Reactions induites par des protons de 155 MeV sur des noyaux legers

Production cross sections of light positron emitters ¹¹C (t_1/2=20.36 min), ¹³N (t_1/2=9.97 min) and ¹⁵O (t_1/2=122 s) have been measured for proton-induced reactions on the main relevant constituents of the human body (C, N and O) from threshold up to 200 MeV of proton incident energy. This has been accomplished by means of two complementary experiments at the clinical proton treatment centers WPE (Essen) and HIT (Heidelberg), both in Germany. In the first case, the multi-foil activation technique was combined with PET imaging, a methodology previously developed at CNA (Seville, Spain). In the second experiment, aimed at energies below 50 MeV, the activation of single foils was measured with conventional LaBr₃ detectors. In both cases the IAEA monitor reaction ^natCu(p,x)⁶³Zn data was used for validation purposes.

The most complete and accurate set of excitation functions for the production of light-positron emitters ¹¹C, ¹³N and ¹⁵O in proton induced reactions on human tissue up to 200 MeV is presented. These data are needed for nuclear reaction model development and are extremely important to improve the accuracy of the PET range verification in proton radiotherapy. Seven excitation functions have been measured: ¹²C(p,x)¹¹C, ¹⁴N(p,x)¹¹C, ¹⁴N(p,x)¹³N, ¹⁴N(p,γ)¹⁵O, ¹⁶O(p,x)¹¹C, ¹⁶O(p,x)¹³N and ¹⁶O(p,x)¹⁵O up to 200 MeV of proton incident energy.

An IAEA research project was dedicated to the compilation, evaluation and recommendation of cross-section data for the accelerator production of ¹¹C, ¹³N, ¹⁵O, ¹⁸F, ⁶⁴Cu, and ¹²⁴I positron-emitting radionuclides clinically used for PET imaging, and for the accelerator production of two gamma emitters, ⁸¹Rb and ¹²³I, used in SPECT imaging. Cross sections for 19 charged-particle induced reactions that can be employed for radionuclide accelerator production were evaluated including uncertainties. The resulting reference cross-section data were obtained from Padé fits to selected and corrected experimental data, and integral thick target yields were subsequently deduced. Uncertainties in the fitted results were estimated via a Padé least-squares method with the addition of a 4% assessed systematic uncertainty to the estimated experimental uncertainty. Experimental data were also compared with comprehensive predictions available from the TENDL library that reflects the current status of theoretical modelling. All of the numerical reference cross-section data with their corresponding uncertainties and deduced integral thick target yields are available on-line at the IAEA-NDS medical portal www-nds.iaea.org/medportal/ and also at the IAEA-NDS web page www-nds.iaea.org/medical/.

In this work, we present measurements of nuclear activation cross sections from carbon. Carbon can be used for proton fluence calculation in proton therapy by measurements of the activity after irradiation. The cross sections are measured on a high-precision level with systematic uncertainties of approx. 2.8% for proton beam energies between 100 and 220 MeV. The targets are activated at the West German Proton Therapy Centre Essen. The determination of the activity is based on gamma-ray spectrometry measurements at the Dortmund Low Background Facility. For the ^natC(p,x)¹¹C-reaction the results of this experimental study are comparable with several measurements reported in the literature. For the ^natC(p,x)⁷Be-reaction, the results calculated by several groups are reproduced by these measurements up to 180 MeV. Above 180 MeV, the observed energy dependence is weaker than the one measured by other groups. The reproducibility of the experimental setup is demonstrated with a measurement of the cross sections from aluminum.

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 ${}^{12}\mathrm{C}(p,pn){}^{11}\mathrm{C}$ and ${}^{12}\mathrm{C}(p,p2n){}^{10}\mathrm{C}$ 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 ${}^{12}\mathrm{C}(p,pn){}^{11}\mathrm{C}$ is in good agreement with existing experimental data. The cross section of ${}^{12}\mathrm{C}(p,p2n){}^{10}\mathrm{C}$ is reported for the first data in the low-energy range of 67.6–10.5 MeV near the Bragg peak of proton beam.

In proton therapy, positron emitters are induced from ¹²C and ¹⁶O nuclei by protons on the beam path in the patient. Many studies for monitoring positron emitters with beam-induced PET technique have been performed by various groups to verify the proton beam range and the dose in the patient for quality assurance (QA). The QA methods proposed by some groups require accurate production cross sections of the positron emitters produced by protons, especially in the low-energy region. The aim of this study was to develop a method for measuring the production cross sections of positron emitters using standard equipment for proton therapy, and to measure the cross sections of positron emitters produced by low-energy protons and verify them in comparison with data of previous experiments. An 80-MeV proton beam was produced by a synchrotron, and the energy was degraded by polyethylene blocks to obtain various beam energies. The number of protons was estimated from the charge induced in a parallel-plate ionization chamber by protons. Low-energy protons of 14–70 MeV were used to bombard ¹²C-rich and ¹⁶O-rich target materials: namely, polyethylene and gelatinous water. The time-activity curve was then measured by a high-sensitivity PET scanner to extract the number of positron emitters produced in the target. The production cross sections for four reaction channels: ¹⁶O(p, pn)¹⁵O, ¹⁶O(p, 3p3n)¹¹C, ¹⁶O(p, 2p2n)¹³N, and ¹²C(p, pn)¹¹C were then measured. The cross sections for the ¹⁶O(p, pn)¹⁵O reaction channel were consistent with data of previous experiments within the uncertainties, while those of ¹²C(p, pn)¹¹C were generally lower than data of previous experiments. These results suggested that further measurements of the production cross sections will be necessary.

The total cross-section for emission of ⁷Be particles in proton induced reactions has been parameterized for the proton beam energy from the threshold up to ∼20 GeV and for target mass from A = 12 to 238. The phenomenological parameterization allows to predict the cross-sections of ⁷Be emission with accuracy comparable to that achieved up to now in the experiments. Such an accuracy enables one to use the parameterization for reliable extrapolation of available measured values to targets and energies that are not easily accessible to experiments. Thus it allows to apply the (p,⁷Be) cross-sections as benchmarks for theoretical models of the reaction mechanism as well as monitor or reference cross-sections for investigation of other proton induced reactions.