Assessment of The Relation Between Energy Of Primary Protons And Undesired Neutron Dose During Proton Therapy By Monte Carlo Method

Introduction: High-energy beams of protons offer significant advantages for the treatment of deepseated local tumors. Their physical depth-dose distribution in tissue is characterized by a small entrance dose and a distinct maximum -Bragg peak- near the end of range with a sharp fall-off at the distal edge. Alongside its advantages there are some point that they need to meticulous attention. Producing dose due to secondary particles is one of the important challenges in proton therapy.
 
Materials and Methods: In the first stage the head was simulated by a cylindrical water phantom with length of 19cm and diameter of 19cm with 0.5 cm thickness of plexiglass. Then proton characteristics such as depth-dose distribution were investigated. In the next stage to evaluate the effect of variation of target density on depth-dose distribution, density of phantom materials varied. Increasing tissue density by 5% proton dose was decreased. Then a spherical tumor with diameter of 1cm in the phantom was considered and calculation of dose performed in the tumor and phantom. We have applied the MCNPX version of 2.6.0 code for proton beam energies ranging from 150 to 160 MeV, with steps of 1MeV, to obtain the ionization values, which are related to the cell damage or dose, in the target. MCNPX code is a general purpose radiation transport simulation code which is capable to simulate proton beams. This code requires an input file data that defines the geometry, the physical parameters and the tallies of the simulated problem.
 
Results: results have good agreement with results of TRIM package. Protons with 160 MeV energy have bragg peak in 17.1 cm and it has value of 1.237e-11 Gy/ source particles . we found that, for each 5 MeV increase for energy of protons, dose increase about 4.28%. after calculating dose for 11 steps between 150 and 160 MeV, proton by 153 MeV energy have the best dose distribution because it has maximum dose in tumor area and minimum dose for healthy tissue. Again simulation with TRIM code confirm this result. And also we found secondary neutron dose was found to be 100 orders lower than primary proton dose. Further providing evidence that secondary dose is relatively small in proton therapy.
 
Conclusion: Energy of entranced protons has significant effect on production of neutrons so it is helpful to use optimum energy for proton therapy especially for sensitive part of the body like brain, because neutron dose during proton therapy may increase the risk of metastases cancer.