bluuebunny/crossref_metadata_2025 huggingface dataset:
SU‐E‐T‐522: Analysis of SCERMA‐to‐KERMA Ratio for Megavoltage Photons
Authors: Bridges D , Zhu T
Published: June 2012

Purpose: To examine the relationship between the primary SCERMA, Sp, and the primary collision KERMA, Kcp, as a function of depth for clinically relevant energy spectra, and to accurately model the SCERMA to KERMA ratio (SKR) for clinical photon beams. Method and Materials: Sp, Kcp, Sp / Kcp (=SKR) for the energy spectra of Cobalt‐60 (Co‐60), and Mohan 4 MV, 6 MV, 10 MV, 15 MV, and 24 MV photons are analytically calculated over depths from 0 to 40 centimeters in water. The Sp and Kcp are fitted to exponential functions, Sp0exp(−μ′d(l−η′d)) and Kp0exp(−μd(l‐ηd)), respectively, with depth d, linear attenuation coefficient μ and beam hardening coefficient η; μ′ and η′ are the corresponding quantities for Sp. The relationships between μ′, η′, and SKR vs. μ are examined. Trends between the fitting parameters and μ were also determined, and the results applied to model the SKR of 6× and 15× clinical beams as functions of only μ, η, and depth. Results: SKR decreases with depth for all spectra. We found μ′ = (0.80496 + 4.8748μ)μ + 0.005736 and η′ = (−0.13076 + 2.6571μ)μ + 0.0036151 for 0.0273/cm<=μ<0.0392/cm, and μ′ = 0.87718μ + 0.010864 and η′ =(−0.009 – 0.50122μ)μ + 0.0037 for 0.0392/cm <= μ <= 0.0667/cm. This model predicts the normalized SKR/(SKR(d=0)) with a relative deviation of 0.1% and max deviation of 1%. SKR/(SKR(d=0)) from clinical beams 6× and 15× is examined to be in agreement with analytic computation from spectral data with 1% and 0.5% maximum error, respectively. Conclusion: Knowledge of normalized SKR is a necessary to calculate scatter dose accurately. We have developed an empirical model to calculate the normalized SKR to be used for clinical (scatter) dose calculation and consequently improve dose calculation accuracy.
SU‐E‐T‐523: Modeling Beam Data for Flattening Filter Free (FFF) Photon Beams
Authors: Zhu TC , Liang X
Published: June 2012

Purpose: For flattening‐filter free (FFF) photon beams, conventional algorithm based on equivalent‐square to calculate dose per MU is invalid because of the non‐uniform profile. In this study, an empirical algorithm is developed to calculate the dose accurately, which can be used for secondary MU check for IMRT using FFF beams. Methods: A kernel‐based algorithm based on three parameters (a0, w0, d0) is used to quantify the phantom scatter characteristics of the photon beam. The model is modified to quantify the shape of the FFF at off‐axis locations by fitting the primary off‐axis ratio (POAR) by a linear function 1 ‐ br, where b is a constant and r is the radial distance. The resulting parameters are used in a kernel‐based dose calculation algorithm for dose calculation. Results: It is found that the proposed model can fit the product of the fractional depth doses (FDD) and phantom scatter factors (Sp) for field sizes between 2 and 40 cm and depth between 0 and 40 cm to a max and standard deviations of 1.7% and 0.01% and 1.8% and 0.01%, respectively, for 6 and 10 MV FFF beams. The value of b is 0.025 and 0.0323 for 6 MV and 10 MV photons, respectively, from fitting the POAR. The resulting phantom scatter parameters are consistent with those obtained from MC simulation. If the slope is not taken into account (b = 0), then the model cannot fit the central‐axis Sp*FDD accurately and resulted in a maximum error of 3% and 4% for 6 and 10 MV, respectively. Conclusions: We have demonstrated that the shape of POAR from FFF beam will impact on the dose calculation on the central‐axis. Conventional equivalent square law concept will not be applicable for dose calculation for FFF beams.
SU‐E‐T‐527: Assessment of Field‐Junction Dosimetry: Pinnacle V9.0 versus Film (EBT2)
Authors: Melancon A , Gillin M , Tailor R
Published: June 2012

Purpose: To investigate dose agreement of Pinnacle V9.0 with measurements across the junction between abutting photon and electron fields. Methods: Photon‐photon, photon‐electron, and electron‐electron field junctions of energies 6 MV and 9 MeV were evaluated with 0, 5, and 10 mm gaps between the field edges at depths of 2, 15, and 30 mm with Gafchromic film (EBT2). Photon and electron fields (10×10 cm) were setup to deliver 300 cGy in their field centers to 30 mm depth. Film optical density was measured every millimeter across the field‐junction with a densitometer (light beam 2 mm). Optical Density to dose conversion was accomplished using a calibration curve specific to the batch of this film. For comparison, all field combinations were generated in Pinnacle, and doses were calculated with collapsed‐cone convolution algorithm employing calculation‐grid sizes 2, 3, and 4 mm. Results: Measured doses at junction are as much as 20% different than Pinnacle calculation. 3 profiles with zero gap, 1 profile with 0.5 cm gap, and 0 profiles with 1.0 cm gap showed a disagreement greater than 12%. Measurement showed a bias towards higher dose in the junction when compared to Pinnacle calculation. With dose‐calculation grid reduced below 4 mm, Pinnacle calculations showed improved agreements, but only a few percent at most. Pinnacle versus measured dose disagreements do not show clear trends with field separation or depth. Conclusions: Doses calculated by Pinnacle V9.0 in the field‐junction region may not be accurate for abutting photon and electron fields, particularly when a coarse dose‐calculation grid is utilized. Lack of clear trends in dose‐agreement with field separation and depth suggests that uncertainties in other factors such as jaw positions and phantom set‐ups may be additional contributors. Significant inaccuracies in doses reported by the treatment‐planning system in field‐junction region should be considered while making clinical decisions.
SU‐E‐T‐529: Dosimetric Evaluation with Heterogeneity in Acuros XB Advanced Dose Calculation Algorithm and Anisotropic Analytical Algorithm (AAA)
Authors: Rana S , Rogers K
Published: June 2012

