Imaging of produced light in water during high energy electron beam irradiations from a medical linear accelerator

Optical imaging of lithium-containing zinc sulfate plate in water during irradiation of neutrons from boron neutron capture therapy (BNCT) system

PURPOSE

Optical imaging of ionizing radiation is a possible method for dose distribution measurements. However, it is not clear whether the imaging method is also applicable to neutrons. To clarify this, we performed the imaging of neutrons in water from boron neutron capture therapy (BNCT) systems. Such systems require efficient distribution measurements of neutrons for quality assessment (QA) of the beams.

METHOD

A water-filled phantom was irradiated from the side with an epithermal neutron beam, in which a lithium-containing zinc sulfate (Li-ZnS(Ag)) plate was set in the beam direction, and during this irradiation the scintillation of the plate was imaged using a cooled CCD camera. In the imaging, Li-6 in the Li-ZnS(Ag) plate captures neutrons and converts them to alpha particles (He-4) and tritium (H-3), while ZnS(Ag) in the Li-ZnS(Ag) plate produces scintillation light in the plate. We also conducted Monte Carlo simulation and compared its results with the experimental results.

RESULTS

The image of the emitted light from the Li-ZnS(Ag) plate was clearly obtained with an imaging time of 0.5 s. The depth and lateral profiles of the measured image using the Li-ZnS(Ag) plate showed the same shapes as the neutron distributions measured with gold foil, within a difference of 8%. The destructive effect of neutrons on the CCD camera increased ∼3 times, but the unit was still working after the measurement.

CONCLUSION

The optical imaging of neutrons in water is possible, and it has the potential to be a new method for efficient QA as well as for research on neutrons. This article is protected by copyright. All rights reserved.

Possibility evaluation of the optical imaging of proton mini-beams

Proton therapy using mini-beams is a promising method to reduce radiation damage to normal tissue. However, distribution measurements of mini-beams are difficult due to their small structures. Since optical imaging is a possible method to measure high-resolution two-dimensional dose distribution, we conducted optical imaging of an acrylic block during the irradiation of mini-beams of protons. Mini-beams were made from a proton pencil beam irradiated to 1 mm slits made of tungsten plate. During irradiation of the mini-beams to the acrylic block, we measured the luminescence of the acrylic block using a charge-coupled device camera. With the measurements, we could obtain slit beam images that have slit shapes in the shallow area while they were uniform in their Bragg peaks, which was similar to the case of simulated optical images by Monte Carlo simulations. We confirmed that high-resolution optical imaging of mini-beams is possible and provides a promising method for efficient quality assessment of mini-beams as well as research on mini-beam therapy.

Analysis of corrected Cerenkov emission during electron radiotherapy by Monte Carlo method

Cerenkov emission during electron radiotherapy had been emerging as a new dose assessment approach for clinical radiotherapy and could be imaged through a standard commercial camera. The purpose of this work aimed to study the accuracy of corrected Cerenkov emission method during electron radiotherapy by Monte Carlo (MC) method. GAMOS MC software was used to model the physics of electron therapy and calculated dose and Cerenkov photon distribution in water phantom. Compared to ionization chamber and diode measurement, MC simulated dose discrepancy was less than 1% in percentage depth dose (PDD) curves and less than. 2% in crossline profile curves, which was acceptable for clinical criterion. Compared to ionization chamber dose measurement, MC simulated Cerenkov discrepancy was less than 2% in crossline profile distribution, which was acceptable for clinical criterion. However, the Cerenkov PDD curves tended to overestimate the dose at the build-up region and underestimate the dose at the remaining attenuation region. After increasing the Cerenkov distribution depth to 2-3 mm, the discrepancy became well within 1% at the remaining attenuation region, which was acceptable for clinical criterion. Therefore, corrected Cerenkov emission could be used to assess PDD accuracy and crossline profile accuracy during electron radiotherapy.

