Prog Med Phys.  2018 Sep;29(3):92-100. 10.14316/pmp.2018.29.3.92.

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

• Cho JD
• Chun M
• Son J
• An HJ
• Yoon J
• Choi CH
• Kim JI
• Park JM
• Kim JS

• KMID: 2421705
• DOI: http://doi.org/10.14316/pmp.2018.29.3.92

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.

Keyword

Linear accelerator; Target degradation; Target melting; Beam verification; IMRT verification

MeSH Terms

Freezing
Humans
Particle Accelerators*

Figure

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  Fig. 1 The punched tungsten target.

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  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.

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  Fig. 3 Percentage depth doses comparisons between reference (dashed line) and measurement (solid line) for (a) 6 MV, (b) 10 MV.

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  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.

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