bluuebunny/crossref_metadata_2025 huggingface dataset:
SU‐GG‐T‐520: A Fault Tree Model and Analyses for Gamma Knife Stereotactic Radiosurgery
Authors: Watanabe Y
Published: August 2010

Purpose: To develop a fault tree model of Gamma Knife stereotactic radiosurgery (GKSRS) with Model 4C to identify critical errors leading to medical events. Methods and Materials: We constructed a fault tree with a medical event as the top event, i.e., the administered dose being different by more than 10% from the prescription dose. The fault tree consisted of 47 basic events and 30 logic gates (25 OR and 5 AND gates). The basic events were grouped into five types of errors: the hardware (15 events), software (8), human (22), and other (5) errors. The probabilities of basic events were subjectively estimated based on authors' experiences and some published data. By varying the probabilities of the five types of events, we calculated the probabilities of the medical event. Results: For a reference case we assumed that the probabilities of occurrences of hardware, software, human, and other types of errors were 10−7, 10−7, 10−6, and 10−7, respectively. The calculation yielded 20 events per a million treatments as the probability of the medical event. When we decreased the probability of the human errors (e.g., incorrect depth measurements, a failure in identifying treatment site, etc.) to 10−7, we found that the medical event occurred only 4 times per a million procedures. When the probabilities of other three types of errors were changed to 10−9, the medical event probability decreased only slightly to 18 per million. Conclusions: Our initial analysis showed that decreasing the human errors before and during GKSRS is the most effective among potential errors for decreasing the medical event. Most of the human errors can be prevented by performing secondary checks of the treatment parameters and setups. The model is being applied to evaluate the usefulness of patient‐specific secondary dose calculation software for prevention of hazardous errors in GKSRS procedures.
SU‐GG‐T‐521: Analysis of Average Path Length and Inhomogeneity Correction Effect for Cranial SRS Cases
Authors: Kuo L , Mechalakos J , Ballangrud A
Published: August 2010

Purpose: This study is designed to analyze the properties of beam path length at different brain locations and the effect of Inhomogeneity orrection (IC) on isocenter dose of cranial Stereotactic Radiosurgery (SRS) treatment planning. Method: Fifty five clinical cranial SRS plans, with total 63 lesions were analyzed in this study. Lesions were grouped as frontal (16 lesions), parietal (10 lesions), temporal (10 lesions), occipital (10 lesions), cerebellum (10 lesions), and acoustic neuroma (7 lesions). All plans were planned on Brainlab iPlan RT Dose 3.0.2. Pencil beam convolution dose calculation algorithm was used both with and without IC with identical beam parameters. Nine to 11 beams were used for each lesion. Average beam path length, and differences in isocenter dose for each lesion with and without IC were analyzed. Result: The mean and standard deviation (SD) of average beam path length for frontal, parietal, temporal, occipital, cerebellum and acoustic neuroma lesions without IC were 47.54±16.03mm, 45.33±8.69mm, 64.15±12.33mm, 51.27±9.73mm, 78.70±10.89mm and 79.92±7.46mm, respectively. The corresponding results with IC were 56.08±14.86mm, 52.80±8.79mm, 71.85±12.27mm, 59.97±10.20mm, 86.57±11.26mm and 90.07±7.57mm, respectively. The differences in average beam path length between with and without IC were 8.54±1.62mm, 7.47±1.04mm, 7.70±1.66mm, 8.71±1.56mm, 7.87±1.39mm and 10.15±3.52mm respectively. The mean and SD of isocenter dose for frontal, parietal, temporal, occipital, cerebellum, acoustic neuroma and all lesions combined without IC were 3.83±0.72%, 3.35±0.61%, 3.47±0.88%, 3.99±0.76%, 3.69±0.69%, 4.83 ±1.77% and 3.81±0.96% higher then the isocenter dose with IC, respectively. The maximum dose difference was 7.05% for an acoustic neuroma case and minimum dose difference was 1.59% for a frontal lobe lesion. Conclusion: In conclusion, an isodose difference between dose calculation with and without IC of about 4% was found for cranial SRS case. The effect of planning cranial SRS with or without IC increases with the average path length differences.
SU‐GG‐T‐526: Clinical Implementation of a Novel Frameless and Maskless Stereotactic Radiosurgery Treatment
Authors: Cerviño L , Lawson J , Taylor M , Jiang S , Pawlicki T
Published: August 2010

Purpose: To develop and implement the workflow of a novel frameless and maskless stereotactic radiosurgery treatment at the University of San Diego, California, including patient simulation, treatment planning, and treatment delivery. The procedure involves minimal head immobilization and continuous monitoring of the patient with surface imaging. Method and Materials: A head immobilization device made of an expandable foam is used for maximum comfort and minimal immobilization of the patient's head. During treatment, the patient's motion is continuously monitored with a surface‐imaging system that provides motion detection in 6 degrees of freedom. Based on motion information, the treatment is interrupted if motion thresholds are exceeded. End‐to‐end tests were performed to develop an optimal workflow for the treatment. The procedure development included providing instructions for patient compliance, simulation, treatment planning, imaging, and treatment delivery. The workflow was implemented and used to treat the first seven patients. Results: An optimal workflow was developed and implemented. Our first seven patients have been treated with the developed procedure. Some changes are planned based on our initial experience. Conclusion: We have successfully developed the workflow of a frameless and maskless stereotactic radiosurgery procedure. Pre‐planning for the process is important but adjustments are necessary based on actual clinical experience with the procedure. Clinical experience will be discussed as it becomes available.
SU‐GG‐T‐528: Study on MRI Image Distortions Caused by Leksell Gamma Knife Stereotactic Localization Box
Authors: Li W , Yee S , Bassano D , Liu L
Published: August 2010

