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WJCR

World Journal of Cancer Research

2164-9049

Scientific Research Publishing

10.12677/WJCR.2022.123017

WJCR-53367

WJCR20220300000_80116399.pdf

医药卫生

非均整模式下6 MV与10 MV能量X线对直肠癌调强放疗影响 Effect of 6 MV and 10 MV Flattening Filter-Free X-Ray Beams on Intensity Modulated Radiotherapy for Rectal Cancer

彭

圣贤

² ¹ 曹

俊逸

² ¹ 刘

悦

² ¹

自贡市第一人民医院放疗室，四川自贡

null

16 05 2022

12 03 124 130

2014

This work is licensed under the Creative Commons Attribution International License (CC BY). http://creativecommons.org/licenses/by/4.0/

目的：探讨非均整(flattening filter-free, FFF)模式下6 MV X线与10 MV X线对直肠癌容积旋转调强放疗(volumetric modulated arc therapy, VMAT)计划的影响，为直肠癌放疗射线质的选取提供参考。方法：选取20例直肠癌术后患者做回顾性分析，在Eclipse 13.6计划系统上分别设计FFF-6 MV的VMAT计划和FFF-10 MV的VMAT计划。比较两种计划的剂量体积直方图、靶区的适形指数(CI)、均匀性指数(HI)和危及器官膀胱、左右股骨头、小肠、正常组织(B-P)的剂量学参数以及剂量验证通过率、机器跳数、治疗时间的差异。结果：6 MV X线与10 MV X线计划的D2和Dmean差异有统计学意义(P < 0.05)。D98和靶区覆盖率无统计学差异(P > 0.05)，6 MV计划组CI和HI优于10 MV计划组(P < 0.05)。危及器官方面，膀胱Dmean无显著差异(P > 0.05)，V50结果为6 MV小于10 MV计划(P < 0.05)；小肠Dmax，左右股骨头Dmean和V40均无统计学差异(P > 0.05)，正常组织V2和V5为10 MV低于6 MV计划(P < 0.05)。剂量验证通过率和机器跳数、治疗时间均是6 MV计划小于10 MV计划(P < 0.05)。结论：两种计划均能满足临床要求，考虑到大部分危及器官受量无显著差异，且FFF模式下6 M X能量下靶区的适形指数(CI)、均匀性指数(HI)优于10 MV X线，且机器跳数更少，建议推荐FFF模式下用6 MV X能量制定直肠癌VMAT计划。 Objective: To explore the influence of 6 MV X-ray and 10 MV X-ray on the VMAT plan of rectal cancer in flattening filter-free (FFF) mode, and to provide a reference for the selection of radiation quality for rectal cancer radiotherapy. Methods: Twenty patients with rectal cancer after surgery were selected for retrospective analysis, and the VMAT plan of FFF-6 MV and the VMAT plan of FFF-10 MV were designed on the Eclipse 13.6 planning system. The dose volume histograms of the two plans, the conformity index (CI) of the target area, the homogeneous index (HI) and the dosimetry parameters of the bladder, left and right femoral heads, small intestine, and normal tissues (B-P), and the dose verification passed Rate, Monitor unit count, treatment time difference were compared. Results: The D2 and Dmean differences between 6 MV X-ray and 10 MV X-ray plans were statistically significant (P < 0.05). There was no statistical difference between D98 and target volume coverage (P > 0.05). CI and HI in the 6 MV plan group were better than those in the 10 MV plan group (P < 0.05). In terms of organs at risk, there was no significant difference in bladder Dmean (P > 0.05), the result of V50 of 6 MV was less than 10 MV plan (P < 0.05), small intestine Dmax, left and right femoral head Dmean and V40 were not statistically different (P > 0.05), normal tissue V2 and V5 were 10 MV Lower than the 6 MV plan (P < 0.05). The passing rate of dose verification, the number of monitor unit, and the treatment time were all in the 6 MV plan less than the 10 MV plan (P < 0.05). Conclusion: Both plans could meet the clinical requirements, considering that most of the organs at risk have no significant difference, and the conformity index (CI) and uniformity index (HI) of the target area at 6 M X energy in FFF mode are better than 10 MV X-ray, and the number of monitor units is less, it is recommended to use 6 MV X energy to develop a rectal cancer VMAT plan in FFF mode.

