Abstract
Purpose An experimental and mathematical study for determining the effective point of measurement (Peff) for a Farmer-type cylindrical chamber in a carbon ion passive scatter beam is presented. Methods The ionization depth curves measured by the Bragg peak chamber were plotted according to the position of the inner surface of the entrance window, while the Farmer chamber was plotted at the tip of the cylindrical geometric center. The ionization depth curves measured by a cylindrical chamber in the 3D water phantom were then compared with a high-precision parallel-plate PTW Bragg peak chamber for inspecting the upstream shift correction of the cylindrical chamber in the carbon ion beam. A component of the vertical and horizontal integration method and the barrier model, cosφ = 1 − [2αRL/(1 + α − RL)], for analyzing the shift of effective point of measurement in different carbon ion energies and various field sizes, were studied. Results The shift between the maximum peak of the Bragg peak chamber and the Farmer chamber in a field size of 10 cm × 10 cm with an energy of 330 MeV/u of carbon ion is 2.3 mm. This upstream shift corresponds to (0.744 ± 0.07)r, where r is the Farmer chamber inner radius of 3.05 mm. Carbon ion energy from 120 MeV/u to 400 MeV/u with different field sizes show different shifts of effective point of measurement in a range of (0.649 ± 0.02)r to (0.843 ± 0.06)r of 3 cm × 3 cm at an energy of 400 MeV/u and 10 cm × 10 cm at an energy of 120 MeV/u, respectively. The vertical and horizontal scatter analysis by the barrier model can precisely describe the shift of the effective point of measurement at different carbon ion energies with various field sizes. Conclusions We conclude that the Farmer chamber can be used for a patient-specific dose verification check in carbon ion beam treatment if Peff is well calibrated.
Highlights
IntroductionWhen cylindrical chambers are used for electron measurement with the chamber geometric central axis aligned to the isocenter, electrons are focused by the chamber cylindricality causing denser electron clouds upstream from the center of the chamber
The Institute of Modern Physics (IMP), affiliated with the Chinese Academy of Sciences (CAS), was founded in 1957 in Lanzhou, China
The cylindrical ionization chambers are used for the daily calibration of the beam quality assurance and the patientspecific dosimetric verification measurement at the middle of the Spread Out of Bragg Peak (SOBP) in a solid water phantom for comparison with the dose calculated by our home-made ciPlan treatment planning system
Summary
When cylindrical chambers are used for electron measurement with the chamber geometric central axis aligned to the isocenter, electrons are focused by the chamber cylindricality causing denser electron clouds upstream from the center of the chamber. It does present a problem in the carbon ion beam measurement by using the Farmer chamber because the fluency inside the cavity has higher average energy than at the same point in the water since carbon ion beams lose less energy in the cavity’s air. The perturbation effect is much more complicated for carbon ion beam measurement using a cylindrical chamber. The shape of the carbon ion beam fluency spectrum is continuously changing with depth in the chamber cavity. [6,7,8]
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