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 1H, 4He, 7Li, and 12C 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 7Li and 12C 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 7Li beam is only located around the Bragg peak, characterized by a Gaussian-type distribution. Furthermore, the secondary particles produced by high energy 7Li ions in tissuelike media have mainly low LET character both in front of and beyond the Bragg peak.

Influence of nuclear interactions in polyethylene range compensators for carbon-ion radiotherapy

“…In a Monte Carlo study on a pristine 391-MeV carbon-ion beam in water, 33 boron nuclei 10+11 B showed notable dose contribution of about 1/4 of the tail-base dose, which would presumably be further moderated by range modulation. If we assume D B = 0.2 D tail +∆D B for the mid-SOBP boron-nucleus dose with variation ∆D B among the range modulations and ǫ B ≈ ǫ 1 ≈ 1.5 ǫ 2 for the boron-nucleus RBE, the dose ratio and the RBE ratio will vary relatively by…”

“…The Monte Carlo study also showed substantial dose from 10+11 C nuclei. 33 Because all carbon nuclides are equivalent in ionization, the neutron-number variation resulted from neutron removal will only distort the Bragg peak, which will also be moderated by range modulation, and thus may be reasonably excluded from consideration.…”

Lateral spread of dose distribution by therapeutic proton beams in liquid water

“…60 Therefore, the possibility of delivering large and uniform dose dis-61 tributions makes of ion beams a suitable technique for external 62 radiation therapy in which the preservation of organs proximal 63 to the target and the sparing of critical structures distal to the tar-64 get are crucial [4]. 65 In order to achieve high precision in hadron therapy treatment [9], PHITS [10], GEANT4 [11], FLUKA [12], KURBUC [13] or MCHIT 96 [14]. Monte Carlo calculations prove that proton dose distribution 97 not only depends on electromagnetic interactions with target elec-98 trons, but also on nuclear interactions, such as fragmentation reac-99 tions and multiple scattering interactions [14][15][16].…”

A fast numerical method for calculating the 3D proton dose profile in a single-ring wobbling spreading system

“…In this equation the dependence of energy loss per length (dE/dx) on z 2 /v 2 results in the sharp Bragg peak at the end of the particle range, where z and v are charge number and velocity of projectile, respectively. For protons, an increasing depth-dose profile, with small amounts of lateral scatter and lack of any tail in the Bragg curve beyond the peak [3,4] make it possible to deliver a conformal and localised dose to a target volume with lower absorbed doses to the critical organs close to the tumour. Therefore, the therapeutic use of protons is preferable to conventional photon and electron radiotherapy, where a higher dose is delivered to healthy tissue during the treatment of deep-seated tumours.…”