Radiation interaction parameters of dosimetric importance for some commonly used compensators in IMRT using Monte Carlo simulation code.

Radiotherapy is one of the commonly used treatments for cancer. A compensator is a device used in intensity modulated radiotherapy (IMRT) treatments to compensate for the irregularities of the body. One can provide a more uniform distribution of dose for the treatment of tumors using compensators in IMRT to obtain a better therapeutic outcome. A variety of compensators are used in IMRT. One high Z (cerrobend), two medium Z (brass and steel) and one low Z (lucite) compensators are selected for the present study. The mass attenuation coefficients have been computed in an energy range of 15 keV to 15 MeV using XCOM program and the Monte Carlo N-particle-5 (MCNP5) code. The percentage deviation between the values indicates that the mass attenuation coefficients obtained using both the methods are in good agreement. These values are further used to calculate the linear attenuation coefficients, mass energy absorption coefficients, effective atomic numbers, electron densities, equivalent atomic numbers and energy absorption and exposure buildup factors. All these parameters have been calculated in the energy range of 0.015-15.0 MeV. The variation of parameters has been studied with incident photon energies, chemical composition and penetration depth of the various compensators. It is observed that up to 3 MeV and 14 mean free path (mfp): compensator with lowest values of the effective atomic numbers has the highest value of the buildup factor. Cerrobend has a particularly high value of the buildup factor near 100 keV. Above 3 MeV and 14 mfp: compensator with lowest values of the effective atomic numbers has the lowest value of the buildup factor. Thus a complete reversal in trend is observed at 3 MeV and 14 mfp. All these observation have been explained due to the dominance of the different interaction processes in different energy regions.