bluuebunny/crossref_metadata_2025 huggingface dataset:
SU‐GG‐T‐422: Radiobiological Effectiveness of 3D‐Conformal Radiotherapy, MLC‐Based IMRT and Helical Tomotherapy in Lung Cancer
Authors: Delichas M , Ferreira B , Shi C , Gutiérrez A , Lind B , Papanikolaou N , Mavroidis P
Published: July 2008

Purpose: To investigate the clinical efficacy and effectiveness of 3D‐Conformal radiotherapy, Intensity Modulated Radiotherapy (IMRT) using Multileaf Collimators (MLC) and Helical Tomotherapy (HT), by evaluating dosimetric and radiobiological measures of a lung cancer case. Method and Materials: A typical case of lung cancer has been investigated by developing a 3D‐Conformal treatment plan, a linac MLC‐based step‐and‐shoot IMRT plan and a Helical Tomotherapy plan. The treatment plans of the 3D‐Conformal and the MLC‐based IMRT were developed on the Philips treatment planning station using the Pinnacle 7.6 software release while the dedicated Tomotherapy treatment planning station was used for the HT plan. With the use of the complication‐free tumor control probability, P+ and the biologically effective uniform dose, D̿ concept as the common prescription point of the plans, the three different treatment plans were compared based on radiobiological measures. Results: The applied plan evaluation method shows that in this lung cancer case the MLC‐based IMRT and the HT treatment plans are almost equivalent over the clinically useful dose prescription range, whereas the 3D‐Conformal plan appears to be quite inferior than the other two modalities. More specifically, the 3D‐Conformal, MLC‐based IMRT and HT treatment plans give a P+ of 55.4%, 72.9% and 66.9%, for a D̿ to the target volume of 57.0Gy, 66.9Gy and 64.0Gy, respectively. If a higher than 5% risk for complications could be allowed, the complication‐free tumor control could be increased by almost 5% compared to the initial dose prescription. Conclusion: In comparison to 3D‐Conformal radiotherapy, both the MLC based‐IMRT and HT can better encompass the often large ITV while minimizing the volume of the OARs receiving high dose. The use of P ‐ D̿ diagrams can compliment the traditional tools of evaluation such as DVHs, in order to compare and effectively evaluate different treatment plans.
SU‐GG‐T‐427: Dose Painting to Combat Tumor Hypoxia While Sparing Urethra in Prostate IMRT: A Biologically‐Based Adaptive Approach Accounting for Setup Uncertainties and Organ Motion
Authors: Yin L , Moiseenko V , Otto K , Liu M , North J
Published: July 2008

Purpose: To explore means of incorporating prostate motion and setup uncertainties into biologically‐based adaptive IMRT optimization for Volumetric Modulated Arc Therapy (VMAT). Method and Materials: IMRT planning objectives were set to produce a dose distribution, which, after accounting for setup uncertainties and prostate motion, meets the following requirements. 1) The CTV is covered with the desired dose; 2) Hypoxia volumes arbitrarily drawn inside CTV are boosted; 3) Desired urethra sparing is achieved. The objective function was based on equivalent uniform dose (EUD). Repositioning data from five prostate patients with implanted fiducial markers were used. Systematic and random uncertainties for each patient were incorporated into an in‐house treatment planning platform for VMAT. During the optimization, the dose matrix was shifted based on the systematic error, then convolved with a pre‐calculated Gaussian kernel to account for random errors. For each of these patients seven plans were generated using uncertainty data accumulated in 5 fraction increments. Results: The urethra EUD increased by >5Gy when the dose distribution from the static plan was propagated to account for geometrical uncertainties. In contrast, if uncertainties were accounted for in planning, urethra EUD was reduced without compromising CTV coverage (EUD decreased <1.5Gy). For four of five patients, the repositioning data from the first five fractions was sufficient to account for uncertainty in planning. In the proposed biologically‐based optimization inclusive of uncertainties, the PTV is not required. Appropriate dose coverage of the CTV is intrinsically produced by the optimization procedure. Because this coverage is typically tight and patient specific, improved normal tissue sparing was achieved. The rectal EUD was lowered by up to 23Gy compared to static plans that utilize the generic PTV concept. Conclusion: Our approach allows the inclusion of geometrical uncertainties into biologically‐based optimization of IMRT. Partial uncertainty data are sufficient to produce an optimal plan.
SU‐GG‐T‐428: Incorporating Biological and Spatial Dose Information Can Change the Predicted Outcome of Radiotherapy Treatment Plans
Authors: Zhao B , Joiner M , Huang Y , Liao Y , Burmeister J
Published: July 2008

