Development and characterization of a dedicated dose monitor for ultrahigh-dose-rate scanned carbon-ion beams

Abstract

The current monochromatic beam mode (i.e., uHDR irradiation mode) of the scanned carbon-ion beam lacks a dedicated dose monitor, making the beam control challenging. We developed and characterized a dedicated dose monitor for uHDR-scanned carbon-ion beams. Furthermore, a simple measurable dose rate (dose rate per spot (DR_spot)) was suggested by using the developed dose monitor and experimentally validating quantities relevant to the uHDR scanned carbon-ion beam. A large plane-parallel ionization chamber (IC) with a smaller electrode spacing was used to reduce uHDR recombination effects, and a dedicated operational amplifier was manufactured for the uHDR-scanned carbon-ion beam. The dose linearity of the IC was within ± 1% in the range of 1.8-12.3 Gy. The spatial inhomogeneity of the dose response of the IC was ± 0.38% inside the ± 40-mm detector area, and a systematic deviation of approximately 2% was measured at the edge of the detector. uHDR irradiation with beam scanning was tested and verified for different doses at the corresponding dose rates (in terms of both the average dose rate and DR_spot). We confirmed that the dose monitor can highlight the characteristics (i.e., dose, dose rate, and dose profile) of uHDR-scanned carbon-ion beams at several dose levels in the monochromatic beam mode.

Figure 1

Linearity of the operational amplifier. The linearity of the output frequency as a function of input current was within ± 1% in the examined range. The plot of (a) output frequency and (b) conversion factor against the input current. The upper horizontal axis corresponds to the estimated equivalent MU rate. The equivalent MU rate was calculated from the output frequency assuming that the IC was connected. The absolute values of the input current are used for display purposes.

Figure 2

Characterization of the IC for uHDR irradiations: (a) saturation measurements of the ion collection efficiency with varying voltages of the IC under uHDR conditions. The absolute values of the voltage are used for display purposes. (b) Dose linearity of the IC obtained using the uHDR carbon-ion beam (208.3 MeV/u). The plot of the dose against the input MU. (c) Spatial inhomogeneity of the dose response of the IC.

Figure 3

Measured beam current of the extracted beam as a function of time. The uHDR extraction with doses of (a) 1.6, (b) 5.4, and (c) 9.7 Gy. (d) Histogram between the black dashed lines shown in (c) for an uHDR irradiation at 9.7 Gy. The intensities were assigned to 20 different bins. The height of the bins corresponded to the relative duration of the particle fluence with these intensities during the spill. The red line shows the fitted curve with a Poisson distribution.

Figure 4

Characterized uHDR scanned carbon-ion beam with the developed IC. The spot timer against the spot index at (a) 1.6, (b) 5.4, and (c) 9.7 Gy. ADR and DR_spot against the spot index for (d) 1.6, (e) 5.4, and (f) 9.7 Gy. (g) Doses of uHDR irradiations for a series of tests (same plan file for a given dose) conducted in a day.

Figure 5

Measured lateral dose distributions for uHDR carbon-ion irradiations using the developed IC at 208.3 MeV/u for doses of 9.7 (a and b), 5.4 (c and d), and 1.6 Gy (e and f). The left and right pictures show the Gafchromic (EBT3) film measurement and the measured lateral dose profiles in the x and y directions for each panel. In the left picture, the dashed lines indicate the section for the profiles in the measured film. In the right picture, lateral dose profiles (x and y directions in red and yellow, respectively) for uHDR irradiations are shown. The axes are the beam definitions.

Figure 6

Schematic of the IC. The active volume of the large, plane-parallel IC is 120 mm × 120 mm × 3 mm.

Figure 7

Setup for uHDR experiments. (a) Monochromatic beam mode in a treatment room. The carbon-ion beam did not pass through the nozzle. The advanced Markus chamber (AMC) and Gafchromic film were used to measure the absolute dose and the field size, respectively, at the plateau depth (b) Scanning pattern used in this study. The cross (in green) indicates the starting point of the scan. The carbon-ion beam was scanned once to create a field within the extraction time. The axes show the beam coordinates. (c) Measured positions for the spatial inhomogeneity of the dose response of the IC.