Automatic phase space generation for Monte Carlo calculations of intensity modulated particle therapy.

Monte Carlo (MC) is generally considered as the most accurate dose calculation tool for particle therapy. However, a proper description of the beam particle kinematics is a necessary input for a realistic simulation. Such a description can be stored in phase space (PS) files for different beam energies. A PS file contains kinetic information such as energies, positions and travelling directions for particles traversing a plane perpendicular to the beam direction. The accuracy of PS files plays a critical role in the performance of the MC method for dose calculations. A PS file can be generated with a set of parameters describing analytically the beam kinematics. However, determining such parameters can be tedious and time consuming. Thus, we have developed an algorithm to obtain those parameters automatically and efficiently. In this paper, we presented such an algorithm and compared dose calculations using PS automatically generated for the Shanghai Proton and Heavy Ion Center (SPHIC) with measurements. The gamma-index for comparing calculated depth dose distributions (DDD) with measurements are above 96.0% with criterion 0.6%/0.6 mm. For each single energy, the mean difference percentage between calculated lateral spot sizes at 5 different locations along beam direction and measurements are below 3.5%.

A track repeating algorithm for intensity modulated carbon ion therapy.

The fast dose calculator (FDC), a track repeating Monte Carlo (MC) algorithm was initially developed for proton therapy. The validation for proton therapy has been demonstrated in a previous work. In this work we presented the extension of FDC to the calculation of dose distributions for ions, particularly for carbon. Moreover the code algorithm is validated by comparing 3D dose distributions and dose volume histograms (DVH) calculated by FDC with Geant4. A total of 19 patients were employed, including three patients of prostate, five of brain, three of head and neck, four of lung and four of spine. We used a gamma-index technique to analyze dose distributions and we performed a dosimetric analysis for DVHs, a more direct and informative quantity for planning system assessment. The gamma-index passing rates of all patients discussed in this paper are above 90% with the criterion 1%/1 mm, above 98% with the criterion 2%/2 mm and over 99.9% with the criterion 3%/3 mm. The root mean square (RMS) of percent difference of dosimetric indices D 02, D 05, D 50, D 95 and D 98 are 0.75%, 0.70%, 0.79%, 0.83% and 0.76%. All the differences are within clinically accepted norms.

Fixed- versus Variable-RBE Computations for Intensity Modulated Proton Therapy.

PURPOSE: To evaluate how using models of proton therapy that incorporate variable relative biological effectiveness (RBE) versus the current practice of using a fixed RBE of 1.1 affects dosimetric indices on treatment plans for large cohorts of patients treated with intensity modulated proton therapy (IMPT). METHODS AND MATERIALS: Treatment plans for 4 groups of patients who received IMPT for brain, head-and-neck, thoracic, or prostate cancer were selected. Dose distributions were recalculated in 4 ways: 1 with a fast-dose Monte Carlo calculator with fixed RBE and 3 with RBE calculated to 3 different models-McNamara, Wedenberg, and repair-misrepair-fixation. Differences among dosimetric indices (D02, D50, D98, and mean dose) for target volumes and organs at risk (OARs) on each plan were compared between the fixed-RBE and variable-RBE calculations. RESULTS: In analyses of all target volumes, for which the main concern is underprediction or RBE less than 1.1, none of the models predicted an RBE less than 1.05 for any of the cohorts. For OARs, the 2 models based on linear energy transfer, McNamara and Wedenberg, systematically predicted RBE >1.1 for most structures. For the mean dose of 25% of the plans for 2 OARs, they predict RBE equal to or larger than 1.4, 1.3, 1.3, and 1.2 for brain, head-and-neck, thorax, and prostate, respectively. Systematically lower increases in RBE are predicted by repair-misrepair-fixation, with a few cases (eg, femur) in which the RBE is less than 1.1 for all plans. CONCLUSIONS: The variable-RBE models predict increased doses to various OARs, suggesting that strategies to reduce high-dose linear energy transfer in critical structures should be developed to minimize possible toxicity associated with IMPT.

Comparison of Monte Carlo and analytical dose computations for intensity modulated proton therapy.

To evaluate the effect of approximations in clinical analytical calculations performed by a treatment planning system (TPS) on dosimetric indices in intensity modulated proton therapy. TPS calculated dose distributions were compared with dose distributions as estimated by Monte Carlo (MC) simulations, calculated with the fast dose calculator (FDC) a system previously benchmarked to full MC. This study analyzed a total of 525 patients for four treatment sites (brain, head-and-neck, thorax and prostate). Dosimetric indices (D02, D05, D20, D50, D95, D98, EUD and Mean Dose) and a gamma-index analysis were utilized to evaluate the differences. The gamma-index passing rates for a 3%/3 mm criterion for voxels with a dose larger than 10% of the maximum dose had a median larger than 98% for all sites. The median difference for all dosimetric indices for target volumes was less than 2% for all cases. However, differences for target volumes as large as 10% were found for 2% of the thoracic patients. For organs at risk (OARs), the median absolute dose difference was smaller than 2 Gy for all indices and cohorts. However, absolute dose differences as large as 10 Gy were found for some small volume organs in brain and head-and-neck patients. This analysis concludes that for a fraction of the patients studied, TPS may overestimate the dose in the target by as much as 10%, while for some OARs the dose could be underestimated by as much as 10 Gy. Monte Carlo dose calculations may be needed to ensure more accurate dose computations to improve target coverage and sparing of OARs in proton therapy.

Evidence of superspin-glass behavior in Zn0.5Ni0.5Fe2O4 nanoparticles.

We have used dc-magnetization and ac-susceptibility to investigate the superspin dynamics in 9 nm average size Zn(0.5)Ni(0.5)Fe(2)O(4) magnetic particles at temperatures (T) between 3 and 300 K. Dc-magnetization M versus T data collected in a H = 50 Oe magnetic field using a field-cooled-zero-field-cooled protocol indicate that the onset of irreversibility occurs in the vicinity of 190 K. This is confirmed by M versus H|(T) hysteresis loops, as well as by frequency- and temperature-resolved ac-susceptibility data. We demonstrate that this magnetic event is not due to the blocking of individual superspins, but can be unequivocally ascribed to their collective freezing in a spin-glass-like fashion. Indeed, the relative variation (per frequency decade) of the in-phase susceptibility peak temperature is ∼0.032, critical dynamics analysis of this peak shift yields an exponent zν = 10.0 and a zero-field freezing temperature T(g) = 190 K, and, in a magnetic field, Tg(H) is excellently described by the de Almeida-Thouless line δT(g) = 1 - T(g)(H)/T(g) ∝ H(2/3). In addition, out-of-phase susceptibility versus temperature datasets collected at different frequencies collapse on a universal dynamic scaling curve. Finally, memory imprinting during a stop-and-wait magnetization protocol confirms the collective freezing nature of the state below 190 K.