A thin target approach for portal imaging in medical accelerators

Area of Science:

• Medical Physics
• Radiological Imaging

Background:

• Low-energy photons used in portal imaging are often absorbed by thick targets (e.g., Copper, Tungsten) in medical linear accelerators.
• Photoelectric absorption in high-Z materials significantly attenuates low-energy photons, limiting image quality.

Purpose of the Study:

• To develop and evaluate a new thin-target method for improved portal imaging using low-energy photons.
• To optimize target thickness and material to maximize low-energy photon fluence for enhanced image detail.

Main Methods:

• Utilized EGS4 Monte Carlo simulations to determine optimal target thickness for maximum photon fluence.
• Experimental validation using a 1.5 mm Copper target and a 5 mm Aluminum target (Copper mass equivalent).
• Acquired portal images using a Rando anthropomorphic phantom and a mammographic film sensitive to low-energy photons.

Main Results:

• A 1.5 mm Copper target yielded maximum photon fluence for a 4 MeV electron beam, significantly thinner than standard targets.
• The new thin-target method demonstrated a marked improvement in sharpness and contrast of anatomical details in portal images.
• Using a low-Z target (e.g., Aluminum) showed comparable results to Copper, while further reduction to Carbon or Beryllium offered no significant benefit.

Conclusions:

• A thin-target approach is effective for enhancing low-energy photon portal imaging in medical linear accelerators.
• Optimizing target thickness and material reduces self-absorption, leading to superior image quality.
• This method offers a practical improvement for diagnostic accuracy in radiotherapy imaging.