TITLE:
A Quality Assurance Approach for Linear Accelerator Mechanical Isocenters with Portal Images
AUTHORS:
Qiyong Fan, Sumin Zhou, Yu Lei, Sicong Li, Mutian Zhang
KEYWORDS:
Linear Accelerator, Isocenter, Mechanical Check, Quality Assurance, Spoke Shot
JOURNAL NAME:
International Journal of Medical Physics, Clinical Engineering and Radiation Oncology,
Vol.7 No.1,
February
27,
2018
ABSTRACT: Purpose: With
usually a millimeter-level PTV margin, stereotactic radiosurgery (SRS) and
stereotactic body radiation therapy (SBRT) pose a stringent requirement on the
isocentricity of the Linac. This requirement is partly fulfilled by routine
isocenter quality assurance (QA) test to verify the size and location of the
isocenter. The current common QA methods such as spoke shot were developed before
SBRT/SRS became popular and when IGRT was largely absent and hence
have their limitations. In this work, we describe an isocenter QA approach
based on portal imaging to provide the community with a superior alternative. Methods: The proposed approach
utilizes a BrainLab ball bearing (BB) phantom in conjunction with an electronic portal imaging devices (EPID) imager.
The BB phantom was first aligned with a calibrated room laser system. Portal
images were then acquired using 6 MV beam with a 2 × 2 cm2 open field and a 15 mm cone on a Varian TrueBeam STx machine. The gantry,
collimator, and table were rotated separately at selected angles to acquire a
series of portal images in order to determine the isocenter of each rotating
system. The location and diameter of these isocenters were determined by
calculating the
relative displacement of either BB or open field edge between the acquired EPID images. The demonstration
of the reproducibility and robustness of this EPID-based approach was carried out by repeating measurements 10 times
independently for each rotating system and simulating clinical scenarios of
asymmetric jaws and misalignment of BB phantom, respectively. Results: For
our TrueBeam STx machine, the isocenter diameter derived from open-field EPID
images was roughly 0.15 mm, 0.18 mm, 0.49 mm for the collimator, table, and
gantry, respectively. For the collimator and gantry, images taken with the cone
gave considerably smaller isocenter diameter. Results remained almost unchanged
despite the presence of simulated BB misalignment and asymmetric jaws error,
and between independent measurements. Isocenter location and diameter derived
from images obtained at a limited number of angles (≤11) were adequately
accurate to represent those derived from images of densely sampled angles. Conclusions: An EPID-based isocenter QA approach is described
and demonstrated to be accurate, robust, and reproducible. This approach
provides a superior alternative to conventional isocenter QA methods with no additional
cost. It can be implemented with convenience for any linear accelerator with
an EPID imager.