bluuebunny/crossref_metadata_2025 huggingface dataset:
SU‐GG‐T‐353: Validation of Calculation for Electrons and Feasibility Study of Penumbra Generator Utilized Electron Monte Carlo Algorithm
Authors: Kuo H , Yaparpalvi R , Hong L , Mah D , Lee J
Published: July 2008

Purpose: To evaluate the performance of a commercial TPS equipped with Monte Carlo algorithm for electron beam calculations. This study also tests the feasibility of the design of penumbra generator by the Monte Carlo algorithm. Method and Materials: Different size cutouts (circular aperture with diameters range 3.8–10cm) and circular inserts (block shield with diameters range 3.8–8cm) were created for the cutout factors, off axis point dose, PDD, electron beam energy profiles (9MeV and 16MeV) and varying SSD (100cm and 115cm). We measured in water tank for those cutouts and block inserts for PDD and profiles for 9MeV and 16MeV on Varian's Trilogy. Measurements were done with 100cm and 115cm SSD. Monte Carlo calculations with Eclipse were compared with measurements. Different materials (range 0HU–2600HU) were designed for simple step wedge shape and combined with air to simulate penumbra generators through the Monte Carlo calculation. Results: For cutout <4cm, the dose output calculation inaccuracies were up to ± 7%. For cutout >=6cm, the output accuracies were within ±2%. The point dose accuracy for off axis positions with different inserts were within 3.5%. At extended SSD (115cm) the calculation slightly increases the error by 1%. The calculated Vs measured profiles at different depths for different inserts agreed well. At 115cm SSD, the profiles within fields still matched well, profiles around or under the blocks were slightly off (<5%). The PDD beyond dmax matched within 1mm. The calculated surfaces dose was slightly lower. Different materials with air have different penumbra effects, which show the feasibility of the MC simulation for the design of penumbra generator. Conclusion: The current beam model needs to be modified for cutouts smaller than 4cm. For larger cutouts, Monte Carlo electron dose algorithms produced reasonable results compared with measurements. Further measurement is warranted for the validation of the penumbra generator.
SU‐GG‐T‐354: Monte Carlo Simulation On the Percentage Depth Dose Ratio for the Surface Dosimetry Using Tangential Photon Beams
Authors: Chow J , Grigorov G
Published: July 2008

Purpose: To investigate the characteristics of the relative dose profile in the phantom skin layer for oblique tangential‐like photon beams with different energies and field sizes using Monte Carlo simulations. Method and Materials: EGSnrc‐based codes (BEAMnrc and DOSXYZnrc) were used to predict the relative depth doses for the half (HPG) and full phantom geometry (FPG) for the 6 and 15 MV photon beams with field sizes of 4×4 and 10×10 cm2 produced by a Varian 21EX Linac. Monte Carlo phase‐space beams were verified by measurements using films. The HPG is the experimental setup for the beam central axis (CAX) parallel to the phantom‐air interface (gantry angle = 270 deg), while FPG is the setup with the air region of the interface filled with the phantom material (Solid Water). The PDD ratios, defined as the relative depth dose from the HPG to the relative depth dose from the FPG at a given depth along the phantom skin, were calculated. Results: The PDD ratio was found very sensitive when the tangential beam was turned slightly from zero to 1 deg clockwise, and the ratio increased with depth. For the same photon beam energy, when a 10×10 cm2 field size was used, the increase of the PDD ratio was less than that for a 4×4 cm2 field. While for the same field size, 6 MV photon beam had a greater increase in the PDD ratio with depth compared to 15 MV. Conclusion: The PDD ratio study based on Monte Carlo simulation shows that the surface dose varies with the energy and field size of the tangential‐like photon beams. The data in this study provides useful information to predict the surface dose variation for a clinical tangential photon beam with changes of beam obliquities, energies and field sizes.
SU‐GG‐T‐355: External Electron Beam Comparison of Monte Carlo Codes (BEAMnrc, MCNP4C) in Homogeneous Phantom and Interfaces
Authors: Owrangi M , Masudiefar M , Nedaie H , Anjomani Z , Mosleh‐Shirazi M
Published: July 2008