Purpose: To incorporate structure radiosensitivity (α/β) and spatial dose variations (cell migration and daily setup uncertainty) into treatment plan evaluation for the Varian Eclipse Treatment Planning System (VETPS). Method and materials: Treatment plans were created using the VETPS from which Dicom RT files were exported to a Matlab‐based interface for analysis. Spatial variation was modeled using a Dose Convolution Filter which applies Gaussian dose smoothing. Its effect on static dose distribution was evaluated theoretically by creating virtual dose matrices. 3D physical dose was then converted to equivalent dose in 2Gy fractions (EQD2), thus incorporating relative radiosensitivity. Generalized equivalent uniform dose (gEUD) was then derived for each structure. Tumor control probability (TCP) and normal tissue complication probability (NTCP) were then calculated using Poisson and logistic models, respectively. Results: Conversion of physical dose using DCF and EQD2 can increase or decrease TCP or NTCP depending on the dose distribution and structure radiosensitivity, thus potentially changing the ranking order between plans. DCF application alone could result in changes of approximately 10% in TCP and/or NTCP. After converting static dose matrices to include spatial dose information and biological information (using the DCF, EQD2, and gEUD formalisms), significant changes of 60% in TCP and 25% in NTCP were observed, respectively. Plans with similar DVHs may respond to the application of DCF and EQD2 quite differently, thus significantly altering predicted TCP or NTCP and plan ranking between rival plans. Conclusion: Treatment plan evaluation based on DVHs alone excludes both spatial and biological information about the dose distribution. Incorporating these spatial and biological parameters may significantly alter predicted TCP and NTCP and could also alter the ranking order of rival treatment plans. Conflict of interest: Research partially supported by Varian Medical Systems.
SU‐GG‐T‐429: Dose‐Mass‐Histogram (DMH) Vs. Dose‐Volume Histogram (DVH) in Predicting Lung Complications
Authors: Mavroidis P , Plataniotis G , Gorka M , Hyodynmaa S , Papanikolaou N , Lind B
Published: July 2008

Purpose: In radiotherapy for breast cancer, it is very important to estimate the expected lung complications. In this study, it is examined whether the Dose Mass Histogram (DMH) concept can be better associated with the expected lung complications than the widely used Dose Volume Histogram (DVH) concept. Method and Mataerials: The problem was investigated theoretically by applying two hypothetical dose distributions (Gaussian and Semi‐Gaussian shaped) on two lungs of uniform and varying densities. Furthermore, a group of breast cancer patients was used to clinically quantify the difference between DVHs and DMHs. These patients were treated with resection and irradiation with two tangential fields. The influence of the deviation between DVHs and DMHs on the clinical endpoint of radiation pneumonitis is estimated by using the Relative Seriality and LKB models. The biological equivalent of the corresponding difference in their mean doses was estimated by the Biologically Effective Uniform Dose (BEUD) and Equivalent Uniform Dose (EUD) concepts, respectively. Results: The relation of the DVHs and DMHs varies depending on the underlying cell density distribution and the applied dose distribution. The range of their deviation in terms of expected clinical outcome was large both for the theoretical and the clinical studies. For the group of patients, according to the DVH, lung receives 9.82 Gy, whereas according to the DMH it receives 8.06 Gy (1.76 Gy difference). Interpreting these figures in terms of lung response, DVH gives 0.30% probability for lung complications compared to 0.10% given by DMH for the Relative Seriality model. Similarly, for the LKB model the corresponding values are 2.62% and 1.85%, respectively. Conclusion: Concluding, the expected lung complications appear to be overestimated when using the DVH concept, whereas the effectiveness of the dose distribution is closer related to the radiation effects when using the DMH concept.
SU‐GG‐T‐431: Radiobiological Evaluation On the Four‐Dimensional Stereotactic Radiotherapy Plan
Authors: Su F , Shi C , Mavroidis P , Papanikolaou N
Published: July 2008

Purpose: To evaluate the four‐dimensional (4D) stereotactic radiotherapy plan for the lung patient using biologically effective uniform dose (BEUD) and to compare this 4D treatment plan with MLC‐based intensity modulated radiation therapy (IMRT) plans for fixed respiratory phases. Method and Materials: One patient with lung cancer, who was previously treated with stereotactic body radiotherapy (SBRT), was chosen to re‐planned with the 4D stereotactic radiotherapy technique using the Pinnacle3 treatment planning system. The 4D treatment plan was developed based on a set of 4DCT images divided into ten respiratory phases. Doses to the target were prescribed as 3 fractions of 16 Gy to 95% of planning target volume (PTV). BEUD calculation was performed to evaluate the final composite 4D dose distribution. Results: For this dose prescription, the complication‐free tumor control probability, P+ value, is 78.6% for the 4D stereotactic plan with a mean dose of 49.9 Gy to PTV and the BEUD of 49.9 Gy as well. The total control probability, PB, is 98.5% and the total complication probability, PI, is 20% due to high maximum dose to the lung, which is proximal to the PTV. When compared with fixed‐phase IMRT plans, the dose distributions were very similar; the P+ value of the 4D plan, however, was higher, but was not substantially different from values of fixed‐phase plans. Conclusion: From our radiobiological analysis of this lung SBRT case, the 4D plan was superior, but not significant improved from MLC‐based IMRT plans for fixed phases. Not only the mean doses to PTV of these plans were very close, but also were P+ and BEUDs of these plans. Dose distributions of the 4D plan can be further optimized to a maximum P+ of 86.5% when achieving BEUDB of 42.95 Gy by sparing the lung to make PI reach as low as 7.2%.
SU‐GG‐T‐432: Dose‐Volume Histogram Modeling Using the Gaussian Error Function in Radiotherapy
Authors: Chow J , Markel D , Jiang R
Published: July 2008

