Medical Physics Research Training Network Research Articles

SU‐D‐206‐02: Evaluation of Partial Storage of the System Matrix for Cone Beam Computed Tomography Using a GPU Platform

Purpose:Iterative reconstruction algorithms in computed tomography (CT) require a fast method for computing the intersections between the photons’ trajectories and the object, also called ray‐tracing or system matrix computation. This work evaluates different ways to store the system matrix, aiming to reconstruct dense image grids in reasonable time.Methods:We propose an optimized implementation of the Siddon's algorithm using graphics processing units (GPUs) with a novel data storage scheme. The algorithm computes a part of the system matrix on demand, typically, for one projection angle. The proposed method was enhanced with accelerating options: storage of larger subsets of the system matrix, systematic reuse of data via geometric symmetries, an arithmetic‐rich parallel code and code configuration via machine learning. It was tested on geometries mimicking a cone beam CT acquisition of a human head. To realistically assess the execution time, the ray‐tracing routines were integrated into a regularized Poisson‐based reconstruction algorithm. The proposed scheme was also compared to a different approach, where the system matrix is fully pre‐computed and loaded at reconstruction time.Results:Fast ray‐tracing of realistic acquisition geometries, which often lack spatial symmetry properties, was enabled via the proposed method. Ray‐tracing interleaved with projection and backprojection operations required significant additional time. In most cases, ray‐tracing was shown to use about 66 % of the total reconstruction time. In absolute terms, tracing times varied from 3.6 s to 7.5 min, depending on the problem size. The presence of geometrical symmetries allowed for non‐negligible ray‐tracing and reconstruction time reduction. Arithmetic‐rich parallel code and machine learning permitted a modest reconstruction time reduction, in the order of 1 %.Conclusion:Partial system matrix storage permitted the reconstruction of higher 3D image grid sizes and larger projection datasets at the cost of additional time, when compared to the fully pre‐computed approach.This work was supported in part by the Fonds de recherche du Quebec ‐ Nature et technologies (FRQ‐NT). The authors acknowledge partial support by the CREATE Medical Physics Research Training Network grant of the Natural Sciences and Engineering Research Council of Canada (Grant No. 432290).

SU-F-R-28: Correction of FCh-PET Bladder Uptake Using Virtual Sinograms and Investigation of Its Impact On the Quantification of Prostate Textural Characteristics

Purpose: Textural biomarkers as a tool for quantifying intratumoral heterogeneity hold great promise for diagnosis and early assessment of treatment response in prostate cancer. However, spill-in counts from the bladder uptake are suspected to have an impact on the textural measurements of the prostate volume. This work proposes a correction method for the FCh-PET bladder uptake and investigates its impact on intraprostatic textural properties. Methods: Two patients with PC received pre-treatment dynamic FCh-PET scans reconstructed at four time points (interval: 2 min), for which prostate and bladder contours were obtained. Projection bins affected by bladder uptake were determined by forward-projection. For each time point and axial position, virtual sinograms were obtained and affected bins replaced by a weighted combination of original values and values interpolated using cubic spline from non-affected bins of the current and adjacent projection angles. The process was optimized using a genetic algorithm in terms of minimization of the root-mean-square error (RMSE) within the bladder between the corrected dynamic time point volume and a reference initial uptake volume. Finally, the impact of the bladder uptake correction on the prostate region was investigated using two standard SUV metrics (1) and three texture metrics (2): 1) SUVmax, SUVmean; 2) Contrast, Homogeneity, Coarseness. Results: Without bladder uptake correction, SUVmax and SUVmean were on average overestimated in the prostate by 0%, 0%, 33.2%, 51.2%, and 3.6%, 6.0%, 2.9%, 3.2%, for each time point respectively. Contrast varied by −9.1%, −6.7%, +40.4%, +107.7%, and Homogeneity and Coarseness by +4.5%, +1.8%, −8.8%, −14.8% and +1.0%, +0.5%, −9.5%, +0.9%. Conclusion: We proposed a method for FCh-PET bladder uptake correction and showed an impact on the quantification of the prostate signal. This method achieved a large reduction of intra-prostatic SUVmax while minimizing the impact on SUVmean. Further investigation is necessary to interpret changes in textural features. SL acknowledges partial support by the CREATE Medical Physics Research Training Network grant of the Natural Sciences and Engineering Research Council (Grant number: 432290).

