The dependence of iododeoxyuridine (IUdR) radiosensitization on photon energy and dose rate in the range of interest to brachytherapy was investigated by irradiating Chinese hamster cells in vitro under aerobic conditions. The radiosensitization produced by 10-5 and 10-4 M IUdR for 28-keV (average) photons from125 I, 60-keV photons from241 Am, and 830-keV (average) photons from226 Ra was measured at nominal dose rates of 0.17, 0.30, 0.57, and 0.73 Gy/h. Radiosensitization factors for IUdR were essentially independent of dose rate from 0.30 to 0.73 Gy/h for all cases except for 10-4 M IUdR plus241 Am, in which case the radiosensitization factor increased from 2.5 ± 0.2 to 3.0 ± 0.1. In all cases, the radiosensitization factor decreased significantly as the dose rate was lowered from 0.30 to 0.17 Gy/h, e.g., the radiosensitization factor for241 Am dropped to 1.9 ± 0.2 at a dose rate of 0.17 Gy/h. Moreover, at 0.17 Gy/h the radiosensitization factors were essentially the same for all three photon energies. As the dose rate increased from 0.17 to 0.73 Gy/h, the difference between the radiosensitization factors for the three photon energies became larger; radiosensitization factors for241 Am were higher than those for226 Ra and125 I. In temporary brachytherapy the tumor is irradiated at the higher dose rate of about 0.50-0.70 Gy/h, while the normal tissues are irradiated at lower dose rates; the dose rate dependence of the radiosensitization factor may therefore lead to an improvement in the therapeutic ratio for brachytherapy in combination with IUdR.

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