6 MV光子束放射治疗中发泡胶的剂量学影响-《中国医学物理学杂志》

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Title:: Dosimetric effects of styrofoam in 6 MV photon beam radiotherapy

文章编号:: 1005-202X（2021）04-0397-07

作者:: 李师; 罗焕丽; 谭霞; 蒙万里; 李博; 李广; 申正文; 钟明松; 王颖; 靳富; 重庆大学附属肿瘤医院肿瘤放射治疗中心，重庆 400030

Author(s):: LI Shi; LUO Huanli; TAN Xia; MENG Wanli; LI Bo; LI Guang; SHEN Zhengwen; ZHONG Mingsong; WANG Ying; JIN Fu; Radiation Oncology Center, Chongqing University Cancer Hospital, Chongqing 400030, China

关键词:: 发泡胶; 皮肤剂量; 固定装置; 剂量衰减; 放射治疗

Keywords:: Keywords: styrofoam skin dose immobilization device dose attenuation radiotherapy

分类号:: R815.6

DOI:: DOI:10.3969/j.issn.1005-202X.2021.04.001

文献标志码:: A

摘要:: 【摘要】目的：评估发泡胶在6 MV光子束放疗中的剂量学影响。方法：制作10块2 cm厚的发泡胶模体，获取CT影像、导入放疗计划系统（TPS），计算其平均线性衰减系数，并用电离室进行测量验证。在固体等效水上表面叠加上述模体成2、4、6、8、10 cm厚度，计算并测量不同厚度发泡胶的剂量学影响。随机选取8例由发泡胶辅助固定的鼻咽癌患者，制定9野均分调强计划，分别计算包含与不包含发泡胶的剂量分布。结果：TPS计算得到的平均线性衰减系数为0.16×10-2 cm-1。发泡胶厚度>4 cm时，γ通过率（1%/1 mm）<90%，且厚度越厚通过率越低。相较于剂量跌落区，发泡胶对剂量建成区的影响更大，仅加入2 cm发泡胶时，表面剂量由16.22%增加到63.73%。发泡胶使靶区PTVnd+nx、PTV1、PTV2的平均剂量分别从（72.61±0.98） Gy减少到（72.12±1.00） Gy、（69.11±0.79） Gy减少到（68.72±0.77） Gy、（62.61±1.04） Gy减少到（62.22±1.03） Gy，100%处方剂量靶区覆盖率分别从0.97%±0.01%降低到0.95%±0.03%、0.98%±0.01%降低到0.97%±0.01%、0.94%±0.04%降低到0.92%±0.05%，总剂量γ通过率（1%/1 mm）为94.06%±0.86%。结论：发泡胶厚度越厚对剂量扰动越大，并导致建成区和表面剂量的显著增加，其衰减作用降低了靶区的平均剂量和处方剂量覆盖率。在临床应用时建议发泡胶厚度不超过4 cm，并应将发泡胶勾画在外轮廓中以评估皮肤剂量是否安全。

Abstract:: Abstract: Objective To evaluate the dosimetric effects of styrofoam in 6 MV photon beam radiotherapy. Methods Ten pieces of styrofoam phantoms with 2 cm thick were made, and their CT images were imported into treatment planning system for calculating the average linear attenuation coefficient which was then verified by ionization chamber measurement. The styrofoam phantoms of 2, 4, 6, 8, 10 cm thick which were formed by superimposing one over one another was placed on the surface of a solid equivalent water phantom to estimate the dosimetric effects caused by Styrofoam of different thicknesses. Furthermore, the doses distributions of 9-field intensity-modulated radiotherapy plan for 8 nasopharyngeal cancer patients immobilized with Styrofoam were calculated in the case of body structure contour with styrofoam or not. Results The average linear attenuation coefficient calculated by treatment planning system was (0.16×10-2) cm-1. The gamma passing rate (1% /1 mm) was less than 90% when styrofoam thickness was larger than 4 cm, and the thicker the Styrofoam was, the lower the gamma passing rate was. Furthermore, styrofoam had larger effects on dose built-up area than dose-drop area, and the relative surface dose was increased from 16.22% to 63.73% when only a piece of Styrofoam of 2 cm thick was placed on the surface of a solid equivalent water phantom. Due to styrofoam, the mean doses of PTVnd+nx, PTV1 and PTV2 were reduced from (72.61±0.98) Gy to (72.12±1.00) Gy, (69.11±0.79) Gy to (68.72±0.77) Gy, (62.61±1.04) Gy to (62.22±1.03) Gy, respectively, and the corresponding 100% prescription dose coverage rate was decreased from 0.97%±0.01% to 0.95%±0.03%, 0.98%±0.01% to 0.97±0.01%, 0.94%±0.04% to 0.92%±0.05%, respectively. The gamma passing rate (1%/1 mm) of overall dose was 94.06%±0.86%. Conclusion The dosimetric effects is increasing with the increasing of styrofoam thickness, which led to a significant dose increase in dose built-up area and surface dose. The attenuation effects of styrofoam reduce both mean dose sand 100% prescription dose coverage rates at target areas. It is recommended that the thickness of styrofoam should no exceed 4 cm, and that styrofoam should be included in body structure contour to assess whether the skin dose is safe.

《中国医学物理学杂志》[ISSN:1005-202X/CN:44-1351/R]

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