[Comments on evaluation method for equivalent square fields to the collimator scatter factors of rectangular fields]

Area of Science:

• Medical Physics
• Radiation Oncology
• Radiotherapy Dosimetry

Context:

• Accurate determination of collimator scatter factors (Sc) is essential for precise dose calculations in external beam radiotherapy.
• Previous methods for measuring Sc and deriving equivalent square fields have limitations.
• The use of mini-phantoms and specific chamber orientations impacts measurement reliability.

Purpose:

• To evaluate the influence of chamber axis orientation (parallel vs. perpendicular to the beam) on Sc measurements using an acrylic mini-phantom.
• To assess the impact of build-up caps on Sc measurements.
• To compare three methods for obtaining equivalent square fields and determine the most accurate approach for 4 MV and 10 MV X-rays.

Summary:

• Measurements of 4 MV and 10 MV X-ray collimator scatter factors (Sc) using an acrylic mini-phantom showed no significant difference with chamber axis parallel or perpendicular to the beam.
• Contaminant electrons did not affect chamber readings when the chamber axis was parallel to the beam.
• The study found that the geometrical weight factor (k) for field-mapping methods is most accurately determined using measured Sc data, considering the radiation head's structure (flattening filter, collimators) and chamber position.

Impact:

• Identifies the most accurate method for calculating equivalent square fields, enhancing the precision of radiation treatment planning.
• Provides guidance on optimal measurement techniques for collimator scatter factors, reducing uncertainties in dose delivery.
• Contributes to improved quality assurance in radiotherapy by refining dosimetry protocols.