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ORIGINAL ARTICLE
Year : 2014  |  Volume : 10  |  Issue : 4  |  Page : 896-902

A Monte Carlo approach to lung dose calculation in small fields used in intensity modulated radiation therapy and stereotactic body radiation therapy


1 Department of Medical Physics, Medical School, Tabriz University of Medical Sciences; Department of Radiation Oncology, Imam Hospital, Tabriz, Iran
2 Department of Medical Physics, Medical School, Tabriz University of Medical Sciences, Tabriz, Iran
3 Department of Radiology, Paramedical School, Tabriz University of Medical Sciences, Tabriz, Iran
4 Department of Radiation Oncology, Imam Hospital, Tabriz, Iran

Correspondence Address:
Asghar Mesbahi
Department of Medical Physics, Medical School, Tabriz University of Medical Sciences, Attare Neishabouri Street, Tabriz
Iran
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Source of Support: None, Conflict of Interest: None


DOI: 10.4103/0973-1482.137989

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Aims: In the current study, the effect of electronic disequilibrium on lung dose with small photon beams was verified. Materials and Methods: The central axis absorbed dose in lung phantom was calculated by Monte Carlo (MC) method. The 6 and 18 MV photon beams of Varian Clinac 2100EX were simulated using MCNPX MC Code (Los Alamos national lab, USA). The MC model was used to calculate the depth doses water and low density water resembling the soft-tissue and lung, respectively. Four small field sizes including 0.5 cm 2 × 0.5 cm 2 , 1 cm 2 × 1 cm 2 , 2 cm 2 × 2 cm 2 , and 3 cm 2 × 3 cm 2 were used in this study. Results: Percentage of dose reduction in lung region relative to homogenous phantom for 6 MV photon beam were 44.6%, 39%, 13%, and 7% for 0.5 cm 2 × 0.5 cm 2 , 1 cm 2 × 1 cm 2 , 2 cm 2 × 2 cm 2 , and 3 cm 2 × 3 cm 2 fields, respectively. For 18 MV photon beam, the results were found to be 82%, 69%, 46%, and 25.8% for the same field sizes, respectively. The solid tumor dose inside lung was reduced considerably between 17% and 35% for 18 MV beam, while there was only 9% dose reduction for tumor dose for 0.5 and 1 cm field sizes. Conclusion: Our study showed that the dose reduction with small fields in the lung was very enormous. Thus, inaccurate prediction of absorbed dose inside lung and also lung soft-tissue interfaces with small photon beams may lead to critical consequences for treatment outcome.

Abstract in Chinese

蒙特卡罗法计算小野调强放疗和立体定向放疗的肺剂量 摘要 目的:验证小光子束电子不平衡对肺剂量的影响。 材料与方法:中心轴在肺部吸收剂量是通过Monte Carlo计算模型(MC)方法算出的。瓦里安医用直线加速器2100EX使用MCNPX MC代码模拟产生了6MV和18 MV光子束(洛斯阿拉莫斯国家实验室,美国)。MC模型被用来分别计算水、低密度似水软组织和肺的深度剂量。研究中使用四小野,尺寸包括0.5 cm2 × 0.5 cm2, 1 cm2 × 1 cm2, 2 cm2 × 2 cm2, 3 cm2 × 3 cm2。 结果:肺区域相对于同质的虚构区域,剂量降低百分比分别为:6 MV光子束为44.6%、39%、13%和7%,对应野大小分别为0.5 cm2 × 0.5 cm2, 1 cm2 × 1 cm2, 2 cm2 × 2 cm2 和3 cm2 × 3 cm2。18 MV光子束,结果是82%、69%、46%和25.8%,对应相同野。肺内实体瘤剂量大大减少,18 MV光束为17%-35%之间,而0.5和1cm野的剂量减少仅为9%。 结论:在肺内小野照射的剂量减少是非常多的。因此,对于小光子束在肺内和肺软组织间吸收剂量的不准确预测,可能导致危险的治疗结果。 关键词:电子不平衡,蒙特卡罗计算,放射治疗肺癌,小场剂量学



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