Inclusion of compensator-induced scatter and beam filtration in pencil beam dose calculations

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 (μ(eff)) 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 μ(eff) 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 μ(eff) 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 μ(eff) varied by 16.5% when the compensator thickness was increased from 0.5-6 cm. However, the variation of the μ(eff) 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 μ(eff) 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 μ(eff) 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.

Commissioning of modulator-based IMRT with XiO treatment planning system

This article describes the procedures for correction of the modulator thickness and commissioning of the XiO treatment planning system (TPS) for modulator-based intensity modulated radiation therapy (M-IMRT). This modulator manufacturing system adopts a method in which the modulator is milled using a floor-type computer-aided numerical control milling machine (CNC-mill) with modulator data calculated by XiO TPS. XiO TPS uses only effective attenuation coefficients (EAC) for modulator thickness calculation. This article describes a modified method for assessing modulator thickness. A two-dimensional linear attenuation array was used to correct the modulator thickness calculated by XiO. Narrow-beam geometry was used for measuring the linear attenuation coefficient (LAC) at off-axis positions (OAP) for varying brass thicknesses. An equation for the two-dimensional LAC ratio (2D-LACR) can be used to calculate the corrected modulator thickness. It is assumed that the broad beam EAC of a small field varies with the brass thickness and the OAP distance in the same way as that of LACR, so the two-dimensional EAC (2D-EAC) is equal to the EAC corrected using the LACR. The dose distribution was evaluated for three geometric patterns and one clinical case on low energy x ray (4 MV) with a large field size (20 x 20 cm2). The results using the proposed correction method of modulator thickness showed a good agreement between the measured dose distributions and the dose distributions calculated by TPS with the correction. Hence, the method is effective to improve the accuracy of M-IMRT in XiO TPS. An important problem for the brass modulator is the milling condition, such as the drill diameter and the cutting pitch size. It is necessary to improve the accuracy of M-IMRT for the "softening" and "hardening" effects of the beam to be considered in dose calculation in patients and the modulator profile design.