An isodose shift technique for obliquely incident electron beams

Area of Science:

• Medical Physics
• Radiation Oncology

Background:

• Electron beam therapy requires accurate dose distribution.
• Oblique incidence of electron beams causes depth dose curve shifts.
• Simple geometric calculations are insufficient due to scatter effects.

Purpose of the Study:

• To develop a method for accurately calculating isodose line depths for obliquely incident electron beams.
• To provide a table of isodose shift factors for practical clinical use.

Main Methods:

• Developed a semi-empirical functional relationship for isodose shift.
• Tabulated isodose shift factors based on beam energy (6-22 MeV) and isodose level (10%-90%).
• Conducted extensive measurements using a Varian Clinac 2500, parallel-plate chamber, and polystyrene phantom.

Main Results:

• The developed method accurately predicts isodose line depths for incident angles up to 60 degrees.
• Isodose shift factors were found to be relatively independent of beam size and angle (<60 degrees).
• The scaling factor for the polystyrene phantom was determined to be 1.00.

Conclusions:

• The semi-empirical method and shift factors provide an easy and accurate way to calculate isodose depths for oblique electron beams.
• This improves the precision of radiation therapy planning, especially for complex beam angulations.
• The findings are applicable to clinical electron beam radiotherapy settings.