Electronic Disequilibrium Research Articles

Comparative assessment and QA measurement array validation of Monte Carlo and Collapsed Cone dose algorithms for small fields and clinical treatment plans.

Many studies have demonstrated superior performance of Monte Carlo (MC) over type B algorithms in heterogeneous structures. However, even in homogeneous media, MC dose simulations should outperform type B algorithms in situations of electronic disequilibrium, such as small and highly modulated fields. Our study compares MC and Collapsed Cone (CC) dose algorithms in RayStation 12A. Under consideration are 6 MV and 6 MV flattening filter-free (FFF) photon beams, relevant for VMAT plans such as head-and-neck and stereotactic lung treatments with heterogeneities, as well as plans for multiple brain metastases in one isocenter, involving highly modulated small fields. We aim to investigate collimator angle dependence of small fields and performance differences between different combinations of ArcCHECK configuration and dose algorithm. Several verification tests were performed, ranging from simple rectangular fields to highly modulated clinical plans. To evaluate and compare the performance of the models, the agreements between calculation and measurement are compared between MC and CC. Measurements include water tank measurements for test fields, ArcCHECK measurements for test fields and VMAT plans, and film dosimetry for small fields. In very small or narrow fields, our measurements reveal a strong dependency of dose output to collimator angle for VersaHD with Agility MLC, reproduced by both dose algorithms. ArcCHECK results highlight a suboptimal agreement between measurements and MC calculations for simple rectangular fields when using inhomogeneous ArcCHECK images. Therefore, we advocate for the use of homogeneous phantom images, particularly for static fields, in ArcCHECK verification with MC. MC might offer performance benefits for more modulated treatment fields. In ArcCHECK results for clinical plans, MC performed comparable to CC for 6 MV. For 6 MV FFF and the preferred homogeneous phantom image, MC resulted in consistently better results (13%-64% lower mean gamma index) compared to CC.

Toward a multi-layer micro-structured detector for high-energy electron radiotherapy.

The use of electron beams has been rekindled by the advent of ultra-high-dose rate radiotherapy (FLASH) and very high energy electrons (VHEE). The need for development of novel technology for beam monitoring and dosimetry of such beams is of paramount importance prior to their clinical translation. In this work we explore the potential of a multi-layer nanoporous aerogel High-Energy-Current (HEC) detector as a dosimeter for electron beam. The detector does not suffer from radiation damage or signal saturation, making it suitable for very-high-dose-rate applications. Standard dose rates and energies are used to establish reference for FLASH and VHEE. We explore detector response to electron energy and residual range both experimentally and computationally. Multilayer HEC detectors were constructed using 1×-10× basic modules of Aluminum(Al)_aerogel(A)_Tantalum(Ta) with 10-70 µm layer thicknesses. Signals are collected from all electrodes (3-21, depending on module multiplicity) with zero external voltage bias. Measurements are acquired as a function of depth(z) in water equivalent plastic using Varian TrueBeam for energies E=6,9,12,15 MeV (SAD=105cm, 6×6 cone, 1000 MU/min). Computational simulations of identical detector geometries are performed using the 1D deterministic code CEPXS/ONEDANT. Additionally, percent-depth-doses PDD(z), measured with diode in water, are used to explore the response of HEC for various energies and residual ranges. The current measured from Ta electrodes resembles the shape of deposited charges in water and it is proportional to the derivative of the clinical PDD corrected for contribution from photon contamination. The signal is positive on the surface, and it decreases with depth reaching a negative local minimum at z=R50, before increasing again, reaching zero at about the practical range z=Rp. In contrast, the signal from Al electrodes is shaped like the electron PDD(z) shape but with lower signal at the surface and higher bremsstrahlung tail. By subtracting the signal from Ta and Al electrodes we obtained a curve resembling PDD(z,E) after Bremsstrahlung contamination correction. Multi-layer HEC sensors exhibit characteristic responses to electron beams that are unlike responses of ion chambers or diodes. Since the sensor structures are sensitive to electronic disequilibrium, high-Z electrodes give a signal proportional to the charge deposition pattern and can be modeled using the derivative of PDD(z).

