Arman Sarfehnia

Evaluation of a commercial MRI Linac based Monte Carlo dose calculation algorithm with geant 4

PURPOSE This paper provides a comparison between a fast, commercial, in-patient Monte Carlo dose calculation algorithm (GPUMCD) and geant4. It also evaluates the dosimetric impact of the application of an external 1.5 T magnetic field. METHODS A stand-alone version of the Elekta™ GPUMCD algorithm, to be used within the Monaco treatment planning system to model dose for the Elekta™ magnetic resonance imaging (MRI) Linac, was compared against GEANT4 (v10.1). This was done in the presence or absence of a 1.5 T static magnetic field directed orthogonally to the radiation beam axis. Phantoms with material compositions of water, ICRU lung, ICRU compact-bone, and titanium were used for this purpose. Beams with 2 MeV monoenergetic photons as well as a 7 MV histogrammed spectrum representing the MRI Linac spectrum were emitted from a point source using a nominal source-to-surface distance of 142.5 cm. Field sizes ranged from 1.5 × 1.5 to 10 × 10 cm(2). Dose scoring was performed using a 3D grid comprising 1 mm(3) voxels. The production thresholds were equivalent for both codes. Results were analyzed based upon a voxel by voxel dose difference between the two codes and also using a volumetric gamma analysis. RESULTS Comparisons were drawn from central axis depth doses, cross beam profiles, and isodose contours. Both in the presence and absence of a 1.5 T static magnetic field the relative differences in doses scored along the beam central axis were less than 1% for the homogeneous water phantom and all results matched within a maximum of ±2% for heterogeneous phantoms. Volumetric gamma analysis indicated that more than 99% of the examined volume passed gamma criteria of 2%-2 mm (dose difference and distance to agreement, respectively). These criteria were chosen because the minimum primary statistical uncertainty in dose scoring voxels was 0.5%. The presence of the magnetic field affects the dose at the interface depending upon the density of the material on either sides of the interface. This effect varies with the field size. For example, at the water-lung interface a 33.94% increase in dose was observed (relative to the Dmax), by both GPUMCD and GEANT4 for the field size of 2 × 2 cm(2) (compared to no B-field case), which increased to 47.83% for the field size of 5 × 5 cm(2) in the presence of the magnetic field. Similarly, at the lung-water interface, the dose decreased by 19.21% (relative to Dmax) for a field size of 2 × 2 cm(2) and by 30.01% for 5 × 5 cm(2) field size. For more complex combinations of materials the dose deposition also becomes more complex. CONCLUSIONS The GPUMCD algorithm showed good agreement against GEANT4 both in the presence and absence of a 1.5 T external magnetic field. The application of 1.5 T magnetic field significantly alters the dose at the interfaces by either increasing or decreasing the dose depending upon the density of the material on either side of the interfaces.

Experimental evaluation of a GPU-based Monte Carlo dose calculation algorithm in the Monaco treatment planning system

