bluuebunny/crossref_metadata_2025 huggingface dataset:
SU‐E‐T‐217: Characterization of the Respisens™ Motion Sensor Device for Intra‐Fraction Patient Tracking
Authors: Gutierrez A , Stathakis S , Esquivel C , Papanikolaou N
Published: November 2011

Purpose: To assess the accuracy and response characteristics of the novel Respisens™ intra‐fraction motion monitoring device. Methods: The Respisens™ (CIVCO, Orange City IA) device is a highly sensitive, real time (20Hz sampling frequency) magnetic distance sensor used for patient motion tracking during radiotherapy—i.e. intra‐fraction motion monitoring. The device consists of both a miniaturized magnetic transmitter and receiver which measure the coplanar distance between the two probes real‐time. The device was evaluated for five different characteristic prior to clinical implementation: (1) Distance accuracy, (2) Distance range, (3) Off‐plane (non‐coplanar) response, (4) Angular response, and (5) Displacement detection accuracy. Results: The Respisens™ showed a distance tracking range from 7.5 to 23.5cm. The distance accuracy was <1.0mm for distances 7.5 to 16.0cm—above 16.0cm deviations were 2.0mm. The off‐plane (non‐coplanar) response was highly dependent on displacement due to the shape of the magnetic field. Overall, the off‐plane response was minimized with small off‐plane displacements (<1.0cm) and increased distance. The distance reading was also sensitive to angular displacement of the receiver relative to the transmitter. Relative to baseline—receiver perpendicular to transmitter, an over‐response of <10.0% was noted with angular deviations of up to 40°. Above 40°, the over‐response increases exponentially to 200% (89°). With regards to displacement detection accuracy based on initial position, the Respisens™ showed accurate response (<0.6%) for a 5.0cm displacement when the initial position was <16.5cm. When the initial position was >16.5cm, a varying response of up to 2.0% was noted. Conclusions: Based on the preliminary findings, the Respisens™ intra‐fraction motion monitoring device can be used to accurately measure patient motion realtime. Precaution must be taken to ensure that the probes are coplanar and not rotated relative to one another.
SU‐E‐T‐218: In Vivo 3D Dose Verification for IMRT Using Electronic Portal Imaging Device (EPID)
Authors: Chang C , Anderson NG , Zhu T C
Published: November 2011

Purpose: To provide in vivo 3D dose verification for IMRT treatment sessions using an FFT‐based dose calculation method for heterogeneous media. Methods: Dosimetric calibration of the EPID is performed using an ionization chamber with mini‐phantom. Optical scattering within the EPID is corrected to minimize systemic error. The dose‐response of EPID, i.e. EPID reading as a function of dose, is linear with dose. A fast ray‐tracing algorithm is developed to calculate the full 3D radiological length distribution within the heterogeneous phantom/patient and up to the EPID active imaging plane. Attenuation of the primary beam as a function of radiological length is approximated by a linear quadratic exponential function and its off‐axis variation is determined experimentally by transmittance measurements in order to account for the polyenergetic MV beams. An iterative algorithm is used to find a self‐consistent solution separating the total dose measured by the EPID into primary and scatter contributions through a phantom/patient. Calculation of the scatter contribution from phantom to EPID is achieved by first deconvolving the effect of EPIDˈs internal lateral scattering, followed by the convolution of a 2D scatter kernel accounting for the patient scatter to EPID. Results: The dose‐response for the amorphous‐silicon (aSi) EPID has been obtained. The fast ray‐tracing algorithm has been developed and tested on both phantom and Rando phantom patient geometry. The linear quadratic parameters for our clinical 6MV beam have been estimated both on the central axis and off‐axis, and used for initial validation of this exit dose verification method. First result has shown that the estimated patient dose is within 5% of that calculated by the treatment planning system. Conclusions: With a fast kernel‐based 3D dose calculation algorithm, aSi EPID can potentially be used for dose verification as a routine patient QA procedure.
SU‐E‐T‐219: Ion Chamber Dosimetry Modification under Strong Magnetic Field Conditions; Air and Liquid Filled Chambers Study
Authors: Koren S , Price R , Tafo A Guemnie , Veltchev I , Fourkal E , Ma C
Published: November 2011

Purpose: To examine the dose response of ion chambers in an external high magnetic field, conducting measurements and Monte Carlo simulations. To examine a liquid filled ion chamber (LION) as a low B field dependant on‐time dosimeter. Methods: This work investigates the response of a 0.6cm3 Farmer chamber to a 11.17 mCi 137Cs source under the influence of a 1.5 T magnetic field. A special setup allowed measurements for transverse and longitudinal field directions inside a 1.5T MRI bore. Monte Carlo simulations were conducted using FLUKA, a particle physics transport package. A PTW 0.002 cm3 MicroLion chamber is offered as a non‐correctional on‐line B field measurement alternative, since it introduces a small cavity and no air phantom voids. Results: Higher Farmer chamber currents were obtained when a transverse (to chamber central axis) magnetic field was applied, compared to zero B field conditions. Lower readings were obtained when a longitudinal field was present. Measurements have shown that the electric field between the electrodes has a very small impact on the ion collection, and can be neglected for simulation purposes. A microLion chamber was found not to have response impact due to the presence of a magnetic field. Conclusions: The dose response of a Farmer chamber in an external magnetic field varies with the average charged particle trajectory length. The average trajectory depends on the B field magnitude and direction, and on the energy spectrum of the primary photon beam. Thus, an alignment with the B field direction would be essential. A liquid‐filled chamber is offered as an alternative due to its homogeneous structure and small cavity that results in no B field correction needed.
SU‐E‐T‐220: Towards Automatic Quality Assurance of Kv‐CBCT Based IGRT Systems: A Time Series Analysis of KV‐MV Isocentre Coincidence
Authors: Naisbit M , Viard R , Sykes J
Published: November 2011

