Correction method for in-air output ratio for output variations occurring with changes in backscattered radiation

Area of Science:

• Medical Physics
• Radiation Oncology

Background:

• The in-air output ratio (S(c)) is crucial for accurate radiation therapy dose calculations.
• Current methods using equivalent square fields for rectangular fields introduce slight inaccuracies.
• Existing correction methods for monitor-backscatter effects can be complex.

Purpose of the Study:

• To propose a simpler correction method for the in-air output ratio (S(c)) in rectangular fields.
• To address the discrepancies between measured S(c) and values obtained from equivalent square field formulas.
• To refine dose calculations in radiation therapy for rectangular treatment fields.

Main Methods:

• Assumed output variation as a product of variations from collimator jaw backscatter areas.
• Developed a correction factor as the ratio of output variations between rectangular and equivalent square fields.
• Utilized telescope measurements to determine output variation.

Main Results:

• The proposed correction method reduced differences between measured and corrected S(c) to within -0.20% to 0.28% for symmetric rectangular fields.
• The correction factor proved effective for both symmetric and asymmetric rectangular fields.
• Demonstrated improved accuracy of S(c) calculations for rectangular fields.

Conclusions:

• A straightforward method is presented to correct S(c) values derived from equivalent square field formulas.
• The study provides a means to determine output variation for fields defined by collimator jaws.
• The proposed technique enhances the accuracy of radiation dose calculations in clinical practice.