Depth absorbed dose and LET distributions of therapeutic , , , and beams

Abstract

The depth absorbed dose and LET (linear energy transfer) distribution of different ions of clinical interest such as , , , and 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 . 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 can generally be regarded as low LET and sparsely ionizing like electrons and photons. The high LET region may be assumed to start at 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 . 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 and 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 beam is only located around the Bragg peak, characterized by a Gaussian‐type distribution. Furthermore, the secondary particles produced by high energy ions in tissuelike media have mainly low LET character both in front of and beyond the Bragg peak.