A new GPU-based Monte Carlo dose calculation algorithm (GPUMCD), developed by the vendor Elekta for the Monaco treatment planning system (TPS), is capable of modeling dose for both a standard linear accelerator and an Elekta MRI linear accelerator. We have experimentally evaluated this algorithm for a standard Elekta Agility linear accelerator. A beam model was developed in the Monaco TPS (research version 5.09.06) using the commissioned beam data for a 6 MV Agility linac. A heterogeneous phantom representing several scenarios - tumor-in-lung, lung, and bone-in-tissue - was designed and built. Dose calculations in Monaco were done using both the current clinical Monte Carlo algorithm, XVMC, and the new GPUMCD algorithm. Dose calculations in a Pinnacle TPS were also produced using the collapsed cone convolution (CCC) algorithm with heterogeneity correction. Calculations were compared with the measured doses using an ionization chamber (A1SL) and Gafchromic EBT3 films for 2×2 cm2,5×5 cm2, and 10×2 cm2 field sizes. The percentage depth doses (PDDs) calculated by XVMC and GPUMCD in a homogeneous solid water phantom were within 2%/2 mm of film measurements and within 1% of ion chamber measurements. For the tumor-in-lung phantom, the calculated doses were within 2.5%/2.5 mm of film measurements for GPUMCD. For the lung phantom, doses calculated by all of the algorithms were within 3%/3 mm of film measurements, except for the 2×2 cm2 field size where the CCC algorithm underestimated the depth dose by ∼5% in a larger extent of the lung region. For the bone phantom, all of the algorithms were equivalent and calculated dose to within 2%/2 mm of film measurements, except at the interfaces. Both GPUMCD and XVMC showed interface effects, which were more pronounced for GPUMCD and were comparable to film measurements, whereas the CCC algorithm showed these effects poorly. PACS number(s): 87.53.Bn, 87.55.dh, 87.55.km.A new GPU‐based Monte Carlo dose calculation algorithm (GPUMCD), developed by the vendor Elekta for the Monaco treatment planning system (TPS), is capable of modeling dose for both a standard linear accelerator and an Elekta MRI linear accelerator. We have experimentally evaluated this algorithm for a standard Elekta Agility linear accelerator. A beam model was developed in the Monaco TPS (research version 5.09.06) using the commissioned beam data for a 6 MV Agility linac. A heterogeneous phantom representing several scenarios — tumor‐in‐lung, lung, and bone‐in‐tissue — was designed and built. Dose calculations in Monaco were done using both the current clinical Monte Carlo algorithm, XVMC, and the new GPUMCD algorithm. Dose calculations in a Pinnacle TPS were also produced using the collapsed cone convolution (CCC) algorithm with heterogeneity correction. Calculations were compared with the measured doses using an ionization chamber (A1SL) and Gafchromic EBT3 films for 2×2 cm2,5×5 cm2, and 10×2 cm2 field sizes. The percentage depth doses (PDDs) calculated by XVMC and GPUMCD in a homogeneous solid water phantom were within 2%/2 mm of film measurements and within 1% of ion chamber measurements. For the tumor‐in‐lung phantom, the calculated doses were within 2.5%/2.5 mm of film measurements for GPUMCD. For the lung phantom, doses calculated by all of the algorithms were within 3%/3 mm of film measurements, except for the 2×2 cm2 field size where the CCC algorithm underestimated the depth dose by ∼5% in a larger extent of the lung region. For the bone phantom, all of the algorithms were equivalent and calculated dose to within 2%/2 mm of film measurements, except at the interfaces. Both GPUMCD and XVMC showed interface effects, which were more pronounced for GPUMCD and were comparable to film measurements, whereas the CCC algorithm showed these effects poorly. PACS number(s): 87.53.Bn, 87.55.dh, 87.55.km

Backscatter dose effects for high atomic number materials being irradiated in the presence of a magnetic field: A Monte Carlo study for the MRI linac.

PURPOSE To quantify and explain the backscatter dose effects for clinically relevant high atomic number materials being irradiated in the presence of a 1.5 T transverse magnetic field. METHODS Interface effects were investigated using Monte Carlo simulation techniques. We used gpumcd (v5.1) and geant4 (v10.1) for this purpose. gpumcd is a commercial software written for the Elekta AB, MRI linac. Dose was scored using gpumcd in cubic voxels of side 1 and 0.5 mm, in two different virtual phantoms of dimensions 20 × 20 × 20 cm and 5 × 5 × 13.3 cm, respectively. A photon beam was generated from a point 143.5 cm away from the isocenter with energy distribution sampled from a histogram representing the true Elekta, MRI linac photon spectrum. A slab of variable thickness and position containing either bone, aluminum, titanium, stainless steel, or one of the two different dental filling materials was inserted as an inhomogeneity in the 20 × 20 × 20 cm phantom. The 5 × 5 × 13.3 cm phantom was used as a clinical test case in order to explain the dose perturbation effects for a head and neck cancer patient. The back scatter dose factor (BSDF) was defined as the ratio of the doses at a given depth with and without the presence of the inhomogeneity. Backscattered electron fluence was calculated at the inhomogeneity interface using geant4. A 1.5 T magnetic field was applied perpendicular to the direction of the beam in both phantoms, identical to the geometry in the Elekta MRI linac. RESULTS With the application of a 1.5 T magnetic field, all the BSDFs were reduced by 12%-47%, compared to the no magnetic field case. The corresponding backscattered electron fluence at the interface was also reduced by 45%-64%. The reduction in the BSDF at the interface, due to the application of the magnetic field, is manifested in a different manner for each material. In the case of bone, the dose drops at the interface contrary to the expected increase when no magnetic field is applied. In the case of aluminum, the dose at the interface is the same with and without the presence of the aluminum. For all of the other materials the dose increases at the interface. CONCLUSIONS The reduction in dose at the interface, in the presence of the magnetic field, is directly related to the reduction in backscattered electron fluence. This reduction occurs due to two different reasons. First, the electron spectrum hitting the interface is changed when the magnetic field is turned on, which results in changes in the electron scattering probability. Second, some electrons that have curved trajectories due to the presence of the magnetic field are absorbed by the higher density side of the interface and no longer contribute to the backscattered electron fluence.