Purpose: A mathematical method, containing pair of error and complimentary error functions, is proposed to characterize cumulative dose‐volume histogram (cDVH). The model was illustrated by 5‐beam prostate IMRT plans with shifted clinical target volumes (CTVs) representing the interfraction organ motion. Method and Materials: A phantom including prostate, seminal vesicle, rectum and bladder was created with planning target volume (PTV) 0.5 cm margin from the prostate (CTV). The CTV was shifted 1 cm to the anterior, posterior, left, right, superior and inferior directions. Pinnacle3 TPS was used for IMRT plans with each shifted CTV. The calculated cDVHs for the plans were exported to MATLAB for modeling using the mathematical formula: , where (a1, b1 and c1) and (a2, b2 and c2) are parameters for the error and complimentary error function, respectively. D and V are the dose and volume of the cDVH, respectively. Results: It is found that parameters a1,2, vary with the relative volume of the cDVH. The slope of the cDVH after the curve drop‐off can be adjusted by b1,2, while a variation of c1,2 can change where the cDVH drops off from the normalized volume close to 1. Corresponding cDVHs calculated using the model agree well with those calculated from Pinnacle3. Parameters a1 and b1 are almost constant with variations of a2 and b2. However, parameter c1 is only constant when the CTV displacement is within the PTV, but changes with c2 when the CTV is shifted outside the PTV. It shows that cDVH variation due to parameter c would happen when the margin between the CTV and PTV is underestimated. Conclusion: This error function model for the cDVH provides a new view to study the dose‐volume relationship of a region of interest in radiotherapy.
SU‐GG‐T‐434: Submillimeter XKnife End‐To‐End Alignment Accuracy Using SAlinac and the Lucy Phantom
Authors: Grimm J , Grimm S , Jacob D , Sarkar A , Simpson L , Steffy E , Brubaker P
Published: July 2008

Purpose: To achieve submillimeter end‐to‐end alignment accuracy on a conventional linac with stereotactic cones using the Stereotactic Alignment for Linac (S. A. linac) system. Method and Materials: Previous conference publications have already shown that the SAlinac system can enable the user to align the Winston‐Lutz radiopaque ball and the room lasers to within 0.2 mm of the isocenter. In this study, the Lucy phantom from Standard Imaging has been used to show that submillimeter end‐to‐end alignment accuracy can also be achieved with the SAlinac system. In the 20 years since the seminal paper by Winston and Lutz, we have not seen any other system that can achieve submillimeter end‐to‐end alignment accuracy on a conventional gantry mounted linear accelerator. The SAlinac system captures live images of the laser targets with digital cameras and combines this information with analysis of the Winston‐Lutz test to provide near‐real time alignment advice. As the user repositions the Winston‐Lutz ball, the room lasers, or the head frame, the SAlinac system continuously captures and processes live digital images to provide updated repositioning advice. Results: To prepare for this study, two years ago the SAlinac system was used to correct a 0.5 mm gantry skew on the Siemens Mevatron MXE2 at Christiana Hospital, and four months ago the ceiling and wall lasers were remounted to minimize laser divergence. In this study we CT scanned the Lucy phantom, generated a treatment plan, and delivered the plan to expose the Gafchromic film inside the phantom. The final end‐to‐end results were x=−0.73, y=0.27, z=−0.41 mm, for a total 3D error of 0.87 mm. Conclusion: Submillimeter end‐to‐end alignment accuracy has been achieved — it is now the Submillimeter Knife (SKnife).
SU‐GG‐T‐445: Dosimetric Analysis of Attenuation in the Leksell Gamma Knife PERFEXION Calibration Phantom Adaptor
Authors: Bhatnagar J , Novotny J , Quader M , Huq M
Published: July 2008

Purpose: The purpose of this study was to evaluate the attenuation of 16 mm beams caused by the ELEKTA calibration phantom adaptor used for the new Leksell Gamma knife PERFEXION (PFX). Method and Materials: For the calibration of PFX, Elekta AB provides a 160 mm diameter polystyrene phantom with an aluminum adaptor of C‐shape which has stainless steel fixation screws for holding the phantom in the irradiation position in PFX. An Exradin A16 ion chamber (collecting volume 0.007 cm3) and a Standard Imaging MAX 4000 electrometer were used to measure radiation output at the center of the 160 mm diameter polystyrene phantom for each of eight identical sectors and 16 mm collimator. Results: While studying the uniformity of output from each of the 8 sectors it was noticed that for sector No. 3 (270 degree) and sector No. 7 (90 degree), the outputs measured were lower by about 2% as compared to other sectors. On detailed investigation with Kodak EDR2 film, we found that for these two sectors, there were 2 beams of 16 mm diameter passing through the aluminum adaptor and the stainless steel fixation screws which caused attenuation of these two beams and hence a reduction in output. Conclusion: The attenuation by phantom adaptor introduces a systematic error in output calibration of PFX. The overall reduction in output measured for all 8 sectors combined is about 0.5%. We suggest that until Elekta AB redesigns a new phantom adaptor for PFX calibration, physicists should be aware of the attenuation of 16 mm beams by the adaptor and incorporate this effect in the output calibration of the PERFEXION.
SU‐GG‐T‐452: Attenuation of Single Beam in Dose Calculation of Gamma Knife (GK) Treatment Planning
Authors: Li K , Wang J , Lo S , Mayr N
Published: July 2008