Purpose: To evaluate accuracy of Acuros XB Advanced Dose Calculation Algorithm and Anisotropic Analytical Algorithm (AAA) with measurements in predicting doses beyond air gaps.Methods and Materials:Three virtual phantoms with layers were created in Eclipse Treatment Planning System. Each layer in phantoms was assigned in terms of water (top), air (middle) and water (bottom) medium. Central axis depth dose in water (bottom medium) beyond 2, 4 and 6 cm air gaps were computed for 5 cm water equivalent material positioned before air gaps. Dose computation was performed for 6MV photon beam with 3×3 and 5×5 cm2 field sizes at 100 cm SSD to surface of phantoms using AAA_10.0.28 and Acuros XB_10.0.28. Next, solid water and Styrofoam were manufactured to mimic virtual phantoms. By keeping identical field, beam parameters, and geometries that were used for dose computation, 100 MUs were delivered to phantoms, and measurements at selected depths were acquired with cylindrical ionization chamber. Measured central axis depth doses were compared against calculated central axis depth doses computed from Acuros XB and AAA. Results: Acuros XB predicted doses within ±2% of measured doses except at 1cm depth in phantoms with 4 cm air gap (−2.4%) and 6 cm air gap (−2.8%) for field size 3×3cm2. Acuros XB showed better dose prediction compared to AAA at all measured depths. Smallest test field size in phantom with largest air gap produced highest range (between depths 1 and 5 cm) in percentage dose difference (AAA: −2.9% to −9.9% and Acuros XB: −2.8% to 1.5%). Improper modeling of primary beam attenuation or lateral scatter (or combination of both) within air gap may have resulted into dose discrepancies. Conclusions: The findings suggest Acuros XB is more accurate to use in dose predictions when tumor is located beyond small air cavity (heterogeneity) within the patient.
SU‐E‐T‐531: Verification of Acuros Dose Calculation Accuracy in Lung SBRT
Authors: Wang L , Bush K , Xing L , Mok E
Published: June 2012

Purpose: The AcurosXB Advanced Dose algorithm (Varian Medical Systems) was reported to give more accurate dose calculation in heterogeneous medium compared with the AAA algorithm. In this work, a validation of AcurosXB dose calculation accuracy is performed for use in lung SBRT. Methods: A CIRS (Computerized Imaging Reference Systems, Inc.) lung phantom was used with a tissue equivalent 2cm circular target inserted into the lung. The phantom was customized to hold a Gafchromic EBT2 film through the center of the target. The phantom was CT scanned, and single field (3cm by 3cm anterior/lateral) plans, as well as VMAT plans were created for 6 and 10 MV energies with different delivery modalities (with/without a flattening filter). All plans delivered 500 cGy to the PTV which was the target plus 0.5 cm uniform margin. Plans were delivered on a Varian TrueBeamTM STx using image guidance to locate the isocenter. The film dose was compared with Eclipse v10.0 dose calculated with AAA and Acuros algorithms using FilmQA software (3cognition). Monte Carlo simulations (BEAMnrc/DOSXYZnrc) were performed to further validate AcurosXB calculations. Results: Within the GTV, AAA and Acuros algorithms were in good agreement with the film measurements to within 2% mean dose difference. AAA calculation consistently overestimated the dose to the lung surrounding the GTV by 5–9% for the 10MV beam and 2–4% for the 6MV beam. Acuros and MC calculations were found to be in good agreement with EBT2 film measurement within 2% at 6MV including the build up and build down regions. Conclusions: The CIRS customized lung phantom with EBT2 films was found to be an excellent tool in validating the dose calculation algorithm for lung SBRT application. AcurosXB was found to be more accurate than AAA algorithm in lung and near lung/tissue interfaces.
SU‐E‐T‐533: Evaluating Effective Source Position Corrections during Modeling with Pinnacle Version 9.2 Software for Flattening Free (FFF) Small Field Treatments
Authors: Philbrook S , Hasson B , Hand C , Sullivan P
Published: June 2012

