Irregular field calculation on the central beam axis of photon beams using sector-integration

Area of Science:

• Medical Physics
• Radiotherapy Physics
• Radiation Dosimetry

Background:

• Accurate calculation of irregular field factors is crucial for precise radiation dose delivery in radiotherapy.
• Existing methods may not fully account for all beam components, including scattered photons, in complex field geometries.

Purpose of the Study:

• To propose and validate a new method for calculating irregular field factors on the central beam axis in a homogeneous medium for X-ray beams.
• To introduce the irregular field factor as a ratio of output with and without blocking on the central beam axis.

Main Methods:

• The algorithm employs the sector-integration method, calculating circular field quantities from in-phantom measurements.
• Circular field quantities are derived from data measured for rectangular, block-shaped fields, incorporating all beam components.
• The method was validated for 6 MV photon beams, comparing calculated irregular field factors against measured values for various field shapes and blocking densities.

Main Results:

• The proposed method demonstrated excellent agreement with measured values, with discrepancies within 0.7%, even for heavily blocked fields (e.g., 40x40 cm2 to 5x5 cm2).
• The sector-integration approach effectively incorporates primary and secondary beam components, including photons scattered from blocks and the phantom.
• The method's accuracy was tested across different source-to-surface distances, depths, phantom compositions, and source-to-block distances.

Conclusions:

• The sector-integration method provides a robust and accurate approach for calculating irregular field factors in X-ray beams.
• This method simplifies the incorporation of scattered radiation, improving the precision of dose calculations in complex treatment fields.
• Further measurements are required to determine input data for applying this method under different setup conditions.