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Cho, Chun, Son, An, Yoon, Choi, Kim, Park, and Kim: Efficient Verification of X-ray Target Replacement for the C-series High Energy Linear Accelerator
Original Article

Progress in Medical Physics 2018;29(3):92-100.

Published online: 19 September 2018

DOI: https://doi.org/10.14316/pmp.2018.29.3.92

Efficient Verification of X-ray Target Replacement for the C-series High Energy Linear Accelerator

Jin Dong Cho∗, †, ‡, Minsoo Chun∗, Jaeman Son∗, †, Hyun Joon An∗, †, Jeongmin Yoon∗, Chang Heon Choi∗, †, §, Jung-in Kim∗, †, §, Jong Min Park∗, ΙΙ, †, §, Jin Sung Kim‡,

∗Department of Radiation Oncology, Seoul National University Hospital, Seoul, Korea

†Biomedical Research Institute, Seoul National University Hospital, Seoul, Korea

‡Department of Radiation Oncology, Yonsei University College of Medicine, Seoul, Korea

§Institute of Radiation Medicine, Seoul National University Medical Research Center, Seoul, Korea

ΙΙCenter for Convergence Research on Robotics, Advance Institutes of Convergence Technology, Suwon, Korea

Corresponding author Jin Sung Kim (jinsung@yuhs.ac) Tel: 82-2-2228-8110 Fax: 82-2-2227-7823

Received 5 September 201814 September 2018       Accepted 17 September 2018

Copyright © 2018 Korean Society of Medical Physics

This is an Open Access article distributed under the terms of the Creative Commons Attribution Non-Commercial License (http://creativecommons.org/licenses/by-nc/3.0) which permits unrestricted non-commercial use, distribution, and reproduction in any medium, provided the original work is properly cited.

Abstract

The manufacturer of a linear accelerator (LINAC) has reported that the target melting phenomenon could be caused by a non-recommended output setting and the excessive use of monitor unit (MU) with intensity-modulated radiation therapy (IMRT). Due to these reasons, we observed an unexpected beam interruption during the treatment of a patient in our institution. The target status was inspected and a replacement of the target was determined. After the target replacement, the beam profile was adjusted to the machine commissioning beam data, and the absolute doses-to-water for 6 MV and 10 MV photon beams were calibrated according to American Association of Physicists in Medicine (AAPM) Task Group (TG)-51 protocol. To verify the beam data after target replacement, the beam flatness, symmetry, output factor, and percent depth dose (PDD) were measured and compared with the commissioning data. The difference between the referenced and measured data for flatness and symmetry exhibited a coincidence within 0.3% for both 6 MV and 10 MV, and the difference of the PDD at 10 cm depth (PDD10) was also within 0.3% for both photon energies. Also, patient-specific quality assurances (QAs) were performed with gamma analysis using a 2-D diode and ion chamber array detector for eight patients. The average gamma passing rates for all patients for the relative dose distribution was 99.1%±1.0%, and those for absolute dose distribution was 97.2%±2.7%, which means the gamma analysis results were all clinically acceptable. In this study, we recommend that the beam characteristics, such as beam profile, depth dose, and output factors, should be examined. Further, patient-specific QAs should be performed to verify the changes in the overall beam delivery system when a target replacement is inevitable; although it is more important to check the beam output in a daily routine.

Keywords: Linear accelerator, Target degradation, Target melting, Beam verification, IMRT verification