Purpose: Regular measurement of the coincidence of CBCT image with the MV treatment beam isocentre is an important component of quality assurance for CBCT based IGRT. In this work the coincidence of the imaging isocentre (kV‐CBCT) with the treatment isocentre measured using the EPID (MV‐EPID) has been measured over an extended period using an algorithm developed to perform the measurement automatically. Methods: The treatment isocentre was computed by analysing EPID images of the PentaGuide phantom (MODUS Medical Devices) for gantry angles of 0°, 90°, 180° and 270° with collimator angles of 90° and 270°. The position of the centre of the PentaGuideˈs central air cavity was calculated relative to the radiation isocentre using the EPID images and relative to the CBCT isocentre in the CBCT images. The vector difference between these two measurements gave the coincidence of the MV radiation isocentre and kV‐CBCT image origin. Processing was performed on the development platform of the ARTISCAN software (AQUILAB, Lille, France) which utilised image processing to automate these calculations directly from the DICOM images. The software was first validated using simulated MV and CBCT images of the phantom with known offsets. Site validation was performed with the Synergy system (ELEKTA, Crawley, UK) and using the Hexapod system (ELEKTA) to offset the phantom. In addition, 10 measurements were performed on two Synergy systems over a 17 months period during site validation. Results: No deviations were observed using the simulated images, and phantom position measurements were confirmed with the offset phantom during on‐site validation with minor deviations of: absolute mean error MV and kV was 0.33 mm and 0.35 mm respectively. The distance between isocentres was less than 1 mm over the period of testing (max=0.53, mean=0.26, std=0.16). Conclusions: The algorithms developed show efficiency and reliability for automatic QA of the isocentre position.
SU‐E‐T‐221: An In‐House Developed Resettable MOSFET Dosimeter for Radiotherapy
Authors: Van Vaerenbergh S , Verellen D , Van De Vondel I , Heuninckx K , Tournel K , Reynders T , Duchateau M , Gevaert T , Leysen K , Depuydt T , Poels K , De Ridder M
Published: November 2011

Purpose: To report the feasibility and clinical validation of an in‐house developed MOSFET dosimetry system and describe an integrated nondestructive reset procedure. Methods: Off‐the‐shelf MOSFETs are connected to a PC using an 18 bit/analogue‐input and 16 bit/output data acquisition card. A reading algorithm was developed defining the zero‐temperature‐coefficient point (ZTC) to determine the threshold voltage. The reset procedure consists of an internal circuit generating a local heating induced by an electrical current. Sensitivity has been investigated as a function of bias voltage (0—9 V) to the gate. Dosimetric properties have been evaluated for 6 MV and 15 MV clinical photon beams and in vivo benchmarking was performed against TLD for conventional treatments and total body irradiation (TBI). Results: Sensitivity of 0.08 mV cGy−1 can be obtained for 200 cGy irradiations at 5 V bias voltage. Ten consecutive measurements at 200 cGy yield a SD of 2.08 cGy (1.05%). Increasing the dose in steps from 5 cGy to 1000 cGy yields a 1.00 Pearson correlation coefficient and agreement within 2.0%. Dose rate dependence (160–800 cGy min−1) was within 2.5%, temperature dependence within 2.0% (25–37° C). Dose response is stable up to 50 Gy (saturation occurs at approximately 90 Gy), which is used as threshold dose before resetting the MOSFET. An average measured‐over‐calculated dose ratio within 1.05 (SD: 0.04) has been obtained in vivo. TBI midplane‐dose assessed by entrance and exit dose measurements agreed within 1.9% with ionization chamber in phantom, and within 1.0% with TLD in vivo. Conclusions: An in‐house developed resettable MOSFET‐based dosimetry system is proposed. The system has been validated and is currently used for in vivo entrance dose measurement in clinical routine for open field treatment configurations.
SU‐E‐T‐226: Image Acquisition and Processing Characteristics of a Siemens EPID: Potential Problems for EPID Dosimetry
Authors: Deshpande S , Vial P , Holloway L , Metcalfe P
Published: November 2011

