Electronic Disequilibrium Research Articles

SU‐A‐BRA‐03: Creative Stimulation: A Flexible Hands‐On Approach to Building a Deeper Understanding of Critical Concepts in Radiation Physics

Vic Montemayor ‐ No one has been more passionate about improving the quality and effectiveness of the teaching of Medical Physics than Bill Hendee. It was in August of 2008 that the first AAPM Workshop on Becoming a Better Teacher of Medical Physics was held, organized and run by Bill Hendee. This was followed up in July of 2010 with a summer school on the same topic, again organized by Bill.There has been continued interest in alternate approaches to teaching medical physics since those initial gatherings. The momentum established by these workshops is made clear each year in the annual Innovation in Medical Physics Education session, which highlights work being done in all forms of medical physics education, from one‐on‐one residencies or classroom presentations to large‐scale program revisions and on‐line resources for international audiences.This symposium, presented on behalf of the Education Council, highlights the work of three finalists from past Innovation in Education sessions. Each will be presenting their approaches to and innovations in teaching medical physics. It is hoped that audience members interested in trying something new in their teaching of medical physics will find some of these ideas and approaches readily applicable to their own classrooms.Rebecca Howell ‐ The presentation will discuss ways to maximize classroom learning, i.e., increasing the amount of material covered while also enhancing students’ understanding of the broader implications of the course topics. Specifically, the presentation will focus on two teaching methodologies, project based learning and flip learning. These teaching methods will be illustrated using an example of graduate medical physics course where both are used in conjunction with traditional lectures. Additionally, the presentation will focus on our experience implementing these methods including challenges that were overcome.Jay Burmeister ‐ My presentation will discuss the incorporation of active learning techniques into a traditional medical physics classroom course. I will describe these techniques and how they were implemented as well as student performance before and after implementation. Student feedback indicated that these course changes improved their ability to actively assimilate the course content, thus improving their understanding of the material.Shahid Naqvi ‐ My talk will focus on ways to help students visualize crucial concepts that lie at the core of radiation physics. Although particle tracks generated by Monte Carlo simulations have served as an indispensable visualization tool, students often struggle to resolve the underlying physics from a simultaneous jumble of tracks. We can clarify the physics by “coding” the tracks, e.g., by coloring the tracks according to their “starting” or “crossing” regions. The regionally‐coded tracks when overlaid with dose distributions help the students see the elusive connection between dose, kerma and electronic disequilibrium. Tracks coded according to local energy or energy‐loss rate can illustrate the need for stopping power corrections in electron beams and explain the Bragg peak in a proton beam. Coding tracks according to parent interaction type and order can clarify the often misunderstood distinction between primary and scatter dose. The students can thus see the “whole” simultaneously with the “sum of the parts,” which enhances their physical insight and creates a sustainable foundation for further learning.After the presentations the speakers and moderator will be open to questions and discussion with the audience members. Learning Objectives: Be able to explain Project‐Based Learning and how can it be incorporated into a Medical Physics classroom. Be able to explain Flipped Learning and how can it be incorporated into a Medical Physics classroom. Be able to explain active‐learning strategies for the teaching of Medical Physics. Be able to explain how Monte Carlo simulations can be used to deepen a student's understanding of radiation physics and dosimetry.

SU‐A‐BRA‐00: Education Council Symposium: Revitalizing Your Medical Physics Classroom: Some Examples and Thoughts from the Trenches

