bluuebunny/crossref_metadata_2025 huggingface dataset:
SU‐E‐T‐303: Neutron Measurements for the Monarch‐250 Proton Accelerator
Authors: Chen K , Klein E , Bloch C , Rosenthal S
Published: November 2011

Purpose: The Monarch‐250 proton accelerator, a unit utilizing a superconducting magnet synchrocyclotron, is currently undergoing testing (Still River Systems, MA). The purpose of this study was to evaluate neutron exposure at various locations around the accelerator. Methods: Neutron dose equivalents per therapeutic dose (H/D) were investigated for a 14cm‐diameter small field (SF) and a 25cm‐diameter large field (LF). The effect of stopping the beam in solid brass apertures was also studied. Two types of neutron detectors were used: a SWENDI‐2 detector (Thermo Scientific) and BD‐PND™ dosimeters (Bubble Technology). Measurements were performed at three locations: 1) ∼1m from central axis along the patient plane (P1), 2) ∼1m below the beamline between the patient and accelerator (P2) and 3) ∼1m behind the accelerator (P3). An ionization chamber (IBA) was used to determine the delivered therapeutic dose in a water phantom. Results: Using the SF beam delivered to a water tank, the measured H/D using the SWENDI‐2 were 0.39, 0.36, 0.14mSv/Gy at P1–P3, respectively. Stopping the beam in brass increased neutron dose to 0.51, 0.54mSv/Gy at P1, P2 respectively. For the LF the H/D values were 1.93, 2.18, 0.56mSv/Gy for P1–P3, respectively. When the LF beam was stopped in brass, the H/D was 2.16mSv/Gy at P1. The measured H/D using BD‐PND™ at P2 was 0.31mSv/Gy with the SF beam stopping in a water tank, and 0.66 mSv/Gy at P2 with brass in place. Conclusions: The H/D values obtained using the BD‐PND™ were not significantly different from the SWENDI‐2 values. The greater H/D with brass in place was consistent with excess neutrons created in a high‐Z material. Preliminary results indicate that the measured neutron H/D from the Monarch‐250 are below the 1mSv/Gy guideline for radiation leakage (IEC 60601‐2‐1 average leakage for 10×10 field size). Supporting calculations are currently being performed using MCNPX2.6.
SU‐E‐T‐307: A Dummy Eye Shield for Electron Treatment Planning
Authors: Kang SK , Park S , Hwang T , Cheong K , Lee M , Kim K , Oh D , Bae H
Published: November 2011

Purpose: An electron treatment for the eyelid is routinely executed with the radiation shield accessory. However, the incorporation of the shield into the dose calculation is not satisfactory, mainly because of the metal artifacts from the shield. Here, we present the use of an acrylic dummy eye shield for tungsten shield and evaluate the procedure and accuracy. Methods: Both the 2 mm thickness tungsten eye shield and the acrylic eye shield machined with the same size were used for the CT scan and beam delivery. Considering the tungsten, steel knob and thin aluminum cover, Monte Carlo simulation and the Pinnacle (version, 8.0m; Phillips Medical Systems, Madison, WI, USA) plan were made for the electron 6 MeV deliveries. The results were confirmed with the metal oxide semiconductor field effect transistor (MOSFET) detectors and Gafchromic EBT2 film measurements. Results: From the MC and EBT2 film measurements, both the maximum dose below the tungsten shield and the eyelid dose above the shield agreed respectively to each other within 1.7%. The Pinnacle plan based on the CT number modified images of the acrylic shield showed the maximum dose agreed with the MC within 0.3%; however, the eyelid dose were under calculated by 19.3%. Conclusions: Using the dummy eye shield for the electron treatment plan on the Pinnacle, contouring of the tungsten shield body and steel knob only, and the following density adjustment was sufficient for the evaluation of the dose distribution below the eye shield. The eyelid dose, however, was underestimated by 19.3%.
SU‐E‐T‐311: Comparison of Two Imaging Setups for Small Animal Cone Beam CT
Authors: Yang Y , Armour M , Siewerdsen J , Iordachita I , Wong J
Published: November 2011

Purpose: To support non‐coplanar irradiation, cone beam CT (CBCT) on our small animal radiation research platform (SARRP) is acquired in a “pancake” setup where the prone animal is rotated between the stationary x‐ray source and flat‐panel detector. This study compares the performance of the pancake setup with conventional CBCT of a stationary animal. Methods: CBCT images were acquired on the SARRP using the pancake and a “tubular” setup with upright phantoms to mimic conventional CBCT. PMMA cylindrical phantoms of mouse dimension (3‐cm wide and 7.5‐cm long) embedded with 5‐mm wide rods of teflon, delrin, acrylic and polyethylene inhomogeneities were employed. The study included a single phantom, three side‐by‐side phantoms, and a euthanized mouse. Images were acquired at 65 kVp, 0.1‐mm brass filtration for: (1) imaging dose at 1x, 2x, and 4x of 1.6 cGy; (2) inclined “pancake” object at 14 degree and 28 degree. Comparison of zero‐frequency signal‐to‐noise ratio (SNR) and contrast‐to‐nose ratio (CNR) were made. Results: At 1.6 cGy, both SNR (19.4–23.0 vs 23.2–48.5) and CNR (−0.11%–0.57% vs −0.15%–0.88%) of the pancake setup were lower than those of the tubular setup for the 4 inhomogeneities. Similar results were obtained for the 3‐phantom study. Doubling the dose for the pancake setup improved these metrics and approached those of tubular setup at 1x dose. The difference in SNR and CNR between the pancake and tubular setups were reduced with increasing inclination. Pancake CBCT of an inclined mouse at 28 degree was virtually identical to that of the tubular setup. Conclusions: CBCT image quality of the pancake setup is compromised in favor of the ease of non‐coplanar irradiation. Image quality can be recovered with a modest increase in imaging dose to 3 cGy. Notwithstanding, pancake CBCT is adequate to support image guidance, while providing other irradiation advantages.

