The

Monte Carlo method is a computer tool used to solve problems through

statistical processes. Based on these basic principles, various teams and

laboratories around the world have developed Monte Carlo applications capable

of handling the problems of particle transport at different levels of accuracy.

In the wide range of Monte Carlo applications developed, the codes most

frequently used are: BeaMnrc, MCNP, PENELOPE and Geant4. Indeed GEANT4, like

most Monte Carlo simulation codes, is a category II code in the Berger

classification Berger, 1963; that is, the code simulates the transport of

charged particles using a multicast theory. This method condenses the

particle’s history by dividing it into several transport stages during which

the low energy loss and low deflection interactions are grouped together.

Geant4 is surely very powerful, but also much too complex. As part of its

research activities, the PCSV team worked extensively on the validation of the

GATE (Geant Application for Tomographic Emission) simulation platform as part of the OpenGATE1

collaboration. GATE is a simulation tool for medical physics applications based

on the generic GEANT4 toolkit originally developed for high energy physics. In

order to extend the qualities and benefits of GATE to dosimetry applications,

the OpenGATE collaboration has set up a working group to specifically study the

capabilities of the simulation platform for dosimetry.

In this

context, we try to exploit the capabilities of the Monte Carlo Geant4/Gate code

to simulate the physical phenomena generated by the Varian Clinac. Which a

precise modeling of a 6 MV beam delivered by the treatment head was carried out

using the latest version (Gate v. 8.0) of Monte Carlo Geant4/Gate by adjusting

all the parameters related to 6 MeV electrons emitted by a virtual source

accelerated and deflected before reaching the target. The simulated beam of

mega-photons was used to calculate the dosimetric functions in a homogeneous

water phantom. These functions as a percentage of depth dose (PDD) and

cross-beam profile were compared to those measured using the gamma index

comparison method. The tolerance value assigned to the relative dose was set at

2% and the tolerance value for the measured positions was considered to be 1mm

. Working on our new Super Computer , the parallel calculation method has been

chosen in a targeted way to reduce a significant CPU time that will probably be

consumed by a Monte Carlo simulation of all the physical phenomena involved by

the particles traveling through the complex geometries of a Clinac processing

head.