Purpose:
The aim of this study was to develop a method for the direct measurement of
electron beam width and distribution at the scattering foil on the carrousel in
a medical linear accelerator gantry head, which differs from an existing
indirect method for measuring the focal spot size using a camera or metallic
slit located outside the gantry head. Methods: The electron beam emitted by the
linear accelerator was used to irradiate radiochromic film mounted on the
scattering foil on the carrousel, which was not used for clinical treatment.
The electron beam width at the scattering foil position was then evaluated using
the full width at half maximum of the Gaussian distribution approximated from
each one dimensional distribution of the irradiated radiochromic film. Results:
The electron beam width at the scattering foil position was found to be 3.1 to
6.4 mm in the crossline direction and 2.8 to 5.5 mm in the inline direction
with electron energy of 4 to 16 MeV. The two-dimensional distribution of the
electron beam was therefore elliptical or distorted in shape, not circular. Conclusions:
Direct measurement of the electron beam width at the scattering foil in the
carrousel of a medical linear accelerator is possible, though the use of lower
sensitivity film in addition to indirect methods is expected to bring about
better results. However, as this method does not allow for direct measurement
of the incident angle of the accelerated electron beam, further improvements
and refinements are still needed.
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