容积调强弧形治疗技术全身照射的初次临床应用及剂量学验证

目的 介绍香港大学深圳医院初次使用容积调强弧形治疗技术(VMAT)行全身照射(TBI)患者的计划设计及剂量学验证方法。方法 在头、脚位两套定位图像上共同确定全身计划靶体积,处方剂量12Gy分6次,设计含5个中心15个全弧的TBI计划。优化时先在脚位图像中进行,并以此为剂量基础进行头位计划优化,最后两段综合剂量累加并评估。多种剂量学验证方式:Delta 4模体验证单等中心VMAT计划剂量;EBT 3胶片验证两相邻中心射野衔接处剂量分布;PinPoint电离室测量两段图像衔接区点剂量;MOSFET剂量仪实时监测患者体表剂量。另对计划结果参数、治疗时间等进行分析。结果 患者两段靶区的平均剂量分别为12.45Gy和12.37Gy,肺平均剂量为10.8Gy。每次治疗总机器跳数2883 MU,出束时间平均约24.3min,床旁平均总时间约121min。与计划计算相比:单中心VMAT计划绝对剂量3%/3mmγ通过率平均为(99.74±0.42)%;射野衔接区域绝对剂量5%/5mmγ通过率平均为(90.11±2.72)%;头、脚位图像衔接区域点剂量平均偏差(3.6±0.4)%;实时监测患者体表8个点,各部位每次剂量在1.57~2.04Gy范围内。结论 基于多中心VMAT技术的TBI计划及剂量学验证结果显示能可靠实施于临床,但还需不断改进、改善剂量分布和测量结果,提高治疗效率。

First clinical application and dosimetric verification of total body irradiation with volumetric modulated arc therapy

Objective To investigate the implementation procedures and dosimetric verification of the first patient treated with total body irradiation (TBI) based on volumetric modulated arc therapy (VMAT). Methods Two sets of CT images were acquired under the head-in first and foot-in first to contour the planning target volume (PTV) of the cranial and caudal segments to accomplish the treatment of the whole body length, on which two interrelated plans of 5 subsequent isocenters with a total of 15 VMAT fields were performed to cover all PTVs. The plans were prescribed to ensure 90% PTV dose coverage with a total dose of 12Gy in 6 fractions. Firstly, a dose optimization was performed on the caudal CT images, then the cranial CT images were optimized based on the dose distribution of the caudal CT images. The evaluation of the final treatment plan was carried out based on a plan sum of both two sets of images. The parameters of PTV and organs at risk (OARs) were measured by dose volume histograms from the accumulated plan. The quality assurance comprised the verification of the VMAT plans for each individual isocenter via Delta4 phantom. The dose distribution in the overlapped region between two adjacent central fields was verified with EBT3 film. The absolute dose at the overlapped region between two images was measured via Pinpoint chamber. In vivo dosimetry on the patient′s skin was monitored by MOSFET dosimeters. The results of planning parameters and treatment duration were analyzed. Results The mean doses of two segments of PTVs were 12.45Gy and 12.37Gy. The mean dose for the lung was 10.8Gy. The machine unit (MU) and mean treatment delivery time were 2883 MU and 24.3min, and the mean total time per fraction was 121min. The mean 3%/3mmγ-analysis pass rate for each isocenter VMAT plan was (99.74±0.42)%, and the mean 5%/5mmγ-analysis pass rate for the overlapped region was (90.11±2.72)%. The average deviation of absolute dose in the overlap region of the caudal and cranial images was (3.6±0.4)%.In vivo measurement of 8 points on the patient showed that the dose of each region was ranged from 1.57Gy to 2.04Gy. Conclusion According to the results of dosimetric verification, TBI based on multi-isocenter VMAT can be applied in clinical practice, which remains to be improved in terms of dose distribution, measurement results and clinical efficiency.