Purpose: To investigate the behavior of the imaging control software on a Siemens EPID and the resulting impact on EPID based IMRT dosimetry Method and Materials:Four ‘simple’ and four ‘clinical’ IMRT fields were created. Each segment of the ‘simple’ field had a constant field size of 10 × 10 cm2. The ‘simple’ fields had 2, 4, 5, and 10 segments with 10, 5, 4, and 2 MU per segment respectively. The ‘clinical’ fields all consisted of 20 segments with constant MU per segment of 2, 4, 5, and 10 MU. The fields were measured simultaneously with a Siemens EPID and ImRT MatriXX ionisation chamber array (iba) by placing the MatriXX beneath EPID. The integrated EPID dose response was determined from the frame average pixel value multiplied by the reported number of frames. Reproducibility of central axis response, beam profiles, and pixel‐by‐pixel response (2D) was compared from 10 repeat measurements. Results: The percentage standard deviation of the integrated EPID response for the ‘simple’ case was within 1% for the 2 and 10 MU per segment fields but more than 10% for the 4 and 5 MU per segment fields. EPID crossplane profiles also yielded large variations in amplitude and shape for the 4 and 5 MU per segment cases only. No such variations were observed by concurrent MatriXX measurements. A similar but less obvious trend was observed for the ‘clinical’ case. In both cases the number of frames per segment reported for the 2 MU and 10 MU per segment fields remained constant. The number of frames reported for the 4 MU and 5 MU per segment fields varied and appeared unreliable. Conclusions: EPID images of IMRT fields displayed large variations in reproducibility depending on the MU per segment. Poor reproducibility correlated with inconsistent reporting of the number of frames.

Cancer Institute NSW Research Equipment Grant 10/REG/1‐20
SU‐E‐T‐227: Patient Treatment Quality Assurance for a Uniformly Scanned Proton Therapy Beam
Authors: Baillie N , Lee T , Duvvuri S , Shahnazi K , Athar B , Long C , Butuceanu C , Nazaryan V , Keppel C
Published: November 2011

Purpose: To describe the main quality assurance (QA) procedures undertaken at the Hampton University Proton Therapy Institute (HUPTI) in preparation for patient treatment via the uniform scanning (US) proton beam delivery technique using two source to axis distances (SADs). Methods and Materials: HUPTI currently uses a uniformly scanned proton beam for patient treatments. Patient specific apertures and compensators (properly scaled to account for the dual SADs) are used to shape the field laterally and to conform to the target volume longitudinally. Machined part QA is performed by using in‐house software to convert the DICOM output from Eclipse into images which represent the aperture and compensator shapes after correcting for the dual SADs. These images are matched to the physical shapes of the apertures and compensators to ensure correct machining. The beam QA measurements include the absorbed dose (output factor) at the center of the Spread‐Out Bragg Peak (Mid‐SOBP) in accordance with the International Atomic Energy Agencyˈs Report TRS‐398; a verification of the lateral planar dose profile at isocenter measured with a 2D ion chamber array (IBA‐MATRIXX); and the depth dose profile measured along isocenter with a multi‐layer ion chamber (MLIC‐IBA‐ZEBRA). Results: The QA measurements for approximately 30 patients who have been treated at HUPTI utilizing the US technique will be presented. Analysis of the lateral and depth dose profiles exhibits agreement with the Varian Eclipse Proton Treatment Planning System within acceptable limits (less than 2%). Dose measurements are within the 5% accuracy recommended by International Commission on Radiation Units and Measurements (ICRU) and are well described by empirical modeling. Conclusions: The Quality Assurance Program at HUPTI has demonstrated that the prescription of proton radiation dose designed by the radiation oncology team is being delivered to each cancer patient under our care with the highest precision and accuracy possible.
SU‐E‐T‐234: Use of GafChromic RTQA Film for Head Leakage Tests
Authors: Deufel C , Remmes N , Niedermayr T , Harms T
Published: November 2011

Purpose: To evaluate the suitability of GafChromic RTQA film for use in head leakage tests and to summarize the advantages/disadvantages of using radiochromic film over traditional radiographic film. Methods: Head leakage tests were conducted on a recently installed Varian 21iX treatment machine. 50 sheets (5 boxes) of 14″ by 17″ GafChromic RTQA film (International Specialty Products) were affixed to the accelerator head with the clear top substrate facing outwards. Leakage tests were performed with a 6 MV photon beam and the smallest possible field, ∼2 mm × ∼2 mm. GafChromic film self‐develops, and areas of leakage were identified without removing the film or pre‐determining the number of monitor units. The film was evaluated after delivery of 10,000 MU, 30,000 MU, and 60,000 MU (100 Gy, 300 Gy and 600 Gy respectively to isocenter). Results: Evidence of leakage radiation was faintly visible after 10,000 MU and clearly visible after 30,000 MU. Survey meter measurements confirmed that the highest leakage occurs at the regions identified by the GafChromic film. The comparison between GafChromic RTQA film and traditionally‐used photographic film can be summarized as an exchange between cost, time, and simplicity. Total retail cost for the GafChromic film used in these measurements was approximately $1200, which is substantially more expensive than radiographic film (approximately $100). Time savings of the GafChromic method was approximately one full workday. GafChromic film was the simpler option for head leakage testing. GafChromic film provides immediate feedback and requires no darkroom, no MU estimation, and no need for film numbering or indexing. Conclusions: This work indicates that, based on time savings and the ease of use, GafChromic RTQA film is a suitable alternative to radiographic film in identifying areas of head leakage.
SU‐E‐T‐236: Evaluation of Electronic Portal Imaging Device in the Quality Assurance of Volumetric Modulated Arc Therapy
Authors: Liu Z , Chan P , Li S , Micaily B , Miyamoto C
Published: November 2011

