bluuebunny/crossref_metadata_2025 huggingface dataset:
SU‐E‐T‐197: Normoxic Polymer Gel Dosimetry for Intensity Modulated Radiation Therapy
Authors: Kwon S , Cho Y , Cho K , Lee S , Ji Y , Jung H , Kim K B , Kim K M
Published: November 2011

Purpose: To evaluate a tissue equivalent normoxic polymer gel dosimeter as a tool for 3‐dimensional dose quality assurance for intensity modulated radiation therapy (IMRT) by measuring the dose characteristics of therapeutic radiation by Magnetic Resonance Imaging (MRI). Methods: Polymer gel under normoxic conditions was synthesized using gelatin, methyacrylic acid (MAA), hydroquinone (HQ) and tetrakis hydroxymethyl phosphonium chloride (THPC), and polymer gel dosimeter was fabricated by placing it into each phantom for the appropriate experiments. Co‐60 gamma‐rays and a 6 MV photon beam were used for the dose reaction experiments. The dose range was 0∼20 Gy. The dose characteristics of the polymer gel dosimeter were compared with those of the ion chamber and Gafchromic EBT2 film using 3.0T MRI. Results: The linear correlation coefficient was r2=0.970, r2=0.999 and r2=0.972 for the polymer gel dosimeter, ion chamber and film, respectively. The percentage depth dose at a depth of 50 mm was 85.8% and 84.6% for the polymer gel dosimeter and ion chamber, respectively. The flatness and symmetry of the gel dosimeter on the cross‐plane were 3.80% and 3.58%, respectively. After applying the gel dosimeter to IMRT, the dosages measured in the 100% dose domain of therapeutic planning for the axial, sagittal and coronal planes were 96.2%, 103.0% and 100%, respectively. Conclusions: The results show a relatively high level of precision in the high dose range in IMRT, which is important in clinic sites. The possibility of this as a 3‐dimensional dose quality assurance tool of a polymer gel dosimeter was confirmed

This work was supported by National Research Foundation of Korea Grant funded by the Korean Government.
SU‐E‐T‐201: Issues Encountered in Film‐Based IMRT QA
Authors: Mallah J , Mihailidis D
Published: November 2011

Purpose: To investigate characteristics of EDR2 film‐based IMRT QA with an eye towards moving from isodose line comparisons to more objective gamma statistical analysis. Methods: IMRT QA was performed on Siemens Linacs using a traditional ion‐chamber‐and‐film phantom setup for step‐and‐shoot IMRT plans. All beams were delivered at their actual gantry angles and to the same film. Fiducial beams were used to fix film location for comparison with calculated dose. Dose was calculated on the Pinnacle planning system (8.0m). The EDR2 films were developed using a Kodak PP‐XOMAT film processor that has been remanufactured by Picker. Films were scanned with a VIDAR VXR‐16 Dosimetry Pro film digitizer using RIT 113 v5.2 software. Calibration films were created using crossed step wedges with doses calculated by Pinnacle. Gamma was calculated using the standard software settings of 3% / 4 mm. Results: For each QA film, the normalization doses (target to measured), mean gamma, and percentage of points failing the gamma limit (gamma < 1) were recorded. These percentages were found to be considerably higher in many cases than the often‐used limit of 5%. This problem was reduced when a new calibration film was used every week and developed together with the QA films; however, the mean fraction of points failing the gamma criterion was still 7%, with 45% of films failing. The mean gamma was 0.4. The product of (target dose) * (mean gamma) * (3%/100%) was relatively constant and averaged 2.3 cGy. Conclusions: Various factors were found to be relevant to the deviations observed, especially film calibration and the target dose (which sets the dose scale for gamma). In many cases, the problem was due to a poorly functioning film processor, resulting in films with streaks and artifacts; this problem was solved by service but recurs on occasion.
SU‐E‐T‐203: Sensitivity Study on the Use of Computed Radiography for Linac Routine Quality Assurance ‐ Light/radiation Field Coincidence
Authors: Sheu R , Svoboda A , Dumane V , Lo Y
Published: November 2011

Purpose: To investigate the accuracy of using computed radiography (CR) system for the light/radiation field coincidence test and to verify that the accuracy level can be achieved by using this technique as a linac routine quality assurance check. Methods: Kodak 2000RT CR system together with a Kodak Flexible Phosphor plate was used in this study. The phosphor plate was exposed with background radiation (cover the entire plate), radiation field and then field light. All measurements were carried out by using 6 MV X‐rays (Varian 21iX) and 100 cm SSD setup. The room lights were off during the entire measure procedure to control the amount of light field discharge on the CR plate. Results: Delivering 1 and 2 MUs to background and radiation fields, respectively and shining the field light for 35 seconds can provide a good quality of contrast to notify discrepancy (darker or lighter stripe along field edges) between light and radiation fields. Users can visually indicate the discrepancy approximately 1 mm without using any tool. For discrepancy less than 1 mm in the penumbra region between light and radiation fields, by analyzing the light/radiation field edge profiles of CR plate response, accuracy of less than 1 mm can be achieved. Conclusions: The CR system can be used as a replacement of the film to perform the linac routine QA procedures. For light/radiation coincidence test, the result can be directly determined by eyes if using TG‐40 tolerance (i.e., < 2mm). Compared to conventional marking film technique, our method does not involve any intervention between light/radiation fields, which makes it possible to reach higher accuracy level (<1 mm).
SU‐E‐T‐210: Precise Gantry Angle Determination for EPID Images during Rotational IMRT
Authors: McCowan P , Rickey D , Rowshanfarzad P , Ansbacher W , Greer P , McCurdy B
Published: November 2011