Purpose: Advanced treatment delivery techniques require more robust treatment planning system (TPS) models for accurate dose planning and computation. As stereotactic body radiation therapy (SBRT) and volumetric‐modulated arc therapy (VMAT) treatments become more prevalent, the ability of the TPS to accurately calculate dose delivered from small fields becomes of greater importance. An additional level of complexity in photon beam modeling is added with the inclusion of flattening filter free (FFF) treatment beams, due to steep in field dose gradients and non uniform penumbra regions. The purpose of this work is to examine the ability of ADAC/Pinnacle TPS to accurately model and calculate dose from small field FFF treatment beams. Methods: Photon beam data was first modeled using Pinnacle v9.0. Flattened and FFF beam models were generated for 6MV and 10MV energies. Models were fine‐tuned using a 5×5 cm2 field as the standard field in order to place more emphasis on the accuracy of the model for the small fields. An upgrade to Pinnacle v9.2 allowed for comparison between version 9.0 and version 9.2. Results: Accurately modeling small fields using Pinnacle v9.0 proved difficult, particularly in the lower penumbra region. An asymmetry appeared in the small field models for both the flattened and FFF models. The Pinnacle v9.2 upgrade eliminated the small field asymmetries, and allowed for more accurate penumbra region calculations. Preliminary results have shown that Pinnacle v9.2 is capable of developing accurate beam models when an emphasis is placed on the small fields. In addition, patient specific dosimetric information for version 9.0 and version 9.2 was also calculated and examined for prostate, head & neck, and lung treatments. Conclusions: Pinnacle v9.2 showed improved accuracy in small field FFF dose modeling over version 9.0. Comparison of patient dosimetric information reflected the improvement in the small field modeling.
SU‐E‐T‐545: Dose Comparison between Intravenous Contrast‐Enhanced CT and Non Contrast CT in Treatment Planning
Authors: Xiong W , Huang D , Gewanter R , Burman C
Published: June 2012

Purpose: With increasing concern for patient dose from CT scan, we are trying to reduce CT scan and use intravenous contrast‐enhanced CT (contrast CT) in treatment planning. This study is to investigate dose calculation accuracy using contrast CT in treatment planning for lung, esophagus and pancreas cancer. Methods: We analyzed treatment plans for 8 patients for whom CT simulation was performed both with and without intravenous contrast agent (CA) (non‐contrast CT). IMRT/3D plans were generated with inhomogeneity correction on the non‐contrast CT scan. Contrast CTs were fused to the non‐contrast studies and all contours and plans were copied to the contrast CT scans. For each patient, we analyzed dose‐volume histograms (DVH) for planning volumes (PTV) and the organs‐at‐risk (OAR), comparing the doses generated on non‐contrast CT scans with those generated on contrast CT scans. Results: Maximum doses ratio Dmax(contrast)/Dmax (non‐contrast) in PTVs was 1.0009±0.0013. The ratio of D05 (contrast)/D05 (non‐contrast) was 0.996±0.005. The ratio of mean PTV dose Dmean(contrast)/Dmean(non‐contrast) was 0.990±0.005%. The ratio of minimum dose Dmin(contrast)/Dmin(non‐contrast) and D95(contrast)/D95(non‐contrast) were 0.970±0.030 and 0.984±0.009, respectively. Contrast CT raised cord dose slightly. The ratio of cord Dmax was 1.005±0.026. However there were two cases the ratio of cord Dmax were 1.035. Conclusions: The PTV D95 is usually normalized to prescription dose and the D95 differences between contrast and regular CT were within 2%. In most cases, the contrast CT could be used to treatment planning clinically. However more attention should be paid to maximum cord dose if it is already close to criteria limit.
SU‐E‐T‐548: Case Study of HDR Vs IMRT in Pediatric Sarcomas
Authors: Chinsky B , Vatner R , Narayana A , Becker SJ
Published: June 2012

Purpose: To compare the dose to critical structures using HDR brachytherapy versus IMRT in pediatric tumors. Method: Two sarcoma cases, for which both HDR and IMRT could be options, were selected for this case study. Case 1 targeted a volume on the posterior neck/upper back with an HDR prescription of 600cGy × 5 fractions, 3000cGy total. The IMRT prescription used was 180cGy × 25 fractions, 4500cGy total with a boost of 180cGy × 6 fractions, 1080cGy. The IMRT PTV used was a 0.5cmexpansion of the HDR PTV; the boost PTV was identical to the HDR treatment plan. Case 2 targeted a volume on the palette of the mouth with an HDR prescription of 300cGy × 12 fractions, 3600cGy total. The IMRT prescription used was 180cGy × 28 fractions, 5040cGy total. A 0.5cm expansion of the HDR PTV was used for the IMRT PTV. In both cases, for both HDR and IMRT, at least 95% of the PTV received 100% prescription dose, following Children's Oncology Group protocols. Results: Case 1 : the mean doses to the body, brachial plexus, esophagus, spinal cord and thyroid were higher using IMRT than HDR. Integral dose was 6.6 times higher using IMRT than HDR. Case 2: the mean doses to the body, mandible, and parotids were higher using IMRT than HDR. Integral dose was 2.9 times higher using IMRT than HDR. Conclusion: This study shows the potential benefit of treating with HDR compared to IMRT in select pediatric cases. These results suggest that HDR can be superior to IMRT in the sparing of critical structures and in delivering less integral dose to the patient while still achieving adequate target coverage. However, lower mean doses to critical structures must also be weighed against the possibility of complications from HDR's very high hotspots.:
SU‐E‐T‐556: Integration of Lung Blocks in the Inverse Planning Process of Modulated Arc Total Body Irradiation Using Cone Beam CT
Authors: Morin O , Held M , Kirby N , Perez‐Andujar A , Chuang C , Pouliot J
Published: June 2012