Purpose: Study if the Gamma‐Knife stereotactic frame can cause image distortions due to chemical shifts at material interfaces and magnetic susceptibility differences of the fiducials and the studied objects as observed with BANG gel dosimeter studies. Method and Materials: A phantom was constructed using one side of the Gamma‐Knife localization box. Several representative materials were fixed to the frame; these include fish oil, grapes and a potato wedge. The phantom was put inside a head coil and adjusted so the diagonal fiducial sat in the horizontal plane. The MRI was done using Philips 1.5T Intera system. Five overlapped slices with thickness of 5 mm and spacing of 3 mm were obtained using the T1‐weighted spin echo (SE) with TR=100 ms, TE=15 ms, Nex=2. FOV was 140 mm × 140 mm and acquisition matrix was 512 (frequency encoding steps) × 435 (phase encoding steps) using the SENSE‐head coil, which was later reconstructed into a 512×512 grids with in‐plane resolution of 0.273 mm × 0.273 mm. The receiver bandwidth per pixel was set to 93.9 Hz, which corresponds to 0.631 mm for the fat‐water chemical shift of 3.4 ppm. The frequency encoding direction was alternated in opposite directions for a pair of scans so that the pixel displacement caused by chemical shift and susceptibly difference, if any, can be doubled when subtracted. Results: The chemical shifts of fat‐water (fish oil) were clearly observed in the image while there were no shifts in grapes or potato wedge. The fiducial markers showed about one pixel size shift in the plane, which is ∼ 0.27 mm. Considering the FE direction was switched in two images, the fiducial marker shifts in one image was ∼0.13 mm from its true position. Conclusion: The fiducials in Gamma‐knife localization box did no have any significant shifts in MRI image.
SU‐GG‐T‐530: Comparison of Coplanar and Non‐Coplanar Intensity Modulated Arc Techniques for Treatment of Intracranial Multi‐Focal Stereotactic Radiosurgery
Authors: Wang Z , Kirkpatrick J , Chang Z , Cai J , Zhuang T , Yin F
Published: August 2010

Objectives: Intensity modulated arc therapy (IMAT) can potentially be a useful technique for the treatment of multi‐focal intracranial stereotactic radiosurgery (SRS) with a single isocenter setup. This study compares the effects of coplanar and non‐coplanar beams on IMAT SRS plans. Methods: A Novalis TX linear accelerator system with a high‐definition multi‐leaf collimator (HDMLC, lead width 2.5 mm at isocenter) was used for treatment. RapidArc with beam modulation using HDMLC at each position along the arcs was used for treatment planning. Cone‐beam CT (CBCT) images were used for precise localization prior to treatment delivery. Patients with 2 to 5 lesions that were treated with conventional SRS plans using dynamic arc and conformal beams were retrospectively planned with RapidArc. Single‐arc IMAT plans are compared to 5‐arc IMAT plans for target coverage, conformity index (CI), and the volume within the low dose (5 Gy) isodose line. Results: The conformality index of non‐coplanar 5‐arc IMAT plans is better than that of the single‐arc IMAT plans. The CI's for a four lesion SRS case are 1.38, 2.29, 1.33, 1.51 for the non‐coplanar 5‐arc IMAT plan and 1.82, 2.83, 1.62, 1.97 for the single‐arc IMAT plan respectively. Figure 1 shows the comparison of the isodose distribution for non‐coplanar 5‐arc and single‐arc IMAT plans. The volume within the 5 Gy isodose line of non‐coplanar 5‐arc IMAT plans is smaller than that of single‐arc IMAT plans. Conclusion: IMAT can be used for stereotactic radiosurgery of multiple intracranial lesions with a single isocenter setup to significantly reduce the treatment time. Non‐coplanar IMAT plans are better than coplanar IMAT plans in terms of conformality and dose to the normal tissue.
SU‐GG‐T‐531: A Unique Method for End‐To‐End Quality Assurance Test for Trigeminal Neuralgia Treatments in CyberKnife
Authors: Prabhu A , Saini D , Patadiya B , Shankar R
Published: August 2010

Purpose: To develop an End‐to‐End QA test for Trigeminal‐Neuralgia in CyberKnife. Method and Materials: Functional treatments like Trigeminal‐Neuralgia are treated with small collimators(5mm–10mm) at high doses.Currently all End‐to‐End tests performed in CyberKnife utilize larger collimators(20mm–30mm).There is no specific QA for trigeminal‐path using small collimators.It is critical to have specific End‐to‐End test for this path as it lacks correction for rotational shifts during treatments.A styrofoam insert was designed to retrofit the mini‐ball cube replacing the regular‐ball cube inside the stereotactic head phantom.The insert was designed to maintain precise geometry similar to original ball cube.The head phantom was CT‐scanned with precision cut films positioned inside the mini ball cube in anterior‐superior and anterior‐left planes.A spherical target of 5mm contoured in center of mini ball cube simulates the volume of Trigeminal Nerve.A isocentric treatment plan was developed with 7.5mm collimator with trigeminal‐path.A maximum dose of 3000cGy to target is planned using RPC protocol,with 80% isodose line covering entire target volume.The plan is then delivered on Cyberknife machine with rotational limits reduced to 0.5mm during treatment delivery.The films are then scanned using Epson Flat bed scanner and analyzed. Results: The total targeting error for Trigeminal End‐to‐End test was found to be 0.3±0.1mm.An analysis of films using RPC protocol for stereotactic QA shows that the 80% isodose widths measured in the sagittal‐axial,axial‐coronal,sagittal‐coronal plane and treated volume are in close agreement with the plan.A comparison and analysis of separate films without center cut is also presented. Conclusion: This specific End‐to‐End test with small collimators provides a unique method for proper verification of trigeminal‐path.It is recommended that this test can be implemented with other End‐to‐End tests.Because of small collimator size,the passing criteria recommended for targeting error is < 0.5 mm.End‐to End test with separate films without center cut,enhances the accuracy and precision of the test.
SU‐GG‐T‐533: Remote PC Application Access to a Stereotactic Treatment Planning System SQL Database
Authors: Zhao T , Drzymala R
Published: August 2010