Purpose: Utilization of an aSi EPID to develop an in vivo patient dose verification system for rotational IMRT (rIMRT) delivery requires accurate knowledge of gantry angle as a function of time. Currently the accuracy of the gantry angle stamp in the header of the EPID image is limited to approximately +/−3 degrees. This work investigates several unique methods for a more accurate determination of the gantry angle during rIMRT. Methods: Gantry angles were determined using: (1) an incremental rotary encoder attached to the rotational axis of the gantry, (2) a direct analogue‐to‐digital measurement of the gantry potentiometer, and (3) through EPID image analyses of an in‐house phantom (manufactured at sub‐millimeter precision). The phantom consists of a cylindrical acrylic frame with one wire wrapped helically around its surface and one straight wire traversing its central axis. This design creates EPID images with unique and identifiable wire intersection points as a function of gantry orientation. Analysis of the treatment console log files was compared to the above methods. Results: The gantry potentiometer is considered the most accurate gantry angle but is unavailable during treatment. The ClinacLog produced discrepancies of up to ±2 degrees, the DynaLog up to ±1 degrees, and the encoder up to ±0.5 degrees with respect to the potentiometer. Preliminary analysis comparing our phantom‐determined gantry angles with the encoder gantry angles showed agreement within ±0.5 degrees of each other for 85% of the data and differed at most by 1.3 degrees from each other. Conclusions: We have developed several techniques to determine gantry angle as a function of time during rIMRT. We have shown a strong agreement in gantry determination by our phantom and encoder. This investigation of gantry angle is critical to develop an accurate in vivo patient dose verification system for rIMRT delivery.
SU‐E‐T‐222: Performance Comparison of In‐Vivo Dosimeters, including TLDs, MOSFETs, and OSLDs for Patients Receiving Total Body Irradiation
Authors: Pillai S , Laub W , He T
Published: November 2011

Purpose: To compare accuracy of in vivo dosimeters such as TLDs, MOSFETs & OSLDs for patients receiving Total Body Irradiation. Methods: Three types of dosimeters were considered for this study, LiF Thermoluminescent Dosimeters, Modified Field Effect Transistors(MOSFET ‐Thomson&Nielsen TN‐502RD) and Optically Stimulated Luminescence Dosimeters(OSLD ‐ Landuerˈs 2% screened nanoDOTs). All three dosimeters were calibrated under full bulildup conditions at 100cm SSD, dmax = 3.4cm for 18MV photon energy and appropriate correction factors were applied for TBI treatment condition. This study is performed on a 30×30×22cm solid water phantom and a rando phantom at the TBI treatment distance. The separations at different locations for phantom were measured and MUs were calculated to deliver 100cGy at the midplane . Dosimeters were placed on the entrance side of the phantoms to measure dmax dose. Each type of dosimeters were irradiated at five locations in the rando phantom under the same treatment setup. The reproducibility of the dosimeters was verified by solidwater phantom measurements. Ten TBI cases, which were monitored using each dosimeter type were selected and tabulated the percentage difference of measured to expected dose for 5 different patient locations. Results: The reproducibility for TLDs, MOSFETs and OSLDs were 118.3cGy +/− 2.7%(1SD), 118.3cGy +/−2.1%(1SD) and 118.3cGy +/− 1.8%(1SD) resp after 10 consecutive measurements in solidwater phantom (Table 1). From rando phantom measurements, maximum percentage variation of the measured to expected dose with 1SD values are TLDs (1.4 +/− 1.4), MOSFETs (3.9+/− 2.2) and OSLDs (1.6 +/− 1.8) (Table 2). Percentage difference of the measured dose to expected dose for 10 clinical cases were TLDs (2.3+/− 2.9), MOSFETs (3.7+/− 4.9) and OSLDs (1.8+/− 2.9%) (Table 3). Conclusions: Measured values were within +/−5% tolerance level for all dosimeters. OSLDs can give more accurate and reproducible results with an added advantage of ease of use and shorter readout period.
SU‐E‐T‐223: Characterization of a Large Area Ion Chamber Used for On‐Line Monitoring of Radiotherapy
Authors: Wang Y , Islam M , Norrlinger B , Heaton R
Published: November 2011

Purpose: To characterize a spatially sensitive large area ion chamber used for on‐line entrance beam monitoring of radiotherapy. Methods: A large area (26.5 ×26.5 cm2) ion chamber has been constructed using 2 mm thick aluminum plates, with the electrode separation varying linearly in one direction from 2 to 23 mm. The chamber has been characterized for (i) dosimetric performance and (ii) effect on the beam in terms of surface dose in the presence of immobilization devices. The ionization as a function of voltage at various dose rates was measured to determine the optimum operating voltage. The charge collection efficiencies and polarity effects also have been evaluated at various dose rates. The spatial sensitivity was determined by placing the chamber on the treatment couch at 100 cm source‐to‐detector distance and measuring the response due to small beamlets. The surface dose in the presence of an immobilization device was measured first, and then the chamber was introduced in the beam to evaluate the changes in the dose under various clinically relevant conditions. Results: For a chamber operating voltage of 500 Volts the charge collection efficiency for 6 MV photon beams was found to be approximately 97%. The polarity effects were found to be negligible: −0.02% to 0.1% as the dose rate changes from 62 to 496 MU/min. The chamber response was found to be independent of dose rates. The spatial sensitivity relative to the chamber center varied from 0.45 to 1.37 for locations −10 cm to 10 cm in the gradient direction. The chamber did not significantly increase the surface dose in the presence of an immobilization device. Conclusion: The operating conditions and the performance of the chamber were found to be suitable for use as an online beam monitor.
SU‐E‐T‐224: An Assessment of EBT2 Gafchromic Film as a Potential Means for Depth Dose Measurement of Radiotherapy Beams
Authors: Arjomandy B , Tailor R , Zhao L , Devic S
Published: November 2011