Vic Montemayor ‐ No one has been more passionate about improving the quality and effectiveness of the teaching of Medical Physics than Bill Hendee. It was in August of 2008 that the first AAPM Workshop on Becoming a Better Teacher of Medical Physics was held, organized and run by Bill Hendee. This was followed up in July of 2010 with a summer school on the same topic, again organized by Bill.There has been continued interest in alternate approaches to teaching medical physics since those initial gatherings. The momentum established by these workshops is made clear each year in the annual Innovation in Medical Physics Education session, which highlights work being done in all forms of medical physics education, from one‐on‐one residencies or classroom presentations to large‐scale program revisions and on‐line resources for international audiences.This symposium, presented on behalf of the Education Council, highlights the work of three finalists from past Innovation in Education sessions. Each will be presenting their approaches to and innovations in teaching medical physics. It is hoped that audience members interested in trying something new in their teaching of medical physics will find some of these ideas and approaches readily applicable to their own classrooms.Rebecca Howell ‐ The presentation will discuss ways to maximize classroom learning, i.e., increasing the amount of material covered while also enhancing students’ understanding of the broader implications of the course topics. Specifically, the presentation will focus on two teaching methodologies, project based learning and flip learning. These teaching methods will be illustrated using an example of graduate medical physics course where both are used in conjunction with traditional lectures. Additionally, the presentation will focus on our experience implementing these methods including challenges that were overcome.Jay Burmeister ‐ My presentation will discuss the incorporation of active learning techniques into a traditional medical physics classroom course. I will describe these techniques and how they were implemented as well as student performance before and after implementation. Student feedback indicated that these course changes improved their ability to actively assimilate the course content, thus improving their understanding of the material.Shahid Naqvi ‐ My talk will focus on ways to help students visualize crucial concepts that lie at the core of radiation physics. Although particle tracks generated by Monte Carlo simulations have served as an indispensable visualization tool, students often struggle to resolve the underlying physics from a simultaneous jumble of tracks. We can clarify the physics by “coding” the tracks, e.g., by coloring the tracks according to their “starting” or “crossing” regions. The regionally‐coded tracks when overlaid with dose distributions help the students see the elusive connection between dose, kerma and electronic disequilibrium. Tracks coded according to local energy or energy‐loss rate can illustrate the need for stopping power corrections in electron beams and explain the Bragg peak in a proton beam. Coding tracks according to parent interaction type and order can clarify the often misunderstood distinction between primary and scatter dose. The students can thus see the “whole” simultaneously with the “sum of the parts,” which enhances their physical insight and creates a sustainable foundation for further learning.After the presentations the speakers and moderator will be open to questions and discussion with the audience members. Learning Objectives: Be able to explain Project‐Based Learning and how can it be incorporated into a Medical Physics classroom. Be able to explain Flipped Learning and how can it be incorporated into a Medical Physics classroom. Be able to explain active‐learning strategies for the teaching of Medical Physics. Be able to explain how Monte Carlo simulations can be used to deepen a student's understanding of radiation physics and dosimetry.

SU‐A‐BRA‐05: Panel Discussion

Vic Montemayor ‐ No one has been more passionate about improving the quality and effectiveness of the teaching of Medical Physics than Bill Hendee. It was in August of 2008 that the first AAPM Workshop on Becoming a Better Teacher of Medical Physics was held, organized and run by Bill Hendee. This was followed up in July of 2010 with a summer school on the same topic, again organized by Bill.There has been continued interest in alternate approaches to teaching medical physics since those initial gatherings. The momentum established by these workshops is made clear each year in the annual Innovation in Medical Physics Education session, which highlights work being done in all forms of medical physics education, from one‐on‐one residencies or classroom presentations to large‐scale program revisions and on‐line resources for international audiences.This symposium, presented on behalf of the Education Council, highlights the work of three finalists from past Innovation in Education sessions. Each will be presenting their approaches to and innovations in teaching medical physics. It is hoped that audience members interested in trying something new in their teaching of medical physics will find some of these ideas and approaches readily applicable to their own classrooms.Rebecca Howell ‐ The presentation will discuss ways to maximize classroom learning, i.e., increasing the amount of material covered while also enhancing students’ understanding of the broader implications of the course topics. Specifically, the presentation will focus on two teaching methodologies, project based learning and flip learning. These teaching methods will be illustrated using an example of graduate medical physics course where both are used in conjunction with traditional lectures. Additionally, the presentation will focus on our experience implementing these methods including challenges that were overcome.Jay Burmeister ‐ My presentation will discuss the incorporation of active learning techniques into a traditional medical physics classroom course. I will describe these techniques and how they were implemented as well as student performance before and after implementation. Student feedback indicated that these course changes improved their ability to actively assimilate the course content, thus improving their understanding of the material.Shahid Naqvi ‐ My talk will focus on ways to help students visualize crucial concepts that lie at the core of radiation physics. Although particle tracks generated by Monte Carlo simulations have served as an indispensable visualization tool, students often struggle to resolve the underlying physics from a simultaneous jumble of tracks. We can clarify the physics by “coding” the tracks, e.g., by coloring the tracks according to their “starting” or “crossing” regions. The regionally‐coded tracks when overlaid with dose distributions help the students see the elusive connection between dose, kerma and electronic disequilibrium. Tracks coded according to local energy or energy‐loss rate can illustrate the need for stopping power corrections in electron beams and explain the Bragg peak in a proton beam. Coding tracks according to parent interaction type and order can clarify the often misunderstood distinction between primary and scatter dose. The students can thus see the “whole” simultaneously with the “sum of the parts,” which enhances their physical insight and creates a sustainable foundation for further learning.After the presentations the speakers and moderator will be open to questions and discussion with the audience members. Learning Objectives: Be able to explain Project‐Based Learning and how can it be incorporated into a Medical Physics classroom. Be able to explain Flipped Learning and how can it be incorporated into a Medical Physics classroom. Be able to explain active‐learning strategies for the teaching of Medical Physics. Be able to explain how Monte Carlo simulations can be used to deepen a student's understanding of radiation physics and dosimetry.