Purpose: Understanding the limitation of the Monte Carlo codes is essential in order to avoid systematic errors in simulations, and to suggest further improvement of the codes. The purpose of this study was to simulate the treatment head of Elekta (Synergy™) linear accelerator with two different Monte Carlo codes called MCNP4C and BEAMnrc and compare the calculated data of both codes with each other and with experimental measurements. Method and Materials: MCNP4C and BEAMnrc Monte Carlo codes commonly used in medical physics, were compared and evaluated against electron depth dose, beam profile and experimental total scatter factor results obtained using clinical radiotherapy beams. Different physical models and algorithms used in the codes give different depth dose curves, beam profile and total scattering factors. Results: The maximum difference for percentage depths dose curves in the Build‐up region was about 1.1% and it was less than 1mm in the dmax. These differences for the dose profile curves in the flat area was at last 0.9% and for the penumbra was about 1.5mm and in the out field regions which only the scatter radiation reach that area, the maximum difference was about 0.2%. Conclusion: In conclusion, the differences in cross sections used for bremsstralung effect, different multiple scattering theory and different electron transport algorithms used by MCNP4C and BEAMnrc will lead to discrepancy in depth dose and beam profile curves, which is negligible for both codes if the simulations were well tuned. The timing study is also shows that BEAMnrc is generally faster than MCNP4C and use of large number of scoring voxels dramatically slows down the MCNP calculation. However, use of a large number of geometry voxels in MCNP only slightly affects the speed of the calculation.
SU‐GG‐T‐357: Geant4 Benchmarking for IMRT Dose Calculations
Authors: Hancox C , Seco J , Paganetti H
Published: July 2008

Purpose: The purpose of this work is to test the accuracy of the Geant4 release 9.0 for use in IMRT patient dose calculations. This is of importance since major changes were made in the multiple scattering engine starting with Geant4 release 8.0, but the package has not since been extensively tested for low energy photons relevant for simulating patient treatments. We compare calculated dose distributions in homogeneous and heterogeneous multilayer phantoms with experiments and with results from a well‐benchmarked Monte Carlo toolkit, DPM (Dose Planning Method). Method and Materials: We investigated the ability of Geant4 to reproduce experimentally measured lateral and depth dose profiles in a homogeneous water phantom irradiated with 10×10 cm2, 30×30 cm2 and 40×40 cm2 open fields. The results were compared with both DPM and experimental data. Next, Geant4 was used to calculate dose distributions in heterogeneous phantoms consisting of slabs of water, bone and lung, paying particular attention to the handling of material interfaces. The results were compared with DPM. Results: We find that Geant4 and DPM give nearly identical results for each of the heterogeneous phantoms, agreeing to better than 2% in the depth dose through each material and the lateral dose profiles across all material boundaries. The agreement between Geant4, DPM and experimental data in the homogeneous water phantom is also quite good, with all three agreeing to within 2% in depth dose (neglecting the buildup region), and in lateral dose profiles taken at 5 and 10 cm depths (neglecting the beam penumbra). Conclusion: Homogeneous and heterogeneous phantom studies show that Geant4 yields accurate dose distributions for clinically relevant photon fields.
SU‐GG‐T‐359: Investigation of Optimal Margins for Proton Therapy of Lung Cancer
Authors: Tafo A Guemnie , Li J , Fan J , Fourkal E
Published: July 2008

