bluuebunny/crossref_metadata_2025 huggingface dataset:
SU‐E‐T‐158: Evaluation of the Sensitivities of Patient Specific IMRT QA Dosimeters
Authors: McKenzie E , Balter P , Jones J , Followill D , Stingo F , Pulliam K , Kry S
Published: July 2013

Purpose: To analyze the sensitivity of patient‐specific IMRT QA devices to detecting plan failures when compared to multiple ion chamber measurements. Methods: Four methods of IMRT QA were chosen: film (Kodak EDR2) and ion chamber (Wellhofer cc04) in an I' mRT body phantom, MapCheck2 (Sun Nuclear) with fields delivered AP, and MapCheck2 in the MapPhan phantom with rotational fields. Additionally, an in‐house designed multiple ion chamber phantom with an insert containing 5 ion chambers (Exradin A1SL 0.057cc) that can rotate to eight positions was used as a gold standard. Twenty plans previously failing film and ion chamber IMRT QA at our institution and five passing plans were delivered to all devices. The plans' performance on the in‐house phantom sorted them into true fails and passes. The sensitivity to detect true failures was assessed across devices with acceptance criteria of 3%/3mm and 90% of pixels passing for planar, and 3% dose difference for point measurements. Additionally, specificity was analyzed. Results: The sensitivities among all devices were less than 0.70 (poor), indicating a deficiency in identifying plans that failed versus multiple ion chamber measurements. The specificities were generally high. The AP‐delivered MapCheck has the lowest sensitivity (0.27), while the rotationally‐delivered Mapcheck MapPhan had the lowest specificity (0.70) due the directional dependence of diodes. Film was the most sensitive (0.60) dosimeter under investigation, while the single ion chamber had the highest specificity (1.00). Conclusion: The different sampling methods of each device lead to a difference in the ability to detect true failures as determined by multiple ion chamber readings. The overall low sensitivities among all devices indicates that failing plans are passing, leading to a potential lack of error detection in patients' IMRT plans. This work was supported by Public Health Service grants CA010953, CA081647, and CA21661 awarded by the National Cancer Institute, United States Department of Health and Human Services.
SU‐E‐T‐159: Development of An Independent, Monte Carlo, Dose Calculation, Quality Assurance Tool for Clinical Trials
Authors: Faught A , Davidson S , Fontenot J , Kry S , Etzel C , Ibbott G , Followill D
Published: July 2013

Purpose: To commission a multiple‐source Monte Carlo model of Elekta linear accelerator beams of nominal energies 6MV and 10MV. Methods: A three source, Monte Carlo model of Elekta 6 and 10MV therapeutic x‐ray beams was developed in a two‐step process. Energy spectra of each of three sources, a primary source corresponding to photons created in the target, an extra‐focal source corresponding to photons originating from scattered events in the linac head, and an electron contamination source, were determined. The two photon sources were determined by an optimization process that fit the relative fluence of 0.25 MeV energy bins to the product of Fatigue‐Life and Fermi functions to match calculated percent depth dose (PDD) data with that measured in water for a 10×10cm2 field. Off‐axis effects were modeled by fitting the off‐axis fluence to a piece‐wise linear function through optimization of relative fluence to match calculated dose profiles with measured dose profiles for a 40×40cm2 field. A 3rd degree polynomial was used to describe the off‐axis half‐value layer as a function of off‐axis angle. The model was then commissioned by comparing calculated PDDs and dose profiles for field sizes ranging from 3×3cm2 to 30×30cm2 to those obtained from measurements. Results: Agreement between calculated and measured data was evaluated using 2%/2mm global gamma criterion for field sizes of 3×3, 5×5, 10×10, 15×15, 20×20, and 30×30cm2. Along the central axis of the beam 99.5% and 99.6% of all data passed the criterion for 6 and 10MV models, respectively. Dose profiles at depths of dmax, 5, 10, 20, and 25cm agreed with measured data for 95.4% and 99.2% of data tested for 6 and 10MV models, respectively. Conclusion: A Monte Carlo multiple‐source model for Elekta 6 and 10MV therapeutic x‐ray beams has been developed as a quality assurance tool for clinical trials. This work was supported by Public Health Service grants CA010953, CA081647, and CA21661 awarded by the National Cancer Institute, United States Department of Health and Human Services.
SU‐E‐T‐161: Assessment of Phantom Positioning Accuracy in IMRT Quality Assurance: Insert Design and Implementation
Authors: Ren J , McNiven A , Letourneau D
Published: July 2013

Purpose: A multi‐centre program for intensity modulated radiation therapy (IMRT) quality assurance (QA) has previously been established. In this work, a phantom insert has been designed, in conjunction with an image analysis algorithm, to determine phantom positioning accuracy as part of program development. Methods: An insert was designed for the ArcCheck phantom that includes four BBs (located outside the sensitive area of the phantom) that are visualized using portal images (PI). A procedure has been developed to acquire PI at 8 gantry angles (2 field sizes per angle) for offline analysis. The image analysis method includes three steps: 1) cross correlation analysis is used to automatically detect the BB position with respect to the radiation isocentre in each PI; 2) 2D BB coordinates are reconstructed to 3D using the projected images; 3) the phantom positioning accuracy is obtained by a 3D rigid body transformation analysis of the BB coordinates. To evaluate the reliability of the image analysis method, the PI procedure was completed with the phantom aligned to the lasers (reference position) and with known shifts in phantom position (translational and rotational). Results: The measured change in phantom position, was compared to the known phantom shifts. Good agreement was found between the measured shift and known shift (within 0.2 mm or 0.2°). For the most complicated shift applied (5° yaw, 3 mm lateral translation and 4 mm vertical translation) the agreement was within 0.1° and 0.1 mm for rotations and translations respectively. Conclusion: A phantom insert has been designed to determine the accuracy of phantom positioning with respect to the radiation isocenter. Initial work illustrates that the analysis method can accurately characterize known positioning errors, which will help to isolate the impact of phantom misalignment in the context of IMRT QA. Supported in part by Cancer Care Ontario
SU‐E‐T‐162: Prospective Treatment Plan Modeling To Guide Patient‐Specific Treatment Setups for Hypofractionated Intracranial and Spinal Radiosurgery
Authors: Ma L , Wang B , Hossain S , Sahgal A
Published: July 2013

