Positron-emitting Nuclei Research Articles

Experimental investigation of the characteristics of radioactive beams for heavy ion therapy.

This work has two related objectives. The first is to estimate the relative biological effectiveness of two radioactive heavy ion beams based on experimental measurements, and compare these to the relative biological effectiveness of corresponding stable isotopes to determine whether they are therapeutically equivalent. The second aim is to quantitatively compare the quality of images acquired postirradiation using an in-beam whole-body positron emission tomography scanner for range verification quality assurance. The energy deposited by monoenergetic beams of C at 350MeV/u, O at 250MeV/u, C at 350MeV/u, and O at 430MeV/u was measured using a cruciform transmission ionization chamber in a water phantom at the Heavy Ion Medical Accelerator in Chiba (HIMAC), Japan. Dose-mean lineal energy was measured at various depths along the path of each beam in a water phantom using a silicon-on-insulator mushroom microdosimeter. Using the modified microdosimetric kinetic model, the relative biological effectiveness at 10% survival fraction of the radioactive ion beams was evaluated and compared to that of the corresponding stable ions along the path of the beam. Finally, the postirradiation distributions of positron annihilations resulting from the decay of positron-emitting nuclei were measured for each beam in a gelatin phantom using the in-beam whole-body positron emission tomography scanner at HIMAC. The depth of maximum positron-annihilation density was compared with the depth of maximum dose deposition and the signal-to-background ratios were calculated and compared for images acquired over 5 and 20min postirradiation of the phantom. In the entrance region, the was 1.2±0.1 for both C and C beams, while for O and O it was 1.4±0.1 and 1.3±0.1, respectively. At the Bragg peak, the was 2.7±0.4 for C and 2.9±0.4 for C, while for O and O it was 2.7±0.4 and 2.8±0.4, respectively. In the tail region, could only be evaluated for carbon; the was 1.6±0.2 and 1.5±0.1 for C and C, respectively. Positron emission tomography images obtained from gelatin targets irradiated by radioactive ion beams exhibit markedly improved signal-to-background ratios compared to those obtained from targets irradiated by nonradioactive ion beams, with 5-fold and 11-fold increases in the ratios calculated for the O and C images compared with the values obtained for O and C, respectively. The difference between the depth of maximum dose and the depth of maximum positron annihilation density is 2.4±0.8mm for C, compared to -5.6±0.8mm for C and 0.9±0.8mm for O vs -6.6±0.8mm for O. The values for C and O were found to be within the 95% confidence interval of the RBEs estimated for their corresponding stable isotopes across each of the regions in which it was evaluated. Furthermore, for a given dose, C and O beams produce much better quality images for range verification compared with C and O, in particular with regard to estimating the location of the Bragg peak.

Comparative study of alternative Geant4 hadronic ion inelastic physics models for prediction of positron-emitting radionuclide production in carbon and oxygen ion therapy

The distribution of fragmentation products predicted by Monte Carlo simulations of heavy ion therapy depend on the hadronic physics model chosen in the simulation. This work aims to evaluate three alternative hadronic inelastic fragmentation physics options available in the Geant4 Monte Carlo radiation physics simulation framework to determine which model most accurately predicts the production of positron-emitting fragmentation products observable using in-beam PET imaging. Fragment distributions obtained with the BIC, QMD, and INCL + + physics models in Geant4 version 10.2.p03 are compared to experimental data obtained at the HIMAC heavy-ion treatment facility at NIRS in Chiba, Japan. For both simulations and experiments, monoenergetic beams are applied to three different block phantoms composed of gelatin, poly(methyl methacrylate) and polyethylene. The yields of the positron-emitting nuclei 11C, 10C and 15O obtained from simulations conducted with each model are compared to the experimental yields estimated by fitting a multi-exponential radioactive decay model to dynamic PET images using the normalised mean square error metric in the entrance, build up/Bragg peak and tail regions. Significant differences in positron-emitting fragment yield are observed among the three physics models with the best overall fit to experimental 12C and 16O beam measurements obtained with the BIC physics model.

