摆位稳健性对IMRT与VMAT剂量学影响研究

Effects of positioning robustness on dosimetry for intensity-modulated radiation therapy and volumetric modulated arc therapy

目的比较摆位稳健性对头颈部肿瘤IMRT与VMAT剂量分布的影响程度，评价两者对图像引导需求。方法纳入30例IMRT鼻咽癌患者按临床上剂量要求设计VMAT计划，AAA法计算剂量。每患者两计划分别沿原始x、y、z轴各移动等中心±1．0、±3．0、±5．0mm模拟左右、上下、前后方向摆位误差对剂量分布影响，分析60个参考计划与1080个再计划DVH参数。配对t检验差异。结果误差为1mm时，GTV D98、CTVD95、HI、PGTV V95平均偏差〈0．5％；误差为3mm时，GTV与CTV剂量平均变化〈1．0％，且VMAT高于IMRT（GTV D98,P=0．00；CTV D95,P=0．00），PGTV剂量偏差要大，IMRT与VAMT中PGTVnx与PGTVndV95平均偏差分别为[1．64％比1．95％（P=0．01）]、[1．73％比2．63％（P=0．00）]。误差增大各指标偏差变大，且VMAT高于IMRT（GTV D98，P=0．00；CTV D95，P：0．00；CTV HI，P=0．00；PGTV V95,P=0．01）。相比靶区，脊髓、脑干Dmax变化更大，但IMRT与VMAT问差异不明显。结论误差较小时（〈3mm），IMRT与VMAT计划均较稳健。VMAT对摆位误差更敏感，主要体现在靶区剂量，随摆位误差增加两者间差异变大。建议行VMAT患者增加图像引导频次。

Objective To compare the effects of positioning robustness on dose distribution between intensity-modulated radiation therapy （IMRT） and volumetric modulated arc therapy （VMAT） for head and neck tumor, and to evaluate their needs for image-based guidance. Methods Thirty patients with nasopharyngeal carcinoma undergoing IMRT were enrolled as subjects. The VMAT plans were designed according to the clinical dosimetric requirements and the dose calculation was made by the AAA method. For the two plans in each patient, the isocenters were shifted by ± 1.0, ±3.0, and ±5.0 mm along the original x, y, and z axes to simulate the impacts of positioning errors in left-right （LR） , superior-inferior （SI）, and anterior-posterior （AP） directions, respectively, on dose distribution. The dose-volume histogram parameters were analyzed in 60 references and 1 080 re-calculated plans. Comparison was made by paired t-test. Results When the error was I ram, the average deviations of gross tumor volume （GTV） D98, clinical target volume （CTV） D95 and heterogeneity index, and planning gross tumor volume （PGTV） V95 were〈0. 5%. When the error was 3 mm, the average dose deviations of GTV and CTV were〈 1.0% and significantly larger in VMAT than in IMRT ; the average close deviation of PGTV was large （ GTV D98, P = 0. 00 ; CTW D95 , P = 0. 00） ; the average deviations of PGTVnxV95 and PGT Vnd V95 were significantly smaller in IMRT than in VMAT （ 1.64% vs. 1.95%, P=0. 01 ;1.73% vs. 2.63%, P= 0. 00）. The deviations of parameters became larger with the increasing positioning error and were significantly larger in VMAT than in IMRT （GTV D98, P= 0. 00;CTV D95 , P = 0. 00 ; CTV HI ; P = 0. 00 ; PGTV V95 ; P = 0. 01 ）. Compared with the target volume, Dmax to the spinal cord and brain stem had larger deviations. However, there were no significant differences in Dmax to the spinal cord and brain stem between IMRT and VMAT. Conclusions The IMRT and VMAT plans are both robust when the positioning error is small （〈3 ram）. Compared with IMRT, VAMT is more sensitive to the positioning error, mainly in the target volume. The difference between the two plans becomes larger with the increasing positioning error. An increase in the frequency of image-based guidance is recommended for patients undergoing VMAT.