bluuebunny/crossref_metadata_2025 huggingface dataset:
SU‐E‐T‐381: Experimental Measurements of 3D Dose Distribution for a Moving Target Treated with IMRT and VMAT
Authors: Yan H , Thomas A , Oldham M , Yin F
Published: June 2012

Purpose: Organ motion during radiotherapy may cause under‐dose of target and over‐dose on surrounding normal tissues. To investigate its effect, a phantom study was performed to assess the dosimetric uncertainty caused by organ motion during IMRT and VMAT. Methods: A motion phantom equipped with high‐precision step motor was employed to mimic organ motion in anterior‐posterior (AP) direction. A cylindrical Presage dosimeter was attached to the moving organ, in which a planning target volume (PTV=GTV+0.5cm GTV along longitudinal axis) was contoured. Multiple sets of computerize tomography (CT) data were acquired: stationary CT for the phantom without organ motion and 4‐dimensional CT (4DCT) with organ motion using two different breathing patterns. A 9‐field conventional intensity modulated radiotherapy (IMRT) plan and a partial‐arc volumetric modulated arc therapy (VMAT) plan were generated based on the stationary CT set. It is designed to deliver prescription doses into the PTV in the Presage dosimeter. After the phantom was setup at treatment couch, onboard CBCT images and fluoroscopic images were taken to verify the positioning accuracy of each Presage dosimeter. The same IMRT and VMAT plans were delivered the phantom three times each with a new Presage dosimeter: one for stationary, two for different motion patterns. The irradiated Presage dosimeters were scanned using an optical scanning system and the 3D dose distributions were reconstructed from the scanned projections. The reconstructed 3D doses were compared with the calculated doses from a planning system to evaluate the delivery accuracy of each treatment. Results: DVHs measured and 2D dose distributions from Presage dosimeters showed comparable dose coverage for GTV between stationary and motion targets. Considerable dose degradation was observed for PTV of moving targets. Conclusions: Breathing‐induced motion will cause considerable loss of dose coverage of PTV for both IMRT and VMAT treatments.
SU‐E‐T‐383: Pulsed Low Dose Rate Radiotherapy Using Advanced Treatment Methods: A Novel Technique for Patient Re‐Irradiation
Authors: Lin M , Price R , Koren S , Li J , Ma C
Published: June 2012

Purpose: Pulsed low dose‐rate radiotherapy (PLDR) re‐irradiation has the potential to reduce late normal tissue toxicity while still providing significant tumor control for recurrent cancers. In contrast to conventional treatments delivered at dose‐rates of 400–600cGy/min, PLDR treatments deliver 20cGy pulses separated by 3‐minute intervals to achieve an effective‐dose‐rate of 6.7cGy/min. This work aims to investigate the planning strategy and delivery quality of PLDR treatment using IMRT and RapidArc techniques. Methods: Twenty cases (10 treated with PLDR IMRT, 10 for evaluation purposes) were recruited in this study including prostate, pancreas, lung, head‐and‐neck, breast and pelvis. IMRT and the RapidArc treatment plans were generated using the Eclipse TPS. For IMRT treatment, each plan consisted of 10 fields to achieve a daily dose of 200cGy. The breast IMRT and the RapidArc plans consisted of two fields/arcs, respectively (40cGy/plan) and were delivered 5 times. The dose contribution from each field to the planning target volume (PTV) was analyzed to evaluate the feasibility for PLDR treatment. Machine‐operation‐ dose‐rate and plan quality was also investigated. Dose delivery accuracy was assessed using a cylindrical diode array. Results: Throughout the six treatment sites, the mean PTV dose ranged from 16.1 to 26.1cGy/arc for RapidArc plans and 10.3 to 36.7cGy/field for IMRT plans. For IMRT, the PTV dose contribution from each field strongly depends on the beam arrangement and optimization parameters. With very low dose for a full rotation (∼ 20cGy/arc), the machine‐operation‐dose‐rate of RapidArc plans significantly affects plan quality and deliverability. A machine‐operation‐dose‐rate of 100 MU/min results in superior delivery accuracy (>97.7% gamma‐passing‐rate for 3%/3mm criteria) for both IMRT and RapidArc plans. Conclusions: PLDR radiotherapy using IMRT and RapidArc techniques Result in both dosimetric and radiobiological benefits, which may have great potential for those previously‐irradiated patients who have historically done poorly.
SU‐E‐T‐384: An Easy Use Anthropomorphic Phantom for Lung IMRT QA
Authors: Fan E , Ellerbusch D , Qin L
Published: June 2012

