Skin dose in longitudinal and transverse linac‐MRIs using Monte Carlo and realistic 3D MRI field models

Keyvanloo, Amir; Burke, B; Warkentin, Brad; Tadic, Tony; Rathee, S; Kirkby, Charles; Santos, D. M.; Fallone, B. G.

doi:10.1118/1.4754657

Cited by 58 publications

(63 citation statements)

References 34 publications

(64 reference statements)

Supporting

Mentioning

Contrasting

Order By: Relevance

“…To implement the magnetic field in EGSnrc, the macro packages dosxyznrc_user_macros.mortran and emf_macros.mortran were modified as previously described . These two macros are called at the end of a charged particle transport step performed in the absence of an electromagnetic field.…”

Section: Methodsmentioning

confidence: 99%

“…However, to date, EGSnrc depth dose calculations in phantom or patient in the presence of a parallel magnetic field have not been verified by experiment. The purpose of the current study is to experimentally explore the accuracy of EGSnrc calculated depth doses in a homogeneous tissue‐like phantom, with a slight modification to the EGS code required to read in the 3D magnetic field map and use it in the standard electromagnetic field macros as previously described . This is achieved by verifying the agreement between the EGS calculated percent depth doses (PDDs) and the measurements performed using a parallel plate ion chamber in a polystyrene phantom placed inside the bore of a solenoidal electromagnet and irradiated using a clinical linac.…”

Section: Introductionmentioning

confidence: 94%

See 1 more Smart Citation

Experimental verification of EGSnrc Monte Carlo calculated depth doses within a realistic parallel magnetic field in a polystyrene phantom

et al. 2017

View full text Add to dashboard Cite

Purpose: Integrating a linac with a magnetic resonance imager (MRI) will revolutionize the accuracy of external beam radiation treatments. Irradiating in the presence of a strong magnetic field, however, will modify the dose distribution. These dose modifications have been investigated previously, mainly using Monte Carlo simulations. The purpose of this work is to experimentally verify the use of the EGSnrc Monte Carlo (MC) package for calculating percent depth doses (PDDs) in a homogeneous phantom, in the presence of a realistic parallel magnetic field. Methods: Two cylindrical electromagnets were used to produce a 0.207 T magnetic field parallel to the central axis of a 6 MV photon beam from a clinical linac. The magnetic field was measured at discrete points along orthogonal axes, and these measurements were used to validate a full 3D magnetic field map generated using COMSOL Multiphysics. Using a small parallel plate ion chamber, the depth dose was measured in a polystyrene phantom placed inside the electromagnet bore at two separate locations: phantom top surface coinciding with top of bore, and phantom top surface coinciding with center of bore. BEAMnrc MC was used to model the linac head which was benchmarked against the linac's commissioning measurements. The depth dose in polystyrene was simulated using DOSXYZnrc MC. For the magnetic field case, the DOSXYZnrc code was slightly modified to implement the previously calculated 3D magnetic field map to be used in the standard electromagnetic macros. Results: The calculated magnetic field matched the measurements within 2% of the maximum central field (0.207 T) with most points within the experimental uncertainty (1.5%). For the MC linac head model, over 93% of all simulated points passed the 2%, 2 mm c acceptance criterion, when comparing measured and simulated lateral beam and depth dose profiles. The parallel magnetic field caused a surface dose increase, compared to the no magnetic field case, due to the Lorentz force confining contaminant electrons within the beam. The surface dose increase was measured to be approximately 10% (relative to no field D max ) when the phantom surface coincided with the top of the electromagnet's bore. This effect was enhanced by moving the phantom surface to the center of the magnet's bore in relatively high magnetic field (> 0.13 T). The surface dose for this setup increased by 30% and the entire buildup region was affected. When the dimensions and composition of the ion chamber air cavity and entrance window were included, EGSnrc was able to accurately simulate these dose increases, both at the surface and in the buildup region. All the simulated points were within 1% of the measurements for both setups. The ferromagnetic linac head was determined to have a negligible effect on the final PDD comparison. Conclusions: Irradiating in the presence of a parallel magnetic field causes measurable surface and buildup depth dose increases. We have experimentally verified that the EGSnrc Monte Carlo package is able to accurately ...

show abstract

Section: Methodsmentioning

confidence: 99%

Section: Introductionmentioning

confidence: 94%

Experimental verification of EGSnrc Monte Carlo calculated depth doses within a realistic parallel magnetic field in a polystyrene phantom

et al. 2017

View full text Add to dashboard Cite

show abstract

“…These may be above the patient, or scattered from the patient on the exit side. 41,42,43,44,45,46,47,48,49,50 With MRPT, the therapeutic proton beam (being charged particles), is subject to the Lorentz deflection force when transporting towards the patient at the center of the MRI, as well as within the patient while slowing down. The planning process is best broken down into two components as described in Fig.…”

