Technical note: Determining equivalent squares of high-energetic photon fields.

Area of Science:

• Medical Physics
• Radiation Oncology
• Dosimetry

Background:

• Accurate calculation of equivalent square fields is crucial for radiation therapy dose calculations.
• Existing methods like the Sterling equation and geometric mean have limitations in their validity range.
• Flattening Filter Free (FFF) beams are increasingly used, necessitating accurate dosimetry for these fields.

Purpose of the Study:

• To develop and validate a physical pencil beam model for accurate equivalent square calculations.
• To assess the method's performance across various definitions of equivalent squares, beam types (with/without flattening filter), photon energies, and depths.
• To provide a versatile and accurate tool for clinical dosimetry.

Main Methods:

• Utilized an analytical pencil beam model to integrate pencil beam kernels.
• Defined equivalent squares based on equal dose at the beam axis and equal depth dose (Tissue Phantom Ratio at 20 and 10 cm).
• Validated results against experimental measurements, Sterling equation, geometric mean, and published data (BJR Suppl. 25).

Main Results:

• The pencil beam model demonstrated the highest accuracy compared to measurements.
• Observed energy and depth dependencies for specific field configurations (small fields, elongated fields).
• The method showed agreement with previous studies for Flattening Filter Free (FFF) to With Flattening Filter (WFF) square field assignments.

Conclusions:

• The validity of the geometric mean and Sterling equation is limited; the pencil beam method offers broader applicability.
• The developed pencil beam method is valid for all aspect ratios, including elongated fields, in the primary dose-dominated region.
• The method is publicly available as a Python library and GUI, supporting quality assurance tasks like monitor unit checks.