Purpose: In Gamma Knife (GK) stereotactic radio‐surgery, highest accuracy of dose calculation is important. The attenuation of each single beam in GK is determined by their paths. The path, where each beam passes through the skull to the tumor, is determined by the measurement of skull radius. To estimate the path attenuation effect on the dose calculation is important for clinicians in deciding selection of the gamma angle, blocking beam and helmet size, etc. The purpose of this study was to design and test a new method to assess the beam path attenuation effects on dose computation in GK. Method and Materials: A simulation of GK C‐model was employed for this study. The skull was initially assumed to be a spherical bulb. Shot locations in the center and in the periphery of the bulb were tested. Shot center dose and each beam's contribution was calculated using the tissue maximum ratio (TMR) formula for each path. The simulation of the inhomogeneity calculation is an approach similar the procedure of finding the different paths from different beams. The dose point at different beam contribution is quantitatively calculated with variation difference. Results: The data from several typical plug locations showed that the path length difference could be over 50% and ensuring TMR changes could be as much as 20% of the average value. The single beam profiles at the shot center were variable; however, the composite profiles from the total beams did not show significant changes. Conclusion: Our method to calculate the different path length of single beam from GK treatment planning simulation model shows that the beam path parameters affect GK treatment planning. The beam profile changes based on path length variations. These findings suggest the necessity of considering dose contributions to the normal tissue from different beam paths.
SU‐GG‐T‐456: Simulation of Intrafraction Motion and Overall Geometrical Accuracy of a Frameless Intracranial Radiosurgery Process
Authors: Walker L , Feygelman V , Chinnaiyan P , Yu M , Forster K
Published: July 2008

Purpose: Evaluate the clinical accuracy of an image‐guided frameless intracranial radiosurgery system including the positioning accuracy of the 6D ExacTrac System. A novel method was evaluated to prospectively determine an optimal PTV margin. Method and Materials: The overall system isocentric accuracy was tested with a rigid anthropomorphic phantom containing a hidden target. Additionally, a Rando® Phantom was utilized for testing the positioning uncertainties in the 6D ExacTrac System where tilts and rotations were physically applied and then adjusted by the Robotic Table Top and ExacTrac Software. Intrafraction motion was simulated in five healthy volunteers immobilized with head and shoulders reinforced thermoplastic masks. The subjects were placed in a treatment position for 15 minutes (the maximum expected time between repeated isocenter localizations) and the six‐degree‐of‐freedom target displacements were recorded with high frequency by tracking infrared markers. The markers were placed on a customized piece of thermoplastic secured to the head independently of the immobilization mask. Results: The hidden target test confirmed overall system isocentric accuracy of ⩽1 mm (total 3D displacement). The final accuracy of the rotational setup of the cranial isocenter was 0.14 ± 0.09 degrees Yaw, −0.33 ± 0.17 degrees Roll and −0.43 ± 0.08 degrees Pitch. The subjects exhibited different patterns and ranges of head motion during the mock treatment. The total displacement vector encompassing 95% of the positional points varied from 0.4 to 2.9 mm. Conclusion: Pre‐planning motion simulation with optical tracking was tested on volunteers and appears promising for determination of patient‐specific PTV margins. Further patient study is necessary and is planned. In the meantime, system accuracy is sufficient for confident clinical use with 3 mm PTV margins.
SU‐GG‐T‐458: Improved Accuracy and Potentially Faster CyberKnife Synchrony Treatments by Tightening Constraints
Authors: Grimm J , Jacob D , Simpson L , Robinson C , Mento J , Bragg T
Published: July 2008

Purpose: To improve the accuracy and delivery time of CyberKnife Synchrony treatments. Method and Materials: The CyberKnife Synchrony system enables state of the art tumor tracking and targeting during stereotactic lung treatments. Unfortunately, for difficult cases some CyberKnife users have resorted to turning off all except one fiducial and loosening whatever constraints necessary to somehow complete every treatment. Is it possible to do better? We have discovered that ironically, judiciously tightening certain constraints can improve the correlation model and hence possibly decrease treatment time, while improving delivery accuracy. When tracking a single fiducial, the CyberKnife system has no way to realize when it has missed the fiducial and falsely locked to something else. This essentially can fill the correlation model with corrupted results that cause the algorithm to miss even worse on subsequent images. Alternatively, for these difficult cases if we still use multiple fiducials, loosen only the necessary parameters, while keeping the “Extraction Confidence Level” and “dxAB” threshold reasonably tight, the algorithm can determine when it has false locked. In this presentation we outline a simple procedure whereby the user can give the algorithm a “hint”, and hence salvage some of the potential misses to improve the correlation model rather than degrading it with corrupted estimates. Results: We present the values of parameters which enabled us to continue the treatments using at least three fiducials in selected case studies. Conclusion: The CyberKnife Synchrony system provides the tools to achieve unparalleled accuracy in delivering radiation to moving targets. However, the parameter set is so flexible and treatments are so intensely interactive, that attaining the full potential of the system is very dependent on the skill level and dedication of the staff.3
SU‐GG‐T‐460: Multiple Gating for Lung Stereotactic Body Radiotherapy Treatment
Authors: Li T , Yang Y , Li X , Li F , Heron D , Huq M
Published: July 2008

