Electron beams

Skin collimation in electron therapy ensures sharper penumbra and maximal protection to adjacent critical structures. It also provides a better clinical dose to the target and avoids recurrences at the periphery. The thickness of the electron skin collimation must be adequate for shielding purposes, not too thick to cause discomfort to the patient and be conformal to the skin. This study assessed the clinical potential of machined brass skin collimation with variable thickness. Brass transmission factors for 6, 9, and 12 MeV electron beams were measured and used to determine the skin collimation clinically acceptable thickness. Dosimetric performance of the variable thickness skin collimation was evaluated for 9 MeV electrons within a rectilinear water-equivalent phantom and a water-filled head phantom. Results showed the variable thickness skin collimation is dosimetrically equivalent to the uniform thickness collimation. Favorable dosimetric advantages for brass skin collimation for small electron fields were achieved.

A coherent nonlinear wave-wave coupling effect which is consistent with
a more complete description of damping effects of a plane wave disturbance
of a finite plasma initially spatially uniform and Maxwellian in velocity
space is considered. A smeared out ion background is assumed and the
coupling between the ions and electrons is neglected. A self consistent
field and collective coordinate approach is used to obtain a dispersion
relation for mode coupling in a plasma. The equations for the amplitudes
and the frequencies are solved numerically both by direct time integration
and by a perturbation method for two and three modes. The perturbation
method solutions are obtained for the n mode equations. The perturbation
equations for two modes are solved analytically. For the two mode case
the resulting coupling shows that the energy oscillates between the modes
and that the periodicity of the amplitudes and the frequencies is associated
with the initial parameters. Energy feeding between the modes is also
observed for three or more modes. However, phase mixing occurs for more
than two modes.