Trend of brain tumor incidence by histological subtypes in Japan: Estimation from the Brain Tumor Registry of Japan, 1973-1993

In order to estimate the risk of primary brain tumor (PBT), we attempted to estimate the national incidence rates of PBT by histological subtypes using the Brain Tumor Registry of Japan (BTR). The number of deaths due to PBT in a certain year is the sum of the deaths among patients diagnosed in different years. Registered cases in the BTR represent incident cases of PBT in the whole country multiplied by a cover rate. The cover rate is defined as the proportions of PBT cases that the Registry counts in relation to all the cases in the country in a given year. If the survival experience among the registered cases represents the survival experience of all cases, then the rate of registered deaths represents all deaths due to PBT in Japan. By this logic, we estimated the cover rates and incidence rates from 1973 to 1993 using the BTR and National Vital Statistics data. Our estimates showed three patterns of time trends: (1) a gradual linear increasing trend before the 1980s followed by a plateau (total PBT, gliomas, meningioma, and hemangioblastoma), (2) a trend with a step-up increase in the 1980s followed by a plateau (germ cell tumor and pituitary tumor), and (3) a linear increasing trend throughout the observation period with no plateau (malignant lymphoma and neurinoma). Furthermore, obvious sex differences in time trends were observed in rates of meningioma, germ cell tumor, and pituitary tumor. The results of this study demonstrated several distinctive patterns in time trends, which give us insight into the possible etiologies of brain tumors. Further epidemiological study is needed to elucidate these findings.