Purpose: To implement remote access using Matlab to a PostgreSQL database, which supports the Leksell GammaPlan (LGP) version 8.1 treatment planning system. Method and Materials: A postgreSQL Java Database Connectivity (JDBC) driver was downloaded from the official website of postgreSQL. The class path that specifies a series of files and directories, pointing to classes accessible to Matlab, must be edited to include the database driver. Table patients maintains information on patients. Table examinations maintains information on each examination. Table skulls maintains information on skull coordinates. Table plans minal information on each target . Table targets maintains information on each target. Table shots maintains information on each shot. lann examinations is related to table patients through examination ID. Table skulls is related to table examinations through examination ID. Table plans is also related to table examinations through examination ID. Table targets is related to table plans through plan ID. Table shots is related to table targets through target ID. Results: An application was implemented in Matlab. By entering the patient ID, any information about any treatment performed on any specific patient, from patient's name and birthday to the shot time of each shot, could be retrieved remotely on a PC from the database in a well‐maintained data structure. The retrieved data were further used to verify LGP treatment plans through independent dose calculation and display. Conclusion: A remote access to PostgreSQL database behind LGP is feasible in Matlab. Similar application could also be implemented in other environments like Visual C#. It is of great convenience for users who desire to develop their own applications that use information found in the LGP SQL database.
SU‐GG‐T‐537: Dosimetric Effect of Residual Tumor Motion in Phase and Amplitude‐Based Gated Lung Stereotactic Body Radiotherapy
Authors: Zhao B , Yang Y , Heron D , Huq M
Published: August 2010

Purpose To evaluate dosimetric effect of residual tumor motion in lung Stereotactic Body Radiotherapy(SBRT) with two gating methods using external markers: amplitude‐(AG) and phase‐based(PG). Methods and Materials The 3D tumor motion data were obtained from the treatment logs of 6 lung patients with various target size, each treated with 3 fractions of SBRT using Cyberknife Robotic Radiosurgical System and Synchrony™(Accuray, Sunnyvale CA). Amplitude and phase‐based gating treatments were simulated with a gating window of 5 mm. Planning target volume(PTV) was defined as physician‐contoured clinical target volume(CTV) surrounded by isotropic 5 mm margin. Each patient was prescribed with 60Gy/3fractions prescribed to 80% isodose line which covered at least 95% of the PTV. In order to reconstruct the dose with motion, the multileaf collimator(MLC) leave positions at each segment were resorted according to the residual tumor motion. The newly created MLC file was imported back to treatment planning system(Eclipse™; Varian Medical Systems, Palo Alto, CA) for dose recalculation. Results In either AG or PG, with a 5‐mm margin, CTV coverage is not affected as much as PTV. With residual tumor motion constrained within gating window(5 mm), both gating methods result in an insignificant decrease(<1%) in dose coverage on PTVs and CTVs. With baseline shift(2–10 mm), the residual motion during PG has higher probability of excursion outside gating window. The PTV D95 decreases by 9–14% with PG and 5–9% with AG. Overall, AG appears to offer greater target dose coverage compared to PG, however at the expense of reduction of the duty cycle. Compared with PG, AG increases the fractional treatment time by up to 46%, which may be undesirable in gated lung SBRT treatments. Conclusion The ideal gating method for lung SBRT requires further investigation and optimization but may be individualized dependent on the empirical pattern of motion observed in each case.
SU‐GG‐T‐540: Intensity‐Modulated Arc Therapy for Stereotactic Radiotherapy of Spinal & Paraspinal Tumors
Authors: Li X , Yang Y , Li T , Burton S , Bednarz G , Heron D , Huq M
Published: August 2010

Purpose: Stereotactic radiosurgery of metastatic spinal tumor with intensity‐modulated radiotherapy (IMRT) technique requires a long treatment time due to an extensive monitor units (MU) resulting from multiple highly intensity‐modulated beams in order to sparing adjacent spinal cord and other critical structures. This study investigates the feasibility of using intensitymodulated arc therapy (IMAT) as an alternative modality with a shorter treatment time while maintaining a compatible dosimetric performance as IMRT technique. Methods/Materials: 8 patients with spinal or paraspinal tumor were recruited in this study. All those patients were previously treated with IMRT technique, in which 18Gy or 24Gy doses were delivered in a single fraction with 11 to 13 coplanar radiation beams. Single arc and 2‐arc IMAT plans were retrospectively generated for each patients using RapidArcTM treatment planning system (Varian Medical System, Sunnyvale, CA). The previous delivered IMRT plans were chosen as a reference. The differences of following parameters between IMAT and IMRT plans were used to evaluate the plan performance: the volumes of PTV receiving 95% and 100% of prescribed dose(V95, V100), the maximum spinal cord dose (MSPDOSE) and the total monitor units (TMU). Results: For all 8 patients, the differences of V95 and V100 between single arc IMAT and IMRT plans are −5.3%±4.8% and −9.3%±7.8% , while the difference of MSPDOSE is 0.23Gy±0.87Gy. In contrary, the differences of V95 and V100 between 2‐arc IMAT and IMRT plans are −0.67%±2.01%, −1.1%±2.23% , while the difference of MSPDOSE is 0.38Gy±0.47GY. The ratios of TMU of single arc and 2‐arc IMAT plans over IMRT plan are 55%±19% and 65%±17%. Conclusion: For stereotactic radiosurgery of spinal tumor, IMRT plan provide better dose coverage than single arc IMAT plan, but 2‐arc IMAT plan is capable of providing a compatible dosimetric performance as IMRT plan while significantly reducing the treatment time.
SU‐GG‐T‐542: Accurate Spine SBRT Setup with Properly Aligned MV and CBCT Image Systems
Authors: Yang J , Du W , Li H , Luo D , McAleer M , Martel M , Gillin M , Chang E
Published: August 2010

