On the initial angular variances of clinical electron beams

Area of Science:

• Medical Physics
• Radiation Oncology
• Accelerator Physics

Background:

• Electron beam radiotherapy requires accurate characterization of electron beams for treatment planning.
• Current pencil beam models rely on parameters like initial energy and angular variance.
• Advanced models need detailed phase space information, including primary and scatter components.

Purpose of the Study:

• To determine the suitability of standard values for initial angular variance in electron beam radiotherapy.
• To investigate the impact of using standard values on dose calculation accuracy.
• To recommend standard input parameters for pencil beam dose calculation models.

Main Methods:

• Derived initial angular variance (sigma2theta(x)) from penumbra measurements in air using film dosimetry.
• Measured at various distances from the lower collimator for different accelerators, energies, and field sizes.
• Calculated sigma2theta(x)/T(E) values, where T(E) is the ICRU-35 linear angular scattering power in air.

Main Results:

• For most modern radiotherapy accelerators, sigma2theta(x)/T(E) values around 13 cm were found, suitable as a standard for primary beam initial angular variance.
• An effective sigma2theta(x)/T(E) of 20 cm is recommended for pencil beam models with small air gaps (5-10 cm).
• The Elekta SL15 showed a 50% higher effective sigma2theta(x)/T(E) compared to other accelerators.

Conclusions:

• Standard values for initial angular variance can be adopted for primary beam characterization with minimal loss of accuracy.
• Recommended effective angular variance values improve dose calculation for pencil beam models in clinical scenarios.
• Accelerator-specific adjustments may be necessary for optimal accuracy, as seen with the Elekta SL15.