Jefferson Sorriaux

Extension of PENELOPE to protons: simulation of nuclear reactions and benchmark with Geant4.

PURPOSE Describing the implementation of nuclear reactions in the extension of the Monte Carlo code (MC) PENELOPE to protons (PENH) and benchmarking with Geant4. METHODS PENH is based on mixed-simulation mechanics for both elastic and inelastic electromagnetic collisions (EM). The adopted differential cross sections for EM elastic collisions are calculated using the eikonal approximation with the Dirac-Hartree-Fock-Slater atomic potential. Cross sections for EM inelastic collisions are computed within the relativistic Born approximation, using the Sternheimer-Liljequist model of the generalized oscillator strength. Nuclear elastic and inelastic collisions were simulated using explicitly the scattering analysis interactive dialin database for (1)H and ICRU 63 data for (12)C, (14)N, (16)O, (31)P, and (40)Ca. Secondary protons, alphas, and deuterons were all simulated as protons, with the energy adapted to ensure consistent range. Prompt gamma emission can also be simulated upon user request. Simulations were performed in a water phantom with nuclear interactions switched off or on and integral depth-dose distributions were compared. Binary-cascade and precompound models were used for Geant4. Initial energies of 100 and 250 MeV were considered. For cases with no nuclear interactions simulated, additional simulations in a water phantom with tight resolution (1 mm in all directions) were performed with FLUKA. Finally, integral depth-dose distributions for a 250 MeV energy were computed with Geant4 and PENH in a homogeneous phantom with, first, ICRU striated muscle and, second, ICRU compact bone. RESULTS For simulations with EM collisions only, integral depth-dose distributions were within 1%/1 mm for doses higher than 10% of the Bragg-peak dose. For central-axis depth-dose and lateral profiles in a phantom with tight resolution, there are significant deviations between Geant4 and PENH (up to 60%/1 cm for depth-dose distributions). The agreement is much better with FLUKA, with deviations within 3%/3 mm. When nuclear interactions were turned on, agreement (within 6% before the Bragg-peak) between PENH and Geant4 was consistent with uncertainties on nuclear models and cross sections, whatever the material simulated (water, muscle, or bone). CONCLUSIONS A detailed and flexible description of nuclear reactions has been implemented in the PENH extension of PENELOPE to protons, which utilizes a mixed-simulation scheme for both elastic and inelastic EM collisions, analogous to the well-established algorithm for electrons/positrons. PENH is compatible with all current main programs that use PENELOPE as the MC engine. The nuclear model of PENH is realistic enough to give dose distributions in fair agreement with those computed by Geant4.

Experimental assessment of proton dose calculation accuracy in inhomogeneous media

PURPOSE Proton therapy with Pencil Beam Scanning (PBS) has the potential to improve radiotherapy treatments. Unfortunately, its promises are jeopardized by the sensitivity of the dose distributions to uncertainties, including dose calculation accuracy in inhomogeneous media. Monte Carlo dose engines (MC) are expected to handle heterogeneities better than analytical algorithms like the pencil-beam convolution algorithm (PBA). In this study, an experimental phantom has been devised to maximize the effect of heterogeneities and to quantify the capability of several dose engines (MC and PBA) to handle these. METHODS An inhomogeneous phantom made of water surrounding a long insert of bone tissue substitute (1×10×10 cm3) was irradiated with a mono-energetic PBS field (10×10 cm2). A 2D ion chamber array (MatriXX, IBA Dosimetry GmbH) lied right behind the bone. The beam energy was such that the expected range of the protons exceeded the detector position in water and did not attain it in bone. The measurement was compared to the following engines: Geant4.9.5, PENH, MCsquare, as well as the MC and PBA algorithms of RayStation (RaySearch Laboratories AB). RESULTS For a γ-index criteria of 2%/2mm, the passing rates are 93.8% for Geant4.9.5, 97.4% for PENH, 93.4% for MCsquare, 95.9% for RayStation MC, and 44.7% for PBA. The differences in γ-index passing rates between MC and RayStation PBA calculations can exceed 50%. CONCLUSION The performance of dose calculation algorithms in highly inhomogeneous media was evaluated in a dedicated experiment. MC dose engines performed overall satisfactorily while large deviations were observed with PBA as expected.