TU-AB-BRC-07: Efficiency of An IAEA Phase-Space Source for a Low Energy X-Ray Tube Using Egs++

Purpose: To extend the capability of the EGSnrc C++ class library (egs++) to write and read IAEA phase-space files as a particle source, and to assess the relative efficiency gain in dose calculation using an IAEA phase-space source for modelling a miniature low energy x-ray source. Methods: We created a new ausgab object to score particles exiting a user-defined geometry and write them to an IAEA phase-space file. A new particle source was created to read from IAEA phase-space data. With these tools, a phase-space file was generated for particles exiting a miniature 50 kVp x-ray tube (The INTRABEAM System, Carl Zeiss). The phase-space source was validated by comparing calculated PDDs with a full electron source simulation of the INTRABEAM. The dose calculation efficiency gain of the phase-space source was determined relative to the full simulation. The efficiency gain as a function of i) depth in water, and ii) job parallelization was investigated. Results: The phase-space and electron source PDDs were found to agree to 0.5% RMS, comparable to statistical uncertainties. The use of a phase-space source for the INTRABEAM led to a relative efficiency gain of greater than 20 over the full electron source simulation, with an increase of up to a factor of 196. The efficiency gain was found to decrease with depth in water, due to the influence of scattering. Job parallelization (across 2 to 256 cores) was not found to have any detrimental effect on efficiency gain. Conclusion: A set of tools has been developed for writing and reading IAEA phase-space files, which can be used with any egs++ user code. For simulation of a low energy x-ray tube, the use of a phase-space source was found to increase the relative dose calculation efficiency by factor of up to 196. The authors acknowledge partial support by the CREATE Medical Physics Research Training Network grant of the Natural Sciences and Engineering Research Council (Grant No. 432290).

TH-CD-201-06: Experimental Characterization of Acoustic Signals Generated in Water Following Clinical Photon and Electron Beam Irradiation

Purpose: Previous work has demonstrated the detectability of acoustic waves induced following the irradiation of high density metals with radiotherapy linac photon beams. This work demonstrates the ability to experimentally detect such acoustic signals following both photon and electron irradiation in a more radiotherapy relevant material. The relationship between induced acoustic signal properties in water and the deposited dose distribution is explored, and the feasibility of exploiting such signals for radiotherapy dosimetry is demonstrated. Methods: Acoustic waves were experimentally induced in a water tank via the thermoacoustic effect following a single pulse of photon or electron irradiation produced by a clinical linac. An immersion ultrasound transducer was used to detect these acoustic waves in water and signals were read out on an oscilloscope. Results: Peaks and troughs in the detected acoustic signals were found to correspond to the location of gradients in the deposited dose distribution following both photon and electron irradiation. Signal amplitude was linearly related to the dose per pulse deposited by photon or electron beams at the depth of detection. Flattening filter free beams induced large acoustic signals, and signal amplitude decreased with depth after the depth of maximum dose. Varying the field size resulted in a temporal shift of the acoustic signal peaks and a change in the detected signal frequency. Conclusion: Acoustic waves can be detected in a water tank following irradiation by linac photon and electron beams with basic electronics, and have characteristics related to the deposited dose distribution. The physical location of dose gradients and the amount of dose deposited can be inferred from the location and magnitude of acoustic signal peaks. Thus, the detection of induced acoustic waves could be applied to photon and electron water tank and in vivo dosimetry. This work was supported in part by CIHR grants MOP-114910 and MOP-136774. S.H. acknowledges support by the NSERC CREATE Medical Physics Research Training Network grant 432290.

SU‐F‐P‐20: Predicting Waiting Times in Radiation Oncology Using Machine Learning

Purpose:Waiting times remain one of the most vexing patient satisfaction challenges facing healthcare. Waiting time uncertainty can cause patients, who are already sick or in pain, to worry about when they will receive the care they need. These waiting periods are often difficult for staff to predict and only rough estimates are typically provided based on personal experience. This level of uncertainty leaves most patients unable to plan their calendar, making the waiting experience uncomfortable, even painful. In the present era of electronic health records (EHRs), waiting times need not be so uncertain. Extensive EHRs provide unprecedented amounts of data that can statistically cluster towards representative values when appropriate patient cohorts are selected. Predictive modelling, such as machine learning, is a powerful approach that benefits from large, potentially complex, datasets. The essence of machine learning is to predict future outcomes by learning from previous experience. The application of a machine learning algorithm to waiting time data has the potential to produce personalized waiting time predictions such that the uncertainty may be removed from the patient's waiting experience.Methods:In radiation oncology, patients typically experience several types of waiting (eg waiting at home for treatment planning, waiting in the waiting room for oncologist appointments and daily waiting in the waiting room for radiotherapy treatments). A daily treatment wait time model is discussed in this report. To develop a prediction model using our large dataset (with more than 100k sample points) a variety of machine learning algorithms from the Python package sklearn were tested.Results:We found that the Random Forest Regressor model provides the best predictions for daily radiotherapy treatment waiting times. Using this model, we achieved a median residual (actual value minus predicted value) of 0.25 minutes and a standard deviation residual of 6.5 minutes. This means that the majority of our estimates are within 6.5 minutes of the actual wait time.Conclusion:The goal of this project was to define an appropriate machine learning algorithm to estimate waiting times based on the collective knowledge and experience learned from previous patients. Our results offer an opportunity to improve the information that is provided to patients and family members regarding the amount of time they can expect to wait for radiotherapy treatment at our centre.AJ acknowledges support by the CREATE Medical Physics Research Training Network grant of the Natural Sciences and Engineering Research Council (Grant number: 432290) and from the 2014 Q+ Initiative of the McGill University Health Centre.