Generation, validation, and benchmarking of a commercial independent Monte Carlo calculation beam model for multi-target SRS

BackgroundDosimetric validation of single isocenter multi-target radiosurgery plans is difficult due to conditions of electronic disequilibrium and the simultaneous irradiation of multiple off-axis lesions dispersed throughout the volume. Here we report the benchmarking of a customizable Monte Carlo secondary dose calculation algorithm specific for multi-target radiosurgery which future users may use to guide their commissioning and clinical implementation. PurposeTo report the generation, validation, and clinical benchmarking of a volumetric Monte Carlo (MC) dose calculation beam model for single isocenter radiosurgery of intracranial multi-focal disease. MethodsThe beam model was prepared within SciMoCa (ScientificRT, Munich Germany), a commercial independent dose calculation software, with the aim of broad availability via the commercial software for use with single isocenter radiosurgery. The process included (1) definition & acquisition of measurement data required for beam modeling, (2) tuning model parameters to match measurements, (3) validation of the beam model via independent measurements and end-to-end testing, and finally, (4) clinical benchmarking and validation of beam model utility in a patient specific QA setting. We utilized a 6X Flattening-Filter-Free photon beam from a TrueBeam STX linear accelerator (Siemens Healthineers, Munich Germany). ResultsIn addition to the measured data required for standard IMRT/VMAT (depth dose, central axis profiles & output factors, leaf gap), beam modeling and validation for single-isocenter SRS required central axis and off axis (5 cm & 9 cm) small field output factors and comparison between measurement and simulation of backscatter with aperture for jaw much greater than MLCs. Validation end-to-end measurements included SRS MapCHECK in StereoPHAN geometry (2%/1 mm Gamma = 99.2% ± 2.2%), and OSL & scintillator measurements in anthropomorphic STEEV phantom (6 targets, volume = 0.1–4.1cc, distance from isocenter = 1.2–7.9 cm) for which mean difference was −1.9% ± 2.2%. For 10 patient cases, MC for individual PTVs was −0.8% ± 1.5%, −1.3% ± 1.7%, and −0.5% ± 1.8% for mean dose, D95%, and D1%, respectively. This corresponded to custom passing rates action limits per AAPM TG-218 guidelines of ±5.2%, ±6.4%, and ±6.3%, respectively. ConclusionsThe beam modeling, validation, and clinical action criteria outlined here serves as a benchmark for future users of the customized beam model within SciMoCa for single isocenter radiosurgery of multi-focal disease.

Evaluation of Radiation Treatment Planning Algorithms in IMRT and VMAT: A Comparative Dosimetric Study in Lung Equivalent Heterogeneous Medium.

In Radiotherapy, computation of dose is important since in a small field with heterogeneity, dose is usually computed with discrepancies. The present study was aimed to evaluate the dosimetry of treatment planning algorithms in lung equivalent heterogeneous medium for Volumetric Modulated Arc Therapy (VMAT) with step and shoot Intensity-Modulated Radiation Therapy (ss-IMRT), and dynamic Intensity-Modulated Radiation Therapy (d-IMRT). In this experimental study, Computerized Imaging Reference System (CIRS) phantom was used with an inhomogeneous Racemosa wood cylinder for two types of tumors, namely, Left Lung Central Tumor (LCT) and Left Lung Peripheral Tumor (LPT) in the CIRS left lung cavity. The computed tomography (CT) datasets were employed with the generation of VMAT, d-IMRT and ss-IMRT plans for the LCT and LPT irradiated with 6 MV photon beams. In this study, the accuracy and efficacy of two algorithms: Monte Carlo (MC) and the Pencil Beam (PB), from the Monaco treatment planning system (TPS), were tested by using Gafchromic EBT3 films and CIRS thorax phantom. Regardless of treatment techniques, both algorithms exhibited higher divergence in LPT than LCT. In both LCT and LPT, the highest deviation was near the tumor-lung junction. However, the deviation was higher in the PB algorithm than MC algorithm, with a minimally acceptable variation of -0.8%. The MC algorithm shows more consistency for EBT3 measured dose in lung equivalent heterogeneous medium. However, accurate dose predictions are complicated due to electronic disequilibrium within and at the interface of inhomogeneity. These constraints may cause variations from the anticipated outcomes of the treatments.

Accuracy Evaluation of Collapsed Cone Convolution Superposition Algorithms for the Nasopharynx Interface in the Early Stage of Nasopharyngeal Carcinoma.