SU‐A‐BRA‐04: Incorporating Active Learning Into Medical Physics Education

Vic Montemayor ‐ No one has been more passionate about improving the quality and effectiveness of the teaching of Medical Physics than Bill Hendee. It was in August of 2008 that the first AAPM Workshop on Becoming a Better Teacher of Medical Physics was held, organized and run by Bill Hendee. This was followed up in July of 2010 with a summer school on the same topic, again organized by Bill.There has been continued interest in alternate approaches to teaching medical physics since those initial gatherings. The momentum established by these workshops is made clear each year in the annual Innovation in Medical Physics Education session, which highlights work being done in all forms of medical physics education, from one‐on‐one residencies or classroom presentations to large‐scale program revisions and on‐line resources for international audiences.This symposium, presented on behalf of the Education Council, highlights the work of three finalists from past Innovation in Education sessions. Each will be presenting their approaches to and innovations in teaching medical physics. It is hoped that audience members interested in trying something new in their teaching of medical physics will find some of these ideas and approaches readily applicable to their own classrooms.Rebecca Howell ‐ The presentation will discuss ways to maximize classroom learning, i.e., increasing the amount of material covered while also enhancing students’ understanding of the broader implications of the course topics. Specifically, the presentation will focus on two teaching methodologies, project based learning and flip learning. These teaching methods will be illustrated using an example of graduate medical physics course where both are used in conjunction with traditional lectures. Additionally, the presentation will focus on our experience implementing these methods including challenges that were overcome.Jay Burmeister ‐ My presentation will discuss the incorporation of active learning techniques into a traditional medical physics classroom course. I will describe these techniques and how they were implemented as well as student performance before and after implementation. Student feedback indicated that these course changes improved their ability to actively assimilate the course content, thus improving their understanding of the material.Shahid Naqvi ‐ My talk will focus on ways to help students visualize crucial concepts that lie at the core of radiation physics. Although particle tracks generated by Monte Carlo simulations have served as an indispensable visualization tool, students often struggle to resolve the underlying physics from a simultaneous jumble of tracks. We can clarify the physics by “coding” the tracks, e.g., by coloring the tracks according to their “starting” or “crossing” regions. The regionally‐coded tracks when overlaid with dose distributions help the students see the elusive connection between dose, kerma and electronic disequilibrium. Tracks coded according to local energy or energy‐loss rate can illustrate the need for stopping power corrections in electron beams and explain the Bragg peak in a proton beam. Coding tracks according to parent interaction type and order can clarify the often misunderstood distinction between primary and scatter dose. The students can thus see the “whole” simultaneously with the “sum of the parts,” which enhances their physical insight and creates a sustainable foundation for further learning.After the presentations the speakers and moderator will be open to questions and discussion with the audience members. Learning Objectives: Be able to explain Project‐Based Learning and how can it be incorporated into a Medical Physics classroom. Be able to explain Flipped Learning and how can it be incorporated into a Medical Physics classroom. Be able to explain active‐learning strategies for the teaching of Medical Physics. Be able to explain how Monte Carlo simulations can be used to deepen a student's understanding of radiation physics and dosimetry.