Purpose: Spine SBRT requires high accuracy and precision in treatment setup. It is not well known how to achieve the required patient setup accuracy. We hypothesized that with properly aligned MV EPID and CBCT systems, highly accurate spine setup can be achieved reliably with a three‐stage image‐guided setup process. Method: We used Trilogy with OBI/CBCT imagers for the spine SBRT treatment. At treatment setup, we first took AP and Lat KV images to align the patient with DRR images to bring the planning isocenter near the radiation isocenter. Then we acquired CBCT images, to ensure no excessive patient rotation occurred and to determine the shifts aligned with planning CT using 3D match tools. After the shifts were made, we took AP and Lat MV portal images using EPID with a graticule plate to determine if the patient setup was correct compared to reference DRR. Both CBCT isocenter and the physical graticule were aligned within about 0.5 mm from radiation isocenter. Periodic QA tests were done to monitor the imager alignments. Prior to planning, the planning isocenter was placed strategically so that bony landmarks were aligned with centimeter tick marks in AP and lateral DRR images, where the graticule BB are located in MV images. The CBCT and MV EPID setup were performed with the assumption that abnormalities might occur in each step. Results: A total of 37 consecutive ambulant spine patients were treated in 64 sessions. The discrepancies between MV and CBCT setups were (x, y, z in mm): 0.0±0.5, −0.3±0.6, −0.1±0.4. The maximum deviation was 2 mm in each direction. Conclusion: With properly aligned imaging systems and the three‐stage setup process, we can achieve agreement of within 1 mm along each axis at about 95% confident index between MV and CBCT setups.
SU‐GG‐T‐543: The Effect of Respiratory Motion on the Dose Distribution of Free‐Breathing Stereotactic Body Radiation Therapy for Spinal Tumors
Authors: Ding C , Medin P , Timmerman R , Solberg T
Published: August 2010

Purpose: Stereotactic Body Radiation Therapy (SBRT) has been used increasingly for the management of spinal lesions. This study was performed to investigate the effect of respiratory motion on the dose distributions of spine SBRT patients. Method: 4D CT and treatment planning data for thoracic spinal lesions (T5 and T8) in two linac‐based SBRT patients was studied retrospectively. Bony structures of the spine were visually inspected in all 10 phases of the 4D CT images and there was no spinal motion observed. Tumors were contoured by radiation oncologists on the 50% phase of the 4D CT images, which corresponded to end exhalation. A single‐fraction dose of 20Gy and 14Gy was prescribed to CTV20 (gross tumor) and CTV14 (contiguous marrow cavity) respectively. Intensity modulated treatment plans were constructed on the 50% 4DCT phase and copied to all other phases of the 4D data set for comparative dose calculations. The DVHs of CTV20, CTV14, and the spinal cord were compared between actual dose, which is the sum of dose from all phases, and planned dose, which was done on 50% phase of 4D CT images. Results: The impacts of chest wall and diaphragm motion on the dose distribution were analyzed. Treatment plans that incorporated all respiratory phases resulted in improved CTV20 coverage less than 2% but the maximum dose within spinal cord was increased 3∼4%. Conclusion: Respiratory motion has a modest impact on the dose distribution of spine SBRT treatments including a 3–4% increase in cord dose. Motion control methods should be considered to reduce the uncertainties of dose delivery when motion amplitude is large.
SU‐GG‐T‐545: Analysis of Biological Effective Doses for 4D‐SBRT Using a Model Based Simulation
Authors: Oita M , Tateoka K , Tahara S , Inamura K , Kato H , Kuroda M , Katayama N , Takemoto M , Kanazawa S
Published: August 2010

Purpose: Four‐dimensional radiotherapy(4DRT) using respiratory gating system and other techniques to control patient breathing are useful for stereotactic body radiotherapy. However, treatment time in 4DRT also will be elongated by the degree of synchronization. In such a situation, the synchronization, internal margin and the target localization might have effects on the target as to tumor control probability. The purpose of this study was to investigate the relationship of biological effective dose(BED) between respiratory motion patterns, gate parameters and target positioning accuracies using a model based simulation. Method and Materials: A virtual square of dimension 512×512 matrices of interest FOVs were used in this simulation. The pseudo tumor was set on the virtual matrices. The motion of the sphere was modeled by a simple cosine curve, and a sinusoidal breathing curve simulated by mathematical equation. A minimal time interval and various motion parameters such as amplitudes, target volumes, and gate parameters were set in this simulation. Then the probability densities of the target in each parameters ware calculated. Using calculated data sets, BEDs for the target were determined by changing two modeled curves of motion, margins, and gate parameters, respectively. Results: Various patterns such as tumor size, motion function, amplitude, gate parameters and size of margins of BEDs for the target were calculated. The result showed that it depended upon target volume and gate parameter. The approximate equation of optimal internal margins for target volume of 99% BED coverage was also determined. Conclusion: In this study, we have calculated BED based optimal internal margins of the moving target. This study also showed a method of 4DRT taking into account BED. Further research is in progress to evaluate the effect of elongation of patient irradiation and shape of the target by gate parameter settings.
SU‐GG‐T‐552: Commissioning an Anthropomorphic Spine and Lung Phantom for Remote Quality Assurance of Spinal Radiosurgery
Authors: Caruthers D , Ibbott G , Shiu A , Chang E , White R , Followill D
Published: August 2010