Purpose: The objective of this work was to determine the optimal margin for proton therapy of lung cancer by investigating the effect of the lung density change on the coverage of the target volume. Discrepancies in the SOBP region for uniform water and heterogeneous patient anatomy were also studied. Method and Materials: Monte Carlo simulations were performed to investigate the distal edge shift for different phases of the respiratory cycle (e.g., inhale and exhale). Several Monte Carlo code systems were used in this study for dose comparison. Simulation geometries were built from patient CT data with proper materials and mass densities. Different positions of the tumor in the beam direction were set up to investigate the maximum effects. Results: Examination of the distal edges shows a shift induced by the density change in lung during the respiratory cycle. This motion has been quantified to a few millimeters (up to 1 cm) depending on the position of the tumor relative to the chest wall. A change in the shape of the SOBP (distal and proximal) has occurred and has been correlated to the thickness (and density) of the lung layer before the tumor. These results are consistent with previous Monte Carlo simulations. The margin required to account for the lung density change must be combined with the margin needed to account for respiratory motion to give an optimal margin for proton therapy in the thoracic region. Conclusion: This work indicates that extra margins have to be taken into account during the treatment of lung cancer with protons due to the lung density change. This study shows the importance of using a gating technique for lung tumor. Monte Carlo dose calculation plays an important role in accurate dose determination for proton therapy in the presence of heterogeneities.
SU‐GG‐T‐423: Radiobiological Comparison of Helical Tomotherapy and MLC‐Based IMRT for Brain and Cranio‐Spinal Tumors
Authors: Esquivel C , Mavroidis P , Shi C , Ferreira B , Lind B , Gutiérrez A , Stathakis S , Papanikolaou N
Published: July 2008

Purpose: In order to estimate better the clinical effectiveness of the Helical Tomotherapy and MLC‐based IMRT radiation modalities a radiobiological assessment was performed. This was achieved by employing additional physical and radiobiological criteria. Method and Materials: Two brain and cranio‐spinal cancers were investigated. For each cancer type, a linac MLC‐based step‐and‐shoot IMRT plan and a Helical Tomotherapy plan were developed. The treatment plans of the MLC‐based IMRT were developed on the Philips treatment planning station (Pinnacle 7.6 software release), whereas the dedicated Tomotherapy treatment planning station was used for the HT plans. The different treatment plans were compared using dosimetric criteria and the biologically effective uniform dose (BEUD) together with the complication‐free tumor control probability (P+). Results: The applied plan evaluation method shows that in the brain cancer case the HT treatment gives similar results with the MLC‐based IMRT (P+ of 66.0% and 63.4%, respectively). The total control probabilities, PB are 79.5% and 76.6%, whereas the total complication probabilities, PI are 13.4% and 13.1%. In both cases, the dose limiting tissue is the Speech Area. In the cranio‐spinal cancer case, the HT treatment plan is much better than the MLC‐based IMRT plan with P+ values of 84.1% and 28.4%, respectively. The PB values are 86.4% and 51.3%, whereas the PI values are 2.3% and 23.0%. In the case of HT, the lung is the dose limiting tissue (1.7%) whereas in the case of MLC‐based IMRT the dose limiting tissues are the lung (8.1%) and the left kidney (14.2%). Conclusion: Both MLC based‐IMRT and HT are suitable for treating often large PTVs while minimizing the volume of the organs at risk receiving high dose. However, in certain more complex clinical cases the HT radiation modality may show clearly better properties in producing very conformal and clinically effective dose distributions.
SU‐GG‐T‐425: Radiobiologically Quantified Treatment Setup Uncertainties Based On Megavoltage (MV) CT Imaging
Authors: Swanson G , Shi C , Mavroidis P , Stathakis S , Su F , Ramer R , Sarkar V , Ferreira B , Komisopoulos G , Giantsoudi D , Papanikolaou N
Published: July 2008

