The influence of beam optics asymmetric distribution on dose in scanning carbon-ion radiotherapy.

PURPOSE: To quantify the influence of beam optics asymmetric distribution on dose. METHODS: Nine reference cubic targets and corresponding plans with modulation widths (M) of 3, 6, and 9 cm and with center depths (CDs) of 6, 12, and 24 cm were generated by the treatment planning system (TPS). The Monte Carlo code FLUKA was used for simulating the dose distribution from the aforementioned original plans and the dose perturbation by varying ±5%, ±15%, ±20%, ±25%, and ±40% in spot full width half maximum to the X-direction while keeping consistent in the Y-direction. The dosimetric comparisons in dose deviation, γ-index analysis, lateral penumbra, and flatness were evaluated. RESULTS: The largest 3D absolute mean deviation was 15.0% ± 20.9% (mean ± standard deviation) in M3CD6, whereas with the variation from -15% to +20%, the values were below 5% for all cube plans. The lowest 2D γ-index passing rate was 80.6% with criteria of 2%-2 mm by a +40% variation in M3CD6. For the M9CD24 with a -40% variation, the maximum 1D dose deviations were 5.6% and 15.7% in the high-dose region and the edge of the radiation field, respectively. The maximum deviations of penumbra and flatness were 3.4 mm and 11.4%, respectively. CONCLUSIONS: The scenario of beam optics asymmetric showed relatively slight influence on the global dose distribution but severely affected dose on the edge of the radiation field. For scanning carbon-ion therapy facilities, beam spot lateral profile settings in TPS base data should be properly handled when beam optics asymmetry variation is over 15%.

Development of a Monte Carlo beam model for raster scanning proton beams and dosimetric comparison.

PURPOSE: To develop a Monte Carlo (MC) beam model for raster scanning proton beams for dose verification purposes. METHODS AND MATERIALS: MC program FLUKA was used in the model. The nominal energy, momentum spread and beam angular distribution in the model were determined by matching the simulation profiles with the measured integral depth dose (IDD) and in air spot size. Dosimetric comparison was done by comparing the measured and simulated dose distributions. The 1 D dose profile of cubic Spread Out Bragg Peak (SOBP) plans, and the 2 D dose distribution of previously treated breast cancer patients' clinical plans were measured by using Pinpoint chambers and 2 D array ionization chambers, respectively. Corresponding DICOM plan information was utilized for MC simulation. RESULTS: The MC results showed good agreement with measurements for the SOBP plans. The absolute comparison of the absorbed dose difference between the MC and the measurement was 0.93%±0.88%. For the patient plans, the overall passing rate of the gamma index analysis (γ-PR) between the MC simulation and measurement with the 2%-2 mm criteria was 97.78%, and only 1 case had a γ-PR less than 90%. With the 3%-3 mm criteria, γ-PR was never below 99% for all cases with and without the range shifter. CONCLUSIONS: This work described a method for adapting a MC simulation model for a raster scanning proton beam. The good concordance between the simulations and measurements shows that the MC model is an accurate and reliable method. It has the potential to be used for patient specific quality assurance (PSQA) to reduce the beam time for the measurements in water.

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%.

Analysis of ion beam teletherapy patient-specific quality assurance.

The objective of this study was to evaluate the procedures for patient-specific quality assurance measurements using modulated scanned and energy stacked beams for proton and carbon ion teletherapy. Delivery records from 1734 portal measurements were analyzed using a 3-point pass criteria: more than 22 of 24 chambers in a water phantom (WP) had to have a measured dose difference from the planned portal doses less than or equal to 3%, or the distance from the measurement point location to a point location in the plan having the same dose had to be less than or equal to 3 mm (distance to agreement [DTA]), and the mean dose deviation of all chambers had to be less than 3%. Stratification of results showed some associations between measurement parameters and pass rates. For proton portals, pass rates were high at all measurement depths, but for carbon ion portals, pass rates decreased as a function of increasing measurement depth. Pass rates of both proton and carbon ion portals with 1 WP were slightly lower than those with a second WP. The total pass rates were 97.7% and 91.9% for proton and carbon ion patient portals, respectively. In general, the measured doses exhibited good agreement with the treatment planning system (TPS) calculated doses. When the chamber position was deeper than 150 mm in carbon ion beams, a lower pass rate was observed, which may have been caused by ion chamber array setup uncertainty (lateral and depth) in highly modulated portals or incorrect modeling of scatter by the TPS. These deviations need further investigation.