Measurement of changes in linear accelerator photon energy through flatness variation using an ion chamber array

Area of Science:

• Medical Physics
• Radiation Oncology
• Accelerator Physics

Background:

• Megavoltage linear accelerators are crucial in radiation therapy.
• Accurate monitoring of photon beam energy is essential for effective cancer treatment.
• Traditional methods like percent depth dose (PDD) have limitations in detecting energy variations.

Purpose of the Study:

• To compare the efficacy of flatness metrics versus PDD in identifying photon beam energy shifts.
• To evaluate the sensitivity of different flatness parameters, including relative flatness (Flat) and dose along diagonals normalized to central axis dose (FDN).
• To determine the most reliable metric for quality assurance in linear accelerator operations.

Main Methods:

• Photon beam energy was altered by adjusting the bending magnet current (±15%) on a linear accelerator.
• Flatness metrics (Flat and FDN) were measured using an ionization chamber array.
• PDD was measured in water for various field sizes and depths using a cylindrical chamber.

Main Results:

• PDD showed greater sensitivity to energy increases than decreases, with limited sensitivity to reductions below nominal energy for 18-MV beams.
• The Flat metric was more sensitive to energy decreases than increases for 18-MV beams.
• FDN demonstrated sensitivity to both energy increases and reductions for 6-MV and 18-MV beams, outperforming PDD.

Conclusions:

• Flatness-based metrics, particularly FDN, are more sensitive to photon beam energy changes than PDD.
• FDN is the most sensitive metric for detecting energy variations in 6-MV and 18-MV photon beams.
• The ionization chamber array facilitates convenient measurement of FDN for routine accelerator quality assurance.