Purpose: To evaluate and compare the expected effectiveness of the planned and delivered dose distributions in prostate cancer radiotherapy. The average setup uncertainties were determined by using the on‐board megavoltage computed tomography (MVCT) capabilities of the tomotherapy HiArt unit. Method and Materials: Co‐registrations between daily MVCT and planning CT were used. Before the delivery of each treatment an MVCT image was acquired. The therapists registered the MVCT image with the planning kilovoltage computed tomography (kVCT) images. The registration criteria were based on bony anatomy and contoured regions of interest. For a typical prostate cancer patient the dose distributions of the Helical Tomotherapy plan and that of the average shifted delivery registration were used. The dose distributions were compared based on dosimetric criteria and the biologically effective uniform dose (BEUD) together with the complication‐free tumor control probability (P+). Results: The average shifts that were observed are 10.7 mm for the vertical, 4.0 mm for the longitudinal and 0.5 mm for the lateral directions, respectively. At the optimum dose levels of the planned and delivered dose distributions, the P+ values are 84.7% and 84.0%, respectively. The total control probabilities, PB are 93.0% and 92.9%, whereas the total complication probabilities, PI are 8.3% and 8.9%. More specifically, the response probabilities of the different tissues are 95.6% and 95.5% for the GTV, 97.4% and 97.3% for the seminal vesicles, 0.8% and 0.7% for bladder and 7.6% and 8.3% for rectum. Conclusion: It is shown that the intra‐fraction movement of tumors can be reduced by using the MVCT co‐registration. This appears to be a powerful tool, which can result in improved sparing of critical structures, while delivering high doses to the target. The use of P+ — BEUD diagrams to compare similar treatment plans may show that in radiobiological terms they may be quite different.
SU‐GG‐T‐426: Evaluation of the Interfraction Dose Variation of MammoSite HDR Brachytherapy Using the Concept of Biological Effective Dose
Authors: Kim Y , Werts E , Trombetta M , Parda D , Miften M
Published: July 2008

Purpose: To evaluate the interfraction dose variations due to changes of balloon shape and location in MammoSite HDR brachytherapy using the concept of biological effective dose (BED). Method and Materials: A method was developed to account for non‐uniform dose distribution in the BED calculation. A prescription dose of 34 Gy was delivered twice daily in 10 fractions to 19 patients. The BED value was computed by two different Method: one using the same dose distribution of fraction 1 over fraction 2–10 (constant case) and one using the actual delivered dose distribution for each fraction 1–10 (interfraction variation case — Fx case). The α/β values were adopted from literature and ranged from 2 to 11 based on specific clinical endpoints: acute effects (erythema, desquamation), late effects (fibrosis, telangiectasia) and tumor control. The α value of 0.3 Gy−1 and Tpot value of 13 days were used for cell proliferation. Results: The average difference in BED values between the constant and Fx cases was less than 0.1 Gy over the range of α/β ratios. The maximum difference was larger for late effects (fibrosis, telangiectasia) than acute effects (erythema, desquamation): −3.3 Gy and −2 Gy versus −1.5 Gy and −1.4 Gy corresponding to α/β ratio of 2 and 4 versus 8 and 11, respectively. The negative value means the BED value of Fx case is smaller than that of constant case. By disregarding high inhomogeneity in HDR brachytherapy, the simple calculation assuming uniform dose distribution tends to overestimate BED for fibrosis while underestimating BED for erythema and desquamation. Conclusion: The BED calculation accounting for non‐uniform dose distribution is more clinically relevant compared to the BED calculation which assumes uniform dose distribution across the target volume. Because the mean interfraction variation in dose was within 0.1 Gy across the target volume, the biological effect was clinically insignificant.
SU‐GG‐T‐430: Radiobiological Investigation of the Dosimetric Effects of Daily Megavoltage CT Registration Techniques On Adaptive Radiotherapy with Helical Tomotherapy
Authors: Mavroidis P , Su F , Giantsoudi D , Stathakis S , Komisopoulos G , Shi C , Swanson G , Papanikolaou N
Published: July 2008

