I am an assistant professor in the Department of Human Oncology. My primary role is to provide clinical medical physics services in the Department of Radiation Oncology at the UW Health University Hospital. In the clinic, I perform tasks to ensure that patients are being treated safely and accurately on each day of their treatment. This includes making sure radiation-producing machines are operating correctly and that the quality of a patient’s treatment is maintained from the day they first arrive in our department to the day of their last treatment. I am the lead physicist for the TomoTherapy service and among the primary physics contacts for our radiosurgery program, treatment planning systems, and image processing software.
The radiotherapy process requires a diverse team of physicians, physicists, dosimetrists, therapists, nurses and others. Much of my efforts are focused on improving the ways in which this team can come together and care for patients. I have detailed knowledge of clinical operations along with the hardware and software tools we use on a daily basis. I do things like develop processes to better achieve treatment goals, learn and implement new technology, and design and execute quality assurance tests that ensure our equipment is functioning properly.
Outside the clinic, I am a rotation mentor for our Radiation Oncology Physics Residency program and occasionally teach various physics courses to residents, graduate students, and trainees. I am involved in several research projects in adaptive therapy, particle therapy, and image guidance. These projects span departments within our university as well as at other institutions. I am fortunate to serve in a position where I can combine my love for solving engineering problems with my knowledge and research in radiation physics to ensure patient safety and continually advance patient care.
Resident, University of Iowa, Radiation Oncology (2014)
Postdoctoral Fellow, Washington University in St. Louis, Radiation Oncology (2012)
PhD, University of Wisconsin–Madison, Medical Physics (2011)
MS, University of Wisconsin–Madison, Nuclear Engineering and Engineering Physics (2009)
MS, University of Wisconsin–Madison, Medical Physics (2007)
BS, University of Wisconsin–Madison, Nuclear Engineering (2005)
Assistant Professor, Human Oncology (2014–pres.)
Selected Honors and Awards
Executive Education Grant, University of Wisconsin Department of Engineering Professional Development (2016)
Physicist Training Scholarship, American Brachytherapy Society (2013)
NIH Training Fellowship (2009–2011)
Vilas Travel Grant (2009, 2010)
Tau Beta Pi Engineering Honor Society (2005)
Boards, Advisory Committees and Professional Organizations
University of Wisconsin TomoTherapy Service Improvement Committee (2017–pres.)
University of Wisconsin Medical Physics Residency Program Oversight Committee (2016–pres.)
American Association of Physicists in Medicine North Central Chapter Executive Committee (2016–pres.)
American Society for Radiation Oncology (2012–pres.)
American Association of Physicists in Medicine (2005–pres.)
Adaptive Radiotherapy, Image Registration, Informatics and Information Systems, Radiosurgery, Treatment Planning Systems, Workflow Automation
Development and validation of the Dynamic Collimation Monte Carlo simulation package for pencil beam scanning proton therapy Medical physics
Nelson NP, Culberson WS, Hyer DE, Geoghegan TJ, Patwardhan KA, Smith BR, Flynn RT, Yu J, Rana S, Gutiérrez AN, Hill PM
2021 Mar 19. doi: 10.1002/mp.14846. Online ahead of print.
PURPOSE: The aim of this work was to develop and experimentally validate a Dynamic Collimation Monte Carlo (DCMC) simulation package specifically designed for the simulation of collimators in pencil beam scanning proton therapy (PBS-PT). The DCMC package was developed using the TOPAS Monte Carlo platform and consists of a generalized PBS source model and collimator component extensions.
METHODS: A divergent point-source model of the IBA dedicated nozzle (DN) at the Miami Cancer Institute (MCI) was created and validated against on-axis commissioning measurements taken at MCI. The beamline optics were mathematically incorporated into the source to model beamlet deflections in the X and Y directions at the respective magnet planes. Off-axis measurements taken at multiple planes in air were used to validate both the off-axis spot size and divergence of the source model. The DCS trimmers were modeled and incorporated as TOPAS geometry extensions that linearly translate and rotate about the bending magnets. To validate the collimator model, a series of integral depth dose (IDD) and lateral profile measurements were acquired at MCI and used to benchmark the DCMC performance for modeling both pristine and range shifted beamlets. The water equivalent thickness (WET) of the range shifter was determined by quantifying the shift in the depth of the 80% dose point distal to the Bragg peak between the range shifted and pristine uncollimated beams.
