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 Table of Contents  
ORIGINAL ARTICLE
Year : 2022  |  Volume : 8  |  Issue : 1  |  Page : 25

Comparison of dosimetric parameters and three-dimensional dosimetric verification of three intensity-modulated radiotherapy plans for thymoma based on the dose–volume histogram and ArcCHECK-3DVH system


Department of Cancer Center, Daping Hospital, Army Medical University, Chongqing, China

Date of Submission22-Mar-2022
Date of Decision10-Jun-2022
Date of Acceptance13-Jun-2022
Date of Web Publication21-Oct-2022

Correspondence Address:
Mingying Geng
Department of Cancer Center, Daping Hospital, Army Medical University, No. 10 Changjiang Zhilu, Yuzhong District, Chongqing 400042
China
Xuan He
Department of Cancer Center, Daping Hospital, Army Medical University, No. 10 Changjiang Zhilu, Yuzhong District, Chongqing 400042
China
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Source of Support: None, Conflict of Interest: None


DOI: 10.4103/digm.digm_11_22

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  Abstract 


Objective: To compare the dosimetric parameters of step-shoot intensity-modulated radiotherapy (sIMRT), dynamic intensity-modulated radiotherapy (dIMRT), and volume-modulated arc therapy (VMAT) in thymoma and to study the feasibility of the ArcCHECK-3DVH system in three intensity-modulated radiotherapy plans to choose a more appropriate intensity-modulated radiotherapy for thymoma. Materials and Methods: Seventeen patients with thymoma were enrolled in this study. Treatment plans of sIMRT, dIMRT, and VMAT for each patient were based on the Monaco treatment planning system (TPS). Dosimetric verification was performed via the ArcCHECK-3DVH system. We compared and analyzed the 3D γ pass rates of the TPS dose calculation and ArcCHECK-3DVH system dose reconstruction with the three gamma criteria (3 mm/3%, 2 mm/2%, and 1 mm/1%) with a threshold of 10%. Dose–volume histogram analysis was used to compare the dose parameters for target volumes, and organs at risk (OARs), such as D98%, D50%, D2%, Dmax, V20, and V5. Monitor units (MUs) and delivery time were also compared. Results: There were significant differences in the three intensity-modulated radiotherapy plans. For target volume, VMAT showed the highest planning target volume (PTV) D98% and the lowest PTV D50% compared with sIMRT or dIMRT. The PTV D2% of VMAT was lower than that of sIMRT and higher than that of dIMRT, and VAMT demonstrated the highest conformity index and MU, lowest homogeneity index, and shortest treatment delivery time. For the OARs, VMAT is not inferior to sIMRT and dIMRT in OARs protection. For the dosimetric verification, the entire area, PTV, lungs, heart, and spinal cord of VMAT showed the highest γ pass rates than the other two techniques under the gamma 3 mm/3% criteria, which was even more pronounced when the stricter gamma criteria of 2 mm/2% and 1 mm/1% were applied. Conclusion: VMAT can be applied to radiotherapy of thymoma, and the accuracy of treatment plan execution can be guaranteed through the ArcCHECK-3DVH system.

Keywords: ArcCHECK volume-modulated arc therapy, Dosimetric parameters, Dosimetric verification, Intensity-modulated radiotherapy, Thymoma


How to cite this article:
Zhou P, Luo J, Xiao H, Geng M, He X. Comparison of dosimetric parameters and three-dimensional dosimetric verification of three intensity-modulated radiotherapy plans for thymoma based on the dose–volume histogram and ArcCHECK-3DVH system. Digit Med 2022;8:25

How to cite this URL:
Zhou P, Luo J, Xiao H, Geng M, He X. Comparison of dosimetric parameters and three-dimensional dosimetric verification of three intensity-modulated radiotherapy plans for thymoma based on the dose–volume histogram and ArcCHECK-3DVH system. Digit Med [serial online] 2022 [cited 2022 Nov 29];8:25. Available from: http://www.digitmedicine.com/text.asp?2022/8/1/25/359354




