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

Application of Cherenkov radiation in tumor imaging and treatment

Cherenkov radiation (CR) is the characteristic blue glow that is generated during radiotherapy or radioisotope decay. Its distribution and intensity naturally reflect the actual dose and field of radiotherapy and the location of radioisotope imaging agents in vivo. Therefore, CR can represent a potential in situ light source for radiotherapy monitoring and radioisotope-based tumor imaging. When used in combination with new imaging techniques, molecular probes or nanomedicine, CR imaging exhibits unique advantages (accuracy, low cost, convenience and fast) in tumor radiotherapy monitoring and imaging. Furthermore, photosensitive nanomaterials can be used for CR photodynamic therapy, providing new approaches for integrating tumor imaging and treatment. Here the authors review the latest developments in the use of CR in tumor research and discuss current challenges and new directions for future studies.

Correcting angular dependencies using non-polarized components of Cherenkov light in water during high-energy X-ray irradiation

OBJECTIVE

Dose distribution measurements of high-energy X-rays from medical linear accelerators (LINAC) in water are important for quality control (QC) of the system. Although Cherenkov-light imaging is a useful method for measuring the high-energy X-ray dose distribution, depth profiles have an underestimated dose at increased depths due to the angular dependency of the Cherenkov light generated in water. In this study, we use a linear polarizer to separate the majority of polarized components from the majority of unpolarized components of Cherenkov-light images in water and then use this information to correct for angular dependencies.

METHODS

A water phantom, a cooled charge-coupled device (CCD) camera, and a polarizer were installed in a black box. Then, the water phantom was irradiated from the upper side with 6 MV or 10 MV X-rays, and the Cherenkov light generated in water was imaged with the polarizer axis at both parallel and perpendicular orientations to the beam. By using these images from the two orientations relative to the beam, we corrected the angular dependency of the Cherenkov light.

RESULTS

By subtracting the images measured with the polarizer perpendicular to the beams from the images measured with the polarizer parallel to the beams, we could obtain images with only the polarized components. Using these images, we could calculate the images with non-polarized components that had similar depth profiles to those calculated with a planning system. The average difference between corrected depth profiles and those calculated with the planning system was less than 1%, while that between uncorrected depth profiles and the planning system was more than 8.3% in depths of water from 20 mm to 100 mm.

CONCLUSION

We conclude that use of the polarizer has the potential to improve the accuracy of dose distribution in Cherenkov-light imaging of water using high-energy X-rays. This article is protected by copyright. All rights reserved.

Computational modeling for the evaluation of suppressed scintillation yields in plastic scintillators using Geant4

The yield of scintillation photons emitted from scintillators is considered to be proportional to the LET (linear energy transfer) which is energy distribution per unit length, in the low-LET domain, but not proportional in the high LET domain due to the suppression yield from the so-called quenching effect. Ogawa et al. proposed a computational method to estimate scintillation yield using Monte Carlo simulations considering the principle of the FRET (fluorescence resonance energy transfer) process, which is a phenomenon of energy transfer between fluorescent molecules. In their study, the track structure simulations could reproduce measured yields of scintillation. However, Ogawa et al.'s model was not suitable for estimating the scintillation yields when the particle energy was low when using condensed history simulations. Therefore, we propose a new method for estimating scintillation yields more accurately using Geant4 to improve the model calculations based on condensed history simulations. We simulated the local energy deposition pattern in a NE102A plastic scintillator to calculate the number of excitors in the microscopic volume for various nuclides (helium to argon ions). The suppressed scintillation yields were estimated using the model calculations of sequential FRET processes while considering the inactivation of the excitors selected as donors of the FRET process. The model calculations successfully reproduced the experimental scintillation yields within 10% error for the lighter ions up to neon. However, when the analysis was repeated for silicon and argon, the maximum error in the scintillation yields increased up to 27%. The proposed computational model for the evaluation of the suppressed scintillation yields emitted from NE102A scintillator irradiated with heavy ions using sequential FRET calculations with condensed history method returned simulated scintillation yields.

Increase in the intensity of an optical signal with fluorescein during proton and carbon-ion irradiation

Purpose

Although the imaging of luminescence emitted in water during irradiation of protons and carbon ions is a useful method for range and dose estimations, the intensity of the images is relatively low due to the low photon production of the luminescence phenomenon. Therefore, a relatively long time is required for the imaging. Since a fluorescent dye, fluorescein, may increase the intensity of the optical signal, we measured the luminescence images of water with different concentrations of fluorescein during irradiation of protons and carbon ions and compared the results with those by measurements with water.

Methods

A cooled charge‐coupled device (CCD) camera was used for imaging a water phantom with different concentrations of fluorescein from 0.0063 to 0.025 mg/cm³, in addition to a water phantom without fluorescein during irradiation of 150‐MeV protons and 241.5‐MeV/n carbon ions.