直肠癌，固定野调强，非均整，剂量学，10 MV, Rectal Cancer Fixed Field Intensity Modulation Radiotherapy Flattening Filter-Free Dosimetry 10 MV

摘要

关键词

直肠癌，固定野调强，非均整，剂量学，10 MV

Effect of 6 MV and 10 MV Flattening Filter-Free X-Ray Beams on Intensity Modulated Radiotherapy for Rectal Cancer<sup> </sup>

Shengxian Peng, Junyi Cao, Yue Liu

Zigong First People’s Hospital, Zigong Sichuan

Received: Jun. 14^th, 2022; accepted: Jun. 26^th, 2022; published: Jul. 6^th, 2022

ABSTRACT

Objective: To explore the influence of 6 MV X-ray and 10 MV X-ray on the VMAT plan of rectal cancer in flattening filter-free (FFF) mode, and to provide a reference for the selection of radiation quality for rectal cancer radiotherapy. Methods: Twenty patients with rectal cancer after surgery were selected for retrospective analysis, and the VMAT plan of FFF-6 MV and the VMAT plan of FFF-10 MV were designed on the Eclipse 13.6 planning system. The dose volume histograms of the two plans, the conformity index (CI) of the target area, the homogeneous index (HI) and the dosimetry parameters of the bladder, left and right femoral heads, small intestine, and normal tissues (B-P), and the dose verification passed Rate, Monitor unit count, treatment time difference were compared. Results: The D₂ and D_mean differences between 6 MV X-ray and 10 MV X-ray plans were statistically significant (P < 0.05). There was no statistical difference between D₉₈ and target volume coverage (P > 0.05). CI and HI in the 6 MV plan group were better than those in the 10 MV plan group (P < 0.05). In terms of organs at risk, there was no significant difference in bladder D_mean (P > 0.05), the result of V₅₀ of 6 MV was less than 10 MV plan (P < 0.05), small intestine D_max, left and right femoral head D_mean and V₄₀ were not statistically different (P > 0.05), normal tissue V₂ and V₅ were 10 MV Lower than the 6 MV plan (P < 0.05). The passing rate of dose verification, the number of monitor unit, and the treatment time were all in the 6 MV plan less than the 10 MV plan (P < 0.05). Conclusion: Both plans could meet the clinical requirements, considering that most of the organs at risk have no significant difference, and the conformity index (CI) and uniformity index (HI) of the target area at 6 M X energy in FFF mode are better than 10 MV X-ray, and the number of monitor units is less, it is recommended to use 6 MV X energy to develop a rectal cancer VMAT plan in FFF mode.

Keywords:Rectal Cancer, Fixed Field Intensity Modulation Radiotherapy, Flattening Filter-Free, Dosimetry, 10 MV

This work is licensed under the Creative Commons Attribution International License (CC BY 4.0).

http://creativecommons.org/licenses/by/4.0/

1. 引言

直肠癌是我国常见的恶性肿瘤之一 [ 1 ]，在消化道肿瘤的发病率中高居第2位，近年来其发病率呈上升趋势。放射治疗技术作为直肠癌的一种重要的治疗方式，目前已越来越广泛地应用于临床 [ 2 ] [ 3 ] [ 4 ]。容积旋转调强放疗(Volumetric Modulated Arc Therapy, VMAT)技术作为调强放疗(IMRT)技术的延伸 [ 5 ]，能够在机架旋转的过程中出束，因而VMAT技术在靶区适形度和均匀性，危及器官受量、机器调数、治疗时间等方面更有优势 [ 6 ]。随着调强放疗技术的发展，非均整(Flattening Filter Free, FFF)模式与传统的均整(Flattening Filter, FF)模式相比，FFF模式的光子线照射剂量率更高、散射更少、漏射辐射更低，因此近年来成为研究的热点 [ 7 ] [ 8 ] [ 9 ]。关于医用直线加速器的FFF模式的剂量学特性研究已有不少的报道 [ 8 ] [ 9 ]，相关研究说明相比于FF模式光子能谱，FFF模式下能谱“软化” [ 8 ] [ 9 ]。FFF模式下高能量与低能量X线放疗物理剂量方面的参数特点少见相关报道。基于此，本研究以直肠癌VMAT计划为例，分析FFF模式下6 MV与10 MV的X线剂量学特点。

