@misc{oai:repo.qst.go.jp:00068536,
 author = {Imaoka, Tatsuhiko and Nishimura, Mayumi and Kakinuma, Shizuko and Ohmachi, Yasushi and Nishimura, Yukiko and Shimada, Yoshiya and 今岡 達彦 and 西村 まゆみ and 柿沼 志津子 and 大町 康 and 西村 由希子 and 島田 義也},
 month = {Nov},
 note = {[Objective] Epidemiology on A-bomb survivors has clarified that childhood exposure confers high breast cancer risk. But the underlying mechanism is unclear. Using Sprague-Dawley rats, we attempted to verify if childhood exposure (1) results in increased development of mammary cancer and (2) affects susceptibility to a subsequent carcinogen. <br>
[Methods] Rats were (1) &gamma;-irradiated (2 Gy) or treated with methylnitrosourea (MNU; 20 mg/kg, i.p.) at 3 or 7 weeks (w) of age; or (2) irradiated at 3 w and then MNU-treated at either 3 or 7 w. They were observed until 50 w. <br>
[Results] (1) Spontaneous palpable mammary cancer multiplicity was 0.07 &plusmn; 0.04 (tumors/rat, mean &plusmn; SEM). Rats irradiated at 3 w developed significantly less tumors than those irradiated at 7 w (0.28 &plusmn; 0.11 vs. 0.90 &plusmn; 0.24, respectively). (2) Contrarily, rats treated with MNU at 3 w developed significantly more tumors than those treated at 7 w (0.56 &plusmn; 0.20 vs. 0.11 &plusmn; 0.08, respectively). After irradiation at 3 w, MNU treatment at 3 w resulted in an additive effect (0.76 &plusmn; 0.20), while treatment at 7 w elicited a synergistic effect (0.71 &plusmn; 0.18). <br>
[Discussion] Relatively low susceptibility to mammary cancers after prepubertal exposure in rats contradicts with epidemiologic data on A-bomb survivors. However, our data suggest that childhood exposure deposits a long-term effect that becomes apparent in combination with subsequent carcinogen exposure. The nature of this long-term effect remains to be elucidated., 第48回日本放射線影響学会／第1回アジア放射線研究会議},
 title = {Mammary carcinogenesis after prepubertal radiation exposure in rats: effect of age at exposure to gamma-rays, 1-methyl-1-nitrosourea and their combination},
 year = {2005}
}