Characterization of PTW-31015 PinPoint ionization chambers in photon and proton beams

Area of Science:

• Medical Physics
• Radiation Oncology
• Dosimetry

Background:

• Increasing use of complex radiotherapy techniques necessitates precise dosimetry.
• Micro-ionization chambers are crucial for small-field beam characterization.
• Accurate quality assurance (QA) is vital for patient safety in proton therapy.

Purpose of the Study:

• To characterize a large set of PTW-TM31015 PinPoint micro-ionization chambers.
• To evaluate their performance in proton and high-energy photon beams.
• To assess chamber-to-chamber consistency for radiotherapy applications.

Main Methods:

• Characterization included imaging, stability checks, ion recombination, and polarity effect measurements.
• Cross-calibration against Farmer-type chambers in absorbed dose to water.
• Theoretical derivation of beam quality correction factors.

Main Results:

• Chamber stability was excellent, with <0.5% drift over one year.
• Proton beam correction factors agreed well with theoretical data.
• Photon beam correction factors showed a small but significant deviation from theoretical values (IAEA TRS-398).
• Ion recombination and polarity correction factors were highly consistent across all chambers.

Conclusions:

• The PTW-TM31015 chambers demonstrate high consistency, making them suitable for proton pencil beam scanning (PBS) commissioning and patient-specific QA.
• Discrepancies in photon beam correction factors highlight potential issues with current theoretical data.
• The large sample size validates the representativeness of findings for this chamber type.