Purpose: The sizing and placement of lung blocks for total‐body irradiation (TBI) is critical to prevent lung toxicities and maintain effective treatments. During modulated‐arc TBI (MATBI) treatment, the patient is stationary near the floor while open‐field beams with varying exposures are delivered. The inverse planning process currently aims for a uniform dose to the body, without accounting for the presence of lung blocks. This study investigates the possibility of including the effect of these blocks in the MATBI optimization process. Methods: Dosimetric comparisons were performed using a water tank and a simple stack of solid water slabs. Lungs blocks made of cerrobend were fabricated and imaged using on‐board megavoltage CBCT (MVCBCT). The reconstructed MVCBCT images were precisely registered with the reference CT for inverse planning. The cerrobend blocks were contoured in the planning system and the density was overridden to 9.3 g/cm3. Simulated doses in Pinnacle were compared to ion chamber, diode array and gaf‐chromic film measurements obtained at 1.0, 5.0, 10.0 and 20.0 cm depths. Specific optimization objectives on the lungs were tested on 5 patients including a lung re‐treatment. Results: The maximum difference between ion chamber measurements and the treatment planning predictions was 2.4%. The measurements profiles with the diode array correlated reasonably well (<5%) with predictions. Gaf‐chromic films demonstrated good accuracy at depth but large differences (>10%) on the surface. Lung blocks reconstructed with MVCBCT were structuraly accurate without significant metal artifacts. A comparison of MATBI plans on patients shows that inclusion of lung blocks during optimization can reduce hot and cold areas in the lungs and the sternum. Conclusion: Reasonable predictions of the lung block transmission can be obtained following the developed technique using megavoltage CBCT. Thus, lung blocks can be included in the MATBI inverse planning process, which can help prevent complications and local failure.
MO‐F‐213CD‐07: Implementation of Dose Monitoring in a Cardiology Department with Independent Medical Reporting Systems
Authors: Zamora D , Dickinson R , Kanal K , Stewart B
Published: June 2012

Purpose: Not all clinical service sections within a large hospital system are incorporated into the main radiology information system (RIS) and picture archiving and communication system (PACS); attaining and analyzing dose data for a cardiology department at our institution is challenging. The aim is to implement a dose monitoring program in a cardiology department whose medical recording systems are independent of the institutional systems. Methods: During a cardiac case the technologist documents the activities (catheterization, biometrics, biopsy, valve replacement, etc.) performed by the medical staff. Physics worked with the cardiology staff to identify what standard notations would indicate the procedure type, and where to reliably enter cumulative air kerma (AK). This information was compiled into a report, and the data was manually classified into seven pertinent procedure types. For a particular procedure and quarter, cases exceeding a z_score of 5 (indicating >5 SD above mean) were excluded and catalogued. Basic statistics (mean/SD/median/percentiles/min/max) were calculated for the purpose of observing longitudinal trends. A list of flagged studies ‐ cases exceeding the 95 th percentile for a given data subset ‐ was generated for the purpose of clinical review. Result: Preliminary analysis shows that common low dose cardiac procedures such as heart biopsies, intra‐arterial balloon pump insertions, and right heart catheterizations (RHC) exhibit stable median AK of approximately 200 mGy over the last two quarters. Higher dose procedures include left heart catheterizations (LHC), combination LHC/RHCs (both with median AK approximately 900 mGy), and the more complex interventional LHC (median AK approximately 3000 mGy). Cases exceeding the 95th percentile for a given procedure are currently being used to develop a follow‐up process for potential deterministic effects. Conclusions: A robust system of analyzing dose data from an RIS/PACS‐independent cardiology system has been developed. The results are being used to improve physician training and fluoroscopic practice.
MO‐F‐217BCD‐01: Assessment of Image Quality in the New CT
Authors: Siewerdsen J , Fessler J , Myers K
Published: June 2012

The emergence of fully volumetric CT imaging systems and statistical / iterative reconstruction methods present an immediate need for rigorous methods of image quality assessment. Major themes arising in the widespread utilization of such technologies include: 1.) the reduction of radiation dose in a manner that maintains imaging performance with respect to the imaging task; 2.) the need for quantitative standards in image quality assessment appropriate to various vendor hardware and reconstruction systems; 3.) imaging performance metrics that provide meaningful descriptors of fully 3D spatial resolution and noise characteristics; 4.) the dependence of these metrics on acquisition technique, reconstruction parameters, and their applicability (or lack thereof) to novel reconstruction methods that may defy conventional metrics founded in assumptions of linearity and shift invariance; and 5.) methods for physical measurement (e.g., phantoms) and theoretical analysis of image quality for these new and emerging CT technologies and their implications for dose reduction.

Learning Objectives:

1. Understand the methods and metrics of image quality assessment for fully volumetric (cone‐beam) CT imaging using 3D filtered backprojection, including analysis of the 3D NPS, NEQ, and detectability index.

2. Understand the limitations and assumptions of such metrics with respect to linearity, stationarity, and novel reconstruction techniques.

3. Gain an understanding of how rigorous assessment of imaging performance can guide technique optimization, dose reduction, and the development of new CT technologies and applications.

4. Understand the factors of image quality assessment in nonlinear statistical / model‐based 3D reconstruction techniques, including PSF and covariance estimation in nonstationary systems.

