Shepard Andrew J, Mittauer Kathryn E, Bayouth John E, Yadav Poonam
Department of Human Oncology, School of Medicine and Public Health, University of Wisconsin-Madison, Madison, WI, USA.
J Appl Clin Med Phys. 2020 Dec;21(12):54-61. doi: 10.1002/acm2.13059. Epub 2020 Oct 29.
Dark current radiation produced during linac beam-hold has the potential to lead to unplanned dose delivered to the patient. With the increased usage of motion management and step-and-shoot IMRT deliveries for MR-guided systems leading to increased beam-hold time, it is necessary to consider the impact of dark current radiation on patient treatments.
The relative dose rate due to dark current for the ViewRay MRIdian linac was measured longitudinally over 15 months (June 2018-August 2019). Ion chamber measurements were acquired with the linac in the beam-hold state and the beam-on state, with the ratio representing the relative dark current dose rate. The potential contribution of the dark current dose to the overall prescription was retrospectively analyzed for 972 fractions from 83 patients over the same time period. The amount of time spent in the beam-hold state was combined with the monthly measured relative dark current dose rate to estimate the dark current dose contribution.
The relative dark current dose rate compared to the beam-on dose rate was 0.12% ± 0.027%. In a near worst-case estimation, the dark current dose contribution accounted for 0.90% ± 0.67% of the prescription dose across all fractions (3.61% maximum). Gantry and MLC motion between segments accounted for 87% of the dark current contribution, with the remaining 13% attributable to gating during segment delivery. The largest dark current contributions were associated with plans delivering a small dose per treatment segment.
The dark current associated with new clinical treatment units should be considered prior to treatment delivery to ensure it will not lead to dosimetric inaccuracies. For the MRIdian linac system investigated in this work, the contribution from dark current remained relatively low, though users should be cognizant of the larger potential dosimetric contribution for plans with small doses per segment.
直线加速器束流保持期间产生的暗电流辐射有可能导致对患者进行非计划的剂量输送。随着运动管理和用于磁共振引导系统的步进式调强放疗（IMRT）交付方式的使用增加，导致束流保持时间延长，有必要考虑暗电流辐射对患者治疗的影响。
对ViewRay MRIdian直线加速器因暗电流产生的相对剂量率进行了为期15个月（2018年6月至2019年8月）的纵向测量。在直线加速器处于束流保持状态和束流开启状态时进行电离室测量，其比值代表相对暗电流剂量率。在同一时期，对83例患者的972个分次进行回顾性分析，以评估暗电流剂量对总体处方剂量的潜在贡献。将束流保持状态下花费的时间与每月测量的相对暗电流剂量率相结合，以估算暗电流剂量贡献。
与束流开启剂量率相比，相对暗电流剂量率为0.12%±0.027%。在近乎最坏情况的估计中，暗电流剂量贡献在所有分次中占处方剂量的0.90%±0.67%（最大为3.61%）。各段之间的机架和多叶准直器运动占暗电流贡献的87%，其余13%归因于段交付期间的门控。最大的暗电流贡献与每个治疗段输送小剂量的计划相关。
在进行治疗之前，应考虑与新临床治疗单元相关的暗电流，以确保其不会导致剂量测定不准确。对于本研究中调查的MRIdian直线加速器系统，暗电流的贡献仍然相对较低，不过用户应认识到对于每个段剂量较小的计划，其潜在的剂量测定贡献更大。
