Electron contamination in ⁶⁰ Co gamma-ray beams

Area of Science:

• Medical Physics
• Radiation Oncology
• Radiotherapy Physics

Background:

• Radiotherapy photon beams contain secondary electrons from various sources.
• These electrons influence critical dosimetry parameters like skin sparing and dose buildup.
• Understanding electron contamination is crucial for accurate dose delivery in radiation therapy.

Purpose of the Study:

• To quantify the impact of secondary electrons on dose buildup curves from a 60Co source.
• To evaluate the effectiveness of different filters in reducing electron contamination.
• To assess the contribution of air-scattered electrons to the overall dose.

Main Methods:

• Dose buildup curves were measured in polystyrene using a flat ion chamber.
• Experiments utilized a 60Co source at varying source-to-surface distances (SSD).
• Various filters (Lucite, Cu, Pb-loaded acrylic, Ba/Pb-loaded glass) and a helium bag were employed to study electron generation and attenuation.

Main Results:

• Different filters significantly altered dose buildup curves by modifying electron contamination.
• Electron generation within the air path was investigated and quantified.
• The lowest achievable relative dose in a clear 60Co gamma-ray beam was estimated at approximately 12%.

Conclusions:

• Electron contamination significantly impacts radiotherapy dose distributions.
• Filter selection and beam path management are important for controlling secondary electrons.
• Accurate dosimetry requires accounting for electron scatter and filter interactions in 60Co beams.