Design of metallic electron beam cones for an intraoperative therapy linear accelerator.
A set of circular collimators and treatment cones from 5 to 12 cm diameter has been designed for an intraoperative accelerator (6-18 MeV) that has an optical docking system. Electron beam scattering theory has been used to minimize their weight while minimizing leakage radiation. Both acrylic and brass were evaluated as possible materials; however, because of substantial electron leakage through the lateral cone wall for acrylic, we have concluded that 2 mm thick brass walls are more desirable than acrylic walls. At 18 MeV, isodose measurements beneath the cones showed hot spots as great as 120% for both materials. The placement and dimension of an internal trimmer ring inside the brass cone was studied as a method for reducing the hot spots, and it was found this could only be accomplished at the expense of decreasing coverage of the 90% isodose surface. The effects of 1 degree cone misalignment on the dose distribution has been studied and found to generate changes of less than 5% in the dose and 3 mm in position of the 90% isodose surface. In a study of the contribution of the cone and its matching collimator assembly to x-ray room leakage, it was noted that although the treatment cone had a negligible contribution, the upper annuli of the upper collimator assembly contributed as much as 80% of the leakage at 16 MeV for the 5-cm cone.
Hogstrom
K R
KR
Department of Radiation Physics, University of Texas M. D. Anderson Cancer Center, Houston 77030.
Boyer
A L
AL
Shiu
A S
AS
Ochran
T G
TG
Kirsner
S M
SM
Krispel
F
F
Rich
T
T
D006801:Humans; D007432:Intraoperative Period; D010315:Particle Accelerators / Q000295:instrumentation*; D011835:Radiation Protection / Q000295:instrumentation*; D011874:Radiometry; D014965:X-Rays