WE‐C‐TOUR‐T‐01: Microdosimeters for Therapy

Tour Leader: Indra Das, NYU Langone Medical Center, New York, NYTour Guides:Hsui Ai, Indiana University School of Medicine, Indianapolis, INPaulina Galvis, NYU Langone Medical Center, New York, NYOlga Volotoskova, NYU Langone Medical Center, New York, NYParticipating Vendors:IBAPTW – New YorkRTI Electronics, Inc.Standard Imaging, Inc.Sun Nuclear CorporationSmall fields are increasing used in specialized radiation treatments such as Gammaknife, Cyberknife, Tomotherapy, IMRT, VMAT, SRS and SBRT. Due to small field size electron transport creates lateral electronic disequilibrium and thus dosimetry could be very difficult. Microdetectors are used for small field dosimetry which will be discussed in preface of this tour as below: Understanding small field e.g. meaning and definition of small field IAEA definition and approach Characteristics of microdetectors in terms of perturbation, recombination, correction Suitability of microdetectors in small field dosimetry

WE‐C‐TOUR‐T‐00: Exhibit Hall Guided Tours‐Microdosimeters for Therapy (Wednesday) WE‐C‐TOUR‐T‐01

Tour Leader: Indra Das, NYU Langone Medical Center, New York, NYTour Guides:Hsui Ai, Indiana University School of Medicine, Indianapolis, INPaulina Galvis, NYU Langone Medical Center, New York, NYOlga Volotoskova, NYU Langone Medical Center, New York, NYParticipating Vendors:IBAPTW – New YorkRTI Electronics, Inc.Standard Imaging, Inc.Sun Nuclear CorporationSmall fields are increasing used in specialized radiation treatments such as Gammaknife, Cyberknife, Tomotherapy, IMRT, VMAT, SRS and SBRT. Due to small field size electron transport creates lateral electronic disequilibrium and thus dosimetry could be very difficult. Microdetectors are used for small field dosimetry which will be discussed in preface of this tour as below: Understanding small field e.g. meaning and definition of small field IAEA definition and approach Characteristics of microdetectors in terms of perturbation, recombination, correction Suitability of microdetectors in small field dosimetry

TU‐E‐TOUR‐T‐01: Microdosimeters for Therapy

Tour Leader: Indra Das, NYU Langone Medical Center, New York, NYTour Guides:Hsui Ai, Indiana University School of Medicine, Indianapolis, INAaron Andersen, Indiana University School of Medicine, Indianapolis, INOlga Volotoskova, NYU Langone Medical Center, New York, NYParticipating Vendors:IBAPTW – New YorkRTI Electronics, Inc.Standard Imaging, Inc.Sun Nuclear CorporationSmall fields are increasing used in specialized radiation treatments such as Gammaknife, Cyberknife, Tomotherapy, IMRT, VMAT, SRS and SBRT. Due to small field size electron transport creates lateral electronic disequilibrium and thus dosimetry could be very difficult. Microdetectors are used for small field dosimetry which will be discussed in preface of this tour as below: Understanding small field e.g. meaning and definition of small field IAEA definition and approach Characteristics of microdetectors in terms of perturbation, recombination, correction Suitability of microdetectors in small field dosimetry

TU‐E‐TOUR‐T‐00: Exhibit Hall Guided Tours‐Microdosimeters for Therapy (Tuesday)

Tour Leader: Indra Das, NYU Langone Medical Center, New York, NYTour Guides:Hsui Ai, Indiana University School of Medicine, Indianapolis, INAaron Andersen, Indiana University School of Medicine, Indianapolis, INOlga Volotoskova, NYU Langone Medical Center, New York, NYParticipating Vendors:IBAPTW – New YorkRTI Electronics, Inc.Standard Imaging, Inc.Sun Nuclear CorporationSmall fields are increasing used in specialized radiation treatments such as Gammaknife, Cyberknife, Tomotherapy, IMRT, VMAT, SRS and SBRT. Due to small field size electron transport creates lateral electronic disequilibrium and thus dosimetry could be very difficult. Microdetectors are used for small field dosimetry which will be discussed in preface of this tour as below: Understanding small field e.g. meaning and definition of small field IAEA definition and approach Characteristics of microdetectors in terms of perturbation, recombination, correction Suitability of microdetectors in small field dosimetry

Probing in Space and Time the Nuclear Motion Driven by Nonequilibrium Electronic Dynamics in Ultrafast Pumped N2

An ultrafast electronic excitation of N2 in the vacuum ultraviolet creates a nonstationary coherent linear superposition of interacting valence and Rydberg states resulting in a net oscillating dipole moment. There is therefore a linear response to an electrical field that can be queried by varying the time delay between the pump and a second optical probe pulse. Both the pump and probe pulses are included in our computation as part of the Hamiltonian, and the time-dependent wave function for both electronic and nuclear dynamics is computed using a grid representation for the internuclear coordinate. Even on an ultrafast time scale there are several processes that can be discerned beyond the expected coherence oscillations. In particular, the coupling between the excited valence and Rydberg states of the same symmetry is very evident and can be directly probed by varying the delay between pulse and probe. For quite a number of vibrations the nuclear motion does not dephase the electronic disequilibrium. However, the nuclear motion does modulate the dipolar response by taking the wave packet in and out of the Franck-Condon region and by its strong influence on the coupling of the Rydberg and valence states. A distinct isotope effect arises from the dependence of the interstate coupling on the nuclear mass.

