Luminescence phenomena from various materials in ultra-high dose rate proton beam irradiation: preliminary dose distribution measurement study

doi:10.21203/rs.3.rs-3864891/v1

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Luminescence phenomena from various materials in ultra-high dose rate proton beam irradiation: preliminary dose distribution measurement study

https://doi.org/10.21203/rs.3.rs-3864891/v1

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This research aimed to identify materials capable of emitting visible light useful for dose management at ultra-high dose rate (uHDR). Various materials were irradiated with proton beams at a normal dose rate (NDR) and uHDR, and the resulting surface luminescence was captured using a high-sensitivity camera. The luminescence images were compared with the corresponding dose distributions. The luminescence of Tough Water Phantoms (Kyoto Kagaku Co. Ltd.) with various thicknesses was also observed to evaluate the depth distributions. Dose distributions were measured using two-dimensional ionization chamber detector arrays. The Tough Bone Phantom (Kyoto Kagaku Co. Ltd.) exhibited the strongest luminescence among the materials, followed by the Tough Water Phantom. The metals exhibited relatively weak luminescence. The luminescence profiles of the Tough Water Phantom, water, the Tough Lung Phantom (Kyoto Kagaku Co. Ltd.), and an acrylic were similar to the dose profiles. The luminescence distribution of the Tough Water Phantom in the depth direction was similar to that of the dose distributions. The luminescence at uHDR and NDR were approximately equivalent. The Tough Water Phantom was found to be a suitable material for dosimetry, even at uHDR. More detailed measurement data, such as wavelength data, must be collected to elucidate the luminescence mechanism.

Particle therapy using mainly protons and carbon is a type of radiotherapy. Particle therapy is known for its Bragg Peak property, which allows for a more focused dose distribution. Although protons have biological effects similar to those of X-rays, carbon ions are more effective than protons against radioresistant tumors. There are now over 100 particle therapy centers worldwide.¹ As of 2022, over 360,000 people have been treated with particle therapy.²

In the field of radiation oncology, there is growing interest in the phenomenon known as the FLASH effect. This effect is triggered by ultra-high dose rate (uHDR) irradiation of over 40 Gy/s with an irradiation time of below 500 ms. It has been reported that compared to normal dose-rate radiotherapy, FLASH has an equivalent anti-tumor effect but provides higher protection to normal tissues.³ This protective effect is confirmed in treatments at various sites in in-vivo studies,^4–7 and uHDR irradiation on humans has been already conducted.⁸

Accurate dose measurement is essential for radiotherapy under all conditions. Conventionally, radiation detectors include ionization chambers, chemical detectors such as radiochromic films, and luminescence detectors that use light or heat luminescence; each detector has its set of advantages and disadvantages. Luminescence detectors exhibit excellent spatial distribution, time resolution, and dose linearity, even for uHDR. Hence, this type of dosimeter is expected to play a role not only in simple detectors but also in monitoring and three-dimensional dosimetry.⁹ Favaudon et al. used this type of dosimeter for the online monitoring of electron FLASH.¹⁰

Detectors that use visible-light emissions include scintillation and Cherenkov detectors. Scintillation is a phenomenon in which electrons excited by radiation emit light when they return to their ground state. Materials used in detectors include inorganic scintillators such as CsI(Tl), NaI(Tl), and BGO, and organic scintillators, which are made of organic compounds and exhibit high energy transfer efficiency and luminescence. These detectors are already in practical use worldwide.