Purpose: Pre‐treatment patient repositioning in highly conformal radiotherapy modalities is a prerequisite for reducing setup uncertainties. A MVCT image set has to be acquired to account for daily changes in the patient's internal anatomy and setup position. This study aims to investigate the expected effect of patient setup correction with the Hi‐Art tomotherapy system by using radiobiological measures. Method and Materials: Using the Planned Adaptive module (tomotherapy, Inc, Madison, WI), eventual dose discrepancies can be evaluated and corrected. In this study, patients treated for lung, pancreas and prostate carcinomas by Helical Tomotherapy plans are evaluated. In each cancer case, two dose distributions with and without patient setup correction were calculated using the MVCT image sets and they were evaluated using the complication‐free tumor control probability (P+) and biologically effective uniform dose (BEUD) concepts. Results: For the lung cancer case, at the optimum dose levels of the two dose distributions, the P+ value is 57.6% and 56.8% for a BEUDITV of 64.8 Gy, respectively. The respective total control probabilities, PB are 78.1% and 78.1%, whereas the corresponding total complication probabilities, PI are 20.5% and 21.3%. For the pancrease cancer case, the P+ value is 95.0% for a BEUDITV of 67.9 Gy and 97.5% for a BEUDITV of 69.9 Gy, respectively. The respective PB values are 97.8% and 98.7%, whereas the PI values are 2.9% and 1.2%. For the prostate cancer case, the P+ value is 57.7% for a BEUDITV of 90.2 Gy and 55.9% for a BEUDITV of 90.6 Gy, respectively. The respective PB values are 83.7% and 84.7%, whereas the PI values are 26.1% and 28.8%. Conclusion: According to our results, patient setup corrections during registration are adequate to produce the necessary target localization. Clinical cases, which look dosimetrically similar, can be quite different in radiobiological terms.
SU‐GG‐T‐433: Simple Plan Evaluation Methods for Three Dimensional Conformal Radiotherapy
Authors: Prabhakar R , Julka P , Rath G
Published: July 2008

Aim: Dose volume histograms (DVHs) play a vital role in determining the optimal plan for radiotherapy treatment delivery. The current concepts of conformality index (CI), equivalent uniform dose (EUD) derived from dose volume histogram (DVH) does not provide any spatial information. In this study, slice‐based evaluation methods have been proposed for spatially analyzing the radiotherapy treatment plans. Method and Materials: A case of prostate cancer has been selected for demonstrating the proposed tools for evaluating the dose distribution. Three dimensional conformal radiotherapy treatment planning (3D‐CRT) was performed with three fields employing multileaf collimator and the treatment plan was evaluated with DVH. The dose maximum point, conformality index, PTV coverage index (PCI) and equivalent uniform dose were evaluated for every single slice along the cranio‐caudal direction for all the planning target volume (PTV) contours and plotted against slice location. Results: The dose maximum point plotted against the slice position allows in identifying those slices where dose maximum point is outside the target volume. The plot of conformality index gives the information about location of those slices where excess of surrounding normal tissues is encompassed inside the prescription dose. Similarly, the plot of PCI and EUD with slice position gives the information about those where the tumor is not covered adequately. Conclusion: The methods proposed in this study will reduce the time consumed for analyzing the treatment plans and forms as a simpler way to assess the spatial distribution of the dose inside the target volume.
SU‐GG‐T‐436: Setup Accuracy of a Thermoplastic Mask System Using Two‐Dimensional (2D) On‐Board Imager (OBI) for Fractionated Stereotactic Radiotherapy (FSRT)
Authors: Kuo L , Mechalakos J , Hunt M , Chang J
Published: July 2008