Purpose: An inherent problem in treatment of lung cancers with stereotactic body radiotherapy (SBRT) is target motion. Multiple‐phase‐based gating refers to treating the target at two or more phase windows with plans optimized for each window individually. It closely approximates the four‐dimensional (4D) tracking technique when large number of gating windows is used. Compared to conventional single gating window technique, multiple‐phase‐based gating can improve the normal tissue sparing by taking advantage of the location change of the target. In addition, the duty cycle may be further improved. Compared to 4D tracking, the dose conformity of the multiple‐gating technique may be inferior. However, its implementation may be more robust because of the allowance of residual error inside each gating window. In this study, we evaluate the dose performances for different techniques including static, single gated, dynamic 4D tracking, and two‐window‐gated SBRT plans. Method and Materials: A motion phantom was scanned with 4DCT and planned using the above techniques. The motion was 1D sinusoidal with amplitude of 1cm. For any optimized plan, 95% of the PTV was covered by the prescription dose of 60Gy. Film dosimetry was performed to compare the doses delivered in each plans. The above techniques were also compared for a lung patient who was undergoing SBRT treatment, prescribed to 60Gy in 3 fractions. Results: Both the phantom and the patient studies showed similar results. If normalized to the static plan, the V20's were decreased by 56%,22%,16%, and the maximum doses in “lung” were changed to 102%,58%,45%, for single window gating, two‐window gating, and 4D tracking, respectively. Conclusion: Multiple‐gating technique has significant improvement in lung dose sparing compared with static or single‐gating techniques. Its difference from 4D plan is relatively small. The dose performance also highly depends on the size of tumor and the extent of target motion.
SU‐GG‐T‐461: Beam Properties and Beam Modeling of a Radiosurgical Variable Aperture Collimator
Authors: Purwar A , Ankrah S , Muniruzzaman M , Nilsson J , Thomson L , Lee M
Published: July 2008

Purpose: The CyberKnife® Robotic Radiosurgery System (Accuray, Inc.) was developed with twelve fixed secondary collimators which deliver circular beams from 5 to 60 mm in diameter at 800 mm SAD. The effective beam diameters vary as actual SADs can range from approximately 650 to 1000 mm depending on the particular treatment. Accuray recently developed the Iris™ Variable Aperture Collimator to improve the ability to use multiple field sizes in a treatment. Tests were performed to compare the beam properties of the Iris Collimator with the fixed collimators. Method and Materials: The Iris Collimator consists of two stacked banks of rotationally offset hexagonal apertures. The resulting beam has a 12‐sided shape and a field size that can be varied between 5 and 60 mm at 800 mm SAD. The standard (non‐Monte Carlo) CyberKnife dose calculation algorithm assumes a circularly symmetric beam model. Beam data acquired with stereotactic diodes was processed to generate average dose profiles that are representative of equivalent circular beams for the Iris Collimator. These beam profiles were uploaded to the treatment planning system and the calculated dose distributions were compared with radiochromic film measurements. Results: Gamma histogram film analyses and water phantom measurements show that Iris Collimator beams are substantially equivalent to fixed circular collimator beams with respect to beam quality, collimator transmission and transverse profiles. Maximum leakage dose through the Iris Collimator <0.2%, RMS deviation of the 50% isodose curve from circularity <2%, and the average profile 20–80% penumbra is typically a fraction of a millimeter wider than that of equivalent fixed circular collimator. Conclusion: The beam properties of the 12‐sided Iris™ Collimator are sufficiently similar to those of the fixed circular collimators that the same beam modeling approach and standard dose calculation algorithm used for fixed collimators can be used with the Iris Collimator.
SU‐GG‐T‐462: Intra‐Fractional Motion of Bony Targets in the Spine and Pelvis During Extra‐Cranial SRS Treatment
Authors: Lovelock D , Wang P , Amols H , Bilsky M , Yamada Y
Published: July 2008

Purpose: to quantify intra‐fractional motion of bony targets with respect to the immobilization cradle during extra‐cranial SRS treatments. Method and Materials: Pre and post treatment cone beam scans were acquired of 64 patients undergoing either single fraction or hypo‐fractionated therapy to tumors in the torso. The median dose delivered in a treatment session was 24 Gy. In the cone beam scans, both the target and the immobilization cradle are visible. Target shifts were determined by computing the difference in the target to target 3D registration, and the cradle to cradle 3D registration. Cradle to cradle registration was done manually. The registration target was either the involved vertebra, or the vertebra at the level of the actual target. Target registration was done automatically using a region of interest encompassing the target vertebra and extending midway into the adjacent vertebrae. Patients were monitored throughout treatment using a stereoscopic infra‐red tracking system. Results: The mean and standard deviation of the shift in target position in the left‐right, posterior‐anterior, and superior‐inferior directions were 0.0±1.4, 0.0±1.0, and 0.2±1.2 mm respectively. The rotations about the corresponding axes, determined from a separate 6D fit, were 0.0±0.7, 0.0±0.5, and 0.0±0.7 degrees, respectively. The length of time between the pre and post treatment scans was 48±15 minutes (mean±std.dev). No correlation was seen between the magnitude of the target shift and the length of time between scans, or between the magnitude of shift and the vertebral level. No indications of radiation induced myelopathy have been seen. Outliers (shifts > 3mm in at least one direction) were seen in 8 patients. Conclusion: Although this form of immobilization works well for most patients, the existence of outliers indicates the importance of infra‐red surveillance.
SU‐GG‐T‐464: Comparison of Dose Gradient in The Critical Normal Structures Between CyberKnife and IMRT Treatment of Prostate Cancer
Authors: Hossain S , Gottschalk A , Huang K , Xia P , Descovich M , Chuang C , Ma L
Published: July 2008

