Azzi A, Ryangga D, Pawiro S A
MSc, Department of Physics, Faculty of Mathematics and Natural Sciences, University of Indonesia, Depok, West Java, 16424, Indonesia.
MSc, Department of Radiotherapy, Pasar Minggu Regional General Hospital, South Jakarta, Jakarta, 12550, Indonesia.
J Biomed Phys Eng. 2021 Feb 1;11(1):17-28. doi: 10.31661/jbpe.v0i0.2004-1096. eCollection 2021 Feb.
Many authors stated that cavities or air-gaps were the main challenge of dose calculation for head and neck with flattening filter medical linear accelerator (Linac) irradiation.
The study aimed to evaluate the effect of air-gap dose calculation on flattening-filter-free (FFF) small field irradiation.
In this comparative study, we did the experimental and Monte Carlo (MC) simulation to evaluate the presence of heterogeneities in radiotherapy. We simulated the dose distribution on virtual phantom and the patient's CT image to determine the air-gap effect of open small field and modulated photon beam, respectively. The dose ratio of air-gaps to tissue-equivalent was calculated both in Analytical Anisotropic Algorithm (AAA) and MC.
We found that the dose ratio of air to tissue-equivalent tends to decrease with a larger field size. This correlation was linear with a slope of -0.198±0.001 and -0.161±0.014 for both AAA and MC, respectively. On the other hand, the dose ratio below the air-gap was field size-dependent. The AAA to MC dose calculation as the impact of air-gap thickness and field size varied from 1.57% to 5.35% after the gap. Besides, patient's skin and oral cavity on head and neck case received a large dose discrepancy according to this study.
The dose air to tissue-equivalent ratio decreased with smaller air gaps and larger field sizes. Dose correction for AAA calculation of open small field size should be considered after small air-gaps. However, delivered beam from others gantry angle reduced this effect on clinical case.
许多作者指出，对于使用带有均整器的医用直线加速器（Linac）对头颈部进行照射时，腔隙或气隙是剂量计算的主要挑战。
本研究旨在评估气隙剂量计算对无均整器（FFF）小射野照射的影响。
在这项对比研究中，我们进行了实验和蒙特卡罗（MC）模拟，以评估放射治疗中不均匀性的存在情况。我们分别在虚拟体模和患者的CT图像上模拟剂量分布，以确定开放小射野和调制光子束的气隙效应。在解析各向异性算法（AAA）和MC中均计算了气隙与组织等效物的剂量比。
我们发现，气隙与组织等效物的剂量比倾向于随着射野尺寸增大而降低。对于AAA和MC，这种相关性均为线性，斜率分别为-0.198±0.001和-0.161±0.014。另一方面，气隙下方的剂量比取决于射野尺寸。气隙厚度和射野尺寸对AAA与MC剂量计算的影响在气隙之后从1.57%变化到5.35%。此外，根据本研究，头颈部病例中患者的皮肤和口腔接受的剂量存在较大差异。
气隙与组织等效物的剂量比随着气隙减小和射野尺寸增大而降低。对于开放小射野尺寸的AAA计算，在小气隙之后应考虑剂量校正。然而，来自其他机架角度的出射束减少了这种对临床病例的影响。
