TRENDS IN THE INCIDENCE RATE AND RISK-FACTORS FOR BREAST-CANCER IN JAPAN

The incidence rate of breast cancer in Japan rose more than two-fold f rom 1959-60 to 1983-87. To assess to what extent this increase can be explained by changes in the prevalence of four major risk factors of b reast cancer (i.e. age at menarche, age at first birth, age at menopau se, and parity), we estimated the probability of developing breast can cer based on the joint distribution and relative risks of these four r isk factors. The age-specific incidence rate during 1959-60 reported b y the Miyagi Prefectural Cancer Registry was used to estimate the base line hazard rate for women without the four risk factors in the same a ge group. Assuming that the baseline hazard rate is constant during al l periods, we calculated the expected incidence rates during the perio ds of 1959-60, 1962-64, 1968-71, 1973-77, 1978-81, and 1983-87 for eac h age group. Large discrepancies were noted between the observed and e xpected incidence rates during 1983-87 in all age groups. The change i n the joint distribution of the four risk factors accounted for less t han 40% of the increase observed from 1959-60 to 1983-87, suggesting t he effects of other powerful risk factors.