Section: B Dose Planning In Magnetic Fieldsmentioning

confidence: 99%

Future of medical physics: Real‐time MRI‐guided proton therapy

et al. 2017

View full text Add to dashboard Cite

With the recent clinical implementation of real-time MRI-guided x-ray beam therapy (MRXT), attention is turning to the concept of combining real-time MRI guidance with proton beam therapy; MRIguided proton beam therapy (MRPT). MRI guidance for proton beam therapy is expected to offer a compelling improvement to the current treatment workflow which is warranted arguably more than for x-ray beam therapy. This argument is born out of the fact that proton therapy toxicity outcomes are similar to that of the most advanced IMRT treatments, despite being a fundamentally superior particle for cancer treatment. In this Future of Medical Physics article, we describe the various software and hardware aspects of potential MRPT systems and the corresponding treatment workflow. Significant software developments, particularly focused around adaptive MRI-based planning will be required. The magnetic interaction between the MRI and the proton beamline components will be a key area of focus. For example, the modeling and potential redesign of a magnetically compatible gantry to allow for beam delivery from multiple angles towards a patient located within the bore of an MRI scanner. Further to this, the accuracy of pencil beam scanning and beam monitoring in the presence of an MRI fringe field will require modeling, testing, and potential further development to ensure that the highly targeted radiotherapy is maintained. Looking forward we envisage a clear and accelerated path for hardware development, leveraging from lessons learnt from MRXT development. Within few years, simple prototype systems will likely exist, and in a decade, we could envisage coupled systems with integrated gantries. Such milestones will be key in the development of a more efficient, more accurate, and more successful form of proton beam therapy for many common cancer sites.

show abstract

“…Intensity of the magnetic field was calculated inside and outside of this model. In the similar research, to calculate the magnetic field, other software of finite element method are used (19); however, arrangement and dimensions of the magnet and also its applications have been different. In most studies, tissue was in the magnetic field like designing the MRI-radiotherapy system; therefore, precise calculation of interactions in the tissue was necessary in the presence of the external field.…”

Section: Discussionmentioning

confidence: 99%

Simulation of therapeutic electron beam tracking through a non-uniform magnetic field using finite element method

et al. 2017

View full text Add to dashboard Cite

IntroductionIn radiotherapy, megaelectron volt (MeV) electrons are employed for treatment of superficial cancers. Magnetic fields can be used for deflection and deformation of the electron flow. A magnetic field is composed of non-uniform permanent magnets. The primary electrons are not mono-energetic and completely parallel. Calculation of electron beam deflection requires using complex mathematical methods. In this study, a device was made to apply a magnetic field to an electron beam and the path of electrons was simulated in the magnetic field using finite element method.MethodsA mini-applicator equipped with two neodymium permanent magnets was designed that enables tuning the distance between magnets. This device was placed in a standard applicator of Varian 2100 CD linear accelerator. The mini-applicator was simulated in CST Studio finite element software. Deflection angle and displacement of the electron beam was calculated after passing through the magnetic field. By determining a 2 to 5cm distance between two poles, various intensities of transverse magnetic field was created. The accelerator head was turned so that the deflected electrons became vertical to the water surface. To measure the displacement of the electron beam, EBT2 GafChromic films were employed. After being exposed, the films were scanned using HP G3010 reflection scanner and their optical density was extracted using programming in MATLAB environment. Displacement of the electron beam was compared with results of simulation after applying the magnetic field.ResultsSimulation results of the magnetic field showed good agreement with measured values. Maximum deflection angle for a 12 MeV beam was 32.9° and minimum deflection for 15 MeV was 12.1°. Measurement with the film showed precision of simulation in predicting the amount of displacement in the electron beam.ConclusionA magnetic mini-applicator was made and simulated using finite element method. Deflection angle and displacement of electron beam were calculated. With the method used in this study, a good prediction of the path of high-energy electrons was made before they entered the body.

show abstract

Skin dose in longitudinal and transverse linac‐MRIs using Monte Carlo and realistic 3D MRI field models

Cited by 58 publications

References 34 publications

Experimental verification of EGSnrc Monte Carlo calculated depth doses within a realistic parallel magnetic field in a polystyrene phantom

Experimental verification of EGSnrc Monte Carlo calculated depth doses within a realistic parallel magnetic field in a polystyrene phantom

Future of medical physics: Real‐time MRI‐guided proton therapy

Simulation of therapeutic electron beam tracking through a non-uniform magnetic field using finite element method

Contact Info

Product

Resources

About