TU-AB-BRC-05: Creation of a Monte Carlo TrueBeam Model by Reproducing Varian Phase Space Data

Purpose: To create a Varian TrueBeam 6 MV FFF Monte Carlo model using BEAMnrc/EGSnrc that accurately reproduces the Varian representative dataset, followed by tuning the model's source parameters to accurately reproduce in-house measurements. Methods: A BEAMnrc TrueBeam model for 6 MV FFF has been created by modifying a validated 6 MV Varian CL21EX model. Geometric dimensions and materials were adjusted in a trial and error approach to match the fluence and spectra of TrueBeam phase spaces output by the Varian VirtuaLinac. Once the model's phase space matched Varian's counterpart using the default source parameters, it was validated to match 10 × 10 cm2 Varian representative data obtained with the IBA CC13. The source parameters were then tuned to match in-house 5 × 5 cm2 PTW microDiamond measurements. All dose to water simulations included detector models to include the effects of volume averaging and the non-water equivalence of the chamber materials, allowing for more accurate source parameter selection. Results: The Varian phase space spectra and fluence were matched with excellent agreement. The in-house model's PDD agreement with CC13 TrueBeam representative data was within 0.9% local percent difference beyond the first 3 mm. Profile agreement at 10 cm depth was within 0.9% local percent difference and 1.3 mm distance-to-agreement in the central axis and penumbra regions, respectively. Once the source parameters were tuned, PDD agreement with microDiamond measurements was within 0.9% local percent difference beyond 2 mm. The microDiamond profile agreement at 10 cm depth was within 0.6% local percent difference and 0.4 mm distance-to-agreement in the central axis and penumbra regions, respectively. Conclusion: An accurate in-house Monte Carlo model of the Varian TrueBeam was achieved independently of the Varian phase space solution and was tuned to in-house measurements. KO acknowledges partial support by the CREATE Medical Physics Research Training Network grant of the Natural Sciences and Engineering Research Council (Grant number: 432290).

TH-CD-BRA-10: Towards Reference Dosimetry of MR-Linacs Using a Clinical Probe-Format Calorimeter

Purpose: To evaluate the influence of a 1.5 T magnetic field (B-field) on the response of a small-scale graphite calorimeter probe (GPC) developed for use as a novel clinical reference dosimeter. Characterization of the GPC was also assessed in a hybrid MRI-linac (MRL) clinical prototype by performing absolute dosimetry in multiple detector orientations. Methods: B-field influence was characterized using a variable-strength electromagnet system located 280 cm from the source of a clinical linac. The GPC was used to perform a total of 160 absolute dose measurements (6 MV, 920 MU/min) in a water phantom placed between the poles of the electromagnet. The magnitude of the B-field between the poles was varied in the range of 0 – 1.5 T. The relative response of the GPC was determined and compared to that of a thimble type ionization chamber (Exradin A1SL, Standard Imaging). Next, 65 dose measurements were performed using the GPC in a clinical MRL field (7 MV, 620 MU/min) to quantify the rotational dependence of the detector in the presence of a 1.5 T B-field. The GPC was rotated in steps of 90° inside a graphite phantom (SSD 140 cm, depth 2.5 cm) for two detector orientations (parallel and perpendicular to the B field). Results: Relative to the zero B-field condition, the A1SL chamber exhibited an average overresponse of +1.2 % ± 0.03 % at a B-field of 1.5 T, while the GPC under-responded on average by −0.5 % ± 0.9 %. For the MRL measurements, no significant differences were observed between the parallel and perpendicular orientations. In both cases, a rotational dependence of approximately ±1 % was measured. Conclusion: This work suggests that the B-field has minimal influence on the response of the GPC, making it a potentially attractive solution for clinical MRL reference dosimetry. This work has been supported in part by the CREATE Medical Physics Research Training Network NSERC grant RGPIN 432290, as well as NSERC grants RGPIN 298191 & 435608. JR is a scholar from The Terry Fox Foundation Strategic Training Initiative for Excellence in Radiation Research for the 21st Century (EIRR21).

MO‐DE‐BRA‐04: The CREATE Medical Physics Research Training Network: Training of New Generation Innovators

Purpose:Over the past century, physicists have played a major role in transforming scientific discovery into everyday clinical applications. However, with the increasingly stringent requirements to regulate medical physics as a health profession, the role of physicists as scientists and innovators has become at serious risk of erosion. These challenges trigger the need for a new, revolutionized training program at the graduate level that respects scientific rigor, attention for medical physics‐relevant developments in basic sciences, innovation and entrepreneurship.Methods:A grant proposal was funded by the Collaborative REsearch and Training Experience program (CREATE) of the Natural Sciences and Engineering Research Council (NSERC) of Canada. This enabled the creation of the Medical Physics Research Training Network (MPRTN) around two CAMPEP‐accredited medical physics programs. Members of the network consist of medical device companies, government (research and regulatory) and academia. The MPRTN/CREATE program proposes a curriculum with three main themes: (1) radiation physics, (2) imaging & image processing and (3) radiation response, outcomes and modeling.Results:The MPRTN was created mid 2013 (mprtn.com) and features (1) four new basic Ph.D. courses; (2) industry participation in research projects; (3) formal job‐readiness training with involvement of guest faculty from academia, government and industry. MPRTN activities since 2013 include 22 conferences; 7 workshops and 4 exchange travels. Three patents were filed or issued, nine awards/best papers were won. Fifteen journal publications were accepted/published, 102 conference abstracts. There are now 13 industry partners.Conclusion:A medical physics research training network has been set up with the goal to harness graduate student's job‐readiness for industry, government and academia in addition to the conventional clinical role. Two years after inception, significant successes have been booked, but the true challenge will be to demonstrate that with this training philosophy CREATE scholars gain access to a much broader job market.Supported by the Natural Sciences and Engineering Research Council (NSERC) Canada

