A widely tested model for head scatter influence on photon beam output

Area of Science:

• Medical Physics
• Radiation Oncology
• Radiotherapy Physics

Background:

• Accurate monitor unit (MU) verification is crucial for safe and effective radiation therapy.
• Scattering within the treatment head can introduce significant errors in MU calculations.
• Existing models may lack generalizability or require extensive data for implementation.

Purpose of the Study:

• To develop and validate a semi-analytical model for predicting treatment head scatter effects on MU verification.
• To implement the model using minimal experimental data, specifically output factors in air (OFair).
• To incorporate geometry-dependent uncertainty estimation into the model.

Main Methods:

• Constructed a semi-analytical model incorporating primary energy fluence, extra-focal scatter, secondary collimator scatter, and monitor chamber backscatter.
• Utilized basic treatment head geometry and 10 measured OFair values for square fields as model input.
• Validated the model across 19 photon beams (4-50 MV) from nine manufacturers, testing various field configurations and positions.

Main Results:

• The model achieved errors below 1% (2 S.D.) in typical clinical scenarios for all tested beams.
• Uncertainties increased to approximately 2% in exceptional cases involving unconventional designs or distant dosimetry points.
• A method for predicting uncertainties under varying treatment conditions was developed based on result analysis.

Conclusions:

• A general, easily implementable head scatter model for computerized MU verification has been successfully developed.
• The model demonstrates adequate performance across all commercially available treatment units.
• The model provides a valuable tool for estimating uncertainties in MU verification.