Prog Med Phys.  2024 Jun;35(2):52-57. 10.14316/pmp.2024.35.2.52.

Simulating the Effect of Junction Setup Error in Dual-Isocentric Volumetric Modulated Arc Therapy for Pelvic Radiotherapy with a Large Target

• Lee H
• Kim DW
• Joo JH
• Ki Y
• Kim W
• Park D
• Nam J
• Kim DH
• Jeon H

• KMID: 2557169
• DOI: http://doi.org/10.14316/pmp.2024.35.2.52

Abstract

Purpose
The use of two adjacent radiation beams to treat a lesion that is larger than the maximum field of a machine may lead to higher or lower dose distribution at the junction than expected. Therefore, evaluation of the junction dose is crucial for radiotherapy. Volumetric modulated arc therapy (VMAT) can effectively protect surrounding normal tissues by implementing a complex dose distribution; therefore, two adjacent VMAT fields can effectively treat large lesions. However, VMAT can lead to significant errors in the junction dose between fields if setup errors occur due to its highly complex dose distributions.
Methods
In this study, setup errors of ±1, ±3, and ±5 mm were assumed during radiotherapy for treating large lesions in the lower abdomen, and their effects on the treatment dose distribution and target coverage were analyzed using gamma pass rate (GP) and homogeneity index (HI). All studies were performed using a computational simulation method based on our radiation treatment planning software.
Results
Consequently, when the setup error was more than ±3 mm, most GP values using a 3%/3-mm criterion decreased by <90%. GP was independent of the direction of the field gap (FG), whereas HI values were relatively more affected by negative values for FG.
Conclusions
Therefore, the size and direction of setup errors should be carefully managed when performing dual-isocentric VMATs for large targets.

Keyword

Large target; Radiation therapy; Volumetric modulated arc therapy; Setup error; Homogeneity index

Figure

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  Fig. 1 Description of field gap (FG) simulation between fields A and B. (a) Negative FG, (b) golden standard, and (c) positive FG.

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  Fig. 2 Comparison of calculated dose distribution with five examples for patient 1 having different field gap (FG) values in the dose range of 80%–110%. (a) –5.0 mm (FG), (b) –3.0 mm, (c) 0 mm, (d) 3.0 mm, and (e) 5.0 mm.

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  Fig. 3 Gamma pass rates of all plans using the gamma criteria of (a) 3%/3 mm, (b) 2%/2 mm, and (c) 1%/1 mm with different setup errors.

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  Fig. 4 Homogeneity indices of all plans with different setup errors.

Reference

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