The genesis of breast cancer is a two-step phenomenon. I. Differential effects of aging on the cancer incidence in the United Kingdom and Japan.

The present study investigated comparatively the relation between host age and age-specific incidence rate (ASIR) of breast cancer in the United Kingdom and Japan. Comparison was also made between breast cancer and 5 non-mammary neoplasias as regards the cancer incidence/age profile. The results obtained are as follows: 1) the ASIR profile of breast cancer in the age range of 22.5 years to 47.5 years was characterized by a rapid rise versus age which was common between the United Kingdom and Japan. By use of the log-log plot, the growth of breast cancer risk was found to be quasi-exponential, and the above time range was termed the stage of exponential growth. 2) From 47.5 years of age on, the stage of stagnation ensued: the ASIR for the United Kingdom kept on rising with a much reduced inclination, whereas that for Japan followed the way of gradual decline. In the log-log plot, the junction between the 2 stages was recognized as a distinct break point. 3) A similar two-stage structure was found in the ASIR profiles of cancers of the stomach, lung, cervix uteri, and corpus uteri. The break points for the first 2 neoplasias were located at senescent ages, and those for cervical cancer and endometrial cancer were located at the pre- and peri-menopausal ages respectively (27.5-32.5 and 52.5 years). The profile of lymphoid leukemia gave 2 distinct surges at the ages of infancy and senescence, regardless of the country. The significance of the break point in the ASIR profile of breast cancer was discussed in relation to the two-step carcinogenesis theory.