8639 &copy; Saturday 25 March 1989 INCREASED RISK OF BREAST CANCER AFTER LOW-DOSE IRRADIATION BARUCH MODAN ESTHER ALFANDARY ANGELA CHETRIT LEAH KATZ Department of Clinical Epidemiology, Chaim Sheba Medical Centre, and Tel Aviv University School of Medicine, Tel Hashomer 52621, Israel Summary A significant increase in the risk of breast cancer has been found for the most recent 5-year period of a long-term follow-up study of children subjected to scalp irradiation, in whom a carcinogenic effect was previously only apparent in the head and neck. This increased risk was found among women aged 5 to 9 years at exposure. The breast had been exposed to a low radiation dose of approximately 16 mGy. Introduction WE have described an increased risk of head and neck tumours in children who had scalp irradiation for tinea capitis between 1949 and 1959.1 In particular, a four-fold increase in the incidence of thyroid cancer has been found after an exposure dose of only 90 mGy. Before 1982, no excess of tumours below the neck had been observed. The potential carcinogenic effect of radiation doses of approximately 1 rad (10 mGy) is still under debate, despite studies of nuclear industry employees, antenatal irradiation,9 participants in nuclear weapon tests,1(}-12 and residents living near nuclear installations in the UK.13 Some of the findings of these studies have been ascribed to deficient methodology, incomplete dosimetry, or to confounding exposures to chemicals and other potential carcinogens. Nevertheless, it has been suggested that existing radiation protection limits should be lowered." The radiosensitivity of breast tissue has been shown by studies of the incidence of breast cancer in atom bomb survivors,ls and women subjected to multiple fluoroscopies,16 or radiotherapy. 17 We describe the risk of breast cancer in a cohort of children whose breast tissue was exposed to a low radiation dose of approximately 16 mGy (1-6 6 rad). Patients and Methods The original cohort included 10 834 irradiated children, 10 834 matched population controls, and 5392 sibling controls. There were equal numbers of boys and girls, aged 1 to 15 years; about half were 6 to 8 years old. Irradiation was given on immigration to Israel from North Africa or the Middle East between 1949 and 1959, and followed the Adamson-Kienbock technique of a five-field treatment of the scalp, with lead shielding of the face and neck. Patients were irradiated with 70-100 kVp superficial X-ray machines with a 0-5 mm aluminium filter and 1 0 mm aluminium half-value layer; a typical course of therapy was about 3-5-3 75 Gy in air to each field. Dosimetry was estimated retrospectively on a specially designed phantom. Total radiation doses would have been 90 mGy to the thyroid, 48-66 mGy to the pituitary, and 16 mGy to the breast. The vital status of the population was determined from the Central Population Registry. The only subjects who might have been lost to follow-up were those who left the country (1 9% of both cases and controls). Patients with cancer from subject and control groups were identified from the records of the Israel Cancer Registry up to Dec 31, 1986, without knowledge of their previous radiation exposure, and diagnosis was individually checked against hospital pathology records. Further details of the protocol have been previously described.1-5 The decision to update the follow up for 5 additional years was technical, made a priori, and independent of any of the findings. We used 90% confidence intervals when stated, since we did not expect a protective anti-carcinogenic effect of radiation. Results Table i presents the number of malignant neoplasms by major site and follow-up period. An excess of head and neck malignancies, with a four-fold incidence of thyroid cancer, has previously been described for the children who received scalp irradiation. However, the figures for 1982-1986 show, for the first time, an increased risk of breast cancer in women. Table II shows the relative risks for the same sites; during 1982-1986 there was an increased relative risk of 2 11 for breast cancer (p <0’01). The relative risk of breast cancer by age at entry and exposure category is shown in table ill: the significantly increased risk of breast cancer only occurs in women who had been irradiated between 5 and 9 years of age. Further breakdown by years since treatment confines the increased