RESULTS: A source model of the IBA DN system was successfully commissioned against on- and off-axis IDD and lateral profile measurements performed at MCI. The divergence of the source model was matched through an optimization of the source-to-axis distance and comparison against in-air spot profiles. The DCS model was then benchmarked against collimated IDD and in-air and in-phantom lateral profile measurements. Gamma analysis was used to evaluate the agreement between measured and simulated lateral profiles and IDDs with 1%/1 mm criteria and a 1% dose threshold. For the pristine collimated beams, the average 1%/1 mm gamma pass rates across all collimator configurations investigated were 99.8% for IDDs and 97.6% and 95.2% for in-air and in-phantom lateral profiles. All range shifted collimated IDDs passed at 100% while in-air and in-phantom lateral profiles had average pass rates of 99.1% and 99.8%, respectively. The measured and simulated WET of the polyethylene range shifter was determined to be 40.9 and 41.0 mm, respectively.
CONCLUSIONS: We have developed a TOPAS-based Monte Carlo package for modeling collimators in PBS-PT. This package was then commissioned to model the IBA DN system and DCS located at MCI using both uncollimated and collimated measurements. Validation results demonstrate that the DCMC package can be used to accurately model other aspects of a DCS implementation via simulation.
PMID:33740253 | DOI:10.1002/mp.14846
View details for PubMedID 33740253
Investigation of tumor and vessel motion correlation in the liver Journal of applied clinical medical physics
Jupitz SA, Shepard AJ, Hill PM, Bednarz BP
2020 Aug;21(8):183-190. doi: 10.1002/acm2.12943. Epub 2020 Jun 13.
Intrafraction imaging-based motion management systems for external beam radiotherapy can rely on internal surrogate structures when the target is not easily visualized. This work evaluated the validity of using liver vessels as internal surrogates for the estimation of liver tumor motion. Vessel and tumor motion were assessed using ten two-dimensional sagittal MR cine datasets collected on the ViewRay MRIdian. For each case, a liver tumor and at least one vessel were tracked for 175 s. A tracking approach utilizing block matching and multiple simultaneous templates was applied. Accuracy of the tracked motion was calculated from the error between the tracked centroid position and manually defined ground truth annotations. The patient's abdomen surface and diaphragm were manually annotated in all frames. The Pearson correlation coefficient (CC) was used to compare the motion of the features and tumor in the anterior-posterior (AP) and superior-inferior (SI) directions. The distance between the centroids of the features and the tumors was calculated to assess if feature proximity affects relative correlation, and the tumor range of motion was determined. Intra- and interfraction motion amplitude variabilities were evaluated to further assess the relationship between tumor and feature motion. The mean CC between the motion of the vessel and the tumor were 0.85 ± 0.11 (AP) and 0.92 ± 0.04 (SI), 0.83 ± 0.11 (AP) and -0.89 ± 0.06 (SI) for the surface and tumor, and 0.80 ± 0.17 (AP) and 0.94 ± 0.03 (SI) for the diaphragm and tumor. For intrafraction analysis, the average amplitude variability was 2.47 ± 0.77 mm (AP) and 3.14 ± 1.49 mm (SI) for the vessels, 2.70 ± 1.08 mm (AP) and 3.43 ± 1.73 mm (SI) for the surface, and 2.76 ± 1.41 mm (AP) and 2.91 ± 1.38 mm (SI) for the diaphragm. No relationship between distance and motion correlation was observed. The motion of liver tumors and liver vessels was well correlated, making vessels a suitable surrogate for tumor motion in the liver.
