AbstractID: 8270 Title: A soft x-ray source for high contrast imaging on a conventional
accelerator
A soft x-ray source for high contrast imaging on a conventional accelerator
Unflattened x-rays from a 6 MeV electron beam incident on the 21 MeV primary scattering foil
are a high contrast alternative to flattened, 6 MV x-rays for portal imaging. Methods of achieving
higher contrast with radiotherapy accelerators have required adding a new lower-Z target
(Ostapiak et al, Med Phys 25:1910, 1998, and Tsechanski et al, Phys Med Biol 43:2221, 1998).
This new soft x-ray source may be easily employed clinically since the target is already available
in the treatment head. Energy, primary foil position, flatness and output settings are specific to
this beam. A 3 cm thick plastic graticle is placed in the compensator tray slot to absorb primary
electrons. This beam could either replace the existing "film" beam or used as an alternative. A
central axis depth dose curve and large-field (40 cm x 40 cm at isocenter) profiles were measured
for the soft beam with and without the graticle and compared to measurements done with the
conventional beam. The graticle absorbs the primary electrons. Dose drops by a factor of 4 at the
edge of the soft-beam profile. Depth penetration for both beams is essentially the same.
Radiographic localization films taken of the head of an anthropomorphic phantom show
improved image contrast with the soft beam. To prevent overheating the target a very
conservative, low beam current was used and exposure times were 100 times longer than
conventional imaging, for example, a 15 second exposure time was used to image the head
phantom using localization film.