Purpose: Even though lung IMRT plans are calculated based on patient CT with heterogeneity correction, patient‐specific validations are often performed by using measurements in homogeneous solid media. In this study we developed heterogeneous phantoms for measurement‐based validation of lung IMRT. Methods: Three slabs from thorax region of anthropomorphic RANDO phantom and virtual water (VW) slabs were used to construct two versions of heterogeneous phantom, “Hetero‐1” and “Hetero‐2”. The RANDO slabs are 2.5 cm thick. A two‐dimensional diode array detector, MapCHECK and an ion chamber were used for measurements. “Hetero‐1” consisted of three RANDO slabs placed horizontally on the MapCHECK plus a 3cm VW on the top. Hetero‐2 was constructed by inserting a 2 cm VW with a chamber cavity on top of the first RANDO slab above the MapCHECK. Two lung‐IMRT plans were generated using Pinnacle TPS, including a plan for the RPC Lung‐Phantom. The point doses were compared with ion chamber measurements and planar dose were compared with MapCHECK measurements. Results: The MapCHECK measurements were performed using “Hetero‐1” on a beam‐by‐beam basis for the two IMRT plans. Passing rates under 3%/3mm Gamma criteria of were from 93.5% to 98.5% for all beams. Accumulative measurements were performed using “Hetero‐2” and the passing rates under same criteria for two IMRT plans were 96.9% and 96.3% respectively, and 95.2% and 98.5% for measurements with 5cm lateral‐IsoCenter‐shift including more lung material. The differences between Ion‐chamber‐ measurements and TPS‐calculations were 1.2% and 1.71% for two IMRT plans. In vivo results obtained from RPC‐report for two TLDs inside the PTV were 98% and 99% between measurements and calculations. Conclusions: An easy‐to‐setup heterogonous phantom for Lung IMRT QA can be constructed by adopting thorax region slabs of anthropomorphic‐type phantoms. It can be implemented as a convenient and efficient tool for lung IMRT QA in a routine clinic.
SU‐E‐T‐390: Monitorization of the Dynamic Multi‐Leaf Collimator Performance Using Log Files ‐ a Quality Assurance Tool for Intensity Modulated Radiotherapy
Authors: Silva A Martins da , Pereira A , Barreiros M , Silva R , Faria D
Published: June 2012

Purpose: Commission baseline values of the multi‐leaf collimator (MLC) using Varian's dynalog file viewer software (DFV) and establish tolerance levels to use as a periodic quality assurance tool in intensity modulated radiotherapy (IMRT). Methods: A sliding window test was created to have all leafs moving at a constant high speed of 2.5cm/s. This test was performed for 100 days at gantry angles of 0°, 90° and 270° during the morning quality checks in an accelerator performing RapidArc treatments with a Millennium 120 MLC. The DFV was used to collect the percentage of counts in bins 1 (0.05mm) to 8 (deviations in steps of 0.5mm) and the root mean square error (RMS). These were analyzed statistically and a baseline value and tolerance levels were established. Results: Results show that the greater percentage of counts is found at bin 4 with an average of 97%. Bins 1 to 3 have less than 1 % and bin 5 presents an average of 1.7%. As for bin 6, the average was below 0.1%. Bins 7 and 8 presented no counts. When comparing the results of the different gantry angles, significant differences are only found in bin 4 and 5, with a better performance for the 0° gantry. As for the RMS error, no difference was found between different carriages or gantry angles and the average value was 0.129. A tolerance level was established for bins that correspond to greater deviations ‐ bins 5 to 8, with an upper limit equivalent to twice the standard deviation of each bin. Conclusions: With the introduction of IMRT, the MLC quality assurance became more extensive and thorough. However a quantitative analysis is often difficult to interpret and time consuming. The purposed method allows the monitorization of the stability and performance of the MLC in a quantitative and easy way.
SU‐E‐T‐391: Modelling Peripheral Photon Dose in TomoTherapy Treatments
Authors: Sánchez‐Nieto BB , Romero MT , Nuñez L , Sansaloni F , Lagares JI , Sánchez P , Medrano JC , Garayoa J , Sánchez‐Doblado F
Published: June 2012

Purpose: The delivery of the therapeutic radiation dose to the tumour in photon radiotherapy, also implies dose deposition in distant organs (peripheral dose) related to secondary cancers induction (Hall and Wuu, Int J Radiat Oncol Biol Phys 56:83–88, 2003). Therefore, peripheral dose estimation in MU‐demanding techniques, such as Helical TomoTherapy (HT), becomes relevant. TLD measurements and Monte Carlo modelling were compared by D'Agostino (Strahlenther Onkol 187:693, 2011). The purpose of this work was to find out experimental models predicting the equivalent photon dose as a function of the distance to the isocenter for different treatment types. The prostate case is presented here. Methods: A HT prostate plan was delivered to an anthropomorphic phantom mimicking a male adult. The phantom was made of polyethylene blocks whereas light wood was used for lungs. 16 points distributed along the phantom, covering different depths, were selected (SÃ¡nchez‐Doblado IFMBE, World Congress Med Phys & Biomed Eng, 259–261, 2009). Additionally, a polyethylene sheet was inserted in the phantom to measure the off‐axis dose profile at midplane depth. Measurements were carried out with standard TLD‐100 pairs of dosimeters (calibrated in a 137Cs source). Results: Two‐exponential‐terms curve fitting was carried out to model separately the scatter and leakage contribution (f=a*exp(−b*x)+c*exp(−d*x)). The former resulted predominant in the proximal region (10=x=14cm) and the latter in the distal re gion (x=14cm). Both components equate at 18cm. Scatter contribution becomes negligible for x=23cm. Points at 5cm were not used for the model as they are too close to the isocenter to be considered as peripheral dose. Model fits well experimental data (13% mean deviation). Only depths behind the build‐up region could be properly modelled. Conclusions: Peripheral photon dose profiles in HT treatments have been modelled by a two‐exponential‐terms curve modelling separately scatter and leakage.
SU‐E‐T‐392: A Software Tool of Extracting Dose Plane for Tomotherapy Delivery QA
Authors: Zhang J
Published: June 2012

