Espinoza A, Petasecca M, Fuduli I, Howie A, Bucci J, Corde S, Jackson M, Lerch M L F, Rosenfeld A B
Centre for Medical Radiation Physics, University of Wollongong, New South Wales 2522, Australia.
St George Hospital Cancer Care Centre, New South Wales 2217, Australia.
Med Phys. 2015 Feb;42(2):663-673. doi: 10.1118/1.4905233.
High dose rate (HDR) brachytherapy is a treatment method that is used increasingly worldwide. The development of a sound quality assurance program for the verification of treatment deliveries can be challenging due to the high source activity utilized and the need for precise measurements of dwell positions and times. This paper describes the application of a novel phantom, based on a 2D 11 × 11 diode array detection system, named "magic phantom" (MPh), to accurately measure plan dwell positions and times, compare them directly to the treatment plan, determine errors in treatment delivery, and calculate absorbed dose.
The magic phantom system was CT scanned and a 20 catheter plan was generated to simulate a nonspecific treatment scenario. This plan was delivered to the MPh and, using a custom developed software suite, the dwell positions and times were measured and compared to the plan. The original plan was also modified, with changes not disclosed to the primary authors, and measured again using the device and software to determine the modifications. A new metric, the "position-time gamma index," was developed to quantify the quality of a treatment delivery when compared to the treatment plan. The MPh was evaluated to determine the minimum measurable dwell time and step size. The incorporation of the TG-43U1 formalism directly into the software allows for dose calculations to be made based on the measured plan. The estimated dose distributions calculated by the software were compared to the treatment plan and to calibrated EBT3 film, using the 2D gamma analysis method.
For the original plan, the magic phantom system was capable of measuring all dwell points and dwell times and the majority were found to be within 0.93 mm and 0.25 s, respectively, from the plan. By measuring the altered plan and comparing it to the unmodified treatment plan, the use of the position-time gamma index showed that all modifications made could be readily detected. The MPh was able to measure dwell times down to 0.067 ± 0.001 s and planned dwell positions separated by 1 mm. The dose calculation carried out by the MPh software was found to be in agreement with values calculated by the treatment planning system within 0.75%. Using the 2D gamma index, the dose map of the MPh plane and measured EBT3 were found to have a pass rate of over 95% when compared to the original plan.
The application of this magic phantom quality assurance system to HDR brachytherapy has demonstrated promising ability to perform the verification of treatment plans, based upon the measured dwell positions and times. The introduction of the quantitative position-time gamma index allows for direct comparison of measured parameters against the plan and could be used prior to patient treatment to ensure accurate delivery.
高剂量率（HDR）近距离放射治疗是一种在全球范围内越来越多地被使用的治疗方法。由于所使用的高源活度以及对驻留位置和时间进行精确测量的需求，开发一个完善的用于验证治疗输送的质量保证程序可能具有挑战性。本文描述了一种基于二维11×11二极管阵列检测系统的新型体模，即“魔法体模”（MPh）的应用，用于精确测量计划驻留位置和时间，将其直接与治疗计划进行比较，确定治疗输送中的误差，并计算吸收剂量。
对魔法体模系统进行CT扫描，并生成一个20根导管的计划以模拟一个非特定的治疗场景。该计划被输送到魔法体模，并使用定制开发的软件套件测量驻留位置和时间，并与计划进行比较。原始计划也进行了修改，修改内容未向主要作者透露，然后再次使用该设备和软件进行测量以确定修改情况。开发了一种新的指标，即“位置 - 时间伽马指数”，以量化与治疗计划相比时治疗输送的质量。对魔法体模进行评估以确定最小可测量驻留时间和步长。将TG - 43U1形式主义直接纳入软件允许根据测量的计划进行剂量计算。使用二维伽马分析方法，将软件计算的估计剂量分布与治疗计划以及校准的EBT3胶片进行比较。
对于原始计划，魔法体模系统能够测量所有驻留点和驻留时间，并且发现大多数分别与计划相差在0.93毫米和0.25秒以内。通过测量修改后的计划并将其与未修改的治疗计划进行比较，使用位置 - 时间伽马指数表明所有做出的修改都能很容易地被检测到。魔法体模能够测量低至0.067±0.001秒的驻留时间以及相隔1毫米的计划驻留位置。发现魔法体模软件进行的剂量计算与治疗计划系统计算的值在0.75%以内相符。使用二维伽马指数，发现与原始计划相比，魔法体模平面的剂量图和测量的EBT3的通过率超过95%。
这种魔法体模质量保证系统在HDR近距离放射治疗中的应用已证明基于测量的驻留位置和时间进行治疗计划验证具有良好的能力。定量位置 - 时间伽马指数的引入允许将测量参数与计划进行直接比较，并且可在患者治疗前使用以确保准确输送。
