Ionization produced by electron beams beneath curved surfaces.

Ionization produced by high-energy (6-18 MeV) electrons beneath cylindrical surfaces has been measured and compared to ionization at corresponding locations under a flat surface. Results are reported as ionization ratios over a range of depths, off-axis distances, radii of curvature, and electron energies. Phantoms with cylindrical entrance surfaces (radii 6-15 cm) were utilized to simulate patient contours. Measurements reveal that ionization ratios may depart from unity by as much as 50% and that a simple shifting of isodose curves may be insufficient to compensate for perturbation in electron dose distribution introduced by curved surfaces.