Purpose: To analyze the setup accuracy of a thermoplastic mask system using two‐dimensional (2D) on‐board imager (OBI) and compare the results with the relocatable Gill‐Thomas‐Cosman (GTC) frame for fractionated stereotactic radiotherapy (FSRT). Method and Materials: Residual setup accuracy of 9 patients with brain lesions undergoing image‐guided hypo‐fractionated FSRT immobilized using a thermoplastic mask system were retrospectively studied. For each patient, image‐guided setup was performed for each treatment using orthogonal pairs of 2D OBI radiographs registered with reference digitally reconstructed radiography (DRR). Residual errors in the anterial‐posterior (A/P), right‐left (R/L) and superior‐inferior (S/I) directions were measured using in‐house software by contouring and comparing the location of bony anatomy on the DRRs and the orthogonal pairs of OBI radiographs taken immediately before the radiation treatment. The PTV margins required to ensure minimum dose to CTV is 95% for 90% of patients as proposed by Van Herk were also calculated for each direction. Results: Totally, setup accuracy was successfully analyzed for 45 fractions (5 fractions for each patient). The mean and standard deviation (SD) of the residual errors in the A/P, R/L, and S/I directions were 0.04±0.79 (mean ± 1 SD) mm, 0.3±1.32 mm, and −0.39±0.51mm respectively. The required margin to guarantee adequate PTV coverage were 2.57 mm, 3.98 mm and 1.86 mm in respectively the AP, R/L, and S/I direction. These margins were comparable to the 3mm PTV margin currently used for the hypo‐fractionated treatment using the GTC frame at our institution. Conclusion: The setup accuracy of 2D image‐guided FSRT immobilized with the thermoplastic mask system is compatible to the GTC frame for patients with brain lesions.
SU‐GG‐T‐439: Dosimetric Comparison of Compensator and Multi‐Leaf Collimator (MLC) Based Intensity Modulated Radiosurgery (IMRS)
Authors: Tang X , Tyagi N , Wang J , Rice R , White G , Cashon K , Vanderspek L , Pawlicki T , Murphy K , Jiang S
Published: July 2008

Purpose: Multi‐leaf collimator (MLC) based intensity‐modulated radiosurgery (IMRS) often results in large number of monitor units (MU) for patients with multiple brain lesions. Compensator based IMRS, however, may dramatically reduce MU. The purpose of this study is to quantify the reduction of MU for IMRS of multiple brain lesions using solid tissue compensators. Method and Materials: Patients with multiple brain tumors were selected for our study. For each patient, Varian Eclipse TPS was used to generate an MLC based IMRS plan consisting of 10–11 coplanar beams. The prescription dose for a typical IMRS treatment is 1800–2000 cGy delivered in 1 fraction using a 6 MV photon beam. IMRS plans were generated on 2 patients. The optimal fluence maps from IMRS plan were exported to the compensator generating system to generate compensators for each field. The compensator files are imported back to Eclipse to calculate MUs for the compensator fields. Eclipse TPS was modified to allow compensator based planning and evaluation inside Eclipse. Finally, we compared MLC and compensator plans in terms of MUs and target and normal structure coverage. Results: Compensators offer superior resolution compared to MLCs and are easier and faster to plan. DVH analysis from both patients shows adequate target coverage for both IMRS and compensator plans. More sparing of normal tissues in compensator plan was observed sometimes. The MUs were reduced by factors of more than 3 compared to an MLC based IMRS plan. Conclusion: Compensator based IMRS can dramatically reduce the number of MU needed for multiple brain lesion radiosurgery as compared to an MLC based IMRS plan while preserve prescribed dose coverage. Conflict of Interest: This work is partially supported by .decimal Inc.
SU‐GG‐T‐440: Dosimetric Delivery Verification of Whole Brain Radiotherapy with Hippocampal Avoidance (WBRT‐HA) and Simultaneously Integrated Multiple Metastases Boost (SIB)
Authors: Caldwell K , Gutiérrez A , Papanikolaou N , Tome W
Published: July 2008