Purpose: To develop a software tool to extract dose planes base on on the setup (red) laser positions for Tomotherapy delivery QA(DQA). Methods: We have a develop a simple tool to simplify a time‐consuming step for tomotherapy DQA. The DQA analysis tools in Tomotherapy software was useful for film dosimetry but not convenient for 2D‐array QA devices. Current DQA software requires the user to perform alignments twice: one for setting DQA phantom and the other for extracting dose plane. These two steps can be simplified into one if both alignments are based on the central axis of the detector plane. Once the setup (red) lasers are set for DQA, the dose plane can be extracted from the dicom dose file and the dicom plan file automatically. An open source software was written in C using dicomtk library. The accuracy of the program has been tested by comparing the extracted dose plane to the same dose plane but extracted by the manual method. Results: The maximum dose difference is 0.5% and the mean difference is 0.2%. The differences are probably due to the variation between two manual alignments. The software extracts 2D‐dose plane in a simple and automatic way. This not only cuts workload but also reduce the degree of human error because only one manual alignment is needed. A copy of this open source software is available upon request. We also hope Tomotherapy provides similar function in future upgrade. Conclusions: A software tool has been developed to simplify Tomotherapy DQA procedure.
SU‐E‐T‐396: Beam Angle Optimization for IMPT Comparing Gantries and Fixed Beam Lines
Authors: Bangert M , Titt U , Ziegenhein P , Mohan R , Oelfke U
Published: June 2012

Purpose: To explore the potential of beam angle optimization (BAO) for IMPT and compare fixed beamlines with gantries. Methods: For three patients with challenging intracranial lesions, we generate reference IMPT treatment plans applying three manually selected beam orientations and treatment plans applying three optimized beam orientations considering five scenarios: (1) patients are in supine position and the treatment room features (1.a) a horizontal beamline, (1.b) a horizontal, 45°, and vertical beamline, (1.c) a gantry, (2) patients are in supine or seated position and the treatment room features (2.a) a horizontal beamline, or (2.b) a horizontal, 45°, and vertical beamline. We use a genetic algorithm that considers up to 1,400 non‐coplanar candidate beams and evaluates 10,000 beam ensembles for one BAO. Beam orientations that may compromise the robustness of treatment plans are excluded before the optimization based on an objective measure of existing tissue heterogeneities. Results: The optimized beam ensembles exhibit certain similarities even though the sets of candidate beams differ significantly for the five scenarios. Compared to manually selected beam orientations, they provide improved OAR sparing and equivalent target coverage. Compared to one another, they yield comparable target conformity (deviations of the conformity number <1%), target homogeneity (standard deviations of the target dose <0.8 Gy), and sparing of OARs (deviations of average mean and maximum doses in OARs +/− 1 Gy). Using a gantry, however, the integral dose can be reduced by 5–15% compared to a horizontal beamline with patients in supine position. For the investigated cases comparable reductions can be achieved by also irradiating in seated position with a horizontal, 45°, and vertical beamline. Conclusions: BAO has the potential to provide beneficial IMPT treatment plans. Compared to fixed beamlines, gantries yield only modest effects regarding OAR sparing but may enable a significant reduction of integral dose for individual patients.
SU‐E‐T‐397: Interplay Effect of Gated Lung Stereotactic Body Radiotherapy with RapidArc Delivery
Authors: Zhao B , Yang Y , Huq M , Heron D
Published: June 2012

Purpose: To investigate the dosimetric effect of intrafraction tumor motion during gated RapidArc Stereotactic Body Radiotherapy (SBRT) delivery. Method: The realtime tumor motion data were retrieved from 6 lung patients. Each of them received 3 fractions of stereotactic radiotherapy treatments with Cyberknife Synchrony. Phase gating through an external surrogate was simulated with a gating window of 5 mm. The resulting residual tumor motion curves during gating (beam‐on) were retrieved. RapidArc SBRT was planned on the platform of Varian Truebeam at 6 MV with 1400 MU/min. Planning target volume (PTV) was defined as physician‐contoured clinical target volume (CTV) surrounded by an isotropic 5 mm margin. Each patient was prescribed with 60Gy/3 fractions. The RA plan typically consists of 2 arcs; each contains 90–120 control points. An algorithm was developed to reconstruct the delivered dose with tumor motion. The MLC segment is assumed to move relatively to a static tumor. Each MLC control point, mainly the leaf position were modified according to the probability density function of tumor motion. The newly created MLC control points were written back to the treatment file in the dicom format which was subsequently imported to treatment planning system (Varian Eclipse) for dose recalculation. Results: The magnitude of dose deviation with motion is consistent with the excursion of the residual tumor movement. Overall CTV coverage of the study group is barely affected owing to the 5 mm margin. The fractional PTV dose coverage dropped by 4% at most and that from all fractions by 3%. An examination in the point dose shows an increase of 4% in the maximum dose and decrease of 10% for the minimum dose. Conclusion: With effective gating, interplay effect does not change the target coverage much during gated RapidArc SBRT. However it increases the dose nonuniformity inside target.
SU‐E‐T‐399: Dosimetric and Geometric Evaluation of a Novel Stereotactic Radiotherapy Device for Breast Cancer: The GammaPod
Authors: Mutaf Y , Yu C , Zhang J , Yi B , Prado K , D'Souza W , Regine W , Feigenberg S
Published: June 2012