Beam-on imaging of short-lived positron emitters during proton therapy

In vivo dose delivery verification in proton therapy can be performed by positron emission tomography (PET) of the positron-emitting nuclei produced by the proton beam in the patient. A PET scanner installed in the treatment position of a proton therapy facility that takes data with the beam on will see very short-lived nuclides as well as longer-lived nuclides. The most important short-lived nuclide for proton therapy is 12N (Dendooven et al 2015 Phys. Med. Biol. 60 8923–47), which has a half-life of 11 ms. The results of a proof-of-principle experiment of beam-on PET imaging of short-lived 12N nuclei are presented. The Philips Digital Photon Counting Module TEK PET system was used, which is based on LYSO scintillators mounted on digital SiPM photosensors. A 90 MeV proton beam from the cyclotron at KVI-CART was used to investigate the energy and time spectra of PET coincidences during beam-on. Events coinciding with proton bunches, such as prompt gamma rays, were removed from the data via an anti-coincidence filter with the cyclotron RF. The resulting energy spectrum allowed good identification of the 511 keV PET counts during beam-on. A method was developed to subtract the long-lived background from the 12N image by introducing a beam-off period into the cyclotron beam time structure. We measured 2D images and 1D profiles of the 12N distribution. A range shift of 5 mm was measured as 6 ± 3 mm using the 12N profile. A larger, more efficient, PET system with a higher data throughput capability will allow beam-on 12N PET imaging of single spots in the distal layer of an irradiation with an increased signal-to-background ratio and thus better accuracy. A simulation shows that a large dual panel scanner, which images a single spot directly after it is delivered, can measure a 5 mm range shift with millimeter accuracy: 5.5 ± 1.1 mm for 1 × 108 protons and 5.2 ± 0.5 mm for 5 × 108 protons. This makes fast and accurate feedback on the dose delivery during treatment possible.

A Conway–Maxwell–Poisson (CMP) model to address data dispersion on positron emission tomography

Positron emission tomography (PET) in medicine exploits the properties of positron-emitting unstable nuclei. The pairs of γ- rays emitted after annihilation are revealed by coincidence detectors and stored as projections in a sinogram. It is well known that radioactive decay follows a Poisson distribution; however, deviation from Poisson statistics occurs on PET projection data prior to reconstruction due to physical effects, measurement errors, correction of deadtime, scatter, and random coincidences. A model that describes the statistical behavior of measured and corrected PET data can aid in understanding the statistical nature of the data: it is a prerequisite to develop efficient reconstruction and processing methods and to reduce noise.The deviation from Poisson statistics in PET data could be described by the Conway-Maxwell-Poisson (CMP) distribution model, which is characterized by the centring parameter λ and the dispersion parameter ν, the latter quantifying the deviation from a Poisson distribution model. In particular, the parameter ν allows quantifying over-dispersion (ν<1) or under-dispersion (ν>1) of data. A simple and efficient method for λ and ν parameters estimation is introduced and assessed using Monte Carlo simulation for a wide range of activity values.The application of the method to simulated and experimental PET phantom data demonstrated that the CMP distribution parameters could detect deviation from the Poisson distribution both in raw and corrected PET data. It may be usefully implemented in image reconstruction algorithms and quantitative PET data analysis, especially in low counting emission data, as in dynamic PET data, where the method demonstrated the best accuracy.

Measurement of proton-induced target fragmentation cross sections in carbon

In proton therapy, positron emitter nuclei are generated via the target nuclear fragmentation reactions between irradiated proton and nuclei constituting a human body. The proton-irradiated volume can be confirmed with measurement of annihilation γ-rays from the generated positron emitter nuclei. To achieve the high accuracy of proton therapy, in vivo dosimetry, i.e., evaluation of the irradiated dose during the treatment is important. To convert the measured activity distribution to irradiated dose, cross-sectional data for positron emitter production is necessary, which is currently insufficient in the treatment area. The purpose of this study is to collect cross-sectional data of C12(p,pn)C11 and C12(p,p2n)C10 reactions between the incident proton and carbon nuclei, which are important target nuclear fragmentation reactions, to estimate the range and exposure dose distribution in the patient's body. Using planar-type PET capable of measuring annihilation γ-rays at high positional resolution and thick polyethylene target, we measured cross-sectional data in continuous wide energy range. The cross section of C12(p,pn)C11 is in good agreement with existing experimental data. The cross section of C12(p,p2n)C10 is reported for the first data in the low-energy range of 67.6–10.5 MeV near the Bragg peak of proton beam.