Monte Carlo dose verification for lung SBRT with CMS/XiO superposition algorithm

Purpose. Superposition dose calculation algorithms are known to suffer compromised accuracy in regions of electronic disequilibrium. This has particular relevance for treatment of lung nodules with electronic disequilibrium in their periphery using stereotactic body radiotherapy (SBRT). This study assessed the accuracy of a commercial superposition dose calculation algorithm within the context of SBRT in order to quantify the impact of these potential dosimetric uncertainties particularly in the periphery of the target. Method and materials. Selected lung SBRT 3D-CRT plans with 10–13 beams of 6 MV were originally created on the CMS/XiO treatment planning system with the superposition algorithm. The corresponding Monte Carlo (MC) plans were produced using the same patient geometry and plan parameters and compared with the CMS plans. Electronic disequilibrium was investigated by constructing a 2 mm thick ‘peel’ of the gross target volume (GTV) and planning target volume (PTV). An 18 MV plan was created corresponding to each 6 MV plan. Beam reduction was studied to investigate the dosimetric effect of beam arrangement. Results. The doses calculated by CMS agreed with MC in a water phantom but overestimated in very low density areas and the buildup and builddown regions. In patient plan comparison, for GTVs and GTV peels, CMS agreed with MC in mean dose within 2.4% for the 6 MV plans, and 3.3% for the 18 MV plans. For PTVs and PTV peels, CMS agreed with MC in mean doses within 2.5% for both 6 and 18 MV beams. The ratio in D95 between CMS and MC was close to 1.0 for all 6 MV plans. Reducing beams could result in underdoses. Conclusions. The CMS superposition algorithm is able to perform accurate dose calculations for SBRT lung plans and ensure adequate peripheral dose. Dosimetric uncertainties are correlated to many factors such as density, energy, number of beams, tumor size/location, and density variation of the target and surrounding tissues.

Dose accuracy research on air cavity interface in treatment planning system

Objective To study the accuracy of collapsed cone convolution (CCC) and anisotropic analytical algorithm (AAA) in dosimetric calculation on the air cavity interface. Methods A BEAMnrc/EGSnrc Monte Carlo (MC) simulation was performed on a Varian Trilogy linear accelerator. The IBA Dosimetry blue 3D scanning system was used to verify the accuracy and reliability of the MC simulation. Central axis depth dose distribution and lateral dose profile in a water-equivalent phantom with variously sized air cavities were calculated by CCC and AAA. The obtained depth dose distribution and lateral dose profile were compared with those by MC simulation and EBT2 film, respectively. Results Both CCC and AAA overestimated the dose on the air cavity interface. In spite of some errors, CCC had a higher accuracy than AAA. The errors were mainly related to computational grid, field size, photon energy, cavity size, and the number of fields. Conclusion Electronic disequilibrium on the air cavity interface should be taken into account when CCC and AAA are used for dosimetric calculation in treatment planning system. Key words: Electron disequilibrium; Collapsed cone convolution; Anisotropic analytical algorithm; Monte Carlo algorithm; EBT2 film

Evaluation of MRI-based MAGIC polymer gel dosimeter in small photon fields

Background: Accurate small radiation field dosimetry is essential in modern radiotherapy techniques such as stereotactic radiosurgery (SRS) and intensity modulated radiotherapy (IMRT). Precise measurement of dosimetric parameters such as beam profile, percentage depth doses and output factor of these beams are complicated due to the electron disequilibrium and the steep dose gradients. In the present work the MAGIC polymer gel was used for dosimetry of small circular photon beams. The results of MAGIC were compared with EBT2 measurements and Monte Carlo (MC) calculations. Materials and Methods: Experimental measurements were made by mentioned dosimeters in four small field sizes 5, 10, 20 and 30 mm. The BEAMnrc code based on EGSnrc was used for simulation to calculate dosimetric parameters at these small fields. The phantoms were irradiated in a 6 MV photon beam Varian 2100C linear accelerator at SSD=100 cm. gel readout performed by 3 Tesla MRI scanner. Results: The results showed that the Percent depth dose (PDD) values measured and calculated by EBT2 film and MC had maximum local differences 4% and 5% with PDD values measured by MAGIC for field size of 5mm respectively. These differences decreased for larger field sizes. The measurements of output factor and penumbra (80%-20%) and (90%-10%) showed good agreement between the measurements and MC calculation. Conclusion: This study showed that the MAGIC polymer gel based on high resolution MRI images is useful detector for small field dosimetry but its agreement with MC is less than agreement of EBT2 film with MC.

