Low-energy electron emitters for targeted radiotherapy of small tumours

The possibility of using electron emitters to cure a cancer with metastatic spread depends on the energy of the emitted electrons. Electrons with high energy will give a high, absorbed dose to large tumours, but the absorbed dose to small tumours or single tumour cells will be low, because the range of the electrons is too long. The fraction of energy absorbed within the t umour decreases with increasing electron energy and decreasing tumour size. For tumours smaller than 1 g, the tumour-to-normal-tissue mean absorbed do se-rate ratio, TND, will be low, e.g. for I-131 and Y-90, because of the hi gh energy of the emitted electrons. For radiotherapy of small tumours, radi onuclides emitting charged particles with short ranges (a few im) are requi red. A mathematical model was constructed to evaluate the relation between TND and electron energy, photon-to-electron energy ratio, p/e, and tumour s ize. Criteria for the selection of suitable radionuclides for the treatment of small tumours were defined based on the results of the TND model. In ad dition, the possibility of producing such radionuclides and their physical and chemical properties were evaluated. Based on the mathematical model, th e energy of the emitted electrons should be less than or equal to 40 keV fo r small tumours (< 1000 cells), and the photon-to-electron energy ratio, p/ e, should be <less than or equal to>2 to achieve a high TND. Using the sele ction criteria defined, five low-energy electron emitters were found to be suitable: Co-58m, Rh-103m, Sb-119, Ho-161, and Os-189m. All of these nuclid es decay by internal transition or electron capture, which yields conversio n and Auger electrons, and it should be possible to produce most of them in therapeutic amounts. The five low-energy electron-emitting radionuclides i dentified may be relevant in the radiation treatment of small tumours, espe cially if bound to internalizing radiopharmaceuticals.