Purpose: To evaluate the feasibility of engaging electronic portal imaging device (EPID) in the routine VMAT QA programs. Methods: A small radiation field of 1.6×1.6cm was used to irradiate an 8mm diameter ball bearing positioned precisely at machine isocenter and projected images at different gantry angles were recorded using EPID. A dynamic‐MLC based picket fence pattern was created both in (a) static mode and (b) VMAT mode. In either mode, a total of 40 control points were evenly divided into ten strips of 2cm in width. The ratio of radiation fluence between two neighboring strips was set at 2. Elekta linear accelerators operate with binned dose rates. The dose rate transition is instant and in strict synchronization with the gantry rotation speed and MLC movement. The entire delivery of the picket fence pattern was recorded in accumulation mode with Elekta iView. Image analysis was performed using an in‐house program written in MATLAB. Results: During a full gantry rotation, EPID panel was found to move up to 1.3 mm in the gantry‐target direction but less than 0.5mm in the lateral direction. The MLC speed and positioning were well controlled in both static mode and VMAT mode. At any time point during radiation delivery, the positions of individual leaves in each bank were within 0.5mm of the planned position in static mode. Comparing the picket fence pattern in VMAT mode with that in static mode, we noted minimal discrepancy (<0.5mm) between the corresponding leaf edges. In addition, we observed a negligible impact of gantry rotation speed and MLC travel speed on the accuracy of leaf positioning. Conclusions: Through this study, we demonstrate that EPID is indeed a feasible tool in routine VMAT QA program given that the motion of EPID panel per se is carefully considered.
SU‐E‐T‐242: Quality Assurance for a Novel Translating Bed Total Body Irradiation Technique
Authors: Granville D , Brown D
Published: November 2011

Purpose: To establish a method of quality assurance for a novel aperture modulated translating bed total body irradiation technique currently under development at the Tom Baker Cancer Centre. Methods: This TBI technique uses the MLC on a conventional linear accelerator to dynamically shape the radiation beam as the patient is translated through with 0° gantry angle. The dynamically shaped field allows for the compensation of varying patient composition as the patient translates through the beam, resulting in improved dose homogeneity. This technique requires a high degree of accuracy in the positioning of the translating bed and MLC leaves throughout treatment, as the bed position must be properly correlated with the field shape being delivered. The QA method developed in the present work makes use of the linacˈs EPID to verify the location of the individual leaves of the collimating system throughout treatment delivery. The EPID operates in cine mode, and acquires images of the MLC defined field throughout plan delivery. The MLC defined field edges of each acquired image are found using a simple edge detection algorithm, and the locations of the detected edges are compared to the prescribed MLC positions for the specified number of MUs delivered. The position of the translating bed is also monitored throughout the delivery to ensure proper correlation between MLC leaf positions and bed position. Results: The EPID allows for the accurate detection of bed and MLC leaf positions throughout the delivery of aperture modulated translating bed TBI plans. Quantitative data comparing prescribed and detected positions of the bed and MLC leaves are supplied for the entire duration of plan delivery. Conclusions: The method developed allows for pre‐treatment verification of the novel TBI treatment plans, and has the potential to be extended in order to verify MLC and bed positions during actual patient treatment.
SU‐E‐T‐244: Should Treatment Time Be Included in Assessing the Quality of a Gamma Plan?
Authors: Luo D , Eley J , Du W , Shiu A , Chang E , Brown P , Newhauser W
Published: November 2011

Propose: To establish a method of estimating nominal treatment time for a given treatment volume in order to facilitate the assessment of Gamma Knife treatment plan quality. Methods and Materials: The clinical plans of 90 patients with brain metastases and/or surgical cavities treated on our Leksell Gamma Knife PERFEXION unit between Nov. 2010 and Jan. 2011 were evaluated. Patients with other diagnosis, e.g. acoustic neuroma or meningioma, were not included. Each plan was prescribed by one of the six radiation oncologists using GammaPlan 8.3.1. After excluding the lesions whose dose grid was overlapped with other lesionsˈ, we analyzed 198 tumors from these 90 plans. The planned treatment (TX) time were normalized to the time needed if prescription (Rx) dose and dose rate were 20 Gy and 2.5 Gy/min, respectively. The relationship between the normalized TX time and TX volume was analyzed, and a curve was established where about half of the lesions have lower normalized TX time than the nominal time predicted by the curve. Results: A formula, which is 4.5 times the cubic root of the TX volume, is proposed to estimate the nominal TX time, which, after scaling back by the Rx dose and treatment dose rate, can be used to compare with the planned TX time. After satisfying other quality metrics, a lower planned TX time indicates that the plan is likely to have reached an optimum, while a much higher planned TX time, as determined by the upper threshold, also established by this study, may signify the need for a better plan. Conclusions: A good plan has high Coverage, high conformity, low Gradient Index, and low treatment time. Knowing the bounds of these metrics helps one determine if an optimum has been reached.
SU‐E‐T‐249: Evaluation of the Surface Rendering Patient Localization System for Proton Therapy Facility
Authors: Mamalui‐Hunter M , Li Z
Published: November 2011