SU-F-BRD-15: Quality Correction Factors in Scanned Or Broad Proton Therapy Beams Are Indistinguishable

Purpose: The IAEA TRS-398 code of practice details the reference conditions for reference dosimetry of proton beams using ionization chambers and the required beam quality correction factors (kQ). Pencil beam scanning (PBS) requires multiple spots to reproduce the reference conditions. The objective is to demonstrate, using Monte Carlo (MC) calculations, that kQ factors for broad beams can be used for scanned beams under the same reference conditions with no significant additional uncertainty. We consider hereafter the general Alfonso formalism (Alfonso et al, 2008) for non-standard beam. Methods: To approach the reference conditions and the associated dose distributions, PBS must combine many pencil beams with range modulation and shaping techniques different than those used in passive systems (broad beams). This might lead to a different energy spectrum at the measurement point. In order to evaluate the impact of these differences on kQ factors, ion chamber responses are computed with MC (Geant4 9.6) in a dedicated scanned pencil beam (Q_pcsr) producing a 10×10cm2 composite field with a flat dose distribution from 10 to 16 cm depth. Ion chamber responses are also computed by MC in a broad beam with quality Q_ds (double scattering). The dose distribution of Q _pcsr matches the dose distribution of Q_ds. k_(Q_pcsr,Q_ds) is computed for a 2×2×0.2cm3 idealized air cavity and a realistic plane-parallel ion chamber (IC). Results: Under reference conditions, quality correction factors for a scanned composite field versus a broad beam are the same for air cavity dose response, k_(Q_pcsr,Q_ds) =1.001±0.001 and for a Roos IC, k_(Q_pcsr,Q_ds) =0.999±0.005. Conclusion: Quality correction factors for ion chamber response in scanned and broad proton therapy beams are identical under reference conditions within the calculation uncertainties. The results indicate that quality correction factors published in IAEA TRS-398 can be used for scanned beams in the SOBP of a high-energy proton beam. Jefferson Sorriaux is financed by the Walloon Region under the convention 1217662. Jefferson Sorriaux is sponsored by a public-private partnership IBA - Walloon Region

SU-E-T-464: On the Equivalence of the Quality Correction Factor for Pencil Beam Scanning Proton Therapy

PURPOSE In current practice, most proton therapy centers apply IAEA TRS-398 reference dosimetry protocol. Quality correction factors (kQ) take into account in the dose determination process the differences in beam qualities used for calibration unit and for treatment unit. These quality correction factors are valid for specific reference conditions. TRS-398 reference conditions should be achievable in both scattered proton beams (i.e. DS) and scanned proton beams (i.e. PBS). However, it is not a priori clear if TRS-398 kQ data, which are based on Monte Carlo (MC) calculations in scattered beams, can be used for scanned beams. Using TOPAS-Geant4 MC simulations, the study aims to determine whether broad beam quality correction factors calculated in TRS-398 can be directly applied to PBS delivery modality. METHODS As reference conditions, we consider a 10×10×10 cm3 homogeneous dose distribution delivered by PBS system in a water phantom (32/10 cm range/modulation) and an air cavity placed at the center of the spread-out-Bragg-peak. In order to isolate beam differences, a hypothetical broad beam is simulated. This hypothetical beam reproduces exactly the same range modulation, and uses the same energy layers than the PBS field. Ion chamber responses are computed for the PBS and hypothetical beams and then compared. RESULTS For an air cavity of 2×2×0.2 cm3 , the ratio of ion chamber responses for the PBS and hypothetical beam qualities is 0.9991 ± 0.0016. CONCLUSION Quality correction factors are insensitive to the delivery pattern of the beam (broad beam or PBS), as long as similar dose distributions are achieved. This investigation, for an air cavity, suggests that broad beam quality correction factors published in TRS-398 can be applied for scanned beams. J. Sorriaux is financially supported by a public-private partnership involving the company Ion Beam Applications (IBA).

Consistency in quality correction factors for ionization chamber dosimetry in scanned proton beam therapy

Purpose The IAEA TRS‐398 code of practice details the reference conditions for reference dosimetry of proton beams using ionization chambers and the required beam quality correction factors (kQ). Pencil beam scanning (PBS) systems cannot approximate reference conditions using a single spot. However, dose distributions requested in TRS‐398 can be reproduced with PBS using a combination of spots. This study aims to demonstrate, using Monte Carlo (MC) simulations, that kQ factors computed/measured for broad beams can be used with scanned beams for similar reference dose distributions with no additional significant uncertainty. Methods We consider the Alfonso formalism13 usually employed for nonstandard photon beams. To approach reference conditions similar as IAEA TRS‐398 and the associated dose distributions, PBS must combine many pencil beams with range or energy modulation and shaping techniques that differ from those used in passive systems (broad beams). In order to evaluate the impact of these differences on kQ factors, ionization chamber responses are computed with MC (Geant4 9.6) in three different proton beams, with their corresponding quality factors (Q), producing a 10 × 10 cm2 field with a flat dose distribution for (a) a dedicated scanned pencil beam (Qpbs), (b) a hypothetical proton source (Qhyp), and (c) a double‐scattering beam (Qds). The tested ionization chamber cavities are a 2 × 2 × 0.2 mm³ air cavity, a Roos‐type ionization chamber, and a Farmer‐type ionization chamber. Results and Discussion Ranges of Qpbs, Qhyp, and Qds are consistent within 0.4 mm. Flatnesses of dose distributions are better than 0.5%. Calculated Symbol is 0.999 ± 0.002 for the air cavity and the Farmer‐type ionization chamber and 1.001 ± 0.002 for the Roos‐type ionization chamber. The quality correction factors Symbol is 0.999 ± 0.002 for the Farmer‐type and Roos‐type ionization chambers and 1.001 ± 0.001 for the Roos‐type ionization chamber. Symbol. No Caption available. Symbol. No Caption available. Conclusion The Alfonso formalism was applied to scanned proton beams. In our MC simulations, neither the difference in the beam profiles (scanned beam vs hypothetical beam) nor the different incident beam energies influenced significantly the beam correction factors. This suggests that ionization chamber quality correction factors in scanned or broad proton beams are indistinguishable within the calculation uncertainties provided dose distributions achieved by both modalities are similar and compliant with the TRS‐398 reference conditions.