Purpose: To verify the delivery accuracy and precision of a whole brain radiotherapy hippocampal avoidance (WBRT‐HA) and simultaneously integrated multiple metastases boost treatment plan in a head phantom. Method and Materials: A helical tomotherapy treatment plan was created using the Rando anthropomorphic head phantom. The image set was transferred to Pinnacle3 TPS where structures were segmented. Contours from an actual patient segmentation were applied to retain an accurate representation of the human anatomy. The plan was generated using a field width of 1.0 cm, pitch of 0.215, and modulation factor of 4.0. Dose prescriptions to the various targets were as follows: Met1 & Met2 − 95% of volume to receive 70.8Gy, Met3 − 95% of volume to receive 63Gy, and WB‐CTV − 95% of volume to receive 32.25Gy delivered in 15 fractions. TLD‐100 chips were placed at the center of each metastasis, hippocampus lobe, and the brain. All fifteen fractions were delivered to the head phantom. After each fraction, the phantom was opened, and TLDs were replaced. The head phantom was then repositioned, and a MVCT was acquired for localization. Results: Agreement between calculated and measured doses for the metastases, hippocampus, and whole brain were shown to be within 15, 10, and 3%, respectively. Reasons for these discrepancies aren't exactly known; however, being that the whole brain dose was within 3%, overdosing of the metastases and hippocampus may be due to their small volumes. Furthermore, the relatively small variance associated within each of the structures indicates that precise interfractional repositioning was possible with MVCT imaging. Conclusion: Our results indicate that for small volumes the calculated dose was underestimated by up to 15% using helical tomotherapy. It was noted that the MVCT of tomotherapy is vital for the precise delivery of WBRT‐HA with SIB of multiple metastases.
SU‐GG‐T‐441: A New Method of Measuring the Accuracy of Stereotactic Cones and MLC with Gantry and Collimator Rotation
Authors: Nygren I , Spencer D
Published: July 2008

Precision and accuracy are vital for stereotactic radiosurgery and radiotherapy. We set up the patients with lasers and we treat with radiation fields defined by cones or MLC, therefore the lasers must be accurately aligned with the radiation fields. In addition, the manner in which the beam shapers move with gantry and collimator angles must be reproducible and reliable. The standard method of measuring laser alignment and rotational shifts at the Tom Baker Cancer Centre, the Winston‐Lutz test, is only semi‐quantitative, and the results are subject to inter‐observer variation. We have developed a method of digitally detecting therapy X‐Rays and lasers using a 512 by 1024 photodiode array with a 48μm pixel pitch, the Radeye1 from the Rad‐icon Imaging Corporation to which we added optical density filters and an intensifying screen. We took images of the lasers and the radiation fields at various gantry and collimator angles and analyzed them with in‐house developed Matlab tools. For the 4mm diameter cone we measured an apparent head tilt of 0.14 (±0.02) mm and a gantry sag of 0.39 (±0.02) mm. The lasers were within 0.25 (±0.05) mm of the compromise radiation isocentre. For a 6mm by 6mm MLC we found the center of the radiation field varied by 0.58 (±0.02) mm with collimator rotation. The ceiling laser was 0.52 (±0.02) mm from the centre of the MLC rotation. This system can be used to both monitor and improve machine performance, which may lead to improved outcomes for patient treatment.
SU‐GG‐T‐454: Localization Accuracy of Cone‐Beam CT Guided Radiosurgery as Investigated Utilizing a Geometric Phantom
Authors: Wang Z , Wu Q , Chang Z , Kirkpatrick J , Yin F
Published: July 2008

Purpose: While the advantage of imaging guidance with cone beam CT (CBCT) guided radiosurgery is obvious, the localization accuracy of CBCT guided radiosurgery still remains a concern. This study utilized a geometric phantom with known target positions to investigate the accuracy of CBCT based localization, and to compare its accuracy with conventional frame‐based localization. Method and Materials: Geometric phantom with various target shapes at known positions was scanned with a localizer (Brainlab AG, Germany) attached. Each target was then localized in the treatment room with a target positioning frame. The same target was then localized by using CBCT. The CBCT localized target position was compared with the frame localized position, and also with the known position of the geometric phantom. Results: The differences of target position between CBCT and frame based localization were listed in Table 1 for targets with various shapes. The mean and standard deviations were 0.8 ± 0.5 mm in AP, 0.3 ± 0.5 mm in IS, 0.8 ± 0.5 mm in RL directions. The differences between CBCT localized target and geometric phantom positions were listed in Table 2 with an average of −0.3 ± 0.5 mm in AP, 0.3 ± 0.5 mm in IS, 0.9 ± 0.6 mm in RL directions. Conclusion: CBCT can provide comparable localization accuracy as conventional SRS localization frames. Rigorous quality assurance tests are required to maintain its accuracy.
SU‐GG‐T‐463: Validity of Inhomogeneity Correction in Small Felds Used in CyberKnife
Authors: Das I , Cryan G
Published: July 2008