PMID:32533758 | PMC:PMC7484818 | DOI:10.1002/acm2.12943
View details for PubMedID 32533758
Design of a focused collimator for proton therapy spot scanning using Monte Carlo methods Medical physics
Geoghegan TJ, Nelson NP, Flynn RT, Hill PM, Rana S, Hyer DE
2020 Jul;47(7):2725-2734. doi: 10.1002/mp.14139. Epub 2020 Apr 6.
PURPOSE: When designing a collimation system for pencil beam spot scanning proton therapy, a decision must be made whether or not to rotate, or focus, the collimator to match beamlet deflection as a function of off-axis distance. If the collimator is not focused, the beamlet shape and fluence will vary as a function of off-axis distance due to partial transmission through the collimator. In this work, we quantify the magnitude of these effects and propose a focused dynamic collimation system (DCS) for use in proton therapy spot scanning.
METHODS: This study was done in silico using a model of the Miami Cancer Institute's (MCI) IBA Proteus Plus system created in Geant4-based TOPAS. The DCS utilizes rectangular nickel trimmers mounted on rotating sliders that move in synchrony with the pencil beam to provide focused collimation at the edge of the target. Using a simplified setup of the DCS, simulations were performed at various off-axis locations corresponding to beam deflection angles ranging from 0° to 2.5°. At each off-axis location, focused (trimmer rotated) and unfocused (trimmer not rotated) simulations were performed. In all simulations, a 4 cm water equivalent thickness range shifter was placed upstream of the collimator, and a voxelized water phantom that scored dose was placed downstream, each with 4 cm airgaps.
RESULTS: Increasing the beam deflection angle for an unfocused trimmer caused the collimated edge of the beamlet profile to shift 0.08-0.61 mm from the baseline 0° simulation. There was also an increase in low-dose regions on the collimated edge ranging from 14.6% to 192.4%. Lastly, the maximum dose, D max , was 0-5% higher for the unfocused simulations. With a focused trimmer design, the profile shift and dose increases were all eliminated.
CONCLUSIONS: We have shown that focusing a collimator in spot scanning proton therapy reduces dose at the collimated edge compared to conventional, unfocused collimation devices and presented a simple, mechanical design for achieving focusing for a range of source-to-collimator distances.
PMID:32170750 | PMC:PMC7375903 | DOI:10.1002/mp.14139
View details for PubMedID 32170750
Validation of an MR-guided online adaptive radiotherapy (MRgoART) program: Deformation accuracy in a heterogeneous, deformable, anthropomorphic phantom Radiotherapy and oncology : journal of the European Society for Therapeutic Radiology and Oncology
Mittauer KE, Hill PM, Bassetti MF, Bayouth JE
2020 May;146:97-109. doi: 10.1016/j.radonc.2020.02.012. Epub 2020 Mar 6.
BACKGROUND AND PURPOSE: To investigate deformable image registration (DIR) and multi-fractional dose accumulation accuracy of a clinical MR-guided online adaptive radiotherapy (MRgoART) program, utilizing clinically-based magnitudes of abdominal deformation vector fields (DVFs).
MATERIALS AND METHODS: A heterogeneous anthropomorphic multi-modality abdominal deformable phantom was comprised of MR and CT anatomically-relevant materials. Thermoluminescent dosimeters (TLDs) were affixed within regions of interest (ROIs). CT and MR simulation scans were acquired. CT was deformed to MR for dose calculations. MRgoART was executed on a MR-linac (MRIdian) for 5 Gy/5 fractions. Before each fraction, a deformation was applied. Ground truth was known for ROI volume, TLD position, and TLD dose measured by an accredited dosimetry calibration laboratory. To validate the range of applied deformations, phantom DVFs were compared to DVFs of clinical abdominal MRgoART fractions. MR-MR deformation accuracy was quantified through dice similarity coefficient (DSC), Hausdorff distance (HD), mean distance-to-agreement (MDA), and as mean-absolute-error (MAE) for CT-MR-MR deformation. Arithmetic-summation of calculated dose at respective TLD positions and deform-accumulated dose (MIM) was compared to TLD measured dose, respectively. MR-MR deformation statistics were quantified for MRIdian and MIM.