John Wong has a sponsored research agreement with Xstrahl Ltd. Xstrahl Ltd. has a technology transfer agreement with Johns Hopkins University on the SARRP.
SU‐E‐T‐335: Contrast‐Enhanced Stereotactic Synchrotron Radiation Therapy Clinical Trials: A Dry Run Report
Authors: Tessier A , Vautrin M , Prezado Y , Deman P , Renier M , Brochard T , Nemoz C , Philibert R , Bravin A , Esteve F , Balosso J , Elleaume H , Berkvens P , Giraud J , Adam J
Published: November 2011

Purpose: Contrast‐enhanced stereotactic synchrotron radiation therapy (SSRT) is an innovative technique based on localized dose‐enhancement effects obtained by reinforced photoelectric absorption in the target. Medium energy monochromatic x‐rays (50 –100 keV) are used for irradiating tumors previously loaded with high‐Z elements. SSRT clinical trials are being prepared at the European Synchrotron Radiation Facility (ESRF). The first patients (scheduled in summer–autumn 2011) should be treated at 80 keV, with 10 conformational beams. A dry run has been performed using an anthropomorphic radiosurgery human head phantom (Computerized Imaging Reference Systems, Norfolk, VA, USA). Methods: The phantom was scanned on a dedicated CT‐scanner, with and without a 3 cm diameter latex balloon filled with 3 mg/ml of iodine located in the supratentorial brain area. The PTV as well as the OAR were then contoured. The fisrt patient should receive 5 Gy at the ESRF in one fraction followed by a 6 Gy fraction and two 11 Gy fractions at the university hospital under stereotactic conditions (6 MV). The treatment will be followed by a whole brain irradiation (30 Gy, 10 fractions of 3 Gy, 6 MV). The treatment plan for the conventional stereotactic and whole brain irradiations were performed respectively on the Brainlab‐IPlan system, and on the Varian‐Eclipse TPS. The contrast enhanced SSRT treatment plan is performed on the ESRF dedicated version of Isogray (Dosisoft, Cachan, France) that has been developed for our irradiation technique.Results: The treatment plan was then successfully evaluated (Isodoses, DVHs and ICRU points). The full treatment was then realized on the phantom. Dose verifications were performed using gafchromic films and nPaG polymer gel dosimetry as well as in vivo dosimetry. Conclusions: This contrast enhanced SSRT clinical trial “dry run” was the last step before the phase I/II trial and shows the feasibility and readiness of the whole treatment chain.
SU‐E‐T‐341: Dosimetric Verification of a Wedge Shaped Midline Block for Teletherapy in Cervical Cancers
Authors: Shenoy K , Srinivas C , Kulal B
Published: November 2011

Purpose: Designing a wedge shaped midline block (WMLB) for teletherapy treatments in early cervical cancers, its dosimetric verification and clinical implementation. Methods: The method of designing a WMLB is discussed based on the dose distribution in coronal plane obtained from treatment planning system (TPS) for manual afterloading intracavitary brachytherapy (ICBT). Horizontal profiles are taken with Kodak EDR2 film and 0.6cc ionization chamber (IC) under water equivalent slab & water phantom respectively at isocenter of telecobalt machine with WMLB at tray level. Using these measurements, WMLB was configured in the unit modeling of TPS. Clinical plans (AP/PA; Three fields, Box field techniques) related to the treatment were created with central shielding by conventional rectangular midline block (RMLB) and WMLB in TPS for a field size of 15cm × 15cm in AP/PA & 8cm × 15cm for Lateral field portals under simulated phantom geometry. Dose calculations (3D) were done at isocenter and were verified with IC (absolute & relative) dosimetric measurements. Results: Wedge shaped MLB was found to be symmetrical within 1.5% accuracy which was confirmed through horizontal profiles generated by Film, IC & TPS. The calculated doses from TPS were verified with ionometric measurements (Central and Off Axis) taken at isocenter and were found to be well within +/−3.3%. Conclusions: The use of a RMLB results in in‐homogeneity of dose around the target and should be avoided. A WMLB on the other hand would certainly improve the dose homogeneity around the target thereby minimizing the dose to the critical structures (bladder and rectum).
SU‐E‐T‐352: Comparison of Dose Distributions Between Two Arrangements of a Range Compensator and of An Aperture Collimator in a Passive Scattering Method for Proton Therapy
Authors: Hotta K , Kohno R , Takada Y , Hara Y , Tansho R
Published: November 2011

Purpose: Two arrangements of a range compensator (RC) and an aperture collimator (AC), RC placed in front of AC (ARGT‐1) or AC placed in front of RC (ARGT‐2), are currently used in proton therapy. We studied difference of the dose distributions for two arrangements.Methods: We calculated dose distributions in water formed by a 190‐MeV modulated proton beam (SOBP width 80 mm) passing through two arrangements of a simple geometrical RC and an AC using a Simplified Monte Carlo method. The air gap between the downstream device and water surface was set at 195 mm. Results: Calculated dose distributions were normalized in the same uniform‐dose region. We compared sharpness of the 80%–20% dose falloffs at a depth of 150 mm where protons passing through the thinnest part of RC mainly contribute the dose. The value for the ARGT‐1 is written first and that for the ARGT‐2 next. Dose falloffs were 8.5±0.2 mm / 7.9±0.2 mm near an inner RC edge. Dose falloffs were 7. 1±0.2 mm / 8.0±0.2 near the AC edge region. In addition, dose contributions of edge‐scattered protons scattered in the AC at water surface were 19% / 10% Conclusions: We verified that ARGT‐1 is superior to the other in dose conformation near the lateral target boundary and that ARGT‐2 is superior to the other in that near the distal target boundary. We also noticed that the RC serves to absorb a part of scattered protons in the AC for ARGT‐2.
SU‐E‐T‐359: Forward / Backward Calibration and Clinical Application of a MLIC System for Uniform Scanning Proton Beam
Authors: Lee T , Mertens T
Published: November 2011