Purpose: A dedicated stereotactic irradiation device, the GammaPodTM, was developed to treat early stage breast cancer. This study presents the first description of the dosimetric and geometric characteristics from the prototype unit. Methods: The GammaPod stereotactic radiotherapy device is an assembly of a hemi‐spherical source carrier containing 36 Co‐60 sources, a tungsten collimator, a dynamically controlled treatment table and a breast immobilization cup embedded with a stereotactic coordinate system. The source carrier and the variable‐size collimator rotate synchronously to form 36 non‐coplanar, concentric arcs focused at the isocenter. The treatment table enables motion in three dimensions facilitating continuous dose painting in comparison to a sphere packing approach. Geometric and dosimetric evaluations and a method for absorbed dose calibration are provided. Dosimetric verifications of the dynamically delivered plans are performed for eight patients in hypothetical pre‐op, post‐op and dose painting treatment scenarios. Results: Loaded with a cumulative activity of 4320 Ci, the GammaPod unit delivers 5.31 Gy/min at the isocenter. Due to non‐coplanar beam arrangement and dynamic dose shaping features, the GammaPod delivers uniform doses to the targets with excellent conformity. The spatial accuracy of the device is less than 1 mm. Single shot profiles with the 25 mm collimator are measured with radiochromic film and found to be in good agreement with respect to the MC based calculations (congruence of FWHM less than 1 mm). Dosimetric verifications corresponding to all treatment plans corresponding to three target scenarios for each of the eight patients demonstrated Gamma index pass rates greater than 97%. Conclusions: The first description of the dosimetric and geometric evaluation of the GammaPod was performed. The observed level of agreement between the treatment planning system calculations and dosimetric measurements has confirmed that the system can deliver highly complex treatment plans with remarkable geometric and dosimetric accuracy. C Yu and J Zhang have commercial affiliations with Xcision Medical Systems.
SU‐E‐T‐409: A Comparison of 3D Optical Surface and IR Marker Tracking for Head Motion Compensated Frameless SRS
Authors: Wiersma R , Tomarken S , Grelewicz Z , Kang H
Published: June 2012

Purpose: Both infrared marker and 3D facial surface tracking were performed simultaneously on both a head phantom and healthy volunteers. Accuracies of the two 6D real‐time tracking systems were quantitatively assessed both spatially and temporally. The suitability of 3D facial surface tracking for providing real‐time head positioning to guide a robotic head motion compensation stage was investigated. Methods: A xyz stage was programmed to move a head phantom to approximately 200 points in 10×10×10 mm3 and 100×100×100 mm3 cubic volumes, respectively. Both 3D surface and IR marker tracking were performed simultaneously on the head phantom to allow direct comparison of spatial accuracy. Sinusoidal motion was performed along all three axis directions and recorded under the same clock cycle. Spatial and temporal accuracies on actual deformable facial surfaces were assessed by simultaneous 3D surface and IR marker tracking on human volunteers. 3D surface tracking both with and without the use of robotic head motion compensation was evaluated. Results: For 10 samples at each position, the 10 mm cube had RMSE of 0.30 mm and 0.08 mm for the AlignRT and Polaris, respectively. Temporal lag of the AlignRT system was found to be approximately twice the fps time (t_{lag} ∼ 2/fps). The fps value was found to be strongly correlated with region‐of‐interest (ROI) surface size and could vary from 0.5 – 1.6 fps, leading to a lag time of 1 – 4 s. The Polaris output lagged behind the reference by 0.06 s in all cases. Conclusions: Both 3D surface and IR marker devices demonstrate high spatial accuracy for phantom based studies, however, a slight dimensional warping along the SI direction was observed to occur with the AlignRT. The current ROI dependent lag time of the AlignRT could also pose an issue for image guidance techniques requiring quick response times.
SU‐E‐T‐412: Can Cyberknife SBRT Be an Alternative to Brachytherapy for Cervical Cancer Treatment?
Authors: Podder T , Fried D , Holland B , Rosenman J , Biswas T
Published: June 2012

Purpose: To investigate the effectiveness of stereotactic body radiation therapy (SBRT) with Cyberknife for treatment of squamous cell carcinoma (SCCa) of cervix that are commonly treated with brachytherapy. Methods: SCCa of cervix is routinely treated with external beam radiation therapy (EBRT) followed by brachytherapy. Common practice is to use high‐dose‐ rate (HDR) brachytherapy, mainly with Ir‐192; however, low‐dose‐rate (LDR) brachytherapy with Cs‐137 is also used. Three of our patients with cervical SCCa who were chosen to have LDR brachytherapy (Cs‐137 with tandem and ovoids) could not tolerate the prolonged treatment or applicator placement. All these patients previously received 45Gy (1.8Gy/fratction) from EBRT and well tolerated. Planned LDR treatment dose and time were for patient‐1: 42.63Gy in 73.5hr, patient‐2: 42.34Gy in 73hr, patient‐3: 41.76Gy in 72hr. Delivered LDR dose and time were: 3.75Gy in 6.5hr, OGy in Ohr, and 17.3Gy in 19.8hr, for patient‐1, ‐2 and ‐3, respectively. Two of the three patients tolerated LDR treatment partially; the second patient could not tolerate the applicator, which required immediate removal after placement. Treatments were completed with Cyberknife SBRT (CK‐SBRT)doses of 25Gy, 15Gy and 25Gy for patient‐1, ‐2 and ‐3, respectively; all had 5Gy/fraction and 3fractions/week. Prescriptions were at 80% isododelines; CTV coverages were 96.6%, 99.9% and 100% for patient‐1, ‐2 and ‐3, respectively. Results: Till their last follow‐up in February 2012, all three patients were doing fine clinically without any evidence of disease; none of these patients had any complications that could be related to CK‐SBRT. Conclusions: Appears that CK‐SBRT can be a viable treatment alternative to brachytherapy. CK‐SBRT may also be more appealing to patients and physicians for a variety of reasons such as out‐patient procedure, shorter treatment time, no need for operating room, and no need for applicator insertion and tolerance. However, extensive clinical study is warranted in this regard.
SU‐E‐T‐414: Clinical Implementation of Anthropomorphic Lung Phantom for Patient‐Specific SAbR QA
Authors: Hrycushko B , Bao Q , Medin P , Solberg T
Published: June 2012