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Fig. 1
The punched tungsten target.
pmp-29-92f1.tif
Fig. 2
Profile comparison between reference (dashed line) and measurement (solid line) for (a) 6 MV, (b) 10 MV photon beam. The measurement condition was as follows: source-to-surface distance (SSD) of 100 cm, reference depth of 10 cm, and field size of 35×35 cm2.
pmp-29-92f2.tif
Fig. 3
Percentage depth doses comparisons between reference (dashed line) and measurement (solid line) for (a) 6 MV, (b) 10 MV.
pmp-29-92f3.tif
Fig. 4
Sample gamma analysis graphical user interface (GUI) provided by SNC Patient software (Sun Nuclear Corporation, Melbourne, FL) for verification of intensity-modulated radiation therapy (IMRT) plan.
pmp-29-92f4.tif
Table 1.
Percent depth dose at depth 10 cm (PDD10) comparisons between reference and measurement for 6 MV and 10 MV photon beam.
Field Size (cm2) 6 MV 10 MV
Reference Measured Difference Reference Measured Difference
3×3 61.3% 61.3% 0.0% 71.1% 71.2% 0.1%
6×6 64.6% 64.5% −0.1% 73.0% 73.3% 0.3%
10×10 67.3% 67.3% 0.0% 74.4% 74.6% 0.2%
20×20 70.1% 70.4% 0.3% 75.9% 76.0% 0.1%
40×40 72.7% 72.8% 0.1% 77.3% 77.5% 0.2%
Table 2.
Flatness and symmetry comparison between reference and measurement data for different field sizes and depths for 6 MV and 10 MV photon beams.
Field Size (cm2) Depth (cm) 6 MV 10 MV
Flatness Symmetry Flatness Symmetry
Reference Measured Difference Reference Measured Difference Reference Measured Difference Reference Measured Difference
3×3 d max 8.4% 7.7% −0.7% 0.9% 0.5% −0.4% 10.3% 9.5% −0.8% 1.1% 0.3% −0.8%
5 8.6% 8.0% −0.6% 0.8% 0.5% −0.3% 10.4% 10.0% −0.4% 0.8% 0.5% −0.3%
10 8.5% 7.8% −0.7% 0.9% 0.3% −0.6% 10.3% 9.8% −0.5% 0.9% 0.3% −0.6%
20 8.0% 7.4% −0.6% 1.0% 0.8% −0.2% 9.8% 9.3% −0.5% 0.9% 0.5% −0.4%
30 7.7% 7.0% −0.7% 0.9% 0.6% −0.3% 9.4% 8.9% −0.5% 1.0% 0.7% −0.3%
6×6 d max 1.8% 1.7% −0.1% 0.3% 0.5% 0.2% 3.3% 3.1% −0.2% 0.7% 0.3% −0.4%
5 2.5% 2.4% −0.1% 0.3% 0.6% 0.3% 3.9% 3.7% −0.2% 0.7% 0.4% −0.3%
10 3.0% 3.0% 0.0% 0.3% 0.6% 0.3% 4.1% 4.1% 0.0% 0.7% 0.3% −0.4%
20 3.2% 3.2% 0.0% 0.3% 0.5% 0.2% 4.4% 4.3% −0.1% 0.7% 0.4% −0.3%
30 3.3% 3.4% 0.1% 0.3% 0.3% 0.0% 4.3% 4.4% 0.1% 0.5% 0.5% 0.0%
10×10 d max 1.1% 1.0% −0.1% 0.6% 0.9% 0.3% 1.8% 1.4% −0.4% 0.8% 0.3% −0.5%
5 1.8% 1.7% −0.1% 0.6% 0.7% 0.1% 2.3% 1.9% −0.4% 0.7% 0.3% −0.4%
10 2.6% 2.6% 0.0% 0.5% 0.8% 0.3% 2.9% 2.7% −0.2% 0.7% 0.3% −0.4%
20 3.4% 3.4% 0.0% 0.5% 0.7% 0.2% 3.6% 3.5% −0.1% 0.7% 0.3% −0.4%
30 3.9% 4.0% 0.1% 0.6% 0.8% 0.2% 4.1% 3.9% −0.2% 0.7% 0.5% −0.2%
20×20 d max 1.2% 1.5% 0.3% 0.6% 0.8% 0.2% 0.9% 0.8% −0.1% 0.7% 0.3% −0.4%
5 1.1% 1.2% 0.1% 0.4% 0.9% 0.5% 1.3% 1.0% −0.3% 0.7% 0.3% −0.4%
10 2.3% 2.2% −0.1% 0.6% 0.9% 0.3% 2.3% 2.1% −0.2% 0.7% 0.4% −0.3%
20 4.4% 4.3% −0.1% 0.7% 0.7% 0.0% 4.3% 4.0% −0.3% 0.7% 0.3% −0.4%
30 5.6% 5.5% −0.1% 0.4% 0.8% 0.4% 5.6% 5.5% −0.1% 0.8% 0.6% −0.2%
Table 3.
Output factors comparison between reference and measurement data for 6 MV and 10 MV photon beams.
Field Size 6 MV 10 MV
X (cm) Y (cm) Reference Measured Difference Reference Measured Difference
3 3 0.830 0.832 0.23% 0.849 0.851 0.21%
3 10 0.889 0.894 0.51% 0.905 0.911 0.59%
3 40 0.924 0.930 0.58% 0.936 0.944 0.76%
5 5 0.895 0.895 −0.03% 0.914 0.915 0.14%
5 15 0.960 0.963 0.26% 0.970 0.973 0.30%
7 7 0.945 0.944 −0.10% 0.955 0.956 0.12%
7 20 1.009 1.011 0.21% 1.009 1.011 0.24%
10 3 0.879 0.879 −0.05% 0.894 0.894 −0.02%
10 10 1.000 1.000 0.00% 1.000 1.000 0.00%
10 30 1.066 1.067 0.15% 1.053 1.055 0.19%
15 5 0.947 0.945 −0.20% 0.954 0.953 −0.05%
15 15 1.063 1.062 −0.13% 1.047 1.046 −0.08%
15 40 1.123 1.125 0.22% 1.097 1.097 −0.04%
20 7 0.995 0.994 −0.14% 0.991 0.991 0.03%
20 20 1.105 1.104 −0.07% 1.077 1.076 −0.08%
30 10 1.050 1.050 −0.01% 1.035 1.035 −0.05%
30 30 1.163 1.163 0.00% 1.121 1.121 0.02%
40 3 0.899 0.898 −0.12% 0.909 0.908 −0.11%
40 15 1.107 1.107 −0.04% 1.077 1.077 0.04%
40 40 1.200 1.204 0.38% 1.153 1.154 0.07%
Table 4.
Gamma passing rates for eight patients with two types of detectors.
Patient Energy MapCHECK2 (∗Abs.) MatriXX (†Rel.)
Patient 1 6X 92.9% 98.4%
Patient 2 10X 99.5% 99.9%
Patient 3 10X 96.7% 98.5%
Patient 4 6X 92.9% 98.6%
Patient 5 6X 99.0% 100.0%
Patient 6 6X 99.8% 100.0%
Patient 7 10X 99.8% 100.0%
Patient 8 6X 97.2% 97.3%

∗ The gamma passing rates for absolute dose distribution.

† The gamma passing rates for relative dose distribution.

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