Dose buildup for obliquely incident photon beams

Area of Science:

• Medical Physics
• Radiation Oncology
• Radiotherapy Physics

Background:

• Dose buildup in the buildup region is critical for accurate radiotherapy planning.
• Limited data exists for high-energy photon beams (>6 MV) under oblique incidence.
• Oblique incidence significantly alters dose distribution compared to normal incidence.

Purpose of the Study:

• To measure and characterize dose buildup for obliquely incident high-energy photon beams.
• To determine an obliquity factor and its dependence on various parameters.
• To develop a predictive mathematical model for dose in the buildup region.

Main Methods:

• Ionization chamber measurements in a polystyrene phantom.
• Use of 6-, 10-, 18-, and 24-MV x-ray beams at angles from 0 to 84 degrees.
• Analysis of buildup curves and determination of obliquity factors.

Main Results:

• Obliquity factor is highly dependent on beam energy, angle of incidence, collimator opening, and source-skin distance.
• Buildup curves differ significantly between normal and oblique incidence.
• A mathematical expression was developed to predict dose in the buildup region.

Conclusions:

• The developed mathematical expression accurately predicts dose in the buildup region for obliquely incident high-energy photon beams.
• Understanding obliquity factors is essential for precise dose delivery in complex radiotherapy cases.
• This research provides a valuable tool for improving radiotherapy treatment planning and accuracy.