcomas as second malignant neoplasms in breast cancer patients have been limited by the numbers of patients and relatively low incidence of sarcoma. Using data from the Surveillance, Epidemiology and End Results registries, we evaluated the influence of radiation therapy on the development of subsequent sarcomas in cases with primary breast cancer.

METHODS

Cases with primary invasive breast cancer (n = 274,572) were identified in the Surveillance, Epidemiology and End Results Cancer Incidence Public-Use Database (1973-1997). The database was then queried to determine the cases developing subsequent sarcomas (n = 263). Eighty-seven of these cases received radiation therapy, and 176 had no radiation therapy. The cumulative incidence of developing secondary sarcoma and the survival post developing secondary sarcoma were determined by the Kaplan-Meier method.

RESULTS

The occurrence of sarcoma was low, regardless of whether cases received or did not receive radiation therapy: 3.2 per 1,000 (SE [standard error] = 0.4) and 2.3 per 1,000 (SE = 0.2) cumulative incidence at 15 years post diagnosis, respectively (p = 0.001). Of the sarcomas occurring within the field of radiation, angiosarcoma accounted for 56.8%, compared to only 5.7% of angiosarcomas occurring in cases not receiving radiotherapy. The cumulative incidence of angiosarcoma at 15 years post diagnosis was 0.9 per 1,000 for cases receiving radiation (SE = 0.2) and 0.1 per 1,000 for cases not receiving radiation (SE < 0.1). Overall survival was poor for cases of sarcoma after breast cancer (27-35% at 5 years), but not significantly different between patients receiving or not receiving radiation therapy for their primary breast cancer.

CONCLUSIONS

Radiotherapy in the treatment of breast cancer is associated with an increased risk of subsequent sarcoma, but the magnitude of this risk is small. Angiosarcoma is significantly more prevalent in cases treated with radiotherapy, occurring especially in or adjacent to the radiation field. The small difference in risk of subsequent sarcoma for breast cancer patients receiving radiotherapy does not supersede the benefit of radiotherapy.

Impact of cancer predisposition and radiosensitivity on the population risk of radiation-induced cancers

This paper provides a brief overview of the current evidence for cancer predisposition and for an increased sensitivity of individuals carrying such predisposing mutations to cancers induced by ionizing radiations. We also discuss the use of a Mendelian one-locus, two-allele autosomal dominant model for predicting the impact of cancer predisposition and increased radiosensitivity on the risk of radiation-induced cancers in the population and in relatives of affected individuals using breast cancer due to BRCA1 mutations as an example. The main conclusions are the following: (1) The relative risk ratio of the risks of radiation-induced cancer in a heterogeneous population which has subgroups of normal and cancer-predisposed individuals to the risks in a homogeneous population (i.e., one which does not have these subgroups) increases with increasing dose; however, the dose dependence of the RR decreases at higher doses because of the fact that at high doses, the radiation risk to a homogeneous population will already be high. (2) The attributable risk (the proportion of cancers attributable to increased cancer susceptibility and increased radiosensitivity) follows a similar pattern. (3) When the proportion of cancers due to the susceptible genotypes is small (< 10%), as is likely to be the case for breast cancers in non-Ashkenazi Jewish women, the increases in risk ratios and attributable risks are small, and become marked only when there are very large increases in cancer susceptibility (> 1000-fold) and radiosensitivity (> 100-fold) in the susceptible group. (4) When the proportion of cancers due to the susceptible genotypes is appreciable (> or = 10%), as may be the case for breast cancers in Ashkenazi Jewish women, there may be significant increases in the risk ratios and attributable risk for comparatively moderate increases in cancer susceptibility (> 10-fold) and radiosensitivity (> 100-fold) in the susceptible subpopulation. (5) The ratio of the risk of radiation-induced cancer in relatives to that in unrelated individuals in the population increases with the biological relatedness of the relative, being higher for close than for distant relatives; however, even when the mutant BRCA1 gene frequency and the proportion of breast cancers due to these mutations are high, as in Ashkenazi Jewish women, for values of predisposition strength and radiosensitivity differential < 10, the increase in breast cancer risks is only marginal, even for first-degree relatives.

A note on the relevance of human population genetic variation and molecular epidemiology to assessing radiation health risk for space travellers

We discuss the relevance to space medicine of studies concerning human genetic variation and consequent variable disease susceptibility or sensitivity between individuals. The size of astronaut and cosmonaut populations is both presently and cumulatively small, and despite the launch of the International Space Station, unlikely to increase by orders of magnitude within the foreseeable future. In addition, astronauts-cosmonauts constitute unrepresentative samples of their national populations. While the context of exposure for the astronaut-cosmonaut group is one unlikely to be replicated elsewhere than in space, aspects of specific exposures may be simulated by events such as occupational radiation exposure or radiation therapy. Hence, population-based studies of genetic susceptibility or sensitivity to disease, especially where it is precipitated by events that may simulate consequences of the space environment, likely will prove of value in assessing long-term health risks.