Comparison of the Antitumor Efficacy of Bismuth and Gadolinium as Dose-Enhancing Agents in Formulations for Photon Capture Therapy

The efficacy of photon capture therapy (PCT) is to a significant extent determined by the properties of the formulation containing the element efficiently absorbing external X-irradiation and operating as a dose-enhancing agent (DEA). We report here a comparison of the efficacies of bismuth and gadolinium as DEA in PCT technologies with X-irradiation at 110 kV for the treatment of superficial tumors. Bismuth and gadolinium are comparable in that they are available as the same chemical form – a complex with diethylenetriaminepentaacetic acid, for which the physicochemical properties are similar for both elements. Studies were performed on mice with transplanted B16F10 melanoma as a tumor model. Both DEAwere given by the intratumor route at the same dose of 5 mg of DEAper animal. Irradiation was with an x-ray apparatus with a tension of 110 kV using a dose of 20 Gy. The results showed significantly greater antitumor efficacy for PCT with both gadolinium and bismuth than with short-focus radiotherapy. In terms of the log dead cells (lgN), there was an increase in lgN from 0.78 for short-focus irradiation to 2.5 for PCT using gadolinium or bismuth, with identical dose exposure. There were no significant differences in the antitumor efficacies of PCT with bismuth and gadolinium for x-ray irradiation at 110 kV.