Cherenkov light is emitted when charged particles pass through a transparent material faster than the speed of light in that medium,¹¹ and this light emission has a higher time resolution than that of scintillators. This phenomenon is also observed in X-rays and electrons from radiotherapy systems. Cherenkov lights can be used for dosimetric quality assurance and quality control,^12–16 surface-based monitoring system using these lights is implemented.¹⁷ Although their use for particle therapy is also expected, Cherenkov light is not emitted at the distal range of particles because of thresholds. The energies for direct Cherenkov light emissions of protons and electrons are 482 MeV and 0.262 MeV respectively,¹⁸ and the secondary electron above 0.262 MeV requires proton beams above 120 MeV.¹⁹ The proton energies for radiotherapy are approximately 70–250 MeV; therefore, it is believed that the surface luminescence can only be observed.¹⁹

Recently, several studies have shown that micro-luminescence also occurs when irradiated at energies below the threshold of Cherenkov radiation. Helo et al. reported that Cherenkov light is emitted from electrons originating from prompt gamma X-rays and secondary radioisotopes when water is irradiated with a 60 MeV proton beam.²⁰ Yamamoto et al. reported that the depth profile luminescence images resembled the simulated dose distributions when irradiated with a 100 MeV proton beam.²¹ Yogo et al. reported that luminescence images obtained from irradiated water are practical for uHDR.²²

Because uHDR irradiation transfers the dose instantaneously, the luminescence is also instantaneous. Therefore, it may be possible to observe light luminescence that is not detected at a normal dose rate and discover better luminescent material for dose management using uHDR irradiation.

This study aimed to discover visible light-emitting materials that can be used in uHDR. Various materials were irradiated with proton beams at a normal dose rate (NDR) and uHDR, and the resulting surface luminescence was captured using a high-sensitivity camera. The luminescence images were compared with the dose distributions.

Proton Beam irradiation system

All experiments were performed using proton beams from a proton therapy system (Sumitomo Heavy Industries Ltd., Tokyo, Japan) at the Kouseikai Proton Center (Nara, Japan). The proton beam parameters are listed in Table 1. The beam energy was 230.0 MeV, and the irradiation technique was line scanning. The width of the spread-out Bragg peak (SOBP) was 20.1 mm using a mini-ridge filter. The field size was φ23 mm at the center of the SOBP. The pattern of the proton beam monitored on the profile monitor of the proton therapy system is shown in Fig. 1 (a). The beam pattern was the combination of two circles (φ27 mm and φ3.3 mm) and formed the uniform distribution at the SOBP. The irradiation dose was 7.2 Gy. The uHDR and NDR were 100 Gy/s and 2.06 Gy/s, respectively. The log of the dose monitor of one uHDR irradiation is shown in Fig. 1 (b). Each monitor is used for clinical purposes.

Table 1

Parameters of the proton beams (uHDR: ultra-high dose rate, NDR: normal dose rate).
Proton Beams Parameter
Energy (MeV)		230.0
Technique		Line Scanning
Dose (Gy)		7.20
Dose Rate (Gy/s)	(uHDR)	100
	(NDR)	2.06
Radiation Time (msec)	(uHDR)	72.0
	(NDR)	3488
Field Size (mm)		φ23.0
SOBP Width (mm)		20.1

Luminescence observation system

A schematic diagram of the setup of the device used to observe the luminescence and its appearance is shown in Fig. 2. The Assembly Block Imaging system (ABIs) (Osaka University, Osaka, Japan) and irradiated materials were placed below the nozzle of the proton therapy system. ABIs were combined with flames of ABS resin, boards of aluminum, and the mirror and were colored black to minimize self-light emission and unnecessary light reflections, except for the mirror. The interior of the ABIs was dark enough to set each block without any gaps. The samples were placed at the entrance of the ABIs. An isocentric plane was located at the bottom of the sample. Luminescence from the bottom surface of the sample was observed using a cooled CMOS camera (CS-61M, Bitran Ltd., Saitama, Japan) to reflect the mirror inside the ABIs. The sensitive wavelength band of the camera was 400–1000 nm. The shutter speed of the camera was 30 s, and the gain was 0%. The measurement system was covered with a black curtain to prevent light contamination. All measurements were performed three times.

