Prog Med Phys.  2024 Jun;35(2):45-51. 10.14316/pmp.2024.35.2.45.

Analysis of the Multi-Leaf Collimator Quality Assurance for the Halcyon^TM Linear Accelerator

• Jegal J
• Park H
• Kang S
• Kim Ji
• Choi CH

• KMID: 2557158
• DOI: http://doi.org/10.14316/pmp.2024.35.2.45

Abstract

Purpose
Accurate operation of the multi-leaf collimator (MLC), a key technology in intensity modulated radiation therapy (IMRT), is essential for safe and optimal radiation treatment. The Halcyon^TM linear accelerator has a collimator with low leakage and radiation transmission, making it suitable for IMRT. The limitations of the existing Halcyon^TM MLC quality assurance (QA) method were supplemented with a mathematical method, and the results were analyzed.
Methods
Electric portal imaging device (EPID) images obtained by performing the MLC QA plan on the Halcyon^TM was analyzed using Python. The picket fence tests were performed and compared using the maximum pixel value and mathematical methods. Dose rate, gantry speed, and leaf speed variation plan were performed for dose transmission comparison.
Results
For the maximum pixel value, the minimum distance between leaf junctions was 13.86 mm, and the maximum was 16.06 mm. However, for the mathematical method, the minimum and maximum were 14.54 mm and 15.68 mm, respectively. This suggests that setting the peak value to the highest value may cause an error in interpretation due to the limitations of the pixels of the EPID image. Performing QA on the remaining items confirmed that the measured values were within 3% of tolerance.
Conclusions
The presented analysis method applied to the MLC QA can derive more reasonable and valid values than existing methods, which will help with MLC monitoring by reducing errors in excessive interpretation.

Keyword

Halcyon^TM linear accelerator; Multi-leaf collimator quality assurance; Picket fence test; Electric portal imaging device image

Figure

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  Fig. 1 Electric portal imaging device image for picket fence test written by Python. X and Y axes represent the pixel number. A white vertical line in the figure is a leaf junction.

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  Fig. 2 Fitting curve samples are represented by the blue line, the pixel number (data) is the blue dot, and the red dot is the peak value at junction, where JN (N=1, 2, 3,…, 10) is the junction and LM (M=1, 2, 3,…, 49) is the leaf pair of the picket fence test.

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  Fig. 3 Electric portal imaging device images of open beam and each intensity modulated radiation therapy plan for the multi-leaf collimator quality assurance. (a) and (d) are for the dose rate plan, (b) and (e) are for the gantry speed plan, and (c) and (f) are for the leaf speed plan.

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  Fig. 4 Over- or under-estimation of cases of the general method using pixel value. The blue solid line is the 5th-order polynomial fitting function, and the red dot is the inflection point of the fitting function. The red arrow represents over- or under-estimated cases if the maximum pixel value distance method was used.

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  Fig. 5 Peak values trend as measured by setting the inflection point corresponding to the leaf pair. S, superior; PL, patient left; I, inferior; PR, patient right.

Reference

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