RBE of neutrons generated by 50 MeV deuterons on beryllium for control of artificial pulmonary metastases of a mouse fibrosarcoma

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Abstract
Artificial pulmonary metastases of a mouse fibrosarcoma were produced by the i.v. injection of 104 cells admixed with 2 .times. 106 plastic microspheres into mice preconditioned with 600 rad whole-body irradiation 24 h earlier. Four days after injection of tumor cells, mice were irradiated with neutrons generated by 50 MeV deuterons on Be at the Texas A and M Variable Energy Cyclotron or with 137Cs .gamma.-rays. One, 3 or 6 fractions of radiation were delivered on a 3 h fractionation schedule. Surviving lung metastases were scored macroscopically 16 days after irradiation. The data indicate that: the RBE [relative biological effectiveness] (n/.gamma.) was in the range 1.6-2.6 depending on the size of dose per fraction; the slopes of the .gamma.-ray curves decreased with increasing fraction number (i.e., decreasing fraction size); the slopes of the neutron curves decreased only slightly with increasing fraction number (and decreasing fraction size); no additional sparing was achieved by further fractionating doses of neutrons of 300 rad or less.