SU‐E‐T‐521: Investigation of the Uncertainties Involved in Secondary Neutron/gamma Production in Geant4/MCNP6 Monte Carlo Codes for Proton Therapy Application

Purpose:A major concern in proton therapy is the production of secondary neutrons causing secondary cancers, especially in young adults and children. Most utilized Monte Carlo codes in proton therapy are Geant4 and MCNP. However, the default versions of Geant4 and MCNP6 do not have suitable cross sections or physical models to properly handle secondary particle production in proton energy ranges used for therapy. In this study, default versions of Geant4 and MCNP6 were modified to better handle production of secondaries by adding the TENDL‐2012 cross‐section library.Methods:In‐water proton depth‐dose was measured at the “The Svedberg Laboratory” in Uppsala (Sweden). The proton beam was mono‐energetic with mean energy of 178.25±0.2 MeV. The measurement set‐up was simulated by Geant4 version 10.00 (default and modified version) and MCNP6. Proton depth‐dose, primary and secondary particle fluence and neutron equivalent dose were calculated. In case of Geant4, the secondary particle fluence was filtered by all the physics processes to identify the main process responsible for the difference between the default and modified version.Results:The proton depth‐dose curves and primary proton fluence show a good agreement between both Geant4 versions and MCNP6. With respect to the modified version, default Geant4 underestimates the production of secondary neutrons while overestimates that of gammas. The “ProtonInElastic” process was identified as the main responsible process for the difference between the two versions. MCNP6 shows higher neutron production and lower gamma production than both Geant4 versions.Conclusion:Despite the good agreement on the proton depth dose curve and primary proton fluence, there is a significant discrepancy on secondary neutron production between MCNP6 and both versions of Geant4. Further studies are thus in order to find the possible cause of this discrepancy or more accurate cross‐sections/models to handle the nuclear interactions of protons with energy ranges used for therapy.NSERC‐CRSNG’ CREATE Medical Physics Research Training Network compute calcul CANADA; McGill University health center research institute the fast foundation; LM acknowledges partial support by the CREATE Medical Physics Research Training Network grant of the Natural Sciences and Engineering; Research Council Grant number 432290

SU‐E‐T‐581: On the Value of LET as a Radiation Quality Descriptor for RBE

Purpose:To investigate the relationship between linear energy transfer (LET) and relative biological effectiveness (RBE) for protons and light ions, and the corresponding role of LET as a descriptor of radiation quality of hadron therapy.Methods:Monte Carlo (MC) proton and light ion (He, Li, C) tracks with LET < 30 eV nm‐1 were generated in an event‐by‐event mode. They were overlaid on a cell nucleus model containing 6×109 nucleotide base pairs using an isotropic irradiation procedure that provides electronic equilibrium. Strand breaks (sbs) were scored in the DNA sugar‐phosphate groups and further sub‐classified into single or double sbs (ssbs or dsbs). Distributions of ssbs and dsbs for 2 Gy fractions were calculated to estimate RBE for the induction of initial dsbs with reference to 60Co. Additionally, sbs were classified based on their complexity (i.e. the number of sbs in each cluster).Results:An increase in LET for light ions of the same atomic number or a decrease in atomic number for ions of the same LET resulted in a lower kinetic energy of emitted secondary electrons. The clustering of DNA damage was more pronounced as reflected by the increase in proton RBE from ∼ 1.75 to 4 for LET values of 7 to 28 eV nm‐1. A significant RBE decrease between protons, He, Li and C ions of the same LET was also noticed as function of the atomic number. Significant differences in ssbs and dsbs complexities were also seen for particles with the same LET, potentially supporting a clustering‐based radiation quality descriptor.Conclusion:The LET‐RBE relationships were simulated for proton and light ions and exhibited expected trends, including different RBEs for particles with the same LET but different atomic numbers. A complexity based radiation quality descriptor may allow better differentiation of RBE between radiation fields of similar LET.We would like to acknowledge support from the Fonds de recherche du Quebec Sante (FRQS), from the CREATE Medical Physics Research Training Network grant (number 432290) of NSERC, support from NSERC under grants RGPIN 397711‐11 and RGPIN‐2014‐06475 and support from CIHR under grants MOP‐114910, MOP‐136774 and MOP‐102550.