Purpose: Proton therapy carries a number of dosimetric and radiobiological advantages for cancer patients and calls for rigorous quality assurance of patient registration. This work is a report of evaluation and use of surface rendering patient positioning system (AlignRTTM) in proton facility. Method and Materials: The AlignRT three‐camera system was installed in a specific non‐symmetric geometry, specific to the proton gantry room. Series of phantom shifts/rotations were done to assess the system performance versus shifts/rotations of the 6D PPS (Patient Positioning System) that accounts for proton gantry sag. 4D phantom (CIRS,inc.) was used to evaluate motion tracking capabilities of AlignRT. Finally, AlignRT installation was used for on‐line monitoring of thorax patients (emphasis on breathing motion). Retrospective analysis of breathing data was performed versus latest weekly 4DCT of these patients. Results: In case of the baseline images taken at specific PPS rotation angles, from which shifts were performed, the proton room‐specific 3‐camera AlignRTTM system shows satisfactory results with shifts/rotations within 0.5mm/0.3degrees maximum difference. The accuracy of SSD determination was within 1 mm. In case of treatment planning system‐generated baseline images and baseline images taken at PPS angle of zero degrees, the system performance depended on PPS rotation angle, the fact whether the AlignRT system had the nominal couch angle input, and the size of the shift. At shifts within +/−3cm from isocentric position, AlignRT system performed with accuracy of 2.0mm/0.3degrees maximum deviations for PPS rotation angles within 30degrees from the “golden standard” surface and for PPS rotations 90+/− 10degrees. At intermediate PPS angles (60+/−10degrees), accuracy shown: within 10mm longitudinal/lateral, sub‐millimeter vertical directions. Reproducibility of phantom position was within 0.5mm/0.3degree. The measured amplitude of the patient breathing motion was within 0.5mm from 4DCT. Conclusion: Installation of surface rendering patient positioning system (AlignRTTM) in proton room‐specific geometry was evaluated and used for patient positioning monitoring.
SU‐E‐T‐251: Prototype QA Device for HDR Source Position Simulator
Authors: Jacob D , Simpson L , Chen H
Published: November 2011

Purpose: Design and build prototype device to perform baseline and ongoing, monthly, tests of resistance‐tolerant, smooth, dummy HDR source, position‐measurements under controlled conditions. Methods: Seven helical loops of Nucletron‐compatible catheter were wound onto the surface of a 10″ long, polyethylene tube which was 1.875″ diameter. A Nucletron HDR transfer tube could be connected between a Source Position Simulator and the entrance, 1st top helical loop end (Figure 1). Results: The 2010 SPS was attached via the transfer tube to the SPS QA prototype. The SPS wire was advanced smoothly from the start position (1030mm) to the maximum extension at 1469mm (using the advance yellow edge of the SPS marker). There was minimal cumulative resistance braking of the wire source thru the last 50mm before that max point at 1469mm. Next, the older, heavily‐used, 2004 SPS was attached via the transfer tube to the SPS QA prototype device. This SPS wire would not advance to the limit smoothly. Actually it met some modest resistance at around 1152mm (identical to the conditions of the Medical Event). The SPS wire was more firmly pushed through that zone of resistance and advanced, with increasing resistance to about 1262mm before the decision was made not to push more firmly. Conclusion: It is possible, using cumulative resistance at increased path lengths in the helical catheter, to objectively measure a distance‐to‐stop. Any motion that is not pushably‐smooth and meets any resistance short of the max finish at 1469mm indicates the beginning of damage. That signal should suffice to warn that ‘it is time’ to refurbish or replace the SPS. Results show a proper range of cumulative resistance‐induced retardation of the pushing motion of the wire thru the full motion range in QA simulation. The clinical condition of use of an SPS is accurately emulated in this device.
SU‐E‐T‐258: Comparison of Dosimetric Characteristics Between Elekta Synergy Linear Accelerators with MLCi and Beam Modulator Collimator Heads
Authors: Fabien J , Zheng Y , Anton L , Colussi V , Shields C , Shields G , Wessels B
Published: November 2011

Purpose: Two Elekta Synergy linacs were installed; one with the Elekta MLCi2 collimator and the other was installed with the Elekta Beam Modulator collimator. The concept of beam matching nominally includes the capability for patients to be switched between machines during the course of treatment with nominal replanning. However with the two different MLC designs and treatment head configurations, a precise beam match is not possible and lies outside the scope of this investigation. Nevertheless, we set out to characterize the physical radiation machines parameters between the machines and highlight any significant deviations. Methods: The acceptance scanning was performed with a Blue Phantom2 from IBA Dosimetry. In conjunction with the standard Elekta acceptance testing criteria, dmax values, depth dose curves and beam profiles were acquired for all energies on both machines. The Beam Modulator MLC has a maximum field size of 16×21 cm so the 8×8cm and 16×16cm field sizes were selected for flatness and symmetry measurements. This machine incorporates 4mm leaves and therefore could not produce the standard 10 × 10 cm field of the MCLi2 with its 10mm leaves, hence a 10.4 × 9.6 cm field was used. The resulting data was normalized, smoothed and central‐axis corrected. Results: Corresponding points on the percent depth dose curve differed by up to 0.5% beyond dmax. The dmax values for all energies were within 1 mm of each other. The largest deviation in flatness and symmetry was 1.1% and 0.7% respectively. The maximum discrepancy between the penumbra widths of the profiles was 1.1 mm. Conclusions: The physical characteristics of the photon beams were found to be surprisingly similar for the chosen reference fields even though the treatment heads and MLC design were markedly different. These results were not anticipated by installation engineers, site physicists or the manufacturerˈs specifications.
SU‐E‐T‐261: Daily Quality Assurance for a 6D Robotic Couch System
Authors: Detorie N C , Scinta L , Dragojevic I , Song W
Published: November 2011