Purpose: Residual patient setup shifts such as 1 mm or 1 degree are commonly observed during online treatment setups of hypofractionated radiosurgery (HRS) of spine and intracranial lesions. To effectively guide patient‐specific setups, a prospective treatment planning approach was developed for such treatments. Methods: For a cohort of intracranial and spinal HRS treatments, individualized setup guidelines were prospectively determined via the following approach: (1) systematically shift and rotate 3D patient image data in incremental small steps. (2) For each step such as 0.1 mm or 0.1 degree, dose distributions from original treatment plan were recalculated. (3) Each resulting dose distributions were then iteratively checked against a set of preset dosimetric criteria for compliance. (4) From step 3, maximum tolerable shift levels along the major axes were then determined. Finally, we tested such an approached on a cohort of treatment cases performed at our institutions. Results: For studied treatment sessions (n=63), the mean maximum tolerable shift levels including rotations projected unto the major axes were determined to be 2.0±0.7mm, 2.1±0.9 mm and 1.9±0.8 mm along the lateral, longitudinal, and vertical axis, respectively. Such tolerance levels were significantly (p=0.02) larger than the machine preset tolerance level of 1.0 mm that commonly enforced for all treatment sessions. However, more than 50% of the treatment sessions exhibited sub‐millimeter tolerance level along certain directions to meet critical structure constraints. For example, a 0.6‐mm anterior maximum shift could incur 36% increase in the maximum dose to the brainstem for one patient case. Conclusion: Patient specific online setup guidelines for HRS has been demonstrated to prevent potential deleterious effects from residual shifts while facilitating online setups by providing more room of larger acceptable shifts for a majority of treatment cases.
SU‐E‐T‐163: Reproducibility in the Field of Patient‐Specific IMRT QA
Authors: McKenzie E , Balter P , Jones J , Followill D , Stingo F , Kry S
Published: July 2013

Purpose: To analyze the reproducibility of patient specific IMRT QA results across five different clinical devices and one in‐house detector by quantifying variations in the QA results from delivery‐to‐delivery (without re‐setup, “redelivery”) and variations when set‐up is repeated (“re‐setup”). Methods: Six patient plans were selected from among four treatment sites. Six methods of IMRT QA were chosen: film and cc04 ion chamber in a body phantom, diode array with all fields AP, as well as with fields delivered rotationally (in a MapPhan phantom), and an in‐house designed solid water phantom with an insert containing 5 ion chambers that can rotate to eight positions. Each of the six plans was delivered three times. The system was then re‐setup and redelivered. This last step was repeated, giving three redelivered plans, and three plans that were delivered under independent set ups. Reproducibility was quantified by the coefficients of variation (CV) of the re‐deliveries and the re‐setup deliveries. Results: Of all detectors investigated, film has the highest CV, with an average CV of 1.56% for re‐deliveries, and 2.46% for re‐setup. For the re‐setup and re‐delivery, the lowest average CV's are the AP diode array with 0.25%, and the ion chamber with 0.13%, respectively. Additionally the standard deviation in dose across the ion chambers calculated by the TPS was compared to the standard deviation of the re‐setup measurements for the in‐house phantom, giving a weak correlation. Conclusion: Different IMRT QA devices exhibit different reproducibility; however all CV's were lower than 5%, indicating good consistency in measurement. Film's dependence on the processor and choice of normalization point are possible reasons why this dosimetry system had a higher CV. This information will help quantify the confidence of IMRT QA performed in the clinic and provide valuable information on the reproducibility of various IMRT QA devices. This work was supported by Public Health Service grants CA010953, CA081647, and CA21661 awarded by the National Cancer Institute, United States Department of Health and Human Services.
SU‐E‐T‐165: Effects of Statistical Fluctuation in Monte Carlo Photon Beam Dose Calculation On Gamma Index Evaluation
Authors: Graves Y , Jia X , Jiang S
Published: July 2013

