CELLDOSE: a Monte Carlo code for electron-tracking in biological matter. Applications in targeted radiotherapy

C. CHAMPION; P. ZANOTTI-FREGONARA; M.A. QUINTO; C. MORGAT; E. HIDIE

Moscow

Conferencia; The International Conference on Many Particle Spectroscopy of Atoms, Molecules, Clusters and Surfaces MPS-2016; 2016

Radionuclide therapy is increasingly seen as a promising option to target micro-metastases or minimal residual disease.Copper-67, scandium-47 and terbium-161 have a medium-energy β - emission which is similar to that of lutetium-177. They offer the advantage ofhaving diagnostic partner isotopes suitable for pretreatment imaging. The aim of this study was to compare the effectiveness of these isotopes atirradiating small tumour volumes. Electron dose from uniform isotopedistributions was assessed in spheres of various sizes (from 5 mm down to 10 μm diameter) by using the Monte Carlo code CELLDOSE previously developed by Champion et al [1] for studying the electron dose distribution of 131 I. The latter consists in the modeling of fullslowing-down histories of electrons whose incident energies are taken from the corresponding isotope spectrum and where all the interactions are described by a large set of quantum mechanical cross sections referring to the various electron-induced interactions inbiological matter. As 177 Lu, 67 Cu, 47 Sc and 161 Tb differ inelectron energy per decay, doses were compared assuming 1 MeV released per μm 3 , which would result in 160 Gy if totally absorbed.In a 5 mm sphere, the four isotopes yielded similardose deposits per MeV/μm 3 (145-149 Gy). The absorbed doses decreasedwith decreasing sphere size, thus underscoring the difficulty of irradiating micro-metastases. 161 Tb, however, delivered a higher dosecompared to the other isotopes. For instance, in a 100-μm metastasis, dose deposits were 24.5Gy with 177 Lu, 24.1Gy with 67 Cu, 14.8Gy with 47 Sc and 44.5Gy with 161 Tb. Auger and conversion electrons accounted for 71% of 161 Tb dose in this 100-μm metastasis. The differences between radionuclides further increased in cell-sized spheres. In a 10-μm cell,the dose delivered by 161 Tb was 3.6 times higher than that from 177 Lu, 4.1 times that from 67 Cu and 8.1 times that from 47 Sc.Terbium-161 can effectively target micro-metastases and single tumour cells thanks to its decay spectrum that combines medium-energy β - emission and low-energy conversion and Auger electrons. These results are in agreement with some recent studies on cell cultures and tumor xenografts [2]