TITLE:
Off-Isocenter Winston-Lutz Test for Stereotactic Radiosurgery/Stereotactic Body Radiotherapy
AUTHORS:
Junfang Gao, Xiaoqian Liu
KEYWORDS:
Off-Isocenter Winston-Lutz Test, SRS, SBRT
JOURNAL NAME:
International Journal of Medical Physics, Clinical Engineering and Radiation Oncology,
Vol.5 No.2,
May
26,
2016
ABSTRACT: The
single-isocenter technique in linear accelerator-based stereotactic
radiosurgery/stereotactic body radiotherapy (SRS/SBRT) has been broadly used to
treat multiple lesions. However, quantitative study to verify that the
mechanical field center coincides with the radiation field center when both are
off from the isocenter has never been performed. We developed an innovative
method to measure this accuracy, called the off-isocenter Winston-Lutz test,
and here we provided a practical clinical guideline to implement this
technique. We used ImagePro V.6 to analyze images of a Winston-Lutz phantom
obtained using a Varian 21EX linear accelerator with an electronic portal
imaging device, set up as for single-isocenter SRS/SBRT for multiple lesions.
We investigated asymmetry field centers that were 3 cm and 5 cm away from the
isocenter, as well as performing the standard Winston-Lutz test. We used a
special beam configuration to acquire images while avoiding collision, and we
investigated both jaw and multileaf collimation. For the jaw collimator
setting, at 3 cm off-isocenter, the mechanical field deviated from the
radiation field by about 2.5 mm; at 5 cm, the deviation was above 3 mm, up to
4.27 mm. For the multileaf collimator setting, at 3 cm off- isocenter, the
deviation was below 1 mm; at 5 cm, the deviation was above 1 mm, up to 1.72 mm,
which was 72% higher than the tolerance threshold. These results indicated that
the further the asymmetry field center is from the machine isocenter, the
larger the deviation of the mechanical field from the radiation field, and the
distance between the center of the asymmetry field and the isocenter should not
exceed 3 cm in our clinic. We recommend that every clinic that uses linear
accelerator, multileaf collimator-based SRS/SBRT perform the off-isocenter
Winston-Lutz test in addition to the standard Winston-Lutz test and use their
own deviation data to create planning guideline.