Purpose: Dose calculations for lung stereotactic ablative radiotherapy (SAbR) are challenged by the presence of extremely heterogeneous tissue and small treatment volumes. In this work, an anthropomorphic chest phantom has been constructed for the purpose of commissioning treatment planning systems (TPS) and for patient‐specific SAbR QA. Methods: A CT scan of a realistic chest phantom containing tissue equivalent materials for the spine, ribs, and lungs was imported into a Pinnacle TPS (CCC dose algorithm) where treatment plans were created for right and left‐sided lung lesions. The phantom lungs are unique in that they contain embedded unit density spherical targets (2 and 4cm in diameter) that represent lung lesions. Plans directed to both tumors were designed for PTVs ranging from 2–5cm in diameter using 6, 10, and 15MV beams while passing RTOG 0813 dose and conformality criteria. Each plan was then exported to an Eclipse TPS (AAA dose algorithm) for dose calculation. Plans were delivered with a TrueBeam LINAC corrected for machine output. Point dose measurementswere verified with a 0.015 cc air ionization chamber placed in the center of each tumor. Results: While the majority of plans developed in Pinnacle passed conformality criteria, the dose distribution as calculated in Eclipse failed to meet the RTOG guidelines, particularly for the small tumor at higher photon energies. All point dose measurements matched both TPS within 4%. Both TPS calculated a lower point dose than measured for large PTVs at all energies, improving as PTV size decreased. Conclusions: After comparing TPS and validating calculations with point dose measurements, the phantom was clinically implemented for patient‐specific conformal SAbR QA.
SU‐E‐T‐415: Evaluation of the Effect of the Dose Buildup Region for Flattening Filter Free Beams in IMRT Plans of Lung and Sinuses
Authors: Chanyavanich V , Siddiqi A , Elder E , Dhabaan A
Published: June 2012

Purpose: The surface and buildup dose is known to be greater for flattening‐filter‐free (FFF) beams as compared to flattened beams. We compare the effect of this difference in the dose buildup region between the two beam modes, in the context of IMRT plans for the lung and sinuses. Methods: IMRT plans are delivered using both the standard flattened beam mode (6X) and flattening‐filter‐free mode (6XFFF). We compare the dosimetric differences of these two modes, for IMRT treatments for the lung and sinuses. Radiochromic films and parallel‐plate ionization chamber measurements were taken, and evaluated for the following field sizes: 2×2 cm2, 3×3 cm2, 5×5 cm2, 8×8 cm2, 10×10 cm2, and 20×20 cm2. We evaluated the area under the PDD curve (AUC) for the buildup region for both 6X and 6XFFF beams. Ten SBRT lung and five IMRT sinuses plans were generated using the EclipseTM planning system, commissioned with AAA. Plan evaluation involved the comparison of the isodose distributions between plans of the two delivery modes. Results: For small field sizes, the dose in the buildup region is greater for FFF beam than for flattened beam. The percentage differences between 6XFFF and 6X flattened beams for the following field sizes (2×2 cm2, 3×3 cm2, 5×5 cm2, 8×8 cm2, 10×10 cm2, and 20×20 cm2) are 3.4%,2.2%, 7.2%, 4.9%, 0.9%, and −0.5%, respectively. Plan evaluation of the IMRT sinus cases found approximately 3% higher maximum dose within the PTV for FFF plans. No observable differences were found between two types of SBRT lung plans. Conclusions: For small fields, the dose buildup region of FFF beam is slightly greater than that of flattened beams. This effect is more pronounced in IMRT sinus cases, leading to higher dose within the PTV. There was no difference in the SBRT lung plans between the two beam modes.
SU‐E‐T‐417: Tomotherapy Isocentricity: Is it Good Enough for Stereotactic Radiosurgery?
Authors: Xu Y , Lam S , Ingram G
Published: June 2012

Purpose: To evaluate the isocenter accuracy of radiation beam relative to the Tomotherapy linear accelerator's mechanical axis using a simplified Winston Lutz test, and to determine if Tomotherapy system is accurate for SRS applications. Methods: BrainLab Winston Lutz pointer was setup on the Tomotherapy couch to the isocenter according to the green lasers. Four static plans were created to deliver radiation with 1cm jaw opening, all MLC closed except the central two leaves, and gantry at 0, 90, 180 and 270 degree, respectively. Gafchromic EBT films were taped on a solid water slab and placed on opposite side of gantry at a fixed distance for each plan. After the exposure, the films were scanned using Vidar scanner along with a 1 cm scale drawn (Figure 1‐A, 2‐A). The images were first processed by ImageJ, using Find Edges function to enhance the visibility of the boundaries of the circles, from a metal ball in the pointer, and the rectangles defined by two central leaves and the jaws (Figure 1‐B, 2‐B). Then two diagonal lines were drawn on each rectangle, and a cross on each circle, with the cross sections representing the centers of the rectangles and circles, respectively (Figure 1‐C, 2‐C). Results: The displacements of the centers of the circles from the centers of the rectangles for the first setup (films were 11cm from the isocenter) were 0.50, 0.707, 1.0 and 0.707 millimeters for each gantry angle, respectively, with an average of 0.73mm. The displacements of second setup (films were 25cm from the isocenter) were 0.54, 1.07, 0.87 and 0.54 millimeters, with an average of 0.75mm. Conclusions: The preliminary data show that the radiation isocenter agrees with mechanical isocenter well within one millimeter tolerance required for SRS treatments, which indicated Tomotherapy system is suitable for such applications.
SU‐E‐T‐418: Evaluation of Peripheral Dose for SRS Treatment Radiations with the VIS CyberKnife: A Phantom Study
Authors: Kalantzis G , Lo A , Dieterich S
Published: June 2012

