Monte Carlo simulation of prompt gamma-ray emission in proton therapy using a specific track length estimator

摘要

A Monte Carlo (MC) variance reduction technique is developed for prompt. emitters calculations in proton therapy. Prompt-gamma emitted through nuclear fragmentation reactions and exiting the patient during proton therapy could play an important role to help monitoring the treatment. However, the estimation of the number and the energy of emitted prompt-gamma per primary proton with MC simulations is a slow process. In order to estimate the local distribution of prompt-gamma emission in a volume of interest for a given proton beam of the treatment plan, a MC variance reduction technique based on a specific track length estimator (TLE) has been developed. First an elemental database of prompt-gamma emission spectra is established in the clinical energy range of incident protons for all elements in the composition of human tissues. This database of the prompt-gamma spectra is built offline with high statistics. Regarding the implementation of the prompt-gamma TLE MC tally, each proton deposits along its track the expectation of the prompt-gamma spectra from the database according to the proton kinetic energy and the local material composition. A detailed statistical study shows that the relative efficiency mainly depends on the geometrical distribution of the track length. Benchmarking of the proposed prompt-gamma TLE MC technique with respect to an analogous MC technique is carried out. A large relative efficiency gain is reported, ca. 10(5).