Technical Note: Comprehensive evaluation and implementation of two independent methods for beam monitor calibration for proton scanning beam

Dose-Area Product Determination and Beam Monitor Calibration for the Fixed Beam of the Shanghai Advanced Proton Therapy Facility

Research conducted to-date, makes use of the IBA-Lynx scintillating screen and radiochromic film to analyze the proton field uniformity for dose-area product (DAP) determination. In this paper, the machine log file based reconstruction is proposed to calculate the field uniformity to simplify the measurement. In order to calculate the field uniformity, the dose distribution is reconstructed based on the machine log file with matRad (an open source software for analytical dose calculation in MATLAB). After acquisition of the dose distribution, the field flatness and symmetry are calculated automatically for different proton energies. A comprehensive comparison of DAP determined with Bragg peak chamber (BPC) and Markus chamber (MC) is presented. The actual delivered dose is reconstructed with the log file to analyze the lateral dose distribution of the scanned field. DAP of different energies are calculated ranging from 70.6 MeV to 235 MeV. The percentage difference is calculated, illustrating the DAP discrepancy between the MC and BPC to the mean value. The percentage difference ranges from −0.19% to 1.26%. The variation between DAP measured with the BPC and MC peaks at −2.5%. The log file based reconstruction to calculate field uniformity can be an alternative for DAP determination. The direct method using a large-area Bragg peak chamber is investigated. The two methods to determine DAP and calibrate beam monitor illustrate consistent results.

Reply to comment on 'Lateral response heterogeneity of Bragg peak ionization chambers for narrow-beam photon and proton dosimetry'

Gomà (2020Phys. Med. Biol.) commented on our paper 'Lateral response heterogeneity of Bragg peak ionization chambers for narrow-beam photon and proton dosimetry' (Kuesset al2017Phys. Med. Biol.629189-206) which describes a method to determine the response pattern of large-area ionization chambers using a collimated x-ray beam. Gomà performed a simple Monte Carlo simulation to investigate the energy transferred by secondary electrons within the detector, deducing that our conclusion, that the chamber has a non-uniform response, is not supported by our results. We appreciate the work performed by Gomà very much and believe that the transport of secondary electrons in the chamber is an important contribution to understand the non-uniformity response of large-area chambers in narrow beams. However, we disagree with the conclusions drawn by Gomà that the radial response is homogenous. His simulation actually demonstrates that the response is non-uniform in an x-ray beam.

MR-guided proton therapy: Impact of magnetic fields on the detector response

PURPOSE

To investigate the response of detectors for proton dosimetry in the presence of magnetic fields.

MATERIAL AND METHODS

Four ionization chambers (ICs), two thimble-type and two plane-parallel-type, and a diamond detector were investigated. All detectors were irradiated with homogeneous single-energy-layer fields, using 252.7 MeV proton beams. A Farmer IC was additionally irradiated in the same geometrical configuration, but with a lower nominal energy of 97.4 MeV. The beams were subjected to magnetic field strengths of 0, 0.25, 0.5, 0.75, and 1 T produced by a research dipole magnet placed at the room's isocenter. Detectors were positioned at 2 cm water equivalent depth, with their stem perpendicular to both the magnetic field lines and the proton beam's central axis, in the direction of the Lorentz force. Normality and two sample statistical Student's t tests were performed to assess the influence of the magnetic field on the detectors' responses.

RESULTS

For all detectors, a small but significant magnetic field-dependent change of their response was found. Observed differences compared to the no magnetic field case ranged from +0.5% to -0.7%. The magnetic field dependence was found to be nonlinear and highest between 0.25 and 0.5 T for 252.7 MeV proton beams. A different variation of the Farmer chamber response with magnetic field strength was observed for irradiations using lower energy (97.4 MeV) protons. The largest magnetic field effects were observed for plane-parallel ionization chambers.

CONCLUSION

Small magnetic field-dependent changes in the detector response were identified, which should be corrected for dosimetric applications.

Gradient corrections for reference dosimetry using Farmer-type ionization chambers in single-layer scanned proton fields

PURPOSE

The local depth dose gradient and the displacement correction factor for Farmer-type ionization chambers are quantified for reference dosimetry at shallow depth in single-layer scanned proton fields.