Measurements of luminescence of various materials

Various materials were irradiated with NDR and uHDR proton beams, and the luminescence of the materials was observed. The irradiated materials are shown in Fig. 3. There were 10 materials: a Tough Water Phantom (WD-3010, Kyoto Kagaku Co., Ltd., Kyoto, Japan), a Tough Lung Phantom (Y04444-97T2-734, Kyoto Kagaku Co., Ltd., Kyoto, Japan), a Tough Bone Phantom (Cortical Bone), (BE-H-05, Kyoto Kagaku Co., Ltd., Kyoto, Japan), polyethylene, an acrylic, water in an acrylic case, an aluminum sheet (A1050), (HA0523, Hikari Co., Ltd., Osaka, Japan), a copper Sheet (C1100P), (CZ553, Hikari Co., Ltd., Osaka, Japan), a brass sheet (YZ552, Hikari Co., Ltd., Osaka, Japan), and a stainless steel sheet (SUS430) (SZ554, Hikari Co., Ltd., Osaka, Japan). Measurements were taken in two patterns. One type of measurement was performed by placing each material alone, and the other type was performed by placing the Tough Water Phantom on top of the material so that the total water equivalent thickness was 310 mm; these measurements were defined as the luminescence at the entrance position and the SOBP position, respectively. The luminescence was observed in the absence of any material to account for the influence of the self-light emission from the ABIs, and the value was used as the background.

Measurements of luminescence of Tough Water Phantom at various thicknesses

The measurements were performed using Tough Water Phantoms of different thicknesses. A schematic diagram of this process is shown in Fig. 4 (left). The Tough Water Phantom at the bottom was always 10 mm thick. Eighteen thicknesses of the Tough Water Phantoms were used: 10, 50, 100, 150, 200, 250, 270, 290, 295, 300, 305, 310, 315, 320, 325, 330, 335, and 340 mm.

Measurements of dose distributions

Dose distribution measurements were performed using 2-dimensional ionization chamber detector arrays (MatriXX PT; IBA Dosimetry Co., Ltd., Schwarzenbruck, Germany). Because these detector arrays have a relatively large sensitive volume, measurements may become saturated. Therefore, only NDR was performed.

A schematic diagram is shown in Fig. 4 (right). The isocentric plane was the reference point for the chamber, and Tough Water Phantoms with different thicknesses were placed on top of the detector arrays. The detector arrays have a water equivalent depth of 6.2 mm, and the setup in which the Tough Water Phantom of 4 mm was placed was treated as 10 mm. An isocentric plane was the reference point for the detector arrays. The thickness interval was 5 mm in the SOBP region and 10 mm in the other regions. The measurement was also taken with the Tough Water Phantom that was not placed on the arrays. Because of the wide spacing of the chambers of the detector arrays, measurements were also performed by shifting the position of the arrays so that the distance between the measurement points was 3.8 mm.

Analysis of luminescence images

The obtained luminescence images were analyzed using public domain software (Image J, version 1.53, National Institute of Health). The background image was subtracted from the luminescent image and processed for noise reduction using a median filter. For only the luminescence of water, the luminescent image of the acrylic case was used as the background image.

Luminescence was observed for all materials. The comparison of luminescence intensities is shown in Fig. 5. The Tough Bone Phantom exhibited the strongest luminescence, followed by the Tough Water Phantom. The metals exhibited weak luminescence, and aluminum and stainless steel exhibited relatively strong luminescence. Non-transparent materials such as the Tough Water Phantom, the Tough Bone Phantom, the Tough Lung Phantom, and polyethylene exhibited stronger luminescence at the SOBP position than at the entrance position. In contrast, the luminescence of transparent materials at the SOBP position was weaker than at the entrance position. The intensities of luminescence at uHDR and NDR were approximately equivalent.

The luminescent images at uHDR and the luminescence profiles at the center of each material are shown in Fig. 6. For comparison, each profile displays the dose profile of the 10 mm Tough Water Phantom at the entrance position and 310 mm Tough Water Phantom at the SOBP position. The profiles were normalized by the value at the 0 mm position.

Although the luminescence profiles of the Tough Water Phantom, water, the Tough Lung Phantom, and the acrylic were similar to the dose profiles, those of the Tough Bone Phantom at the entrance position and polyethylene at both positions were different. Because metals have weak luminescence and a high noise level, a dose comparison was not possible. The luminescence profiles at uHDR and NDR were approximately equivalent.