Purpose: This study aims to investigate dose gradient or fall‐off with distance on the critical normal structures using CyberKnife and IMRT delivery for hypofractionated stereotactic prosate radiotherapy. Method and Materials: Eight cases of prostate cancer patient treated with the CyberKnife (CK) were used in this study. Based on the identical contour sets, treatment plan for each patient was generated on IMRT system. To calculate dose fall‐off, CTV (where CTV was prostate itself) contour was expanded 30 mm posteriorly and 50 mm uniformly for all patient cases with CK and IMRT planning systems. Plans were normalized such that 95% of the CTV received 100% of the prescribed dose. DVH parameters were used to analyze and compare doses to CTV and its posterior and uniform extension, bladder, rectum and urethra. Results: All plans satisfied the specified requirements with an exception in CTV dose conformity indices (CI). A number of IMRT plans show prescribed CTV isodose curve bulged out transversely. CI value for Cyberknife plans is 1.18±0.08 and 1.44±0.11 for IMRT. Dose fall‐off study showed that percent dose falloff/ mm (pdf/mm) are 3.75±1.58 and 3.15±1.88 posteriorly, and 3.60±3.97 and 3.62±4.30 in 20 mm uniformly expanded region for CK and IMRT, respectively. An attempt was made to recalculate pdf/mm for 5 mm split regions. For (0–5), (5–10), (10–15), and (15–20) mm regions, pdf/mm are 5.71±1.02, 4.49±0.56, 2.77±0.35, 2.05±0.19, and 5.61±1.26, 3.81±0.53, 2.07±0.46, 1.13±0.16 posteriorly, and 9.01±4.91, 2.67±0.59, 1.58±0.21, 1.13±0.11, and 9.62±5.08, 2.47±0.71, 1.32±0.16, 1.06±0.03 uniformly for CK and IMRT, respectively. Conclusion: CyberKnife and IMRT plans produced equivalent target dose coverage while CyberKnife plans show CTV dose is more conformal than IMRT. Dose fall‐off is slightly faster posteriorly for CyberKnife plans than IMRT plans. Conflict of Interest: Data was presented in CyberKnife User's Meeting 2008, Scottsdale, AZ.
SU‐GG‐T‐468: Image‐Guided Stereotactic Spinal Radiosurgery with a Conventional Linear‐Accelerator
Authors: Wang J , Rice R , Mundt A , Murphy K
Published: July 2008

Purpose: As large experiences of spinal radiosurgery were reported by using CyberKnife, this study evaluates our experiences with spinal radiosurgery given by a conventional linear‐accelerator, utilizing the features of KV‐OBI and CBCT. Method and Materials: Image‐guided stereotactic radiosurgery is used in our clinic to treat spinal metastases with 16–18 Gy in a single‐fraction, or 20–30 Gy in 5‐fractions. Treatment is delivered using nine co‐planar IMRT‐beams in 20–25 minutes. The success of spinal radiosurgery requires sparing of the proximal spinal cord and high precision of dose delivery. With proper beam arrangement and dose optimization, the maximal cord dose can be minimized to 60–65% of the prescription dose. Stereotactic localization of the metastatic vertebrae‐body is achieved by KV‐OBI and CBCT guidance before and during the treatment. The shifts based on OBI♯1 are made from the skin marks after initial setup. Next, CBCT further verifies the patient alignment. Two more OBIs before every three fields ensure patient immobility and allow iso shifts if necessary. Results: Couch shifts were recorded for a sample of 12 patients, a total of 27 fractions in this evaluation. It is found that the ranges of the shifts guided by OBI♯1 from skin marks are: 1.4 cm longitudinal, 0.9 cm vertical, 0.7 cm lateral, and 2° in‐plane couch rotation, while CBCT and consecutive OBI♯2 and ♯3 before every three fields found little patient movements. Conclusion: The conventional linear‐accelerator is able to deliver stereotactic radiosurgery treatment of spinal metastases with a high degree of accuracy, as guided by OBI and CBCT image techniques, in combination with proper patient setup, immobilization, and skin marks. No body‐wrap or fiducials are necessary in our approach. The study also finds that OBI and CBCT are equivalent due to the proper patient setup procedures in our approach. CBCT is beneficial for localizing C7/T1 for treatment.
SU‐GG‐T‐471: Potential Dosimetric Benefits of Four‐Dimensional Radiation Treatment Planning
Authors: Starkschall G , Britton K , McAleer M , Jeter M , Kaus M , Bzdusek K , Mohan R , Cox J
Published: July 2008

Purpose: Four‐dimensional (4D) dose calculations involve computing radiation doses on each phase of a 4D computed tomography (CT) image data set, deforming the dose matrices based on the deformations needed to register each phase with a reference phase, and accumulating the doses on the reference phase based on these deformations. The purpose of the present work is to determine the extent of dosimetric differences between conventional three‐dimensional (3D) dose calculations and 4D dose calculations based on deformation of organ models. Method and Materials: 4D dose calculations were performed on a set of 15 patients with Stage III non‐small‐cell lung cancer using a model‐based deformable registration algorithm on a research version of a commercial radiation treatment planning system. Patients were planned using internal target volumes based on explicit motion of the clinical target volume in the 4D data set and setup margins based on clinical practice. Target volume coverage as well as doses to critical structures calculated using the 4D methodology were compared to those calculated using conventional 3D methodology. Results: For 11 of 15 patients, CTV coverage was comparable in the 3D and 4D calculations, whereas for 7 of 15 patients, PTV coverage was comparable. For the other patients, the 4D calculation indicated a sufficient difference in target volume coverage so that replanning should have been done. No correlations could be established between 3D and 4D differences and GTV size or extent of GTV motion. Negligible differences were observed between 3D and 4D dose‐volume relationships for normal anatomic structures. Conclusion: To ensure that target volumes are not underirradiated when respiratory motion may affect the dose distribution, 4D dose calculations should be performed.