Purpose: : Stereotactic radiosurgery (SRS) procedures are known to deliver a very high dose per fraction and thus, the increased risk of secondary types of cancer due to increased peripheral dose could be a limiting factor for the long term survival of the patients. The aim of this study is to evaluate the peripheral dose (PD) received at preselected anatomical sites in an anthropomorphic phantom for treatments of intracranial lesions with the CyberRnife. Methods: Eight patients treated using the CyberRnife were selected for this study. Organs at risk and target were delineated on volumetric CT data and treatment planning (Multiplan v.4.5.0) was optimized accordingly, in order to achieve the required prescribed target dose and critical structures sparing for each patient. The final treatment plan was delivered with a CyberRnife VIS (Accuray, Inc., Sunnyvale, CA) operating with a dose rate of 1000 MU/min at a flattening filter free mode and upgraded shielding. We performed our measurements using a male anthropomorphic RANDO phantom (Alderson Research Laboratories, Inc., Stamford, CT). Groups of three TLD 100 were placed anteriorly inside RANDO at a depth of 5 cm at locations corresponding to the thyroid, breast or lung, uterus and inferior abdomen for each treatment plan. Results: The average percentage dose normalized to the prescribed dose for the thyroid gland was 0.92+0.23 % with a max of 1.95%. The maximum reduction of the PD (expressed as percentage of the prescribed dose) was 80% between the thyroid gland and the lower pelvic area. Similarly the PD normalized to the number of MU showed an average of 0.84×10−3 (cGy/MU), with a max of 0.0025 (cGy/MU) for the thyroid gland region. Conclusions: It is evident that the PD is proportional to the number of MU as well as to the prescribed dose. These correlations can be utilized to estimate the PD during intracranial treatments.
SU‐E‐T‐420: Development of a Comprehensive Linac‐Based Quality Assurance Procedure for Retrofitted Micro‐MLC SRS System
Authors: Hancock G , Pearson D
Published: June 2012

Purpose: To develop a QA tool and methodology to be used with an Elekta SL 25 Linear Accelerator retrofitted with a 3DLine Micro‐MLC gantry attachment in a room installed with the 3DLine Dynatrac camera localization system Methods: A phantom wa manufactured from clear polyester casting resign. This phantom was designed to fit within the localizing head frame to test the accuracy of the 3DLine camera system in terms of isocenter localization. The phantom was aligned using the imported isocenter coordinates from the ERGO++ TPS. The phantom consists of metallic wires intersecting at the center and extending outwards to the center of each of the six sides of the cube. Gafchromic film was taped to each side and the cube was irradiated at different gantry and couch angles to verify isocenter accuracy to within 1mm in accordance with TG 142. The films were analyzed with RIT113 software, using the 50% isodose curves as the field edges and the center of the field as marked by each of the wiresextending outwards from the isocenter. Results: Prelimina ry results of this isocenter test with the retrofitted Micro‐MLC gantry head attachment show a deviation of .72mm and −1.25 in the × and y directions respectively for the 90° gantry angle. A deviation of −1.27mm and −2.35mm in the x and y directions, respectively were measured for the 270° gantry angle. The additional sides will be irradiated and their results along the results of the additional tests will be presented at the AAPM national conference. Conclusions: After successful completion of the above procedures for SRS QA, it was determined that the phantom made with the localization wires can indeed be used to accurately and efficiently QA a linac‐based SRS system for isocenter location accuracy.
SU‐E‐T‐425: Impact of Model Based Dose Calculation Algorithm for Ir‐192 Intracavitary Brachytherapy with Shielded Applicator
Authors: Mikell J , Klopp A , Price M , Mourtada F
Published: June 2012

Purpose: To determine the impact of a grid based Boltzmann solver (GBBS) on a cohort of cervical cancer patients treated with Ir‐192 intracavitary brachytherapy with shielded applicators. Methods: Retrospective plans were generated using BrachyVision v8.8 (TPS) with GBBS Acuros v1.3.1. The study includes 24 patients that had CT planning images acquired with CT/MR compatible applicators. Using the TPS applicator library, shielded colpostats and tandem (#AL13122005) were virtually positioned to replace the applicators seen on CT. Dwell weights were based on TG43 delivering 6 Gy to point A. Four GBBS calculations were performed to assess differences from the standard of practice TG43. The different GBBS calculations were: 1) no applicator modeled, body= 1 g/cc muscle, 2) applicator modeled, body=1g/cc muscle, 3) applicator modeled, CT‐to‐material mapping with contrast (vaginal packing, rectal, Foley balloon) = 1g/cc muscle, and 4) applicator model, CT‐to‐material mapping without material overrides. The multiple GBBS calculations allow differences from TG43 to be attributed to factors representing the modeling of source and patient boundaries (scatter conditions), tissue heterogeneities, and applicators. Results: Differences between GBBS4 and TG43 at clinical dosimetric points were as follows: [mean ± standard (min, max)], Point A: − 2.5% ± 0.5% (−3.8%, −1.2%), Point B: −1.5% ± 1.0% (−3.2%, 1.1%), ICRU rectum: −8.4% ± 2.5% (−14.0%, −4.1%), D2cc rectum: −6.2% ± 2.6% (− 11.9%, −0.8%), ICRU bladder: −7.2% ± 3.6% (−15.7%, −2.1%); D2cc bladder: −3.4% ± 1.8% (−7.2%, −1.1%). Bar plots comparing the modeling factors previously listed show that applicator modeling is the largest contributor to differences from TG43. Conclusions: Clinically significant dose differences (>5%) relative to TG43 exist when using a model‐based dose calculation algorithm such as the GBBS with shielded applicators. Differences were largely due to applicator modeling, not tissue heterogeneities, source modeling, or patient boundary modeling.
SU‐E‐T‐426: Comparison of HDR Brachytherapy for Cervix Cancer Using an Adaptive Simulated Annealing Program and Oncentra‐ for Simultaneously Integrated Boost
Authors: Yao R , Liao Y , Kiel K , Templeton A , Turian J , Chu J
Published: June 2012