RESULTS: Mean phantom DVFs were 5.0 ± 2.9 mm compared to mean DVF of clinical abdominal patients at 5.2 ± 3.0 mm. Respective mean DSC, HD, MDA was 0.93 ± 0.03, 0.74 ± 0.80 cm, 0.08 ± 0.03 cm for MRIdian and 0.93 ± 0.03, 0.54 ± 0.27 cm, 0.08 ± 0.03 cm for MIM (N = 80 ROIs). Mean MAE was 20.5 HU. Respective mean and median dose differences were 0.3%, -0.3% for arithmetic-summation and 4.1%, 0.6% for deformed-accumulation. Maximum differences were 0.21 Gy (arithmetic-summation), 0.31 Gy (deformed-accumulation).
CONCLUSIONS: MRgoART deformation and dosimetric accuracy has been benchmarked for mean fractional DVFs of 5 mm in a multiple-rigid-body deformable phantom. Deformation accuracy was within TG132 criteria and clinically acceptable end-to-end MRgoART dosimetric agreement was observed for this phantom. Further efforts are needed in validation of deform-accumulated dose.
PMID:32146260 | DOI:10.1016/j.radonc.2020.02.012
View details for PubMedID 32146260
STAT-ART: The Promise and Practice of a Rapid Palliative Single Session of MR-Guided Online Adaptive Radiotherapy (ART) Frontiers in oncology
Mittauer KE, Hill PM, Geurts MW, Costa AD, Kimple RJ, Bassetti MF, Bayouth JE
2019 Oct 22;9:1013. doi: 10.3389/fonc.2019.01013. eCollection 2019.
This work describes a novel application of MR-guided online adaptive radiotherapy (MRgoART) in the management of patients whom urgent palliative care is indicated using statum-adaptive radiotherapy (STAT-ART). The implementation of STAT-ART, as performed at our institution, is presented including a discussion of the advantages and limitations compared to the standard of care for palliative radiotherapy on conventional c-arm linacs. MR-based treatment planning techniques of STAT-ART for density overrides and deformable image registration (DIR) of diagnostic CT to the treatment MR are also addressed.
PMID:31696053 | PMC:PMC6817496 | DOI:10.3389/fonc.2019.01013
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Trimmer sequencing time minimization during dynamically collimated proton therapy using a colony of cooperating agents Physics in medicine and biology
Smith BR, Hyer DE, Flynn RT, Hill PM, Culberson WS
2019 Oct 21;64(20):205025. doi: 10.1088/1361-6560/ab416d.
The dynamic collimation system (DCS) can be combined with pencil beam scanning proton therapy to deliver highly conformal treatment plans with unique collimation at each energy layer. This energy layer-specific collimation is accomplished through the synchronized motion of four trimmer blades that intercept the proton beam near the target boundary in the beam's eye view. However, the corresponding treatment deliveries come at the cost of additional treatment time since the translational speed of the trimmer is slower than the scanning speed of the proton pencil beam. In an attempt to minimize the additional trimmer sequencing time of each field while still maintaining a high degree of conformity, a novel process utilizing ant colony optimization (ACO) methods was created to determine the most efficient route of trimmer sequencing and beamlet scanning patterns for a collective set of collimated proton beamlets. The ACO process was integrated within an in-house treatment planning system optimizer to determine the beam scanning and DCS trimmer sequencing patterns and compared against an analytical approximation of the trimmer sequencing time should a contour-like scanning approach be assumed instead. Due to the stochastic nature of ACO, parameters where determined so that they could ensure good convergence and an efficient optimization of trimmer sequencing that was faster than an analytical contour-like trimmer sequencing. The optimization process was tested using a set of three intracranial treatment plans which were planned using a custom research treatment planning system and were successfully optimized to reduce the additional trimmer sequencing time to approximately 60 s per treatment field while maintaining a high degree of target conformity. Thus, the novel use of ACO techniques within a treatment planning algorithm has been demonstrated to effectively determine collimation sequencing patterns for a DCS in order to minimize the additional treatment time required for trimmer movement during treatment.