2. 材料和方法 2.1. 临床资料

随机选取既往在本院肿瘤科治疗的20例直肠癌术后患者，男女各10例，年龄45~78岁(中位年龄63岁)。20例患者未见有放疗并发症。肿瘤原发灶均经病理证实。KPS评分均在70分以上，没有放疗禁忌症。筛选数据经医院伦理委员会审批。纳入和排除标准：1) 纳入标准。① 有完整的临床病理资料；② 术前病理活检结果为直肠癌；③ 未发现有远处转移。2) 排除标准。有放射治疗禁忌证。

2.2. CT模拟定位

使用热塑膜及专用碳纤维板和枕头固定患者，虽然有临床研究表明 [ 10 ]，采用俯卧位方式对正常组织保护优于仰卧位，但考虑对摆位的重复性和精确度要求较高，一些患者术后伤口疼痛，仰卧的舒适性更好等因素，故本研究统一取仰卧位，在GE Light Speed 64排螺旋CT模拟定位机下增强扫描，CT扫描层厚5 mm，扫描范围为腰3椎体下缘到闭孔下缘。获得CT影像后，通过专用网络传输至Varian Eclipse放疗计划工作站，由高年资医生进行靶区和危及器官的勾画，危及器官限量包括小肠、膀胱、双侧股骨头。

2.3. 计划设计

基于Eclipse v13.6计划系统和Varian True Beam加速器，对每个患者制定两组放疗计划，两组计划分别使用FFF模式下能量为6 MV X线和10 MV X线。两组VMAT计划均采用两个全弧(顺时针181˚~179˚，逆时针179˚~181˚)照射，准直器角度5˚，网格分辨率大小2.5 mm，剂量率为1400 MU/min，60对MLC。处方剂量均为50.4 Gy/28 F。为了便于评价计划，两组计划靶区覆盖率归一到50.4 G覆盖95% (≥95%)的靶区体积。靶区剂量最大值高于110%的处方剂量体积不超过1%。

2.4. 计划评估

根据剂量–体积直方图(Dose-Volume Histogram, DVH)来评价靶区和危及器官的剂量学参数，参考ICRU83号报告 [ 11 ]。靶区的剂量学参数：靶区覆盖率(处方剂量包绕的靶体积与靶体积之比)，D₂、D₉₈、平均剂量D_mean，其中D₂、D₉₈分别为包围靶区体积2%、98%的最小剂量；均匀性指数定义为式(1)：

H I = ( D 2 − D 98 ) / D 50 (1)

其中D₅₀为包围靶区体积50%的最小剂量，HI值越接近0，表明靶区的均匀性越好。

适形度指数定义为式(2)：

C I = ( V T , r e f / V T ) × ( V T , r e f / V r e f ) (2)

其中 V T , r e f 为处方剂量所覆盖的靶区体积，V_T为靶区体积，V_ref为处方剂量所覆盖的总体积，CI值越接近于1，说明靶区的适形度越好。各危及器官的剂量参数为膀胱D_mean、V₅₀，左右股骨头D_mean、D₄₀，小肠D_max，正常组织(Body减去PTV) V₂、V₅。此外，评估各计划的机器跳数(Monitor Unit, MU)。