TU-AB-201-08: Rotating Shield High Dose Rate Brachytherapy with 153Gd and 75Se Isotopes

Purpose: To introduce rotating shield brachytherapy (RSBT) for different cancer sites with 1⁵3Gd and ⁷⁵Se isotopes. RSBT is a form of intensity modulated brachytherapy, using shielded rotating catheters to provide a better dose distribution in the tumour while protecting healthy tissue. Methods: BrachySource, a Geant4-based Monte Carlo dose planning system was developed for investigation of RSBT with 1⁵3Gd and ⁷⁵Se for different cancer sites. Dose distributions from 1⁵3Gd, ⁷⁵Se and 192Ir isotopes were calculated in a 40 cm radius water phantom by using the microSelectron-v2 source model. The source was placed inside a cylindrical platinum shield with 1.3 mm diameter. An emission window coinciding with the active core of the source was created by removing half (180°) of the wall of the shield. Relative dose rate distributions of the three isotopes were simulated. As a proof of concept, a breast cancer patient originally treated with Mammosite was re-simulated with unshielded 192Ir and shielded 1⁵3Gd. Results: The source with the lowest energy, 1⁵3Gd, decreased the dose on the shielded side by 91%, followed by ⁷⁵Se and 192Ir with 36% and 16% reduction at 1 cm from the source. The breast cancer patient simulation showed the ability of shielded 1⁵3Gd to spare the chest wall by a 90% dose reduction when only one emission window angle is considered. In this case, fully covering the PTV would require more delivery angles and the chest wall dose reduction would be less, however, the simulation demonstrates the potential of shielded 1⁵3Gd to selectively isolate organs at risk. Conclusion: Introducing 1⁵3Gd and ⁷⁵Se sources combined with RSBT will allow escalation of dose in the target volume while maintaining low doses in radiation sensitive healthy tissue. Tailoring treatments to each individual patient by treating all parts of the tumour without over-irradiation of normal tissues will be possible. The author acknowledges partial support by the CREATE Medical Physics Research Training Network grant of the Natural Sciences and Engineering Research Council (Grant number: 432290), and the Quebec Fonds de recherche Nature et Technologies.

SU‐C‐304‐03: Experimental Investigation On the Accuracy of Plastic Scintillation Dosimeters in Small Fields

Purpose:To investigate the accuracy of the Exradin W1 (SI) and of an “in‐house” plastic scintillation dosimeter (CHUQ PSD) in small radiation fields.Methods:Output factor (OF) measurements with the W1 and CHUQ PSD were performed for field sizes of 0.5 × 0.5, 1 × 1 and 2 × 2 cm2. Both detectors were placed parallel to the central axis (CAX) in water. The spectrum discrimination calibration method was performed in each set‐up to account for the Cerenkov (CRV) signal created in the fiber. The OFs were compared to the expected field factors in water derived using i) Monte Carlo (MC) simulations of an accurate accelerator model and ii) microLion (PTW) and D1V diode (SI) OFs. MC‐derived correction factors were applied to both the microLion and D1V OFs. For the CHUQ PSD the calibration was repeated in water (// CAX), solid water (perpendicular to CAX) and under a shielded configuration. The signal was collected using a spectrometer (wavelength range = 185–1100 nm). Spectral analysis was performed to evaluate potential changes of the spectral distributions under the various calibration set‐up configurations.Results:The W1 OFs presented an over‐response for the 0.5 × 0.5 cm2 in the range of 3 – 4.1% relative to the expected field factor. The CHUQ PSD presented an under‐response in the range of 1.5 – 2.7%, without accounting for volume averaging. The CRV spectra under the various calibration procedures appeared similar to each other and only minor changes were observed to the respective OFs.Conclusion:The W1 and CHUQ PSD can be used in small fields down to a 1 × 1 cm2 field size. Discrepancies were encountered between the two detectors for the smallest field size of 0.5 × 0.5 cm2 with the CHUQ PSD exhibiting a closer agreement to the expected field factor.Funding sources: 1) Alexander S. Onassis Public Benefit Foundation in Greece and 2) CREATE Medical Physics Research Training Network grant of the Natural Sciences and Engineering Research Council, Grant number: 432290, RGPIN‐2014‐06475

TH‐A‐213‐02: IAEA/AAPM Code of Practice for the Dosimetry of Static Small Photon Fields

Modern radiotherapy techniques such as SRS/SRT, SBRT, IMRT, VMAT as well as specialized machines such as Tomotherapy, CyberKnife and Gamma Knife use small photon fields with at least one dimension <3 cm. Dosimetry in such small fields is challenging because of large detector perturbations due to non‐equilibrium conditions, occlusion of the primary photon source and large dose gradients across the field. Many small‐size dosimeters have been proposed for use in small fields. However, their characteristics especially the volume averaging and fluence perturbations have only recently been adequately understood. An IAEA‐AAPM working group has provided a framework for reference dosimetry in non‐compliant beams and the measurement of field output factors small fields (1). The AAPM TG‐155 (2) has adopted this framework to provide guidelines on relative dosimetry. This course explains the code of practice for absolute dosimetry that is under review and discusses the availability of correction factors to convert detector readings to doses. TG‐155 defines small field conditions, provides recommendations for suitable detectors and recommendations for good working practice for relative dosimetry (PDD, TMR, output factor, etc.) and dose calculations based on the new formulation. It also discusses beam modeling and dose calculations as a critical step in clinical utilization of small field radiotherapy. Alfonso et al, Med Phys 35, 5179–5186 (2008). Das et al, Med Phys (under review, 2014) Learning Objectives: Concepts and recommended procedures in the IAEA‐AAPM code of practice for dosimetry of small fields. Physics of dosimetry in non‐equilibrium conditions and definition of small fields Recommended procedures for relative dosimetry in TG‐1554. Choice of detectors for small field dosimetry, perturbations and corrections for dosimetry Understand the detector properties required for small field measurements Understand the advantages and limitations of commercially available detectors for small field dosimetry Research supported with operating grants from CIHR, NSERC, and BIOWin, a program funded at the Universite Catholique Louvain by the Walloon Government (Belgium). Student stipends supported by Medical Physics Research Training Network funded by the Collaborative Research and Training Experience program of the NSERC.Seuntjens: support by Sun Nuclear Corporation