METHOD

Integrated radial profiles as a function of depth (IRPDs) measured at three proton therapy centers were smoothed by polynomial fits. The local relative depth dose gradient at measurement depths from 1 to 5 cm were derived from the derivatives of those fits. To calculate displacement correction factors, the best estimate of the effective point of measurement was derived from reviewing experimental and theoretical determinations reported in the literature. Displacement correction factors for the use of Farmer-type ionization chambers with their reference point (at the center of the cavity volume) positioned at the measurement depth were derived as a ratio of IRPD values at the measurement depth and at the effective point of measurement.

RESULTS

Depth dose gradients are as low as 0.1-0.4% per mm at measurement depths from 1 to 5 cm in the highest clinical proton energies (with residual ranges higher than 15 cm) and increase to 1% per mm at a residual range of 4 cm and become larger than 3% per mm for residual ranges lower than 2 cm. The literature review shows that the effective point of measurement of Farmer-type ionization chambers is, similarly as for carbon ion beams, located 0.75 times the cavity radius closer to the beam origin as the center of the cavity. If a maximum displacement correction of 2% is deemed acceptable to be included in calculated beam quality correction factors, Farmer-type ICs can be used at measurements depths from 1 to 5 cm for which the residual range is 4 cm or larger. If one wants to use the same beam quality correction factors as applicable to the conventional measurement point for scattered beams, located at the center of the SOBP, the relative standard uncertainty on the assumption that the displacement correction factor is unity can be kept below 0.5% for measurement depths of at least 2 cm and for residual ranges of 15 cm or higher.

CONCLUSION

The literature review confirmed that for proton beams the effective point of measurement of Farmer-type ionization chambers is located 0.75 times the cavity radius closer to the beam origin as the center of the cavity. Based on the findings in this work, three options can be recommended for reference dosimetry of scanned proton beams using Farmer-type ionization chambers: (a) positioning the effective point of measurement at the measurement depth, (b) positioning the reference point at the measurement depth and applying a displacement correction factor, and (c) positioning the reference point at the measurement depth without applying a displacement correction factor. Based on limiting the acceptable uncertainty on the gradient correction factor to 0.5% and the maximum deviation of the displacement perturbation correction factor from unity to 2%, the first two options can be allowed for residual ranges of at least 4 cm while the third option only for residual ranges of at least 15 cm.

Beam monitor calibration of a synchrotron-based scanned light-ion beam delivery system

PURPOSE

This paper presents the implementation and comparison of two independent methods of beam monitor calibration in terms of number of particles for scanned proton and carbon ion beams.

METHODS

In the first method, called the single-layer method, dose-area-product to water (DAP_w) is derived from the absorbed dose to water determined using a Roos-type plane-parallel ionization chamber in single-energy scanned beams. This is considered the reference method for the beam monitor calibration in the clinically relevant proton and carbon energy ranges. In the second method, called the single-spot method, DAP_w of a single central spot is determined using a Bragg-peak (BP) type large-area plane-parallel ionization chamber. Emphasis is given to the detailed characterization of the ionization chambers used for the beam monitor calibration. For both methods a detailed uncertainty budget on the DAP_w determination is provided as well as on the derivation of the number of particles.

RESULTS

Both calibration methods agreed on average within 1.1% for protons and within 2.6% for carbon ions. The uncertainty on DAP_w using single-layer beams is 2.1% for protons and 3.1% for carbon ions with major contributions from the available values of k_Q and the average spot spacing in both lateral directions. The uncertainty using the single-spot method is 2.2% for protons and 3.2% for carbon ions with major contributions from the available values of k_Q and the non-uniformity of the BP chamber response, which can lead to a correction of up-to 3.2%. For the number of particles, an additional dominant uncertainty component for the mean stopping power per incident proton (or the CEMA) needs to be added.

CONCLUSION

The agreement between both methods enhances confidence in the beam monitor calibration and the estimated uncertainty. The single-layer method can be used as a reference and the single-spot method is an alternative that, when more accumulated knowledge and data on the method becomes available, can be used as a redundant dose monitor calibration method. This work, together with the overview of information from the literature provided here, is a first step towards comprehensive information on the single-spot method.