PMID:31484170 | PMC:PMC6995666 | DOI:10.1088/1361-6560/ab416d
View details for PubMedID 31484170
Secondary Neutron Dose From a Dynamic Collimation System During Intracranial Pencil Beam Scanning Proton Therapy: A Monte Carlo Investigation International journal of radiation oncology, biology, physics
Smith BR, Hyer DE, Hill PM, Culberson WS
2019 Jan 1;103(1):241-250. doi: 10.1016/j.ijrobp.2018.08.012. Epub 2018 Aug 14.
PURPOSE: Patients receiving pencil beam scanning (PBS) proton therapy with the addition of a dynamic collimation system (DCS) are potentially subject to an additional neutron dose from interactions between the incident proton beam and the trimmer blades. This study investigates the secondary neutron dose rates for both single-field uniform dose (SFUD) and intensity modulated proton therapy treatments.
METHODS AND MATERIALS: Secondary neutron dose distributions were calculated for both a dynamically collimated and an uncollimated, dual-field chordoma treatment plan and compared with previously published neutron dose rates from other contemporary scanning treatment modalities. Monte Carlo N-Particle transport code was used to track all primary and secondary particles generated from nuclear reactions within the DCS during treatment through a model of the patient geometry acquired from the computed tomography planning data set. Secondary neutron ambient dose equivalent distributions were calculated throughout the patient using a meshgrid with a tally resolution equivalent to that of the treatment planning computed tomography.
RESULTS: The median healthy-brain neutron ambient dose equivalent for a dynamically collimated intracranial chordoma treatment plan using a DCS was found to be 0.97 mSv/Gy for the right lateral SFUD field, 1.37 mSv/Gy for the apex SFUD field, and 1.24 mSv/Gy for the composite intensity modulated proton therapy distribution from 2 fields.
CONCLUSIONS: These results were at least 55% lower than what has been reported for uniform scanning modalities with brass apertures. However, they still reflect an increase in the excess relative risk of secondary cancer incidence compared with an uncollimated PBS treatment using only a graphite range shifter. Regardless, the secondary neutron dose expected from the DCS for these PBS proton therapy treatments appears to be on the order of, or below, what is expected for alternative collimated proton therapy techniques.
PMID:30114462 | DOI:10.1016/j.ijrobp.2018.08.012
View details for PubMedID 30114462
A New Era of Image Guidance with Magnetic Resonance-guided Radiation Therapy for Abdominal and Thoracic Malignancies Cureus
Mittauer K, Paliwal B, Hill P, Bayouth JE, Geurts MW, Baschnagel AM, Bradley KA, Harari PM, Rosenberg S, Brower JV, Wojcieszynski AP, Hullett C, Bayliss RA, Labby ZE, Bassetti MF
2018 Apr 4;10(4):e2422. doi: 10.7759/cureus.2422.
Magnetic resonance-guided radiation therapy (MRgRT) offers advantages for image guidance for radiotherapy treatments as compared to conventional computed tomography (CT)-based modalities. The superior soft tissue contrast of magnetic resonance (MR) enables an improved visualization of the gross tumor and adjacent normal tissues in the treatment of abdominal and thoracic malignancies. Online adaptive capabilities, coupled with advanced motion management of real-time tracking of the tumor, directly allow for high-precision inter-/intrafraction localization. The primary aim of this case series is to describe MR-based interventions for localizing targets not well-visualized with conventional image-guided technologies. The abdominal and thoracic sites of the lung, kidney, liver, and gastric targets are described to illustrate the technological advancement of MR-guidance in radiotherapy.
