부산대학교 도서관

Since Nagasaki city is hilly, some people exposed to the atomic bomb over mountains might be shielded. The people exposed in the shielded areas should have received less radiation dose and will be probable lower in cancer mortality than those exposed in the unshielded areas at the same distance from the hypocenter. We first defined shielded and unshielded areas located 2-5 km from the hypocenter into 5 groups using GIS, i.e., shielded and unshielded areas located at 2-3 km from the hypocenter (denoted by S2-3 and U2-3, respectively), and unshielded areas located at 3-4 km (U3-4) and 4-5 km (U4-5). We compared the cancer mortality among atomic bomb survivors exposed in the respective areas on the basis of the Cox proportional hazard model after adjusting for sex, age at the bombing, and attained age. Of 37,357 survivors who were exposed at the age of less 30 years and were living in Nagasaki city on 1 January 1970, those exposed in areas of U2-3, S2-3, U3-4 and U4-5 were 1663, 2341, 5062 and 3313, respectively. From 1 January 1970 through 31 December 2009, the number (percent) of cancer deaths observed among survivors exposed in areas of U2-3, S2-3, U3-4 and U4-5 were 198 (11.9%), 202 (8.6%), 435 (8.6%) and 267 (8.1%), respectively. The hazard ratio of cancer mortality in S2-3 was 0.76-fold of that in U2-3 (95% confidence interval: 0.63-0.93), that is 24% reduction. Similarly, the hazards of cancer mortality in U3-4 and U4-5 were 0.75-fold (0.64-0.89) and 0.80-fold (0.67-0.97) of that in U2-3, respectively. No significant difference was observed in hazards of cancer was observed among in S2-3, U3-4 and U4-5. The present study demonstrates the terrain shielding effects and suggests that such effects are similar to those by remoteness from the hypocenter for the distance of 3 km or over.