Pre-irradiation dose requirements for reference-class ionization chambers: Quantifying measurement stability.

Area of Science:

• Medical Physics
• Radiation Oncology
• Dosimetry

Background:

• Pre-irradiation of ionization chambers is standard practice to ensure stable output readings in radiation therapy.
• However, specific pre-irradiation requirements for different chamber models under controlled conditions are not well-defined.

Purpose of the Study:

• To quantitatively assess the pre-irradiation dose needed for stabilizing output in various reference-class ionization chambers.
• To minimize uncertainties from environmental and system factors during evaluation.

Main Methods:

• Twelve reference ionization chambers were tested under strict environmental controls (25°C room/water, 50% RH).
• Output stability was measured using a 10 MV photon beam, normalized to a monitor chamber.
• Stability criteria: <0.1% variation without trending. Optimal pre-irradiation doses were determined per chamber.

Main Results:

• Large-volume chambers (Exradin A12, IBA FC65G, PTW 30013) showed <0.05% variation from the first irradiation, suggesting no pre-irradiation needed.
• Middle-volume chambers (Exradin A1SL, IBA CC13, PTW 31021) varied up to 0.25% and required ~21.25 Gy (100 MU x 25) for stability.

Conclusions:

• The necessary pre-irradiation dose for ionization chambers is chamber-dependent.
• Observed output variations were significantly lower than the previously reported 1%, indicating potential for streamlined clinical reference dosimetry.