Purpose: High dose rate (HDR) volumetric brachytherapy is an effective method of treating advanced cervix carcinoma. Local failure is associated with multiple factors including higher maximal standardized uptake value (SUV) values in fluorodeoxyglucose positron emission tomography (FDG‐ PET) scans. The purpose of this study is to evaluate the ability to simultaneously boost regions of high SUV values using an in‐house adaptive simulated annealing (ASA) algorithm and the Oncentra® (Nucletron V.B., Veenendaal, The Netherlands) treatment planning system, thereby potentially improving local control. Methods: Five cervix cancers were evaluated for brachytherapy treatment (tandem/ring and/or interstitial needles). MRI and PET images were obtained post‐implant and fused with treatment planning CTs to define a high‐risk (HR) CTV (cervix and tumor on MRI) and GTV (volume with >50% of the maximum SUV on PET). The prescribed dose was 5–6 Gy to the HR CTV and 7–9 Gy to the GTV. Treatment plans were first generated in Oncentra® with IPSA followed by manual graphic optimization by the physician. Plans were also independently optimized using the ASA program. The two plans were compared side by side and one was chosen for treatment. Dose‐volume parameters including D90, V100 of targets, D2cc to the critical organs, and generalized equivalent uniform dose (gEUD) of all structures were compared between the ASA and the Oncentra plans. Results: Both ASA and Oncentra plans were considered acceptable by the physician in four of five cases. Two ASA plans were chosen due to better critical organ sparing and tumor coverage. Two Oncentra plans were preferred because of lower doses to critical organs. One ASA plan was not accepted because of a higher bowel dose. Conclusions: Both ASA and Oncentra® planning methods produce acceptable treatment plans for optimized brachytherapy of cervix carcinoma. Continued studies are warranted to further determine the relative strength of each method. This study was supported in part by a research grant from Varian Medical System Inc.
SU‐E‐T‐429: Image‐Guided Eye Plaque Brachytherapy Optimization: Implications for Patients at 2‐Year Follow‐up
Authors: Guida K , Niermann K , Malcolm A , Sternberg P , Munn G , Coffey C
Published: June 2012

Purpose: Episcleral eye plaque brachytherapy has been utilized in the treatment of intra‐ocular malignancies, delivering large prescription doses to the apex of the tumor. Advances in dose calculation and image guidance, via calibrated fundus images, enable localization of the tumor and determination of dose to the macula, optic disc, and lens. A two‐year post‐implant study aims to correlate dosimetry with local tumor control and changes in visual acuity, as well as assess the need for plaque optimization with respect to critical structures. Methods: A retrospective, two‐year follow‐up study of 21 patients who have received episcleral eye plaque brachytherapy at our institution was used to correlate dosimetry with clinical outcomes and evaluate the need for eye plaque optimization. BEBIG Plaque Simulator wasused in treatment planning; fundus photographs were registered for tumor localization and the TG43‐U1 formulism enabled dose calculation of I‐125‐ loaded COMS plaques. Doses to the apex, macula, and optic disc were correlated to changes in apex height and visual acuity. Selected patients were replanned using optimization strategies to reduce dose to critical structures. Results: A total of seven patients (33%) noted improved eyesight at two years. 11 (52%) patients lost at least two lines of vision at two years. Two patients saw increases in apical height (9%) within two years. Optimized eye plaque plans were able to reduce optic disc and macular doses (average 68Gy and 80Gy, respectively) by 36% and 25% on the average, while maintaining the prescribed dose. Conclusion: Image guidance and optimization are important tools that can aid in treatment of intra‐ocular malignancies, as these techniques provide physicists with the ability to spare critical structures while delivering the prescription dose, thus increasing the possibility of local control and vision sparing.
SU‐E‐T‐434: Use of Deformable Registration for Cumulated Dose Calculation in Image‐Guided Intracavitary Bracytherapy for Cervical Cancer
Authors: Wu X , Shen S , Whitley A , Kim R , Santos J De Los , Spencer S
Published: June 2012

Purpose: The minimum dose to 2 cc of tissue receiving the highest dose (D2cc) has been suggested as a surrogate measurement for risk of developing toxicity in organs at risk (OARs) in patients receiving image‐guided cervical intracavitary brachytherapy. Conventional D2cc is typically calculated by simple summation, assuming the locations of the hot spots do not vary substantially. This study explores the effect of deformable image registration in determining a more accurate D2cc of rectum, sigmoid colon, bladder and small bowel. Methods: Five patients, with 2 pelvic CT scans each performed at the time of brachytherapy planning, were evaluated. The first scan was performed with an empty bladder (an indwelling catheter was in place) and the second with a full bladder (with 180‐cc sterile water). OARs were contoured on each scan. A symmetric demons algorithm with weighted sub‐area approach was used to register images from different fractions. A pelvic prostate phantom was used to verify the method. After registration, the cumulated D2cc (ND2cc) was compared to the conventional D2cc (OD2cc) without deformable registration. Results: In all cases, the ND2cc was lower than the OD2cc. For full bladder scans, the average difference (range) for rectum, sigmoid colon, bladder and small bowel between ND2cc and OD2cc were 28.9 (1.6 – 56.6)%, 34.7 (5.7 –68.4)%, 12.1 (0.0 –27.7)%, 7.2 (1.1 –22)% respectively. For empty bladder scans, the average differences (range) for the above organs between the ND2cc and the OD2cc were 30.7 (0.75–56.3%, 20.6 (6.3–39.6)%, 7.5 (0.88–13.1)%, 12.2(0.0–22.2)% respectively. Conclusions: A significant difference was observed between the ND2cc and the OD2cc with rectum and sigmoid being the most variable and bladder the least variable. While further studies are needed to evaluate which D2cc method is more useful in predicting late toxicity, we found the conventional method often overestimates dose to OARs.
SU‐E‐T‐441: A Feasibility Study to Replace Electron Cutouts with a Motorized Electron Multileaf Collimator
Authors: Eldib A , Li J , Jin L , Ma C
Published: June 2012

