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Depth absorbed dose and LET distributions of therapeutic H 1 , He 4 , Li 7 , and C 12 beams
Author(s) -
Kempe Johanna,
Gudowska Irena,
Brahme Anders
Publication year - 2007
Publication title -
medical physics
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.473
H-Index - 180
eISSN - 2473-4209
pISSN - 0094-2405
DOI - 10.1118/1.2400621
Subject(s) - bragg peak , absorbed dose , ion , linear energy transfer , dosimetry , atomic physics , monte carlo method , range (aeronautics) , proton , percentage depth dose curve , physics , photon , stopping power , scattering , irradiation , ionization chamber , nuclear physics , materials science , ionization , nuclear medicine , optics , mathematics , medicine , statistics , quantum mechanics , composite material
The depth absorbed dose and LET (linear energy transfer) distribution of different ions of clinical interest such as H1 , He4 , Li7 , and C12 ions have been investigated using the Monte Carlo code SHIELD‐HIT . The energies of the projectiles correspond to ranges in water and soft tissue of approximately 260 mm . The depth dose distributions of the primary particles and their secondaries have been calculated and separated with regard to their low and high LET components. A LET value below 10 eV ∕ nm can generally be regarded as low LET and sparsely ionizing like electrons and photons. The high LET region may be assumed to start at 20 eV ∕ nm where on average two double‐strand breaks can be formed when crossing the periphery of a nucleosome, even though strictly speaking the LET limits are not sharp and ought to vary with the charge and mass of the ion. At the Bragg peak of a monoenergetic high energy proton beam, less than 3% of the total absorbed dose is comprised of high LET components above 20 eV ∕ nm . The high LET contribution to the total absorbed dose in the Bragg peak is significantly larger with increasing ion charge as a natural result of higher stopping power and lower range straggling. The fact that the range straggling and multiple scattering are reduced by half from hydrogen to helium increases the possibility to accurately deposit only the high LET component in the tumor with negligible dose to organs at risk. Therefore, the lateral penumbra is significantly improved and the higher dose gradients of Li7 and C12 ions both longitudinally and laterally will be of major advantage in biological optimized radiation therapy. With increasing charge of the ion, the high LET absorbed dose in the beam entrance and the plateau regions where healthy normal tissues are generally located is also increased. The dose distribution of the high LET components in the Li7 beam is only located around the Bragg peak, characterized by a Gaussian‐type distribution. Furthermore, the secondary particles produced by high energy Li7 ions in tissuelike media have mainly low LET character both in front of and beyond the Bragg peak.