LET: ionizations in 1 μm of water - protons, carbon ions, X rays and alpha particles | Monte Carlo

The energy radiation gives per micrometre of path (LET) differs by about a thousand times between kinds of radiation, and for protons and carbon ions it rises sharply just before they stop. We followed every ionization made by one particle in 1 μm of water and calculated the LET of protons and carbon ions throughout a water tank (Monte Carlo simulation: a method that follows radiation one particle at a time). What you will see • LET (linear energy transfer) = the energy given per micrometre of path (keV/μm); heavier, slower particles ionize more densely and have a higher LET • Ionizations by one particle in 1 μm of water: electron 1 MeV 4, proton 150 MeV 17, carbon 290 MeV/u 291, proton 2 MeV 574, alpha 5 MeV 3259, carbon 10 MeV/u 5947 • LET differs by about a thousand times: X rays (the electrons they set in motion) 120 kV about 3.3, 6 MV about 0.4, electrons 12 MeV about 0.3, alpha 5 MeV about 90-229 keV/μm • Protons 150 MeV: about 0.9 at the entrance to 7.4 keV/μm at the peak; carbon 290 MeV/u: 14 to 207 keV/μm (a sharp rise just before stopping) • Even in a spread-out Bragg peak of stacked energies, the LET grows towards the far end • Radiation protection weights high-LET radiation more: the quality factor Q(L) (ICRP 60) is largest at about 100 keV/μm (beyond that, energy is wasted and it falls); radiation weighting factors 1 for photons and electrons, 2 for protons, 20 for alpha particles and heavy ions, 2.5-20 for neutrons (ICRP 103) Chapters 00:00 Different numbers of ionizations in 1 μm 00:08 Ionizations in 1 μm of water and LET 00:24 Heavier, slower: denser 00:46 LET by kind of radiation 01:02 LET of protons 01:20 LET of carbon ions 01:38 LET along the beam 01:54 LET across the SOBP 02:10 LET in radiation protection 02:28 What dense ionization does 02:44 Summary How it was calculated • An example calculated by Monte Carlo simulation, not a measurement. • The 1 μm pictures follow the particle and every electron it knocks out in a 1 μm cube of water (track structure); each knocked-out electron counts as an ionization • LET: dose-mean LET in water; beams from above into a 40 x 40 x 40 cm water tank (100 cm from the source, 10 x 10 cm equivalent field), within 3 cm of the axis, 1 mm steps • For protons: the LET of the protons (heavy nuclei from nuclear reactions excluded - rare recoils would raise the mean several-fold); colours in the tank show the LET by depth (grey below 2% of the maximum dose) • For X rays: the LET of the electrons they set in motion; alpha 5 MeV: LET in 1 μm steps in water • Spread-out Bragg peaks: 12 proton energies of 120-147.5 MeV and 12 carbon energies of 225-280 MeV/u stacked for 10.5-15 cm (examples; each carbon peak slightly broadened in depth like a therapy ripple filter) Why it matters LET is one measure of the 'quality' of radiation. Dense ionization tends to cluster damage near DNA, so the same dose can have a larger effect on cells (RBE); this underlies the radiation weighting factors of radiation protection and the planning of particle therapy. Subscribe for more The "See the invisible" series shows how radiation spreads, from X-ray imaging, CT and fluoroscopy to natural radiation, calculated the same way and shown in 3D.    / @labfujibuchi8974   Sources and affiliation Quality factor Q(L): ICRP Publication 60 (1990) Radiation weighting factors: ICRP Publication 103 (2007) Fujibuchi Laboratory, Kyushu University Simulation: Monte Carlo / 3D visualization: ParaView / Music: original #LET #Radiation #ParticleTherapy