Therac-25 X AECL 1983 11 1985 1987 6 (1) Therac-25 AECL 6MeV X Therac-6 X 20MeV Therac-20 Therac-20 Therac-6 Therac-25 Therac-20 Therac-25 Therac-25 1983 1985 2 1985 6 7 12 1986 3 4 1987 1 1 (2) Therac-25 Therac-20 Therac-20 AECL Therac-20 Therac-20 nuisance Therac-20 Therac-25 (3) Therac-25 x X 8 8 8 Therac-25 Therac-20 Therac-20 Therac-25 (4) x xTherac-25 8 X 54 Therac-25 (5) AECL 1985 11 FDA AE