5. Gain perspective on image quality assessment standards, including methods for dose and image quality measurement on different vendor platforms and the implications for dose reduction.
MO‐F‐218‐01: Overview of ACR/ASTRO and ACRO Practice Accreditation Programs
Authors: Conway P , Mian T , Kuettel M , Sibata C
Published: June 2012

ACR Practice AccreditationThis session will familiarize participants with the objectives of the Radiation Oncology Practice Accreditation, the ACR‐ASTRO Practice Guidelines and Technical Standards, and the actual process of obtaining ACR‐ASTRO accreditation. The steps in the accreditation process will be reviewed, including the pre‐survey documentation submitted with the application, the on‐site visit activities and the final report contents. ACR‐ASTRO surveyors review of a facility's personnel, equipment, treatment planning and treatment records, as well as its quality control measures, are assessed during the survey. The benefits of ACR‐ASTRO accreditation will be reviewed as well as common reasons for deferral of accreditation and the corrective action process. ACRO Practice AccreditationThe American College of Radiation Oncology (ACRO) developed a Practice Accreditation Program (Program) in 1995, consisting of practice standards for radiation oncology. Accreditation is a voluntary process in which professional peers identify standards indicative of a quality practice, and an audit is conducted to assure that these standards are followed. Since its establishment, the Program has undergone periodic revisions to reflect clinical and scientific advances within the field, providing for the safe and effective practice of radiation therapy. The most recent review (in 2010) resulted in major changes to the process by introducing online case submissions by each practice, online review by disease site specific team members, and an onsite visit. The history and process for ACRO accreditation will be presented. In addition, state activities for requiring accreditation will be discussed. Finally, requirements to become a Medical Physics reviewer will be outlined.
MO‐G‐213AB‐03: Simulations of Real‐Time Geometric and Dosimetic Verification System Using EPID
Authors: Fuangrod T , Woodruff H , VanUytven E , McCurdy B , O'Connor D , Greer P
Published: June 2012

Purpose: To demonstrate a new method for real‐time geometric and dosimetric verification of IMRT and VMAT using synchronization between predicted and measured EPID images. Methods: Predicted EPID images were calculated using a comprehensive physics‐based model. Each predicted image represents the integrated signal expected from the delivery between control points. The measured images are acquired in cine mode and compared to the set of predicted images in real‐time. The system performs geometric verification prior to dosimetric verification. When the measured image is acquired, the algorithm automatically detects the MLC leaf positions. A comparison between the leaf positions of the measured image and control points in the MLC file is made using the cosine similarity technique. The similarity index(SI) provides geometric MLC verification and synchronization between the measured and predicted images, as a uniform dose‐rate cannot be assumed for IMRT or VMAT deliveries. The SI threshold was based on a series of experiments including 21 dynamic‐IMRT fields defining pass/fail boundary(5 brain, 8 H&N, and 8 prostate cases).If geometric verification is successful, dosimetic verification is performed with the Gamma comparison(3%,3mm).The system reports the verification Result in real‐time. Results: The system was simulated by MATLAB/SIMULINK and detected geometric and dosimetric errors during delivery. Both artificially introduced errors and clinical data were used for testing and analysis of the system performance. For a tested prostate field, the cumulative dose comparisons showed the minimum and maximum number of points with Gamma index<1 as 93.5% and 98.5%, respectively. For individual dose comparisons on the same field, the values were 87% and 97%, respectively. Conclusion: This method includes automatic MLC leaf positioning, synchronization, and dosimetric verification. The pass/fail boundary of geometry was calculated based on the experiments. This system is a useful approach to detect unexpected possible errors occurring in the clinical setting and to prevent patient overdoses during radiotherapy especially in complex deliveries such as arc‐IMRT.
MO‐G‐BRA‐01: Optimal Image Filtration Strategies for PET Segmentation
Authors: McGurk R , Bowsher J , Smith V , Lee J , Das S
Published: June 2012

Purpose: Gaussian smoothing reduces noise in PET images but blurs edges impacting the achievable accuracy in radiotherapy target segmentation. Bilateral filters both provide smoothing while simultaneously preserving edges. We compared the accuracy of four methods used to segment objects in filtered images. Methods: Five spherical volumes (1.1–26.5 ml) were imaged for 1, 2 and 5 minutes at high (8:1) and low (4:1) contrast. Four filters were used to smooth images: two Gaussian kernels of 3 mm and 6 mm spatial FWHM, and two bilateral filters with the same FWHM but with an adaptive intensity kernel. Segmentation methods of 40%‐max (40%), adaptive thresholding (ADP), K‐means (KM) and region‐growing (SRG) segmented the volumes. Accuracy was judged comparing segmentations to a known ground truth via Jaccard coefficients (JC) (values closer to 1 better) and symmetric mean average surface distance (SMASD) error. Models describing the impact of filtering and segmentation method were fit using generalized estimating equations. Results: Median JC and SMASD results indicated the 6mm FWHM Gaussian filter was the most accurate across all segmentation techniques (JC=0.72, SMASD=0.68mm). ADP and KM were tied for accuracy across all filters (JC=0.68, SMASD=0.73mm). Model estimated accuracy was significantly higher for each Gaussian filter relative to its corresponding bilateral version and for ADP compared to 40% (p<0.0001). For the 13mm, 4:1 object with 1‐minute scan duration a 6mm FWHM Gaussian kernel with ADP was best (JC=0.59, SMASD=0.95mm). For the 37mm, 8:1 object with 5‐minute scan duration a 6mm FWHM Gaussian kernel with 40% thresholding had the highest JC (0.91) but KM had lower SMASD (0.42mm), vs. 40% (0.5 1mm). Conclusion:Choosing the optimal filter for segmenting radiotherapy targets is a complex relationship between object size, contrast and noise. Object size has the biggest impact. For small low‐contrast objects, Gaussian kernels slightly larger than scanner resolution showed higher accuracy. New Zealand Tertiary Education Commission Top Achiever Doctoral Scholarship
MO‐G‐BRA‐04: Fast and Robust Automated Segmentation of the Cervix‐Uterus Structure in CT‐Images Driven by Patient‐Specific Motion‐Models
Authors: Bondar L , Heijmen B , Hoogeman M
Published: June 2012