Purpose: To systematically investigate the impact of the Monte Carlo (MC) statistical fluctuation on the gamma‐index test results. Methods: Firstly, we build a simplified model that theoretically demonstrates, to the first order approximation, the MC noise tends to overestimate gamma‐index values when existing in the reference dose and underestimate gamma‐index values when existing in the evaluation dose given the original gamma‐index is relatively large for the noise level. Secondly, we validate the theoretical conclusions using real clinical data: a IMRT prostate and a VMAT HN plan. We first conduct a base gamma‐index comparison between the non‐MC dose and the MC dose with zero noise level. We then compute average gamma‐index value, when the non‐MC dose is compared with the MC doses of different noise levels. Furthermore, the gamma passing rates are also reported for each comparison between the non‐MC dose and MC doses. Results: Our numerical experiments show that 1) when performing a gamma‐index test between an MC reference dose and a non‐MC evaluation dose, the average gamma‐index is overestimated and the passing rate decreases with the increase of noise levels in the reference dose; 2) when performing the test between a non‐MC reference dose and an MC evaluation dose, the average gamma‐index is underestimated when they are within the clinically relevant range and the passing rate increases with the increase of noise levels in the evaluation dose; 3) when performing a gamma‐index test between an MC reference dose and an MC evaluation dose, the passing rate is overestimated when the noise in the evaluation dose dominates, and underestimated when the noise in the reference dose dominates. Conclusion: Gamma‐index tests should be used with caution when comparing dose distributions computed with MC simulations.
SU‐E‐T‐166: Clinical Impact of Minor Errors Discovered in Conventional IMRT QA On Patient Treatment
Authors: Chang S , Lawrence M , Cullip T , Wang S , Fried D
Published: July 2013

Purpose: To demonstrate that conventional IMRT QA metrics and passing criteria should be replaced by DVH‐based criteria that can efficiently interpret the clinical impact of minor errors found for head and neck and prostate cancer patient treatments. Methods: 41 prostate cancer and 31 head & neck (H&N) cancer clinical IMRT QAs using MapCheck were retrospectively analyzed using three different tightness of the pass/fail metrics (3%/3mm, 5%/4mm, and 7%/5mm). An in‐house TPS, PLanUNC, was used to reproduce the accelerator‐delivered plan from the beams measured by MapCheck in IMRT QA. The impact of the deviation/error discovered in the per‐field IMRT QA on PTV D95, mean dose and max. dose of major OARs are computed and correlated with the conventional IMRT QA pass rate and tightness of IMRT metrics, and compared with the impact of a 2mm random setup error on patient dosimetry. Results: Statistically there is no correlation between the quality of DVH‐based dosimetry for PTV and OARs and the conventional IMRT QA metrics and the criteria tightness. The standard deviation of errors in PTV D95 for H&N and prostate treatments due to a 2mm random setup error are 0.7% and 2.2%, respectively. The same standard deviation due to errors discovered in IMRT QA is 1.1% and 1.0% for H&N and prostate, respectively. Our data shows that the clinical impact of a given error found in conventional IMRT QA is treatment site dependent. Based on this study, reducing IMRT errors found in H&N patients may have more clinical impact than for prostate patients. Conclusion: While conventional per‐field IMRT QA metrics are effective in catching gross error for patient safety it fails to interpret their clinical impact. A DVH‐based IMRT QA evaluation should be used instead to timely and effectively identify and reduce the minor IMRT QA errors that have clinical significance.
SU‐E‐T‐167: QA of Dose‐Painting Plans: Risk of Overdosage in the High‐Dose Regions?
Authors: Haakansson K , Aznar M , Fog L , Specht L , Rasmussen J , Bentzen S , Vogelius I
Published: July 2013

Purpose: Deliverability of a treatment plan is generally evaluated by the gamma value (γ) and pass rate (PR) describing the fraction of measured points passing a certain γ‐criteria for dose deviation (DD) and distance‐to‐agreement. γ does not distinguish between under/overdosage. When dose‐painting to high doses in head‐and‐neck cancer, excessive dose to small volumes is associated with dose‐limiting toxicity (Madani et al. Radiother Oncol. 2011). Overdosage of high‐dose volumes is therefore a particular concern. Here we focus on the presence and detection of overdosage in high‐dose regions of dose‐painting plans. Methods: 40 dose‐painting head‐and‐neck plans with five prescription levels (52.8, 60, 69.7, 73.1 and 79.7 Gy, all in 34 fractions) were measured with an EPID‐based system. 14 of these plans with the poorest PR were additionally measured with a cylindrical PMMA diode phantom system. PR(γ: 3%/3mm), γ(3%/3mm) and DD were analysed. Correlations between planned dose, DD and γ were quantified by the Spearman rank correlation coefficient (ρ). Results: Mean PR were 98.2% (EPID, range 93.9–100) and 98.4% (PMMA, range 95.5– 99.9). In the PMMA phantom, 262 diodes had a planned dose >2.2 Gy/fractions. Overdosage (between +0.004% and +6.9%) was measured in 248 of these. A high planned dose correlated with high DD (ρ=0.24) and poor γ (ρ=0.19) (p<0.001). The biological effect accentuates the problem; for example, 5% physical overdose at 80 Gy/34 fractions corresponds to +7.3% in biological effect for normal tissue (α/β=3). Conclusion: The dose‐painting plans were considered deliverable based on gamma PR. However, overdosage is common in the high‐dose regions. This cannot be captured with the gamma PR criteria. Unfortunately, the translation of the dose to a phantom diode to a point in the patient is not easily performed. Additional QA may therefore be needed to ensure safe conduct and correct interpretation of the results of prospective dose‐painting trials.
SU‐E‐T‐169: Characterization and Evaluation of Octavius 4D System for Patient Specific Quality Assurance Using 3D Gamma Index Analysis in VMAT
Authors: Dhanabalan R , Vivekanandan N , Prakash J , Prabakar S , Arulpandiyan R
Published: July 2013

