EPID based transit dosimetry to monitor inter-fraction variation in radiation therapy

Bawazeer, O 2018, EPID based transit dosimetry to monitor inter-fraction variation in radiation therapy, Doctor of Philosophy (PhD), Health and Biomedical Sciences, RMIT University.

Document type: Thesis
Collection: Theses

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Title EPID based transit dosimetry to monitor inter-fraction variation in radiation therapy
Author(s) Bawazeer, O
Year 2018
Abstract In radiotherapy treatment, an amorphous silicon electronic portal imaging device (a-Si EPID) is mounted to the linear accelerator and is routinely used to verify patient setup prior to treatment. EPIDs have garnered much research interest for their important clinical applications and advantageous features, such as high resolution and fast acquisition times. As well, EPIDs offer the potential for “free” transit dosimetry and simply detect the treatment beam, meaning that the patient receives no additional dose. However, experience with EPIDs for transit dosimetry applications in the clinical setting is partially limited because of the complexity of reproducing the published methods. Commercial solutions have been presented and are continuing to emerge but are not mature enough for widespread adoption. The goal of this project was to extend the application of EPIDs in two main clinical areas, with the overall goal of providing a simple procedure that uses minimal resources. The EPID was used in transit dosimetry and in measuring beam attenuation through couch and immobilization devices.

For the first investigation, the modern delivery modalities can change the dose rate during delivery or can deliver a low monitor unit (MU). Therefore, the influence of the cine mode dosimetric characteristics of the EPID were examined. Emphasis was placed on delivering a low MU and changing the dose rates. The EPID’s performance was then compared to the well-documented integrated mode and dose measurements using an ionization chamber. The greatest nonlinearity was observed at low MU settings of less than 100 MU with the highest dose per frame. For dosimetric applications, this technique could be avoided by calibrating the EPID with a large number of MU. Despite the observed nonlinearity with low MU, EPID performance showed comparable responses, within 2%, when operated in cine and integrated acquisition modes. These results indicate that for dosimetry applications, no additional corrections are required when the EPID operated in cine compared to integrated mode.

A transit dosimetry method published by Sabet and colleagues in 2014 utilized two-dimensional (2D) transit EPID images normalized to ionization chamber measurements at the depth of dose maximum in water at the EPID plane and included a number of corrections to determine the complete 2D field area. The present work expanded on this idea and simplified the methodology to determine the complete 2D in-field area by applying a single correction rather than several corrections, while maintaining the standard clinical flood field (FF) calibration.The calibration method was assessed by comparing transit doses derived from the EPID versus MapCHECK for thirty dynamic intensity modulated radiation therapy (dIMRT) fields. The mean gamma pass rates in the field area for the examined dIMRT fields were 94% ± 3% (1SD), with a maximum and minimum of 99.7% and 86.7%, respectively. The proposed method is less complicated than previous measurement approaches. The key advantages of this method are the maintenance of the standard clinical FF calibration and the ability to calibrate the whole in-field area with a single correction. In addition, the method does not require any kernel application or Monte Carlo simulation. This 2D transit dosimetry could be a useful tool for monitoring the dose between treatment fractions and for error detection.

The feasibility of using EPID-based transit dosimetry and its sensitivity in detecting patient variations between treatment fractions were examined using gamma analysis and a structural similarity (SSIM) index.The transit EPID dose in the first fraction was considered the reference dose and variations in patient position or weight were then introduced in the subsequent fractions. Theresults indicated that the sensitivity of the EPID transit dosimetry methods depended on the treatment site, the type of delivery technique, and tissue heterogeneities.The factor that optimized the sensitivity of EPID was a reduction in the distance to the agreement criteria. The optimal criterion that could detect the most variations was 3%/1mm. With the SSIM index, the EPID can detect a 2 mm positional variation and 1 cm of lung variation. The findings in the present study indicated a difference between an image-based evaluation method (SSIM) and a typical dose-based evaluation method (gamma analysis) using the EPID. Transit measurements during the course of patient treatment potentially will aid in the detection of variations that could occur between fractions.

For the second investigation, the current practice of one dimensional measurement at the centre of field could not accurately estimate the beam attenuation values for the couch and immobilization devices which are a non-homogenous structure. We propose a simple and fast method to measure beam attenuation through support devices with either 2D (the attenuated image), or 1D (the mean of attenuated image) methods. The proposed method was validated against ionization chamber measurements. Beam attenuation through couches and immobilization devices was then examined using this method. Beam attenuation measurements using an EPID and an ionization chamber agreed to within ± 0.1 to 1.4 % (1 SD). The EPID was found to be able to clearly identify the difference in thickness of the IGRT couch. This study highlights the importance of individualized beam attenuation measurement based on the region of couch irradiated. This method provides the user with beam attenuation data in either two dimensions or one dimension within a few minutes.
Degree Doctor of Philosophy (PhD)
Institution RMIT University
School, Department or Centre Health and Biomedical Sciences
Subjects Medical Physics
Numerical Analysis
Synchrotrons; Accelerators; Instruments and Techniques
Keyword(s) EPID
Cine mode
Inter-fraction variations
Similarity index
Couch and immobilization devices
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Created: Thu, 17 Jan 2019, 11:30:35 EST by Keely Chapman
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