Purpose: The Gafchromic EBT2 film was investigated as a potential mean for depth dose (DD) measurement of broad range of radio‐therapy beam‐modalities and energies. Methods: Gafchromic EBT2 films were exposed to 6 Gy dose from various beams in water for DD measurements. The beams employed for this study comprised of KV x‐rays (75 kVp), 60Co, Mega‐voltage x‐rays (6 &18MV), electrons (7 & 20 MeV), and spot‐scanning proton beams (180 & 150 MeV). Film optical densities (OD) were measured using a flat bed scanner. For each beam, film response measured over the dose range 0.6–6.2 Gy, was employed for OD‐to—dose conversion. Results: for all mega‐voltage photon & electron beams, percentage depth‐doses measured with EBT2 showed an excellent agreement (<1.5%) with those measured with ion chambers. In case of 75 kVp (nominal), a small over‐response is observed at deep depths (d > 2 cm) due to beam hardening. In contrast, an under‐response in the Bragg‐peak region is observed for proton beams due to quenching effects similar to the results published for EBT1. However, the suggested empirical corrections enable good agreement with ion‐chamber data for these beams as well. Conclusion: The results indicate that EBT2 is able to serve as a reliable tool for measuring DDs for all radiotherapy beams in general with a high spatial resolution and reproducibility.
SU‐E‐T‐225: An Evaluation of the Impact of Calypso Electromagnetic Beacons on in Vivo Dosimeter Function in Prostate IGRT
Authors: Winburn M , Gejerman G , Cardel Freddie
Published: November 2011

Purpose: Investigate dosimetric variability between in‐vivo dosimeters (DVS) coupled with real‐time electromagnetic tracking beacons (Calypso) versus daily linear fiducial soft tissue markers (Visicoil). Methods: Single fraction in‐vivo dose measurements were collected from 20 patients that received surgically implanted bilateral dosimeters plus a position localizing device within the prostate. 10 patients were implanted with DVS dosimeters and Visicoil fiducial markers, and 10 with DVS dosimeters and Calypso position tracking beacons. Predicted dose was calculated with Eclipse TPS (Varian) and imported into the DVS dose reader. Each patient completed a full course of IGRT treatment (43 fractions, 180cGy/fraction). 934 DVS readings were collected over a 4‐month period. Percentage dose variability between planned and measured dose was analyzed using a 1 tailed t‐test with unequal variances. 3 frequencies of dose variability were compared (>5%, >7%, >10%). Respective p‐values were calculated to test statistical significance between the two patient cohorts. Results: 934 DVS readings (nVisicoil, DVS = 670, nCalypso, DVS = 264) were analyzed with the following average reading count per patient: n(Visicoil, DVS) = 33.5, n(Calypso, DVS) = 13.2. Average readings showed diverging dose variability frequencies. Visicoil frequencies: 80(11.9%) >5%, 33(4.9%) >7% and 7(1.0%) >10%; Calypso frequencies: 78(29.6%) >5%, 36(13.6%) >7% and 10(3.8%) >10%. P‐values were calculated to show differences between the two cohorts using reading frequencies of two dosimeters (DVS1, DVS2) within the defined parameters (>5%,>7% and >10%). Results exhibited statistical significance within the following sets: >5% DVS1 and DVS2 (pDVS1 = 0.029 pDVS2 = 0.004), >7% DVS2 (pDVS2 = 0.021), and >10% DVS2 (pDVS2 = 0.019). 2 dosimeters did not satisfy statistical significance criteria: DVS1 (>7% pDVS1 = 0.088, >10% pDVS1 = 0.105) Conclusions: This report shows how two coexisting technologies in‐vivo can affect one another. Specifically, DVS dosimeters exhibit significantly different behavior when coupled with Calypso or Visicoil localizing devices.
SU‐E‐T‐246: Practical Quality Assurance for Image‐Guided Robotic Brachytherapy System
Authors: Buzurovic I , Podder T , Yu Y
Published: November 2011