Purpose: To characterize and evaluate Octavius 4D system for patient specific quality assurance using 3D gamma index analysis for patients treated with VMAT technique. Methods: Characterization of the 4D system was carried out by following tests. (i) Verification of calibration factor of each chambers, (ii) reproducibility of all the 729 chambers, (iii) linearity for static and arc mode, (iv) output factor dependency for static and arc mode, (v) dosimetric comparison between measured and TPS calculated (AcurosXB 11.0.21) dose distributions for combination of open fields and/or wedge fields and TG 119 benchmark plans for VMAT. Evaluation of Octavius 4D system is carried out by 3D gamma index analysis (3mm/3% DD) of 102 clinically approved VMAT patient plans of different sites. Results: Average percentage deviation between measured and manufacturer specified calibration factors for all the chambers is −0.28%. Reproducibility measurements of 729 chambers showed maximum standard deviation of 0.25% and 0.2% for 6MV and 15MV respectively. Linearity (test) for static and arc mode, output factor dependency (test) for static and arc mode tests agree very well with semiflux ionization chamber measurements. Average passing rates for combination of open fields and/or wedge fields are, 96.13% and 96.08% for coronal and sagittal plane 2D gamma evaluation respectively and 95.84% for volumetric 3D gamma evaluation. Isocentre dose measurements for TG 119 benchmark IMRT AND VMAT plans are within 2% and average volumetric 3D gamma analysis results are 97.77% and 96.25% for 6 and 15MV respectively. Volumetric global gamma evaluation shows 97.13%, 95.39% and 97.11% passing results for Head and Neck, Thorax and Pelvis sites respectively. Conclusion: Characterization and evaluation results indicate that Octavius 4D system is an efficient tool for 3D patient specific quality assurance.
SU‐E‐T‐170: A Survey of Quality Assurance in Intensity‐Modulated Radiation Therapy in Japan
Authors: Nakamura M , Ono T , Akimoto M , Mukumoto N , Ishihara Y , Utsunomiya S , Shiinoki T , Miyabe Y , Sato S , Monzen H , Hiraoka M
Published: July 2013

Purpose: To investigate the status of the implementation of quality assurance (QA) for intensity‐modulated radiation therapy (IMRT) in Japan using a questionnaire survey. Methods: The questionnaire consisted of six sections: (1) clinical uses of IMRT; (2) IMRT‐related equipment; verification of the (3) absorbed dose; (4) dose distribution; and (5) fluence map; and (6) acceptance criteria for each verification. There were 55 questions in total. Results: The questionnaire was completed for 124 institutions. IMRT was performed for prostate cancer in 120 institutions (96.8%), followed by head and neck cancer in 81 (65.3%), and brain tumors in 67 (54.0%). Although at least four individuals were engaged in IMRT QA in 54.0% of the institutions, the number of full‐time persons involved in IMRT QA was two or less in 110 institutions (88.7%). This indicated that most institutions in Japan have a staff shortage. Dynamic IMRT was the most common beam delivery technique (52.5%) for prostate cancer, followed by segmental IMRT (25.0%) and volumetric modulated arc radiotherapy (15.0%). Similar tendencies were seen for head and neck cancer and brain tumors. More than 90% of the institutions verified both the absorbed dose and dose distribution. In more than 50% of the institutions, these were verified in each and every beam. The acceptance criterion for the absorbed dose verification was set to +/−3% in at least 80% of the institutions. Gafchromic film was used for the majority of dose distribution verification. The acceptance criteria for dose distribution verification mainly involved gamma analysis and a comparison of dose profiles; however, the judgment of acceptance did not depend on the results of the gamma analysis. Conclusion: This survey increases our understanding of how institutions currently perform IMRT QA analysis. This understanding will help to move institutions toward more standardization of IMRT QA in Japan. This research was funded by the Japan Society of Medical Physics.
SU‐E‐T‐171: Pre‐Treatment Radiotherapy Dose Verification Using Monte Carlo Doselet Modulation in a Spherical Phantom
Authors: Townson R , Zavgorodni S
Published: July 2013

Purpose: To design a robust pre‐treatment dose verification tool for non‐coplanar IMRT and VMAT radiotherapy treatments using electronic portal images (EPIs). Methods: The method aims to modulate pre‐calculated Monte Carlo (MC) doselets, leveraging symmetry in a spherical phantom. The patient‐independent phase‐space source MC model of a Varian 21 EX 6MV Clinac was used to transport particles. We utilized cylindrical symmetry of the phase‐space above the jaws and azimuthally rotated its particles to lie along a radial strip. The strip was divided along the radius into 2mm (as projected to isocentre) segments, and the particles within each segment were used for dose calculation in a spherical water phantom via DOSXYZnrc, producing a doselet. For patient‐dependent dose calculation in the spherical water phantom the doselets were redistributed to fill a treatment field, and then modulated based on a fluence maps derived from EPIs. Finally, gantry and couch rotations are applied. The spherical symmetry of the geometry makes this process simple and efficient. Results: A 10x10 cm2 open field was tested against BEAMnrc and DOSXYZnrc (∼1% uncertainty) in a spherical phantom with voxel size 1.25×1.25×1.25 mm3. This passed a 3D gamma‐index test with 98% agreement for 3%/3mm criteria above the 10% isodose, and calculated in 46s. A 7 field IMRT plan was calculated in 3.5 minutes on 2.5×2.5×2.5 mm3 resolution and was compared with results using the phase‐space modulation method our group has published previously. The same gamma‐index test achieved 97% agreement in this comparison. Timing results are based on a single 2.1 GHz processor. Conclusion: Results demonstrate that our pre‐treatment dose verification method can be used for efficient dose verification of non‐coplanar treatments. Significant speed enhancements were achieved by utilizing phantom spherical symmetry in dose reconstruction.
SU‐E‐T‐172: Portal Dosimetry of Gated VMAT; with and Without Gating
Authors: Mynampati D , Yaparpalvi R , Hong L , Kuo H , Tome W
Published: July 2013

