Electron contamination and build-up doses in conformal radiotherapy fields.

Area of Science:

• Medical Physics
• Radiation Oncology
• Radiotherapy Physics

Background:

• The build-up region dose in radiotherapy is influenced by primary photons, patient backscatter, and external contamination.
• Accurate dose calculation in this region is crucial for effective radiation therapy planning.

Purpose of the Study:

• To develop and validate a model for determining build-up dose in arbitrarily shaped treatment fields.
• To improve the accuracy of radiation dose calculations in the build-up region.

Main Methods:

• A model was developed based on measured data, separating build-up dose into primary and contamination components.
• The contamination component was analyzed using 1 cm x 1 cm cross-sectional elements at the isocenter.
• Measured data were fitted to an exponential function to determine component magnitudes, with the exponent varying linearly with depth.

Main Results:

• The model accurately determined build-up dose for various treatment field shapes.
• The exponent in the dose model showed a linear relationship with depth for photon energies from 4 MV to 20 MV.
• The primary component was approximated using a specific function involving primary surface dose (PSD).

Conclusions:

• The proposed model provides a reliable method for calculating build-up dose in complex radiotherapy fields.
• The findings contribute to more precise radiation dose delivery and treatment planning.
• The model's accuracy was confirmed through good agreement with calculated doses for diverse field shapes.