WE-AB-BRB-01: Development of a Probe-Format Graphite Calorimeter for Practical Clinical Dosimetry: Numerical Design Optimization, Prototyping, and Experimental Proof-Of-Concept

Purpose: In this work, the feasibility of performing absolute dose to water measurements using a constant temperature graphite probe calorimeter (GPC) in a clinical environment is established. Methods: A numerical design optimization study was conducted by simulating the heat transfer in the GPC resulting from irradiation using a finite element method software package. The choice of device shape, dimensions, and materials was made to minimize the heat loss in the sensitive volume of the GPC. The resulting design, which incorporates a novel aerogel-based thermal insulator, and 15 temperature sensitive resistors capable of both Joule heating and measuring temperature, was constructed in house. A software based process controller was developed to stabilize the temperatures of the GPC’s constituent graphite components to within a few 10’s of µK. This control system enables the GPC to operate in either the quasi-adiabatic or isothermal mode, two well-known, and independent calorimetry techniques. Absorbed dose to water measurements were made using these two methods under standard conditions in a 6 MV 1000 MU/min photon beam and subsequently compared against TG-51 derived values. Results: Compared to an expected dose to water of 76.9 cGy/100 MU, the average GPC-measured doses were 76.5 ± 0.5 and 76.9 ± 0.5 cGy/100 MU for the adiabatic and isothermal modes, respectively. The Monte Carlo calculated graphite to water dose conversion was 1.013, and the adiabatic heat loss correction was 1.003. With an overall uncertainty of about 1%, the most significant contributions were the specific heat capacity (type B, 0.8%) and the repeatability (type A, 0.6%). Conclusion: While the quasi-adiabatic mode of operation had been validated in previous work, this is the first time that the GPC has been successfully used isothermally. This proof-of-concept will serve as the basis for further study into the GPC’s application to small fields and MRI-linac dosimetry. This work has been supported in part by the CREATE Medical Physics Research Training Network of the Natural Sciences and Engineering Research Council (NSERC) grant 432290, NSERC grants RGPIN 298191 & 435608-13, Canadian Institutes of Health Research doctoral scholarship GSD-121793. This work has also been supported by Sun Nuclear Corporation.

SU‐E‐T‐494: Influence of Proton Track‐Cell Nucleus Incidence Angle On Relative Biological Effectiveness

Purpose:To explain a Monte Carlo (MC) simulation artifact whereby differences in relative biological effectiveness (RBE) in the induction of initial double strand breaks are observed as a function of the proton track incidence angles in a geometric cell nucleus model. Secondly, to offer an alternative isotropic irradiation procedure to mitigate this effect.Methods:MC tracks of 1 MeV protons were generated in an event‐by‐event mode. They were overlaid on a cylindrical model of a cell nucleus containing 6×109 nucleotide base pairs. The tracks incidence angle θ with respect to the cell nucleus's axis was varied in 10 degrees intervals, each time generating one hundred fractions of ∼2 Gy. Strand breaks were scored in the modeled DNA sugar‐phosphate groups and further sub‐classified into single or double strand breaks (ssbs or dsbs). For each angle, an RBE for the induction of initial dsbs with reference to Co‐60 was calculated.Results:Our results show significant angular dependencies of RBE, with maximum values for incidence angles parallel to the nucleus central axis. Further examination shows that the higher cross‐sections for the creation of dsbs is due to the preferential alignment of tracks with geometrical sub‐parts of the cell nucleus model, especially the nucleosomes containing the sugar‐phosphate groups. To alleviate the impact of this simulation artifact, an average RBE was calculated with a procedure based on a weighted sampling of the angular data.Conclusion:This work demonstrates a possible numerical artifact in estimated RBE if the influence of the particle incidence angle is not correctly taken into account. A correction procedure is presented to better conform the simulations to real‐life experimental conditions.We would like to acknowledge support from the Fonds de recherche du Quebec Sante (FRQS), from the CREATE Medical Physics Research Training Network grant (number 432290) of NSERC, support from NSERC under grants RGPIN 397711‐11 and RGPIN‐2014‐06475 and support from the CIHR under grants MOP‐114910, MOP‐136774 and MOP‐102550.