Comparisons of the luminescence profiles of the Tough Water Phantom with various thicknesses and dose profiles are shown in Figs. 7 and 8, respectively. Lateral profiles are shown in excerpts. The depth distributions were normalized by the value at a depth of 150 mm, and the profiles were determined by the value at the center. The depth distribution of luminescence was the list of the values of the region of interest of φ 4 mm to the center of each image. Although the depth distribution of the luminescence was generally comparable to the depth dose distribution, there was a difference in the SOBP region. The luminescence profiles were similar to the dose profiles for all thicknesses. The depth distributions and luminescence profiles for uHDR and NDR were comparable.

Visible light luminescence was observed for all the irradiated materials. This mechanism can be divided into two categories: scintillation and Cherenkov radiation.

The irradiated materials, except water and metals, are plastic. Therefore, these plastic materials are expected to behave similarly to organic scintillators, although their fluorescence quantum yields are different. In an organic scintillator, the π-electrons involved in the conjugated double bands of carbon such as benzene rings are important. The Tough Bone Phantom and Tough Water Phantom, which showed relatively strong luminescence, may also have conjugated double bands, although their structural formulas are unknown. In contrast, polyethylene also exhibits these bands. However, because the level of electronic excitation is higher than that in the visible-light region, it is unlikely that visible-light emission will be observed. Nevertheless, the somewhat strong luminescence of this material is probably due to the influence of the additives. Acrylic does not have a conjugated double band of carbon, but Cherenkov radiation is thought to occur because of its transparent nature. Water and metals are inorganic and therefore do not show the mechanism mentioned above. While water causes Cherenkov radiation, metals are opaque. Atomic or molecular spectra are thought to be the source of this weak luminescence. Any material can show luminescence when electrons excited by radiation fall into the inner shell. The sensitive wavelength band of the camera used in this study is 400–1000 nm, and the energy range corresponding to the wavelength band is about 1–3 eV. This light emission from this energy range may be due to deexcitation when ionized electrons are trapped in orbital electron holes on the outermost or nearby shell of atoms and molecules. Although metal luminescence is rarely studied because it is considerably weak, there have been several reports on photoluminescence in metals such as gold, silver, and copper.^{23, 24} To verify this, the wavelengths of the luminescence should be examined and compared to the emission spectrum of each material.

Cherenkov radiation is important in transparent materials, such as water and acrylic. Because proton beams irradiated at this time have an energy of 230 MeV and the luminescence occurs near the surface of the materials, this luminescence is considered to contain the factor derived from this effect. It has been reported that the luminescence of acrylic irradiated with 2.7 MeV electron beams is mostly due to Cherenkov radiation.²⁵ The result that the luminescence at the SOBP position was very weak compared to that at the entrance position suggests that the main component of the luminescence of transparent materials is Cherenkov radiation. The weak luminescence at the SOBP position, where no Cherenkov radiation is emitted, is considered to be due to the scintillation effect mentioned above. The Tough Bone Phantom, the Tough Lung Phantom, the Tough Water Phantom, polyethylene, and metals are opaque; hence, only the surface luminescence was observed. Therefore, it is assumed that the Cherenkov radiation is rarely observed for these materials. To clarify the source of the luminescence observed in this investigation, it is imperative to conduct an analysis of the wavelength. A comprehensive examination of the luminescence spectra used in this investigation is necessary to assess these hypotheses.