Purpose: To describe the successive calibration steps and applications in clinical environment of a multi‐layered ion chamber (MLIC) system for uniform scanning proton beam. Methods: The MLIC system contains 180 independent planar, parallel plate air vented chambers. The active collective area is defined by a circular surface of 2.5cm diameter. A dedicated electrometer was used to collect readable counts produced in the different chambers. Uniformly scanned proton beams with various ranges have been delivered to the device with field sizes of 10*10 [cm*cm] projected at the iso‐center. The material composition of the device has been chosen in order to present equivalent scattering properties to water. The Water Equivalent Thickness (WET) of the device was fine‐tuned and the channel to channel variation corrected following a forward and backward calibration strategy. Various Pristine and Spread Out Bragg peaks of several ranges and modulations have been measured and compared to reference water scans to validate the calibration method. Results: The characteristic analyses of Pristine and spread‐out Bragg peak measurements show that MLIC measurements are in good agreement with requested ranges and modulations within sub‐millimeter accuracy. Additionally the system allows to choose the appropriate time resolution in order to timestamp each layer individually and to proceed further with a detailed analysis of the irradiation delivery. Conclusions: MLIC measurements with WET and relative variation calibration show accurate and time‐saving depth dose measurements. It can be concluded that the use of such a device in overall reduces the overall time of the commissioning process down to ∼10% of the necessary time with standard Water Phantom equipment.
SU‐E‐T‐360: A Novel Procedure to Determine the Dose Output Change with Spread Out Bragg Peak Width and Range Shift in Passively Scattered Proton Beams
Authors: Wen Z , Sahoo N
Published: November 2011

Purpose: Two of the major factors affecting the change in output of a passively scattered proton beam are the spread‐out Bragg peak width factor (SOBPF) and range shifter factor (RSF), both of which are usually measured during clinical commissioning. The purpose of this study is to determine these two factors by numerically constructing the SOBP from experimentally measured pristine Bragg peak (PBP), which can serve as an independent monitor unit (MU) calculation procedure. Methods: SOBPs were constructed by superposition of PBPs that are shifted by the water‐equivalent thicknesses (WET) of the corresponding steps of the range modulation wheel. A least‐squares fit was used to determine the weightings of the shifted PBPs to form the required SOBPs. With a simplification in modeling that MU monitor in the nozzle receives dose proportional to the surface dose of the SOBP (higher by an inverse‐square factor), we can relate the MU to the dose at the center of SOBP and compute the SOBP factors for different SOBP widths. When the range shifter was added, the SOBP was shifted toward the surface by the WET of the range shifter and the RSF can also be calculated from the change in the surface dose.Results: The SOBP and range shifter factors were calculated for SOBPs with modulation widths between 4 and 16 cm of 250 and 200 MeV proton beams. The calculated SOBPF and RSF agreed within 2%, and 1.5% with the respective measured values. Conclusions: Starting with measured pristine Bragg peaks, SOBPs can be numerically constructed by simulating the range modulation process, and important dosimetry factors dependent on the modulation width and range shifters can be calculated within 2% accuracy.
SU‐E‐T‐369: Comparison of Monte Carlo Calculations Around An Intracavitary Brachytherapy CT‐MR Compatible Fletcher Applicator with Radiochromic Film
Authors: Mathews B , Price M
Published: November 2011

Purpose: To calculate the dose distribution with Monte Carlo (MC) around an intracavitary brachytherapy (ICBT) ovoid type applicator and compare these calculations with radiochromic film (RCF) measurements acquired in a liquid water phantom. Methods: Detailed physical measurements of a CT‐MR compatible Fletcher ovoid applicator (Nucletron Co.) were performed in‐house to facilitate construction of a MC model of the device. MC calculations were simulated utilizing MCNPX version 2.7.b of a microSelectron version 2 (mHDR v2) Ir‐192 source (Nucletron Co.) at multiple dwell‐positions within a single ovoid. Oncentra brachytherapy treatment planning system (Nucletron Co.) was used to determine the dwell‐time required for three active dwell‐positions to deliver 10 Gy to a prescription point taken as the center of each film. RCF measurements were performed in two perpendicular planes, (a) 1 cm beyond the distal end and (b) 3.3 cm from, and parallel to, the long axis of the ovoid. RCF measurements were performed in a liquid water phantom. The ovoid, liquid water phantom, and RCF were physically aligned using in‐room patient setup lasers. The MC model application was confirmed superimposing simulated dose distributions resulting from three active dwell‐positions and comparing with RCF. Results: Brachytherapy measurements could be acquired in a liquid water phantom consistent with AAPM TG‐43 protocol. For both planes, 96% of the absolute simulated and measured dose points agreed within 2% or 2 mm distance‐to‐agreement. Conclusions: The MC model is sufficient to predict measured RCF dose distributions acquired in a liquid water phantom accurately.
SU‐E‐T‐375: The Impact of Heterogeneity Corrections in High Dose Rate Brachytherapy
Authors: Hyer D , Sheybani A , Jacobson G , Kim Y
Published: November 2011