Purpose: To create a daily quality assurance program that would evaluate the positioning accuracy and reproducibility of a new robotic couch system (CIVCO Medical Solutions) for routine clinical use. Methods: Translation accuracy and reproducibility were measured using a rectangular box phantom covered with 1.0 mm spaced graph paper. The couch was programmed to move a set of translations in all three directions. The magnitude and sign of the set translations alternated daily and encompassed the expected range of travel relevant to clinical use. The applied displacements ranged from 2.0 mm – 10.0 mm. An IGRT QA phantom (CIVCO Medical Solutions) was used for evaluating rotation accuracy and reproducibility. Digital levels on top of the phantom measured the rotations. The couch was programmed to move a set of pitch and roll rotations. The magnitude and sign of the set rotations alternated daily. The applied rotations ranged from 0.3 degrees – 1.5 degrees. A combination of translations and rotations were applied with the robotic couch and verified with CBCT. The CBCT was manually registered with a planning CT of the phantom and the shifts were recorded. Results: The maximum translational disagreement between the programmed and measured motions with the graph paper phantom was 0.5 mm. The maximum rotational disagreement between the programmed and measured motions that was identified by the digital levels mounted on the IGRT QA phantom was 0.2 degrees. The maximum deviations between the programmed and measured couch motions identified by CBCT were 2 mm for translations and 0.5 degrees for rotations. Conclusions: A daily quality assurance procedure, which evaluates daily positioning accuracy and reproducibility of a new, commercially available robotic couch, has been implemented. Based on the initial results, the system is accurate, reproducible, and safe for routine clinical use.
SU‐E‐T‐263: Development of a Video Guided Real‐Time Patient Motion Monitoring System
Authors: Hong C , Ju S , Kim J , Shin J , Shin E , Han Y , Ahn Y , Park H , Choi D , Huh W
Published: November 2011

Purpose: Using conventional CCD camera systems, we developed a video image guided real‐time patientˈs motion monitoring (VGRPM) system; a motion phantom was used for evaluation of its performance for clinical use. Methods: The VGRPM system consists of three parts: an image acquisition device, a main control computer, and in‐house developed patient motion analysis software. For development of an intelligent patient motion monitoring system that works only during treatment time and for system automation, the system was designed for synchronization with a beam on/off trigger signal. Patient movement during radiation is detected by real‐time frame difference based in‐house developed analysis software. When the error range exceeds the set criteria, (Cmove), a warning message is generated in the form of light and sound. The described procedure repeats automatically for each patient. A motion phantom, which operates by moving a distance of 0.5, 1, and 2 cm for 1 and 2 sec, respectively, was used for evaluation of system performance. We measured optimal Cmove for clinical use, minimum distance, which can be detected in this system, and response time of the whole system. The stability of the system in a linear accelerator unit was evaluated for a period of 6 months. Results: : As a result of the moving phantom test, the Cmove for detection of all simulated phantom motion was determined to be 0.5%. The system can detect phantom motion as small as 0.5 cm. The measured response time from detection of phantom movement to warning signal through video analysis was 0.1 sec. During this testing period, no significant functional disorder was observed. Conclusions: The VGRPM system can contribute to reduction of treatment error by patientˈs motion and increase the accuracy of treatment dose delivery.
SU‐E‐T‐269: Study IMRT Dosimetry Uncertainty Factors by Simulation
Authors: Jiang Z , Naughton J , Holupka E , Huang K , Chin S
Published: November 2011

Purpose: To analyze IMRT dosimetry uncertainty by simulation, and identify the key error from IMRT verification. Method: IMRT uncertainty factors are convoluted in the routine verification results, and can be from treatment plan, LINAC, and QA tools. With similar dosimetry coverage, several controlled IMRT plans can be generated by varying optimization parameters and algorithms. Some of them are more sensitive to certain perturbation and may fail to pass routine IMRT QA. First we modify the plan parameters to simulate the uncertainties. For example, we change MLC leaf and isocenter positions to simulate leaf and setup errors, and we change machine with different output factors to simulate small field measurement error. Then we import the plan back for dose calculation and comparison. Results: In the high dose region with low gradient, the mean dose is quite robust to positioning perturbation. In prostate case, the mean dose changes are within 2%, even with ±5mm random MLC leaf, or setup error. For HN cases, the mean dose changes are within 3% for most of perturbed plan with same uncertainties. However, large variations for the point dose from each beam are observed. The random MLC leaf error results the distribution broadened in the analysis of planar dose error distribution. As for the small field output factor error, it can cause a typical prostate IMRT case fail to pass standard QA criteria dependent on the plan. The small field error can behave randomly in the distribution of percentage dose error. In the low dose region with low gradient, simulation results indicated systematical MLC leaf position error is important, apart for MLC modeling parameters. Conclusion: IMRT verification is a part of on‐going efforts to improve the IMRT process. By identifying the key factors from QA data to optimizes the IMRT treatment.
SU‐E‐T‐274: Evaluation of Atlas‐Based Segmentation Algorithms: VelocityAI vs. MIMvista
Authors: Morcos M , Sultanem K , Stroian G , Deblois F
Published: November 2011