Purpose: To establish device‐specific daily quality assurance procedure for image‐guided brachytherapy robotic system. To recommend sufficient tests and tolerance levels which would be acceptable for readily identifying and localizing possible deviations from normal system functionalities.Methods: Image‐guided brachytherapy robot consists of three mutually paired functional modules: robotic hardware with sensors, control and planning software, and image acquisition subsystem. Therefore, daily QA procedure should ensure mechanical and software functionality, appropriate system calibration, clinical image acquisition and quality, and verification of system accuracy. Recommended daily QA tests can be summarized into three basic groups: mechanical screening, robotic system functionality, and overall calibration test. Mechanical screening test includes visual check of connectors, controllers, wires, sensors and motors, connectivity test for controllers, motors, encoders and moving parts, sensors readings and sensitivity, and emergency subsystem functionality test, such as emergency button or manual takeover check. Robotic system functionality test incorporates automatic hardware detection and mobility of moving parts, such as probe stage, gantry and insertion mechanism. It also includes image upload and acquisition test. Purpose of overall calibration test is to simulate seed placement into predefined position using captured image sets. Specially designed tool with three markers should be attached to the robot. Tip of each marker has different coordinates. System should drive needle, touch and stop next to each marker. Difference between needle and marker tip should be recorded.Results: First and the second groups of tests are designed to have pass/fail outcome. Tolerance values for third test are ±0.5 mm. Daily QA procedure was developed to minimize test time which is 20min on average Conclusions: Routine daily QA procedure plays crucial role in ensuring overall safety of brachytherapy robotic system for clinical implementation. Recommended procedures are intended to support and supplement development of comprehensive brachytherapy robotic QA program.

Acknowledgement: Supported by NCI‐R01‐CA091763.
SU‐E‐T‐250: Results of a Survey to Assess the Current Status of In‐Vivo Dosimetry in Canada
Authors: Sawakuchi G O , Archambault L , Scullion A , Cygler J E
Published: November 2011

Purpose: To assess the current status of in‐vivo dosimetry in the Canadian cancer centers. Methods: A survey on the use of in‐vivo dosimetry was performed between July and September 2010. The survey was sent to 39 Canadian cancer centers and it was composed of 16 questions including questions on the use of in‐vivo dosimetry as well as demographics of the centers. A total of 34 centers completed the survey. Results: The survey showed that the Ontario and Nova Scotia have the largest number staff per clinic (99 and 75, respectively). Alberta and Manitoba have the largest number of medical physicist per clinic (15 and 12, respectively). The majority of the centers (79%) answered that they perform in‐vivo dosimetry to some extent. However, none of the centers perform daily or weekly in‐vivo dose measurements for individual patients, except for total body irradiation and total skin irradiation treatments. Most of the centers (74%) reported that they use a tolerance level of 5% or higher in their in‐vivo dosimetry programs. The majority of the centers (85%) answered that in‐vivo dose measurements are performed by the medical physicists other than physics assistants, dosimetrists and therapists. As pointed out by the centers, the major drawbacks and difficulties involved in the use of in‐vivo dosimetry included increased treatment and staff time. Conclusion: We assessed the current status of in‐vivo dosimetry in the Canadian cancer clinics. The results of this survey will serve as a documentation of the current status of the practice of in‐vivo dosimetry in Canada. Then, in the future such results will serve as a reference to assess further changes, developments and improvements in the field of in‐vivo dosimetry in Canada.
SU‐E‐T‐253: Checklist: A Tool for Safety in Radiation Oncology
Authors: Hand C , Sullivan P , Hasson B , Yeager C
Published: November 2011

Since its inception as a discipline in medicine, radiation oncology has had safety as its cornerstone. There are three separate areas of safety that exist within any department: patient, staff, and general public safety. Many national and international organizations have collaborated to provide a framework for a quality assurance program that will provide the backbone for safety in radiation oncology. As the complexity of treatments and delivery systems has increased, the ability of quality assurance (QA) programs to maintain safety in all three areas has been challenged. Purpose: QA programs need to be reassessed and adjusted continuously to provide the highest level of confidence. Wikipedia defines a checklist as “a type of information job aid used to reduce failure by compensating for potential limits of human memory and attention”1. This project produced checklists in our department which provide a focused approach to initial physics plan and weekly checks, dosimetry plan checks, and treatment machine resumption checks. Methods: Based on the approach used by the World Health Organization and Dr. Gawande on the appropriate types of checks to be used, our existing checklist were streamlined to include the most essential checks performed2,3. Each of the three checklists has “Do‐Confirm” and “Read‐Do” checks. Results: The inception of the new physics plan and weekly checklist has assisted in discovering multiple types of potential errors. The errors included wrong prescription, no signed consent, IVD measurements not performed, wrong DRR points, and no request for conebeam imaging noted. The new dosimetry checklist has provided more relevant checks associated with more complex treatment plans. The treatment resumption checklist has created a virtual “Timeout” before a treatment is resumed. Conclusions: Checklist provide an important QA tool for overall safety by tailoring them to specific critical checks, and changing the culture by insisting all operators “work the checklist”.
SU‐E‐T‐259: Implementation of An Automated Workflow Auditing and Notification System for Radiation Oncology
Authors: Chong S , Anderson N , Finlay J
Published: November 2011

Purpose: To develop an automated notification and reporting system that uses a series of database queries to monitor and audit the Radiation Oncology workflow, and provide feedback to users via e‐mail notification to reduce human errors and improve consistency in the department workflow. Methods: A set of SQL queries was developed to monitor elements of the workflow that correspond directly with data recorded in the database. These queries were automated using a combination of third‐party querying and formatting tools including iSQL and Infomaker (Sybase, Inc), Acrobat Professional (Adobe), and Windows XP batch programming and task scheduling. The output of these queries was stored in the form of PDF or plain text results files. An interpreter was written in Visual Basic 2010 (Microsoft) to read in a text files created by a SQL database query, sort out appropriate recipients, and send out reminders via e‐mail based on e‐mail addresses stored in Aria. Results: The automated system was first used to replace manual notification of physicians of upcoming simulation appointments with missing documentation. In a survey of recipients, the feedback was positive, indicating acceptance of the system in the clinic. A retrospective audit of the workflow showed no change in the rate of on‐time completion of documentation, indicating successful replacement of the manual process. Conclusions: The software package for the automated notification/reporting system has been implemented successfully, and its feedback was very positive. Application to additional parts of the workflow is in development.
SU‐E‐T‐262: Quality Assurance on Field Size Shaped by Iris Collimator of the CyberKnife
Authors: Wong F , Chow T
Published: November 2011