采用SunNuclear公司的Map Check I，以3%/3 mm的误差标准对所有计划进行二维剂量验证，分析其gamma通过率。

2.5. 统计学方法

所有数据采用SPSS 22.0软件进行统计处理，数据以均值 ± 标准差表示，经检验数据服从正态分布，采用配对t检验分析两组计划，P < 0.05为差异有统计学意义。

3. 结果 3.1. 靶区剂量学参数比较

表1所示是FFF模式下6 MV与10 MV两组计划靶区剂量学参数比较结果。由表1看出，两种能量下靶区D₂和D_mean差异有统计学意义(P < 0.05)。D₉₈和靶区覆盖率无统计学意义(P > 0.05)。6 MV能量的计划组CI和HI优于10 MV计划组(P < 0.05)。图1是6 MV与10 MV结果的DVH图对比，从图中可以看出，6 MV的均匀性好于10 MV，正常组织受量6 MV计划高于10 MV计划结果。

Table 1 Comparison of dosimetric parameters of target volume ( x ¯ ± s

	PTV
组别	D₂/cGy	D₉₈/cGy	D_mean/cGy	CI	HI	靶区覆盖率/%
6 MV	5193.27 ± 57.31	4977.76 ± 68.11	5222.56 ± 20.22	0.818 ± 0.028	0.091 ± 0.016	95.01 ± 0.21
10 MV	5260.83 ± 61.27	4950.89 ± 119.92	5250.80 ± 18.79	0.813 ± 0.027	0.097 ± 0.029	95.00 ± 0.22
P	0.002	0.089	0.008	0.034	0.003	0.896

表1. 靶区剂量学参数比较( x ¯ ± s )

图1. 6 MV计划与10 MV计划DVH对比

3.2. 危及器官剂量学参数比较

表2为两组计划危及器官剂量学结果。两组计划膀胱D_mean无统计学差异(P > 0.05)、V₅₀结果为6 MV小于10 MV计划组(P < 0.05)、左右股骨头的D_mean和V₄₀均无显著差异(P > 0.05)。小肠的D_max无显著差异(P > 0.05)。正常组织V₂、V₅结果为10 MV计划组低于6 MV计划组(P < 0.05)。

Table 2 Dosimetric parameters of organs at risk ( x ¯ ± s

	OAR
组别	膀胱		左股骨头		右股骨头		小肠	正常组织
	D_mean/Gy	V₅₀/%	D_meant/Gy	V₄₀/%	D_meant/Gy	V₄₀/%	D_max/Gy	V₂/%	V₅/%
6 MV	41.54 ± 1.91	22.62 ± 4.14	22.62 ± 4.14	0.82 ± 1.43	22.48 ± 2.14	0.62 ± 1.44	5352.62 ± 44.14	61.62 ± 5.31	51.79 ± 9.17
10 MV	41.26 ± 1.93	24.45 ± 5.32	21.88 ± 3.77	0.79 ± 1.71	21.71 ± 3.12	0.58 ± 1.61	5348.33 ± 50.14	56.31 ± 5.45	49.31 ± 8.05
P	0.126	0.010	2.122	0.460	2.090	0.363	0.229	0.009	0.008

表2. 危及器官剂量学参数 ( x ¯ ± s )

3.3. 剂量验证通过率、机器跳数、治疗时间比较

表3所示为两组计划剂量验证通过率、机器跳数、治疗时间比较，剂量验证通过率结果10 MV高于6 MV (P < 0.05)，机器跳数和治疗时间结果都为6 MV计划组小于10 MV计划组(P < 0.05)。

Table 3 Comparison of pass rate of dose verification, number of machine hops, and treatment time comparison of target dosimetry parameter

组别	剂量验证通过率/%	机器跳数/MU	治疗时间/S
6 MV	98.27 ± 0.81	617.7 ± 38.1	70.5 ± 5.2
10 MV	98.57 ± 0.65	637.9 ± 44.9	71.8 ± 5.9
P	0.012	0.009	0.008

