Measurements of basic parameters in wedged high-energy photon beams using a mini-phantom

Area of Science:

• Medical Physics
• Radiation Oncology
• Radiotherapy Physics

Background:

• Accurate dosimetry is crucial for effective radiotherapy.
• Specifying dosimetric quantities for wedged beams is complex.
• Understanding beam quality variations is essential for treatment planning.

Purpose of the Study:

• To measure basic dosimetric quantities for wedged photon beams.
• To investigate beam quality changes with wedges at different energies.
• To develop a method for calculating output ratios in irregular wedged fields.

Main Methods:

• Measurements performed in a narrow coaxial mini-phantom for 6, 18, and 25 MV photon beams.
• Beam quality assessed using attenuation coefficient (mu) at varying depths.
• Wedge factors and output ratios measured as a function of field size at 10 cm water-equivalent depth.

Main Results:

• Beam softening observed in open beams with increasing distance from the collimator axis.
• Beam hardening detected at 6 MV with wedges, decreasing at 18 MV and becoming softening at 25 MV.
• Output ratios varied more with field size in wedged beams compared to open beams.

Conclusions:

• An equivalent square formula accurately predicts head-scatter factors for rectangular wedged fields.
• A proposed method for irregular wedged fields calculates the output ratio-wedge factor product with 1% agreement.
• The study provides essential data for accurate radiotherapy treatment planning with wedged beams.