WE-F-105-02: Fano Cavity Test of the Monte Carlo Codes GEANT4 and PENH for Proton Transport

PURPOSE In the scope of reference dosimetry of radiotherapy beams, Monte Carlo (MC) simulations are widely used to compute ionization chamber dose response accurately. Uncertainties related to the transport algorithm can be verified performing self-consistency tests, e.g. the so-called Fano cavity test. The Fano cavity test is based on the Fano theorem, which states that under charged particle equilibrium (CPE) conditions, the charged particle fluence is independent of the mass density of the media as long as the cross-sections are uniform. Such tests have not been performed yet for MC codes simulating proton transport. The objectives of this communication are 1) presenting a new methodology for Fano cavity test of MC codes for protons and other charged particles; 2) applying the methodology for two MC codes: GEANT4 and PENELOPE extended to protons (PENH). METHODS The geometry considered is a 10×10 cm2 parallel virtual field and a cavity (2×2×0.2 mm3) in a water phantom with dimensions large enough to ensure CPE. Virtual particles of energy E and attenuation coefficient μ are transported. During each interaction, the virtual particle triggers a proton with kinetic energy E and is then regenerated. Assuming no nuclear reactions and no generation of other secondaries, we theoretically demonstrate that the computed cavity dose should equal μE/ρ times the incident fluence. Simulations satisfying those assumptions were implemented in GEANT4 and PENH. RESULTS For conservative user-inputs (small step sizes), both GEANT4 and PENH pass the FANO cavity test within 0.1%. However, differences of 0.6% were observed for PENH using larger step sizes. The difference was attributed to the random-hinge method that introduces an artificial energy straggling if step size is not small enough. CONCLUSION Using safe user-inputs, both PENH and GEANT4 pass the Fano cavity test for proton transport. Our methodology is valid for any type of charged particle. Jefferson Sorriaux is sponsored by a public-private partnership IBA - Region Wallonne.

WE‐E‐141‐05: Ion Recombination for Ionization Chamber Dosimetry in a Pencil Beam Scanning Proton Therapy Beam

PURPOSE IAEA TRS-398 provides recipes and formulas to compute ion recombination correction factors for continuous and pulsed broad proton beams. However, those formulas may not be optimal for pencil beam scanning modalities (PBS). This work aims at evaluating appropriately ion recombination correction for different ionization chamber types for PBS delivery. METHODS Ion chamber measurements were performed in a water phantom (BluePhantom2 , IBA Dosimetry GmbH) irradiated by a 10×10 cm2 uniform field (2.5mm spot spacing, IBA PBS dedicated nozzle) for Extradin T1, FC65-G, CC01 and PPC05 at different beam energies, beam current and polarizing voltages. The Boutillon formalism was used in order to separate the contributions from initial and general recombination. The recombination correction factor was computed using the two-voltage method for continuous, pulsed, and pulsed-scanned beams as well. Chamber-dependent conditions such as depth and relative position to spot mapping were also evaluated. RESULTS The formulas for continuous beams in TRS-398 lead to an underestimation of the correction factors (ks) of 0.4% compared to Boutillon analysis for EXTRADIN T1. An overestimation of 0.2% is observed considering the beam as pulsed. For PPC05 using the two-voltage methods, (ks) difference of 0.3% is found compared to Boutillons value. For FC65-G using the two-voltage correction (ks) is 0.4% underestimated in continuous beam and can be 3% overestimated using pulsed beam formula. (ks) values is computed for various depth positions, energies and beam currents. Plotting inverse ionizing charge versus inverse squared voltage shows that initial recombination is not negligible for Extradin T1, FC65-G and PPC05 at low residual range. CONCLUSION We have determined recombination correction factors for 4 ion chambers using various practical and yet accurate methods in the specific case of PBS delivery. Significant differences in recombination correction factors can appear if recipes from IAEA TRS-398 are applied blindly for proton PBS delivery. Jefferson Sorriaux is financed by the Walloon Region under the project name InVivoIGT, convention number 1017266. Jefferson Sorriaux is sponsored by a public-private partnership IBA - Walloon Region.