Purpose: To evaluate the dosimetric impact of heterogeneity corrections on both conventional and volume‐optimized high dose rate (HDR) tandem‐and‐ovoid (T&O) brachytherapy treatment plans. Methods: A retrospective study of 8 CT‐based T&O plans from 5 different patients with FIGO stage IB2‐IIIA cervical cancer was conducted. Each plan featured a titanium Fletcher‐Suit‐Delclos‐style T&O applicator (Varian Medical Systems, Palo Alto, CA). In the original clinical plans, dose was prescribed to Point‐A (5.5–6 Gy per fraction). For the purposes of this study, volume‐optimized plans were retrospectively created in addition to the original Point‐A based clinical plans. In the volume‐optimized plans, the clinical target volume (CTV) and organs‐at‐risk (OAR; rectum, bladder, and sigmoid) were contoured on the CT datasets by a single physician. Dose calculations with and without heterogeneity corrections were then performed for both the conventional Point‐A based plans and the volume‐optimized plans using Brachyvision v8.9 (Varian) along with Acuros: a heterogeneity correction application available in Brachyvision. Results: For the conventional plans, the change in dose to Point‐A (left and right) and ICRU 38 defined rectum and bladder points was assessed after applying the heterogeneity correction with Acuros. It was found that the dose to the ICRU bladder decreased the most (− 2.2±0.9%) while ICRU rectum (−1.7±0.8%), Point‐A right (−1.1±0.4%), and Point‐A left (−1.0±0.3%) also showed decreases. For the volume‐optimized plans, dose‐volume‐histogram (DVH) parameters (D90 of CTV and D2cc of rectum, bladder, and sigmoid) were evaluated before and after applying the heterogeneity correction with Acuros. It was found that D90 of the CTV decreased by −1.9±0.7% and D2cc decreased by −2.6±1.4%, −1.0±0.4%, and −2.0±0.6% for the rectum, bladder and sigmoid, respectively. Conclusions: Heterogeneity corrections on T&O brachytherapy plans were found to have only a small dosimetric impact over TG‐43 based dose calculations for both conventional Point‐A and 3D imaging based volume‐optimization plans.
SU‐E‐T‐391: Is 1D Anisotropy Function Recommended by TG‐43U1 Protocol a Good Candidate for Clinical Brachytherapy Dosimetry?
Authors: Abboud F , Scalliet P , Vynckier S
Published: November 2011

Purpose: The effect of the anisotropy on dose distributions for real permanent prostate implants with 125I seeds has been evaluated and compared with Monte Carlo (MC) calculations. Methods: soSeed (Bebig) was used. Dose distribution of the prostate implants was calculated using MCNP5 code and compared to TG43 calculation. A DVH comparison between 2D and 1D approximations was performed for prostate, urethra and rectum for 20 patients. Results: A DVHs comparison between the use of 1D & 2D anisotropy functions are presented for 20 prostates implants. For prostate, Dmean, D90 and D98 are underestimated by 1D approximation by about 3%, 1.5% and 1% respectively. It leads also to inconsistent results for Dmin and D100. 1D approximation overestimates the dose for about of 15% in planes situated at the two ends of implant containing no implanted seeds while it underestimates dose about 5% in the other planes. This yields a compensating effect in the resulting DVHˈs. These over dosage and under dosage must be attributed to an averaging effect between 2D and 1D anisotropy function. It can be noticed that the effect of 1D approximation on D98 and Dmin of prostate varies from patient to another. This is due to the many factors, for example, prostate volume, number of seeds and seeds positions relative to prostate…etc. D90 is always underestimated by 1D approximation where for 70% of patients D90 exceeds 185 Gy. For the urethra, 1D approximation underestimates the Dmax in which 60% of patients, their maximum dose for the urethra exceeds the limits. Finally, 1D approximation underestimates V100 about 50% les than 2D approximation Conclusions: The use of the 2D anisotropy function is preferred to the 1D anisotropy function as this approximation yields more consistent results for the 20 patients studied. The observed overestimation and the variability among the patients can influence dose—outcome.
SU‐E‐T‐398: 3D Dosimetry Verification for Prostate Implant Treatment Plans
Authors: Wu X , Popple R , Duan J , Shen S , Brezovich I
Published: November 2011

Purpose: A 3D dose calculation program (PIDOSE) for prostate implant was developed for comparison to treatment plans from a commercial treatment planning system (Variseed V7.1, Varian Medical System, Palo Alto, CA). Methods: Although a simple dose verification (one source) was done when Variseed was commissioned, we believe more complicated treatment plans should also be verified. These verifications include 3D dose distributions, dose histograms, isodose lines, maximum, median and minimal doses as a second hand check. A file with needle‐source information was exported from the Variseed planning system to PIDOSE. A 3D dose matrix was created using a tri‐exponential fitting function for the seed model (1). All single‐source doses were summed and selected isodose lines were superimposed on the ultrasound image slices together with those from Variseed. Dose‐volume histograms for prostate, urethra and partial rectum were calculated and compared. Results: : Although maximum dose, mean dose and standard deviation were slightly different, there were no large deviations from typical isodose lines (< 1 mm), minimum dose (<2%), median dose (<2%) and DVHs. Conclusions: The 3D dose verification as a second hand check confirms the accuracy of dose calculations provided by the Variseed Planning System. The small differences between maximum dose, mean dose and standard deviation are due to the differences in dose calculation resolution, dose volume interpolation, and radial dose fitting functions. The maximum doses from PIDOSE at some calculation points near the seeds were high due to the exponential fitting function.
SU‐E‐T‐402: Modulated Electron Radiotherapy Dose Delivery Using Photon Multileaf Collimators
Authors: Jin L , Li JS , Wang L , Fan J , Price R , Ma C
Published: November 2011