This work has been supported in part by a Sponsored Research Agreement with Philips Healthcare.
SU‐GG‐T‐475: Volumetric Analysis of Moving Structures in Radiation Treatment Planning
Authors: Mirkovic D , Vedam S , Chang J
Published: July 2008

Purpose: The purpose of this study is to design and validate a set of metrics for volumetric analysis of moving structures in radiation treatment planning. Method and Materials: 4D CT based treatment planning in thoracic radiation oncology requires a specific set of metrics for quantitative comparison of moving treatment planning volumes. For each pair of reference and test volumes axial overestimation and underestimation areas as functions of z coordinate are defined as the areas bounded by the set of contours in each axial slice. The axial underestimation and overestimation volumes are defined as integrals of corresponding areas along the z‐coordinate. The sagittal and coronal under/overestimation areas and volumes are defined analogously. An efficient and robust implementation for polygonal contours was obtained by using the Delaunay triangulation. The above metrics were used to compare different methods for determination of demonstrable respiratory motion of the GTV (iGTV) in 4‐D CT datasets. 27 patients with non‐small‐cell lung cancer (stage I: 17, stage III: 10) who underwent 4‐D CT simulation were studied. The iGTV was determined manually by (a) combining the GTV contours from 10 phases (GTVAllPhases) (b) combining the GTV contours from two extreme respiratory phases (0 and 50%) (GTV2Phases) (c) contouring using the MIP (GTVMIP) and (d) contouring using the MIP followed by modification with visual verification in individual respiratory phases (GTVMIP_Modified). For comparison of different approaches, GTVAllPhases was used as the reference. Results: The proposed metrics were successfully applied to 27 patients to show that iGTV as determined from GTVMIP and GTV2Phases underestimated iGTV as determined from the reference GTVAllPhases. Modification of iGTV determined from GTVMIP based on visual verification of contours through each individual phase reduced the degree of underestimation. Conclusion: The constructed metrics provide very good tools for volumetric analysis of moving structures.
SU‐GG‐T‐486: Characteristics of Motion Induced Dose Reduction for Prostate Cancer in Proton Therapy
Authors: Yoon M , Shin D , Park S , Kwak J , Kim D , Lim Y , Kim J , Shin J , Lee S , Park S , Cho K
Published: July 2008

Purpose: To quantify proton radiotherapy dose reduction in the prostate target volume due to the three‐dimensional movement of the prostate based on an analysis of dose‐volume histograms (DVH). Method and Materials: Twelve prostate cancer patients were scanned in a supine position and a target contour was delineated for each using a proton treatment planning system. To simulate target movement, the contour was displaced from 3 mm to 15 mm in 3 mm intervals in the superior to inferior (SI), inferior to superior (IS), anterior to posterior (AP), posterior to anterior (PA), and left to right (LR) directions. Results: For both intra‐ and interfractional movements, the average coverage index and conformity index of the target were reduced in all directions. For interfractional movements, the magnitude of dose reduction was greater in the LR direction than in the AP, PA, SI and IS directions. While the reduction of target dose was proportional to the magnitude of intrafractional movement in all directions, a proportionality between dose reduction and the magnitude of interfractional target movement was clear only in the LR direction. Like the coverage index and conformity index, the equivalent uniform dose (EUD) and homogeneity index showed similar reductions for both types of target movements. Conclusion: Small target movements can significantly reduce target proton radiotherapy dose during treatment of prostate cancer patients. Attention should be given to interfractional target movement along the longitudinal direction since image guided radiotherapy may be ineffective if margins are not sufficient.
SU‐GG‐T‐491: Dose Summation for Multi‐Phase Helical Tomotherapy Plans
Authors: Turian J , Chen S , Coleman J , Khelashvili G , Chu JCH
Published: July 2008

Purpose: Develop a method to combine dose matrices from multi‐phase Tomotherapy plans to facilitate dose escalation clinical trials. Method and Materials: For many clinical sites it is customary to use treatment plans consisting of 2–3 phases. Currently the Tomotherapy treatment planning system doesn't provide a method to combine these phases, thus hampering the ability of clinicians to review dose volume information. We propose a simple method to create a composite plan which could be evaluated on a tertiary treatment planning system (TPS). Dose matrices from approved Tomotherapy plans for each phase are extracted and exported to a temporary location. A software algorithm was developed to read each distribution and perform a pixel by pixel summation. The final dose matrix was reviewed and placed in a format compatible with Pinnacle‐TPS. The dose matrix was then registered with the image set in Pinnacle facilitating composite plan evaluation. The accuracy of the method was tested using ion chamber and film measurements. Film dosimetry was performed using EBT‐Gafchromic film, Epson flatbed scanner and our in‐house film dosimetry protocol. The measured dose distributions were compared with values calculated by Tomotherapy at the location of the film and summed using this method. Dose distributions were compared using the γ‐index. Results: The x,y,z registration errors between the composite dose matrix and the image set of the TPS were found to be 0.6mm for x‐y axes and 1.5mm for z‐axis. Ion chamber measurements were within 1–2%. Good agreement (low gamma, high conformity index) was recorded using film dosimetry Conclusion: This summation method provides a fast, accurate, and simple way to evaluate multiphase composite treatment plans. The method can be used to sum dose distributions provided by different treatment planning system or to perform dosimetric comparisons between plans from these systems.
SU‐GG‐T‐507: 4D Planning: Dose Calculation That Accounts for Moving Beams and Tissue Deformation
Authors: West JB , Maurer CR , Dooley JR
Published: July 2008

