B. Clasie

Massachusetts General Hospital, Boston, Massachusetts, United States

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Publications (26)67.19 Total impact

  • N. Depauw · B. Clasie · T. Madden · A. Rosenfeld · H. Kooy
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    ABSTRACT: This work presents the CPU implementation of GMC ['gimik]: a fast yet accurate one-variable Monte Carlo dose algorithm for proton therapy to be incorporated into our in-house treatment planning system, Astroid. GMC is based on a simple mathematical model using the formulated proton scattering power and tabulated data of empirical depth-dose distributions. These Bragg peaks determine the energy deposited along the particle's track. The polar scattering angle is based on the particle's local energy and the voxel's density, while the azimuthal component of that scattering angle is the single variable in GMC, uniformly distributed from 0 to 2π. The halo effect of the beam, currently not implemented, will consider large scattering angles and secondary protons for a small percentage of the incident histories. GMC shows strong agreement with both the empirical data and GEANT4-based simulations. Its current CPU implementation runs at ~300 m.s−-1, approximately ten times faster than GEANT4. Significant speed improvement is expected with the upcoming implementation of multi-threading and the portage to the GPU architecture. In conclusion, a one-variable Monte Carlo dose algorithm was produced for proton therapy dose computations. Its simplicity allows for fast dose computation while conserving accuracy against heterogeneities, hence drastically improving the current algorithms used in treatment planning systems.
    Journal of Physics Conference Series 02/2014; 489(1). DOI:10.1088/1742-6596/489/1/012010
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    ABSTRACT: Purpose: To verify a clinical pencil (PB) beam dose calculation algorithm for scanned beam intensity modulated proton therapy (IMPT), using TOPAS (TOol for PArticle Simulation), a GEANT4 based Monte Carlo (MC) simulation system. Methods: Seven patients, previously treated with IMPT for various treatment sites and prescriptions, were selected from our patient database. Proton fluence maps of the treated plans were exported for each field from our clinical treatment planning system (ASTROID) and imported to TOPAS along with the patient and beam geometry. The absolute dose distribution of each individual beam was calculated and compared to the PB algorithm‐based calculation from ASTROID. Results: The differences observed in mean and median target doses were less than ±1% for all cases, while D02 and D98 (surrogates for maximum and minimum dose values respectively) differed by less than ±3% for the majority of beams. Differences in the mean dose for the organs at risk (OARs) ranged from −8.9% to 3.7%, with reference to MC calculation, with an average over all the OARs of −0.1%, indicating no systematic over‐or under‐estimation of the dose by the PB algorithm. 3D gamma analysis (2%/2mm) for the PB to MC dose comparison resulted in an average 95.2% (±5.0) of the target volume having an absolute gamma value equal or less than 1 and 99.2% (±1.2%) equal or less than 2. For the healthy tissue receiving at least 5% of the target mean dose, the corresponding percentages were 99.6% (±0.3%) and 99.9% (±0.1%). Conclusion: We have clinically implemented MC for IMPT plan recalculation. Our PB calculation algorithm for IMPT was found to be in overall good agreement with MC calculations. Clinically significant deviations in OAR mean dose can be attributed to lung tissue or bone anatomy in the beam path.
    Medical Physics 06/2013; 40(6):323. DOI:10.1118/1.4814941 · 3.01 Impact Factor
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    ABSTRACT: In order to quantify the dose from neutrons to a patient for contemporary radiation treatment techniques, measurements inside phantoms, representing the patient, are necessary. Published reports on neutron dose measurements cover measurements performed free in air or on the surface of phantoms and the doses are expressed in terms of personal dose equivalent or ambient dose equivalent. This study focuses on measurements of local neutron doses inside a radiotherapy phantom and presents a field calibration procedure for PADC track etch detectors. An initial absolute calibration factor in terms of Hp(10)Hp(10) for personal dosimetry is converted into neutron dose equivalent and additional calibration factors are derived to account for the spectral changes in the neutron fluence for different radiation therapy beam qualities and depths in the phantom. The neutron spectra used for the calculation of the calibration factors are determined in different depths by Monte Carlo simulations for the investigated radiation qualities. These spectra are used together with the energy dependent response function of the PADC detectors to account for the spectral changes in the neutron fluence. The resulting total calibration factors are 0.76 for a photon beam (in- and out-of-field), 1.00 (in-field) and 0.84 (out-of-field) for an active proton beam and 1.05 (in-field) and 0.91 (out-of-field) for a passive proton beam, respectively. The uncertainty for neutron dose measurements using this field calibration method is less than 40%.The extended calibration procedure presented in this work showed that it is possible to use PADC track etch detectors for measurements of local neutron dose equivalent inside anthropomorphic phantoms by accounting for spectral changes in the neutron fluence.
