Research on the correction method for radiotherapy verification plans based on displaced electronic portal imaging device

Our plain-English summary of Research on the correction method for radiotherapy verification plans based on displaced electronic portal imaging device, by J. Guo, L. Zhou, H. Zeng (Journal of Applied Clinical Medical Physics, 2024).

The study

• Authors: J. Guo, L. Zhou, H. Zeng
• Journal: Journal of Applied Clinical Medical Physics
• Published: 2024;25(8):e14401
• Cited by: 1 article, as counted by NIH iCite
• Read it: PubMed · Publisher · Free full text

Abstract

Background: It has been observed that under the single isocenter conditions, the potential shifts of the electronic portal imaging devices (EPID) may be introduced when executing portal dosimetry (PD) plans for bilateral breast cancer, pleural mesothelioma, and lymphoma. These shifts are relative to the calibration positions of EPID and result in significant discrepancies in the plan verification results.

Purpose: To explore methods including correction model and specific correction matrices to revise the data obtained from displaced EPID.

Methods: Two methods, the correction model and the specific correction matrices, were applied to correct the data. Five experiments were designed and conducted to build correction model and to validate the effectiveness of these two methods. Gamma passing rates were calculated and data profiles along X-axis and Y-axis were captured.

Results: The gamma passing rates for the EPID-displaced IMRT validation plans after applying correction model, along with the application of specific correction matrices to VMAT and IMRT validation plans, exhibit results that are comparable to the cases with non-displaced EPID. Except for the VMAT plans applied correction model which showed larger discrepancies (0.041 ± 0.028, 0.049 ± 0.030), the other three exhibit minimal differences in discrepancy values. In all profiles, the corrected data from displaced EPID exhibit a high level of agreement with data obtained from non-displaced EPID. Good consistency is observed in actual application of the correction model and the specific correction matrices between gamma passing rates of data corrected and those of non-displaced data.

Conclusions: The proposed methods involving correction model and specific correction matrices can correct the data collected from the displaced EPID, and the gamma passing rates of the corrected data show results that are comparable to some extent with those of non-displaced data. Particularly, the results corrected by specific correction matrices closely resemble the data from non-displaced EPID.

© 2024 The Author(s). Journal of Applied Clinical Medical Physics published by Wiley Periodicals LLC on behalf of American Association of Physicists in Medicine.

In plain English

This was a technical radiotherapy physics study. It did not study asbestos exposure or patients' health. It concerned the electronic portal imaging device (EPID), a panel on a radiotherapy machine used to check treatment plans. The authors reported that the panel can shift from its calibrated position when checking plans for conditions including bilateral breast cancer, pleural mesothelioma and lymphoma, which distorts the check results. Using a Varian VitalBeam machine and 20 patient plans carried out in their radiotherapy department, the authors ran five experiments with test fields and patient plans. They tested two ways to correct data from a shifted panel: a correction model and custom 'specific correction matrices'. With the correction model, VMAT plans still differed significantly from unshifted results, with average differences of 0.041 and 0.049 in passing rate. With specific matrices, average differences were 0.004 for VMAT and 0.005 for IMRT, though the IMRT difference was statistically significant. The authors concluded that both methods can correct the data and that the specific matrices came closest to unshifted results. They noted the methods apply only to static-jaw plans, and the model only to square fields.

We wrote this plain-English summary from the full article with the help of AI, and a second AI model checked each sentence against the article. It is not the authors' wording; the abstract and the article are their own account.

What the study did and found

• Design: Experimental technical (medical physics) study of two correction methods for data from a displaced EPID panel, using five experiments on a linear accelerator
• Who was studied: Not people but radiotherapy plans and equipment. Twenty patient plans carried out in the authors' radiotherapy department (eleven VMAT and nine IMRT) were selected, from September 2021 to November 2022, plus open-field test plans. Experiment No.5 used 18 past patients (ten VMAT, eight IMRT). Equipment was a Varian VitalBeam accelerator with an aSi-1200 MV EPID. The authors' affiliations are two Soochow University hospitals in Suzhou, China; the article does not name which department carried out the work.
• Exposure: No asbestos exposure was studied. The study tested EPID panel displacements (0 to 4.5 cm in model building; 2.2, 3.7, 4.0 and 4.2 cm in validation) during verification of 6 MV x-ray plans.
• Main results:
  • In open-field tests with the panel shifted 2.2 cm, correction matrices from the model agreed closely with actual correction matrices. The lowest gamma passing rate was 0.968, for the 12.5 cm × 12.5 cm field.
  • For VMAT verification plans corrected with the correction model, gamma passing rates differed significantly from non-displaced data (t = 5.721 and t = 6.594, both p less than 0.05).
  • For IMRT verification plans corrected with the correction model, the difference from non-displaced data was significant at 2.2 cm displacement (p = 0.026). It was not significant at 3.7 cm (p = 0.052).
  • Average differences in gamma passing rate between corrected displaced and non-displaced data:
• VMAT plans with the correction model: 0.041 ± 0.028 (2.2 cm) and 0.049 ± 0.030 (3.7 cm).
• The other three conditions (IMRT with the correction model; VMAT and IMRT with specific matrices): 0.005 or less.
  • With specific correction matrices at 3.7 cm displacement:
• VMAT plans: no significant difference from non-displaced data (t = 1.45, p = 0.175).
• IMRT plans: a significant difference (t = 7.907, p less than 0.05).
  • In actual patient plans (Experiment No.5), the average difference from non-displaced gamma passing rates was 0.036 with the correction model and 0.002 with specific correction matrices. Bland-Altman points fell basically within the 95% consistency interval.
  • The authors suggested that the remaining VMAT discrepancy with the correction model may come from a bias in the PDIP algorithm's predicted reference dose for that VMAT plan.
• The authors' conclusion: The authors concluded that both the correction model and the specific correction matrices can correct data from a displaced EPID, with results comparable to some extent with non-displaced data. They found that results corrected by specific correction matrices closely resembled non-displaced data.
• Limitations the authors state:
  • The methods are applicable to plans using static jaws, not to jaw-tracking plans.
  • The correction model works only for square fields, which limits its use in IMRT plans.
  • In Experiment No.5, field sizes used for IMRT plans with the correction model were smaller than the targets.

Topics

Medical terms in this study, explained in our Lexicon: Bilateral, Pleural Mesothelioma.

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