TH‐A‐213‐00: Small Field Dosimetry: Overview of AAPM TG‐155 and the IAEAAAPM Code of Practice

Modern radiotherapy techniques such as SRS/SRT, SBRT, IMRT, VMAT as well as specialized machines such as Tomotherapy, CyberKnife and Gamma Knife use small photon fields with at least one dimension <3 cm. Dosimetry in such small fields is challenging because of large detector perturbations due to non‐equilibrium conditions, occlusion of the primary photon source and large dose gradients across the field. Many small‐size dosimeters have been proposed for use in small fields. However, their characteristics especially the volume averaging and fluence perturbations have only recently been adequately understood. An IAEA‐AAPM working group has provided a framework for reference dosimetry in non‐compliant beams and the measurement of field output factors small fields (1). The AAPM TG‐155 (2) has adopted this framework to provide guidelines on relative dosimetry. This course explains the code of practice for absolute dosimetry that is under review and discusses the availability of correction factors to convert detector readings to doses. TG‐155 defines small field conditions, provides recommendations for suitable detectors and recommendations for good working practice for relative dosimetry (PDD, TMR, output factor, etc.) and dose calculations based on the new formulation. It also discusses beam modeling and dose calculations as a critical step in clinical utilization of small field radiotherapy. Alfonso et al, Med Phys 35, 5179–5186 (2008). Das et al, Med Phys (under review, 2014) Learning Objectives: Concepts and recommended procedures in the IAEA‐AAPM code of practice for dosimetry of small fields. Physics of dosimetry in non‐equilibrium conditions and definition of small fields Recommended procedures for relative dosimetry in TG‐1554. Choice of detectors for small field dosimetry, perturbations and corrections for dosimetry Understand the detector properties required for small field measurements Understand the advantages and limitations of commercially available detectors for small field dosimetry Research supported with operating grants from CIHR, NSERC, and BIOWin, a program funded at the Universite Catholique Louvain by the Walloon Government (Belgium). Student stipends supported by Medical Physics Research Training Network funded by the Collaborative Research and Training Experience program of the NSERC.Seuntjens: support by Sun Nuclear Corporation

MO‐FG‐303‐02: BEST IN PHYSICS (THERAPY): Cherenkov Emission Dosimetry: Feasibility for Electron Radiotherapy

Purpose:To investigate from first principles, corroborated by Monte Carlo simulations and experimental measurements, the feasibility of developing a relative Cherenkov emission (CE) dosimetry protocol for electron beam radiotherapy.Methods:Monte Carlo (MC) simulations of mono‐energetic electrons incident on water were carried out in Geant4. Percent depth Cherenkov emission (PDCE) and dose (PDD) distributions were scored for incidence energies of 4, 6, 9, 12, 15, and 18 MeV. PDCE‐to‐PDD analytical conversion models were developed from least‐squares data fits generated for PDD as a function of PDCE at the same depth and at different depths. Experimental techniques for validation of these models are examined.Results:Same‐depth PDD versus PDCE data fits indicate that although the relationship is linear to first order (correlation r > 0.9 for all energies), it is much more accurately approximated by separate linear and quadratic models for the build‐up and drop‐off regions, respectively (r > 0.999), which is theoretically underpinned. To understand the source of this relationship and its basis for developing robust conversion models, an approximate quadratic first‐principles model was derived and found in agreement with MC/measured data (20% deviation at worst). Conversely, data fits of PDD versus different‐depth PDCE unveiled a depth‐invariant effective point of measurement of 1.5–2.1 mm downstream with 4–18 MeV incidence, respectively (r > 0.999 in the drop‐off region). We present an analytical first‐principles justification for this shift. This method led to errors of <1% in drop‐off region PDD (<2% for PDD<20% with 4 MeV incidence) and <0.2 mm in practical range prediction.Conclusion:We present robust quantitative prediction models, derived from first‐principles and supported by simulation and measurement, for relative dose from Cherenkov emission by high‐energy electrons. This constitutes a major step towards development of protocols for routine clinical quality assurance as well as real‐time in vivo Cherenkov dosimetry in radiotherapy.The authors acknowledge partial support by Fonds de recherche du Quebec ‐ Nature et technologies (FRQNT), CREATE Medical Physics Research Training Network grant of the Natural Sciences and Engineering Research Council of Canada (NSERC), CREATE Integrated Sensor Systems grant of NSERC, the Canadian Institutes of Health Research (CIHR), and NSERC.

