A modified method of calculating the lateral build-up ratio for small electron fields

Area of Science:

• Medical Physics
• Radiation Oncology
• Dosimetry

Background:

• Accurate dose calculation is crucial for effective radiation therapy.
• Small electron fields present unique challenges in dose calculation due to scatter effects.
• Existing methods may not fully account for lateral scatter variations in small fields.

Purpose of the Study:

• To introduce and validate an improved method for calculating dose per monitor unit (MPU) for small electron fields.
• To enhance the accuracy of MPU calculations by refining the use of the lateral build-up ratio (LBR).
• To address the limitations of current methods in accounting for lateral scatter in small radiation fields.

Main Methods:

• Developed a modified method to obtain the lateral build-up ratio (LBR) directly from measured surface-normalized electron beam percentage depth dose curves.
• Utilized the modified LBR within Khan's established dose per monitor unit formula.
• Calculated MPU values for various small circular and triangular electron fields across different energies.
• Compared calculated MPU values against experimentally measured data.

Main Results:

• The modified method demonstrated improved accuracy in accounting for lateral scatter changes with decreasing field size.
• Calculated MPU values showed agreement within 3.5% of measured values for all circular fields and energies tested.
• Testing on a small triangular field yielded a maximum difference of 4.8% compared to measured data.

Conclusions:

• The improved method provides a more accurate calculation of dose per monitor unit for small electron fields.
• Direct measurement of LBR from depth dose curves enhances the precision of MPU calculations.
• This method offers a valuable tool for optimizing treatment planning in radiation oncology, particularly for small field dosimetry.