Purpose: To study duty cycle and respiratory period influence on VMAT delivery using portal dosimetry. Methods: For this study, we selected 12 ARC fields from four different gated VMAT plans with Varian, TrueBeam 6 MV photon beam and HD120 MLC. For Planning and portal dose prediction we used Varian, Eclipse (Version 10.0) AAA and PDIP algorithms respectively. We acquired integrated portal images with and without gated arc fields delivered to EPID at SID 140cm. We simulated respiratory periods with Quasar, Motion Phantom. For gated delivery, we used planned duty cycle for patient respiratory trace, and 6 sec per breath (SPB) and duty cycles 40%, 30%, 20% and 10% with 3SPB. Eclipse, Portal dosimetry software was used for Gamma analyses of measured and predicted portal dose images. In addition, we compared with and without gated portal dose images of same arc. MLC log file analyses of delivered arcs were done with Mobius, Dose lab software. Gamma passing criteria is to have gamma<1 for greater than 90% data points. Results: For all arcs, predicted vs. measured dose distributions passed with criteria ΔD = 3% & DTA = 3mm, excluding few 10% duty cycle measurements. For these, pass criteria is ΔD = 5% & DTA = 4mm. Similarly, with and without gated portal dose image comparison passed the criteria of ΔD = 1% & DTA = 1mm excluding few 10% duty cycle measurements. For these pass criteria is ΔD = 2% & DTA = 2mm. For all delivered arcs, Leaf error 95 percentile is < 0. 1mm and error RMS is below 0.05mmConclusion: Except 10 % duty cycle, the gating influence on VMAT delivery is minimal. Therefore, gated VMAT QA can be done without gating. Duty cycles larger than 20% are recommended to minimize delivery errors.
SU‐E‐T‐176: Commissiong and Initial Clinical Experience with Dosimetry Check, a Commercial Software for in Vivo Volumetric Dosimetry
Authors: Gimeno‐Olmos J , Pujades‐Claumarchirant MC , Garcia T , Meseguer V Carmona , Lliso‐Valverde F , Palomo R , Ballester F , Perez‐Calatayud J
Published: July 2013

Purpose: The aim of this work is to study the differences of reference point doses between the commercial EPID based dosimetry software Dosimetry Check (DC) and TPS to establish an accuracy level of the system, to evaluate its use in clinical routine and to obtain results for the first patients. Methods: We used DC v.3.8 (Math Resolutions), two Varian Clinac iX accelerators equipped with EPID aS1000 and Eclipse v.10.0 with AAA algorithm. Several plans with and without air gap were generated over the phantoms: MP1 Water Tank and Solid Octavius 4D (PTW). 5 head and neck, 3 lung and 4 prostate cases were selected for clinical evaluation. We compared the calculated dose at the isocenter and in a set of 9 points distributed in the PTV and made a gamma analysis of the 3D volume in pre‐treatment mode and during 3 CBCT‐guided sessions in transit mode. Results: In the study with phantoms, total dose differences in the isocenter between DC and TPS are less than 2%, but single field contributions achieve greater values. In transit mode, DC does not consider properly the couch attenuation, especially when there is an air gap between phantom and couch. For patients, better results are observed for prostate than for lung (heterogeneities) or head and neck (heterogeneities, field size). Information is lost in edges of large fields due to the limitation in EPID size and in the event of an interruption during treatment. Conclusion: Tests carried out with phantoms suggest that the accuracy of DC achieves 2% for total dose. For off‐axis dose distribution, and also for patients, this uncertainty results significantly higher. In any case, the possibility of this in vivo evaluation and the potentiality of this new system have a very positive impact on improving patient QA.
SU‐E‐T‐182: The Effect of Multi Leaf Collimator Characteristics in Treatment Planning Systems Calculation Errors as Detected in Phantom and in Patient QA
Authors: Bakhtiari M , Schmitt J , Sedaghat A , Aroumougame V , Sarfaraz M , Rodgers J
Published: July 2013

Purpose: We performed patient specific VMAT quality assurance (QA) with a 3D array dosimetry device and a software system that estimates 3D dose distribution in patient anatomy from 3D measurements in phantom. The correlation between the dose errors from phantom measurements and the corresponding volumetric (3D) estimated dose in patients were studied. We used the gamma pass‐rates to determine if either method depends on MLC characteristics. Methods: The plan and dose were calculated by the Eclipse TPS (VARIAN) and the VMAT treatments were delivered to an ArcCHECK diode phantom (Sun Nuclear). The gamma pass‐rates were calculated using the data measured using the ArcCHECK and the verification plan calculated in Eclipse. The 3DVH software (Sun Nuclear) was used to estimate the 3D dose in patient using Plan Dose Perturbation algorithm, based on the ArcCHECK measurements. In‐house software was developed to calculate the percentage of the leaf pairs with a gap less than 1 cm for each treatment along with the percentage of the beams areas that was blocked by MLCs for the entire treatment. For each patient, any dependency on pass‐rates and the MLC characteristics was investigated. Results: There was no correlation between the phantom and the 3D patient QA gamma pass‐rates. The Phantom gamma pass‐rates did not correlate with the percentage of the leaf pairs with 1 cm gaps or smaller. With 3D QA in patient anatomy, it was found that when the percentage of the leaf pairs with smaller gaps increases, the gamma pass‐rate drops. There was no correlation between phantom gamma‐pass rate and the field area blocked by MLCs. Increasing the MLC‐blocked area corresponds to decrease in 3D gamma pass‐rate in patient QA. Conclusion: It was found that 3D QA in patient anatomy more efficiently reflects the errors caused by MLC characterization in the treatment planning software.
SU‐E‐T‐183: Lateral Scattering Penumbra for Radiation Therapy with High Energy Electrons Using Small Cutouts
Authors: Lief E , Marsden C , Ruff E , Gianadda R , Lee H , Stevens R
Published: July 2013

