The evaluation of a 2D diode array in “magic phantom” for use in high dose rate brachytherapy pretreatment quality assurance

Abstract

. (2015). The evaluation of a 2D diode array in "magic phantom" for use in high dose rate brachytherapy pretreatment quality assurance. Medical Physics, 42 (2), 663-673.The evaluation of a 2D diode array in "magic phantom" for use in high dose rate brachytherapy pretreatment quality assurance Abstract Purpose: 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 x 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. Methods: 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. Results: 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. Conclusions: 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 paramete...

Design and testing of a phantom and instrumented gynecological applicator based on GaN dosimeter for use in high dose rate brachytherapy quality assurance

“…The measured dwell position deviations of our systems are comparable with those from systems based on silicon diode and IC array. 18,31,34 The dwell time deviations are also comparable with those from systems based on silicon diode. 18,31 In terms of functionality, our systems allow not only dwell position and time verification but also source activity monitoring for HDR-BT QA (and in vivo QC with the instrumented applicator).…”

“…one hand, the measured results are compared with planned data on dwell position and dwell time. On the other, we propose to adopt a modified gamma index (MGI) in dwell position and time based upon the original concept proposed by Espinoza et al 31 In the formalism of Espinoza, the gamma index for each measured dwell position is calculated over all planned dwell positions in the treatment protocol, and a pass/fail decision is made upon its minimum value (passed if the minimum gamma index is lower than unity). However, for early error detection, the modified gamma index is calculated on sequentially paired dwell positions in measured and planned datasets.…”

“…The MGI is calculated with clinical relevant distance-to-agreement (DTA = 1.3 mm) and time-toagreement (TTA = 0.3 s) criteria. 31 We also propose to evaluate the success rate to detect the errors on dwell position and time. These errors are intentionally introduced by modifying parameters of the treatment plan, such as ejection length, dwell step, and dwell time to simulate possible human and mechanical errors in the treatment delivery.…”

HDR brachytherapy in vivo source position verification using a 2D diode array: A Monte Carlo study

“…This study has shown that similar source position verification accuracy to EPIDs may be achieved with the MP system. Furthermore, the MP system has been shown to have a superior timing resolution of less than 1 ms . As such, the MP delivers a dedicated, inexpensive HDR brachytherapy in vivo source position verification system with superior timing resolution that can easily be mass produced and is practical for routine clinical use.…”

“…This study aims to investigate the feasibility of using a two‐dimensional (2D) diode array, the “Magic Plate” (MP), developed at the Centre for Medical Radiation Physics (CMRP), University of Wollongong (UoW), Australia, for in vivo source position verification of a HDR prostate brachytherapy treatment. Previous studies performed with the MP in homogeneous phantom media and have reported source localization accuracy of less than 1 mm and temporal resolution of 1 ms, making it an ideal device for real‐time source position verification. It is hypothesized that the introduction of heterogeneous media associated with the patient geometry may compromise the accuracy of source localization using this device.…”