New Directions in Radionuclide Sources for Brachytherapy

Brachytherapy continues to play an important role in the management of cancers of several sites, including the head and neck, brain, uterine cervix, endometrium, and prostate. The physical advantages of brachytherapy result from a superior localization of dose to the tumor volume. In brachytherapy, as radiation is continuously delivered over a period of time, repair of sublethal and potentially lethal damage, proliferation, and other cell kinetic effects modify the response of tumor and normal tissues, resulting in complex dose rate effects that also influence the therapeutic ratio for brachytherapy. There are two different forms of brachytherapy. There are two different forms of brachytherapy: intracavitary irradiation using radioactive sources that are placed in body cavities in close proximity to the tumor, and interstitial brachytherapy using radioactive seeds implanted directly into tumor volume. Intracavitary radiation is always temporary and usually takes about 1 to 4 days. On the other hand, interstitial brachytherapy can be temporary or permanent. Radioactive sources emtting higher energy photons have better tissue penetration but are more difficult to shield for radiation protection purposes than sources emitting lower energy photons. Longer-lived radioactive sources can be used several times in different patients, which reduces their effective cost compared with shorter-lived radioisotopes. However, for permanent implants, shorter-lived radionuclides can provide higher initial dose rate, which may have biological advantages in some cases. These characteristics of various forms of brachytherapy dictate the choice of radioactive sources for a specific treatment. Recently, a number of new radionuclide sources have been developed, making a wide variety of half-lives and photon energies available to the brachytherapy community. Conventional sources for brachytherapy in the United States are cesium 137, iridium 192, iodine 125, and gold 198. The physical characteristics of these sources and of the newly developed sources, americium 241, palladium 103, samarium 145, and ytterbium 169, are described.