Purpose: To determine lateral scatter penumbra margins for treatments of small lesions with electrons. These margins could be different for small and normal‐sized cutouts due to the lack of lateral scatter equilibrium when the cutout dimension is disproportionally small for a given electron energy. Methods: Round cutouts with diameters of 2 and 3 cm were compared with a standard electron 4*4 cm2 square insert in a 6*6 cm2 electron cone on a Varian Trilogy linac. Measurements were done in a scanning M3 water tank and analyzed using Mephysto v.1.5 program from PTW. 6, 9, 12, 15, 18, and 22 MeV electron beams were measured with a small‐size semiconductor diode TN60012 from PTW. Depth‐dose scanning was performed with the diode in vertical position, while the transverse dose profiles were taken with a horizontal diode. Results: There was a noticeable reduction in depths of maximum dose and 90% dose for smaller cutouts, especially noticeable for higher electron energies. Depth of 90% dose on the central axis can be twice smaller for 2 cm cutout than for a standard 10*10 cm2 open electron field. The lateral scattering 50%–90% penumbra was higher for smaller electron energies, reaching 8 mm for 6 MeV beams. Although results depend on cutout dimensions, measurement geometry and methods, they give a practical measure of possible lateral scatter penumbra for cutout design in electron treatments of small lesions. Conclusion: Choice of scatter‐based margins is essential for radiation therapy with electrons. Special measurements or calculations should be done for small size cutouts. For off‐axis cutout locations care should be taken when light guidance is used for patient positioning because the effective electron source position does not coincide with the light source position which leads to light‐radiation field misalignment, especially noticeable at extended distance.
SU‐E‐T‐184: Practical Method of Scanner Stability Compensation for Film Dosimetry
Authors: Ishikawa M , Kojima H , Tachibana H , Tanabe S , Suzuki R , Minemura T , Tohyama N , Narita Y , Nishio T , Ishikura S
Published: July 2013

Purpose: In high‐accuracy radiation therapy such as intensity‐modulated radiotherapy (IMRT), dosimetric quality assurance is required to ensure the accuracy of the dose delivery. Although film dosimetry, associated with absolute dose measurement, is often used for this purpose, the uncertainty of film dosimetry is strongly influenced by the stability of the scanning device. This paper proposes a practical correction method for compensating scanner instability when making dose response curves. Methods: Gafchromic EBT3 films were irradiated with 6 MV X‐rays to 0, 10, 50, 100, 200 and 300 cGy. All films were scanned 1,000 times for 45 hours with about 2.7 minutes interval by a flat‐bed type image scanner (ES‐10000G, Seiko Epson Corp.). Pixel values for 1,000 images were automatically measured using a macro function of Image J (National Institutes of Health) to analyze the same area for each ROI, stored as the average of 50 x 50 pixel values. The stability of film measurement was assessed by analyzing optical density (OD) and corrected optical density, calculated by summing the OD of unirradiated film and net optical density (ODnet). Results: OD changes were significantly corrected, including post‐exposure density growth, compared to the original OD changes. This means that highly accurate film dosimetry, independent of the condition of the scanner device, may be possible by correcting OD with ODnet. Conclusion: In making dose response curves, dose uncertainty due to scanner device instability can be minimized by correcting OD values using the ODnet value. By using the presented method, a reduction of uncertainty in film dosimetry can be achieved. Furthermore, the procedure is simple and practical for clinical implementation. This research was supported by a grant for Clinical Cancer Research (10103757) from Ministry of Health, Labour and Welfare, Japan.
SU‐E‐T‐186: Treatment Planning Dose Accuracy of Whole Breast Irradiation Using Field‐In‐Field Technique
Authors: Choi D , Nookala P , Patyal B , Yoon J
Published: July 2013