Optical imaging of muons

Optical imaging of particle beams is a promising method for range and width estimations. However it was not clear that optical imaging was possible for muons. To clarify this, we conducted optical imaging of muons, since high-intensity muons are now available at J-PARC. We irradiated positive muons with different momenta to water or plastic scintillator block, and imaged using a charge-coupled device (CCD) camera during irradiation. The water and plastic scintillator block produced quite different images. The images of water during irradiation of muons produced elliptical shape light distribution at the end of the ranges due to Cherenkov-light from the positrons produced by positive muon decay, while, for the plastic scintillator block, we measured images similar to the dose distributions. We were able to estimate the ranges of muons as well as the measurement of the asymmetry of the direction of the positron emission by the muon decays from the optical images of the water, although the measured ranges were 4 mm to 5 mm larger than the calculated values. The ranges and widths of the beams could also be estimated from the optical images of the plastic scintillator block. We confirmed that optical imaging of muons was possible and is a promising method for the quality assessment, research of muons, and the future muon radiotherapy.

Estimation and correction of Cerenkov-light on luminescence image of water for carbon-ion therapy dosimetry

PURPOSE

The luminescence images of water during the irradiation of carbon-ions provide useful information such as the ranges and the widths of carbon-ion beams. However, measured luminescence images show higher intensities in shallow depths and wider lateral profiles than those of the dose distributions. These differences prevent the luminescence imaging of water from being applied to a quality assurance for carbon-ion therapy. We assumed that the differences were due to the contaminations of Cerenkov-light from the secondary electrons of carbon-ions as well as the prompt gamma photons in the measured image. In this study, we applied a correction method to a luminescence image of water during the irradiation of carbon-ion beams.

METHODS

We estimated the distribution of the Cerenkov-light in water during the irradiation of carbon-ions by Monte Carlo simulation and subtracted the simulated Cerenkov-light from the depth and lateral profiles of the measured luminescence image for 241.5 MeV/u-carbon-ions.

RESULTS

With these corrections, we successfully obtained depth and lateral profiles whose distributions are almost identical to the dose distributions of carbon-ions. The high intensities in the shallow depth areas decreased and the Bragg peak intensity increased. The beam widths of the measured images approached those of the ionization chamber.

CONCLUSIONS

These results indicate that the luminescence imaging of water with our proposed correction has potential to be used for dose distribution measurements for carbon-ion therapy dosimetry.

Estimation of the fractions of luminescence of water at higher energy than Cerenkov-light threshold for various types of radiation

Although the luminescence of water at a lower energy than the Cerenkov-light (CL) threshold has been found for various types of radiation, the fractions of the luminescence of water to the total produced light have not been obvious for radiations at a higher energy than the CL threshold because it is difficult to separate these two types of light. Thus, we used a Monte Carlo simulation to estimate the fractions of the luminescence of water for various types of radiation at a higher energy than the CL threshold to confirm the major component of the produced light. After we confirmed that the estimated light production of the luminescence of water could adequately simulate the experimental results, we calculated the produced light photons of this luminescence and the CL from water for protons (170 MeV), carbon ions (330 MeV/n), high-energy x-ray (6 MV) from a linear accelerator (LINAC), high-energy electrons (9 MeV) from LINAC, positrons (F-18, C-11, O-15, and N-13), and high-energy gamma photon radionuclides (Co-60). For protons, the major fraction of the produced light was the luminescence of water in addition to the CL from the prompt gamma photons produced by the nuclear interactions. For carbon ions, the major fraction of the produced light was the luminescence of water and the CL produced by the secondary electrons in addition to the prompt gamma photons produced by the nuclear interactions. For high-energy x-ray and electrons from LINAC, the fractions of luminescence of water were ∼0.1  %   to 0.2%. The fractions of luminescence of water for positrons were 0.2% to 1.5% and that for Co-60 was 0.4%. We conclude that the major fractions of light produced from x-ray and electrons from LINAC, positron radionuclides, and the Co-60 source are CL, with fractions of the luminescence of water from <0.1  %   to 1.5%.