Purpose: Fabrication of electron beam cutouts not only is a time consuming process but also involves the handling of cerrobend which is a toxic material. Hospital workers involved in cutout construction can actually be exposed to toxic fumes that are usually generated during the process. The aim of this work is to study the feasibility of replacing electron cutouts with our prototype motorized electron multileaf collimator (eMLC). Methods: Electron beams collimated by an eMLC have very similar penumbra to those collimated by applicators and cutouts as we already demonstrated in a previous study. However undulation of the isodose curves is expected due to the finite size of the eMLC. This may be a problem when the field edge is close to critical structure. Thus ten different breast cases that were previously treated with an electron boost were selected from our database. An inhouse Monte Carlo based treatment planning system were used for dose calculation using the patients CTs. For each patient two plans were generated one with electron beams collimated using the applicator/cutout combination and the other plan with beams collimated only by the eMLC. Treatment plan quality was compared for each patient based on dose distribution and dose volume histogram. In order to determine the optimal position of the leaves, the impact of the different leaf positioning strategies were investigated. Results: Results have shown that target coverage and critical structure sparing can be effectively achieved by electron beams collimated by eMLC. Preliminary results have shown that the out‐of‐field strategy is most conservative and would be the recommended method to define the actual leaf position for the eMLC defined field. Conclusion: The eMLC represents an effective time saving and pollution free device that can completely eliminate the need for patient specific cutouts. This work has been supported by a UICC American Cancer Society Beginning Investigators Fellowship funded by the American Cancer Society
SU‐E‐T‐442: Assessing Vertex Beam with its Dosimetric Relevance for Intracranial Tumours: A Computational Exploration
Authors: Adurkar P , Rathod S , Shah R , Jalali R , Deshpande D
Published: June 2012

Purpose: To assess the selection of the minimal vertex beam angle for the avoidance of organs at risk (OARs) in radiotherapy planning of brain tumors. Methods: Seventy patients with intracranial tumors were studied. Three field conformal plans with co‐planar or non co‐planar beam arrangements with an anterior vertex beam were used. Two methods were studied for determining the vertex angle for avoidance of eye as organ at risk. In the standard technique, the beam's eye view (BEV) was used to determine need of vertex beam. For the vertex beam, the angle was approximated to avoid eyes. In the second method, the angle from the baseplate to the posterior surface of the head and the angle from the posterior surface of the head to the inferior‐most extent of the head were measured from sagital view. The minimum vertex angle required was calculated as the complement of the sum of these angles. The dose volume histogram parameters were maintained. Results: Depending on the spatial location of the planning target volume with reference to the eyes, patients were classified into 4 types: no overlap; overlap with eye anteriorly; overlap with eye posteriorly, overlap with eye anteriorly and posteriorly. No additional angulation, positive vertex and negative vertex angles were needed for type 1, 2 and 3 respectively. The angle subtended by the isocenter to the neck rest apex was the required vertex angle for type 4. Of the 36 type 2, 3 and 4 patients the planned vertex angle was more than the calculated vertex angle by over 5 degrees in 10 patients, and less by over 5 degrees in 4 patients. Conclusions: A simple method for deriving the minimal vertex beam angle for OAR avoidance in radiotherapy planning of intracranial tumors was described and validated.
SU‐E‐T‐445: A Dosimetric Comparison of Gamma Knife Plans with Measurement Based and Image Based Skull Definitions
Authors: Zhao Z , Wang X , Yang J
Published: June 2012

Purpose: To compare gamma knife treatment plans that use skull scaling instrument measurements for skull definition vs. CT based skull definition. Methods: Eight previously treated patients who had head CT scans were randomly selected. Their CT images were imported into Leksell GammaPlan. For each patient, three dose plans were developed: 1. the reference plan. It was planned based on the standard clinical treatment planning guidelines. Measurements were used for skull definition and TMR10 for dose calculation algorithm. 2. CT TMR10 plan, a recalculation of the reference plan with CT images used for skull definition 3. CT Convolution plan, a recalculation of the TMR10 plan using convolution algorithm. The treatment times for CT TMR10 plan and CT convolution plan were set to be the same as the reference plan by adjusting the prescription dose level. Prescription dose and shots were left unchanged. Plan comparisons were carried out using minimum dose, target coverage, gradient index and selectivity. Results: On average, the minimum dose for CT TMR10 and CT Convolution plans are 1.8% and 6.5% lower than that of the reference plan. The target coverage, gradient index are comparable for these three plans. The selectivity indices for CT TMR10 and CT Convolution plans were found to be 3.1% and 9.8% higher than that of reference plan. Conclusions: This work indicates the measurement based skull definition could Result in inaccurate calculation in minimum dose to the target. The target coverage and gradient index are however uncompromised. The selectivity index was actually underestimated.