With respect to the correlation between luminescence and dose, luminescence distributions of the Tough Water Phantom, the Tough Lung Phantom, water, and the acrylic were comparable to the dose distributions. In particular, the luminescence intensity of the Tough Water Phantom was relatively strong, i.e., approximately 11 times stronger than that of the tap water and approximately 45 times stronger than that of the Tough Lung Phantom and the acrylic. The depth distributions of the Tough Water Phantom were similar to those of the doses. However, a difference between the two is observed in the SOBP region. This can be attributed to the quenching of the luminescence. The Tough Bone Phantom and polyethylene also exhibited strong luminescence; however, the distributions differed from the dose distributions. The Tough Bone Phantom included P and Si, whereas the Tough Water Phantom did not. Polyethylene may also contain these additives. However, if the proportion of the luminescence originating from secondary particles or secondary radioisotopes is large, the luminescence distributions may not differ from the dose distributions. The intensities and distributions of luminescence were similar for NDR (2.06 Gy/s) and uHDR (100 Gy/s). However, the luminescence profiles at uHDR in the area of the valley showed slight differences from the dose distribution. The dose monitor and flatness monitor are common to both NDR and uHDR and the position and dose feedback work. However, it has not been fully verified that these monitors work equally well for uHDR; therefore, their accuracy may be lower compared to that for NDR. These results indicate that the Tough Water Phantom has strong visible light luminescence, which is similar to the dose distribution of proton beams with high dose rates. The luminescence of the Tough Water Phantom can be used to provide accurate dosimetry for proton irradiations at different dose rates. However, plastic materials such as those in the Tough Water Phantom are damaged by repeated irradiation. This may change the dose-luminescence scale or luminescence distributions. In addition, radioactivation also occurs. The Tough Water Phantom is composed of a greater number of elements than polyethylene and water, so more attention should be paid to this problem. The luminescence observation system used in this study emitted visible light without any material placed on it. This was considered to be caused by luminescence originating from the mirror in this system. This luminescence is considered to include air luminescence. However, air luminescence is much weaker than the luminescence of water²⁶ and therefore almost impossible to measure with the system used in this study. Although this effect is suppressed by the image subtraction process, this was considered as one of the factors of uncertainty in this study.

In FLASH proton therapy with uHDR proton irradiation, information of the light emission distribution can be used to monitor the proton irradiation area on the patient’s body surface. In the future, we plan to perform experiments for imaging of the luminescence distribution on annimal skin by uHDR proton irradiation.

We observed the visible light luminescence of various materials when irradiated with proton beams at NDR and uHDR with a high-sensitivity camera and compared their luminescence images with the dose distributions. Visible-light luminescence was observed in all irradiated materials, and the Tough Water Phantom was found to be a suitable material for dosimetry. Water-equivalent phantoms such as the Tough Water Phantom exist in almost all radiotherapy facilities, and this dose management system can be established relatively easily in existing facilities. More detailed measurement data such as wavelength data must be collected to elucidate the luminescence mechanism.

Acknowledgment

We would like to thank the staff members of the Department of Radiology of Kouseikai Takai Hospital for their help, the members of Sumitomo Heavy Industries Ltd. for the control of proton beams at the ultra-high dose rate, and the members of SHI Accelerator Service Ltd. for the operation of the proton apparatus.

This work was supported by JSPS KAKENHI Grant Number JP23H00553 and JST ERATO Grant Number JPMJER2102.

We would like to thank Editage (www.editage.jp) for English language editing.

Author contributions

R.Y. performed measurements and wrote the manuscript. T.N. conceived the project, supervised the study and helped with writing of the manuscript. D.K., T.T. and M.O. helped with measuring and analyzing. J.K., T.N., H.Y., Y.T., D.K., T.T., M.O. and R.Y. participated in discussions. All authors have read and approved the manuscript.

Funding information

Japan Society for the Promotion of science, KAKENHI, Grant Number: JP23H00553

Japan Science and Technology Agency, ERATO, Grant Number: JPMJER2102

Conflict of Interest

The authors have no relevant conflicts of interest to disclose.

Data availability

The datasets used and/or analysed during the current study available from the corresponding author on reasonable request.

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You are reading this latest preprint version

Luminescence phenomena from various materials in ultra-high dose rate proton beam irradiation: preliminary dose distribution measurement study

Status:

Version 1

Abstract

Figures

Introduction

Materials and Methods

Proton Beam irradiation system

Luminescence observation system

Measurements of luminescence of various materials

Measurements of luminescence of Tough Water Phantom at various thicknesses

Measurements of dose distributions

Analysis of luminescence images

Results

Discussion

Conclusion

Declarations

References

Additional Declarations

Status:

Version 1