Estimation of Skin doses for Retrofit Prototype Multileaf Collimators Designed for Telecobalt Therapy Machine.

Area of Science:

• Medical Physics
• Radiation Oncology
• Radiotherapy Equipment

Background:

• Patient skin dose is critical for assessing late radiation effects in teletherapy.
• Multileaf collimators (MLCs) are essential for shaping radiation beams.
• Retrofit prototype MLCs require thorough dose evaluation for safe clinical implementation.

Purpose of the Study:

• To assess the skin dose distribution using prototype multileaf collimators (MLCs) designed for a cobalt-60 teletherapy unit.
• To determine the impact of source-to-surface distance (SSD) and field size on skin dose.
• To establish guidelines for acceptable skin dose in radiotherapy using these MLCs.

Main Methods:

• Measurements were conducted using EBT3 radiochromic film in a solid water phantom with a Phoenix cobalt-60 teletherapy machine.
• Skin dose was measured at various source-to-surface distances (60-90 cm) and field sizes (2x2 to 14x14 cm).
• Dose readings were normalized to the dose at maximum depth (dmax) and analyzed using custom software.

Main Results:

• Skin dose generally increased with field size and decreased with increasing source-to-surface distance (SSD).
• Measured surface doses ranged from 20% to 60% of the dose at dmax within the investigated SSD range.
• No significant difference in surface dose was observed between the two prototype MLCs evaluated.

Conclusions:

• Prototype MLCs offer good skin sparing, even at shorter source-to-surface distances (SSDs).
• Skin dose remains below 50% of the dose at dmax for SSDs greater than 65 cm.
• A minimum gap of 5 cm between the MLC and the patient's skin is recommended for acceptable skin dose levels.