Small field output factors: Comparison of measurements with various detectors and effects of detector orientation with primary jaw setting

Accurate dosimetry in small photon fields used in modern radiotherapy is a challenging task due to electronic disequilibrium, steep dose gradients, source occlusion and size of the sensitive volume of the detector. These challenging effects and the lack of metrological dosimetric reference instigated an investigation on the acquisition of output factor with various detectors in parallel and perpendicular orientations. Small field output factor measurements of tertiary collimators such as BrainLab circular cones, BrainLab mMLC and Millennium MLC were carried out in this study. The data acquired show the differences between output factor results with different detectors for all collimating systems. Good agreement in output values was observed in field sizes greater than ∼2 × 2 cm2 for all detectors and all tertiary collimators. For smaller fields when compared to electron field diode (EFD), 0.125 cm3 ion chamber underestimates the output by up to −11.1% and −20.4% and pinpoint ion chamber underestimates the output by up to −1.5% and −6.1% in their parallel and perpendicular orientation, respectively. In contrast, PTW SRS diode and photon field diode (PFD) overestimate the output factor by up to 2.5% and 6.9% respectively in its parallel orientation. The investigated data for the effect of jaw position (0.25 × 0.25 cm2, 0.5 × 0.5 cm2 and 1 × 1 cm2) away from the field edge generated by different tertiary collimating systems inferred that the opening of X–Y jaw highly influences the small field output factors. The orientation of the detectors and the position of the jaws could influence the output factors considerably in small fields.

Breakdown of Bragg–Gray behaviour for low-density detectors under electronic disequilibrium conditions in small megavoltage photon fields

In small photon fields ionisation chambers can exhibit large deviations from Bragg–Gray behaviour; the EGSnrc Monte Carlo (MC) code system has been employed to investigate this ‘Bragg–Gray breakdown’. The total electron (+positron) fluence in small water and air cavities in a water phantom has been computed for a full linac beam model as well as for a point source spectrum for 6 MV and 15 MV qualities for field sizes from 0.25 × 0.25 cm2 to 10 × 10 cm2. A water-to-air perturbation factor has been derived as the ratio of total electron (+positron) fluence, integrated over all energies, in a tiny water volume to that in a ‘PinPoint 3D-chamber-like’ air cavity; for the 0.25 × 0.25 cm2 field size the perturbation factors are 1.323 and 2.139 for 6 MV and 15 MV full linac geometries respectively. For the 15 MV full linac geometry for field sizes of 1 × 1 cm2 and smaller not only the absolute magnitude but also the ‘shape’ of the total electron fluence spectrum in the air cavity is significantly different to that in the water ‘cavity’. The physics of this ‘Bragg–Gray breakdown’ is fully explained, making reference to the Fano theorem. For the 15 MV full linac geometry in the 0.25 × 0.25 cm2 field the directly computed MC dose ratio, water-to-air, differs by 5% from the product of the Spencer–Attix stopping-power ratio (SPR) and the perturbation factor; this ‘difference’ is explained by the difference in the shapes of the fluence spectra and is also formulated theoretically. We show that the dimensions of an air-cavity with a perturbation factor within 5% of unity would have to be impractically small in these highly non-equilibrium photon fields. In contrast the dose to water in a 0.25 × 0.25 cm2 field derived by multiplying the dose in the single-crystal diamond dosimeter (SCDDo) by the Spencer–Attix ratio is within 2.9% of the dose computed directly in the water voxel for full linac geometry at both 6 and 15 MV, thereby demonstrating that this detector exhibits quasi Bragg–Gray behaviour over a wide range of field sizes and beam qualities.

