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A Varian Clinac 2100C/D linac equipped with a
Mark II EPID (Varian Inc, Palo Alto, CA) and modified by an attachable micromultileaf
collimator (m3, BrainLAB AG, Heimstetten,
used. A 5mm diameter radiopaque ball (WL Phantom Pointer, BrainLAB) was
centered in the “isocenter” defined by the room lasers and portal test shots
for selected gantry, collimator and couch angles combinations were taken with a
m3 collimated beam (30x30mm, 6MV).
The software Rodeo1.1 (written in Fortran) detects the
centers of the radiation field and the ball shadow in every 2D image taken. The
algorithm calculates the field center from its penumbra by extracting squares
profiles and the pointer center position is determinated by means of a
summation filter. From all portal images, the 3D position of the radiation
isocenter can be computed and compared with the isocenter defined by the room
lasers. The distance between them is a measure of the alignment error.
The accuracy of the procedure described was investigated by applying known
shifts to the ball and recording the displacements detected by the mentioned
For comparison purpose, twenty gantry isocenter alignment verifications (270,
0, 90 and 180º angles) were performed using EPID and film based methods,
resulting in a total of 160 images to analyze (80 corresponding to each one).
The Bland-Altman statistic approach was used to assess the agreement between
them, in order establish the EPID measuring technique as the routine method and
replacing the old film based procedure. Besides, differences intra and
inter-observer was inspectioned for the film case.