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Direct calibration in megavoltage photon beams using Monte Carlo conversion factor: validation and clinical
Tracy Wright1, Jessica E Lye, Ganesan Ramanathan
1Australian Radiation Protection and Nuclear Safety Agency, 619 Lower Plenty Rd, Yallambie, Victoria 3085, Australia. School of Chemistry and Physics, University of Adelaide, Adelaide, Australia.
The Australian Radiation Protection and Nuclear Safety Agency (ARPANSA) developed a new ionization chamber calibration method using megavoltage photon beams. This advanced technique reduces calibration uncertainty compared to the previous cobalt-60 calibration method.
Area of Science:
- Medical Physics
- Radiation Dosimetry
- Metrology
Background:
- Ionization chamber calibration is crucial for accurate radiation therapy dose delivery.
- Existing methods using cobalt-60 beams and TRS-398 have limitations in uncertainty.
- Megavoltage photon beams offer potential for improved calibration accuracy.
Purpose of the Study:
- To establish and validate a new method for ionization chamber calibration using megavoltage photon beams at ARPANSA.
- To reduce calibration uncertainty compared to the current cobalt-60 calibration protocols.
- To assess the performance of the new method through international comparisons.
Main Methods:
- Utilized a graphite calorimeter and Monte Carlo (MC) simulations for dose conversion.
- Employed EGSnrc software to model the linear accelerator head and dose distributions.
- Validated the linac model by comparing measured and simulated Percentage Depth Doses (PDDs) and profiles.
- Participated in the Bureau International des Poids et Mesures (BIPM) key comparison BIPM.RI(I)-K6.
Main Results:
- Achieved relative standard uncertainties of 0.47% (6 MV), 0.51% (10 MV), and 0.46% (18 MV).
- Results from the BIPM key comparison were within 1σ of the participant average for all tested beam energies.
- Measured kQ values for an NE2571 Farmer chamber were lower than TRS-398 values but consistent with other studies.
Conclusions:
- The new ARPANSA calibration method using megavoltage photon beams significantly reduces calibration uncertainty.
- The method is validated through international comparisons and provides accurate absorbed dose to water determination.
- Users may observe a calibration factor shift of 0.4-1.1% for NE2571 chambers compared to the current method.