This study combined the use of radiation dosimeteric measurements and a custom-made anthropomorphic phantom in order to evaluate the accuracy of therapeutic dose calculations at the nasopharyngeal air-tissue interface. The doses at the nasopharyngeal air-tissue interface obtained utilizing the Pinnacle and TomoTherapy TPS, which are based on collapsed cone convolution superposition (CCCS) algorithms, were evaluated and measured under single 10 × 10 cm2, 2 × 2 cm2, two parallel opposed 2 × 2 cm2 and clinical fields for early stage of nasopharyngeal carcinoma by using EBT3, GR-200F, and TLD 100. At the air-tissue interface under a 10 × 10 cm2 field, the TPS dose calculation values were in good agreement with the dosimeter measurement with all differences within 3.5%. When measured the single field 2 × 2 cm2, the differences between the average dose were measured at the distal interface for EBT3, GR-200F, and TLD-100 and the calculation values were -15.8%, -16.4%, and -4.9%, respectively. When using the clinical techniques such as IMRT, VMAT, and tomotherapy, the measurement results at the interface for all three techniques did not imply under dose. Small-field sizes will lead to dose overestimation at the nasopharyngeal air-tissue interface due to electronic disequilibrium when using CCCS algorithms. However, under clinical applications of multiangle irradiation, the dose errors caused by this effect were not significant.

The effects of intestinal air cavity on dose distribution of volume modulated arc therapy for cervical cancer.

This study explored the effects of air cavity on the dose distribution of radiotherapy in patients after extensive hysterectomy. In patients who have an air cavity in the intestines near the planning target volume (PTV), the photon beams may interact with the air cavity to cause electron disequilibrium (ED), resulting in a reduction in the absorbed dose by the surrounding tissues. In this paper, the electron density assignment (EDA) of the air cavity was used to simulate the disappearance of the intestinal gas, and the effects of the intestinal air cavity on the volume modulated arc therapy (VMAT) results were examined. A total of 21 patients who underwent VMAT after extensive hysterectomy were enrolled in this retrospective analysis. The dose parameters from the selected treatment plan were used the experimental reference. The treatment plan of the reference group was copied, and the intestinal air cavity structure was identified using the computed tomography (CT) simulation image. The electron density value of the intestine near the cavity was measured and averaged according to the intestinal electron density value recommended by International Commission Radiological Units (ICRU) report No. 46. The averaged value was assigned to the air cavity structure. Subsequently, the treatment plan was re-calculated without changing other parameters, and the resulting treatment plan was defined as the experimental group. The dose parameters of the PTV and organs at risk (OAR) in the 2 groups were assessed, and the influence of the intestinal air cavity on the VMAT dose distribution in cervical cancer patients was analyzed. The minimum dose (D98) and the maximum dose (D2) of the PTV was significantly different between the experimental group and the control group (P<0.05), however, mean dose of the PTV was comparable between the 2 groups. The dose parameters of OARs were not significantly different between the two groups except the bone structural organs. When the intestinal air cavity is large and related to the target area, the intestinal air cavity should be intervened, and the patient should be treated with radiotherapy after the intestinal gas decreases.

Boosting radiotherapy dose calculation accuracy with deep learning.

In radiotherapy, a trade‐off exists between computational workload/speed and dose calculation accuracy. Calculation methods like pencil‐beam convolution can be much faster than Monte‐Carlo methods, but less accurate. The dose difference, mostly caused by inhomogeneities and electronic disequilibrium, is highly correlated with the dose distribution and the underlying anatomical tissue density. We hypothesize that a conversion scheme can be established to boost low‐accuracy doses to high‐accuracy, using intensity information obtained from computed tomography (CT) images. A deep learning‐driven framework was developed to test the hypothesis by converting between two commercially available dose calculation methods: Anisotropic analytic algorithm (AAA) and Acuros XB (AXB). A hierarchically dense U‐Net model was developed to boost the accuracy of AAA dose toward the AXB level. The network contained multiple layers of varying feature sizes to learn their dose differences, in relationship to CT, both locally and globally. Anisotropic analytic algorithm and AXB doses were calculated in pairs for 120 lung radiotherapy plans covering various treatment techniques, beam energies, tumor locations, and dose levels. For each case, the CT and the AAA dose were used as the input and the AXB dose as the “ground‐truth” output, to train and test the model. The mean squared errors (MSEs) and gamma passing rates (2 mm/2% & 1 mm/1%) were calculated between the boosted AAA doses and the “ground‐truth” AXB doses. The boosted AAA doses demonstrated substantially improved match to the “ground‐truth” AXB doses, with average (± s.d.) gamma passing rate (1 mm/1%) 97.6% (±2.4%) compared to 87.8% (±9.0%) of the original AAA doses. The corresponding average MSE was 0.11(±0.05) vs 0.31(±0.21). Deep learning is able to capture the differences between dose calculation algorithms to boost the low‐accuracy algorithms. By combining a less accurate dose calculation algorithm with a trained deep learning model, dose calculation can potentially achieve both high accuracy and efficiency.

