Prog Med Phys.  2024 Dec;35(4):178-204. 10.14316/pmp.2024.35.4.178.

Development of an Instantaneously Interpretable Real-Time Dosimeter System for Quality Assurance of a Medical Linear Accelerator

• Lee D
• Kim SJ
• Cheon W
• Cho H
• Han Y

• KMID: 2564157
• DOI: http://doi.org/10.14316/pmp.2024.35.4.178

Abstract

Purpose
Modern radiotherapy delivers radiation doses to targets within a few minutes using intricate multiple-beam segments determined with multi-leaf collimators (MLC). Therefore, we propose a scintillator-based dosimetry system capable of assessing the dosimetric and mechanical performance of intensity-modulated radiotherapy (IMRT) and volumetric-modulated arc therapy (VMAT) in real time.
Methods
The dosimeter was equipped with a scintillator plate and two digital cameras. The dose distribution was generated by applying deep learning-based signal processing to correct the intrinsic characteristics of the camera sensor and a tomographic image reconstruction technique to rectify the geometric distortion of the recorded video. Dosimetric evaluations were performed using a gamma analysis against a two-dimensional array and radiochromic film measurements for 20 clinical cases. The average difference in the MLC position measurements and machine log files was tested for the applicability of the mechanical quality assurance (QA) of MLCs.
Results
The agreement of the dose distribution in the IMRT and VMAT plans was clinically acceptable between the proposed system and conventional dosimeters. The average differences in the MLC positions for the IMRT/VMAT plans were 1.7010/2.8107 mm and 1.4722/2.7713 mm in banks A and B, respectively.
Conclusions
In this study, we developed an instantaneously interpretable real-time dosimeter for QA in a medical linear accelerator using a scintillator plate and digital cameras. The feasibility of the proposed system was investigated using dosimetric and mechanical evaluations in the IMRT and VMAT plans. The developed system has clinically acceptable accuracy in both the dosimetric and mechanical QAs of the IMRT and VMAT plans.

Keyword

Dosimeter; Real-time; Deep learning; Dose rate; Multi-leaf collimators

Figure

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  Fig. 1 Experimental setup of the developed dosimeter with a scintillator plate and complementary metal-oxide-semiconductor based digital cameras.

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  Fig. 2 Simplified flow chart of the signal processing in the developed system. Output correction is made in the signal processing. IMRT, intensity-modulated radiotherapy; VMAT, volumetric-modulated arc therapy; FT, Fourier transformed.

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  Fig. 3 Extracted mask from the recorded videos of the (a) IMRT and (b) VMAT plans by the thresholding method. The mask of the IMRT plan was extracted from the segmented image accumulated during the beam-on time, whereas that of the VMAT plan was obtained from the single-frame image corresponding to the CP. IMRT, intensity-modulated radiotherapy; VMAT, volumetric-modulated arc therapy; CP, control point.

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  Fig. 4 (a) Comparison of the output factors between the ionization chamber and proposed system before the output correction and (b) correction curve of the proposed dosimeter in the solid water phantom thickness of 5 cm.

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  Fig. 5 The linear response between the computed and actual field sizes. The computed square root field size was calculated from the extracted masks of the recorded videos.

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  Fig. 6 A simplified diagram of mask extraction in the VMAT plan’s recorded video images employing the cycleGAN model. IMRT, intensity-modulated radiotherapy; TPS, treatment planning system; DICOM-RT, digital imaging and communication in medicine of radiotherapy; VMAT, volumetric-modulated arc therapy.

• 

  Fig. 7 Comparison of the following signals measured by the proposed dosimeter among the different dose rate settings of the TrueBeam STx (Varian Medical Systems): (a) periodically attenuated signals due to the interference of the sampling frequency between the linear accelerator and cameras, (b) response to the dose rates in the single frame and accumulated frames, and (c) calibrated signal. MU, monitor unit.

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  Fig. 8 Flowchart of the iterative Fourier transform (FT)-based aliasing correction algorithm. DC, direct current.

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  Fig. 9 (a) Examples of the recorded video with geometric distortion (left) and corresponding dose map corrected with the tomographic image reconstruction technique (right). The geometric distortion of the recorded video was corrected by considering the proposed dosimeter as a (b) digital synthesis model.

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  Fig. 10 An example of the multi-leaf collimators (MLC) mask generation in the intensity-modulated radiotherapy (IMRT) plan.

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  Fig. 11 An example of the multi-leaf collimators (MLC) mask generation in the volumetric-modulated arc therapy (VMAT) plan. CP, control point; WaveUNet, wavelet-assisted UNet.

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  Fig. 12 A simplified diagram representing the network structure of the WaveUNet for extracting the multileaf collimator mask from the control point dose image in the volumetric modulated arc therapy case.

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  Fig. 13 A comparison of the (a) correcting methods for geometric distortion and dose profiles in field sizes of (b) 5×5, (c) 10×10, and (d) 20×20 cm2 measured along AB¯¯.

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  Fig. 14 A comparison of the (a) reconstruction algorithms for geometric distortion and dose profiles in field sizes of (b) 5×5, (c) 10×10, and (d) 20×20 cm2 measured along CD¯¯. FBP, filtered back projection; MLEM, maximum-likelihood estimation–maximization.

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  Fig. 15 A comparison of the characteristics between the proposed dosimeter, ionization chamber, and EBT3 film analyzed in terms of the (a) output factor, (b) dose linearity, and (c) dose rate response. MU, monitor unit.

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  Fig. 16 (a) A comparison of mask extraction using UNet and proposed algorithm, and (b) enlarged images inside box A. DICOM-RT, digital imaging and communication in medicine of radiotherapy.

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  Fig. 17 A comparison of the (a) single-segment dose maps from the intensity-modulated radiotherapy plans and dose profiles measured along EF¯¯ in fields (b) 1, (c) 2, and (d) 3. 2D, two-dimensional.

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  Fig. 18 Examples of the measured and predicted dose rates from the developed dosimeter: (a) measured dose and estimated equivalent field sizes of the pelvic IMRT case, (b) predicted dose rates of LINAC in the pelvic IMRT case, (c) measured dose and estimated equivalent field sizes in the pelvic VMAT case, and (d) predicted dose rates of LINAC in the pelvic VMAT case. IMRT, intensity-modulated radiotherapy; LINAC, linear accelerator; VMAT, volumetric-modulated arc therapy; MU, monitor unit.

Reference

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