Influence of compensator thickness, field size, and off-axis distance on the effective attenuation coefficient of a cerrobend compensator for intensity-modulated radiation therapy

Intensity-modulated radiation therapy (IMRT) can be performed by using compensators. To make a compensator for an IMRT practice, it is required to calculate the effective attenuation coefficient (μeff) of its material, which is affected by various factors. We studied the effect of the variation of the most important factors on the calculation of the μeff of the cerrobend compensator for 6-MV photon beams, including the field size, compensator thickness, and off-axis distance. Experimental measurements were carried out at 100 cm source-to-surface distance and 10 cm depth for the 6-MV photon beams of an Elekta linac using various field size, compensator thickness, and off-axis settings. The field sizes investigated ranged from 4 × 4 to 25 × 25 cm2 and the cerrobend compensator thicknesses from 0.5-6 cm. For a fixed compensator thickness, variation of the μeff with the field size ranged from 3.7-6.8%, with the highest value attributed to the largest compensator thickness. At the reference field size of 10 × 10 cm2, the μeff varied by 16.5% when the compensator thickness was increased from 0.5-6 cm. However, the variation of the μeff with the off-axis distance was only 0.99% at this field size, whereas for the largest field size, it was more significant. Our results indicated that the compensator thickness and field size have the most significant effect on the calculation of the compensator μeff for the 6-MV photon beam. Therefore, it is recommended to consider these parameters when calculating the compensator thickness for an IMRT practice designed for these beams. The off-axis distance had a significant effect on the calculation of the μeff only for the largest field size. Hence, it is recommended to consider the effect of this parameter only for field sizes larger than 25 × 25 cm2.