The impact of a magnetic field on the dose distribution using the Bebig60Co HDR sources

Objective To evaluate the impact of an external magnetic field on the dose distribution and electronic disequilibrium region around a Bebig type 60Co HDR brachytherapy source and to judge the feasibility of applying MRI scanner during brachytherapy. Methods The source model was established based on the Monte Carlo package Geant4 software. The simulated geometries consisted of the 60Co source inside a water phantom of 10.0cm×10.0cm×10.0cm in size. The magnetic field strength of the 0T, 1.5T and 3.0T was considered, respectively. The voxels with a size of 0.2 mm, 0.5 mm and 0.5 mm were established along the x-, y-and z-axis. The influence of the magnetic field on the Kerma (kinetic energy released to matter) distribution and dose distribution within the 10.0mm region from the source center was evaluated. Furthermore, the ratio of the Kerma to dose as a function of the distance to the center source was acquired. Results The 1.5T magnetic field exerted no effect on the dose distribution adjacent to 60Co HDR brachytherapy source, whereas3.0T magnetic field caused significant increase in the dose distribution within r<6 mm from the source center. The dose distribution was increased by 40% at r=5.4 mm from the source center. The ratio of Kerma to dose was less than 1 within the region of 1.2 mm<r<6.0 mm, suggesting that 3.0T magnetic field can lead to electronic disequilibrium within a larger region from the source center. Conclusions For Bebig 60Co HDR brachytherapy source, it is safe and reliable to apply1.5T external magnetic field. Nevertheless, 3.0T magnetic field can cause high dose risk. Consequently, safety assessment and verification should be delivered prior to clinical application. Key words: Magnetic field; Brachytherapy; Dose; Kerma

The effect of SSD, Field size, Energy and Detector type for Relative Output Factor measurement in small photon beams as compared with Monte Carlo simulation

Abstract Introduction: Small fields photon dosimetry is associated with many problems. Using the right detector for measurement plays a fundamental role. This study investigated the measurement of relative output for small photon fields with different detectors. It was investigated for three-photon beam energies at SSDs of 90, 95, 100 and 110 cm. As a benchmark, the Monte Carlo simulation was done to calculate the relative output of these small photon beams for the dose in water. Materials and Methods: 6, 10 and 15 MV beams were delivered from a Synergy LINAC equipped with an Agility 160 multileaf collimator (MLC). A CC01 ion chamber, EFD-3G diode, PTW60019 microdiamond, EBT2 radiochromic film, and EDR2 radiographic film were used to measure the relative output of the linac. Measurements were taken in water for the CC01 ion chamber, EFD-3G diode, and the PTW60019. Films were measured in water equivalent RW3 phantom slabs. Measurements were made for 1 × 1, 2 × 2, 3 × 3, 4 × 4, 5 × 5 and a reference field of 10 × 10 cm2. Field sizes were defined at 100cm SSD. Relative output factors were also compared with Monte Carlo (MC) simulation of the LINAC and a water phantom model. The influence of voxel size was also investigated for relative output measurement. Results and Discussion: The relative output factor (ROF) increased with energy for all fields large enough to have lateral electronic equilibrium (LEE). This relation broke down as the field sizes decreased due to the onset of lateral electronic disequilibrium (LED). The high-density detector, PTW60019 gave the highest ROF for the different energies, with the less dense CC01 giving the lowest ROFs. Conclusion: These are results compared to MC simulation, higher density detectors give higher ROF values. Relative to water, the ROF measured with the air-chamber remained virtually unchanged. The ROFs, as measured in this study showed little variation due to increased SSDs. The effect of voxel size for the Monte Carlo calculations in water does not lead to significant ROF variation over the small fields studied.