SU-D-BRF-02: In Situ Verification of Radiation Therapy Dose Distributions From High-Energy X-Rays Using PET Imaging

Purpose: To study the possibility of in situ verification of radiation therapy dose distributions using PET imaging based on the activity distribution of 11C and 15O produced via photonuclear reactions in patient irradiated by 45MV x-rays. Methods: The method is based on the photonuclear reactions in the most elemental composition {sup 12}C and {sup 16}O in body tissues irradiated by bremsstrahlung photons with energies up to 45 MeV, resulting primarily in {sup 11}C and {sup 15}O, which are positron-emitting nuclei. The induced positron activity distributions were obtained with a PET scanner in the same room of a LA45 accelerator (Top Grade Medical, Beijing, China). The experiments were performed with a brain phantom using realistic treatment plans. The phantom was scanned at 20min and 2-5min after irradiation for {sup 11}C and {sup 15}, respectively. The interval between the two scans was 20 minutes. The activity distributions of {sup 11}C and {sup 15}O within the irradiated volume can be separated from each other because the half-life is 20min and 2min for {sup 11}C and {sup 15}O, respectively. Three x-ray energies were used including 10MV, 25MV and 45MV. The radiation dose ranged from 1.0Gy to 10.0Gy per treatment. Results: It was confirmed thatmore » no activity was detected at 10 MV beam energy, which was far below the energy threshold for photonuclear reactions. At 25 MV x-ray activity distribution images were observed on PET, which needed much higher radiation dose in order to obtain good quality. For 45 MV photon beams, good quality activation images were obtained with 2-3Gy radiation dose, which is the typical daily dose for radiation therapy. Conclusion: The activity distribution of {sup 15}O and {sup 11}C could be used to derive the dose distribution of 45MV x-rays at the regular daily dose level. This method can potentially be used to verify in situ dose distributions of patients treated on the LA45 accelerator.« less

SU‐E‐T‐519: Emission of Secondary Particles From a PMMA Phantom During Proton Irradiation: A Simulation Study with the Geant4 Monte Carlo Toolkit

Purpose:Proton therapy exhibits several advantages over photon therapy due to depth‐dose distributions from proton interactions within the target material. However, uncertainties associated with protons beam range in the patient limit the advantage of proton therapy applications. To quantify beam range, positron‐emitting nuclei (PEN) and prompt gamma (PG) techniques have been developed. These techniques use de‐excitation photons to describe the location of the beam in the patient. To develop a detector system for implementing the PG technique for range verification applications in proton therapy, we studied the yields, energy and angular distributions of the secondary particles emitted from a PMMA phantom.Methods:Proton pencil beams of various energies incident onto a PMMA phantom with dimensions of 5 x 5 x 50 cm3 were used for simulation with the Geant4 toolkit using the standard electromagnetic packages as well as the packages based on the binary‐cascade nuclear model. The emitted secondary particles are analyzed .Results:For 160 MeV incident protons, the yields of secondary neutrons and photons per 100 incident protons were ~6 and ~15 respectively. Secondary photon energy spectrum showed several energy peaks in the range between 0 and 10 MeV. The energy peaks located between 4 and 6 MeV were attributed to originate from direct proton interactions with 12C (~ 4.4 MeV) and 16O (~ 6 MeV), respectively. Most of the escaping secondary neutrons were found to have energies between 10 and 100 MeV. Isotropic emissions were found for lower energy neutrons (&lt;10 MeV) and photons for all energies, while higher energy neutrons were emitted predominantly in the forward direction. The yields of emitted photons and neutrons increased with the increase of incident proton energies.Conclusions:A detector system is currently being developed incorporating the yields, energy and angular distributions of secondary particles from proton interactions obtained from this study.

Application of activity pencil beam algorithm using measured distribution data of positron emitter nuclei for therapeutic SOBP proton beam

