An assessment of the factors involved in effective attenuation coefficient of the compensator material for the treatment with 6MV photons using intensity modulated radiation therapy method

One of the intensity modulated radiation therapy (IMRT) methods is based on using compensators. The most important factor in designing a compensator is the accurate calculation of its thickness to achieve the intensity modulation of interest. To achieve that, the exact attenuation coefficient of compensator materials must be calculated. However, there are several parameters that are effective in calculating the attenuation coefficient of compensator materials. In this research, the effects of dosimeter and phantom type as well as irradiation dose and measurement depth in the calculation of this compensator characteristic were assessed.

Using two types of dosimeters (RK & FC65G) and phantoms (RFA300plus & SP34), the effects of radiation dose and measured depth on the estimation of the effective attenuation coefficient was investigated for a 6MV linear accelerator. The value of applied radiation dose was 100, 200, 300 and 400 cGy, and the measured depths were 2, 5, 10, 15 and 20 cm. The measurements were carried out at the reference field size (10×10 cm2) and for a thickness of 1 cm of the compensator.

The results indicated that radiation dose has no significant effect in calculating the effective attenuation coefficient of compensator materials. However, altering measured depth from 2 to 20 cm resulted in a change of more than 5% in the calculations. In addition, the type of the dosimeter and phantom used in this study had no significant effect on the calculations.

Based on these findings, it is recommended that for more accurate estimation of the effective attenuation coefficient of a compensator material, it is necessary to measure the attenuation coefficient at different depths of the treatment field.