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.Using MapCHECK 2 model 1177 and phantom (SP34). We studied the effect of compensator thickness and field size on the calculation of the effective attenuation coefficient (EAC) of the brass compensator for 6-MV photon beams.

Experimental measurements were carried out at 100 cm source-to-surface distance and 5 cm depth for the 6-MV photon beams of an Elekta  linac using various field size and compensator thickness. The field sizes investigated ranged from 1×1 cm² to 20×20 cm² and the brass compensator thickness from 0.5-6 cm. Our results indicated that the compensator thickness and field size have the most significant effect on the calculation of the compensator EAC for the 6-MV photon beam and also these parameters can reduce the error due to delivered dose to target volume and organs at risk.