Purpose: Different designs of multileaf collimators (MLC) have been investigated for dose delivery for energy and intensity modulated electron radiotherapy (MERT). This work aims at verifying MERT dose delivery accuracy on clinical accelerators, Siemens Artiste and Varian Clinac iX, equipped with conventional photon multileaf collimators (pMLC). Method and Materials:Expensive measurements and Monte Carlo (MC) simulations were performed to acquire the electron beam data for Monte Carlo‐based MERT treatment planning. The calculated percentage depth dose (PDD) and dose profiles were benchmarked with measurements. Differences of electron beam characteristics for fields shaped by the Artiste and Clinac iX pMLC were examined, demonstrating their strengths and weaknesses in MERT dose delivery. Results: MC simulated PDDs and dose profiles for electron energies, 6, 9, 12, 15 (Artiste) and 16 (Clinac iX) MeV with fields shaped by either applicators or MLCs agreed well (2%/2mm) with ion chamber measurements. For pMLC‐shaped fields, no significant changes in PDDs were observed as SSD varied from 61 cm to 85 cm while the flatness and penumbra of dose profiles showed obvious deterioration as SSD increased. In comparison, the Clinac iX pMLC resulted in flatter electron beams with smaller penumbra than those collimated the Artiste pMLC most likely due to the differences of the pMLC locations. Conclusions: Both Siemens Artiste and Varian Clinac iX are capable of delivering energy and intensity modulated electron dose distributions efficiently using their existing pMLC. The electron beam properties shaped by pMLCs were better suited for MERT at smaller SSDs (with smaller gaps between the pMLC and the patient surface). For the same SSD, the Clinac iX pMLC manifested superior beam flatness and penumbra compared to the Artiste pMLC.
SU‐E‐T‐405: Evaluation of IMRT QA Using 3DVH, a 3D Patient Dose and Verification Analysis Software
Authors: Jarry G , Martin D , Lemire M , Ayles M , Pater P
Published: November 2011

Purpose: 3DVH (Sun Nuclear Corporation) is a software that uses individual field QA data to obtain a three‐dimensional dose distribution in the patient. The purpose of this poster is to evaluate 3DVH as an IMRT QA tool; mainly to present our validation results, the TPS model improvements they lead to and preliminary passing rate results on IMRT plans analysed using 3DVH. Methods: The validation was performed using simulated Mapcheck (Sun Nuclear Corporation) QA results obtained from the TPS. The accuracy of 3DVH in calculating errors was verified by introducing errors in the plans and comparing these plans with the dose 3DVH predicted. IMRT plans were verified using the 3DVH software and the results were analysed by looking at DVH values. The 3DVH results were compared to Mapcheck QA results. Results: The validation shows that 3DVH can predict errors within 3%, with discrepancies tending to increase as the induced errors become greater. The retrospective analysis of IMRT plans resulted in a TPS model modification as it brought to light the occasional under dosing of the PTVs. The rounded leaf tip model was modified to decrease the field size for fields defined by the MLC only. This change to the model was validated using picket fence measurements. The IMRT plans were recalculated using the new model; as a result, the 3DVH passing rate for a 3%/3mm gamma between the expected dose and the 3DVH dose increased from 93% to 98% on average. Further analysis of IMRT plans has shown that 3DVH is more sensitive to dose discrepancies than individual field QA. Conclusions: 3DVH is a useful tool for IMRT QA which supplements the planar dose information collected using 2D arrays or film. The analysis of IMRT plans has shown that 3DVH can detect dose differences that are not detected by individual field analysis alone.
SU‐E‐T‐425: Plan‐Specific Gamma Analysis Acceptance Criteria for Multi‐Chamber Arrays
Authors: Price R , Veltchev I , Ma C
Published: November 2011

Purpose: Gamma analysis acceptance criteria for patient‐specific IMRT QA are subject to many variables including the measurement device, plan complexity and the threshold value used for the dose distribution. In this work we demonstrate plan‐specific gamma analysis using the IBA Matrix Evolution phantom. Methods: A series of previously treated prostate patient plans were selected to represent moderately complex dose distributions. The plans were delivered to the same phantom plane containing the ionchamber array and again with EDR2 film. Gamma criteria of 3%/3mm were evaluated using thresholds of 10% and a meaningful, plan‐specific threshold of 50%. Differences in number of pixels passing were evaluated for both detectors and thresholds using film as the “gold standard”. All plans were delivered in a composite manner using treatment gantry angles. Passing rates for both detectors were compared to determine detector‐specific acceptance criteria. The clinical results from the last 329 prostate patients were compared with these criteria.Results: The passing rates for the 10% threshold between film and the ionchamber array were 99.2% and 92.1%, respectively. Passing rates for the 50% threshold were 99.5% and 96.7%, respectively. These rates represent an approximate 7% and 3% absolute reduction in passing criteria for detector and plan‐specific analysis. The average passing rate for the initial 329 clinical prostate cases measured with the array using the 50% threshold was 94.6% Conclusions: AAPM TG‐119 suggests a passing rate of 88–90% for composite distributions using film. It is important to understand the relationship between film (high resolution) and the detector array and dose threshold used clinically. The 3% drop between film and array in points passing using a plan‐specific threshold of 50% for our test cases indicates an acceptable range of 85–87%. Our clinical passing rates ranged between 82.0% to 99.7% with 97.6% of cases meeting the aforementioned range.
SU‐E‐T‐426: Assessment of the Limitations of Intensity Modulated Radiation Therapy Quality Control Procedure
Authors: Girard F , Lacroix F
Published: November 2011

