Transit and non‐transit 3D EPID dosimetry versus detector arrays for patient specific QA

Abstract

Purpose Despite their availability and simplicity of use, Electronic Portal Imaging Devices (EPIDs) have not yet replaced detector arrays for patient specific QA in 3D. The purpose of this study is to perform a large scale dosimetric evaluation of transit and non‐transit EPID dosimetry against absolute dose measurements in 3D. Methods After evaluating basic dosimetric characteristics of the EPID and two detector arrays (Octavius 1500 and Octavius 1000SRS), 3D dose distributions for 68 VMAT arcs, and 10 IMRT plans were reconstructed within the same phantom geometry using transit EPID dosimetry, non‐transit EPID dosimetry, and the Octavius 4D system. The reconstructed 3D dose distributions were directly compared by γ‐analysis (2L2 = 2% local/2 mm and 3G2 = 3% global/2 mm, 50% isodose) and by the percentage difference in median dose to the high dose volume (%∆HDVD50). Results Regarding dose rate dependency, dose linearity, and field size dependence, the agreement between EPID dosimetry and the two detector arrays was found to be within 1.0%. In the 2L2 γ‐comparison with Octavius 4D dose distributions, the average γ‐pass rate value was 92.2 ± 5.2%(1SD) and 94.1 ± 4.3%(1SD) for transit and non‐transit EPID dosimetry, respectively. 3G2 γ‐pass rate values were higher than 95% in 150/156 cases. %∆HDVD50 values were within 2% in 134/156 cases and within 3% in 155/156 cases. With regard to the clinical classification of alerts, 97.5% of the treatments were equally classified by EPID dosimetry and Octavius 4D. Conclusion Transit and non‐transit EPID dosimetry are equivalent in dosimetric terms to conventional detector arrays for patient specific QA. Non‐transit 3D EPID dosimetry can be readily used for pre‐treatment patient specific QA of IMRT and VMAT, eliminating the need of phantom positioning.

Evaluation of automated pre-treatment and transit in-vivo dosimetry in radiotherapy using empirically determined parameters

“…Moreover, it is often not possible to use these in combination with Intensity Modulated Radiation Therapy (IMRT) or Volumetric Modulated Arc Therapy (VMAT) for which verification is limited to pre-treatment solutions. Since Electronic Portal Imaging Device (EPID) images contain dose information, many groups have investigated their use for radiotherapy dose measurement [12] , [13] , [14] , [15] , [16] , [17] , [18] , [19] , [20] , [21] , [22] . Pre-treatment dose verification and in-vivo dosimetry using EPIDs has become routine practice in a growing number of clinics in recent years.…”

Simulation of x‐ray‐induced acoustic imaging for absolute dosimetry: Accuracy of image reconstruction methods

“…For example, thermoluminescent detectors (TLDs) are only capable of point measurement (measuring of the dose on the patient's skin) and films are limited to two‐dimensional (2D) measurements. Recent studies have demonstrated the use of electronic portal imaging devices (EPIDs) as 3D patient‐specific dosimetry tools; however they are associated with uncertainties in the calibration process along with uncertainties in various steps of the dose reconstruction process . Therefore, the implementation of adaptive radiotherapy is limited by the lack of a real‐time device that can accurately measure the delivered dose to the patient …”

“…Recent studies have demonstrated the use of electronic portal imaging devices (EPIDs) as 3D patient-specific dosimetry tools; 8,9 however they are associated with uncertainties in the calibration process along with uncertainties in various steps of the dose reconstruction process. 10,11 Therefore, the implementation of adaptive radiotherapy is limited by the lack of a real-time device that can accurately measure the delivered dose to the patient. 3,7,12 The x-ray acoustic (XA) effect has recently been proposed as a method for real-time 3D in-vivo dosimetry using x-ray acoustic computed tomography (XACT).…”

Portal dosimetry in radiotherapy repeatability evaluation

“…Measurement of a dose or fluence map, taking into account the patient's body, is called transit dosimetry. 11 The aforementioned common use of portal matrices allows to check the dose distribution by measuring fluence maps before the therapeutic session without the patient. [12][13][14] The parameter adopted for comparing two data sets, including dose distributions or fluence maps, is the gamma coefficient.…”