Purpose: The aim was to develop and test a novel automated intra‐patient segmentation method of the cervix‐uterus structure in CT‐scans of cervical cancer patients to facilitate the online selection of the best plan‐of‐the‐day based on in‐room acquired CT‐scans. Current automated segmentation methods for pelvic organs, which use statistical shape models and require a large training set, are unsuitable for cervix‐uterus due to large inter‐patient variability in shape and position. Methods: An automated segmentation method was implemented that adapts a closely initialized surface of the cervix‐uterus to boundaries in the new image. The novel idea was to use patient‐specific motion‐models derived from only two pretreatment CT‐scans to initialize and drive the segmentation process. The cervix‐uterus surface was initialized by using a 3D patient‐specific cervix‐ uterus model that predicts the shape and position of cervix‐uterus based on bladder volume, a 1D model predicting the bladder volume based on a manually marked bladder top, and implanted markers. The segmentation method was tested on 13 patients that had 9‐10 CT–scans acquired at pretreatment and after 40 Gy. For each patient, two pretreatment CT‐scans (full and empty bladder) were used for model construction and others were used for testing. The overlap between manually delineated and automatically segmented cervix‐uterus structures was quantified by the Dice coefficient. Results: Marking the bladder top and markers required minimal user intervention (<1 min). The overlap between the manually delineated and the initialized cervix‐uterus structures was 82±7% for pretreatment and 71±11% for after 40 Gy data. The automatic adaptation of the initialized structure to image boundaries increased the overlap to 87±3% for pretreatment and 80±13% for after 40 Gy. The automatic segmentation method required 2±0.5 min. Conclusions: A fast and robust automated segmentation method was developed that could support plan selection in online adaptive radiotherapy for cervical cancer patients. Luiza Bondar was funded by the Dutch Cancer Society (grant number 2007–3777).
TU‐A‐217BCD‐01: Introductory CT Dosimetry (and What We Can Learn from Making Measurements on Cylinders)
Authors: Bakalyar Donovan
Published: June 2012

A variety of methods are used to describe and control the radiation output of a CT scanner. Different approaches may serve to emphasize one facet of a particular scan over another, favor the lexicon and style of a specific manufacturer or simply reflect history. The multiplicity of terms often adds confusion to what should be a simple task: to characterize the output of a CT machine in terms of the radiation dose to a cylinder. We will begin by examining radiation dose to cylinders with an emphasis on isolating superfluous parameters.

We start, as in TG111, with an infinite helical scan of an infinitely long cylindrical phantom. During such a scan the dose at any point in the cylinder will asymptotically build up to its ultimate value Deq. The spatial average of this value, Deq, can serve as an indicator of scanner output and along with the total number of rotations N determine Etot, the total energy absorbed by the cylinder. If a finite cylinder is long enough (≥ Leq), a scan through it from one end to the other will yield these values in its central plane.

The approach to Deq can conveniently be described by h(L), a robust function of scan length L which depends only weakly on z axis collimation and tube potential. This function facilitates the simple calculation of the lower dose to the central plane resulting from a finite scan. If the length of the cylinder is finite, as it is for the standard CTDI phantom, the dose is reduced further in a predictable fashion resulting in good correlation between Deq,bar and CTDIvol which in turn has been shown to be a useful tool in equating the radiation output between scanners differing in model and vendor.

Though CTDIvol (and Deq,bar) are useful for characterizing CT machine output, we need to include the effect of patient size in assessing patient dose. TG204 has used measurements on phantoms of varying size to determine a size specific dose estimate (SSDE) which serves to accomplish this task. The SSDE has also proven to be very helpful in designing protocols for patients of varying size, especially pediatric patients.

Learning Objectives:

1. To describe the radiation dose to a cylinder in uncomplicated terms, starting with the particularly simple case of an infinite scan to an infinite cylinder.

2. To extend the description to finite scans to infinite cylinders in terms of the h(L) function and then to the standard CTDI phantoms.

3. To describe some of the subtleties in making reliable CTDI measurements.

4. The demonstrated fact that dose indices (e.g., CTDIvol) provide a reliable indicator of machine output will be used to show, for example, how patient scans from machines of varying model and manufacturer can be compared.

5. To describe how characterizing the dose to cylinders of varying size can be used both to provide a reasonable index of patient dose and as a useful tool in the design of pediatric protocols.
TU‐A‐BRB‐02: Temporal Signal Stability of KCl:Eu2+ Storage Phosphor Dosimeters
Authors: Xiao Z , Chen H , Hansel R , Li H
Published: June 2012