Purpose: To evaluate the planned dose accuracy of whole breast irradiation using field‐in‐field technique by comparing with 2D array detector measurements and Monte Carlo simulations. Methods: The ion chamber based Octavius 2D‐array detector (PTW, Freiburg, Germany) was used to measure the absolute dose and dose distributions of tangential field‐in‐field breast irradiation. A commercially available treatment planning system (Eclipse V10.0, Varian, Palo Alto CA USA) was used to calculate the absolute dose and dose distributions. Phantom geometry was simplified for the measurements and for Monte Carlo simulations, but its size and shape were similar to typical breast. The dose accuracy of the treatment planning system was evaluated and the desirable points for monitor unit calculation were recommended based on the results of the study. Results: The discrepancy between measured and planned dose distribution depended on the location of the monitor unit calculation point. The differences were larger when monitor unit calculation point was closer to the irradiated edges of phantom. The doses calculated by the treatment planning system near the center of the irradiated field were about 2% higher than measured doses. There was good agreement between the measured doses and those predicted by Monte Carlo methods. Conclusion: Because of inadequate treatment of scatter in the irradiation of breast with tangential ports, the dose predictions by a treatment planning system do not always agree with the actual measurements. The treatment planning algorithm evaluated in this study over‐compensates for the loss of scatter in tangential ports glancing the sloping surface of a typical breast. The choice of the prescription point can have significant effect on the dose prediction and the actual dose delivered. Keeping the prescription point away from the field edges will help improve the agreement between the calculated and measured dose.
SU‐E‐T‐188: An Effective Quality Assurance Method for TomoTherapy Craniospinal Irradiation Patients with A 3D Semiconductor Phantom
Authors: Wang S , Burkhardt K , Chang S , Lian J
Published: July 2013

Purpose: This study aims to develop an effective and efficient approach to perform delivery quality assurance (DQA) to any large length spine in Tomo craniospinal irradiation (CSI) with a 3D semiconductor phantom. Methods: It is challenging to use conventional method to perform DQA to an entire large length of spine because the Tomo machine limits the maximum longitudinal distance between red and green laser (=18 cm). In order to overcome this limit, three DQA plans for each CSI patient were generated for covering the entire treatment site, in which we intentionally overlapped the red and green laser in DQA calculation, then we measured the distance (d) between the centers of the phantom and lasers. When using SunNucelar QA software to compute the plannar dose, we set the center of the cylindrical surface by applying d to the IEC Z coordinate. In phantom setup, we first align the center of phantom with green laser then manually shift the phantom superiorly or inferiorly d. With this new approach, we measured five CSI patients using a commercial 3D semiconductor phantom, the ArcCHECK (SunNuclear). The measured and computed dose is compared with gamma analysis and (3%, 3 mm) agreement criteria. Results: For all CSI patients and sections measured, the acquired dose distribution covered the complete treatment length and matched the computed data very well. All the DQA passed the gamma criteria with an average passing rate of 99.1%. The total delivery time of DQA using the new method was 66.7% of the time using the conventional method. Conclusion: The measured DQA results from the five CSI patients demonstrate that this new method overcomes the distance limitation of 18 cm between the red and green laser in the conventional DQA method. The DQA delivery becomes more efficient with 33.3% reduction of beam‐on time.
SU‐E‐T‐189: A Protocol for 3D IMRT Quality Assurance of Prostate Radiotherapy
Authors: Gueorguiev G , Sharp G , Mahd M , Turcotte J , Crawford B
Published: July 2013

Purpose: Define a set of statistical parameters and plan structures that will characterize the pass/fail criteria for a 3D prostate pre‐treatment IMRT quality assurance protocol. Methods: For this study, 3D IMRT QA measurements were performed on thirteen IMRT prostate patients. All plans were for an initial treatment course to the prostate and the seminal vesicles with prescription dose of 45Gy, and planned with step‐and‐shoot IMRT. All patients plans passed traditional QA methods, including point measurement with ion chamber and 2D gamma analysis at 3mm/3%. A total of 75 3D IMRT QA measurements were performed on all patients, on different days and on different beam matched linear accelerators. IBA COMPASS system was used for 3D QA measurements. For each patient, nine structures of interest were selected: femoral heads, two planning target volumes (PTV1, PTV2), prostate, seminal vesicles, anterior and posterior rectal wall and bladder. For each structure the following statistical parameters were evaluated: average dose, 3D gamma test, volume at 4% and 6% difference, and volume that receives between 5%‐105% of the prescription dose. Two QA protocols are proposed, a conservative with pass/fail QA thresholds the maximum of these 75 passing measurements; standard protocol with pass/fail QA threshold value at the maximum of the 75 measurements +50% increase. Results: Out of the 153 statistical parameters investigated, 65 were excluded from the QA protocol. The reason is that their values were constant for all 75 measurements. Assessment of the validity of the conservative protocol and standard protocol are ongoing. Conclusion: 3D IMRT QA is a powerful and versatile tool for pre‐treatment prostate IMRT QA, with ability to perform variety of statistical tests and overlay dose and anatomy. However, because 3D QA provides more information than single ion chamber measurement and/or 2D gamma analysis, a protocol is needed to establish pass/fail thresholds.
SU‐E‐T‐191: Transit Dose Based Quality Assurance of Intensity Modulated Radiotherapy with Two Dimensional Array Detector
Authors: Baek T , Yoon M
Published: July 2013

Purpose: The purpose of this work is to evaluate the possibility of the transit dose based verification of the accuracy in dose delivery to the patient with two dimensional array dosimeter. Methods: Five intensity modulated radiotherapy (IMRT) plans were selected from lung cancer patients and irradiated on a homogeneous solid water phantom and an inhomogeneous anthropomorphic phantom. To simulate error in patient positioning, the anthropomorphic phantom was displaced from 5 mm to 10 mm in the superior to inferior (SI), inferior to superior (IS), right to left (RL), and left to right (LR) directions. The transit dose distribution was measured with MAPCHECK2 and was compared to the computed dose map based on gamma index analysis. Results: While the average passing rate based on gamma index with a 3% dose and a 3 mm distance‐to‐dose agreement tolerance limit was 98.6 % for the transit dose with homogeneous phantom, it was reduced to 95.5 % for the transit dose with inhomogeneous anthropomorphic phantom. The transit dose analysis shows that the average passing rate decreases up to 11.5% and 16.6% in gamma index for 5 mm and 10 mm setup error, respectively. Conclusion: Our feasibility study suggests that the transit dose based quality assurance can provide the information about the accuracy of the inhomogeneity correction algorithm and patient positioning during treatment, which can be used to verify the accuracy in actual dose delivery to the patient in treatment room.
SU‐E‐T‐192: Investigating the Importance of Phantom Geometry in Intensity‐Modulated Radiation Therapy Quality Assurance
Authors: Hamilton B , Langner U
Published: July 2013