Evaluation of Full scatter convolution algorithm based Treatment Planning System performance in the presence of inhomogeneities using three-dimensional film dosimetry

Introduction: Inclusion of inhomogeneities such as air-filled cavities in the head and neck treatment fields may result in potential dosimetric disagreement which was caused by electronic disequilibrium. Most of treatments planning systems (TPS) are not able to predict dose distribution of inhomogeneous regions accurately. EBT2 films are used frequently in radiotherapy quality assurance testing to evaluate the performance of dose delivery systems and TPS. The development and usage of 3D film dosimetry method may be useful. Materials and Methods: 9 pieces of EBT2 films were embedded in a rectangular inhomogeneous head and neck phantom. Films were exposed with 6 MV X-ray photons with parallel-opposed fields and three approximate small field sizes were examined. A homemade computer code was developed in MATLAB for creation of 3D dose map of irradiated films and calculation of enclosed volumes which were surrounded by isodose lines of films. Calculated dose volumes should be compared with Dose Volume Histogram (DVH) which was derived from TPS. Results: our results showed significant differences between the results of the film dosimetry and TPS. Maximum difference of calculated and measured values was observed in volumes surrounded by 95% isodose curves and 3x3cm2 field size (P=0.035) and minimum difference was observed in volumes surrounded by 95% isodose curves and 5x5cm2 field size (P=0.047). Conclusion: The results of this study demonstrated overestimated results in volumetric calculation of TRGRT dose calculation algorithm in the presence of inhomogeneities and approxi small fields sizes. Also, these results confirmed that utilization of an accurate and comprehen method such as 3D film dosimetry can be useful for TPS verification.

Radiotherapy of lung cancers: FFF beams improve dose coverage at tumor periphery compromised by electronic disequilibrium

The purpose of this work was to investigate radiotherapy underdosing at the periphery of lung tumors, and differences in dose for treatments delivered with flattening filter-free (FFF) beams and with conventional flattened (FF) beams. The true differences between these delivery approaches, as assessed with Monte Carlo simulations, were compared to the apparent differences seen with clinical treatment planning algorithms AAA and Acuros XB.Dose was calculated in a phantom comprised of a chest wall, lung parenchyma, and a spherical tumor (tested diameters: 1, 3, and 5 cm). Three lung densities were considered: 0.26, 0.2, and 0.1 g cm−3, representing normal lung, lung at full inspiration, and emphysematous lung, respectively. The dose was normalized to 50 Gy to the tumor center and delivered with 7 coplanar, unmodulated 6 MV FFF or FF beams. Monte Carlo calculations used EGSnrc and phase space files for the TrueBeam accelerator provided by Varian Medical Systems. Voxel sizes were 0.5 mm for the 1 cm tumor and 1 mm for the larger tumors. AAA and Acuros XB dose calculations were performed in Eclipse™ with a 2.5 mm dose grid, the resolution normally used clinically.Monte Carlo dose distributions showed that traditional FF beams underdosed the periphery of the tumor by up to ~2 Gy as compared to FFF beams; the latter provided a more uniform dose throughout the tumor. In all cases, the underdosed region was a spherical shell about 5 mm thick around the tumor and extending into the tumor by 2–3 mm. The effect was most pronounced for smaller tumors and lower lung densities. The underdosing observed with conventional FF beams was not captured by the clinical treatment planning systems.We concluded that FFF beams mitigate dose loss at tumor periphery and current clinical practice fails to capture tumor periphery underdosing and possible ways to mitigate it.

MLC positioning verification for small fields: a new investigation into automatic EPID-based verification methods.

