Haghparast Abbas, Hashemi Bijan, Eivazi Mohammad Taghi
Department of Medical Physics, Faculty of Medical Sciences, Tarbiat Modares University, Tehran, Islamic Republic of Iran.
Med Dosim. 2013 Spring;38(1):25-9. doi: 10.1016/j.meddos.2012.06.001. Epub 2012 Jul 25.
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.
调强放射治疗（IMRT）可通过使用补偿器来实现。为制作用于IMRT实践的补偿器，需要计算其材料的有效衰减系数（μ(eff)），而该系数会受到多种因素的影响。我们研究了最重要的因素变化对用于6兆伏光子束的铈钨补偿器μ(eff)计算的影响，这些因素包括射野大小、补偿器厚度和离轴距离。使用Elekta直线加速器的6兆伏光子束，在源皮距100厘米和深度10厘米处，针对各种射野大小、补偿器厚度和离轴设置进行了实验测量。所研究的射野大小范围为4×4至25×25平方厘米，铈钨补偿器厚度范围为0.5 - 6厘米。对于固定的补偿器厚度，μ(eff)随射野大小的变化范围为3.7 - 6.8%，最高值出现在最大补偿器厚度时。在10×10平方厘米这个参考射野大小下，当补偿器厚度从0.5厘米增加到6厘米时，μ(eff)变化了16.5%。然而，在这个射野大小下，μ(eff)随离轴距离的变化仅为0.99%，而对于最大射野大小，这种变化更为显著。我们的结果表明，补偿器厚度和射野大小对6兆伏光子束补偿器μ(eff)的计算影响最为显著。因此，在为这些光束设计的IMRT实践中计算补偿器厚度时，建议考虑这些参数。离轴距离仅对最大射野大小的μ(eff)计算有显著影响。因此，建议仅在射野大小大于25×25平方厘米时考虑该参数的影响。