  Introduction Top


Thymoma is a common mediastinal tumor that is difficult to diagnose early due to hidden onset. The location of the lesion is adjacent to the great blood vessels of the heart, and the boundary between the tumor and peripheral blood vessels is not clear in advance thymoma, which is difficult to cure completely by surgery alone. Therefore, postoperative adjuvant radiotherapy is essential to reduce local recurrence and distant metastasis.[1],[2],[3],[4],[5]

At present, intensity-modulated radiotherapy (IMRT) is one of the most common radiotherapy techniques for thymoma and can be divided into step-shoot IMRT (sIMRT) and dynamic IMRT (dIMRT). For sIMRT, the beam is delivered with the specific dose rate, while gantry angle and multileaf collimators (MLC) position are fixed. For dIMRT, MLC position and dose rate are all variable during the irradiation process.[6],[7],[8] With the rapid development of radiotherapy techniques, volume-modulated arc therapy (VMAT), as a new modulated radiotherapy technique, can continuously change the gantry angle, MLC position, and dose rate during the irradiation process. VMAT improves the conformity of the dose distribution to the target volume and the irradiation efficiency and can also better protect organs at risk (OARs) from radiation. Therefore, VMAT is widely used in the clinic.[8],[9],[10],[11] However, the dynamic change in multiple parameters and the complex dose distribution in VMAT make the implementation of radiotherapy more uncertain, and patient-specific quality assurance (QA) is necessary. The most frequently used method of QA is to compare the calculated dose of treatment planning system (TPS) and the measured dose of the dosimetric verification phantom.[12],[13] Gamma analysis is usually used to determine if the dose distribution of the measured result is appropriate for the calculated dose distribution. It consists of two concepts: dose difference and distance to agreement (DTA). The American Association of Medical Physics in Medicine recommended that the criteria of a 3% dose difference and 3 mm DTA (3 mm/3%) are commonly used in clinical dosimetric research.[14],[15],[16]

Therefore, VMAT used in postoperative adjuvant therapy of thymoma is rarely reported at home and abroad. To explore the differences in the application of sIMRT, dIMRT, and VMAT in thymoma, we compared the dosimetric parameters, conducted dosimetric verification using the ArcCHECK-3DVH system, and analyzed their dosimetric differences to provide a basis for clinical application.


  Materials and Methods Top


Patient selection

Seventeen thymoma patients who received postoperative adjuvant radiotherapy in Army Special Medical Center from January 2017 to January 2020 were enrolled in this study, including 11 males and 6 females, aged 24–64 years, with a median age of 45 years. All patients were confirmed by pathological examination to have lesions located in the anterior superior mediastinum, and there were no contraindications to radiotherapy. All patients signed the informed consent form during the experiment.

Computed tomography simulation-based localization

All the patients were supine with their heads in their hands and fixed with thermoplastic body film. After a thermoplastic membrane was formed, lead points were placed in the anterior midline and on both sides of the body under laser beam guidance. Under calm breathing, the coverage of the contrast-enhanced computed tomography (CT) scan was extended from the cricoid cartilage to the bottom of the lung using the Philips 16-slice CT locator (Philips, Eindhoven, Netherlands) and reconstruction with a layer spacing of 5 mm. The obtained CT images were transmitted to the Monaco TPS (Version 5.11.01, Elekta AB, Stockholm, Sweden) and reconstructed 3D images.

Target volume delineation and organs at risk

The gross tumor volume and clinical target volume (CTV) were delineated according to the experience of our hospital and the guidelines of the International Commission on Radiation Units and Measurements. The planning target volume (PTV) was generated by expanding 3 mm outward on the basis of CTV and delineating the lungs, heart, spinal cord, and other OARs. The limits for OARs were as follows: for the left and right lungs, they were V5 ≤ 55%, V10 ≤ 45%, V20 ≤ 30%, and V30 ≤ 20%; for the heart, they were V20 ≤ 50%, V30 ≤ 40%, and V40 ≤ 30%; and for the spinal cord, it was Dmax ≤ 40 Gy.