Purpose: To develop a simple method to measure the x‐ray beam field size shaped by the iris collimator of the CyberKnife. Methods: The profile of the 12‐sided polygonal beam shaped by iris collimator of the Cyberknife (Accuray) was obtained using a single array beam profiler (Sun Nuclear, Model: 1170, spatial resolution = 5 mm). To improve the resolution of the profile, we moved the x‐ray beam along the array of diodes on the profiler and obtained beam profiles for every 1 mm. The beam profiles were then shifted according to their positions relative to the starting position and created an integrated beam profile. Since there are 6 different axes on the 12‐sided polygonal beam, we can obtain the beam profiles on the other five axes in a similar fashion, by simply rotating the head of the CyberKnife. This method was also examined by replacing the beam profiler with an IˈmRT MatriXX (IBA Dosimetry). Results: By using the beam profiler, we measured the field sizes of different iris collimator setting (from 10 to 60 mm). We found that the measured field sizes are within +/− 0.4 mm compared to the expected field size. Also, we found that the variation of measured field sizes is within +/− 0.2 mm between measurements. By using the IˈmRT MatriXX, we found that the uncertainty of the measured field size now increases to +/− 1 mm. Conclusions: The method of using the beam profiler is a feasible and easy way to check the consistency of the field size shaped by the iris collimator, and reduces the reliance on using radiochromic film and water phantom. We are also investigating the feasibility of using dose area product (DAP) measured by parallel plate chamber to verify field sizes in daily QA.
SU‐E‐T‐267: A Comprehensive QA Method of MLC Accuracies for Dynamic IMRT
Authors: Oita M , Sasaki M , Yamada M , Aoyama H , Tateoka K
Published: November 2011

Purpose: The purpose of this study was to investigate the usefulness of our original quality assurance (QA) program and patientsˈ treatment of MLC data analysis for dynamic IMRT as routine practice. Method and Materials: As to the QA program, we have made three MLC sweep test patterns by changing MLC travel speed of 0.5, 1.0 and 2.0 cm/sec, respectively. All three fields were irradiated by three gantry angles of 0, 90 and 270 degree as daily QA. Each MLC logs of the QA program and patientsˈ treatment were recorded during Aug 2009 to Nov 2010. Actual MLC position errors (Bank A, Bank B) and gap errors compared with planned position were analyzed by a program (Matlab, Cybernet Inc.). Systematic and random errors of each MLC and gap were determined by everyday and every month. And then, the temporal data were also evaluated. Results: The accuracy of MLC position was depended upon its travel speed and gantry angle. Our data showed that the MLC errors in the QA program got worse as twice if the gantry angle was 270 deg compared to other angles. From patientsˈ treatment data analysis, we could monitor the accuracy of MLC during the whole treatment courses. Conclusion: In this study, we have found that our original QA program and patientsˈ treatment of MLC data analysis could be useful for clinical routine MLC QA for dynamic IMRT to identify MLC position accuracies by time‐line.
SU‐E‐T‐270: A “Rolling Average” Adaptive Planning Technique to Compensate Target Volume Changes in Prostate Cancer Radiotherapy
Authors: Liu H , Wu Q
Published: November 2011

Purpose: In external beam radiotherapy of prostate cancer, the inter‐fractional motion can have several forms: changes in position, shape and volume. The changes in position can be corrected by online image guidance, while the shape and volume changes are typically handled through the patient‐population based margins in treatment planning. In this study, we propose a rolling‐average (RA) adaptive replanning method to account for the volume variation of the target. The effectiveness of RA is evaluated by comparing with the standard online image guidance (IG) only strategy and a single replanning hybrid strategy. Methods: Twenty‐eight (28) patients with at least 16 helical CT scans each were included. Treatment of both low‐risk patient (LRP, CTV=prostate) and intermediate‐risk patient (IRP, CTV=prostate+seminal vesicles) were simulated. A new cumulative index of target volume, CITV, was proposed for the comparison of RA and other strategies. Two hypothetical scenarios: Type Ascending and Descending, simulated by sorting the CTV volumes of actual patient data, were also included in the investigation for a better evaluation of the techniques. Results: Modest target volume shrinkages were observed in our patient group. The CITV comparison demonstrates that the RA is superior to the online IG and hybrid techniques. For LRP, RA is beneficial if there is significant time trend in target volume. The optimal number of fractions necessary for the internal target volume construction is 2 for LRP and 3 for IRP. Conclusions: The RA is effective in compensating the target volume changes during the treatment course. However, the magnitude of improvement depends on how significantly and rapidly the changes in the patient population. The RA requires multiple replanning and is significantly more complex than the online only and hybrid strategies. Effectiveness and efficiency should be balanced in the implementation of RA.
SU‐E‐T‐272: In‐Vivo Proton Dose Reconstruction from PET Images
Authors: Remmele S , Hesser J , Paganetti H , Bortfeld T
Published: November 2011

