Prog Med Phys.  2019 Dec;30(4):128-138. 10.14316/pmp.2019.30.4.128.

Segmental Analysis Trial of Volumetric Modulated Arc Therapy for Quality Assurance of Linear Accelerator

• Rahman MM
• Kim CH
• Huh HD
• Kim S

• KMID: 2468174
• DOI: http://doi.org/10.14316/pmp.2019.30.4.128

Abstract

PURPOSE
Segmental analysis of volumetric modulated arc therapy (VMAT) is not clinically used for compositional error source evaluation. Instead, dose verification is routinely used for plan-specific quality assurance (QA). While this approach identifies the resultant error, it does not specify which machine parameter was responsible for the error. In this research study, we adopted an approach for the segmental analysis of VMAT as a part of machine QA of linear accelerator (LINAC).
METHODS
Two portal dose QA plans were generated for VMAT QA: a) for full arc and b) for the arc, which was segmented in 12 subsegments. We investigated the multileaf collimator (MLC) position and dosimetric accuracy in the full and segmented arc delivery schemes. A MATLAB program was used to calculate the MLC position error from the data in the dynalog file. The Gamma passing rate (GPR) and the measured to planned dose difference (DD) in each pixel of the electronic portal imaging device was the measurement for dosimetric accuracy. The eclipse treatment planning system and a MATLAB program were used to calculate the dosimetric accuracy.
RESULTS
The maximum root-mean-square error of the MLC positions were <1 mm. The GPR was within the range of 98%-99.7% and was similar in both types of VMAT delivery. In general, the DD was <5 calibration units in both full arcs. A similar DD distribution was found for continuous arc and segmented arcs sums. Exceedingly high DD were not observed in any of the arc segment delivery schemes. The LINAC performance was acceptable regarding the execution of the VMAT QA plan.
CONCLUSIONS
The segmental analysis proposed in this study is expected to be useful for the prediction of the delivery of the VMAT in relation to the gantry angle. We thus recommend the use of segmental analysis of VMAT as part of the regular QA.

Keyword

VMAT; Quality assurance; Segmental arc; Dynalog file; Portal dosimetry

MeSH Terms

Calibration
Particle Accelerators*
Radiotherapy, Intensity-Modulated*
Research Design

Figure

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  Fig. 1 The full arc and its 12 segments. The segmental arcs were created by splitting one full arc into sub-arcs. Each sub-arc subtended an angle of 30°. All 24 arcs were created for two full arcs, one in the clockwise and one in the counter clockwise arc direction.

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  Fig. 2 Schematic of the dose distribution data matrix extracted from the portal dose distribution. Column and row numbers represent the X and Y coordinates respectively, as determined from the size of a pixel (0.392×0.392 mm2) on the electronic portal imaging device (EPID) at a source-to-surface distance (SSD) of 1000 mm. Therefore, the value at the ith column and jth row of the matrix represents the dose in a pixel of the EPID. This representation was used to compare the predicted and measured doses at each dose point on the EPID at an SSD of 1000 mm.

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  Fig. 3 Maximum root-mean-square (maxRMS) error of the multileaf collimator (MLC) position (mm) in the full and segmented arc portal dose quality assurance (PDQA) plans for the head and neck quality assurance (QA) plan with a pcsr class. The X-axis shows the names of the arc/arc segments, and the Y-axis shows the scale of the maxRMS value (in mm). Each arc segment represents the 30° arc range. The maxRMS values for the full arc and segmented arc deliveries were less than 1 mm. Seg, segment.

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  Fig. 4 Gamma passing rate (GPR) (%) in full arc and segmented arc portal dose quality assurance (PDQA) plans for the pcsr QA plans with a head and neck class. The X-axis shows the name of the arc/arc segments, and the Y-axis shows the scale of GPR (%). Each arc segment subtends an angle of 30°. Seg, segment.

• 

  Fig. 5 Dose differences in calibration units (CU) in the segments of arc1. Subpart of this figure represent the dose difference in segment 1–12, respectively. The color bars were set to ±15 dose difference ranges based on considerations of the maximum and minimum dose differences among all segments.

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  Fig. 6 Dose differences in calibration units (CUs) in the case of the full arc portal dose QA (PDQA) plan delivery. The subfigures on the left column (a, c) show the dose differences for arc1 and arc2 when the dose was delivered continually. The subfigures on the right column (b, d) show the dose differences accumulated from the segments of arc1 (b) and arc2 (d). The ranges of the color bars were set to ±40 CUs according to the maximum and minimum dose differences among all of the four dose difference distributions.

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  Fig. 7 Root-mean-square (RMS) dose difference in CUs in the full and segmented-arc PDQA plans intended for the Plan-Class Specific Reference (pcsr) QA plan with a head and neck class. RMS dose difference in full and segmented arc PDQA plans intended for the pcsr QA plan with a head and neck class. The X-axis shows the name of the arc/arc segments, and the Y-axis shows the scale of the RMS dose difference (in CUs). Each arc segment subtends an angle of 30°. Seg, segment.

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