Monte Carlo-Based Radiobiological Investigation of the Most Optimal Ion Beam Forming SOBP for Particle Therapy.

Proton (p) and carbon (C) ion beams are in clinical use for cancer treatment, although other particles such as He, Be, and B ions have more recently gained attention. Identification of the most optimal ion beam for radiotherapy is a challenging task involving, among others, radiobiological characterization of a beam, which is depth-, energy-, and cell type- dependent. This study uses the FLUKA and MCDS Monte Carlo codes in order to estimate the relative biological effectiveness (RBE) for several ions of potential clinical interest such as p, 4He, 7Li, 10Be, 10B, and 12C forming a spread-out Bragg peak (SOBP). More specifically, an energy spectrum of the projectiles corresponding to a 5-cm SOBP at a depth of 8 cm was used. All secondary particles produced by the projectiles were considered and RBE was determined based on radiation-induced Double Strand Breaks (DSBs), as calculated by MCDS. In an attempt to identify the most optimal ion beam, using the latter data, biological optimization was performed and the obtained depth-dose distributions were inter-compared. The results showed that 12C ions are more effective inside the SOBP region, which comes at the expense of higher dose values at the tail (i.e., after the SOBP). In contrast, p beams exhibit a higher DSOPB/DEntrance ratio, if physical doses are considered. By performing a biological optimization in order to obtain a homogeneous biological dose (i.e., dose × RBE) in the SOBP, the corresponding advantages of p and 12C ions are moderated. 7Li ions conveniently combine a considerably lower dose tail and a DSOPB/DEntrance ratio similar to 12C. This work contributes towards identification of the most optimal ion beam for cancer therapy. The overall results of this work suggest that 7Li ions are of potential interest, although more studies are needed to demonstrate the relevant advantages. Future work will focus on studying more complex beam configurations.

Theoretical and experimental determination of scaling factors in electron dosimetry for 3D-printed polylactic acid.

PURPOSE: Plastic phantoms are commonly used in daily routine for dosimetric tasks in radiation therapy. Although water is the reference medium according to the dosimetric protocols, measurements with nonwater phantoms are easier to be performed. To succeed absorbed dose determination, certain scaling factors have to be applied to the acquired measurements. Taking into account the increased availability of three-dimensional (3D) printing, we attempted to obtain scaling factors for polylactic acid (PLA), a commonly used thermoplastic material for 3D printing. METHODS: Measurements were performed with a custom-made phantom from PLA material, which was designed and constructed using 3D printing technology. Depth and fluence scaling factors were obtained within the range of 6 to 20 MeV. Moreover, Monte Carlo simulations were performed to verify the measured results. RESULTS: Experimental and Monte Carlo (MC) values showed a good agreement, especially in lower energies. Mean value of depth scaling factor (cpl ) over the whole range of energies was 0.946, while mean fluence scaling factor (hpl ) was found to be 1.050. For energies below 10 MeV, the corresponding mean values for cpl and hpl were 0.946 and 1.054, respectively. CONCLUSIONS: PLA phantoms could be constructed and used for electron beam nonreference measurements, reproducing even more complex geometries, from simple quality assurance devices to geometrically complicated anthropomorphic phantoms.

Stopping power for particle therapy: the generic library libdEdx and clinically relevant stopping-power ratios for light ions.

PURPOSE: Stopping-power data enter at a number of different places in particle therapy and their uncertainties have a direct impact on the accuracy of the therapy, e.g., in treatment planning. Furthermore, for clinical quality assurance, the particle beam stopping-power ratios (STPR) have to be known accurately for dosimetry. METHODS: An open-source computer library called libdEdx (library for energy loss per unit path length, dE/dx, calculations) is developed, providing stopping-power data from data tables and computer programs as well as a stopping-power formula comprising a large list of target materials. Calculations of STPR in the case of spread-out Bragg-peaks (SOBP) are performed with the Monte Carlo transportation code SHIELD-HIT (SHIELD-Heavy Ion Transport) using different ions relevant for particle therapy. RESULTS: For SOBP the water-to-air STPR depends on the residual range and is qualitatively very similar for different ions; however, small quantitative differences exist between the considered ion species. CONCLUSIONS: libdEdx allows for a convenient and efficient treatment of stopping powers in numerical applications. It can be applied to estimate the dependence on the accuracy of the stopping power and to provide data for an extended number of target materials. The STPR for SOBP for different ions are found to be qualitatively the same which may allow for an analytical description valid for all ions.

Comparison of optimized single and multifield irradiation plans of antiproton, proton and carbon ion beams.

BACKGROUND AND PURPOSE: Antiprotons have been suggested as a possibly superior modality for radiotherapy, due to the energy released when antiprotons annihilate, which enhances the Bragg peak and introduces a high-LET component to the dose. However, concerns are expressed about the inferior lateral dose distribution caused by the annihilation products. METHODS: We use the Monte Carlo code FLUKA to generate depth-dose kernels for protons, antiprotons, and carbon ions. Using these we then build virtual treatment plans optimized according to ICRU recommendations for the different beam modalities, which then are recalculated with FLUKA. Dose-volume histograms generated from these plans can be used to compare the different irradiations. RESULTS: The enhancement in physical and possibly biological dose from annihilating antiprotons can significantly lower the dose in the entrance channel; but only at the expense of a diffuse low dose background from long-range secondary particles. Lateral dose distributions are improved using active beam delivery methods, instead of flat fields. CONCLUSIONS: Dose-volume histograms for different treatment scenarios show that antiprotons have the potential to reduce the volume of normal tissue receiving medium to high dose, however, in the low dose region antiprotons are inferior to both protons and carbon ions. This limits the potential usage to situations where dose to normal tissue must be reduced as much as possible.