Recently, much research on imaging the clinical proton-irradiated volume using positron emitter nuclei based on target nuclear fragment reaction has been carried out. The purpose of this study is to develop an activity pencil beam (APB) algorithm for a simulation system for proton-activated positron-emitting imaging in clinical proton therapy using spread-out Bragg peak (SOBP) beams. The target nuclei of activity distribution calculations are (12)C nuclei, (16)O nuclei, and (40)Ca nuclei, which are the main elements in a human body. Depth activity distributions with SOBP beam irradiations were obtained from the material information of ridge filter (RF) and depth activity distributions of compounds of the three target nuclei measured by BOLPs-RGp (beam ON-LINE PET system mounted on a rotating gantry port) with mono-energetic Bragg peak (MONO) beam irradiations. The calculated data of depth activity distributions with SOBP beam irradiations were sorted in terms of kind of nucleus, energy of proton beam, SOBP width, and thickness of fine degrader (FD), which were verified. The calculated depth activity distributions with SOBP beam irradiations were compared with the measured ones. APB kernels were made from the calculated depth activity distributions with SOBP beam irradiations to construct a simulation system using the APB algorithm for SOBP beams. The depth activity distributions were prepared using the material information of RF and the measured depth activity distributions with MONO beam irradiations for clinical therapy using SOBP beams. With the SOBP width widening, the distal fall-offs of depth activity distributions and the difference from the depth dose distributions were large. The shapes of the calculated depth activity distributions nearly agreed with those of the measured ones upon comparison between the two. The APB kernels of SOBP beams were prepared by making use of the data on depth activity distributions with SOBP beam irradiations that were made from the depth activity distributions with MONO beam irradiations and sorted in terms of energy, SOBP width, and thickness of FD. The data on APB kernels of SOBP beams were determined as installment data for the simulation system using the APB algorithm for SOBP beam irradiations. A method of obtaining the depth activity distributions and the APB algorithm for clinical use of SOBP beams have been developed. It is suggested that the simulation system for imaging the clinical irradiated volume with the APB algorithm can be used in clinical proton therapy using SOBP beams by preparing and investigating the data on APB kernels of SOBP beams.

MO‐G‐137‐01: Proton Therapy Range Verification Using Prompt Gamma Rays: A Simulation Study with the Geant4 Monte Carlo Toolkit

Purpose: To simulate a coincidence detector system for in vivo range verification using prompt gamma (PG) emission technique for proton radiation therapy and to compare the results with the alternative technique utilizing the positron‐emitting nuclei (PEN). Methods: A coincidence detector system for characterizing both emitted and registered prompt gamma spectrum was simulated by Geant4 Monte Carlo toolkit version 9.4.9 p02. The interaction of 110 MeV protons with target phantom of PMMA (5 cm × 5 cm × 50 cm) was used in the simulation. A virtual detector (5 cm × 5 cm × 1 mm) was replicated 500 times to form a bank of detectors. A lower bank was placed 1 cm above the phantom while another bank was placed 1 cm above the lower bank. A coincidence requirement was met when a gamma had passed through both a bottom detector segment and the corresponding top detector segment directly above it. Our investigation used the built‐in physics list consisting of electromagnetic as well as nuclear interactions. Results: The depths corresponding to the maximum prompt gamma counts were found to be in close proximity to the range of protons as determined by the Bragg Peak. We noted that the PEN yields had their maximum and distal 50% yield more superficially than the Bragg Peak. This is clearly different compared to PG yields that showed their maximum in closer proximity to the Bragg Peak. The 50% distal PEN yields also lied shallower to the Bragg peak whereas the 50% distal PG yields lied deeper with respect to the Bragg Peak. Conclusion: This work shows clearly that the range of the incident protons can be measured in vivo via a suitable detector system utilizing the PG technique. More realistic simulations by implementing and optimizing various detector materials are now being planned.

A new PET prototype for proton therapy: comparison of data and Monte Carlo simulations

Ion beam therapy is a valuable method for the treatment of deep-seated and radio-resistant tumors thanks to the favorable depth-dose distribution characterized by the Bragg peak. Hadrontherapy facilities take advantage of the specific ion range, resulting in a highly conformal dose in the target volume, while the dose in critical organs is reduced as compared to photon therapy. The necessity to monitor the delivery precision, i.e. the ion range, is unquestionable, thus different approaches have been investigated, such as the detection of prompt photons or annihilation photons of positron emitter nuclei created during the therapeutic treatment. Based on the measurement of the induced β+ activity, our group has developed various in-beam PET prototypes: the one under test is composed by two planar detector heads, each one consisting of four modules with a total active area of 10 × 10 cm2. A single detector module is made of a LYSO crystal matrix coupled to a position sensitive photomultiplier and is read-out by dedicated frontend electronics. A preliminary data taking was performed at the Italian National Centre for Oncological Hadron Therapy (CNAO, Pavia), using proton beams in the energy range of 93–112 MeV impinging on a plastic phantom. The measured activity profiles are presented and compared with the simulated ones based on the Monte Carlo FLUKA package.

