Electron-beam characteristics at extended treatment distances

Area of Science:

• Medical Physics
• Radiation Oncology
• Radiotherapy Physics

Background:

• Accurate dose delivery in electron beam radiotherapy necessitates understanding delivered dose, central axis depth dose, and beam flatness, especially at extended source-to-surface distances (SSD).
• Extended SSD treatments can lead to lateral tissue underdosage due to reduced beam flatness, impacting treatment efficacy.

Purpose of the Study:

• To investigate the impact of extended source-to-surface distances (SSD) on electron beam characteristics, specifically depth dose and beam flatness.
• To provide data on beam characteristics at extended treatment distances for clinical application in electron beam therapy.

Main Methods:

• Depth dose and beam flatness were measured for electron beam energies from 6 to 20 MeV and field sizes ranging from 3x3 cm² to 15x15 cm² at various SSDs.
• Beam flatness was quantified using the Target Coverage Factor (TCF), defined as the ratio of the width of a specified isodose line to the geometrical field width.

Main Results:

• Depth dose changes were minimal at extended SSDs, except in the buildup region, with surface dose generally decreasing as SSD increased.
• Beam flatness, assessed by TCF, showed significant degradation for smaller fields, higher isodose lines, and lower energies.
• Variations in SSD had minimal impact on relative beam flatness changes for field sizes larger than 8x8 cm².

Conclusions:

• Extended SSD treatments require careful consideration of beam flatness, particularly for smaller fields and lower energies, to avoid lateral underdosage.
• The Target Coverage Factor (TCF) offers a clinically practical method for assessing and managing beam uniformity loss at extended SSDs.
• The presented data on depth dose and TCF at extended treatment distances aid in optimizing electron beam radiotherapy planning and delivery.