Buildup/surface dose and exit dose measurements for a 6-MV linear accelerator

Area of Science:

• Medical Physics
• Radiation Oncology

Background:

• Accurate dose distribution is crucial for effective radiation therapy.
• Understanding surface and buildup doses is essential for minimizing skin toxicity.
• The Varian Clinac 6/100 is a widely used linear accelerator for radiotherapy.

Purpose of the Study:

• To measure the central axis dose distribution in the buildup region for a 6-MV x-ray beam.
• To investigate the impact of field size, blocking tray, and metal filters on surface and buildup doses.
• To quantify dose perturbations at the phantom exit due to lack of backscatter and evaluate methods to restore scatter conditions.

Main Methods:

• Utilized a polystyrene phantom and a 0.5 cm3 parallel-plate ionization chamber.
• Measured surface dose for various field sizes (5x5 cm to 40x40 cm) at a 100-cm source-skin distance.
• Assessed the influence of a polycarbonate blocking tray and metal filters on dose distributions.
• Conducted exit dose measurements and evaluated the effect of adding backscattering material.

Main Results:

• Surface dose varied from approximately 8% (5x5 cm field) to 36% (40x40 cm field) of the maximum dose.
• Blocking tray and metal filters affected surface and buildup doses.
• Exit dose measurements showed a 15% reduction without backscattering material.
• Adding 5-10 mm of unit density material restored near full scatter conditions at the exit surface.

Conclusions:

• Surface dose in the buildup region is highly dependent on field size for 6-MV x-rays.
• External components like blocking trays can alter dose distributions.
• Backscattering significantly impacts exit dose, and its absence can be compensated for with appropriate material placement.