Targeted alpha therapy with 212Pb or 225Ac: Change in RBE from daughter migration

Targeted α-therapy (TAT) could be delivered early to patients who are at a high-risk for developing brain metastases, targeting the areas of the vasculature where tumor cells are penetrating into the brain. We have utilized a Monte Carlo model representing brain vasculature to calculate physical dose and DNA damage from the α-emitters ²²⁵Ac and ²¹²Pb. The micron-scale dose distributions from all radioactive decay products were modeled in Geant4, including the eV-scale interactions using the Geant4-DNA models. These interactions were then superimposed on an atomic-scale DNA model to estimate strand break yields. In addition to ²²⁵Ac having a higher dose per decay than ²¹²Pb, it also has a double strand break yield per decay that is 4.7 ± 0.5 times that of ²¹²Pb. However, the efficacy of both nuclides depends on retaining the daughter nuclei at the target location in the brain vasculature. The relative biological effectiveness (RBE) of ²²⁵Ac and ²¹²Pb are similar when the entire decay chains are included, with maxima of 2.7 ± 0.6 and 2.5 ± 0.5 (respectively), and RBE values of about 2 to a depth of 80 μm. If the initial daughter is lost, the RBE of ²¹²Pb is completely reduced to 1 or lower and the RBE of ²²⁵Ac is approximately 2 only for the first 40 μm.

Dosimetric evaluation of radionuclides for VCAM-1-targeted radionuclide therapy of early brain metastases

Brain metastases develop frequently in patients with breast cancer, and present a pressing therapeutic challenge. Expression of vascular cell adhesion molecule 1 (VCAM-1) is upregulated on brain endothelial cells during the early stages of metastasis and provides a target for the detection and treatment of early brain metastases. The aim of this study was to use a model of early brain metastasis to evaluate the efficacy of α-emitting radionuclides, 149Tb, 211At, 212Pb, 213Bi and 225Ac; β-emitting radionuclides, 90Y, 161Tb and 177Lu; and Auger electron (AE)-emitters 67Ga, 89Zr, 111In and 124I, for targeted radionuclide therapy (TRT).

METHODS

Histologic sections and two photon microscopy of mouse brain parenchyma were used to inform a cylindrical vessel geometry using the Geant4 general purpose Monte Carlo (MC) toolkit with the Geant4-DNA low energy physics models. Energy deposition was evaluated as a radial function and the resulting phase spaces were superimposed on a DNA model to estimate double-strand break (DSB) yields for representative β- and α-emitters, 177Lu and 212Pb. Relative biological effectiveness (RBE) values were determined by only evaluating DNA damage due to physical interactions.

RESULTS

177Lu produced 2.69 ± 0.08 DSB per GbpGy, without significant variation from the lumen of the vessel to a radius of 100 µm. The DSB yield of 212Pb included two local maxima produced by the 6.1 MeV and 8.8 MeV α-emissions from decay products, 212Bi and 212Po, with yields of 7.64 ± 0.12 and 9.15 ± 0.24 per GbpGy, respectively. Given its higher DSB yield 212Pb may be more effective for short range targeting of early micrometastatic lesions than 177Lu.

CONCLUSION

MC simulation of a model of early brain metastases provides invaluable insight into the potential efficacy of α-, β- and AE-emitting radionuclides for TRT. 212Pb, which has the attributes of a theranostic radionuclide since it can be used for SPECT imaging, showed a favorable dose profile and RBE.

Monte Carlo simulations support non-Cerenkov radioluminescence production in tissue

There is experimental evidence for the production of non-Cerenkov radioluminescence in a variety of materials, including tissue. We constructed a Geant4 Monte Carlo simulation of the radiation from P32 and Tc99m interacting in chicken breast and used experimental imaging data to model a scintillation-like emission. The same radioluminescence spectrum is visible from both isotopes and cannot otherwise be explained through fluorescence or filter miscalibration. We conclude that chicken breast has a near-infrared scintillation-like response with a light yield three orders of magnitude smaller than BGO.

Cherenkov light production from the α-emitting decay chains of 223Ra, 212Pb, and 149Tb for Cherenkov Luminescence Imaging

Cherenkov Luminescence Imaging (CLI) is a new method to image radioactive therapeutic and diagnostic agents, primarily in preclinical studies. This study used Geant4 and Python to generate the predicted Cherenkov light production as a function of time for a set of isotopic chains of interest for targeted alpha therapy: ²²³Ra, ²¹²Pb, and ¹⁴⁹Tb. All are shown to produce substantial Cherenkov light, though time delays between initial decays and the production of Cherenkov light requires caution in interpreting CLI.

The potential for Cerenkov luminescence imaging of alpha-emitting radionuclides

Targeted α-emitting drugs are promising for cancer therapy, but cannot be effectively imaged by conventional techniques. Cerenkov luminescence imaging (CLI) has previously been shown capable of imaging β(+)- and β(-)-emitting radionuclides in vivo and could have the potential to image α-emitters. Cerenkov light production from α-emitters is through Compton scattering and from farther down the decay chain. This causes the Cerenkov production to vary in time and depend on sample geometry, complicating the interpretation of CLI images. We used the simulation toolkit Geant4 to predict the Cerenkov light output from five α-emitting radionuclides that have therapeutic potential: (225)Ac, (230)U, (213)Bi, (212)Bi and (212)At. We found that (225)Ac, (213)Bi and (212)Bi produced an order of magnitude more Cerenkov light than (18)F. However, the light from (225)Ac is delayed from the initial decay, possibly decreasing the correlation of the drug and light source. This indicates that CLI will not be helpful in the development of some α-emitting drugs.