Purpose: To evaluate the ability of 4D Treatment Optimization and Planning, which is a recently available feature in the MultiPlan® Treatment Planning System for 4D treatment planning for motion tracking with the CyberKnife® Robotic Radiosurgery System, to calculate a dose distribution that takes into account both beam movement due to respiratory motion and soft tissue deformation. Method and Materials: A phantom was developed for 4D planning validation, containing a target that exhibits periodic motion driven by a motor, and a critical structure adjacent to the target. A 4D CT scan of the phantom in motion was obtained, and a treatment plan created on this 4D CT scan using MultiPlan with the 4D Planning module. The target has fiducials embedded in it, and Synchrony® Respiratory Motion Tracking System was used so that the treatment beams followed the target motion. Radiochromic film was placed in the critical structure and the treatment plan was delivered. The film dose after treatment was compared with that calculated by MultiPlan using both the static dose calculation (3D dose) and that calculated using the 4D Planning module. The 4D Planning module uses deformable registration and information about beam movement to calculate a dose distribution that takes into account both target movement and tissue deformation during treatment (4D dose). Results: For the 3D dose in the coronal film, 64.4% of pixels had a disagreement of 3% or less between measured and predicted dose; for the 4D dose distribution, 97.6% of pixels had a disagreement of 3% or less. Using a Gamma function with a passing criterion of 5% dose difference or 3mm distance to agreement, 94.9% of pixels pass for the 3D dose and 100.0% of pixels pass for 4D dose. Conclusion: The 4D Planning module accurately calculated dose in this phantom test for a moving target.
SU‐GG‐T‐508: Evaluation of the Monte Carlo Dose Calculation Engine of Eclipse Treatment Planning System for Electron Beams
Authors: Kamta G Lagmago , Reniers B
Published: July 2008

Method and Materials: Eclipse©'s electron Monte Carlo (eMC) (Varian, Palo Alto, CA) engine is a fast implementation of Monte Carlo (MC) method for radiotherapy treatment planning with electron beams. Several types of phantoms, combining low and high density heterogeneities ranging from air through lung to cortical bone, are used to evaluate eMC by comparing its results with measurements and with full MC simulations using DOSXYZnrc (NRC, Canada). Measurements with physical phantoms are done using EBT Gafchromic© films, leading to absolute dose. The phantoms are reproduced virtually in order to exclude the effect of the CTscan (HU — density conversion curve, noise, artifacts, …) in DOSXYZnrc and in Eclipse© for full MC and eMC calculations, respectively. Results: Absolute dose profiles obtained via the three methods (measurements, eMC and MC) are compared. As expected, MC simulations agree better with measurements. The three methods agree quite well in general, within 5% even in regions of high dose gradients, which is a great improvement compared over the previous algorithms. However, larger discrepancies (as high as 10%) between eMC and measurements are found under the small heterogeneities, as was already reported by Ding et al (Phys. Med. Biol. 51, 2781 (2006)). Our differences are smaller though probably due to the fact that that they use smaller heterogeneities which is harder to model due to scatter. This is still under investigation. The effect of the use of CT‐defined phantoms instead of the virtual phantoms is also part of this study. Conclusion: Our preliminary results using virtually designed phantoms show that eMC simulations give a good agreement with measurements within uncertainties. This study is very encouraging and shows that it should now be possible to treat the electron beams with the same kind of precision as is already done for photon beams.
SU‐GG‐T‐512: CT Resolution for Lung Treatment Planning: An Application of a 2 ½‐D Random Lung Model Using MC Method
Authors: Liang L , Larsen E , Chetty I , Al‐Ghazi M
Published: July 2008

Purpose: To investigate the effects of CT resolution on treatment planning where heterogeneities exist, such as in the lung, using a recently developed, realistic random lung model and Monte Carlo method. Method and Materials: A thoracic phantom with a realistic random lung model embedded was built and two representative realizations with two different sizes of tumors were generated. The MC code DPM was employed to calculate dose distributions in the phantom with different CT resolutions. The three‐field conformal setup used a 6MV photon beam. Both qualitative and quantitative dose evaluation metrics were applied. Results: A reference CT resolution of 1×1 cm2 was established by comparing the CAX depth doses between a detailed lung model and its voxelized version. The fine details revealed in high resolution can be smoothed, especially when the geometrical voxels cross the heterogeneities, hence introducing a potential systematic error. Visible difference, up to 1%, can be seen in the DVHs of the cases with a small tumor. The insensitive relative absolute differential dose shows the DVH's disadvantage of lack of positional information of the dose distribution. Conclusion: A realistic random lung model was applied to show the effect of the accuracy of the geometrical representation on dose distribution in heterogonous sites, such as the lung. Our results show that a CT resolution up to 2×2 mm2 may be sufficient while a 4×4 mm2 could lead to significant perturbations. This may be especially problematic for treatment planning involving small tumors and tissue heterogeneities.