Range verification of proton radiotherapy with prompt gamma rays

In-vivo range verification systems for incident protons recently utilize positron emission tomography (PET) based on the phenomenon of positron-emitting nuclei (PEN). However, recent investigations are suggesting that the range can be verified also from the prompt gamma (PG) photon emissions generated from proton interactions. In this work we investigate using the Geant4 Monte Carlo toolkit the clinical viability of a theoretical sequential detector system to verify the range of protons in proton radiotherapy by the PG method for simple geometries and beam configurations. The results show a correlation between selected emitted PG rays and the incident protons range and suggest that our detector system is capable of in-vivo range monitoring for energies typical of radiation oncology applications. Future work will include implementing more realistic scenarios and optimizing current detector parameters.

SU-E-J-80: A Simulation Study with Geant4 on the Yields of Positron-Emitting Nuclei (10C, 11C, and 15O) Induced by Protons and Carbon Ions.

To study the yields and the depth-distributions of positron-emitting nuclei (PEN) 10C, 11C, and 15O induced by protons and carbon ions in PMMA. An application built with various physics packages was constructed with Geant4 Monte Carlo Toolkit. A phantom of 15 cm × 15 cm × 50 cm consisting of PMMA (C5H8O2, density 1.18 g/cm3 ) was irradiated with 70, 110 MeV protons and 204 A, 212.12 A MeV carbonions. Beam quality of 1 cm FWHM with 0.2% Gaussian energy spread FWHM was used. For each beam energy, simulation of the energy deposited was recorded for every 1 mm increment of depth in the phantom. The resulting PEN and their yields were recorded at the production point and also at the point of decay in increments of 1 mm. The overall percentage yields, depth-distributions, and total percentages of PEN per incident particle for both protons and carbon ions were obtained. Our results found that the predominant PMMA fragment was 11C irrespective of incident particle type or energy. Our yields of PEN are comparable with other simulations (FLUKA and MCHIT) as well as existing experimental data. The present work however overestimates the percentage of PEN in the simulations of incident carbon ions and the percentage of 10C for incident protons compared to the experimental data. This particular application built within the Geant4 Monte Carlo Toolkit can estimate the amount of PEN as well as the depth-distributions of these nuclei in hadron induced radiation therapy.

Yields of positron and positron emitting nuclei for proton and carbon ion radiation therapy: A simulation study with GEANT4

A Monte Carlo application is developed to investigate the yields of positron-emitting nuclei (PEN) used for proton and carbon ion range verification techniques using the GEANT4 Toolkit. A base physics list was constructed and used to simulate incident proton and carbon ions onto a PMMA or water phantom using pencil like beams. In each simulation the total yields of PEN are counted and both the PEN and their associated positron depth-distributions were recorded and compared to the incident radiation's Bragg Peak. Alterations to the physics lists are then performed to investigate the PEN yields dependence on the choice of physics list. In our study, we conclude that the yields of PEN can be estimated using the physics list presented here for range verification of incident proton and carbon ions.

Development of activity pencil beam algorithm using measured distribution data of positron emitter nuclei generated by proton irradiation of targets containing 12 C, 16 O, and 40 Ca nuclei in preparation of clinical application