Purpose: To reconstruct the delivered dose distribution in proton therapy from proton beam induced PET signals using deconvolution methods. Methods: Previous verification strategies in proton therapy compared measured PET signals with estimated PET signals created by a convolution of the planned dose distribution with a specific filter function. We now use this filter function to reconstruct the delivered dose distribution from the PET signals by developing suitable de‐convolution methods. Since deconvolution is an ill‐posed inverse problem, regularization is required to obtain a stable solution and to cope with noise found in measured PET signals. The basic filtering approach is developed for homogeneous media and additional procedures are necessary to generalize the PET estimation to inhomogeneous media. We use this generalization formalism in our deconvolution approach to allow a dose reconstruction from measured patient PET data. In addition, the PET image formation includes a convolution with a system response function which has to be removed in an additional deconvolution. To evaluate our reconstruction method, we detect the edge of the distal fall‐off region in the reconstructed dose and compare it with its nominal/planned position. Results: Various simulations demonstrate the potential and robustness of the dose reconstruction. The detection of the distal fall‐off region shows an average accuracy of 2 mm even for strongly degraded measured PET signals. In addition, we use the deconvolution approach to successfully reconstruct the dose distribution from measured PET data for a head and neck patient. Conclusions: This work indicates the potential of dose reconstruction in proton therapy from measured PET signals. The proposed method is able to correctly reconstruct a dose distribution and to cope with noise in the PET images. The shape of the reconstructed dose distribution of a head and neck patient is in good agreement with the treatment planning dose.
SU‐E‐T‐278: Volume Based Comparison of Deformable Image Registration Algorithms Using Spatial Discrepancy Volume Histograms
Authors: Fatyga M , Dogan N , Wijesooriya K , Sleeman W , Zhang B , Christensen G
Published: November 2011

Purpose: Accurate Deformable Image Registration (DIR) algorithms are essential to clinical implementation of adaptive planning strategies hence finding validation strategies for DIR algorithms remains a pressing concern. Most validation efforts are based on contour or landmark tracking, thus sampling the Deformable Vector Field (DVF) relatively sparsely. The primary purpose of this work is to assess interchangeability of DIR algorithms in dose accumulation, and assess if contour based methods are sufficient to validate the equivalence of DIR algorithms. Methods: We registered peak inhale and peak exhale phases of thirteen lung patients using three DIR algorithms. The DVF maps were pairwise compared through voxel‐by‐voxel subtraction of vector fields. The vector difference maps were analyzed by building volume histograms on regions of interest. This method of analysis is directly relevant to the Dose Volume Histogram accumulation, as vector difference between the maps will be translated into a distance between dose interpolation points. We further compared Jacobian distributions for the three maps, as local derivatives of DVF maps would be important to any algorithm that attempts local density corrections. We performed contour based comparison of the three algorithms, to connect this validation method to prior work. Results: The volume histogram analysis shows that differences between DVFs in 2% tails of volume histogram are in the 1cm – 4cm range, although the contour‐based analysis using Diceˈs Similarity Coefficient (DSC) would suggest that the three algorithms are nearly equivalent. For most structures, spatial differences between maps are below 0.5cm over approximately 70% of structure volume, and exceed 0.5cm over the remainder. Jacobian distributions differ significantly, implying that local density corrections are strongly algorithm dependent. Conclusions: Differences between algorithms are potentially significant for dose accumulation, and such differences are not revealed by contour based comparisons.

Acknowledgments: Supported by NIH Grant P01 CA11602
SU‐E‐T‐286: A Case Study of More Accurate Phase Re‐Assignment for 4DCT Images
Authors: Wang B , Sadinski M , Sarkar V , Ruhnau S , Szegedi M , Anker C , Rassiah‐Szegedi P , Zhao H , Huang Y , Salter B
Published: November 2011

Purpose: We present a case study of an improved phase re‐assignment approach for a challenging 4DCT image data set on a commercial system.Methods: The Varian RPM system was used to produce the respiratory traces for phase binning the raw 4DCT images acquired from a GE LightSpeedRT CT scanner. The RPM system uses a predictive approach to generate a *.vxp file containing details of the respiratory signal for correlation with the raw 4DCT images. The *.vxp file was read in using a program written in C++, and respiratory phases were re‐assigned using an approach which retrospectively detects individual max and min of the respiratory signal. The re‐binned file was used to reprocess the raw 4DCT data and generate a set of new phase binned images, which were compared with those from the original *.vxp files.Results: For one challenging clinical case, we demonstrate how the current predictive method can inadvertently assign a grossly incorrect phase after a highly irregular breathing cycle has occurred. The new image sets were seen to be significantly more accurate in representation of particular smooth internal structures. Seven additional patients were retrospectively studied using the same re‐binning approach, with none of these patients demonstrating significant image quality improvement. This suggests that the vendorˈs currently employed phase assignment method produces reasonable results for the majority of scenarios, but may suffer significantly for unusual cases where extreme breathing anomalies occur. Conclusions: The vendorˈs standard phase assignment method which uses a predictive breathing cycle algorithm can be challenged in instances where highly anomalous breathing cycles of much longer period are encountered. This can introduce a rippling, or ‘piano key’, effect on subsequent images. We presented a simple retrospective method for improving the accuracy of phase binning in these circumstances and demonstrated improved image fidelity on one challenging clinical case.
SU‐E‐T‐288: A Finite‐Element Model to Predict Prostate Displacement and Deformation Due to Bladder Filling, Rectal Distension and Patient Posture during Prostate Brachytherapy
Authors: Mosleh‐Shirazi M , Faghihi R , Bagheri M , Hadad K , Baradaran‐Ghahfarokh M
Published: November 2011

