CALCULATION OF THE STOPPING POWER RATIO WATER-TO- AIR FOR REFERENCE DOSIMETRY IN HADRONTHERAPY.

TESSARO, VERÓNICA; GALASSI, MARIEL ELISA; GERVAIS BENOIT ; MICAEL BEUVE

Santiago de Chile

Conferencia; 24th International Conference on Medical Physics 8th Latin American Congress of Medical Physics 2nd Chilean Congress of Medical Physics; 2019

Accuracy in the Stopping power (S) of liquid water and air is of fundamental interest in reference dosimetry for Hadrontherapy. In particle therapy, ionization chamber dosimetry requires the knowledge of the water-to-air stopping power ratio S W,Air . According to the International Atomic Energy Agency Technical Report TRS-398, the S W,Air for swift proton beams can be calculated using a semi-empirical law as a particle range function. However, due to the complexity of the particle-energy spectrum for heavier ions, the protocol recommends to take a constant value of 1.13 with an estimated uncertainty of 2%. In more recent publications, a parametrization of the water-to-air stopping power ratio is proposed, but it depends strongly on the ionization potentials of water and air used. Other authors propose an approach based on indirect experimental measurements.In the present work, Stopping power of proton, helium, carbon and oxygen ions in liquid water and air are calculated using a distorted-wave model to approximate the ionization cross sections required. Taking into account only the primary particle spectrum (at the entrance channel), the S W,Air can be approximated as the ratio of the stopping powers of water and air. The ratios of theoretical Stopping powers as a function of the incident energy and charge are compared with the obtainedusing the values recommended by ICRU Reports 49 (for proton and helium) and 73 (for carbon and oxygen ions). The dependence of the Stopping power ratios water-to-air with the charge and energy of the incident ions is analyzed. Results are compared with the proposed by TRS-398 and thecalculated by the semi-empirical approaches.