Treatment plan design

SIMRT, dIMRT, and VMAT treatment plans were designed for each patient using the Monaco TPS (v5.11.01), and all the treatment plans were generated for the Elekta Synergy linear accelerator. The prescription dose was 50 Gy/25 F. For the three techniques' treatment plans, the plan isocenter was placed in the center of the PTV. Five fixed gantry fields were created for the sIMRT and dIMRT plans (300°, 330°, 30°, 60°, and 180°). Two fields were created for VMAT plans (the angle between 2 beams ranged from 120° to 230°). All plans were calculated using the Monaco TPS (v5.11.01), and 8 MV photon energy was used. All plans were optimized under the same objective functions and parameters.

Dose delivery and verification

All plans included irradiation with 8 MV of photon energy generated by an Elekta Axesse (Elekta AB, Stockholm, Sweden) linear accelerator. The ArcCHECK-3DVH system (Sun Nuclear Corporation [SNC], Melbourne, Florida, USA) was used for dosimetric verification of all treatment plans, which uses a phantom for data acquisition and SNC patient software and 3DVH software (version 3.2) for comparative analysis. The length of the ArcCHECK phantom is 21 cm, and it is cylindrical. There are 1386 semiconductor probes in a helical distribution in the phantom, the probe spacing is 1 cm, and the effective detection area is 0.8 mm × 0.8 mm. Calibration of the background, matrix, and absolute dose are necessary prior to acquiring data with the ArcCHECK system. All the treatment plans were uploaded into the ArcCHECK phantom to recalculate doses. The RT plan and RT dose files of the calculated results were imported into the SNC patient software in DICOM format and compared with the results obtained from the phantom measurements. The difference between the calculated dose and the measured dose was analyzed using γ pass rates.[17],[18],[19]

Comparison of dose–volume histogram-based parameters

Comparison of the differences between the sIMRT, dIMRT, and VMAT plans for each patient by dose–volume histogram (DVH). All the treatment plans required more than 95% of the target volume covered by the prescription isodose curve. PTV dose (D98%, D50%, and D2%), lung dose (V30, V20, V10, and V5 and mean dose), heart (V30, V20, and V10), and spinal cord doses (maximum dose) were evaluated. The homogeneity index (HI) and the conformity index (CI) of the target volume were also evaluated. CI = VRX2/(TV × VRI), VRX represents the target volume covered by the 95% prescription isodose curve, TV represents the target volume, VRI represents the volume of all areas covered by the 95% prescription isodose curve, and CI is ideally 1. HI = D5/D95, where D5 represents the dose covering 5% of the target volume and D95 represents the dose covering 95% of the target volume; a lower HI value represents better uniformity of the target.[20],[21] The actual treatment delivery time and the monitor units (MUs) were compared between the three techniques.[22],[23]

Analysis of 3D γ pass rates with 3DVH

In our study, the 3D γ pass rates of each region of interest (ROI) were calculated. The 3D γ pass rates of sIMRT, dIMRT, and VMAT for each patient were compared using three gamma criteria (3 mm/3%, 2 mm/2%, and 1 mm/1%) with a threshold of 10%. The difference was analyzed using the Friedman test.[24]

Statistical methods

The data were represented as medians and interquartile range. The differences in all dosimetry parameters among the three groups were evaluated with the Friedman test. Stepwise comparison between any two groups was performed by the Nemenyi test, and the Bonferroni method was used to correct the P value from multiple comparisons. All statistical analyses were performed with R (version 4.1.1), and P < 0.05 was considered statistically significant.


  Results Top


Comparison of target volume dose, monitor units, and treatment delivery time

All the treatment plans conformed to the clinical treatment requirements. The dose distribution of the sIMRT, dIMRT, and VMAT plans on the same cross-section and sagittal section is shown in [Figure 1]. The comparison results of the PTV dose parameters (D98%, D50%, and D2%), MUs, and treatment delivery time for the three techniques are shown in [Table 1] and [Figure 2]. The differences in the D98%, D50%, and D2% of the PTV among the three techniques were statistically significant (P < 0.05). VMAT showed the highest PTV D98% and the lowest PTV D50% compared with sIMRT or dIMRT. For the D2% of the PTV, VMAT was lower than sIMRT and higher than dIMRT. There were also statistically significant differences in CI, HI, MUs, and treatment delivery time among the three techniques (P < 0.05). For the other two techniques, VAMT demonstrated the highest CI and MUs, lowest HI, and shortest treatment delivery time.
Figure 1: Dose distribution of sIMRT, dIMRT, and VMAT plans on the same cross-section and sagittal plane. sIMRT: Step-shoot intensity-modulated radiotherapy, dIMRT: Dynamic intensity-modulated radiotherapy, VMAT: Volume-modulated arc therapy.

