Abstract
To elucidate microscopic details of proton cancer therapy (PCT), we apply the simplest-level electron nuclear dynamics (SLEND) method to H+ + (H2O)1-6 at ELab = 100 keV. These systems are computationally tractable prototypes to simulate water radiolysis reactions—i.e. the PCT processes that generate the DNA-damaging species against cancerous cells. To capture incipient bulk-water effects, ten (H2O)1-6 isomers are considered, ranging from quasi-planar/multiplanar (H2O)1-6 to “smallest-drop” prism and cage (H2O)6 structures. SLEND is a time-dependent, variational, non-adiabatic and direct method that adopts a nuclear classical-mechanics description and an electronic single-determinantal wavefunction in the Thouless representation. Short-time SLEND/6-31G* (n = 1–6) and /6-31G** (n = 1–5) simulations render cluster-to-projectile 1-electron-transfer (1-ET) total integral cross sections (ICSs) and 1-ET probabilities. In absolute quantitative terms, SLEND/6-31G* 1-ET ICS compares satisfactorily with alternative experimental and theoretical results only available for n = 1 and exhibits almost the same accuracy of the best alternative theoretical result. SLEND/6-31G** overestimates 1-ET ICS for n = 1, but a comparable overestimation is also observed with another theoretical method. An investigation on H+ + H indicates that electron direct ionization (DI) becomes significant with the large virtual-space quasi-continuum in large basis sets; thus, SLEND/6-31G** 1-ET ICS is overestimated by DI contributions. The solution to this problem is discussed. In relative quantitative terms, both SLEND/6-31* and /6-31G** 1-ET ICSs precisely fit into physically justified scaling formulae as a function of the cluster size; this indicates SLEND’s suitability for predicting properties of water clusters with varying size. Long-time SLEND/6-31G* (n = 1–4) simulations predict the formation of the DNA-damaging radicals H, OH, O and H3O. While “smallest-drop” isomers are included, no early manifestations of bulk water PCT properties are observed and simulations with larger water clusters will be needed to capture those effects. This study is the largest SLEND investigation on water radiolysis to date.
Highlights
Introduction and backgroundProton cancer therapy (PCT) is an approved type of radiotherapy that utilizes high-energy H+ projectiles to fight cancer [1,2,3,4,5,6,7]
The first property calculated in this investigation is the cluster-to-proton total 1-electron transfers (ETs) integral cross sections (ICSs), σ1−ET, for the H+ + (H2O)n systems, n = 1–6, at ELab = 100 keV at the simplest-level electron nuclear dynamics (SLEND)/6-31GÃ (n = 1–6) and SLEND/6-31GÃÃ (n = 1–5) levels
To model microscopic processes in proton cancer therapy (PCT) [1,2,3,4,5,6,7], the SLEND method was applied to the H+ + (H2O)n systems at ELab = 100 keV with the 6-31GÃ (n = 1–6) and 6-31GÃÃ (n = 1–5) basis sets
Summary
Introduction and backgroundProton cancer therapy (PCT) is an approved type of radiotherapy that utilizes high-energy H+ projectiles to fight cancer [1,2,3,4,5,6,7]. The ultimate effect of this radiation is to damage the DNA of cancerous cells [1,2,3,4,5,6,7]. PCT radiation is applied as collimated beams of H+ projectiles at an initial kinetic energy of 70–250 MeV [1,2,3,4,5,6,7]. As they penetrate the patient’s body, these projectiles lose their energy through molecular interactions until they reach thermal energy in deep tissues. By focusing Bragg peaks on a deep tumor, PCT inflicts a maximum DNA damage on that region and a minimum DNA damage on the surrounding healthy tissues [1,2,3,4,5,6,7]
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