Dose‐to‐water conversion for the backscatter‐shielded EPID: A frame‐based method to correct for EPID energy response to MLC transmitted radiation

Zwan, Benjamin; King, Brian W.; O’Connor, D.J.; Greer, Peter B.

doi:10.1118/1.4890677

Cited by 6 publications

(6 citation statements)

References 13 publications

(16 reference statements)

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“…In the second approach, EPID images were calculated to the absorbed dose at the water. Then the dose estimation at the water was compared to TPS's dose calculation [27,28]. The advantage of the second approach is the potential to directly verify the TPS algorithm's accuracy [29].…”

Section: Discussionmentioning

confidence: 99%

Planar EPID-Based Dosimetry for SRS and SRT Patient-Specific QA

Thongsawad

Chanton

Saiyo

et al. 2021

Life

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The study’s purpose was to develop and validate Electronic Portal Imaging Device (EPID)-based dosimetry for Stereotactic Radiosurgery (SRS) and Stereotactic Radiation Therapy (SRT) patient-specific Quality Assurance (QA). The co-operation between extended Source-to-Imager Distance (SID) to reduce the saturation effect and simplify the EPID-based dosimetry model was used to perform patient-specific QA in SRS and SRT plans. The four parameters were included for converting the image to dose at depth 10 cm; dose-response linearity with MU, beam profile correction, collimator scatter and water kernel. The model accuracy was validated with 10 SRS/SRT plans. The traditional diode arrays with MapCHECK were also used to perform patient-specific QA for assuring model accuracy. The 150 cm-SID was found a possibility to reduce the saturation effect. The result of model accuracy was found good agreement between our EPID-based dosimetry and TPS calculation with GPR more than 98% for gamma criteria of 3%/3 mm, more than 95% for gamma criteria of 2%/2 mm, and the results related to the measurement with MapCHECK. This study demonstrated the method to perform SRT and SRT patient-specific QA using EPID-based dosimetry in the FFF beam by co-operating between the extended SID that can reduce the saturation effect and estimate the planar dose distribution with the in-house model.

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Section: Discussionmentioning

confidence: 99%

Planar EPID-Based Dosimetry for SRS and SRT Patient-Specific QA

Thongsawad

Chanton

Saiyo

et al. 2021

Life

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“…Electronic portal imaging devices (EPIDs) can be considered for IMRT field QA since they provide high-resolution images. However, they show a non-water-equivalent dosimetric response caused by the high-Z material in some layer of the detector, which results in a higher sensitivity to lower-energy photons (Zwan et al 2014), and thus require a conversion procedure to obtain 2D dose distributions. High spatial resolution diode arrays have been proposed for stereotactic QA (Rose et al 2020) and can also be used for QA of MLC-shaped irradiation fields.…”

Section: Introductionmentioning

confidence: 99%

A tomographic reconstruction algorithm for cross-sectional imaging of IMRT beams from six projections