The purpose of this study is to develop a new calculation algorithm that is satisfactory in terms of the requirements for both accuracy and calculation time for a simulation of imaging of the proton-irradiated volume in a patient body in clinical proton therapy. The activity pencil beam algorithm (APB algorithm), which is a new technique to apply the pencil beam algorithm generally used for proton dose calculations in proton therapy to the calculation of activity distributions, was developed as a calculation algorithm of the activity distributions formed by positron emitter nuclei generated from target nuclear fragment reactions. In the APB algorithm, activity distributions are calculated using an activity pencil beam kernel. In addition, the activity pencil beam kernel is constructed using measured activity distributions in the depth direction and calculations in the lateral direction. (12)C, (16)O, and (40)Ca nuclei were determined as the major target nuclei that constitute a human body that are of relevance for calculation of activity distributions. In this study, "virtual positron emitter nuclei" was defined as the integral yield of various positron emitter nuclei generated from each target nucleus by target nuclear fragment reactions with irradiated proton beam. Compounds, namely, polyethylene, water (including some gelatin) and calcium oxide, which contain plenty of the target nuclei, were irradiated using a proton beam. In addition, depth activity distributions of virtual positron emitter nuclei generated in each compound from target nuclear fragment reactions were measured using a beam ON-LINE PET system mounted a rotating gantry port (BOLPs-RGp). The measured activity distributions depend on depth or, in other words, energy. The irradiated proton beam energies were 138, 179, and 223 MeV, and measurement time was about 5 h until the measured activity reached the background level. Furthermore, the activity pencil beam data were made using the activity pencil beam kernel, which was composed of the measured depth data and the lateral data including multiple Coulomb scattering approximated by the Gaussian function, and were used for calculating activity distributions. The data of measured depth activity distributions for every target nucleus by proton beam energy were obtained using BOLPs-RGp. The form of the depth activity distribution was verified, and the data were made in consideration of the time-dependent change of the form. Time dependence of an activity distribution form could be represented by two half-lives. Gaussian form of the lateral distribution of the activity pencil beam kernel was decided by the effect of multiple Coulomb scattering. Thus, the data of activity pencil beam involving time dependence could be obtained in this study. The simulation of imaging of the proton-irradiated volume in a patient body using target nuclear fragment reactions was feasible with the developed APB algorithm taking time dependence into account. With the use of the APB algorithm, it was suggested that a system of simulation of activity distributions that has levels of both accuracy and calculation time appropriate for clinical use can be constructed.

Measurement and verification of positron emitter nuclei generated at each treatment site by target nuclear fragment reactions in proton therapy

The purpose of this study is to verify the characteristics of the positron emitter nuclei generated at each treatment site by proton irradiation. Proton therapy using a beam on-line PET system mounted on a rotating gantry port (BOLPs-RGp), which the authors developed, is provided at the National Cancer Center Kashiwa, Japan. BOLPs-RGp is a monitoring system that can confirm the activity distribution of the proton irradiated volume by detection of a pair of annihilation gamma rays coincidentally from positron emitter nuclei generated by the target nuclear fragment reactions between irradiated proton nuclei and nuclei in the human body. Activity is measured from a start of proton irradiation to a period of 200 s after the end of the irradiation. The characteristics of the positron emitter nuclei generated in a patient's body were verified by the measurement of the activity distribution at each treatment site using BOLPs-RGp. The decay curves for measured activity were able to be approximated using two or three half-life values regardless of the treatment site. The activity of half-life value of about 2 min was important for a confirmation of the proton irradiated volume. In each proton treatment site, verification of the characteristics of the generated positron emitter nuclei was performed by using BOLPs-RGp. For the monitoring of the proton irradiated volume, the detection of (15)O generated in a human body was important.

The Development and Clinical Use of a Beam ON-LINE PET System Mounted on a Rotating Gantry Port in Proton Therapy

To verify the usefulness of our developed beam ON-LINE positron emission tomography (PET) system mounted on a rotating gantry port (BOLPs-RGp) for dose-volume delivery-guided proton therapy (DGPT). In the proton treatment room at our facility, a BOLPs-RGp was constructed so that a planar PET apparatus could be mounted with its field of view covering the iso-center of the beam irradiation system. Activity measurements were performed in 48 patients with tumors of the head and neck, liver, lungs, prostate, and brain. The position and intensity of the activity were measured using the BOLPs-RGp during the 200 s immediately after the proton irradiation. The daily measured activity images acquired by the BOLPs-RGp showed the proton irradiation volume in each patient. Changes in the proton-irradiated volume were indicated by differences between a reference activity image (taken at the first treatment) and the daily activity-images. In the case of head-and-neck treatment, the activity distribution changed in the areas where partial tumor reduction was observed. In the case of liver treatment, it was observed that the washout effect in necrotic tumor cells was slower than in non-necrotic tumor cells. The BOLPs-RGp was developed for the DGPT. The accuracy of proton treatment was evaluated by measuring changes of daily measured activity. Information about the positron-emitting nuclei generated during proton irradiation can be used as a basis for ensuring the high accuracy of irradiation in proton treatment.

