The low‐LET radiation contribution to the tumor dose in diffusing alpha‐emitters radiation therapy

BackgroundDiffusing alpha‐emitters Radiation Therapy (“Alpha DaRT”) is a new technique that enables the use of alpha particles for the treatment of solid tumors. Alpha DaRT employs interstitial sources carrying a few μ$\mu$Ci of 224$^{224}$Ra below their surface, designed to release a chain of short‐lived atoms (progeny of 224$^{224}$Ra) which emit alpha particles, along with beta, Auger, and conversion electrons, x‐ and gamma rays. These atoms diffuse around the source and create—primarily through their alpha decays—a lethal high‐dose region measuring a few millimeters in diameter. PurposeWhile previous studies focused on the dose from the alpha emissions alone, this work addresses the electron and photon dose contributed by the diffusing atoms and by the atoms remaining on the source surface, for both a single Alpha DaRT source and multi‐source lattices. This allows to evaluate the low‐LET contribution to the tumor dose and tumor cell survival, and demonstrate the sparing of surrounding healthy tissue. MethodsThe low‐LET dose is calculated using the EGSnrc and FLUKA Monte Carlo (MC) codes. We compare the results of a simple line‐source approximation with no diffusion to those of a full simulation, which implements a realistic source geometry and the spread of diffusing atoms. We consider two opposite scenarios: one with low diffusion and high 212$^{212}$Pb leakage, and the other with high diffusion and low leakage. The low‐LET dose in source lattices is calculated by superposition of single‐source contributions. Its effect on cell survival is estimated with the linear quadratic model in the limit of low dose rate. ResultsFor sources carrying 3 μ$\umu$Ci/cm 224$^{224}$Ra arranged in a hexagonal lattice with 4 mm spacing, the minimal low‐LET dose between sources is ∼18−30$\sim 18-30$ Gy for the two test cases and is dominated by the beta contribution. The low‐LET dose drops below 5 Gy ∼3$\sim 3$ mm away from the outermost source in the lattice with an effective maximal dose rate of