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Positron range (PR) is one of the important factors that limit the spatial resolution of positron emission tomography (PET) preclinical images. Its blurring effect can be corrected to a large extent if the appropriate method is used during the image reconstruction. Nevertheless, this correction requires an accurate modelling of the PR for the particular radionuclide and materials in the sample under study. In this work we investigate PET imaging with 68Ga and 66Ga radioisotopes, which have a large PR and are being used in many preclinical and clinical PET studies. We produced a 68Ga and 66Ga phantom on a natural zinc target through (p,n) reactions using the 9-MeV proton beam delivered by the 5-MV CMAM tandetron accelerator. The phantom was imaged in an ARGUS small animal PET/CT scanner and reconstructed with a fully 3D iterative algorithm, with and without PR corrections. The reconstructed images at different time frames show significant improvement in spatial resolution when the appropriate PR is applied for each frame, by taking into account the relative amount of each isotope in the sample. With these results we validate our previously proposed PR correction method for isotopes with large PR. Additionally, we explore the feasibility of PET imaging with 68Ga and 66Ga radioisotopes in proton therapy. We acknowledge support from the Spanish MINECO through projects FPA2010-17142, FPA2013-41267-P, CSD-2007-00042 (CPAN), and the RTC-2015-3772-1 grant. We also acknowledge support from Comunidad de Madrid via the TOPUS S2013/MIT-3024 project.

Technical advances towards high resolution PET imaging try to overcome the inherent physical limitations to spatial resolution. Positrons travel in tissue until they annihilate into the two gamma photons detected. This range is the main detector-independent contribution to PET imaging blurring. To a large extent, it can be remedied during image reconstruction if accurate estimates of positron range are available. However, the existing estimates differ, and the comparison with the scarce experimental data available is not conclusive. In this work we present positron annihilation distributions obtained from Monte Carlo simulations with the PeneloPET simulation toolkit, for several common PET isotopes (18F, 11C, 13N, 15O, 68Ga and 82Rb) in different biological media (cortical bone, soft bone, skin, muscle striated, brain, water, adipose tissue and lung). We compare PeneloPET simulations against experimental data and other simulation results available in the literature. To this end the different positron range representations employed in the literature are related to each other by means of a new parameterization for positron range profiles. Our results are generally consistent with experiments and with most simulations previously reported with differences of less than 20% in the mean and maximum range values. From these results, we conclude that better experimental measurements are needed, especially to disentangle the effect of positronium formation in positron range. Finally, with the aid of PeneloPET, we confirm that scaling approaches can be used to obtain universal, material and isotope independent, positron range profiles, which would considerably simplify range correction. We kindly acknowledge support from Comunidad de Madrid (ARTEMIS S2009/DPI-1802), Spanish Ministry of Science and Innovation (grants FPA2010-17142 and ENTEPRASE, PSE-300000-2009-5), by European Regional Funds, by CDTI under the CENIT Programme (AMITProject) and by CPAN, CSPD-2007-00042@Ingenio2010. Part of the calculations of this work were performed in the “Cluster de Cálculo de Alta Capacidad para Técnicas Físicas” funded in part by UCM and in part by UE under FEDER programme. This is a contribution to the Campus of International Excellence of Moncloa Publicado

The positron range, that is, the distance from the positron emission point to the annihilation one in β+ decay, limits the spatial resolution of PET images. Thus, any realistic simulation of PET acquisitions must include this effect as accurately as possible. Positron range distributions have been computed and compared in the literature. Not all authors present it in the same way. For instance, accumulated range distributions, 1D projection of the positrons annihilation coordinates, or angular integrated 3D radial distributions, have been used. Any of these presentations may emphasize a different physical property. Thus it is difficult to perform a meaningful comparison of the results obtained by different authors. The aim of this work is to present a general framework to compare positron range distributions. We use a genetic algorithm to fit the radial, angle integrated, g3D(r) distribution of the annihilation points to any positron annihilation distribution chosen as reference. Therefore, given a positron range distribution presented in a certain way, we can obtain the distributions, first fitting g3D and then using the relations between distributions. Using this procedure, positron range distributions available in the literature are compared with the ones computed with the simulation package PeneloPET

Monte Carlo (MC) methods are a very powerful tool to compute dose in radiotherapy, as all effects that need to be considered, such as material inhomogeneities, back-scatter from large bones, and beam hardening can be properly modeled. Their most important caveat, however, besides computation time, is that they need a realistic and reliable description of the electron and/or photon beam that delivers the dose, and this is not usually available. In this respect, Monte Carlo (MC) methods have been an invaluable tool for realistic modeling of medical therapy accelerators, including electron linear accelerators (LINAC) used in Intra-operative Radiation Therapy (IORT). The purpose of this work is to obtain the radiation beam properties (or phase-space fro the beam or PHSP) at phantom surface based on a set of dose measurements, without the need for a detailed simulation of the accelerator head and/or applicator. An iterative reconstruction algorithm (EM-ML), commonly used in tomographic image reconstruction, has been employed to optimize iteratively all aspects of the PHSP, such as energy spectra, particle type and fluency, and angle of particle emission, required by the MC dose calculation code Dose Planning Method (DPM). Phase space files for IORT have been derived for different energies and applicator diameters, which yield dose in good agreement with the measurements.

Proceeding of: 2011 Nuclear Science Symposium and Medical Imaging Conference, Valencia, España, 23-29 October, 2011 In this work, we investigate PET imaging with 68Ga and 66Ga after proton irradiation on a natural zinc foil. The nuclides 68Ga and 66Ga are ideally suited for off line PET monitoring of proton radiotherapy due to their beta decay halflives of 67.71(9) minutes and 9.49(3) hours, respectively, and suitable fl end point energy. The purpose of this work is to explore the feasibility of PET monitoring in hadrontherapy treatments, and to study how the amount of activity and the positron range affect the PET image reconstruction. Profiting from the low energy reaction threshold for production via (p,n) reactions, both 68Ga and 66Ga gallium isotopes have been produced by activation on a natural zinc target by a proton pencil beam. In this way, it is possible to create detailed patterns, such as the Derenzo inspired one employed here. The proton beam was produced by the 5 MV tandetron accelerator at CMAM in Madrid. The energy of this beam (up to 10 MeV) is similar to the residual energy of the protons used for therapy at the distal edge of their path. The activated target was imaged in an ARGUS small animal PETtCT scanner and reconstructed with a fully 3D iterative algorithm, with and without positron range corrections. This work was supported in part by Comunidad de Madrid (ARTEMIS S2009/DPI 1802), Spanish Ministry of Science and Innovation (grants FPA2010 17142 and ENTEPRASE, PSE 300000 2009 5), by European Regional Funds, by CDTI under the CENIT Programme (AMIT Project), UCM (grupos UCM, 910059) and by CPAN, CSPD 2007 00042@lngenio2010. Publicado