Purpose: To determine the impact of dose delivery errors due to multileaf collimator bank offset and gantry angle offset on the resulting gamma value of an IMRT Quality Control (QC) test as determined by a planar 2D dosimeter. Also to determine the impact of the planar 2D dosimeter array resolution on the gamma. Methods: Systematic errors such as MLC leaf bank offset and gantry angle offset were numerically introduced in IM verification plans. The dose received by a phantom was calculated using anisotropic analytical algorithm with a grid size of 1 mm. A corona plane (30×30 cm2) was selected for each modified plan and compared t original using the geometric gamma approach in absolute or relative using a 3%/3 mm criteria. The plans were then irradiated in a phan composed of 20×20 cm2 slabs of plastic water. The phantom was im using a CT scan and imported into the treatment planning system Ec (Varian inc.). A radiochromic EBT2 (ISP Corp. inc.) film was placed depth of 4 cm. Results: Absolute dose measurements are much m sensitive to MLC bank offsets than relative measurements. In con modifying the gantry angle by 1 degree has little effect on the relativ absolute gamma value. There is a clear increase in the variance of gamma value as the detector spacing is increased beyond 2–3 Conclusions: Absolute dose measurements should be performed maximize QC sensitivity. Ideally, the detector spacing in a 2D planar a should be less than 2–3 mm in order to minimize the uncertainty on gamma.

This work has been supported by the Ministère de la Santé et des Ser Sociaux and Natural Sciences and Engineering Research Council Disco Grant No. 357402.
SU‐E‐T‐434: Improvements in Step‐And‐Shoot Dose Delivery Accuracy on Varian TrueBeam
Authors: Stepaniak C , Li J , Farrey K , Yenice K , Al‐Hallaq H
Published: November 2011

Purpose: To investigate the impact of monitor units (MUs) per segment and dose rate on the dose delivery accuracy of step‐and‐shoot intensity modulated radiation (SS‐IMRT) fields on a TrueBeam LINAC.Methods: A step‐and‐shoot multi‐leaf collimator (MLC) sequence was created consisting of 10 identical 10×10cm segments with identical MUs. Beam holding between segments was achieved by moving one out‐of‐field MLC leaf pair. Measurements were repeated for various combinations of MU/segment ranging from 2–40 and dose rates of either 300 or 600MU/min. All measurements were made with a Farmer (0.6cc) ionization chamber placed at the isocenter in a SolidWater phantom at 10cm depth. The measurements were performed on two Varian LINACs: Trilogy and TrueBeam. Each sequence was delivered three times and the charge readings for the corresponding segments were averaged. The effects of MU/segment, dose rate, and LINAC type on the relative dose variation (Delta_i) was compared using F‐tests.Results: On the Trilogy, large Delta_i was observed in small MU segments: at 2MU/segment, the maximum Delta_i was 20.0%/26.2% at 300/600MU/min, respectively. Also, the first segment of each sequence consistently over‐shot(Delta_i>0), while the last segment consistently under‐shot(Delta_i<0). On the TrueBeam, at 2MU/segment, Delta_i ranged from −1.6% to +2.0% at 300 MU/min and −3.0% to +3.1% at 600MU/min; no obvious over/under‐shoot trend was observed. F‐tests showed statistically significant difference ((1—beta)=1.0000) in 2 MU segments between the Trilogy and the TrueBeam, at either dose rate. No significant difference was observed for large (>10 MU/segment) MU segments Conclusions: Earlier generations of Varian LINACs exhibited large dose variations for small MU segments in SS‐IMRT delivery. Our results appeared to confirm these findings. The dose delivery accuracy in small MU segments and at high dose rate for SS‐IMRT is significantly improved on TrueBeam compared to Trilogy, likely due to the faster sampling rate (100Hz vs. 20Hz).
SU‐E‐T‐441: Sensitivity of a 3D‐Verification System to VMAT Delivery Errors
Authors: Boggula R , Wertz H , Birkner M , Simeonova A , Steil V , Lohr F , Wenz F
Published: November 2011

Purpose: VMAT offers continuous dose delivery in the form of arcs while dynamically changing position of collimators and dose rate. The aim of the present study is to investigate the sensitivity of a 3D‐verification system to different VMAT delivery errors. Methods: Three VMAT plans were generated with MONACO TPS (v2.0, Elekta‐CMS, Maryland Heights, MO) for brain tumor paradigms. With in‐house software, we modified the plans introducing the following errors: (i) One set of leaf bank was opened by 1mm, 2mm and individual leaves were randomly opened (0mm–2mm). (ii) Both sets of leaf banks were shifted by +/−1mm and +/− 2mm. (iii) Gantry angle was shifted by +/−0.5°, +/−1°, +/−2° and randomly shifted (−2° to +2°). All these plans (a total of 45) were delivered to a MatriXX‐based COMPASS system (v2.0, IBA‐Dosimetry,Germany). 3D‐dose distribution was reconstructed onto the patient anatomy based on the measured fluences. Dose—volume parameters for target (D95) and critical organs (Davg) were compared and analyzed. Results: For three reference plans, the maximum dose‐difference (MDD) between MONACO and COMPASS was about 10% (Davg‐lens). However, the absolute dose‐difference (ADD) for the same was 4cGy. For plans with errors in one set of leaf bank, the MDD and ADD were 10% and 13cGy for Davg‐optic nerve, respectively. With errors in both leaf banks, the MDD and ADD were 15% and 21cGy for Davg‐optic nerve, respectively. With gantry angle errors, the MDD and ADD were 50% and 20cGy, respectively. For PTV‐D95, the MDD noticed was 6% and the ADD was 16cGy for different kind of errors. Conclusions: Any increase in aperture size (error in one leaf bank) showed systematic deviations. Shifts in both leaf banks up to 2mm and gantry angle difference (2°) showed considerable deviations. Errors greater than 2mm/2° can be detected with COMPASS system in most of the cases.
SU‐E‐T‐449: Clinical Impact of Couch Top and Couch Rails on Treatment Dose for IMRT and Arc Therapy
Authors: Pulliam K , Howell R , Followill D , Luo D , White R , Kry S
Published: November 2011

