SU-E-T-172: Ion Recombination Correction Factor in Medium Dose Rate for Advanced Markus Ionization Chamber

Area of Science:

• Medical Physics
• Radiation Dosimetry
• Radiotherapy Physics

Background:

• Accurate absorbed dose determination is crucial in radiotherapy.
• Electron beams, particularly for intraoperative radiation therapy, require precise dosimetry.
• The ion recombination correction factor (ks) is essential for accurate measurements with ionization chambers.

Purpose of the Study:

• To evaluate the ion recombination correction factor (ks) using conventional and modified Boag's methods for the Advance Markus ionization chamber (AMC).
• To assess the performance of these methods in medium dose per pulse electron beams (4-41 mGy/pulse).

Main Methods:

• Applied conventional Boag's two-voltage analysis (TVA) and modified Boag's expressions.
• Utilized high-energy electron beams from a linear accelerator for intraoperative radiation therapy.
• Employed radiochromic films (EBT2) as a dose rate-independent reference method.
• Performed dose measurements with AMC in a water phantom.

Main Results:

• Conventional TVA showed acceptable accuracy (1.0% deviation) for dose rates up to 35 mGy/pulse.
• At higher dose rates (>35 mGy/pulse), conventional TVA had a significant deviation (4.2% at 41 mGy/pulse).
• Modified Boag's first expression provided acceptable results (1.3% deviation) in the higher dose rate range.

Conclusions:

• The Advance Markus ionization chamber (AMC) is suitable for absorbed dose determination in medium dose per pulse electron beams.
• The accuracy of the IAEA TRS-398 recommended TVA is limited to dose rates below 35 mGy/pulse.
• Modified Boag's expressions are recommended for accurate ks determination at higher dose rates (>35 mGy/pulse).