Verification of Positron Emitter Nuclei Generated in Human Body by Proton Irradiation

Verification of Positron Emitter Nuclei Generated in Human Body by Proton Irradiation

PET monitoring of cancer therapy with 3He and 12C beams: a study with the GEANT4 toolkit

We study the spatial distributions of β+-activity produced by therapeutic beams of 3He and 12C ions in various tissue-like materials. The calculations were performed within a Monte Carlo model for heavy-ion therapy (MCHIT) based on the GEANT4 toolkit. The contributions from positron-emitting nuclei with T1/2 > 10 s, namely 10,11C, 13N, 14,15O, 17,18F and 30P, were calculated and compared with experimental data obtained during and after irradiation, where available. Positron-emitting nuclei are created by a 12C beam in fragmentation reactions of projectile and target nuclei. This leads to a β+-activity profile characterized by a noticeable peak located close to the Bragg peak in the corresponding depth–dose distribution. This can be used for dose monitoring in carbon-ion therapy of cancer. In contrast, as most of the positron-emitting nuclei are produced by a 3He beam in target fragmentation reactions, the calculated total β+-activity during or soon after the irradiation period is evenly distributed within the projectile range. However, we predict also the presence of 13N, 14O, 17,18F created in charge-transfer reactions by low-energy 3He ions close to the end of their range in several tissue-like media. The time evolution of β+-activity profiles was investigated for both kinds of beams. We found that due to the production of 18F nuclides the β+-activity profile measured 2 or 3 h after irradiation with 3He ions will have a distinct peak correlated with the maximum of depth–dose distribution. We also found certain advantages of low-energy 3He beams over low-energy proton beams for reliable PET monitoring during particle therapy of shallow-located tumours. In this case the distal edge of β+-activity distribution from 17F nuclei clearly marks the range of 3He in tissues.

Experimental verification of the utility of positron emitter nuclei generated by photonuclear reactions for X-ray beam monitoring in a phantom

The utility of positron emitter nuclei generated by photonuclear reactions was verified for X-ray beam monitoring in a phantom. Positron emission tomography-computed tomography (PET-CT) images of a gelatinous water phantom (H(2)O target) and a polyethylene phantom (CH(2) target) were acquired 5 min after delivering a dose of 17 Gy with an X-ray beam energy of 21 MV. Reconstructed PET images and the calculated half-life showed that the positron emitters of (15)O (half-life 122.2 s) in the H(2)O target and (11)C (half-life 20.4 min) in the CH(2) target were generated by photonuclear reactions. A comparison was made between measured activity and dose distributions for each target. The measured times of annihilation gamma rays from the positron emitter nucleus were 10 and 30 min for the (15)O nucleus in the H(2)O target and the (11)C nucleus in the CH(2) target, respectively. The activity distributions of the (15)O and (11)C positron emitter nuclei were similar to the measured dose distributions for both depth and lateral directions except for dose buildup and collimator edge regions. It was confirmed that no activity was detected at an X-ray energy of 14 MV, which was far below the energy threshold for both photonuclear reactions. It was estimated that the PET-CT image acquired from the activity of the (15)O and (11)C positron emitter nuclei might provide the area of X-ray beam irradiation in a phantom.

Experimental verification of proton beam monitoring in a human body by use of activity image of positron-emitting nuclei generated by nuclear fragmentation reaction

Proton therapy is a form of radiotherapy that enables concentration of dose on a tumor by use of a scanned or modulated Bragg peak. Therefore, it is very important to evaluate the proton-irradiated volume accurately. The proton-irradiated volume can be confirmed by detection of pair-annihilation gamma rays from positron-emitting nuclei generated by the nuclear fragmentation reaction of the incident protons on target nuclei using a PET apparatus. The activity of the positron-emitting nuclei generated in a patient was measured with a PET-CT apparatus after proton beam irradiation of the patient. Activity measurement was performed in patients with tumors of the brain, head and neck, liver, lungs, and sacrum. The 3-D PET image obtained on the CT image showed the visual correspondence with the irradiation area of the proton beam. Moreover, it was confirmed that there were differences in the strength of activity from the PET-CT images obtained at each irradiation site. The values of activity obtained from both measurement and calculation based on the reaction cross section were compared, and it was confirmed that the intensity and the distribution of the activity changed with the start time of the PET imaging after proton beam irradiation. The clinical use of this information about the positron-emitting nuclei will be important for promoting proton treatment with higher accuracy in the future.