SU-E-T-647: Quality Assurance of VMAT by Gamma Analysis Dependence On Low-Dose Threshold

Authors

  • Song J,

    1. Department of Biomedical Engineering, Reasearch Institute of Biomedical Engineering, The Catholic University of Korea, Seoul
    2. Department of Radiation Oncology, Uijeongbu St. Mary's Hospital, Gyeonggi-do
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  • Kim M,

    1. Department of Biomedical Engineering, Reasearch Institute of Biomedical Engineering, The Catholic University of Korea, Seoul
    2. Department of Radiation Oncology, Uijeongbu St. Mary's Hospital, Gyeonggi-do
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  • Park S,

    1. Department of Biomedical Engineering, Reasearch Institute of Biomedical Engineering, The Catholic University of Korea, Seoul
    2. Department of Radiation Oncology, Uijeongbu St. Mary's Hospital, Gyeonggi-do
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  • Lee S,

    1. Department of Biomedical Engineering, Reasearch Institute of Biomedical Engineering, The Catholic University of Korea, Seoul
    2. Department of Radiation Oncology, Uijeongbu St. Mary's Hospital, Gyeonggi-do
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  • Lee M,

    1. Department of Biomedical Engineering, Reasearch Institute of Biomedical Engineering, The Catholic University of Korea, Seoul
    2. Department of Radiation Oncology, Uijeongbu St. Mary's Hospital, Gyeonggi-do
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  • Suh T

    1. Department of Biomedical Engineering, Reasearch Institute of Biomedical Engineering, The Catholic University of Korea, Seoul
    2. Department of Radiation Oncology, Uijeongbu St. Mary's Hospital, Gyeonggi-do
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Abstract

Purpose:

The AAPM TG-119 instructed institutions to use low-dose threshold (LDT) of 10% or a ROI determined by the jaw when they collected gamma analysis QA data of planar dose distribution. Also, based on a survey by Nelms and Simon, more than 70% of institutions use a LDT between 0% and 10% for gamma analysis. However, there are no clinical data to quantitatively demonstrate the impact of the LDT on the gamma index. Therefore, we performed a gamma analysis with LDTs of 0% to 15% according to both global and local normalization and different acceptance criteria: 3%/3 mm, 2%/2 mm, and 1%/1 mm.

Methods:

A total of 30 treatment plans—10 head and neck, 10 brain, and 10 prostate cancer cases—were randomly selected from the Varian Eclipse TPS, retrospectively. For the gamma analysis, a predicted portal image was acquired through a portal dose calculation algorithm in the Eclipse TPS, and a measured portal image was obtained using a Varian Clinac iX and an EPID. Then, the gamma analysis was performed using the Portal Dosimetry software.

Results:

For the global normalization, the gamma passing rate (%GP) decreased as the LDT increased, and all cases of low-dose thresholds exhibited a %GP above 95% for both the 3%/3 mm and 2%/2 mm criteria. However, for local normalization, the %GP increased as LDT increased. The gamma passing rate with LDT of 10% increased by 6.86%, 9.22% and 6.14% compared with the 0% in the case of the head and neck, brain and prostate for 3%/3 mm criteria, respectively.

Conclusion:

Applying the LDT in the global normalization does not have critical impact to judge patient-specific QA results. However, LDT for the local normalization should be carefully selected because applying the LDT could affect the average of the %GP to increase rapidly.

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