Scattered radiation from linear accelerator and cobalt-60 collimator jaws

Area of Science:

• Medical Physics
• Radiation Oncology
• Dosimetry

Background:

• Solid state diodes and thermoluminescent dosimeters (TLDs) are commonly used for in vivo dosimetry near radiation fields.
• Their energy-dependent response can lead to uncertain dose interpretations.
• Scattered radiation from linear accelerators and cobalt-60 units is a concern for critical organs.

Purpose of the Study:

• To investigate scattered radiation from linear accelerator collimator jaws and cobalt-60 teletherapy units.
• To document the magnitude, energy composition, and sources of scattered radiation.
• To compare the accuracy of different dosimetry methods in scattered radiation fields.

Main Methods:

• A Markus-type parallel-plate ionization chamber was used in a polystyrene phantom.
• Measurements were taken at various distances from radiation field edges and with/without phantom material.
• Scattered radiation was documented near fields from different manufacturers' treatment units.

Main Results:

• Scattered radiation adjacent to fields ranged from 1% to 15% of the central axis dose.
• Magnitude depended on distance, beam energy, field size, and accessories.
• Manufacturer differences had minimal impact; collimator jaw orientation had no effect.

Conclusions:

• Depth dose curves indicate complex energy spectra from scattered X-rays and electrons.
• Energy-dependent dosimeters (TLDs, diodes) may over- or under-respond in scattered radiation fields.
• Ionization chambers are preferred for accurate scattered radiation dose determination.