Multileaf-collimator (MLC) defined small fields in radiotherapy are used in high dose, ultra-conformal techniques such as stereotactic radiotherapy and stereotactic radiosurgery. Proximity to critical structures and irreversible damage arising from inaccurate delivery mean that correct positioning of the MLC system is of the utmost importance. Some of the existing techniques for MLC positioning quality assurance make use of electronic portal imaging device (EPID) images. However, conventional collimation verification algorithms based on the full width at half maximum (FWHM) fail when applied to small field images acquired by an EPID due to overlapping aperture penumbrae, lateral electron disequilibrium and radiation source occlusion. The objective of this study was to investigate sub-pixel edge detection and other techniques with the aim of developing an automatic and autonomous EPID-based method suitable for MLC positional verification of small static fields with arbitrary shapes. Methods investigated included derivative interpolation, Laplacian of Gaussian (LoG) and an algorithm based on the partial area effect hypothesis. None of these methods were found to be suitable for MLC positioning verification in small field conditions. A method is proposed which uses a manufacturer-specific empirically modified FWHM algorithm which shows improvement over the conventional techniques in the small field size range. With a measured mean absolute difference from planned position for Varian linacs of 0.01 ± 0.26mm, compared with the erroneous FWHM value of 0.70 ± 0.51mm. For Elekta linacs the proposed algorithm returned 0.26 ± 0.25mm, in contrast to the FWHM result of 1.79 ± 1.07mm.

Evaluation of the photon dose calculation accuracy in radiation therapy of malignant pleural mesothelioma.

Photon dose distribution of malignant pleural mesothelioma (MPM) in matched photon-electron technique is influenced by media inhomogeneity, lateral electronic disequilibrium at interfaces and narrow field. These may influence the dose calculation accuracy, calculated by treatment planning systems (TPS). This study aimed to evaluate the dose calculation accuracy of TiGRT TPS in radiation therapy of MPM. 18 MV photon beams of ONCOR Siemens linear accelerator was simulated using EGSnrc Monte Carlo (MC) code. Data of four patients were used to compare TPS and MC results in different regions included: Open and in-field, under shied and out of field regions. Compared to MC results, the TPS overestimated the pleura dose coverage (90% of prescribed dose) about 3-12 mm, and also it overestimated the dose in under the shielded regions of lung (4-74%). While the TPS underestimated the dose profile width about 1-16 mm in low dose region (<50% prescribed dose) as well as the out of field region dose (4-100%). Results showed that TPS underestimated the dose in out of field and overestimated the dose in under the shielded regions. Unlike MC measurements, TPS calculation showed adequate pleural dose coverage. Based on the results, MC calculation can be used in matched photon-electron beam radiation therapy of MPM to modify the TPS photon dose calculations in the presence of heterogeneity, interfaces, and shield in MPM radiotherapy.

Evaluation of dose calculation accuracy of various algorithms in lung equivalent inhomogeneity: Comparison of calculated data with Gafchromic film measured results.

To evaluate dose calculation accuracy of various algorithms in lung equivalent inhomogeneity comprising tumor within it and comparison with Gafchromic film data. Gafchromic film measured central axis absorbed dose in lung insert (-700 Hounsfield unit [HU]), in racemosa wood cylindrical inhomogeneity (-725 HU) and at three surfaces of tumor (-20 HU) created in cylindrical inhomogeneity, put in the cavity of computerized imaging reference systems (CIRS) thorax phantom were compared with convolution (CON), superposition (SP), fast SP (FSP), and X-ray voxel Monte Carlo (XVMC) algorithms calculated dose using 6 MV beams of field size 2 cm × 2 cm, 3 cm × 3 cm, 4 cm × 4 cm, 5 cm × 5 cm, and 8 cm × 8 cm. XVMC was in good agreement with film measured results for all selected field sizes except 3 cm × 3 cm. SP under estimated by 5.7% at the center of the lung insert while deviation up to 6% was found at the cent of wood inhomogeneity in 2 cm × 2 cm. Except CON, increase in dose from proximal to the central surface of the tumor and then dose falloff from central to the distal surface for field size 2 cm × 2 cm to 4 cm × 4 cm was recorded. The change in film measured percentage depth dose from 2 cm × 2 cm to 3 cm × 3 cm field sizes was found -8% however for consecutive field size(s) larger than 3 cm × 3 cm this difference was less. CON and FSP produced overestimated results. Out of four algorithms, XVMC found consistent with measured data. The electronic disequilibrium within and at the interface of inhomogeneity make the accurate dose predictions difficult. These limitations results in deviations from the expected results of the treatments.

An Analytical Method to Calculate Phantom Scatter Factor for Photon Beam Accelerators.

