Depth for dose calibration in high energy photon beams

Area of Science:

• Medical Physics
• Radiation Oncology

Background:

• The standard practice for determining output factors in photon fields often uses the depth of maximum dose.
• At higher photon energies, contaminating electrons significantly influence dose measurements at the maximum depth.
• This electron contamination is highly sensitive to beam geometry parameters, necessitating a deeper reference point.

Purpose of the Study:

• To evaluate the impact of electron contamination on absorbed dose measurements at various depths.
• To determine an optimal reference and normalization depth for photon beams that minimizes electron contamination effects.
• To provide recommendations for consistent and accurate output factor determination in radiation therapy.

Main Methods:

• Utilized a purging magnet to eliminate charged particles from the treatment head.
• Employed a helium bag to reduce secondary electron production between the treatment head and phantom.
• Investigated electron contamination effects across different megavoltage photon beam qualities.

Main Results:

• A depth of 10 cm was found to be beyond the range of contaminating electrons for photon energies up to 20 MV (TPR(20)(10) = 0.772).
• At 50 MV, contaminating electrons contributed 2-3% to the absorbed dose at a 10 cm depth (TPR(20)(10) = 0.810).
• These findings indicate that 10 cm is largely effective in mitigating electron contamination effects.

Conclusions:

• A reference and normalization depth of 10 cm is recommended for all megavoltage photon beam qualities.
• This includes Cobalt-60 and X-rays generated by accelerators up to 50 MV.
• Adopting a 10 cm depth ensures greater accuracy and consistency in output factor determination, crucial for radiation therapy dosimetry.