Purpose: IMRT is driven by volumetric segmentation, thus greater care and accuracy are necessary when contouring structures. The contouring process requires staff experience and ample time. We have evaluated the performance of two commercial atlas‐based segmentation algorithms. Methods: VelocityAI and MIMvista were compared. Twenty‐one IMRT head and neck cases were randomly and retrospectively chosen. These cases included their respective CT scans and physician‐drawn structures. The twenty‐one cases were divided into two sets: one to create the atlas (eleven) and the other to test the atlas on (ten). In MIMvista the atlas was created using the in‐software tool and setting the most representative patient as the template. In VelocityAI the atlas was created using all ten cases to create an average patient atlas. The averaging used on the ten cases for the VelocityAI atlas was created using the STAPLE algorithm (provided by Velocity Medical Solutions).Results: Twelve OARs were compared to physician‐drawn structures using the Dice similarity coefficient (DSC). VelocityAI and MIMvista performed quite well on the brain, brainstem, spinal cord and eyes with mean DSCs ranging between 0.770– 0.947 for VelocityAI and 0.647– 0.978 for MIMvista. Neither program performed too well on the esophagus, larynx, oral cavity, parotids and sphincter muscle with mean DSCs ranging between 0.348– 0.690 for VelocityAI and 0.389–0.709 for MIMvista Conclusions: This work revealed that neither of the software truly outperformed the other. MIMvista did yield slightly better structures, yet the problem is that much modification is required to render the structures valid. VelocityAI and MIMvista both have great potential and can be used to provide a quick draft of structures, which may reduce physician‐contouring time.
SU‐E‐T‐276: Learning Statistical Correlations of Deformation for Adaptive Radiation Therapy of Prostate Cancer
Authors: Shi Y , Shen D
Published: November 2011

Purpose: The goal of this study is to develop a fast registration algorithm for aligning the planning image onto the treatment image of the same patient for adaptive radiation therapy of the prostate cancer. Methods: Our method is implemented by learning the statistical correlation of the deformations between prostate boundaries and internal regions from a population of training patients, as well as the online‐collected treatment images of the current patients. With the learned statistical correlation, the estimated boundary deformations can be used to rapidly predict the regional correspondences between prostates in the planning and treatment images. Initially, the population‐based correlation is used to predict the regional correspondences when the number of treatment images from the current patient is small. When more treatment images are obtained, the patient‐specific statistical correlation can take a more important role to reflect shape changes of prostate during the treatment. Eventually, only the patient‐specific statistical correlation is used to guide the regional correspondence prediction, once a sufficient number of treatment images are acquired from the current patient. Results: To demonstrate the performance of our method, we first evaluate its registration accuracy by comparing the deformation field predicted by our method with the deformation field estimated by TPS on serial CT images of 24 patients. The predictive error on the voxels around the prostate boundary is 0.38 mm by our method. It takes 24.5 seconds to register two prostate images, with comparable performance as TPS which needs 6.7 minutes for registering the same images. Conclusions: A new fast registration method has been presented to align the planning image onto each treatment image of a patient during radiotherapy. It can achieve similar registration accuracy as TPS, but performs much faster than TPS.
SU‐E‐T‐280: DSA as the Primary Image Modality in Frameless SRS for Treatment of Arteriovenous Malformations
Authors: Lu X‐Q , Mathiowitz G , Lin P
Published: November 2011

Purpose: Stereotactic radiosurgery (SRS) is an effective alternative to microsurgical resection or embolization for arteriorvenous malformations (AVMs). Digital Subtraction Angiography (DSA) is the gold standard for diagnosis and characterization of vascular anatomy, but it requires a frame attached to the skull. Proton and the new image‐guided LINAC SRS systems are frameless. These frameless systems represent a major progress in radiotherapy — however, they prevent the usage of DSA. Without DSA as the primary image, the outcome may be less‐than‐optimal and sometimes even impossible to treat. This study intends to solve the problem. Methods: The solution is based on a conceptual breakthrough whereby, in contrast to the previous practice where a localizer was fixed to the skull during angiography, the patient can now be angiographed under standard clinical conditions without frame and localizer, while a newly‐designed localizer box is radiographed separately. Based on several fiducials implanted into the skull, a target volume on DSA can be transferred to the localizer system in 3D, then further transferred to contours in multi‐CT slices. Combined with other imaging modalities, a treatment can be planned and executed by a routine frameless SRS. Results: Phantom tests demonstrated that the uncertainty in transferring a point target to the localizer is less than 0.3 mm. The maximum transformation errors from the localizer to CT system are ∼0.3 mm. The overall uncertainty of placing a point target to CT is 0.4 mm. For volume targets the volume determined is accurate within < 6%. The algorithm and software are robust and accurate. The method has been successfully applied clinically. Conclusions: A new reliable method for frameless Stereotactic Radiosurgery has been developed, which allows using DSA as the primary imaging in AVM treatment. Implanted fiducials are required for the time being, but using non‐implanted fiducials is feasible.