IntroductionOne of the important input factors in the commissioning of the radiotherapy treatment planning systems is the phantom scatter factor (Sp) which requires the same collimator opening for all radiation fields. In this study, we have proposed an analytical method to overcome this issue.MethodsThe measurements were performed using Siemens Primus Plus with photon energy 6 MV for field sizes from 5×5cm2 to 40×40cm2. Phantom scatter factor was measured through the division of total scatter output factors (Scp), and collimator scatter factor (Sc).ResultsThe mean percent difference between the measured and calculated Sp was 1.00% and -3.11% for 5×5, 40×40 cm2 field size respectively.ConclusionThis method is applicable especially for small fields used in IMRT which, measuring collimator scatter factor is not reliable due to the lateral electron disequilibrium.

Optimisation of output factor measurements using the Magic Plate 512 silicon dosimeter array in small megavoltage photon fields

We evaluate the impact of an air gap and optimization of this air gap for the MP512 silicon detector array when operated in dosimetry mode for small photon field measurements in solid water. We present output factor measurements for 6MV and 10 MV photon beams with the square field sizes ranging from 0.5 to 10 cm2. The size of the air gap above the MP512 detector was changed from 0.5, 1.0, 1.2, 2.0 and 2.6 mm. We compare the output factors measurements of the MP512 with EBT3 film and the MOSkin dosimeter. For the two photon energies investigated, we find that the output factor measured by the MP512 reduce with increasing air gap and reducing of field size. The reduction in output factor is most pronounced for the 0.5 and 1 cm2 field sizes. The air gap of 0.5 mm and 1.2 mm showed good agreement with the EBT3 film and MOSkin output factor for 6 and 10 MV photon fields, respectively. The negligible effect on dosimetry for the field sizes larger than 4x4 cm2 demonstrates that the electronic disequilibrium caused by small air gap only influences the dosimetry measurements for small fields. The study shows that the output factor reduction is enhanced by increasing of air gap and demonstrates that the optimal air gap for the MP512 at 6 and 10 MV photon fields is 0.5mm.

An overview on small-field dosimetry in photon beam radiotherapy: Developments and challenges.

Small fields are more repeatedly used for radiation therapy as small segments in intensity-modulated radiotherapy or as in the form of independent fields in stereotactic radiosurgery and other novel equipment such as cyberknife and tomotherapy. Nevertheless, the application of small fields for radiotherapy of lung makes the dose calculation and planning inaccurate due to the existence of electronic disequilibrium and intrinsic deficiencies within most of the analytical dose calculation algorithms. The current review attempts to gather the information in this regard and focuses on the current progresses and retaining issues associated with this type of photon beams.

SU‐A‐BRA‐01: Introduction

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‐F‐T‐12: Monte Carlo Dosimetry of the 60Co Bebig High Dose Rate Source for Brachytherapy

Purpose:The purpose of this work is to obtain the dosimetry parameters in accordance with the AAPM TG‐43U1 formalism with Monte Carlo calculations regarding the BEBIG 60Co high‐dose‐rate brachytherapy. The geometric design and material details of the source was provided by the manufacturer and was used to define the Monte Carlo geometry.Methods:The dosimetry studies included the calculation of the air kerma strength Sk, collision kerma in water along the transverse axis with an unbounded phantom, dose rate constant and radial dose function. The Monte Carlo code system that was used was EGSnrc with a new cavity code, which is a part of EGS++ that allows calculating the radial dose function around the source. The XCOM photon cross‐section library was used. Variance reduction techniques were used to speed up the calculation and to considerably reduce the computer time. To obtain the dose rate distributions of the source in an unbounded liquid water phantom, the source was immersed at the center of a cube phantom of 100 cm3.Results:The obtained dose rate constant for the BEBIG 60Co source was 1.108±0.001 cGyh‐1U‐1, which is consistent with the values in the literature. The radial dose functions were compared with the values of the consensus data set in the literature, and they are consistent with the published data for this energy range.Conclusion:The dose rate constant is consistent with the results of Granero et al. and Selvam and Bhola within 1%. Dose rate data are compared to GEANT4 and DORZnrc Monte Carlo code. However, the radial dose function is different by up to 10% for the points that are notably near the source on the transversal axis because of the high‐energy photons from 60Co, which causes an electronic disequilibrium at the interface between the source capsule and the liquid water for distances up to 1 cm.

SU‐A‐BRA‐02: Making the Most of a One Hour Lecture with Alternative Teaching Methodologies: Implementing Project‐Based and Flipped Learning

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.