Purpose: To evaluate the dosimetric impact of the Varian Exact Couch top and rails on dose and relative volume coverage to target and critical structures for 6 MV IMRT and Arc Therapy. Methods: Five prostate patients were planned with both 6 MV 8‐field IMRT and 2‐field RapidArc per clinical practice at MDACC for target and normal tissue constraints using the Eclipse treatment planning system. The dose distributions were then re‐calculated with inclusion of the couch top and rails in varying configurations. DVH analysis of the target and critical structures for dose and relative volume coverage loss were evaluated and compared to MDACC clinical plan requirements (98% and 95% coverage of prostate and PTV, respectively, at prescription dose). To isolate the effects of the couch top and rails individually, the dose distribution was re‐calculated for the treatment plans with the couch top only. Calculated dose perturbations were verified through measurements in an IMRT QA phantom. Results: The average dose loss to the prostate and relative volume coverage (at prescription dose) were: 3.2 Gy/35% and 1.5 Gy/84% for IMRT with rails out and in, respectively, and 2.4 Gy/18% and 2.2 Gy/17% for RapidArc with rails out and in, respectively. The couch top accounted for 1.5 Gy/84% for IMRT and 1.5 Gy/40% for RapidArc, the remainder was from the rails. Conclusions: Dose and volume coverage loss for IMRT plans is primarily due to the rails while the couch top is the primary cause for RapidArc. Both the couch top and rails contribute to dose and coverage loss to a degree that, if included, would cause the plan to fail clinical approval. Therefore, the couch top and rails should be accounted for in treatment planning.
SU‐E‐T‐450: Can Low Frequency Electric Fields Be Generally Used as a Sensitizer for Radiation Therapy and Chemo‐Therapy?
Authors: Hegseth J
Published: November 2011

Purpose: A low frequency electric field applied to a tumor is called a Tumor Treatment Field (TTF) and works by arresting cells during mitosis (M‐ phase). I have numerically simulated E‐field distributions to determine if a TTF can be applied to arbitrary tumor masses. This modality is currently under clinical trail with chemotherapy as a sensitizer. Because cells in M‐phase are also sensitive to radiation this method may also work as a radiotherapy sensitizer. Methods: Previous published results showed a TTF applied in to an intracranial glioma in a rat brain and subsequent tumor shrinkage. The TTF was numerically modeled. I have also modeled several therapies. The E‐field is calculated in inhomogeneous tissue using the Finite Element Method with normal and tangential boundary conditions applied to the exterior of the patient. These boundary conditions correspond to electrodes that may be manipulated to design treatment plans. Results: An AC electric field of amplitude 1V/cm–2V/cm in the 100kHz–200kHz range was previously found to be the most effective for arresting mitotic cells. In this range, however, the dielectric constant is very large and varies considerably between tissue layers. These large variations cause large impedances in layers of, e.g., fat and bone, i.e., these layers effectively “shield” deeper tissue layers against E‐fields applied in the normal direction. This requires large external voltages on surface electrodes to penetrate and achieve the required 1V/cm–2V/cm at deeper tissue layers, especially in human treatments. These large impedances are avoided with an E‐field applied in the tangential direction. The tangential fields are also patient size independent, greatly simplifying treatment planning. Results from several simulations comparing methods are presented. Conclusions: This work shows that an external applied device can produce an effective TTF at reasonably safe voltages if the applied field is not applied normally to the tissue layers.
SU‐E‐T‐451: Is Rotational IMRT More Susceptible to Delivery Uncertainties than Dynamic IMRT?
Authors: Betzel G T , Niu Y , Yi B , Yu C
Published: November 2011

Purpose: To investigate whether rotational IMRT is susceptible to delivery uncertainties. Methods: Dosimetric effects of variation in dose rate, gantry position and MLC leaf positions were evaluated by incorporating their corresponding uncertainties into recalculated RapidArc treatment plans for four head‐and‐neck (HN) and four prostate test cases via modifications in gantry angles or MLC leaf positions. Total monitor units per aperture and therefore per plan were held fixed. The dose distributions and dose‐value histograms of the original and modified plans were compared using 3D Gamma analysis and dose indices of planned treatment volumes (PTV) and organs at risk (OAR). Results reported below used 2%–2 mm Gamma test pass rate criterion. Results: For a 50% normally distributed random variation in dose rate (sigma = 1.3° in gantry angle), all HN plans had a 99.7% Gamma test pass rate. For HN PTVs, an increase in dose rate variation resulted in a decrease in minimum dose and an increase in maximum dose; for nearly all OARs, no more than a 0.4% dose difference was observed. For HN plans, a 5° rotational shift resulted in a 96.2% average pass rate. Dose distributions were sensitive to systematic variations especially above 1 mm; HN plans modified with a −2 mm shift in the X1 bank resulted in a 97.3% pass rate, and PTVs and OARs received a 3.8% higher dose on average. Uniformly distributed random shifts of +/−2 mm in aperture MLC leaf positions showed no significant change in mean dose (<0.5%) and resulted in a 100% pass rate Conclusions: Rotational IMRT deliveries were found to be very tolerant to random errors in dose rate, gantry position and MLC leaf position. Clinically acceptable treatments may be delivered successfully using RapidArc despite large fluctuations in dose rate and gantry position.
SU‐E‐T‐453: Feasibility of IMRT System Evaluations Using Pre‐Made IMRT Leaf Patterns
Authors: Gordon J , Young S , Dukes B , Hamilton R
Published: November 2011