This work is partially supported by the JCRT Foundation.
SU‐E‐T‐291: How Many Sets of Four‐Dimensional Computed Tomography (4DCT) Images Are Needed to Determine the Internal Target Volume (ITV) for Lung Radiation Therapy?
Authors: Cao J , Cui Y , Werner‐Wasik M , Champ C , Liu H , Xiao Y , Yu Y
Published: November 2011

Purpose: For the treatment of patients with lung cancer, internal target volume (ITV) is frequently determined by contouring of gross tumor volumes (GTV) on 10 phases of a four‐dimensional computed tomography (4DCT) scan. This study investigated the possibility of generating ITV by using selective phases of a 4DCT scan. Methods: The 4DCT scans of 20 patients with lung cancer were included in this study. GTVs were contoured on all 10 phases in Focal4D (CMS, St. Louis, MO). Different ITVs were derived by encompassing volumes of contours from selective phases. ITV10 was derived from contouring GTV on all 10 phases and served as the gold standard. All the other ITVs were smaller and within ITV10. The ratios of the volume of ITVs to ITV10 were calculated and used as a criterion to determine the similarity of ITVs to ITV10. ITV2 represented the ITV derived by using end of inhalation and end of exhalation (0%+50%). ITV3E was derived from contouring the three phases at end‐inhalation, mid‐exhalation, end‐exhalation (0%+20%+50%). ITV3I was derived from contouring the three phases at end‐inhalation, mid‐inhalation, end‐exhalation (0%+70%+50%). ITV4 was created by contouring the fours phases at end‐inhalation, mid‐inhalation, end‐exhalation, mid‐exhalation (0%+20%+50%+70%). ITV6E was derived from contouring the six consecutive phases during exhalation (0%+10%+20%+30%+40%+50%). ITV6I was derived from contouring the six consecutive phases during inhalation (50%+60%+70%+80%+90%+0%). Results: ITV6I showed excellent agreement with ITV10 (Volume ratio ITV6I/ITV10=0.975). ITV4 and ITV6E showed good agreement with ITV10 (ITV6E/ITV10=0.939, ITV4/ITV10=0.944). The volume ratio ITV3I/ITV10 and ITV3E/ITV10 was 0.927 and 0.906 respectively. ITV2 did not agree well with ITV10 (ITV2/ITV10 =0.888). Conclusions: Contouring all phases during inhalation will give a good estimate of the internal target volume, while helping to minimize the clinical load and work time during treatment planning. However, the ITV will be underestimated if only contouring on two extreme phases.
SU‐E‐T‐292: CBCT Imaging and the Assessment of PTV Margin Size for Rectal Cancer Patients Treated Prone on Belly Board
Authors: Cranmer‐Sargison G , Kundapur V , Vachhrajani H , Sidhu NP
Published: November 2011

Purpose: The goal of this work was to use CBCT images taken at the time of treatment to derive PTV margin sizes that would account for inter and intra‐fractional systematic and random errors associated with CTV position for patients treated prone on a new couch top belly board (CDR Systems Inc.). Methods: Twenty‐four patients (12 male and 12 female) were included in this study. CBCT data was acquired once every 5 fractions for a total of 5 images per patient. A 3D‐3D bony anatomy auto‐match was performed offline and the residual difference used as a surrogate for inter‐fractional positional errors of the CTV. Systematic and random variations in CTV position were evaluated in a manner consistent with that of Stroom et al and used in PTVmargin = 1.96Σ + 0.7σ. The influence of hypothetical intra‐fractional motion was included in the margin evaluation by introducing the following values: 1.0, 2.0 and 3.0mm.Results: PTVmargin required to account for inter‐fraction positional errors was found to be (AP, SI, LR) = (5.2 mm, 3.1 mm, 2.8 mm). If we assume any intra‐fractional motion to be similar to that presented by Xu et al, then the required PTVmargin increases to (AP, SI, LR) = (7.0 mm, 5.0 mm, 5.0 mm). A 7.0 mm AP expansion is consistent with that quoted in the “Elective Clinical Target Volumes in Anorectal Cancer: An RTOG Consensus Contouring Atlas“, which recommends a margin between 7.0 and 10.0 mm. However, 7.0 mm is 2.0 mm greater than the 5.0 mm margin specified in the RTOG 0822 trial. Conclusions: A PTVmargin expansion of (AP, SI, LR) = (7.0 mm, 5.0 mm, 5.0 mm) will account for inter and intra‐fractional systematic and random errors associated with CTV position for patients treated prone on a new couch top belly board.
SU‐E‐T‐294: A Method for Dosimetric Verification of Exit Dose Using Deconvolution of Integrated Cone Beam CT Images to Obtain Incident Fluence for Dose Reconstruction
Authors: Sperling N , Parsai E
Published: November 2011

Purpose: To create a method of determining in vivo patient dosimetry from exit EPID images by iterative determination of scatter and primary components of exit patient fluence. Methods: Exit fluence was simulated using a monoenergetic photon beam on an existent machine model in the BEAMnrc monte carlo computation engine. The exit fluence was used as input to a program designed to accept a CT dataset (e.g. from treatment cone beam CT) and exit fluence (e.g. output of an EPID after deconvolving the detector function) and return the fluence at entrance to the phantom. The program uses a superposition convolution algorithm to predict, and iteratively remove, the patient scatter component of the fluence at the detector. The result is then back‐projected through the phantom to generate an input phantom fluence. After suitable variance reduction in subsequent runs, the computed input fluence is accepted as the true fluence and used to compute dose to the phantom. Results: Initial results indicate that the algorithm described is capable of determining the dose to the patient by using exit simulated fluence. Conclusions: It has been demonstrated by others that EPID images may be deconvolved from the detector function to create a map of machine fluence at the plane of the detector. It has also been demonstrated by other researchers that a superposition‐convolution algorithm, when used with computed fluence, may be used to compute accurate dosimetry in a patient phantom (e.g. the CCC algorithm). This work demonstrates that combining these two algorithms and an iterative deconvolution approach can provide a method of determining in vivo dosimetry using only an EPID and a treatment time cone beam CT.