PMID:29872602 | PMC:PMC5985918 | DOI:10.7759/cureus.2422
View details for PubMedID 29872602
Long-term dosimetric stability of multiple TomoTherapy delivery systems Journal of applied clinical medical physics
Smilowitz JB, Dunkerley D, Hill PM, Yadav P, Geurts MW
2017 May;18(3):137-143. doi: 10.1002/acm2.12085. Epub 2017 May 2.
The dosimetric stability of six TomoTherapy units was analyzed to investigate changes in performance over time and with system upgrades. Energy and output were tracked using monitor chamber signal, onboard megavoltage computed tomography (MVCT) detector profile, and external ion chamber measurements. The systems (and monitoring periods) include three Hi-Art (67, 61, and 65 mos.), two TomoHDA (31 and 26 mos.), and one Radixact unit (11 mos.), representing approximately 10 years of clinical use. The four newest systems use the Dose Control Stability (DCS) system and Fixed Target Linear Accelerator (linac) (FTL). The output stability is reported as deviation from reference monitor chamber signal for all systems and/or from an external chamber signal. The energy stability was monitored using relative (center versus off-axis) MVCT detector signal (beam profile) and/or the ratio of chamber measurements at 2 depths. The clinical TomoHDA data were used to benchmark the Radixact stability, which has the same FTL but runs at a higher dose rate. The output based on monitor chamber data of all systems is very stable. The standard deviation of daily output on the non-DCS systems was 0.94-1.52%. As expected, the DCS systems had improved standard deviation: 0.004-0.06%. The beam energy was also very stable for all units. The standard deviation in profile flatness was 0.23-0.62% for rotating target systems and 0.04-0.09% for FTL. Ion chamber output and PDD ratios supported these results. The output stability on the Radixact system during extended treatment delivery (20, 30, and 40 min) was comparable to a clinical TomoHDA system. For each system, results are consistent between different measurement tools and techniques, proving not only the dosimetric stability, but also these quality parameters can be confirmed with various metrics. The replacement history over extended time periods of the major dosimetric components of the different delivery systems (target, linac, and magnetron) is also reported.
PMID:28464517 | PMC:PMC5689853 | DOI:10.1002/acm2.12085
View details for PubMedID 28464517
Dosimetric Comparison of Real-Time MRI-Guided Tri-Cobalt-60 Versus Linear Accelerator-Based Stereotactic Body Radiation Therapy Lung Cancer Plans Technology in cancer research & treatment
Wojcieszynski AP, Hill PM, Rosenberg SA, Hullett CR, Labby ZE, Paliwal B, Geurts MW, Bayliss RA, Bayouth JE, Harari PM, Bassetti MF, Baschnagel AM
2017 Jun;16(3):366-372. doi: 10.1177/1533034617691407. Epub 2017 Feb 7.
PURPOSE: Magnetic resonance imaging-guided radiation therapy has entered clinical practice at several major treatment centers. Treatment of early-stage non-small cell lung cancer with stereotactic body radiation therapy is one potential application of this modality, as some form of respiratory motion management is important to address. We hypothesize that magnetic resonance imaging-guided tri-cobalt-60 radiation therapy can be used to generate clinically acceptable stereotactic body radiation therapy treatment plans. Here, we report on a dosimetric comparison between magnetic resonance imaging-guided radiation therapy plans and internal target volume-based plans utilizing volumetric-modulated arc therapy.
MATERIALS AND METHODS: Ten patients with early-stage non-small cell lung cancer who underwent radiation therapy planning and treatment were studied. Following 4-dimensional computed tomography, patient images were used to generate clinically deliverable plans. For volumetric-modulated arc therapy plans, the planning tumor volume was defined as an internal target volume + 0.5 cm. For magnetic resonance imaging-guided plans, a single mid-inspiratory cycle was used to define a gross tumor volume, then expanded 0.3 cm to the planning tumor volume. Treatment plan parameters were compared.
