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

Artificial pulmonary metastases of a mouse fibrosarcoma were produced by the intravenous injection of 10⁴ cells admixed with 2 x 10⁶ plastic microspheres into mice preconditioned with 600 rad whole-body irradiation 24 hours earlier. Four days after injection of tumor cells, mice were irradiated with neutrons generated by 50 MeV deuterons on Be at the Texas A & M Variable Energy Cyclotron or with ¹³⁷Cs γ rays. One, three or six fractions of radiation were delivered on a three hour fractionation schedule. Surviving lung metastases were scored macroscopically 16 days after irradiation. The data indicate that: the RBE (n/γ) was in the range 1.6-2.6 depending on the size of dose per fraction; the slopes of the γ-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); and no additional sparing was achieved by further fractionating doses of neutrons of 300 rad or less.