表3. 剂量验证通过率、机器跳数、治疗时间比较靶区剂量学参数比较

4. 讨论

目前的研究已有FFF模式治疗直肠癌可行性的报道 [ 12 ] [ 13 ]，大多是基于6 MV X线治疗的研究，6 MV能量与10 MV能量的X线能谱不同，FFF模式下10 MV的能量注量和在水组织不同深度的profile分布不均匀性都比FFF-6MV更明显 [ 8 ]，这说明FFF模式下6 MV能量的X线与10 MV能量的X线理论的物理特性有很大差别，因此对于FFF模式下10 MV与6 MV X射线治疗肿瘤靶区和危及器官的物理剂量参数问题值得探讨。本研究比较了FFF模式下10 MV与6 MV治疗直肠癌的剂量学差异。研究结果表明，FFF模式下10 MV与6 MV两种能量下计划的靶区剂量参数均满足临床要求且无统计学差异，部分危及器官、正常组织受量、剂量验证通过率和治疗时间等方面存在差异。

近年来，国内外关于FFF模式相关报道有很多 [ 14 ] [ 15 ] [ 16 ]，FFF模式的最大优势在于增加的剂量率能够达到1400 MU/min。与FF模式计划相比，6~10 MV的FFF模式计划，距离射野边缘20 cm处的剂量减少23%~31% [ 14 ]。很多报道对FFF模式下加速器束流物理特性的影响进行了研究和FF模式的特性进行了总结，诸如深度剂量变浅、皮肤剂量略微提高、离轴剂量跌落较快等特点。但是关于FFF模式下不同能量下的剂量特性报道较少。早期，Ost等 [ 17 ] 报道了高能与低能X线IMRT和VMAT治疗前列腺癌，高能X线无优势，剂量分布无统计学差异。这与本研究结论基本一致，本研究的结果显示CI与HI高能X线治疗无优势。两种能量的靶区覆盖率无显著差异(P = 0.896)。危及器官方面，本研究膀胱D_mean、左右股骨头、小肠等危及器官受量均无统计学差异，这与Kumar等 [ 18 ] 的研究结论基本一致，大多数危及器官受量无显著差异，膀胱V₅₀有统计学意义(P = 0.01)。Vassiliev等 [ 19 ] 报道与FFF-18MV相比，FFF-6MV治疗前列腺IMRT计划跳数MU增加，而本研究结论则相反，FFF-6MV VMAT计划机器跳数比FFF-10 MV少，原因可能与机器跳数取决于靶区大小、位置、小机头旋转的角度等因素，且10 MV能谱峰值和非均匀束流曲线更高。跳数越少，理论的治疗时间越少，因此，本研究6 MV计划治疗时间少于10 MV计划治疗时间。二维剂量验证通过率方面，两种能量的放疗计划gamma通过率有统计学差异(P = 0.012)，但结果均能达到98%以上。10 MV能量下的正常组织V₂、V₅体积显著减少，这可减低2次癌症复发的风险，这与Kumar等 [ 16 ] 的研究基本一致，但10 MV能量大小会产生中子污染，中子对靶区剂量的贡献不及X光子，且其放射生物学效应更高，对正常组织损害更大，而且给辐射防护带来困难。下一步比较其他能量大小的剂量学特性。本研究的不足之处是该结论仅仅为物理剂量学结果，只为临床提供参考，真正治疗后的临床疗效如何有待考证。

综上所述，在直肠癌FFF VMAT放疗计划中，6 MV能量与10 MV能量X线均能满足临床要求，从剂量学角度而言，6 MV能量能获得更好的均匀性和适形度，建议选用6 MV能量X线。

文章引用

彭圣贤,曹俊逸,刘悦. 非均整模式下6 MV与10 MV能量X线对直肠癌调强放疗影响Effect of 6 MV and 10 MV Flattening Filter-Free X-Ray Beams on Intensity Modulated Radiotherapy for Rectal Cancer[J]. 世界肿瘤研究, 2022, 12(03): 124-130. https://doi.org/10.12677/WJCR.2022.123017

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