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Figure 2: Dosimetry, MU and treatment delivery time comparison of the target among sIMRT, dIMRT, and VMAT. (a) : PTV D98%; (b) : PTV D50%; (c) : PTV D2%; (d) : HI; (e): CI; (f) : MUs; (g) : Treatment delivery time. sIMRT: Step-shoot intensity-modulated radiotherapy, dIMRT: Dynamic intensity-modulated radiotherapy, VMAT: Volume-modulated arc therapy, MUs: Monitor units, PTV: Planning target volume, CI: Conformity index, HI: Homogeneity index.

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Table 1: Dosimetry, monitor units and treatment delivery time comparison of the target among step-shoot intensity-modulated radiotherapy, dynamic intensity-modulated radiotherapy and volume-modulated arc therapy

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Dosimetry comparison of organs at risks

The DVHs of the sIMRT, dIMRT, and VMAT plans are shown in [Figure 3].
Figure 3: The DVH of sIMRT, dIMRT, and VMAT plans. sIMRT: Step-shoot intensity-modulated radiotherapy, dIMRT: Dynamic intensity-modulated radiotherapy, VMAT: Volume-modulated arc therapy, DVH: Dose–volume histogram, PTV: Planning target volume.

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The dosimetry comparison of OARs between each group of the three techniques is shown in [Table 2] and [Figure 4]. Except for no significant difference in the V10 of the left lung between dIMRT and VMAT (P > 0.05), the Dmean, V40, V30, V20, V10, and V5 of the lungs, the Dmean, V40, V30, and V20 of the heart, and the maximum dose of the spinal cord were all significantly different between each of the three technique groups (P < 0.05).
Figure 4: Dosimetric comparison of OARs among sIMRT, dIMRT, and VMAT. (a–e) represents the Dmean, V30, V20, V10, V5 of R-lung; (f–j) represents the Dmean, V30, V20, V10, V5 of L-lung; (k–n) represents the Dmean, V40, V30, V20 of heart; (o) represent Dmax of spinal cord. sIMRT: Step-shoot intensity-modulated radiotherapy, dIMRT: Dynamic intensity-modulated radiotherapy, VMAT: Volume-modulated arc therapy, OARs: Organs at risk.

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Table 2: Dosimetric comparison of organs at risk among step-shoot intensity-modulated radiotherapy, dynamic intensity-modulated radiotherapy and volume-modulated arc therapy

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For the right lung, VMAT showed the lowest V10 and the highest V30 and V20, and the Dmean and V5 were lower than those of sIMRT and higher than those of dIMRT. For the left lung, the V20 and V5 of VMAT were lower than those of the other two techniques, while Dmean and V30 were higher, and the V10 of VMAT was lower than that of sIMRT. For the heart, VMAT demonstrated the lowest Dmean, V30, and V20 and the highest V40. VMAT produced the lowest Dmax of the spinal cord.

Although the difference in dosimetric OARs among the three techniques was statistically significant, it was not clinically significant due to the small differences between each group. Therefore, VMAT is not inferior to sIMRT and dIMRT in OARs protection.