    Nuclear Instruments and Methods in Physics Research Section A Accelerators Spectrometers Detectors and Associated Equipment 12/2012; 694:205–210. DOI:10.1016/j.nima.2012.08.021 · 1.32 Impact Factor
  • Benjamin M Clasie · Jacob B Flanz · Hanne M Kooy
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    ABSTRACT: Treatment planning databases for pencil beam scanning can be large, difficult to manage and problematic for quality assurance when they contain tabulated Bragg peaks at small range resolution. Smaller range resolution, in the absence of an accurate interpolation method, improves the accuracy in dose calculations. In this work, we derive an approximate scaling function to interpolate between tabulated Bragg peaks, and determine the accuracy of this interpolation technique and the minimum number of tabulated peaks in a treatment planning database. With the new interpolation technique, three tabulated mono-energetic Bragg peaks (N = 3) are a suitable lower limit for N to achieve interpolation accuracy better than ±1% of the maximum dose in pristine and spread out Bragg peaks for ranges between 6.8 and 32.1 cm of water.
    Physics in Medicine and Biology 10/2012; 57(21):N405-9. DOI:10.1088/0031-9155/57/21/N405 · 2.92 Impact Factor
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    Benjamin M Clasie · Gregory C Sharp · Joao Seco · Jacob B Flanz · Hanne M Kooy
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    ABSTRACT: The γ-index is used routinely to establish correspondence between two dose distributions. The definition of the γ-index can be written with a single equation but solving this equation at millions of points is computationally expensive, especially in three dimensions. Our goal is to extend the vector-equation method in Bakai et al (2003 Phys. Med. Biol.48 3543-53) to higher order for better accuracy and, as important, to determine the magnitude of accuracy in a higher order solution. We construct a numerical framework for calculating the γ-index in two and three dimensions and present an efficient method for calculating the γ-index with zeroth-, first- and second-order methods using tricubic spline interpolation. For an intensity-modulated radiation therapy example with 1.78 × 10(6) voxels, the zeroth-order, first-order, first-order iterations and semi-second-order methods calculate the three-dimensional γ-index in 1.5, 4.7, 34.7 and 35.6 s with 36.7%, 1.1%, 0.2% and 0.8% accuracy, respectively. The accuracy of linear interpolation with this example is 1.0%. We present efficient numerical methods for calculating the three-dimensional γ-index with tricubic spline interpolation. The first-order method with iterations is the most accurate and fastest choice of the numerical methods if the dose distributions may have large second-order gradients. Furthermore, the difference between iterations can be used to determine the accuracy of the method.
    Physics in Medicine and Biology 10/2012; 57(21):6981-97. DOI:10.1088/0031-9155/57/21/6981 · 2.92 Impact Factor
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    ABSTRACT: Purpose: Patient specific apertures are commonly employed in passive double scattering (DS) proton therapy (PT). This study was aimed at identifying the potential benefits of using such an aperture in pencil beam scanning (PBS). Methods: An accurate Geant4 Monte Carlo model of the PBS PT treatment head at Massachusetts General Hospital (MGH) was developed based on an existing model of the passive double-scattering (DS) system. The Monte Carlo code specifies the treatment head at MGH with sub-millimeter accuracy and was configured based on the results of experimental measurements performed at MGH. This model was then used to compare out-of-field doses in simulated DS treatments and PBS treatments. The PBS treatments were simulated both with and without the patient-specific aperture used in the DS treatment. Results: For the conditions explored, a typical prostate field, the lateral penumbra in PBS is wider than in DS, leading to higher absorbed doses and equivalent doses adjacent to the primary field edge. For lateral distances greater than 10cm from the field edge, the doses in PBS appear to be lower than those observed for DS. Including an aperture at nozzle exit reduces the penumbral width by preventing wide-angle scatter from reaching the patient. This can reduce the dose in PBS for lateral distances of less than 10cm from the field edge by over an order of magnitude and allow better dose conformity. Conclusions: Placing a patient-specific aperture at nozzle exit during PBS treatments can potentially reduce doses lateral to the primary radiation field by over an order of magnitude. This has the potential to further improve the normal tissue sparing capabilities of PBS. The magnitude of this effect depends on the beam spot size of the scanning system and is thus facility dependent.