Purpose: It is known that the PSL (photo‐stimulated luminescence) signal fades with time after x‐ray irradiation and eventually reaches a plateau. The purpose of this work is to determine whether x‐ray generated electrons stored in non‐dosimetric shallow traps can be liberated prior to dose readout. If so, we would achieve a rapid signal stabilization so the wait time could be shortened to, for example, one hour or less. Methods: Pellet‐style KCl: Eu2+ dosimeters were fabricated in‐house for this study. PSL signal stability after x‐ray irradiation was studied by interfering with a thermal annealing and optical annealing with near infra‐red light, respectively prior to dosimetric readout. Results: Without optical annealing, PSL faded with time at a rate of 13.6%/hr for the first 2 hrs and 6%/hr for the next 3 hrs after irradiation and eventually reached a fading rate of about 0.5%/hr after 12 hrs. PSL intensity decreased with increasing NIR optical annealing time; however, there was only marginal improvement in the PSL stabilization rate, indicating that the shallow electron traps that are responsible for the fast‐fading for the first few hours were not efficiently emptied. 15‐minute thermal annealing treatments at either 60 or 70°C stabilized the PSL to a slow decay rate of approximately 0.4%/hr over a time period of 24 hrs post irradiation. Moreover, 90% of PSL sensitivity can be retained. More stable PSL signal, 0.18%/hr and 0.08%/hr decay rate for 90 and 120°C thermal annealing, respectively, can be achieved at the expense of PSL sensitivity. Conclusions: Thermal annealing immediately after x‐ray irradiation is an effective way to remove electrons stored in the shallow traps. This study is expected to guide the future design of storage phosphor dosimeter readers, which could incorporate a thermal annealing pre‐read cycle. As a Result, a long wait time would be avoided.
TU‐A‐BRA‐07: New High Resolution T2‐FFE MR Imaging of Axillary Lymph Nodes for Real Time Radiation Treatment Guidance
Authors: Philippens M , Bongard H , Asselen B , van Ulpen M , Lagendijk J
Published: June 2012

In breast cancer patients with pathologically involved lymph nodes, wide field axillary +/− periclavicular lymph node radiotherapy (RT) is performed. A new T2‐FFE MRI method has been developed which provides an excellent tool for direct lymph node localization. Its application might Result in extreme small target volumes and thus improved sparing of normal tissues. It opens the possibility of stereotactic targeting accuracy of individual lymph nodes using the MRI linac design. The method may also be applied to most other lymph node regions.Presently direct visualization of lymph nodes with MRI is generally performed with T1 weighted MRI (T1w MRI) or diffusion weighted MR (DW‐MRI). In T1w MRI, lymph nodes close to blood vessels might be obscured and DW‐MRI suffers from geometrical distortions, and is characterized by low resolution. We introduce a novel 3D high‐resolution MRI technique for direct lymph node imaging, T2 weighted Fast Field Echo (T2‐FFE). The method was tested on a 1.5T Philips Ingenia in healthy volunteers in supine RT position. The Flexcoverage anterior receive coil was positioned on a PMMA support to prevent deformation of the outer body contour. We applied a 3D T2‐FFE using binomial RF pulses (ProSet 121) for water‐selective excitation. The T2 contrast in this sequence is enhanced by the intrinsic diffusion weighting produced by imaging gradients particularly in the read‐out direction. The resolution was 0.7×0.7×1 mm3 and the field of view covered the entire breast, axillary, periclavicular and cervical region up to mandible in a scan time of 8m40s. For clear visualization, 7 mm thick maximum intensity projections (MIPs) were constructed in three orthogonal directions.T2‐FFE showed excellent visualisation of lymph nodes (LN) in the axillary, periclavicular and the deep cervical region. Excellent fat suppression and black blood imaging was achieved. Besides lymph nodes also the brachial plexus was visualized in detail. Funding of this research line has been provided by both Elekta and Philips
TU‐A‐BRA‐08: Integration of Optical Tracking for Organ Motion Compensation in Scanned Ion‐Beam Therapy
Authors: Fattori G , Saito N , Pella A , Kaderka R , Seregni M , Constantinescu A , Cerveri P , Steidl P , Riboldi M , Baroni G , Bert C
Published: June 2012

Purpose: To integrate and test an optical tracking platform for motion detection in scanned ion beam tracking for organ motion compensation. Methods: An infra‐red optical tracking system (IR‐OTS) has been integrated into the treatment control system to test beam tracking for organ motion compensation in the framework of a nearly clinical scenario. The IR‐OTS provides the 3D localization of infrared reflective fiducials with sub‐millimetric accuracy at 100 Hz frame rate. A dedicated modular software application has been designed to assist in‐room motion capture and to encapsulate external‐internal correlation models for tumor motion estimation. Digital UDP protocol has been selected for data communication. Delays caused by motion detection process and data communication have been measured experimentally and compensated by means of time prediction. The three‐dimensional motion mitigation capability of the integrated system has been assessed by measuring the dosimetric deviations between static and motion compensated irradiations. As detector, an array of pinpoint ionization chambers was mounted on the target which was moved by a robotic arm (amplitude: 20 mm peak‐to‐peak). Results: Time delays for motion detection and data communication were estimated around 30 msec. Compensating a regular motion resulted in dosimetric deviations, with respect to the static case, of lower than 7% (neural‐network estimated compensation, no time prediction) and 2% (direct compensation, 30 msec time prediction). Dedicated experimental testing after code optimization allowed assessing overall detection and communication delays of 16.5±7 msec (median±quartile), to be used for the fine tuning of the time predictor. Conclusion: An optical tracking device for tumor motion estimation has been integrated into the experimental setting for treating moving targets with scanned ion beams. Future activities will focus on the optimization of the IR‐OTS integration and on the testing of neural estimators with retraining capabilities for managing irregular clinical like breathing patterns. This work has been supported by the ULICE EU FP7 Program.
TU‐G‐BRB‐05: Dose to Mass in Lung Cancer IMRT Optimization
Authors: Watkins WT , Moore JA , Dial C , Hugo GD , Siebers JV
Published: June 2012

