CELLDOSE: A Monte Carlo code to assess electron dose distribution - S values for ₁₃₁I in spheres of various sizes

Monte Carlo simulation can be particularly suitable for modeling the microscopic distribution of energy received by normal tissues or cancer cells and for evaluating the relative merits of different radiopharmaceuticals. We used a new code, CELLDOSE, to assess electron dose for isolated spheres with radii varying from 2,500 μm down to 0.05 μm, in which ¹³¹I is homogeneously distributed. Methods: All electron emissions of ¹³¹I were considered, including the whole β2 ¹³¹I spectrum, 108 internal conversion electrons, and 21 Auger electrons. The Monte Carlo track-structure code used follows all electrons down to an energy threshold E_cutoff = 7.4 eV. Results: Calculated S values were in good agreement with published analytic methods, lying in between reported results for all experimental points. Our S values were also close to other published data using a Monte Carlo code. Contrary to the latter published results, our results show that dose distribution inside spheres is not homogeneous, with the dose at the outmost layer being approximately half that at the center. The fraction of electron energy retained within the spheres decreased with decreasing radius (r): 87.1% for r = 2,500 μm, 8.73% for r = 50 μm, and 1.18% for r = 5 μm. Thus, a radioiodine concentration that delivers a dose of 100 Gy to a micrometastasis of 2,500 μm radius would deliver 10 Gy in a cluster of 50 μm and only 1.4 Gy in an isolated cell. The specific contribution from Auger electrons varied from 0.25% for the largest sphere up to 76.8% for the smallest sphere. Conclusion: The dose to a tumor cell will depend on its position in a metastasis. For the treatment of very small metastases, ¹³¹I may not be the isotope of choice. When trying to kill isolated cells or a small cluster of cells with ¹³¹I, it is important to get the iodine as close as possible to the nucleus to get the enhancement factor from Auger electrons. The Monte Carlo code CELLDOSE can be used to assess the electron map deposit for any isotope.