Abstract: The disclosure details a novel method of use of the radioactive isotope Xenon-133 (Xe-133) in the radiotherapy of cancer. Xe-133 gas is infused directly into the core of the tumor and diffuses as microbubbles of gas toward the tumor boundaries where it is eventually taken up into patent capillaries and excreted by the lungs. During its infusion and diffusion throughout the tumor the Xe-133 exposes the cancerous tissue to very high fluxes of beta rays which penetrate tissue at about one millimeter increments. This limited penetration is very distructive to tissue while the Xe-133 gas is contained within the tumor. However, when the Xe-133 reaches the surface of the tumor, normal capillary blood flow carries it away and thereby minimizes exposure to normal tissues. Xenon-133 represents the only radioactive gas isotope which is suitable to practice the invention.