et al. 2023

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Objective: Patient-specific Quality Assurance (QA) measurements are of key importance in radiotherapy for safe and efficient treatment delivery and allow early detection of clinically relevant errors. Such QA processes remain challenging to implement for complex IMRT radiotherapy fields delivered using a multileaf collimator (MLC) which often feature small open segments and raise QA issues similar to those encountered in small field dosimetry. Recently, detectors based on long scintillating fibers have been proposed to measure a few parallel projections of the irradiation field with good performance for small field dosimetry. The purpose of this work is to develop and validate a novel approach to reconstruct MLC-shaped small irradiation fields from six projections.Approach: The proposed field reconstruction method uses a limited number of geometric parameters to model the irradiation field. These parameters are iteratively estimated with a steepest descent algorithm. The reconstruction method was first validated on simulated data. Real data were measured with a water-equivalent slab phantom equipped with a detector made of 6 scintillating-fiber ribbons placed at 1m from the source. A radiochromic film was used to acquire a reference measurement of a first dose distribution in the slab phantom at the same source-to-detector distance and the treatment planning system (TPS) provided the reference for another dose distribution. In addition, simulated errors introduced on the delivered dose, field location and field shape were used to evaluate the ability of the proposed method to efficiently identify a deviation between the planned and delivered treatments.Main results: For a first small IMRT segment, 3%/3mm, 2%/2mm and 2%/1mm gamma analysis conducted between the reconstructed dose distribution and the dose measured with radiochromic film exhibited pass rates of 100%, 99.9% and 95.7%, respectively. For a second and smaller IMRT segment, the same gamma analysis performed between the reconstructed dose distribution and the reference provided by the TPS showed pass rates of 100%, 99.4% and 92.6% for the 3%/3mm, 2%/2mm and 2%/1mm gamma criteria, respectively. Gamma analysis of the simulated treatment delivery errors showed the ability of the reconstruction algorithm to detect a 3% deviation between the planned and delivered doses, as well as shifts lower than 7mm and 3mm when considering an individual leaf and a whole field shift, respectively.Significance: The proposed method allows accurate tomographic reconstruction of IMRT segments by processing projections measured with six scintillating-fiber ribbons and is suitable for water-equivalent real-time small IMRT segments QA.

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“…Calibration methodologies that convert raw EPID images, which are not directly representative of absorbed dose, to absorbed dose to water have been established by several groups. [12][13][14] These methodologies rely on a series of corrections factors and kernels to model the dosimetric characteristics of EPID response, the most common of which account for dead pixels, dark current, pixel sensitivity (PS), the off -axis response (OAR) of EPID relative to water, aperture-driven energy dependence, and penumbra correction. 12,15 Once corrected, a calibration coefficient is then applied to convert pixel values to absorbed doses to water, D w , the clinical dosimetric quantity of interest.…”

Section: Introductionmentioning

confidence: 99%

Assessing the accuracy of electronic portal imaging device (EPID)‐based dosimetry: II. Evaluation of a dosimetric uncertainty budget and development of a new film‐in‐EPID absorbed dose calibration methodology

Renaud

Muir

2022

Medical Physics

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The aim of this study is to reduce the uncertainty associated with determining dose-to-water using an amorphous silicon electronic portal imaging detector (EPID) under reference conditions by developing a direct calibration formalism based on radiochromic film measurements made within the EPID panel and detailed Monte Carlo simulations. To our knowledge, this is the first EPID-based dosimetry study reporting an uncertain budget Methods: Pixel sensitivity and relative off -axis response were mapped by simultaneously irradiating film contained within the imager panel and acquiring an EPID image set. The detector panel was disassembled for the purpose of modeling the EPID in detail using the EGSnrc DOSXYZnrc usercode, which was in turn used to calculate dose-to-film in the EPID and dose-to-water in water conversion factors Results: A direct comparison of the two correction methodologies investigated in this work, the previously established empirical method and the proposed simultaneous measurement approach involving in-EPID film dosimetry, produced an agreement with an RMS deviation of 1.4% overall. A combined standard relative uncertainty of 3.3% (k = 1) was estimated for the determination of absorbed dose to water at the position of the EPID using the proposed calibration methodology Conclusions: This work describes a direct method of calibrating EPID response in terms of absorbed dose to water requiring fewer measurements than other empirical approaches, and without 2D spatial interpolation of correction factors.

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Dose‐to‐water conversion for the backscatter‐shielded EPID: A frame‐based method to correct for EPID energy response to MLC transmitted radiation

Cited by 6 publications

References 13 publications

Planar EPID-Based Dosimetry for SRS and SRT Patient-Specific QA

Planar EPID-Based Dosimetry for SRS and SRT Patient-Specific QA

A tomographic reconstruction algorithm for cross-sectional imaging of IMRT beams from six projections

Assessing the accuracy of electronic portal imaging device (EPID)‐based dosimetry: II. Evaluation of a dosimetric uncertainty budget and development of a new film‐in‐EPID absorbed dose calibration methodology

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