TU‐CD‐304‐06: Using FFF Beams Improves Tumor Control in Radiotherapy of Lung Cancers

Purpose:Electron disequilibrium at the lung‐tumor interface results in an under‐dosage of tumor regions close to its surface. This under‐dosage is known to be significant and can compromise tumor control. Previous studies have shown that in FFF beams, disequilibrium effects are less pronounced, which is manifested in an increased skin dose. In this study we investigate the improvement in tumor dose coverage that can be achieved with FFF beams. The significance of this improvement is evaluated by comparing tumor control probabilities of FFF beams and conventional flattened beams.Methods:The dosimetric coverage was investigated in a virtual phantom representing the chest wall, lung tissue and the tumor. A range of tumor sizes was investigated, and two tumor locations – central and adjacent to the chest wall. Calculations were performed with BEAMnrc Monte Carlo code. Parallel‐opposed and multiple coplanar 6‐MV beams were simulated. The tumor control probabilities were calculated using the logistic model with parameters derived from clinical data for non‐small lung cancer patients.Results:FFF beams were not entirely immune to disequilibrium effects. They nevertheless consistently delivered more uniform dose distribution throughout the volume of the tumor, and eliminated up to ∼15% of under‐dosage in the most affected by disequilibrium 1‐mm thick surface region of the tumor. A voxel‐by‐voxel comparison of tumor control probabilities between FFF and conventional flattened beams showed an advantage of FFF beams that, depending on the set up, was from a few to ∼9 percent.Conclusion:A modest improvement in tumor control probability on the order of a few percent can be achieved by replacing conventional flattened beams with FFF beams. However, given the large number of lung cancer patients undergoing radiotherapy, these few percent can potentially prevent local tumor recurrence for a significant number of patients.

WE‐G‐BRD‐05: Inline Magnetic Fields Enhance Tumor Dose for Small Lung Cancers

Purpose:To report on significant dose enhancement effects caused by magnetic fields aligned parallel to 6MV photon beam radiotherapy of small lung tumors. Findings are applicable to future inline MRI‐guided radiotherapy systems.Methods:9 clinical lung plans were recalculated using Monte Carlo methods and external inline (parallel to the beam direction) magnetic fields of 0.5 T, 1.0 T and 3 T were included. Three plans were 6MV 3D‐CRT and six were 6MV IMRT. The GTV's ranged from 0.8 cc to 73 cc, while the PTV ranged from 1 cc to 180 cc.Results:The inline magnetic field has a moderate impact in lung dose distributions by reducing the lateral scatter of secondary electrons and causing a small local dose increase. Superposition of multiple small beams acts to superimpose the small dose increases and can lead to significant dose enhancements, especially when the GTV is low density. Two plans with very small, low mean density GTV's (<1 cc, ρ(mean)<0.35g/cc) showed uniform increases of 16% and 23% at 1 T throughout the PTV. Three plans with moderate mean density PTV's (3–13 cc, ρ(mean)=0.58–0.67 g/cc) showed 6% mean dose enhancement at 1 T in the PTV, however not uniform throughout the GTV/PTV. Replanning would benefit these cases. The remaining 5 plans had large dense GTV's (∼ 1 g/cc) and so only a minimal (<2%) enhancement was seen. In general the mean dose enhancement at 0.5 T was 60% less than 1 T, while 5–50% higher at 3 T.Conclusions:A paradigm shift in the efficacy of small lung tumor radiotherapy is predicted with future inline MRI‐linac systems. This will be achieved by carefully taking advantage of the reduction of lateral electronic disequilibrium withing lung tissue that is induced naturally inside strong inline magnetic fields.

Photon small-field measurements with a CMOS active pixel sensor

In this work the dosimetric performance of CMOS active pixel sensors for the measurement of small photon beams is presented. The detector used consisted of an array of 520 × 520 pixels on a 25 µm pitch. Dosimetric parameters measured with this sensor were compared with data collected with an ionization chamber, a film detector and GEANT4 Monte Carlo simulations. The sensor performance for beam profiles measurements was evaluated for field sizes of 0.5 × 0.5 cm2. The high spatial resolution achieved with this sensor allowed the accurate measurement of profiles, beam penumbrae and field size under lateral electronic disequilibrium. Field size and penumbrae agreed within 5.4% and 2.2% respectively with film measurements. Agreements with ionization chambers better than 1.0% were obtained when measuring tissue-phantom ratios. Output factor measurements were in good agreement with ionization chamber and Monte Carlo simulation. The data obtained from this imaging sensor can be easily analyzed to extract dosimetric information. The results presented in this work are promising for the development and implementation of CMOS active pixel sensors for dosimetry applications.

