DNA double strand breaks induced by low dose mammography X-rays in breast tissue: A pilot study

Breast tissue is very sensitive to ionizing radiation due to the presence of reproductive hormones, including estrogen. In the present pilot study, the efficiency of mammography X-rays to induce DNA double strand breaks (DSB) in mammary epithelial cells was investigated. For this, freshly resected healthy breast tissue was irradiated with 30 kV mammography X-rays in the dose range 0-500 mGy (2, 4, 10, 20, 40, 100 and 500 mGy). Breast specimens were also irradiated with identical doses of ⁶⁰Co γ-rays as a radiation quality standard. With the γH2AX-foci assay, the number of DNA DSB induced by radiation were quantified in the mammary epithelial cells present in breast tissue. Results indicated that foci induced by 30 kV X-rays and γ-rays followed a biphasic linear dose-response. For 30 kV X-rays, the slope in the low dose region (0-20 mGy) was 8.71 times steeper compared with the slope in the higher dose region (20-500 mGy). Furthermore, compared with γ-rays, 30 kV X-rays were also more effective in inducing γH2AX-foci. This resulted in a relative biological effectiveness (RBE) value of 1.82 in the low dose range. In the higher dose range, an RBE close to 1 was obtained. In conclusion, the results indicated the existence of a low dose hypersensitive response for DSB induction in the dose range representative for mammography screening, which is probably caused by the bystander effect. This could affect the radiation risk calculations for women participating in mammography screening.