RESULTS: Planning tumor volumes trended larger for volumetric-modulated arc therapy-based plans, with a mean planning tumor volume of 47.4 mL versus 24.8 mL for magnetic resonance imaging-guided plans ( P = .08). Clinically acceptable plans were achievable via both methods, with bilateral lung V20, 3.9% versus 4.8% ( P = .62). The volume of chest wall receiving greater than 30 Gy was also similar, 22.1 versus 19.8 mL ( P = .78), as were all other parameters commonly used for lung stereotactic body radiation therapy. The ratio of the 50% isodose volume to planning tumor volume was lower in volumetric-modulated arc therapy plans, 4.19 versus 10.0 ( P < .001). Heterogeneity index was comparable between plans, 1.25 versus 1.25 ( P = .98).
CONCLUSION: Magnetic resonance imaging-guided tri-cobalt-60 radiation therapy is capable of delivering lung high-quality stereotactic body radiation therapy plans that are clinically acceptable as compared to volumetric-modulated arc therapy-based plans. Real-time magnetic resonance imaging provides the unique capacity to directly observe tumor motion during treatment for purposes of motion management.
PMID:28168936 | PMC:PMC5616053 | DOI:10.1177/1533034617691407
View details for PubMedID 28168936
Gadoxetate for direct tumor therapy and tracking with real-time MRI-guided stereotactic body radiation therapy of the liver Radiotherapy and oncology : journal of the European Society for Therapeutic Radiology and Oncology
Wojcieszynski AP, Rosenberg SA, Brower JV, Hullett CR, Geurts MW, Labby ZE, Hill PM, Bayliss RA, Paliwal B, Bayouth JE, Harari PM, Bassetti MF
2016 Feb;118(2):416-8. doi: 10.1016/j.radonc.2015.10.024. Epub 2015 Nov 25.
SBRT is increasingly utilized in liver tumor treatment. MRI-guided RT allows for real-time MRI tracking during therapy. Liver tumors are often poorly visualized and most contrast agents are transient. Gadoxetate may allow for sustained tumor visualization. Here, we report on the first use of gadoxetate during real-time MRI-guided SBRT.
PMID:26627702 | DOI:10.1016/j.radonc.2015.10.024
View details for PubMedID 26627702
A method for modeling laterally asymmetric proton beamlets resulting from collimation Medical physics
Gelover E, Wang D, Hill PM, Flynn RT, Gao M, Laub S, Pankuch M, Hyer DE
2015 Mar;42(3):1321-34. doi: 10.1118/1.4907965.
PURPOSE: To introduce a method to model the 3D dose distribution of laterally asymmetric proton beamlets resulting from collimation. The model enables rapid beamlet calculation for spot scanning (SS) delivery using a novel penumbra-reducing dynamic collimation system (DCS) with two pairs of trimmers oriented perpendicular to each other.
METHODS: Trimmed beamlet dose distributions in water were simulated with MCNPX and the collimating effects noted in the simulations were validated by experimental measurement. The simulated beamlets were modeled analytically using integral depth dose curves along with an asymmetric Gaussian function to represent fluence in the beam's eye view (BEV). The BEV parameters consisted of Gaussian standard deviations (sigmas) along each primary axis (σ(x1),σ(x2),σ(y1),σ(y2)) together with the spatial location of the maximum dose (μ(x),μ(y)). Percent depth dose variation with trimmer position was accounted for with a depth-dependent correction function. Beamlet growth with depth was accounted for by combining the in-air divergence with Hong's fit of the Highland approximation along each axis in the BEV.
RESULTS: The beamlet model showed excellent agreement with the Monte Carlo simulation data used as a benchmark. The overall passing rate for a 3D gamma test with 3%/3 mm passing criteria was 96.1% between the analytical model and Monte Carlo data in an example treatment plan.
CONCLUSIONS: The analytical model is capable of accurately representing individual asymmetric beamlets resulting from use of the DCS. This method enables integration of the DCS into a treatment planning system to perform dose computation in patient datasets. The method could be generalized for use with any SS collimation system in which blades, leaves, or trimmers are used to laterally sharpen beamlets.
PMID:25735287 | PMC:PMC5360162 | DOI:10.1118/1.4907965
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Patrick Hill, PhD
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