Dosimetrie Framework for Therapeutic

Alpha-particle emitters are currently being considered for the treat ment of metastatic disease. However, the dosimetry of alpha-particle emitters is a challenge because the dimensions of subcel lular targets (e.g., the cell nucleus) are of the same order of magnitude as the range of the particle. Hence, a single dose value is often not representative of the dose delivered to a cell population. Here, we propose a dosimetry system that combines the calcula-tional ease of the Medical Internal Radiation Dosimetry (MIRD) system with the additional information provided by microdosimetry. Methods: Beginningwith the microdosimetric,single-event specif ic-energy spectrum, we derived expressions for the first and second moments. Using the MIRD S-factor along with these moments, we determined not only the mean absorbed dose but also the s.d. and the fraction of cells receiving zero (or any number of) hits. Results: Using the formalism developed here, we have generated tables for a simple example to demonstrate the use of this method. Conclu sion: We have developed a formalism for the rapid determination of not only the mean absorbed dose but also the s.d. and fraction of cells receiving zero hits. These parameters are potentially useful in analyzing the biological outcome for cells exposed to alpha-particle irradiation. Key Words: alpha-particle emitters; dosimetry; microdosimetry; radiation therap