Purpose: The RPC has developed a new phantom to ensure comparable and consistent radiation administration in spinal radiosurgery clinical trials. This study assesses the phantom's dosimetric utility. Methods: The ‘spine phantom’ is a water filled thorax with pertinent anatomy encountered in spinal radiosurgery: tumor, vertebral column, spinal canal, esophagus, heart, and lungs. The dose to the tumor volume is measured with axial and sagittal planes of radiochromic film and thermoluminescent dosimeters (TLD). The relative dose distribution is measured with the radiochromic film, and is calibrated to the absolute dose measured by the TLD. The dosimetric utility was assessed by administering a series of irradiations to the phantom. Three irradiations were administered: a four angle box plan, a seven angle conformal plan, and a nine angle IMRS plan. In each plan, at least 95% of the defined tumor volume received 8 Gy. The first two unmodulated plans were not clinically applicable; rather, they were used for dosimetric verification without intensity modulation. The third plan was tuned to the clinical specifications for organ tolerances delineated by the RTOG protocol 0631. The planned and administered doses were then registered via pinpricks, and compared using point dose measurements, dose profiles, isodose distributions, and gamma analyses. Results: The minimum number of pixels passing the gamma analyses was set at 95% at 5% dose difference and 3 mm distance to agreement, based on previous experience at the RPC. Each irradiation showed acceptable conformity in the qualitative assessments and exceeded the 95% passing rate at the 5% / 3mm criteria. Conclusions: Treatment planning and measurement in the new spinal phantom demonstrated acceptable conformality at the 5% / 3mm criteria, and the spine phantom should be a useful tool for the remote assessment of other institution's treatment planning and dose delivery regimen.
SU‐GG‐T‐553: Comparison of RTOG 0618, 0813, 0915, and Accuray STARS SBRT Lung Protocols
Authors: Grimm J , Xue J , Asbell S , Pahlajani N , Chen Y , Croce R , Chew M , Karasick T , LaCouture T
Published: August 2010

Purpose: Recently several stereotactic body radiation therapy (SBRT) lung protocols have been developed. To gain insight into the dose tolerance limits and the fractionation schemes, this study is a comparison of the RTOG 0618, 0813, 0915, and Accuray STARS SBRT lung protocols. Method and Materials: To make a fair, unbiased comparison, all selected patients were evaluated according to all six protocol regimens. The DVH Evaluator software tool, which can evaluate a treatment plan according to all selected protocols, was used. Results: When comparing the various dose schemas and normal tissue tolerance limits utilized in currently available protocols, the Accuray STARS protocol incorporates radiobiologically higher dose tolerance limits. There is some discordance in the use of radiobiological equivalent doses as demonstrated with spinal cord limits heavily dependent upon fractionation schema and lung dose limits very similar to standard fractionated target doses. When we compare and contrast the various protocols RTOG 0618 incorporates the highest effective dose, yet some normal tissue tolerance targets are more strict than in other similar trials. Comparative analysis of included patients illustrates that three patients would have received treatment in full compliance with three of six protocols. Another patient was noted to exceed the spinal cord tolerance only for the single fraction arm of RTOG 0915 and skin limit for RTOG 0618, while meeting the esophageal dose limits in three of six protocols. Conclusion: This opens a dialogue regarding SBRT dose limits for the potential delivery of higher effective target doses utilizing the available ranges of acceptable normal tissue tolerance levels for the development of individualized treatment maximizing the risk‐benefit ratio of normal tissue to target dose delivery in an attempt to provide improved outcome for patients.
SU‐GG‐T‐554: A Computer Aided Approach to Accurately Cutting Custom Bolus
Authors: Sheu R , Svoboda A , Lo Y
Published: August 2010

Purpose: To refine the custom bolus making procedure and improve the conformity of custom bolus on any irregular 3D surface. Method and Materials: A computer‐aided system was developed to extract the custom bolus created in treatment planning system and unwrap/map it into a 2‐D plane. Our 3D surface unwrapping technique works for any arbitrary 3D surface. The number of vertices and polygons were user‐defined variable to determine the resolution of the 3D surface approximates custom bolus designed in treatment planning system. The custom design bolus can be made by cutting along the print‐out and folding back into 3D. Three different types of custom boluses (larynx, node, and full scalp) were tested in this study. The results were evaluated by comparing CT images of planning and actual bolus placed on anthropomorphic humanoid phantom. Results: All three custom boluses can be unwrapped and mapped into cutout patterns onto a 2D plane. The extracted 3D surfaces contain 20, 114, 88 vertices and 18, 180, 126 polygons to approximate larynx, node and full scalp bolus respectively. The resolution of approximation surface depends on the curvature of surface. The result showed a mesh size up to 3 cm was adequate to approximate the most complex case e.g., scalp bolus in this study. The CT scan showed the actual bolus made by using this computer‐aided system matched well with the boluses designed in treatment planning system by planners. Compare to convention way of manually‐made bolus, the computer‐aid 3D to 2D system significantly reduced the discrepancy in shape and size between planning and actual bolus. Excess material causing air gap and non‐uniform thickness were minimized. Conclusion: Computer‐aided system provides an accurate and precise way to customize bolus. With our 3D surface unwrap technique, a conformal bolus can be made without tailoring bolus on patient directly.
SU‐GG‐T‐555: Dosimetric Effect of Couch Position Variation for Intensity Modulated Arc Therapy
Authors: Popple R , Duan J , Shen S , Wu X , Brezovich I
Published: August 2010