Results

For both protons and carbon ions, the intensity of the luminescence images increased as the concentration of fluorescein increased. With a fluorescein concentration of 0.025 mg/cm³, the intensities increased to more than 10 times those of water for both protons and carbon ions. Although the shape of the depth profiles of luminescence images of water with fluorescein appeared similar to that of water for protons, those for carbon ions were different from those of water due to the increase in the Cherenkov light component at shallow depths by the decrease in the angular dependencies of the Cherenkov light.

Conclusion

We confirmed the increase in intensity of the luminescence of water by adding fluorescein for particle ions. With a small amount of Cherenkov light contamination in the images, such as protons, the relative distributions of the luminescence images with fluorescein were similar to that of water and will be used for range or dose determination in a short time.

Energy spread estimation of radioactive oxygen ion beams using optical imaging

Radioactive ion (RI) beams combined with in-beam positron emission tomography enable accurate in situ beam range verification in heavy ion therapy. However, the energy spread of the radioactive beams generated as secondary beams is wider than that of conventional stable heavy ion beams which causes Bragg peak region and distal falloff region broadening. Therefore, the energy spread of the RI beams should be measured carefully for their quality control. Here, we proposed an optical imaging technique for the energy spread estimation of radioactive oxygen ion beams. A polymethyl methacrylate phantom (10.0 × 10.0 × 9.9 cm3) was irradiated with an 15O beam (mean energy = 247.7 MeV u−1, standard deviation = 6.8 MeV u−1) in the Heavy Ion Medical Accelerator in Chiba. Three different momentum acceptances of 1%, 2% and 4% were used to get energy spreads of 1.9 MeV u−1, 3.4 MeV u−1 and 5.5 MeV u−1, respectively. The in-beam luminescence light and offline beam Cerenkov light images were acquired with an optical system consisting of a lens and a cooled charge-coupled device camera. To estimate the energy spread of the 15O ion beams, we proposed three optical parameters: (1) distal-50% falloff length of the prompt luminescence signals; (2) full-width at half maximum of the Cerenkov light signals in the beam direction; and (3) positional difference between the peaks of the Cerenkov light and the luminescence signals. These parameters estimated the energy spread with the respective mean squared errors of 2.52 × 10−3 MeV u−1, 5.91 × 10−3 MeV u−1, and 0.182 MeV u−1. The distal-50% falloff length of the luminescence signals provided the lowest mean squared error among the optical parameters. From the findings, we concluded optical imaging using luminescence and Cerenkov light signals offers an accurate energy spread estimation of 15O ion beams. In the future, the proposed optical parameters will be used for energy spread estimation of other RI beams as well as stable ion beams.

Imaging of polarized components of Cerenkov light and luminescence of water during carbon-ion irradiation

PURPOSE

The luminescence image of water during the irradiation of carbon ions showed higher intensity at shallow depths than dose distribution due to the contamination of Cerenkov light from secondary electrons. Since Cerenkov light is coherent and polarized for the light produced during the irradiation of carbon ions to water, the reduction of Cerenkov light may be possible with a polarizer. In addition, there is no information on the polarization of the luminescence of water. To clarify these points, we measured the optical images of water during the irradiation of carbon ions with a polarizer by changing the directions of the transmission axis.

METHODS

Imaging was conducted using a cooled charge-coupled device (CCD) camera during the irradiation of 241.5 MeV/n energy carbon ions to a water phantom with a polarizer in front of the lens by changing the transmission axis parallel and perpendicular to the carbon-ion beam.

RESULTS

With the polarizer parallel to the carbon-ion beam, the intensity at the shallow depth was ~26% higher than that measured with the polarizer perpendicular to the beam. We found no significant intensity difference between these two images at deeper depths where the Cerenkov light was not included. The difference image of the parallel and perpendicular directions showed almost the same image as the simulated Cerenkov light distribution. Using the measured difference image, correction of the Cerenkov component was possible from the measured luminescence image of water during the irradiation of carbon ions.

CONCLUSION

We could measure the difference of the Cerenkov light component by changing the transmission axis of the polarizer. Also we clarified that there was no difference in the luminescence of water by changing the transmission axis of the polarizer.

Discovery of the luminescence of water during irradiation of radiation at a lower energy than the Cherenkov light threshold

It is widely believed that light is not emitted in water during irradiation of radiation at energies lower than the Cherenkov light threshold. Contrary to this consensus, we discovered that light (luminescence) is emitted in water during irradiation of radiation, and imaging of this luminescence was possible. In this review, the author describes the optical images obtained for various types of radiation, their characteristics and origins, and potential applications of the luminescence of water during irradiation at a lower energy than the Cherenkov light threshold. The author also describes the luminescence of other transparent materials and future prospects of the discovered luminescence.