Purpose: To determine the effect of beam weighting through the phantom geometry on the error detection for phantoms used in intensity modulated radiation therapy quality assurance (IMRT QA) programs. Methods: PTW Octavius II and MapCHECK2 with MapPHAN were evaluated due to their differing phantom geometries. PTW is an octagonal phantom with its central diode at ∼15cm equivalent depth from all directions in its axial plane, whereas MapPHAN is a rectangular phantom. Beams with a field size of 10x10 cm, were delivered onto each phantom at gantry angles of 0, 20, 45, and 70 degrees for varying monitor units. The central axis reading was recorded and normalized to the reading for gantry angle at 0 and 100 monitor units. Errors of different magnitude were introduced in a beam from each gantry angle and compared to the beam with no errors. Results: This work showed that while the response at large angles for MapCHECK2 is still linear (constant response vs. monitor units delivered), the reading per monitor unit falls off by 26% from that of the enface beam. This is caused by increased attenuation because of more material in the beams path.This results in an underestimation of the error in this beam. A test on a plan involving beams from all four angles with a passing criteria of 5%/3mm, passed for an error of 25 percent introduced in the 70 degree beam for MapCheck2, while it failed for PTW. Conclusion: Phantom geometry intrinsically introduces beam weighting for IMRT QA, which might cause some errors to be undetected. A phantom with geometry similar to that of the patient should therefore be chosen when IMRT QA plans are delivered in order to minimize this effect. This will ensure that beams are weighted similarly than for the intended plan.
SU‐E‐T‐194: Radiochromic Film Based Transit Dosimetry for Verification of Dose Delivery with Intensity Modulated Radiotherapy
Authors: Chung K , Yoon M , Son J , Park S , Lee K , Shin D , Lim Y , Lee S
Published: July 2013

Purpose: To evaluate the transit dose based patient specific quality assurance (QA) of intensity modulated radiation therapy (IMRT) for verification of the accuracy of dose delivered to the patient. Methods: Five IMRT plans were selected and utilized to irradiate a homogeneous plastic water phantom and an inhomogeneous anthropomorphic phantom. The transit dose distribution was measured with radiochromic film and was compared with the computed dose map on the same plane using a gamma index with a 3 % dose and a 3 mm distance‐to‐dose agreement tolerance limit. Results: While the average gamma index for comparisons of dose distributions was less than one for 98.9 % of all pixels from the transit dose with the homogeneous phantom, the passing rate was reduced to 95.0 % for the transit dose with the inhomogeneous phantom. Transit doses due to a 5 mm setup error may cause up to a 50 % failure rate of the gamma index. Conclusion: Transit dose based IMRT QA may be superior to the traditional QA method since the former can show whether the inhomogeneity correction algorithm from TPS is accurate. In addition, transit dose based IMRT QA can be used to verify the accuracy of the dose delivered to the patient during treatment by revealing significant increases in the failure rate of the gamma index resulting from errors in patient positioning during treatment.
SU‐E‐T‐195; An Algorithm for the Reconstruction of An Entrance Beam Fluence Using a Simulated Exit Phantom Fluence
Authors: Sperling N , Parsai E
Published: July 2013

Purpose: To design and test an algorithm for reconstructing entrance fluence from an exit fluence through a cylindrical phantom using fluence kernels generated in the BEAMnrc monte carlo code. This work may be extended to replace the simulated fluence with direct measurements on an EPID where dosimetry data from treatment can be acquired and compared with direct measures of fluence from devices used commonly for IMRT verification. Method and Materials: A parameter space array of monte carlo fluence kernels is calculated for each voxel in a 256x256 pixel grid measuring across 16.384cm2 at 60cm SPD, which corresponds to a 40.96cm2 imager at 150cm SPD. An array of coefficients of parameter space elements is then used to calculate a fluence at 150cm SPD, and an iterative minimization solver is configured to minimize the sum of the square of the differences between the calculated fluence and the measured fluence. Results: Comparison between two IMRT beam sequences simulated using the same BEAMnrc model used to generate the parameter space array were performed. The iterative solver was allowed to run for 15 minutes on a 128 core cluster on each phase space generated. A comparison of directly simulated entrance fluence and entrance fluence calculated by the proposed algorithm was performed, with a comparison criterion of the percentage of points in the fluence array with a threshold of 10% of maximum intensity, within 1% of the simulated fluence intensity; passing rates of 94.5% and 97.9% were achieved. Conclusion: These results demonstrate that the algorithm is capable of reconstructing entrance fluence from simulated exit fluence within a reasonable tolerance for verification of delivery. Additionally, patient and beam specific parameter space arrays may be generated, allowing live measurements during treatment to be used for the reconstruction of entrance fluence with a high degree of accuracy.