Purpose: To assess the feasibility of utilizing pre‐made IMRT leaf patterns in IMRT system evaluation. Methods and Materials: A TG1 19 evaluation set consisting of five IMRT test cases were optimized using a pencil‐beam algorithm and delivered on a 30 cm × 30 cm × 15 cm solid water phantom. Per‐field measurements were collected using a diode array, and composite measurements were collected using film and ion chamber. Measurements were compared to calculation to determine confidence limit values per TG1 19 methodology. The phantom image set, associated contours, and leaf patterns were then transferred to a separate treatment planning system utilizing different treatment planning software previously commissioned for the same LINAC. Dose calculations utilizing a convolution‐superposition algorithm were performed on this planning system utilizing the pre‐made IMRT leaf patterns and image set. These calculations were compared to previously collected measurements, and TG1 19 confidence limits were determined. Results: The TG1 19 evaluation utilizing the pencil‐beam treatment planning system resulted in confidence limits of 1.55%, 8.06%, and 3.13% for diode array, film, and ion chamber comparisons, respectively. The TG1 19 evaluation utilizing the same leaf patterns within the convolution‐superposition treatment planning system resulted in confidence limits of 10.5%, 14.9%, and 5.46% for diode array, film, and ion chamber comparisons, respectively. The TG1 19 confidence limit values for the two planning systems differed by 8.95%, 6.84%, and 2.33%.Conclusion: This study has demonstrated the feasibility in utilizing pre‐made leaf patterns to evaluate the dose model for an IMRT system. In this case, the pencil‐beam TPS calculations were closer to measured values, as quantified by the TG119 assessment.
SU‐E‐T‐454: Feasibilty of Image‐Guided Total Marrow Irradiation Using Helical TomoTherapy
Authors: Reynders T , Tournel K , Soete G , Duchateau M , Gevaert T , Depuydt T , Leysen K , Gulyban A , Boussaer M , Poels K , Storme G , Verellen D , De Ridder M
Published: November 2011

Purpose: Investigation of the technical feasibility and dosimetry of image‐ guided total marrow irradiation using helical TomoTherapy in the treatment of relapsed multiple myeloma patients. Irradiation is followed by a hematopietic stemcell transplantation. This study concentrates on planning and dosimetry Methods: Patients are simulated in supine position with H&N mask and pelvic cast. TomoTherapy plans for the upper body (head‐knees) were generated (pitch 0.45, modulation factor 2.0, field width 5cm). Dose prescription was 4x3Gy=12Gy. Dose verification is performed with the IˈmRT MatriXX. Beam‐on‐time is +/−30min. TMI‐stability measurement was done with 2 ICs on different distances in 1.5m solid water equivalent blocks. Lower extremities were planned conventional and with TomoTherapy. Dose in the junction region between upper and lower body plans was verified with EDR2 film for both techniques. Results: It is possible to create clinically acceptable plans for the upper body with TomoTherapy. Verification shows acceptable results in the lung region (gamma 5%, 7mm). Measurement in pelvis region is less good as in lung region. TMI‐stability measurement shows differences in dose till 4% measured/calculated. During TMI a 2.5% dose drift was seen by the detectors. Plans for the extremities were more homogeneous in the junction region with TomoTherapy comparing with the conventional technique. Conclusions: TPS has difficulties to find a good ratio between couch and gantry speed. Differences seen at TMI‐stability measurements can be pointed to drift and stability of one rotation. Differences are depending on system stability (magnetron, target, …). Patient specific QA is necessary before treatment. Differences between the gamma index of the lungs and pelvis can be pointed to the fact that the planning is optimized very well for the lung region. Pitch and modulation factor are optimized for this region. For the junction region tomotherapy plans give a better homogeneity than 1 tomotherapy and 1 conventional plan.
SU‐E‐T‐458: Instability of Electronic Portal Imaging Device Responses for Intensity Modulated Irradiation
Authors: Yeo I , Jung J , Yi B , Piskulich F , Choi D , Nookala P , Patyal B
Published: November 2011

Purpose: To investigate the response of electronic portal imaging device (EPID) in integration mode for intensity modulated beams including step‐ and‐shoot (SS) and sliding window (SW) deliveries. Methods: We evaluated EPID dose response measurements of open, SS, and SW irradiations. We designed three beams using 6MV x‐rays with 10×10 cm2 field size. For the SS irradiation, two MLC leaves with 0.5 cm opening outside 10 × 10 cm2 were discontinuously moved by 10 cm during each segmental irradiation among ten segments of the entire delivery. For the SW beam, the same MLC leaves were continuously moved by 10 cm ten times during the delivery. The employment of the same open beam area ensures the same dose irradiation by the three. Monitor units were varied. We additionally investigated the dose linearity of EPID response. We used EPID‐ADU1000 operated with IAS3 system for the integrated acquisition. Results: The SS delivery showed a deviation greater than 2% from the open beam, if a segment employed less than 10 MUs. If it employed greater than 10 MUs, then a deviation less than 1% was observed. The SW delivery showed a maximum deviation of 1.4% at the lowest MU. The open beam irradiation showed linearity within 0.8%, the SW irradiation 1.2%, and the SS irradiation 1.7%. Conclusions: The SS delivery is associated with a greater error than the SW delivery due to beam instability at the start of acquisition and reading loss during MLC movement. Using large monitor units greater than 10 MUs/segment can minimize the deviation for the SS delivery from open beam. The linearity of EPID response to dose was better with the SW delivery than the SS delivery. This study will help understand dosimetric response of EPID for the SS delivery.

This study was in part supported by Varian Medical Systems, Inc.