Determining the Thickness of a Second-Generation Electron Skin Collimator
Abstract
Purpose
To determine the optimal thickness of an electron skin collimator that minimizes the surface dose inhomogeneity (“hot” spots), which reached up to 23% for previous prototypes, while maintaining the prior penumbral advantages.
Methods
We investigated different collimator thicknesses and shapes using stackable copper plates. Varying the copper thicknesses allowed us to determine whether energy-specific collimators would be beneficial for reducing inhomogeneities for 6, 9, 12, and 15 MeV electron beams. A fixed MU was delivered to radiochromic film with a 10x10 cm field. Copper pieces of varying thickness (0.64 cm and 1.27 cm), including intentionally thicker-than-necessary plates, were positioned at the field edge based on expected electron ranges. Dose inhomogeneity was defined as the percentage increase in dose relative to the central field dose.
Results
Using 1.27 cm copper, dose inhomogeneity was measured at 102.9%, 104.1%, 104.6%, and 103.7% for 6, 9, 12, and 15 MeV electrons, respectively. The 0.64 cm copper was insufficient for 12 and 15 MeV but produced dose inhomogeneities of 106.4% and 109.5% for 6 and 9 MeV electrons. Although 1.27 cm copper provides a larger scattering surface, it achieved lower inhomogeneities for these energies and maintained penumbra performance of the previous prototypes.
Conclusion
Copper thicknesses greater than the minimum required for attenuation do not worsen scatter-induced dose inhomogeneity. These results suggest that future skin collimator designs should prioritize selecting a thickness capable of effectively attenuating the highest-energy electrons, without concern that additional material will degrade performance at lower energies.