Purpose: Noninvasive determination of the location and deformation of pelvic organs in different stages of organs fullness and patient‐related situations encountered during the post‐implant prostate irradiation period. To help perform a more realistic treatment planning, a biomechanical finite‐element (FE) model is presented which provides prospective information on individual patients based on their initial MRI. Methods: Three patients with different prostate sizes (mean 54.6 g, range 32–71 g) were studied. Sequential MR images at different stages of bladder and rectal filling and patient positions (supine and left decubitus) were acquired. The geometry of pelvic organs was 3D reconstructed using MR images of supine patients and then transferred to the ABAQUS software for FE modeling. To tune the FE model, the tissue mechanical properties, distension forces and boundary conditions were adjusted to find the optimum combinations of these parameters, to individually produce the best match with images of each patient in all positions and stages. Finally, the model was tested in a predictive mode, in which the averaged coefficients were used to predict prostate displacement and deformation following bladder and rectal filling for supine and left‐decubitus positions. Results: The tuned model showed prostate displacements/deformations that matched the measurements to within 1 mm. In the supine position, modeling the same bladder and rectal volumes as their image‐based measurements, mean prediction discrepancy for bladder‐filling prostate displacement was 1.5 mm, and 2.9 mm for bladder filling plus rectal distension. The corresponding left‐decubitus discrepancies were 4.3 mm and 4.1 mm. Mean prostate deformation discrepancies due to large rectal distension were 1.0 mm in both supine and left‐decubitus positions. Conclusions: The model predicts prostate displacement and deformation with reasonable accuracy and offers the potential to provide more representative dose distributions. This model can potentially be used to improve organs dosimetry in brachytherapy.
SU‐E‐T‐289: Optimal Slow Scan Technique on a Single Slice CT Scanner
Authors: Markovic A , Jeutang L
Published: November 2011

Purpose: Adoption of slow scan CT (SSCT) for contouring targets affected by respiratory motion artifacts is an accepted practice. The goal of this study was to propose a CT technique which would yield a target shape that would most closely reproduce that of the ideal excursion volume traced by a moving target sphere. Methods: A single‐slice, large‐bore Marconi scanner was used to acquire axial and spiral scans of a moving plastic sphere. 1 cm movement in the superior‐inferior and anterior‐posterior directions was considered separately to isolate directional issues. Scan techniques such as 1 sec fast scan, 4 sec slow scan, over‐scanning and pitch variations were used to produce target volumes of different shapes due to the interplay between the movement of the target and the x‐ray tube. Shapes were compared to the true excursion volume by scoring them with the Paddick conformity index (CI) as well as by visual inspection. Results: For anterioposterior movement, the average CI was 0.777 (STDEV=0.05) and the largest CI occurred for the axial slow scan, yet visual interpretation showed best agreement with Axial SSCT with overscan. For inferior‐superior movement, the average CI was 0.83 (STDEV=0.05) and the largest CI of 0.872, occurred for the spiral scan with pitch 0.5, which did agree with visual inspection. Conclusions: Optimal scanning technique, depended on the direction of movement and the interpretation method. Furthermore, it required either a pitch of 0.5 or an axial overscan technique in order to reduce respiratory motion induced target artifacts.
SU‐E‐T‐293: Accuracy of Elekta ‐ Image Guided Radiation Therapy
Authors: Sutton M , Matthews K , Parker B
Published: November 2011

Purpose: Frameless Stereotactic Radiosurgery requires <1mm positional accuracy. We quantified the accuracy and precision of the IGRT treatment process for the Elekta Infinity using a comprehensive phantom‐based QA procedure. The IGRT process was evaluated for both MV planar imaging and kilovoltage cone beam computed tomography (kVCBCT). Methods: A CIRS Radiosurgery head phantom with a film dosimetry insert was used to measure dose distributions in three anatomic planes. Once the phantom was properly aligned on the treatment couch, a series of offsets were applied to the position of the phantom; intentional misalignments of ±5mm in the lateral, longitudinal, and axial directions were applied. Prior to treatment, one of the IGRT modalities was used, and the calculated realignment values were applied to the phantom position. An isocentric seven‐field treatment plan with a prescription of 3Gy per fraction was delivered to the phantom. The delivered dose distributions were analyzed and compared to the calculated dose distributions from the Pinnacle(v.8.0m) treatment planning system. Two metrics were analyzed from the film: (1) dose profile positional alignment error at the 70% dose level and (2) directional mismatch of dose profile edges at the 80% dose points. Results: The largest positional alignment errors occurred along the inferior‐superior axis [0.64 ± 1.01mm]. This was possibly due to slack in the couch along this direction — the couch tended to shake slightly along this axis. Similarly, the shifts in the 80% dose points were worst along the inferior‐superior axis [80%(Inf) = −0.71 ± 0.91mm; 80%(Sup) = −0.89 ± 1.17mm]. Conclusions: Our evaluation indicates the Elekta Infinity does not have sufficient positional accuracy for frameless Stereotactic Radiosurgery, but its positional accuracy is more than suitable for conventional radiotherapy. For all cases, using kVCBCT for phantom alignment resulted in more conformal dose coverage than using MV Planar imaging.

