An analysis of equivalent fields for electron beam central-axis dose calculations

Area of Science:

• Medical Physics
• Radiation Oncology
• Dosimetry

Background:

• Equivalent field concepts (square, circle) are standard for photon beam dose calculations.
• These methods extend square/circular field data to irregular fields.
• Previous work suggested limited utility of equivalent fields for electron beams due to depth-dependent variations.

Purpose of the Study:

• To analyze the conditions under which equivalent fields might be applicable for electron beam dose calculations.
• To compare Fermi-Eyges theory-derived equivalent fields with existing approximations.
• To validate calculations against published experimental measurements.

Main Methods:

• Derived equivalent square field dimensions and circular field radii using Fermi-Eyges theory.
• Compared these derived fields with approximate equivalent fields used in electron dosimetry.
• Evaluated accuracy by comparing calculations with literature measurements.

Main Results:

• Accuracy of electron dose calculations using approximate equivalent fields decreases with reduced lateral scatter equilibrium.
• Calculations become accurate only when lateral scatter equilibrium is established.
• At equilibrium, central-axis dose is independent of field shape/size, making equivalent fields redundant.

Conclusions:

• Equivalent fields analyzed in this study are not suitable for electron beam dose calculations.
• The utility of equivalent fields is limited by the degree of lateral scatter equilibrium.
• The approach becomes unnecessary and potentially inaccurate under various conditions relevant to electron dosimetry.