    Medical Physics 06/2012; 39(6):3872. DOI:10.1118/1.4735808 · 3.01 Impact Factor
  • Radiotherapy and Oncology 05/2012; 103:S608-S609. DOI:10.1016/S0167-8140(12)71920-5 · 4.86 Impact Factor
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    ABSTRACT: This study is aimed at identifying the potential benefits of using a patient-specific aperture in proton beam scanning. For this purpose, an accurate Monte Carlo model of the pencil beam scanning (PBS) proton therapy (PT) treatment head at Massachusetts General Hospital (MGH) was developed based on an existing model of the passive double-scattering (DS) system. The Monte Carlo code specifies the treatment head at MGH with sub-millimeter accuracy. The code was configured based on the results of experimental measurements performed at MGH. This model was then used to compare out-of-field doses in simulated DS treatments and PBS treatments. For the conditions explored, the penumbra in PBS is wider than in DS, leading to higher absorbed doses and equivalent doses adjacent to the primary field edge. For lateral distances greater than 10 cm from the field edge, the doses in PBS appear to be lower than those observed for DS. We found that placing a patient-specific aperture at nozzle exit during PBS treatments can potentially reduce doses lateral to the primary radiation field by over an order of magnitude. In conclusion, using a patient-specific aperture has the potential to further improve the normal tissue sparing capabilities of PBS.
    Physics in Medicine and Biology 04/2012; 57(10):2829-42. DOI:10.1088/0031-9155/57/10/2829 · 2.92 Impact Factor
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    ABSTRACT: Proton, as well as other ion, beams applied by electro-magnetic deflection in pencil-beam scanning (PBS) are minimally perturbed and thus can be quantified a priori by their fundamental interactions in a medium. This a priori quantification permits an optimal reduction of characterizing measurements on a particular PBS delivery system. The combination of a priori quantification and measurements will then suffice to fully describe the physical interactions necessary for treatment planning purposes. We consider, for proton beams, these interactions and derive a 'Golden' beam data set. The Golden beam data set quantifies the pristine Bragg peak depth-dose distribution in terms of primary, multiple Coulomb scatter, and secondary, nuclear scatter, components. The set reduces the required measurements on a PBS delivery system to the measurement of energy spread and initial phase space as a function of energy. The depth doses are described in absolute units of Gy(RBE) mm² Gp⁻¹, where Gp equals 10⁹ (giga) protons, thus providing a direct mapping from treatment planning parameters to integrated beam current. We used these Golden beam data on our PBS delivery systems and demonstrated that they yield absolute dosimetry well within clinical tolerance.
    Physics in Medicine and Biology 03/2012; 57(5):1147-58. DOI:10.1088/0031-9155/57/5/1147 · 2.92 Impact Factor
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    ABSTRACT: In proton therapy neutrons are introduced to out-of-field regions inside the patient. Clinicians would like to know the absorbed dose being deposited by neutrons separately to that from protons, so as to be able to directly apply their own dose equivalent weighting factors based on their opinion of the biological risk posed by neutrons in this region. The purpose of this study is to investigate a novel approach to experimentally separating the proton and neutron contributions to the absorbed dose in out-of-field regions. The method pairs specially designed silicon PIN diodes with a standard clinical ionization chamber. The sensitivity of the Si diode to non-ionizing energy losses in silicon is exploited, and can be quantified by measuring the shift in forward voltage for a fixed injection current, pre and post irradiation. The mathematical relations that describe the response of the diode and the ionization chamber can be solved simultaneously to give the contributions to the absorbed dose from protons and neutrons separately. Experimental measurements were made at the Loma Linda University Medical Center (LLUMC), Loma Linda, and Massachusetts General Hospital (MGH), Boston, proton therapy facilities. Experimental separation of the partial proton and neutron contributions to the absorbed dose measured at positions lateral to a typical prostate therapy treatment field delivered to a Lucite phantom was successfully performed and compared with results from a GEANT4 simulation. The experimental results matched well with simulation confirming the validity and promise of the novel approach.
    Radiation Measurements 12/2011; 46(12):1368-42. DOI:10.1016/j.radmeas.2011.05.022 · 1.14 Impact Factor
  • Benjamin Clasie · Harald Paganetti · Hanne Kooy
  • Medical Physics 01/2011; 38(6):3657-. DOI:10.1118/1.3612685 · 3.01 Impact Factor
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    ABSTRACT: The aim of this manuscript is to describe the direct measurement of absolute absorbed dose to water in a scanned proton radiotherapy beam using a water calorimeter primary standard. The McGill water calorimeter, which has been validated in photon and electron beams as well as in HDR 192Ir brachytherapy, was used to measure the absorbed dose to water in double scattering and scanning proton irradiations. The measurements were made at the Massachusetts General Hospital proton radiotherapy facility. The correction factors in water calorimetry were numerically calculated and various parameters affecting their magnitude and uncertainty were studied. The absorbed dose to water was compared to that obtained using an Exradin T1 Chamber based on the IAEA TRS-398 protocol. The overall 1-sigma uncertainty on absorbed dose to water amounts to 0.4% and 0.6% in scattered and scanned proton water calorimetry, respectively. This compares to an overall uncertainty of 1.9% for currently accepted IAEA TRS-398 reference absorbed dose measurement protocol. The absorbed dose from water calorimetry agrees with the results from TRS-398 well to within 1-sigma uncertainty. This work demonstrates that a primary absorbed dose standard based on water calorimetry is feasible in scattered and scanned proton beams.