Analysis of 3D γ pass rates with 3DVH

[Table 3] and [Figure 5] show the 3D γ pass rate analysis results for the entire area and each ROI of sIMRT, dIMRT, and VMAT. Except for the fact that the right lung γ pass rate was not significantly different between sIMRT and dIMRT under the gamma criteria of 3 mm/3% and 1 mm/1% (P > 0.05), the differences in γ pass rates between the other groups were statistically significant (P < 0.05). The γ pass rates of VMAT are >95% under the gamma criteria of 3 mm/3% and 2 mm/2%, meeting the clinical demands. For the gamma 3 mm/3% criteria, the entire area, PTV, lungs, heart, and spinal cord of VMAT showed the highest γ pass rates than the other two techniques, which was even more pronounced when the stricter gamma criteria of 2 mm/2% and 1 mm/1% were applied.
Figure 5: γ pass rates of sIMRT, dIMRT, and VMAT. (a–f) represents the γ pass rates of entire area, PTV, R-lung, L-lung, heart, spinal cord under the gamma 3mm/3% criteria; (g–l) represents the γ pass rates under the gamma 2mm/2% criteria; (m–r) represents the γ pass rates under the gamma 1mm/1% criteria. sIMRT: Step-shoot intensity-modulated radiotherapy, dIMRT: Dynamic intensity-modulated radiotherapy, VMAT: Volume-modulated arc therapy, PTV: Planning target volume.

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Table 3: Comparison of γ pass rates of step-shoot intensity-modulated radiotherapy, dynamic intensity-modulated radiotherapy and volume-modulated arc therapy

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  Discussion Top


In recent years, VMAT has been widely used because of its uniform dose distribution and short treatment delivery time, but it also has a shortcomings, as it is difficult to control the dose delivered to the lungs, especially in low-dose areas.[25],[26] Radiopulmonary lesion (RPL) is the main factor affecting the quality of life after thoracic tumor radiotherapy, and the occurrence of severe RPL is closely related to the volume of the low-dose irradiated area of the lungs, especially the V5 of the lungs.[27] Therefore, VMAT is rarely used in the treatment of thoracic tumors. Radiotherapy is one of the comprehensive treatment measures for invasive thymoma, and it is a necessary treatment for thymoma patients with Stage II or III disease diagnosed by pathology.[28–30] Only small clinical studies have reported the use of VMAT in thymoma radiotherapy.[31] In our study, we analyzed and compared the dosimetric parameters and verification results of sIMRT, sIMRT, and VMAT. We provide a reference for the selection of different IMRT for thymoma in the clinic.

The results of this study showed that VMAT was not inferior to sIMRT and dIMRT in thymoma radiotherapy. For the target volume, the dose distribution of VMAT met the clinical requirements and was comparable to those of the other two techniques. VMAT demonstrated the highest CI and the lowest HI than sIMRT and dIMRT, resulting in better conformity and uniformity of target volume. For the OARs, VMAT showed the lowest V20, V10, and V5 of the L-lung, the lowest V30 and V20 of the heart, and the lowest Dmax of the spinal cord compared to sIMRT and dIMRT. For the right lung, VMAT showed the lowest V10, and V5 was lower than sIMRT and higher than dIMRT. VMAT exhibited better protection of the left lung, heart, and spinal cord and had a protective effect on the right lung, especially the low-dose irradiated area. At the same time, VMAT reduced the treatment delivery time, which can reduce the position movement error caused by the discomfort of the patient due to the long treatment time.

Of course, whether a treatment plan can be accurately implemented for treatment is also critical. Multiple variable parameters make the VMAT treatment plan more complex during execution; therefore, the dosimetric verification of VMAT is important. ArcCHECK-3DVH system is a 3D dosimetric verification system that has been used for dosimetric verification in multiple treatment plans.[12],[13] Our study found that VMAT showed the highest γ pass rates than the other two techniques, which was even more pronounced when stricter gamma criteria of 2 mm/2% and 1 mm/1% were applied. From the dose validation results, VMAT outperforms sIMRT and dIMRT.


  Conclusion Top


VMAT can be applied to radiotherapy of thymoma, and the accuracy of treatment plan execution can be guaranteed through the ArcCHECK-3DVH system.

Financial support and sponsorship

This work was supported by the National Key Research and Development Project (2018YFC0114402).

Conflicts of interest

There are no conflicts of interest.



 
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    Figures

  [Figure 1], [Figure 2], [Figure 3], [Figure 4], [Figure 5]
 
 
    Tables

  [Table 1], [Table 2], [Table 3]



 

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