Purpose: To investigate the dosimetric effect of variation in treatment couch positioning relative to isocenter for intensity modulated arc therapy. Method and Materials: 5 RapidArc (Varian Medical Systems, Palo Alto, CA) treatment plans (2 head and neck, 2 prostate, and a lung) were selected that were calculated including the treatment couch with the support rails in the lateral position. Calculations were done for the couch positioned ±3 cm relative to isocenter and for the support rails in the center position. Dose‐volume histograms were compared for the targets and critical structures. Results: For ±3 cm shift, the largest deviation of the minimum, maximum, and mean target doses was 0.2%. For the critical structures, the largest deviation of the minimum, maximum, and mean doses was 0.6%, 0.4%, and 0.4%, respectively. The largest deviations occurred for the head and neck plans. Excluding the head and neck plans, the largest deviation of the minimum, maximum, and mean critical structure doses was 0.2%. For the support rails in the center, largest deviation of the minimum, maximum, and mean target doses from the planned lateral rail position was 1.6%, 1.6%, and 1.1%, respectively. For the critical structures, the largest deviation of the minimum, maximum, and mean doses from the planned lateral rail position plan was 0.9%, 3.8%, and 1.2%, respectively. Conclusion: The effect of couch shifts of up to 3 cm relative to isocenter is small. For positioning systems indexed to the treatment couch, the daily variation of the couch position can be safely ignored. However, for positioning systems that are independent of the couch, care must be taken to limit the daily deviation. The support rail positions have a more significant effect than couch position relative to isocenter.

Conflict of Interest: First author has a sponsored research agreement with Varian.
SU‐GG‐T‐558: Effective MLC Leaf Width for RapidArc Plans: Can Coarse Resolution Give Fine Results?
Authors: Buckey C , Stathakis S , Mihailidis D , Mavroidis P , Papanikolaou N
Published: August 2010

Purpose: To determine the feasibility of using a multileaf collimator (MLC) with leaf widths of 1.0 cm, to obtain dosimetrically similar results to an MLC with 0.5 cm leaf widths, for RapidArc treatments. Method and Materials: The CT data of 10 patients, who had recently completed treatment at our clinic, were loaded into the Eclipse treatment planning system. Two plans were generated for each patient: a single‐arc plan using a 0.5 cm MLC leaf width, and a two‐arc plan using a 1.0 cm MLC leaf width, with the isocenter of one field shifted by 0.5 cm in the direction perpendicular to leaf travel. The optimization parameters used were the same as those employed during the original patient treatment plan. Both plans were optimized by Eclipse's arc optimization algorithm, without any manipulation of constraints or other outside intervention once the process was underway. All plans were normalized such that 95% of the target volume received 100% of the prescribed dose. Results: For all 10 patients, the two‐arc 1.0 cm MLC plans were able to return at least equivalent results for PTV coverage; in all but one instance the two arcs had superior PTV coverage and homogeniety as compared to the single‐arc 0.5 cm MLC plan. Dose to organs at risk (OARs) were not identical, but no trend for higher OAR doses for a particular leaf width can be established. On average, the number of monitor units used for the two‐arc 1.0 cm MLC plans increased by 15%, when compared to the single‐arc 0.5 cm MLC plans. Conclusion: By using two arcs, with an isocenter shift of half a leaf‐width, it is possible for centers using a 1.0 cm MLC leaf‐width to plan RapidArc cases that are dosimetrically similar to those planned with a 0.5 cm leaf width.
SU‐GG‐T‐559: Novel Treatment Technique for Conjunctival Lymphoma of the Eye
Authors: Dikeman K , Mills M
Published: August 2010

Purpose: This study was designed to develop a new technique to better treat conjunctival lymphoma while limiting dose to the lens. Previous texts have suggested using a “hanging block” approach, for example a lead filled straw suspended in the center of the treatment field. The possibility of using a baby nipple has several advantages, including ease and reproducibility of set‐up, safety for the patient (the baby nipple moves with the patient), and an ideal dose distribution due to the shaping and slight attenuation of the radiation beam through the plastic. Materials/Methods: A typical baby bottle nipple (Philips, 3–6 months) was placed in a 4 cm diameter circle Cerrobend cut‐out and placed in the 6×6 electron cone. A lead rod, a Cerrobend rod, and spherical tungsten fishing weights were each placed in the tip of the bottle nipple and measurements were made with radiochromic film in the sagittal and coronal planes for 6 and 9 MeV beams. Dose distributions were evaluated to determine the best block and ideal energy for the patient's disease. Results: The Cerrobend rod did the best job of blocking the radiation in the center of the field while still treating the area corresponding to the white of the eye. It was determined that energy of 6 MeV generated adequate disease coverage. The patient seen at the Brown Cancer Center was prescribed 26 Gy to the 90% isodose line in 13 fractions at 6 MeV for each eye. The bottle nipple was used to block the lens each day and yielded a successful treatment. Conclusions: This technique is a excellent option for certain diseases of the eye. It provides a safe yet simple way to effectively treat the patient while shielding the lens from radiation that even at relatively low doses could form cataracts in the eye.
SU‐GG‐T‐560: Method for Optimizing Geometric Beam Parameters for Volumetric Modulated Arc Therapy of Multiple Brain Metastases
Authors: Kang J , Ford E , Smith K , Wong J , McNutt T
Published: August 2010