SU‐E‐T‐203: Comparison of a Commercial MRI‐Linear Accelerator Based Monte Carlo Dose Calculation Algorithm and Geant4

Purpose: An MRI-linear accelerator is currently being developed by the vendor Elekta™. The treatment planning system that will be used to model dose for this unit uses a Monte Carlo dose calculation algorithm, GPUMCD, that allows for the application of a magnetic field. We tested this radiation transport code against an independent Monte-Carlo toolkit Geant4 (v.4.10.01) both with and without the magnetic field applied. Methods: The setup comprised a 6 MeV mono-energetic photon beam emerging from a point source impinging on a homogeneous water phantom at 100 cm SSD. The comparisons were drawn from the percentage depth doses (PDD) for three different field sizes (1.5 x 1.5 cm2, 5 x 5 cm2, 10 x 10 cm2) and dose profiles at various depths. A 1.5 T magnetic field was applied perpendicular to the direction of the beam. The transport thresholds were kept the same for both codes. Results: All of the normalized PDDs and profiles agreed within ± 1 %. In the presence of the magnetic field, PDDs rise more quickly reducing the depth of maximum dose. Near the beam exit point in the phantom a hot spot is created due to the electron return effect. This effect is more pronounced for the larger field sizes. Profiles selected parallel to the external field show no effect, however, the ones selected perpendicular to the direction of the applied magnetic field are shifted towards the direction of the Lorentz force applied by the magnetic field on the secondary electrons. It is observed that these profiles are not symmetric which indicates a lateral build up of the dose. Conclusion: There is a good general agreement between the PDDs/profiles calculated by both algorithms thus far. We are proceeding towards clinically relevant comparisons in a heterogeneous phantom for polyenergetic beams. Funding for this work has been provided by Elekta.

Sci-Sat AM: Radiation Dosimetry and Practical Therapy Solutions - 10: Towards LET detection: A study on the effects of scintillator doping

Purpose: In radiotherapy, the amount of radiation delivered is determined by optimizing the amount of absorbed dose to the tumor. Dose does not always correlate well with the actual biological effects of radiation. This work seeks to validate the LET-dependence of doped plastic scintillators for use in a radiation beam quality (LET) detector. Methods: The LET spectrum ([Φ]) can be resolved knowing the measured signals of uniquely LET-dependent detectors, [S], and the response of each LET-dependent detector to specific LETs ([R]), through the relation [Φ]=[S][R]−1. Plastic scintillator response is intrinsically LET dependent and can be varied via doping. Initial prototype consists of plastic scintillator and glass taper coupled to an optical fiber; components are housed in black acrylic, reducing effect of ambient light. In order to determine [R], the light response matrix, GEANT4.10.1 Monte Carlo (MC) was used. To validate MC, measurements were done using high energy electrons (9,12,15MeV) and orthovoltage x-rays (100,250kV); scintillator signal was normalized to dose measured simultaneously. Results: Stopping power was varied by changing particle type/energy; measurements indicated that as stopping power increased from 1.9 to 6.6MeV/cm, detector response increased by 263% (+/−29.2%) for 5%Pb-doped scintillator (155% in MC); 52% (+/−7.8%) increase observed when undoped scintillator was used (49% in MC). 5%Pb-doped discrepancy (100kV x-rays) is being investigated. Conclusions: This work validates that doping effects LET/energy response of scintillators; an effect that can be utilized for construction of an LET detector.

SU‐F‐T‐612: Investigation of Acoustic Neuroma Planning for Stereotactic Radiosurgery Utilizing Linac‐Based Cone Collimators

PURPOSE To assess the feasibility of designing clinically-acceptable stereotactic radiosurgery (SRS) plans utilizing linac-based cone collimators for patients presenting with acoustic neuroma. METHODS Five acoustic neuroma patients with gross tumour volumes (GTVs) of sizes from 1.3 to 2.7 cc were studied. The cranial-caudal extent of the GTVs range from 1.1 to 1.7 cm whereas the largest cross-sectional extent of the lesions varied from 2.0 to 2.4 cm. No PTV margin was added. The relevant organs-at-risk (OARs) were the brainstem, brain, lens, eyes and cochlea. The SRS planning system, ERGO (Elekta), was used to design treatment plans with non-coplanar arcs delivered using various stereotactic cone sizes on an Elekta Synergy unit. The prescription dose was 12 Gy to be delivered in a single fraction. The final dose distribution for each target was achieved with two to five isocenters, each consisting of up to five non-coplanar arcs. RESULTS The achieved GTV V12, V11.4, and V11 were 97.6-98.6%, 99.2- 99.8% and 99.6-100%, respectively. The penalty for using multiple isocenters for a single target was a relatively high maximum dose of up to 18 Gy, which equals 150% of the prescription dose. In all cases, the RTOG and Paddick conformity indices fell within the range 1.45-1.70 and 0.57-0.66, respectively. Point maximum dose to the brainstem varied from 12.4 to 14 Gy and its V12 was ≤0.12cc. The point maximum doses to the lens and eyes were ≤80 and ≤110cGy, respectively, and total body V10 was ≤6.2cc. Point maximum dose to ipsilateral cochlea was similar to the prescription dose. CONCLUSION Clinically-acceptable and deliverable dose distributions for acoustic neuroma cases can be achieved with linac-based stereotactic cones system. Up to five isocenters per target are required for GTVs of sizes ≤3cc. The treatment plans meet RTOG protocol requirement on conformity index.

