Surface dose measurements for highly oblique electron beams

Area of Science:

• Medical Physics
• Radiation Oncology
• Dosimetry

Background:

• Clinical electron beam therapy often involves oblique incidence.
• Existing data on surface dose variations is limited for highly oblique beams (>60 degrees).
• Accurate surface dose assessment is critical for treatment planning and patient safety.

Purpose of the Study:

• To measure surface dose variations in highly oblique electron beams.
• To determine obliquity factors (OBF) for electron energies from 4 to 20 MeV.
• To compare TLD and ion chamber measurements and assess suitability for oblique dosimetry.

Main Methods:

• Surface dose measurements using parallel-plate ion chambers and LiF:Mg,Ti (standard and carbon-loaded) TLDs.
• Measurements performed on a solid water phantom at 100 cm SSD without collimation.
• Obliquity factors (OBF) calculated as the ratio of surface dose at oblique incidence to perpendicular incidence.

Main Results:

• OBFs increased with angle of incidence, peaking before a rapid drop-off, with higher energies showing a more rapid increase.
• Maximum OBFs occurred at angles from 73° (4 MeV) to 84° (20 MeV), reaching 130-160% of direct beam doses.
• At 2 mm depth, dose ratios initially increased then decreased; maximum dose at 2 mm exceeded surface dose.

Conclusions:

• Highly oblique electron beams significantly increase surface dose, necessitating careful consideration in clinical practice.
• Specific ion chamber designs (small electrode spacing, large guard ring) are recommended for accurate oblique dose measurements.
• A semi-empirical model was developed to predict surface dose increases for various electron beam energies.