Monte Carlo track-structure for the radionuclide Copper-64: characterization of S-values, nanodosimetry and quantification of direct damage to DNA

TOPAS-nBio was used to simulate, collision-to-collision, the complete trajectories of electrons in water generated during the explicit simulation of ⁶⁴Cu decay. S-values and direct damage to the DNA were calculated representing the cell (C) and the cell nucleus (N) with concentric spheres of 5 μm and 4 μm in radius, respectively. The considered 'target'←'source' configurations, including the cell surface (Cs) and cytoplasm (Cy), were: C←C, C←Cs, N←N, N←Cy and N←Cs. Ionization cluster size distributions were also calculated in a cylinder immersed in water corresponding to a DNA segment of 10 base-pairs in length (diameter 2.3 nm, length 3.4 nm), modeling a radioactive point source moving from the central axis to the edge of the cylinder. For that, the first moment (M₁) and cumulative probability of having a cluster size of 2 or more ionizations in the cylindrical volume (F₂) were obtained. Finally, the direct damage to the DNA was estimated by quantifying double-strand breaks (DSBs) using the clustering algorithm DBSCAN. The S-values obtained with TOPAS-nBio for ⁶⁴Cu were 7.879 × 10^-4 ± 5 × 10^-7, 4.351 × 10^-4 ± 6 × 10^-7, 1.442 × 10^-3 ± 1 × 10^-6, 2.596 × 10^-4 ± 8 × 10^-7, 1.127 × 10^-4 ± 4 × 10^-7 Gy Bq-s^-1 for the configurations C←C, C←Cs, N←N, N←Cy and N←Cs, respectively. The difference of these values, compared with previously reported S-values for ⁶⁴Cu with the code MNCP and software MIRDCell, ranged from -4% to -25% for the configurations N←N and N←Cs, respectively. On the other hand, F₂ was maximum with the source at the center of the cylinder 0.373 ± 0.001, and monotonically decreased until reaching a value of 0.058 ± 0.001 at 2.3 nm. The same behavior was observed for M₁ with values ranging from 2.188 ± 0.004 to 0.242 ± 0.002. Finally, the DBSCAN algorithm showed that the mean number of DNA DSBs per decay were 0.187 ± 0.001, 0.0317 ± 0.0005, and 0.0125 ± 0.0002 DSB-(Bq-s)^-1 for the configurations N←N, N←Cs, and N←Cy, respectively. In conclusion, the results of the S-values show that the absorbed dose strongly depends on the distribution of the radionuclide in the cell, the dose being higher when ⁶⁴Cu is internalized in the cell nucleus, which is reinforced by the nanodosimetric study by the presence of DNA DSBs attributable to the Auger electrons emitted during the decay of ⁶⁴Cu.

Positron range effects of 66Ga in small-animal PET imaging

PURPOSE

Gallium-66 is a non-conventional positron emitter that stands out not only for its high potential to label peptides, proteins and antibodies, but also because it can provide spatio-temporal information of relatively slow physiological processes in the body due to its conveniently long half-life of 9.5 h. However, ⁶⁶Ga emits the most energetic positrons for PET imaging. The lack of information of the positron range effect on spatial resolution for this positron emitter is an issue, particularly in preclinical imaging.

METHODS

The line spread function (LSF) in tissue-equivalent materials with densities between 0.2 and 1.93 g/cm³ was obtained with ⁶⁶Ga and ¹⁸F. A complementary study with the NEMA NU 4-2008 image quality phantom is also included.

RESULTS

High-energy positrons moving in lower density materials produce far-reaching activity distributions. The LSFs were characterized with Lorentzian-Gaussian fits, with spatial resolution (FWHM) in the 2.14-3.2 mm range, and long tails extending a few tens of mm depending on the material type and density. A narrowing of the LSF was observed for lung-equivalent materials, indicating the lack of enough material for the positron annihilation to take place. The NEMA NU 4-2008 image quality phantom produced blurred images, notoriously observed in the hot and cold cylinders used for evaluation of recovery coefficients (RC) and spill-over ratios (SOR), producing very low RC and very large SOR.

CONCLUSIONS

Quantitative PET imaging with the non-conventional ⁶⁶Ga is hampered due to the large range of its high-energy positrons affecting both spatial resolution and activity concentration quantification.

Transplantation of Human Neural Progenitor Cells (NPC) into Putamina of Parkinsonian Patients: A Case Series Study, Safety and Efficacy Four Years after Surgery

Individuals with Parkinson’s disease (PD) suffer from motor and mental disturbances due to degeneration of dopaminergic and non-dopaminergic neuronal systems. Although they provide temporary symptom relief, current treatments fail to control motor and non-motor alterations or to arrest disease progression. Aiming to explore safety and possible motor and neuropsychological benefits of a novel strategy to improve the PD condition, a case series study was designed for brain grafting of human neural progenitor cells (NPCs) to a group of eight patients with moderate PD. A NPC line, expressing Oct-4 and Sox-2, was manufactured and characterized. Using stereotactic surgery, NPC suspensions were bilaterally injected into patients’ dorsal putamina. Cyclosporine A was given for 10 days prior to surgery and continued for 1 month thereafter. Neurological, neuropsychological, and brain imaging evaluations were performed pre-operatively, 1, 2, and 4 years post-surgery. Seven of eight patients have completed 4-year follow-up. The procedure proved to be safe, with no immune responses against the transplant, and no adverse effects. One year after cell grafting, all but one of the seven patients completing the study showed various degrees of motor improvement, and five of them showed better response to medication. PET imaging showed a trend toward enhanced midbrain dopaminergic activity. By their 4-year evaluation, improvements somewhat decreased but remained better than at baseline. Neuropsychological changes were minor, if at all. The intervention appears to be safe. At 4 years post-transplantation we report that undifferentiated NPCs can be delivered safely by stereotaxis to both putamina of patients with PD without causing adverse effects. In 6/7 patients in OFF condition improvement in UPDRS III was observed. PET functional scans suggest enhanced putaminal dopaminergic neurotransmission that could correlate with improved motor function, and better response to L-DOPA. Patients’ neuropsychological scores were unaffected by grafting. Trial Registration: Fetal derived stem cells for Parkinson’s disease https://doi.org/10.1186/ISRCTN39104513Reg#ISRCTN39104513

Positron range in tissue-equivalent materials: experimental microPET studies

In this work an experimental investigation was carried out to study the effect that positron range has over positron emission tomography (PET) scans through measurements of the line spread function (LSF) in tissue-equivalent materials. Line-sources consisted of thin capillary tubes filled with (18)F, (13)N or (68)Ga water-solution inserted along the axis of symmetry of cylindrical phantoms constructed with the tissue-equivalent materials: lung (inhale and exhale), adipose tissue, solid water, trabecular and cortical bone. PET scans were performed with a commercial small-animal PET scanner and image reconstruction was carried out with filtered-backprojection. Line-source distributions were analyzed using radial profiles taken on axial slices from which the spatial resolution was determined through the full-width at half-maximum, tenth-maximum, twentieth-maximum and fiftieth-maximum. A double-Gaussian model of the LSFs was used to fit experimental data which can be incorporated into iterative reconstruction methods. In addition, the maximum activity concentration in the line-sources was determined from reconstructed images and compared to the known values for each case. The experimental data indicates that positron range in different materials has a strong effect on both spatial resolution and activity concentration quantification in PET scans. Consequently, extra care should be taken when computing standard-uptake values in PET scans, in particular when the radiopharmaceutical is taken up by different tissues in the body, and more even so with high-energy positron emitters.