Evaluation of dose perturbation at the interface of two different density medium using GAFCHROMIC film EBT2 and Monte Carlo code EGSnrc for Co-60 beam

Accurate dosimetry at the interface of two different density medium (e.g., air cavity in the head and neck cancers and lungs in thoracic region) is a major cause of concern in external beam radiation therapy. It has been observed that there is dose variation in and around air cavities, which occur as a result of the loss of both longitudinal and lateral electronic equilibrium. Heterogeneous structures with spatial differences in functionality and sensitivity for radiation pose challenge to radiation dosimetry. This study is an attempt to evaluate the dose perturbations produced at the interface of two medium for C0-60 gamma radiation. Low density polyethene foam has been used to mimic air cavity. GAFCHROMIC EBT2 dosimetry film was used for the measurement of dose at different locations. Simulation studies were performed using DOSRZnrc user code that comes with EGSnrc V4 2.4.0. Cylindrical geometry is used for all the simulations. We observed significant variation in dose for smaller fields. There is a dose build down in the backward region and a dose build up in the forward direction. In the region of electronic disequilibrium, dose reduction near interface (proximal end) will have negative impact if target region is embedded there, on the contrary, it would be beneficial if there is normal tissue/critical organ adjacent to it.

Poster - Thur Eve - 62: A Retrospective Assessment of the Prevalence and Dosimetric Effect of Lateral Electron Disequilibrium in a Population of Lung Cancer Patients Treated by Stereotactic Body Radiation Therapy

Stereotactic Body Radiation Therapy (SBRT) is a treatment option for early stage non-small cell lung cancer (NSCLC). SBRT uses tightly conformed megavoltage (MV) x-ray beams to ablate the tumour. However, small MV x-ray fields may produce lateral electron disequilibrium (LED) within lung tissue, which can reduce the dose to tumour. The goal of this work is to estimate the prevalence of LED in NSCLC patients treated with SBRT, and determine dose effects for patients prone or averse to LED. Thirty NSCLC patients were randomly selected for analysis. 4-dimensional CT lung images were segmented into the right and left upper and lower lobes (RUL, RLL, LUL, LLL), and the right middle lobe. Dose calculations were performed using volume-modulated arc therapy in the Pinnacle3 TPS. Most tumours were located in the upper lobes (RUL 53%, LUL 27%) where density was significantly lower (RUL −808±46 HU vs. RLL −743±71 HU; LUL −808 ±56 HU vs. LLL −746±70 HU; p<0.001). In general, the prevalence of LED increased with higher beam energy. Using 6MV photons, patients with a RUL tumour experienced moderate (81 %), and mild (19%) levels of LED. At 18MV, LED became more prominent with severe (50%) and moderate (50%) LED exhibited. Dosimetrically, for patients prone to LED, poorer target coverage (i.e. increased R100 by 20%) and improved lung sparing (i.e. reduced V20 by −46%) was observed. The common location of lung cancers in the upper lobes, coupled with lower lung density, results in the potential occurrence of LED, which may underdose the tumour.

Poster — Thur Eve — 43: Monte Carlo Modeling of Flattening Filter Free Beams and Studies of Relative Output Factors

Flattening filter free (FFF) beams have been adopted by many clinics and used for patient treatment. However, compared to the traditional flattened beams, we have limited knowledge of FFF beams. In this study, we successfully modeled the 6 MV FFF beam for Varian TrueBeam accelerator with the Monte Carlo (MC) method. Both the percentage depth dose and profiles match well to the Golden Beam Data (GBD) from Varian. MC simulations were then performed to predict the relative output factors. The in‐water output ratio, Scp, was simulated in water phantom and data obtained agrees well with GBD. The in‐air output ratio, Sc, was obtained by analyzing the phase space placed at isocenter, in air, and computing the ratio of water Kerma rates for different field sizes. The phantom scattering factor, Sp, can then be obtained from the traditional way of taking the ratio of Scp and Sc. We also simulated Sp using a recently proposed method based on only the primary beam dose delivery in water phantom. Because there is no concern of lateral electronic disequilibrium, this method is more suitable for small fields. The results from both methods agree well with each other. The flattened 6 MV beam was simulated and compared to 6 MV FFF. The comparison confirms that 6 MV FFF has less scattering from the Linac head and less phantom scattering contribution to the central axis dose, which will be helpful for improving accuracy in beam modeling and dose calculation in treatment planning systems.