Validity of equivalent square field concept in small field dosimetry

Area of Science:

• Medical Physics
• Radiation Oncology
• Radiotherapy Physics

Background:

• The equivalent square (ES) concept is standard for traditional radiation fields.
• For small fields, the Sclin concept, based on dosimetric field width (FWHM), was introduced.
• Evaluating the pros and cons of Sclin in small fields is crucial for accurate dosimetry.

Purpose of the Study:

• To compare the traditional ES (4A/P method) with the Sclin concept for small radiation fields.
• To evaluate the validity and limitations of both concepts across different linear accelerator designs.
• To understand the impact of machine design on field output factors and exchange factors in small fields.

Main Methods:

• Measured FWHM for 137 square and rectangular fields (5-50 mm) using a microSilicon detector and scanning water tank.
• Utilized Elekta (Versa) and Varian TrueBeam machines with varying MLC/Jaw configurations.
• Measured field output factors for all fields to assess the effect of the exchange factor.

Main Results:

• Both 4A/P and Sclin methods showed differences, with a linear relationship and machine-specific slopes (Elekta vs. Varian).
• For Elekta, 4A/P < Sclin; for Varian, 4A/P > Sclin for square fields.
• Exchange factors were similar for fields ≥10 mm but higher on Elekta for fields <10 mm.

Conclusions:

• The 4A/P and Sclin concepts are equivalent and can be related by an empirical equation.
• The 4A/P method remains valid for small fields, except for very small fields (≤10 mm) affected by source occlusion.
• The Sclin method is sensitive to measurement uncertainties, while 4A/P is more robust, relying mainly on machine geometry.