Effective source size, yield and beam profile from multi-layered bremsstrahlung targets

Area of Science:

• Medical Physics
• Radiotherapy Technology
• High-Energy Physics

Background:

• Modern conformal radiotherapy utilizes heterogeneous dose delivery with scanned high-energy bremsstrahlung beams.
• Limited target space in radiotherapy necessitates specialized, thin, multi-layered targets when using purging magnets.
• Accurate modeling of photon source characteristics is crucial for precise dose delivery.

Purpose of the Study:

• To develop a computational technique for determining bremsstrahlung beam properties from multi-layer targets.
• To calculate forward yield, angular distributions, and effective/virtual photon source characteristics.
• To optimize target design for limited space in scanned beam radiotherapy.

Main Methods:

• Utilized Gaussian approximation of the electron diffusion equation convolved with bremsstrahlung production.
• Applied deconvolution of electron multiple scattering from experimental beryllium target profiles to determine intrinsic photon profiles.
• Modeled arbitrary emittance electrons impinging on multi-layer targets of varying atomic numbers (1-100 MeV).

Main Results:

• Effective photon source size is approximately 0.1 mm for high-Z targets (e.g., tungsten).
• Effective photon source depth varies with atomic number (3-7 mm for low-Z, tenths of mm for high-Z).
• Optimal two-layer target (9 mm Be + 6 mm W) identified for 15 mm space limitation; thin Be targets yield high-intensity lobes.

Conclusions:

• The developed computational technique accurately models bremsstrahlung beams and photon source properties for multi-layer targets.
• Target design significantly impacts photon source characteristics and beam profiles, crucial for collimator alignment and dose distribution.
• Optimized thin targets, even with minimal electron energy loss, can achieve high forward dose rates, enhancing radiotherapy efficiency.