SU‐E‐T‐72: Influence of Chamber Wall Material On Ionization Chamber Absorbed Dose Energy Response: A Numerical and Experimental Study

PURPOSE To study the energy response of ionization chambers with different wall materials. METHODS The dose inside the cavity of an accurately modeled Exradin A12 ionization chamber was scored with Monte Carlo user code egs++/egs_chamber. The C552 plastic chamber wall material was changed in the simulations with materials of higher atomic number Z (aluminum, copper, molybdenum and tungsten) and simulations were carried out with five beam energies ranging from 120 kVp to 18 MV. The dose scored inside the cavity for each wall material was normalized to that from a C552 plastic wall. The mean secondary electron energy for each beam Ee- was also calculated at the level of chamber cavity using MC user code FLURZnrc. The simulations were experimentally verified by replacing the Exradin A12 chamber C552 wall with Al or Cu walls of identical dimensions. AAPM TG51 and TG61 setups were followed for high and low energy beams, respectively. Large attenuation of kilovoltage photons by high Z wall materials was accounted for by correcting the readings with a CAVRZnrc-calculated chamber wall attenuation and scatter correction Awall . RESULTS The relative readings obtained showed that with the use of higher Z wall materials, the chamber signal increased by up to a factor of 2.96 for MV photons, and 54.71 at 120 kVp. Higher Z walls Result in larger contribution of photoelectrons, and as such changing the wall material significantly affects the absorbed dose energy-dependence of the chambers. Experimental results agree with simulations to within 9.8 %. The discrepancy is largest at kV beams and can be mitigated if the impurities found in each wall material were considered in the MC simulations. CONCLUSION The change in ionization chamber absorbed-dose energy dependence is studied (numerically and experimentally), by replacing the original wall chamber with walls of different Z material, while keeping the wall dimensions identical.

SU‐E‐T‐82: Change of Ionization Chamber Correction Factors (Ppol , Pion, KWall) with Chamber Walls of Different Materials in Continuous and Pulsed Beams

PURPOSE To study the effect of wall material on correction factors for polarity effect Ppol , collection efficiency Pion and scatter and attenuation in the chamber wall/central electrode Kwall , in continuous and pulsed beams. METHODS An Exadin A12 ionization chamber was modified to have geometrically identical chamber walls built from aluminum and copper. Measurements were performed using the different walls in both 60 Co, and Varian-Clinac 21EX 6 MV beams. Ppol was measured using AAPM TG51 protocol. Pion was obtained form measurement data, where the collected charge was measured at 10 voltage settings and used to form Jaffe-plots. Measurements were compared against Monte Carlo simulated data (egs++/egs_chamber). Kwall values were also calculated using CAVRZnrc. RESULTS For all beams and all wall materials, Ppol was found to be less than the 0.3 % limit recommended in TG-51. Saturation charges (extrapolated from the Jaffe-plots) were observed to increase with wall materials of increasing atomic number Z. The breakdown of the Boag-predicted linearity of Jaffe-plots in the near-saturation region was observed for all beam types and wall materials. The ratio of the saturation charges (relative to the C552 wall) were predicted by cavity theory, and agreed with simulations to within 9.8 % for 60 Co and 5.2 % for 6 MV. Pion measured by the two-voltage technique and from Jaffe-plots were all found to be less than the 1.05 accepted upper limit recommended by TG-51. Kwall showed that attenuation and scatter for the Cu wall influences the measured signal by 1.9 % for 60 Co. CONCLUSION Ppol and Pion were measured for an Exradin A12 chamber with C552, Al, and Cu wall materials. The basic behavior of Jaffe-plots is independent of wall material. As expected, Kwall increases drastically for high Z walls with decreasing beam energies.