Purpose: To assess the efficacy of intensity modulated radiotherapy (IMRT) optimization for lung cancer based on dose‐to‐structure mass objectives. Methods: Three patient cases are planned for two radiotherapy deliveries; each optimized using either dose‐volume histogram (DVH) constraints or dose‐mass histogram (DMH) constraints. An inhale‐phase optimization simulates breath hold (BH) treatment; an average CT (aCT) optimization simulates treatment under free breathing. DVH plan objectives include 70 Gy to 98% of the PTV while minimizing lung V20, esophagus V25, heart V30. DMH optimization utilizes the same beam angles with corresponding objectives; 70 Gy to 98% of the PTV mass and minimum dose to relative mass of risk structures. Relative mass at objective dose levels are compared for DVH and DMH optimization. Results: For BH plans, DMH optimization maintains or improves target‐mass coverage (up to 2% increase in mass at 70 Gy) and decreases lung mass by up to 2%, heart mass by up to 4%, and esophagus mass by up to 2% at the objective dose levels. The DMH plans optimized on aCT increase target mass coverage by <:1% in all cases and do not improve risk structure mass sparing compared to DVH based plans. The DVH to DMH optimization differences are patient and radiation path dependent. For a complete set of voxels composing a given volume, irradiated voxels are a subset which can have a different mean density than the entire set. This leads to variations in DVH and DMH solutions for the PTV, lungs, esophagus, and heart. Preferred (reduced) radiation path‐lengths through regions of lower density lung are observed in BH‐DMH optimization. Conclusions: DMH optimization has the potential to improve the therapeutic ratio on well‐defined geometry (for example during BH‐RT). This may have advantages in the heart, however, functional lung sub‐units must be further investigated. Support: P01CA11602 and Philips Medical Systems
TU‐G‐BRB‐08: IMRT Optimization with Fixed Point Iteration
Authors: Tian Z , Jia X , Uribe‐Sanchez A , Gautier Q , Graves Y , Li N , Jiang S
Published: June 2012

Purpose: In IMRT optimization, dose‐deposition coefficient (DDC) matrix is needed to parameterize the contribution of each beamlet to each dose voxel. However, due to the limitation of computer memory and the requirement on computational efficiency, small matrix elements are usually truncated, compromising the resulting plan's quality. Besides, the practice of ignoring multileaf collimator (MLC) transmission in IMRT planning also introduces inaccuracy into the resulting plan. Therefore an IMRT optimization algorithm with fixed point iteration is developed. Methods: For truncation problem, IMRT optimization is implemented as the inner‐loop with truncated DDC matrix. Fixed point iteration, as the outer‐loop, is to recalculate the actual dose with complete DDC matrix and use the difference between the optimized dose and the actual dose as the input for the optimization at the next iteration. MLC transmission can also be incorporated into IMRT planning, by adding this fixed point iteration to optimize the intensity of the already optimized apertures. The convergence and feasibility of this algorithm are mathematically studied using a simplified model. Results: Two head‐and‐neck IMRT cases are used to test our algorithm. It is mathematically proven and experimentally validated on the simplified model that with proper DDC matrix splitting, the fixed point iteration converges, although not to the solution obtained using the complete DDC matrix, but to a solution much closer to it than that from the truncated DDC matrix, in terms of the resulting dose distribution. The experimental results on the patient cases also demonstrate that our algorithm could handle both DDC matrix truncation and MLC transmission problems and give us good plans with comparable dose‐volume histograms and dose distributions. Conclusions: This fixed point iteration scheme can effectively take the DDC matrix truncation and MLC transmission into account during IMRT optimization, and thus solve the efficiency and memory issue while maintaining a reasonable accuracy. This work is supported by Varian Medical Systems through a Master Research Agreement.
TU‐G‐BRA‐03: Reconstructing Motion Manifold from Real‐Time MRI: A Level Set Approach
Authors: Liu W , Gach M , Ruan D
Published: June 2012

Purpose: To reconstruct the motion manifold defined on anatomical interfaces from real‐time MR imaging. Methods∷We have developed a novel method to construct the evolving motion manifold in real‐time by preserving the point correspondence when a level set function is updated to track moving anatomical interfaces. The key to our development is to define a correspondence function in a Eulerian system and advecting it along the level set function under the same velocity. The motion manifold is ‘grown’ by concatenating the point correspondence maps. The proposed method was tested with renal MR image sequences under both synthetic and physiological motion. The synthetic sequence was generated by shifting a static MR image continuously in one dimension to a maximum of 29mm. Performance of the estimation was analyzed by comparing the reconstructed motion manifold and the known truth. Physiological motion manifold was reconstructed from real‐time EPI data acquired from human subjects under heavy breathing condition. Results: In the synthetic motion test, the reconstructed motion manifold translates each particle to closely follow the underlying motion. The mean error along and perpendicular to the motion direction is −2.58mm and 0.03mm respectively. These are reasonable results considering the low signal‐to‐noise ratio of the real‐time MR images with pixel size equals 1.52mm. The motion manifold reconstructed from physiological real‐time renal MR exhibits a semiperiodic pattern that agrees with respiratory behavior. Spatially varying motion pattern from different parts of the kidney interface can be observed. Conclusions: We have proposed a novel level set method to track and estimate anatomical moving interface while preserving the point correspondence during the dynamic evolution. The generated motion manifold provides important knowledge about the characteristics of a particular organ movement and facilitates further model development for retrospective analyzing dose impact of motion or predicting motion for real‐time delivery adaptation. NIH grant 1R01CA159471. PI: Michael Gach.