SU‐E‐T‐524: In‐Vivo Diode Dosimetry Proton Therapy Range Verification Validation Study for Pediatric CSI

Purpose:With the aim of reducing acute esophageal radiation toxicity in pediatric patients receiving craniospinal irradiation (CSI), we investigated the implementation of an in‐vivo, adaptive proton therapy range verification methodology. Simulation experiments and in‐phantom measurements were conducted to validate the range verification technique for this clinical application.Methods:A silicon diode array system has been developed and experimentally tested in phantom for passively scattered proton beam range verification for a prostate treatment case by correlating properties of the detector signal to the water equivalent path length (WEPL). We propose to extend the methodology to verify range distal to the vertebral body for pediatric CSI cases by placing this small volume dosimeter in the esophagus of the anesthetized patient immediately prior to treatment. A set of calibration measurements was performed to establish a time signal to WEPL fit for a “scout” beam in a solid water phantom. Measurements are compared against Monte Carlo simulation in GEANT4 using the Tool for Particle Simulation (TOPAS).Results:Measurements with the diode array in a spread out Bragg peak of 14 cm modulation width and 15 cm range (177 MeV passively scattered beam) in solid water were successfully validated against proton fluence rate simulations in TOPAS. The resulting calibration curve allows for a sensitivity analysis of detector system response with dose rate in simulation and with individual diode position through simulation on patient CT data.Conclusion:Feasibility has been shown for the application of this range verification methodology to pediatric CSI. An in‐vivo measurement to determine the WEPL to the inner surface of the esophagus will allow for personalized adjustment of the treatment plan to ensure sparing of the esophagus while confirming target coverage.A Toltz acknowledges partial support by the CREATE Medical Physics Research Training Network grant of the Natural Sciences and Engineering Research Council (Grant number: 432290).

TH-CD-304-06: Amplitude and Frequency Characteristics of Acoustic Waves Induced Following Radiotherapy Linear Accelerator Irradiation

Purpose: To enhance understanding of the characteristics of acoustic waves induced following irradiation by a linac photon beam through experiments and simulations. Such waves have potential applications in in vivo dosimetry since their properties are related to the dose distribution following a single pulse of irradiation. Methods: The acoustic waves induced following linac photon beam irradiation of different metal blocks suspended in a water tank were investigated. Experimentally, these waves were detected with an ultrasound transducer and amplification electronics. Computationally, the acoustic waves were modeled by combining Monte Carlo dose calculations and acoustic wave transport techniques. The effect of varying metal block size, material, depth, and linac photon beam energy on the amplitude and frequency of the induced acoustic signal was investigated. Results: Experimental and simulated acoustic waveforms indicated that increasing photon beam energy and decreasing metal block depth along the central axis increases signal amplitude by an amount proportional to the increased dose deposited in the block. Decreasing in-plane lead block width increased the frequency of acoustic waves due to the smaller dimension of the lead-induced perturbation in the global dose distribution. Varying the metal block material from lead to aluminum resulted in smaller amplitude acoustic waves due to aluminum’s lower electron density, while aluminum’s higher speed of sound resulted in higher frequency waves. Both simulated and experimental waveforms showed similar trends in signal amplitude and frequency when set-up variables were altered. Conclusion: This study revealed factors that affect the amplitude and frequency of radiation-induced acoustic waves and their relation to the deposited dose characteristics. The agreement between simulated and experimental waveforms verifies the simulation workflow’s ability to model such acoustic waves, and indicates that it could be utilized to explore more complex irradiation scenarios and potentially aid in the development of experimental detection systems for in vivo dosimetry. This work was supported by NSERC and CIHR grants. S.H. acknowledges support by the NSERC CREATE Medical Physics Research Training Network grant 432290.

TH‐A‐213‐01: Small Field Dosimetry: Overview of AAPM TG‐155

Modern radiotherapy techniques such as SRS/SRT, SBRT, IMRT, VMAT as well as specialized machines such as Tomotherapy, CyberKnife and Gamma Knife use small photon fields with at least one dimension <3 cm. Dosimetry in such small fields is challenging because of large detector perturbations due to non‐equilibrium conditions, occlusion of the primary photon source and large dose gradients across the field. Many small‐size dosimeters have been proposed for use in small fields. However, their characteristics especially the volume averaging and fluence perturbations have only recently been adequately understood. An IAEA‐AAPM working group has provided a framework for reference dosimetry in non‐compliant beams and the measurement of field output factors small fields (1). The AAPM TG‐155 (2) has adopted this framework to provide guidelines on relative dosimetry. This course explains the code of practice for absolute dosimetry that is under review and discusses the availability of correction factors to convert detector readings to doses. TG‐155 defines small field conditions, provides recommendations for suitable detectors and recommendations for good working practice for relative dosimetry (PDD, TMR, output factor, etc.) and dose calculations based on the new formulation. It also discusses beam modeling and dose calculations as a critical step in clinical utilization of small field radiotherapy. Alfonso et al, Med Phys 35, 5179–5186 (2008). Das et al, Med Phys (under review, 2014) Learning Objectives: Concepts and recommended procedures in the IAEA‐AAPM code of practice for dosimetry of small fields. Physics of dosimetry in non‐equilibrium conditions and definition of small fields Recommended procedures for relative dosimetry in TG‐1554. Choice of detectors for small field dosimetry, perturbations and corrections for dosimetry Understand the detector properties required for small field measurements Understand the advantages and limitations of commercially available detectors for small field dosimetry Research supported with operating grants from CIHR, NSERC, and BIOWin, a program funded at the Universite Catholique Louvain by the Walloon Government (Belgium). Student stipends supported by Medical Physics Research Training Network funded by the Collaborative Research and Training Experience program of the NSERC.Seuntjens: support by Sun Nuclear Corporation