Microdosimetry for Microbeam Radiation Therapy (MRT): theoretical calculations using the Monte Carlo toolkit

Radiation therapy is widely used in the treatment of very different types of cancer. Recent developments in this field are aiming at delivering high doses to the target volume while sparing the surrounding healthy tissues. The microbeam radiation therapy (MRT) is a new kind of radiotherapy which could be used for treating infantile brain tumors, as other kinds of radiotherapy would be extremely dangerous to the normal brain development. MRT is carried out using an array of parallel microbeams of synchrotron-wiggler-generated X-rays. In this work, Monte Carlo simulations using the Geant4 toolkit are carried out to estimate the dose deposition on a 20-cm-diameter, 20-cm-long cylindrical PMMA phantom, mimicking an infantile head. A set of physics processes is implemented in Geant4 to extend the range of validity of electromagnetic interactions down to 250 eV. The dose distribution in MRT is computed to prepare the treatment planning of preclinical trials. Primary photon histories are simulated for the different experimental setups, scoring the dose in cylindrical shells. We used cylindrical monoenergetic microbeams of 50, 100 and 150 keV and one microbeam with energies sampled from the measured spectrum at the ESRF ID17 beamline. The depth- and lateral-dose profiles have been studied, and for a few typical cases, the simulation results have been compared with those obtained with other codes.