Purpose: Volumetric modulated arc therapy (VMAT) treatment can be a patient friendly delivery choice for brain stereotactic radiosurgery especially in the setting of multiple lesions. Depending on the geometric relationship of the targets, one lesion may “shadow” another during portions of the arc which affects plan quality. With the proper table and collimator angles, however, target shadowing can be reduced or avoided. Our goal is to establish a method to determine the best angles for VMAT treatment. Method and Materials: The method consists of four steps: 1) define the vector of Beam‐Eye‐View(BEV)‐Y axis in the treatment planning CT coordinates which defines the plane of MLC travel. 2) project each lesion onto the BEV y‐axis vector. 3) determine the best table and collimator angle that minimizes MLC leaf sharing between lesions. 4) using these table and collimator angles generate an optimized VMAT plan and evaluate the plan quality. We performed test plans on 3 patients and demonstrate that the proposed method resulted in superior plans for several example cases of targets of various sizes and locations. Depending on the lesions' geometric relationship multiple arcs may be required. Results: We tested the method on three example cases with targets of various locations in the brain and sizes ranging from 1.18 to 17.86 cm3. Applying the optimized geometric parameter to generate VMAT plan, the volume of normal brain receiving 12Gy or more was reduced more than 6.1% for all cases. The plan homogeneity (D2% –D95%) was also improved from 5.88± 1.21Gy to 5.21± 0.93Gy vs. a VMAT plan with the manufacturer recommended table and collimator angles. Conclusion: The projection method is a promising tool for improving VMAT delivery in the setting of multiple brain metastases.
SU‐GG‐T‐561: The Effect of MLC Leaf Width in RapidArc Plan Optimization
Authors: Stathakis S , Papanikolaou N , Buckey C , Mavroidis P , Mihailidis D , Shi C , Gutiérrez A , Esquivel C
Published: August 2010

Purpose: To evaluate the effect of the leaf width during RapidArc optimization Materials and Methods: Twenty patients were randomly selected from out department's database that were previously planned using the Eclipse treatment planning system with the RapidArc optimization capabilities. Prostate, brain, lung, liver and head and neck were the sites that were evaluated in this study. For each patient three plans were generated using linear accelerators with 120 Millennium, 120 High definition and 80 Millennium multileaf collimators. Each plan consisted of 2 arcs of the same span. The collimators of each arc were rotated by 90 degrees with respect to each other. The isocenter location and optimization parameters were kept the same for each plan within each patient. The DVH and isodose distributions were chosen for the evaluation and comparison of the plans. Results: The comparison of the DVHs showed that the high definition MLC produced superior plans with respect to PTV coverage. Also, lower critical organ doses were calculated for the high definition MLC optimized. The millennium 80 MLC produced the most inferior plans for all cases. The plan quality was similar between the 120 and 80 millennium MLC for targets that were lateral with the respect to the central axis of the beam where the MLC leaf width was the same. Conclusions: The MLC leaf width plays an important role during the plan optimization for centrally located targets. A superior plan is achieved with smaller MLC leaf width.
SU‐GG‐T‐562: Dosimetric Comparison of 3D Conformal Proton Therapy and IMRT for Retroperitoneal‐Sarcoma Treatments
Authors: Slopsema R , Swanson E , Louis D , Flampouri S , Li Z , Indelicate D
Published: August 2010

Purpose: To determine the difference in dose to target and critical structures when treating retroperitoneal sarcoma with 3D conformal proton therapy or IMRT, and to evaluate the sensitivity to setup errors for both techniques. Method and Materials: 3D conformal proton, IMRT, and 3D conformal photon plans were made for 8 patients. GTV volumes varied between 277 and 3482cc. A uniform 2cm GTV to CTV margin was applied, followed by a 0.5cm CTV to PTV margin. Laterally the proton plans were conformed to the PTV, while a distal margin equal to 1.5% of the range plus 1mm was applied to the CTV. Range‐compensator smearing of 0.5cm was used. For proton plans 2 to 4 beams were used; 5 to 8 for photon plans. Prescribed dose is 50.4CGE. To evaluate the effect of setup uncertainty on target coverage and dose to critical structures, proton and IMRT plans were re‐calculated for 5mm shifts along the cardinal axes. Pinnacle (Philips) treatment planning system was used to generate photon plans; Eclipse (Varian) to generate proton plans. Results: The bowel V15Gy is lower in the proton plan (19.9% versus 51.7% averaged over all patients), while V45Gy is near identical (6.9% versus 6.3%). Mean dose to liver is 5.8 and 13.2Gy for proton and IMRT respectively. When not involved in the CTV, the V20Gy of the ipsilateral kidney is significantly lower in proton plans (by up to 45%). Both techniques avoid the contralateral kidney giving comparable V20Gy (0.2% and 2.3%). Conclusion: Conformity of the dose to the target is comparable for both techniques. Critical‐structure volumes receiving high doses are comparable, while the volume receiving low dose is reduced significantly in proton plans. IMRT plans are less sensitive to variations in setup.
SU‐GG‐T‐563: System for Treating Tumors with Necrotic Cores: Modeling and Experimental Results
Authors: Wu D. H. , Garrett L. , Miao H. , Wong M. E. , Liu H.
Published: August 2010

