Skin dose estimation for various beam modifiers and source-to-surface distances for 6MV photons

Area of Science:

• Medical Physics
• Radiation Oncology
• Dosimetry

Background:

• Accurate skin dose estimation is crucial in radiation therapy to minimize side effects.
• Beam modifiers and source-to-surface distance (SSD) variations can significantly impact surface dose.
• Understanding these effects is essential for optimizing treatment planning and patient safety.

Purpose of the Study:

• To estimate skin dose for a 6 MV photon beam with various beam modifiers (motorized wedges, acrylic block trays, Multileaf collimators) at different source-to-surface distances (SSDs).
• To evaluate the impact of field size and SSD on skin dose when using different beam modification techniques.
• To compare the skin dose contribution from open fields, wedge fields, and blocked fields.

Main Methods:

• Skin and buildup region doses were measured using an acrylic slab phantom and a Markus 0.055 cc parallel plate ionization chamber.
• Measurements were performed for open fields, motorized wedge fields, and acrylic block tray fields across a range of field sizes (3x3 cm² to 30x30 cm²).
• The effect of cerrobend blocks and Multileaf collimators (MLCs) was assessed for a 20x20 cm² field at 80 cm, 100 cm, and 120 cm SSD.

Main Results:

• Skin dose increased with increasing field size. For a 10x10 cm² open field, skin dose was 14.8-15.5% across tested SSDs.
• Acrylic block trays significantly increased skin dose (e.g., 70.4% for a 30x30 cm² field at 80 cm SSD), particularly at lower SSDs and larger fields.
• Motorized wedge fields generally resulted in lower skin doses than open fields, with SSD having a less significant effect on smaller fields but becoming more pronounced for fields larger than 15x15 cm².

Conclusions:

• Acrylic block trays substantially elevate skin dose, necessitating careful consideration in treatment planning, especially for large fields and shorter SSDs.
• Multileaf collimators (MLCs) demonstrated a weak dependence on surface dose, offering a potentially advantageous alternative for dose reduction compared to other blocking methods.
• Motorized wedges can reduce skin dose compared to open fields, but their effectiveness and SSD dependence vary with field size.