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. 2021 Nov 2:11:782263.
doi: 10.3389/fonc.2021.782263. eCollection 2021.

Evaluation of Daily CT for EPID-Based Transit In Vivo Dosimetry

Bin Feng  1   2   3 , Lei Yu  1   2   3 , Enwei Mo  1   2   3 , Liyuan Chen  1   2   3 , Jun Zhao  1   2   3 , Jiazhou Wang  1   2   3 , Weigang Hu  1   2   3
Affiliations

Evaluation of Daily CT for EPID-Based Transit In Vivo Dosimetry

Bin Feng et al. Front Oncol. .

Abstract

Purpose: The difference in anatomical structure and positioning between planning and treatment may lead to bias in electronic portal image device (EPID)-based in vivo dosimetry calculations. The purpose of this study was to use daily CT instead of planning CT as a reference for EPID-based in vivo dosimetry calculations and to analyze the necessity of using daily CT for EPID-based in vivo dosimetry calculations in terms of patient quality assurance.

Materials and methods: Twenty patients were enrolled in this study. The study design included eight different sites (the cervical, nasopharyngeal, and oral cavities, rectum, prostate, bladder, lung, and esophagus). All treatments were delivered with a CT-linac 506c (UIH, Shanghai) using 6 MV photon beams. This machine is equipped with diagnosis-level fan-beam CT and an amorphous silicon EPID XRD1642 (Varex Imaging Corporation, UT, USA). A Monte Carlo algorithm was developed to calculate the transmit EPID image. A pretreatment measurement was performed to assess system accuracy by delivering based on a homogeneous phantom (RW3 slab, PTW, Freiburg). During treatment, each patient underwent CT scanning before delivery either once or twice for a total of 268 fractions obtained daily CT images. Patients may have had a position correction that followed our image-guided radiation therapy (IGRT) procedure. Meanwhile, transmit EPID images were acquired for each field during delivery. After treatment, all patient CTs were reviewed to ensure that there was no large anatomical change between planning and treatment. The reference of transmit EPID images was calculated based on both planning and daily CTs, and the IGRT correction was corrected for the EPID calculation. The gamma passing rate (3 mm 3%, 2 mm 3%, and 2 mm 2%) was calculated and compared between the planning CT and daily CT. Mechanical errors [ ± 1 mm, ± 2 mm, ± 5 mm multileaf collimator (MLC) systematic shift and 3%, 5% monitor unit (MU) scaling] were also introduced in this study for comparing detectability between both types of CT.

Result: The average (standard deviation) gamma passing rate (3 mm 3%, 2 mm 3%, and 2 mm 2%) in the RW3 slab phantom was 99.6% ± 1.0%, 98.9% ± 2.1%, and 97.2% ± 3.9%. For patient measurement, the average (standard deviation) gamma passing rates were 87.8% ± 14.0%, 82.2% ± 16.9%, and 74.2% ± 18.9% for using planning CTs as reference and 93.6% ± 8.2%, 89.7% ± 11.0%, and 82.8% ± 14.7% for using daily CTs as reference. There were significant differences between the planning CT and daily CT results. All p-values (Mann-Whitney test) were less than 0.001. In terms of error simulation, nonparametric test shows that there were significant differences between practical daily results and error simulation results (p < 0.001). The receiver operating characteristic (ROC) analysis indicated that the detectability of mechanical delivery error using daily CT was better than that of planning CT. AUCDaily CT = 0.63-0.96 and AUCPlanning CT = 0.49-0.93 in MLC systematic shift and AUCDaily CT = 0.56-0.82 and AUCPlanning CT = 0.45-0.73 in MU scaling.

Conclusion: This study shows the feasibility and effectiveness of using two-dimensional (2D) EPID portal image and daily CT-based in vivo dosimetry for intensity-modulated radiation therapy (IMRT) verification during treatment. The daily CT-based in vivo dosimetry has better sensitivity and specificity to identify the variation of IMRT in MLC-related and dose-related errors than planning CT-based.

Keywords: EPID; IMRT verification; Monte Carlo method; portal dosimetry; quality assurance.

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Conflict of interest statement

The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.

Figures

Figure 1
Figure 1
Anatomical variations between planning CT (pCT) and daily CT (dCT). (A) pCT of a rectal patient; (B) dCT of a rectal patient; (C) pCT of an NPC patient; and (D) dCT of an NPC patient. NPC, Nasopharyngeal Carcinoma.
Figure 2
Figure 2
The study workflow.
Figure 3
Figure 3
The CT-Linac 506c.
Figure 4
Figure 4
The quality assurance (QA) phantom, the RW3 slab phantom (PTW, Freiburg).
Figure 5
Figure 5
The cases of pretreatment quality assurance (QA). (A) An electronic portal image device (EPID)-measured image. (B) A phantom model-based calculated image. (C) The corresponding Gamma analysis results. (D) Profiles representing inlines and crosslines are shown in panels (A, B).
Figure 6
Figure 6
The Gamma analysis results of planning CT (pCT) and daily CT (dCT). The red box diagram represents the pCT results. The blue box represents the dCT results.
Figure 7
Figure 7
Gamma analysis results of the daily CT-based vs. planning CT-based methods. The daily CT-based results indicate a higher gamma passing rate; (A) 3 mm 3%; (B) 2 mm 3%; and (C) 2 mm 2%.
Figure 8
Figure 8
The cases of electronic portal image device (EPID)-based in vivo dosimetry during treatment. (A) An EPID-measured image. (B) A planning CT (pCT)-based calculated image. (C) A daily CT (dCT)-based calculated image. (D) The corresponding Gamma analysis results between the pCT and measurement. (E) The corresponding Gamma analysis results between the dCT and measurement. (F) Profiles representing inlines and crosslines are shown in panels (A–C).
Figure 9
Figure 9
Distribution of gamma passing rates for daily CT (dCT)-based plan and a plan with a 3% and 5% scaling in monitor units (MUs) and a 1-, 2-, and 5-mm shift in multileaf collimator (MLC) leaf (left). Dilstribution of gamma passing rates for planning CT (pCT)-based plan and a plan with a 3% and 5% scaling in MU and a 1-, 2-, and 5-mm shift in MLC leaf (right).
Figure 10
Figure 10
Receiver operating characteristic (ROC) curve for a systematic shift to multileaf collimator (MLC) leaves of ±1, ± 2, and ±5 mm (left). ROC curve for scaling of the monitor unit (MU) by 3% and 5% (right).
Figure 11
Figure 11
An example of introducing error on electronic portal image device (EPID)-based in vivo dosimetry during treatment.
Figure 12
Figure 12
(A) The gamma passing rate (3 mm, 3%) between the planning CT (pCT)-based calculated dose and measured dose for a patient with 10 treatment fractions. (B) The gamma passing rate (3 mm, 3%) between the daily CT (dCT)-based calculated dose and measured dose for a patient.
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