    Medical Physics 07/2010; 37(7):3541-50. DOI:10.1118/1.3427317 · 3.01 Impact Factor
  • B. Clasie · T. Madden · Hm Lu · K. Zhang · J. Flanz · H. Kooy
    Medical Physics 06/2010; 37(6). DOI:10.1118/1.3468855 · 3.01 Impact Factor
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    ABSTRACT: Purpose: To develop a novel primary standard for absorbed dose to water DW measurement in scanning proton beams based on 4°C stagnant water calorimetry. Method and Materials: An in‐house built Domen‐type water calorimeter was used to measure the absolute DW directly in water at isocenter in a relatively flat SOBP region (peak‐to‐trough variation < 0.25%) of a scattered (250 MeV proton) and a scanned (15 layers painted with proton energies between 128–150 MeV) proton beam. The heat loss correction factor, defined as the ratio of the temperature in the calorimeter under ideal conditions to realistic conditions, was numerically calculated using COMSOL MULTIPHYSICS™. The calorimeter was modeled, proton beam scanning was simulated, and a time‐dependent heat‐transport module was used. The calorimetry measurements were compared to dose results from an Exradin T1 Mini Shonka following TRS‐398 protocol. Results: The heat loss correction factor was calculated to be 0.4% in scattered and 4.7% in scanned beams. We calculated the total 1‐sigma uncertainty on dose measurements to be 0.38% (scattered) and 0.64% (scanned). This was in contrast to a 1.86% uncertainty on the dose measured using the ionization chamber. The chamber dose results agreed with the absolute dose to water measurements made using water calorimetry well to within uncertainty: A difference of 0.14% in scattered beams and 0.32% in scanned beams was observed between the two techniques. Conclusions: Feasibility of performing water calorimetry in proton therapy and specifically in scanned beam delivery has been shown both numerically and experimentally. By adopting a water calorimetry‐based primary standard and calibrating user ionization chambers directly in proton beams, national dosimetry laboratories can significantly reduce the uncertainty on dose determination in proton beams from the current 1.5%–2% level down to sub‐percent regime similar to the current standard of practice in high energy photon beams.
    Medical Physics 05/2010; 37(6):3095-3096. DOI:10.1118/1.3468012 · 3.01 Impact Factor
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    ABSTRACT: We completed an implementation of pencil-beam scanning (PBS), a technology whereby a focused beam of protons, of variable intensity and energy, is scanned over a plane perpendicular to the beam axis and in depth. The aim of radiotherapy is to improve the target to healthy tissue dose differential. We illustrate how PBS achieves this aim in a patient with a bulky tumor. Our first deployment of PBS uses "broad" pencil-beams ranging from 20 to 35 mm (full-width-half-maximum) over the range interval from 32 to 7 g/cm(2). Such beam-brushes offer a unique opportunity for treating bulky tumors. We present a case study of a large (4,295 cc clinical target volume) retroperitoneal sarcoma treated to 50.4 Gy relative biological effectiveness (RBE) (presurgery) using a course of photons and protons to the clinical target volume and a course of protons to the gross target volume. We describe our system and present the dosimetry for all courses and provide an interdosimetric comparison. The use of PBS for bulky targets reduces the complexity of treatment planning and delivery compared with collimated proton fields. In addition, PBS obviates, especially for cases as presented here, the significant cost incurred in the construction of field-specific hardware. PBS offers improved dose distributions, reduced treatment time, and reduced cost of treatment.
    International journal of radiation oncology, biology, physics 02/2010; 76(2):624-30. DOI:10.1016/j.ijrobp.2009.06.065 · 4.18 Impact Factor
  • Medical Physics 01/2010; 37(7). DOI:10.1118/1.3476169 · 3.01 Impact Factor
  • Medical Physics 01/2009; 36(6). · 3.01 Impact Factor
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    ABSTRACT: A novel technique for separating the out-of-field proton and neutron dose is presented. The technique combines measurements from special pin diodes sensitive to the non ionizing energy losses of protons and neutrons in silicon and standard clinical absorbed dose measurements. Preliminary testing shows good agreement with results from a GEANT4 study.
    IEEE Nuclear Science Symposium conference record. Nuclear Science Symposium 01/2009; DOI:10.1109/NSSMIC.2009.5402219