This work is supported by a research contract from Elekta
SU‐E‐T‐299: Verification of High Density Concrete Transmission Characteristics under User Beams
Authors: Yang C , Dieck G , Edwards J , Liu T , Rajaguru P , He R , Mobit P
Published: November 2011

Purpose: To verify transmission factors of the high density concrete (HDC) used in modification of an existing radiation shielding for a new linear accelerator and to measure transmission factors of other types of HDC under user beams before shielding construction started. Method and Materials:Six HDC 300 PCF (Pounds per Cubic Feet) blocks in size of 6″×6″×12″, randomly picked up from a shipment for radiation shielding at the user site, in addition to one sample of each 288 PCF, 250 PCF and 240 PCF were measured for their transmission factors using a 0.6 cc Farmer chamber and the Dose 1 electrometer. User beams used for measurement includes 6MV, 10 MV and 18 MV photon beams from an Elekta Synergy machine. Transmission factors are measured under both narrow (3×3 cm2)(NBG) and broad beam geometry (40×40 cm2)(BBG) with a close condition to a real shielding geometry. Results: Weight measurement indicates that the actual PCF is higher than stated: 313.3±4.5 (one SD) in the range from 306 to 318 PCF for 300 PCF and 291 PCF for 288 PCF, 251 PCF for 250 PCF, 247 PCF for 240 PCF. Transmission measured at seven spots under NBG (18 MV) for one 300 PCF sample is 0.087±0.002 (Min: 0.084 and Max: 0.089). Transmission measured under NBG (18 MV) for six 300 PCF sample blocks is 0.089±0.004 (Min: 0.086 and Max: 0.096). The first tenth value layer under BBG (18 MV) is determined as 7.4″, which is smaller than 8.5″ used in the shielding design calculation. Conclusions: We have confirmed that the HDC blocks used in constructing the radiation shielding meet and actually succeeded the transmission requirement in the shielding calculation. We provided the commercial company with updated measurement of its shielding materials under our user beams.
SU‐E‐T‐300: Linac Bunker Access Control System for An Exactrac/Novalis Imaging Systems
Authors: Evans M , Larkin J , Siva B , Leger P , Patrocinio H , Parker W
Published: November 2011

Purpose: To implement and assess the radiological safety of a modified Last‐Person‐Out (LPO) interlock circuit in the bunker of a robotic Novalis/Exactrac stereotactic radiosurgery linac allowing the Exactrac imaging system to operate with the bunker door open. Methods: The Exactrac floor mounted X‐ray imaging system is designed to be used with the bunker door open, however access control is a concern in bunkers not designed for open door operation. The door interlock contact in our Exactrac generator was identified and a LPO circuit for the Exactrac imaging system interlocked with the operator console was installed. This allowed for operation of the Exactrac imaging system with the door open while providing access control using a light shower (Banner‐Engineering) installed at the maze entrance. To assess the radiological safety for operators, a large volume ionization chamber (PTW32003) used in conjunction with an electrometer (Keithley6517A) was placed at the location of the bunker door at the entrance to the maze. 100 radiographs (50 from each X‐ray tube) were taken with the Exactrac X‐ray imaging using a cranial standard (100 kVp, 80 mA and 100 ms) and a 20 × 20 × 20 cm3 solid water phantom placed at isocentre to simulate the patient. This corresponds to 2 to 3 times our typical daily load. Results: The Exactrac LPO system runs in parallel to the Novalis Last‐Person‐Out system that remains unaffected by the modification. Our measurements revealed a yearly equivalent dose at the open door of 4 micro‐Sv per year, and a committed dose of 0.5 micro‐Sv per year (assuming one‐eighth occupancy factor). Conclusions: An access control system was implemented on our Novalis/Exactrac system allowing the imaging system to be safely used with the treatment door open. Prior to use, the federal regulator (CNSC) was notified and staff training was provided.
SU‐E‐T‐302: Modular Technology as a Tool to Streamline the Construction and Clinical Readiness of a New Radiotherapy Clinic
Authors: Shepard D
Published: November 2011

Purpose: In February 2011, the Swedish Cancer Institute in Seattle, Washington opened a new 4000 square foot radiation therapy facility incorporating a Tomotherapy Hi‐Art treatment unit. A key differentiator of this clinic is the use of modular construction techniques for both the radiation therapy vault and the clinic. In this work, we report on our experience with shielding design, construction, and clinical operations when utilizing modular technology. Methods: The radiation therapy vault was constructed using 10 steel modules that were filled on site with washed silica based sand from local quarries. The clinic modules were constructed offsite including all plumbing, duct work, and electrical fixtures along with the acceleratorˈs base frame. Results: The use of modular construction techniques made it possible to accelerate the construction timeline. Within 60 days, the vault and the clinic were erected on site, the Tomotherapy Hi‐Art treatment system was installed, acceptance testing and commissioning were completed, and the first patient was treated. Our shielding calculations predicted weekly doses for nine locations in the range of 0.01% to 3% of the allowable weekly dose, and radiation surveys have validated these predictions. The facility was constructed in conformance with the U.S. Green Building Councilˈs Leadership in Energy and Environmental Design (LEED) certification and incorporates green features such as heating the clinic using heat recovered from the cooling of the Tomotherapy Hi‐Art system. While the vault was prepped for the Tomotherapy unit, it can be easily converted for use with any gantry based machine with a swappable base frame system and adjustable shielding. Conclusions: Modular construction provides a rapid path to clinical operation along with the flexibility to move the vault at a later date. Shielding design can be customized to the selected delivery technology and has proven more than adequate for our Tomotherapy Hi‐Art system.