Purpose: Our goal was to develop and refine a method that can target a periphery of the tumor with central necrosis. By translation of a collimated source off‐axis it is possible to deliver an eccentric cylindrical ‘mantle’. Overall requirements to target the necrotic center are decreased resulting in a dose savings to the sensitive normal tissue that surrounds the tumor that would normally be in the path when using techniques as 3DCRT. For radiation therapy planning, we applied DCE‐MRI to evaluate the vascular supply to the interior as an indicator to locate the locality of the necrotic core. Method and Materials: We evaluated through experimentation and Monte Carlo simulation using MCNPx. For empirical validation, we applied a 40 kVp, 20W source for 30 seconds. The X‐ray tube to lead slit = 36 inches, X‐ray tube to center‐of‐object = 39.5 inches and X‐ray tube‐to‐detector surface = 44.75 inches (for demonstration purposes of the technology only) Results: We were able to achieve interrater reliability in spatial locality comparisons from 80.4%, decreasing to 41.3% on a series of MCNP models and experiments. Regions of does were highly dependent on beam collimation and beam focusing. Monte Carlo modeling could be used to enhance targeting. Conclusion: Multi‐source and off‐axis Radiation Therapy Methods have a potential to reduce dose in targets particularly if necrotic cores can be localized. Agreement between experimental results and Monte Carlo Simulation provide ample information for planning. Such techniques have a future with advancing technologies such as Multileaf collimator which can enhance localization potential. Offsetting and multiple sources (not necessary, but can provide additional speed to the therapy) are used not only to spare dose to the center but reduce the dose in the path of the radiation that inefficiently may target regions of necrosis, as seen with such techniques as 3DCRT.
SU‐GG‐T‐571: A Comparative Study between RapidArc and IMRT Techniques Applied to H&N Cancers
Authors: Nes E , Blough M , Mira J
Published: August 2010

Purpose/Objectives RapidArc (RA) is a new technique in which the linear accelerator rotates around the patient while the radiadion is shaped in small beams that are aimed at the tumor from multiple angles. The purpose of this study is to evaluate RA relative to the conventional Intensity‐Modulated Radiotherapy (IMRT) for head and neck (H&N) cancers. Materials/Methods Twelve patients with H&N cancers were selected for the study. All RA and IMRT plans were created on Eclipse 8.6. Photon beams of 6 or 15 MV from a Trilogy machine outfitted with imaging capabilities were used for all the IMRT plans and only 6 MV photon beams for RA plans. The organs at risk (OAR) included parotid glands, brainsteam, cord, mucosa and mandibula. RA plans consisted of one or two full or partial arcs while IMRT plans consisted of 5 or more fields. Evaluation of the plans was performed by means of parameters such as conformity index CI90% and homogeneity index. For OAR evaluation included maximum dose expressed as D1% and mean dose. Results Both RA and IMRT techniques reached planning objective for the PTV with CI90% slightly superior for the IMRT. The hot spots were inside the PTV for RA plans. RA mean and maximum doses for parotids were equal or smaller than the IMRT ones, with relative reductions of up to 20%. IMRT plans failed to achieve the planning objective for parotid glands in case of five of the patients. RA technique proved to be much faster than IMRT, calculated treatment time for RA being below four min. Conclusions RapidArc technique is superior or at least equal to IMRT for H&N cancers. Benefits of RapidArc over IMRT consist of similar PTV coverage, better OAR sparing with hot spots placed inside target, fewer monitor units, shorter treatment times and less patient motion.
SU‐GG‐T‐572: Are Constant Dose Rate VMAT Plans Clinically Acceptable? — A Site‐By‐Site Plan Comparison Study
Authors: Palaniswaamy G , Moore K , Rangaraj D , Santanam L , Low D
Published: August 2010

Purpose: To evaluate the adequacy of constant dose rate (CDR) VMAT plans for clinical use of commonly treated sites. The clinical acceptability of CDR VMAT plans was evaluated based on plan quality comparisons against clinically used IMRT plans. Methods: Several patient treatments planned using IMRT were considered for the study and replanned using Pinnacle 9.0 (SmartArc) treatment planning system. Several clinical sites were considered for this study — prostate and prostate bed (4 patients), pelvis (1), breast (1) and head and neck (H&N) (2). The quality of CDR plans normalized to PTV mean dose was compared against static field IMRT plans based on dose conformality, planning target volume (PTV) coverage and dose volume histograms (DVHs) of organs at risk (OAR). Results: Plan qualities of prostate, prostate bed and pelvis plans were evaluated based on rectum and bladder DVHs, PTV coverage and dose conformality. Prostate and prostate bed: CDR plan was found to be clinically acceptable with better OAR sparing and delivery time. Pelvis: CDR plan was lacking coverage and conformality and didn't have a significant advantage over the IMRT plan. Breast: Plan quality was evaluated based on PTV coverage, isodose conformality and DVHs generated for OARs — lungs, heart and cord. The CDR plan was clinically unacceptable, primarily due to low doses (<10Gy) covering a large volume of the lungs. H&N plans were evaluated based on PTV coverage, dose conformality and dose to parotids, cord and submandibular glands. The CDR plans were clinically unacceptable and didn't show significant advantage over IMRT plans beyond the reduced treatment time. Conclusion: The CDR plans for prostate and prostate bed were clinically acceptable and found to be equivalent, if not superior to IMRT plans. For the other sites investigated ‐ pelvis, breast, and H&N, static field IMRT plans proved to be superior to CDR plans.