Purpose: With simultaneous imaging and tracking target during treatment, CyberKnife has created a niche for the treatment of lung cancer. Dosimetry in small field is challenging that becomes extremely uncertain with low‐Z inhomogeneities. This study is undertaken to dosimetrically evaluate the accuracy of CyberKnife treatment planning for lung treatment. Method and Materials: A lung phantom made out of cork (ρ=0.25 g/cm3) sheet and solid water was used. Gold fiducial markers were placed on the cork slab at different depths for tracking. Three set of scans (depending on the depth of measurements) were acquired that was used for planning using Multiplan version 2.0.5 of the CyberKnife. For each cone, treatment plans were generated with a single beam (QA mode) to deliver 200 cGy to 100% isodose line to the micro‐detector that was chosen as the target. Under image guidance calculated MUs were delivered and the detector readings were converted to dose. Results: The differences between measured and calculated dose at shallower depths and small cones were significantly large (−120%). The differences reduced as the cone size increased. At deeper depths the measured and calculated dose agreed relatively better. However the disagreement was still large (20%). For all depths, the disagreements between measured and calculated dose plateau for cones >2 cm. Such large differences indicate that advance Monte Carlo based algorithm for inhomogeneity correction should be incorporated in treatment planning. Conclusion: It is concluded that Multiplan does not provide accurate dose calculation with inhomogeneities. The range of errors are from −120% to +20% in small fields. Additional investigation is needed to validate these finding with different detectors with known perturbations such as film, and diodes. This study provides a window in the problem of the dosimetry in small fields and has significant clinical implication for the treatment of lung cancer with CyberKnife.
SU‐GG‐T‐469: On the Dosimetric Characterization of Inclined Target Geometries Using Helical Tomotherapy
Authors: Yam M , Gutiérrez A , Shi C , Papanikolaou N
Published: July 2008

Purpose: To investigate whether the overshoot effect or “ballooning” of isodose lines into the rectum observed in tomotherapy patient treatment plans is caused by the sharp inclination of the posterior border of the prostate. Method and Materials: Cylindrical target and OAR volumes were segmented in a solid water phantom and placed adjacent to one another in an anterior‐posterior orientation at the center of the phantom. Individual treatment plans were created for the following angular orientations with respect to the y‐axis (IEC coordinates): Target0/OAR0: 0° above y‐axis, Target15/OAR15: 15° above y‐axis, Target30/OAR30: 30° above y‐axis and Target45/OAR45: 45° above y‐axis. Separation between the target and OAR remained fixed at 5 mm for all angles. Treatment plans were generated with a field width of 2.5cm, pitch of 0.287, and modulation factor of 2.5. For all plans, dose prescription was 98% of the target volume to receive at least 6.0Gy. Four additional treatment plans were created in which the target extended into the OAR. Results: None of the phantom plans showed any overshoot of the isodose lines into the adjacent OAR. In the phantom plans with the target extended into the OAR, the effect was not clearly seen however a slight indication of minimal ballooning was indeed observed. Additionally, it was noted that as the inclination angle increased the mean dose to the OAR increased. Conclusion: This study served to investigate the dependency of the target inclination on the isodose ballooning effect seen in prostate patients. Our phantom study results show that the angle of inclination does not produce a ballooning of the isodose lines but does lead to an increase in mean dose to the OAR as the angle is incremented. Future studies will investigate whether the extension of the target volume into the OAR is the cause.
SU‐GG‐T‐472: Architecture and Development Issues That Effect Treatment Planning Performance
Authors: Moyers MF
Published: July 2008

Purpose: Evaluate and compare treatment planning systems from different vendors. Method and Materials: Prostate, lung, and head and neck treatment plans were generated on systems from three different vendors using identical CT scans, beam arrangements, and margins. Comparisons were made concerning the available tools, ease of tool use, flow of the planning process, and tool integration. Results: The list of available tools was remarkably similar for all three systems however the tools functioned considerably different. Some differences existed with respect to the degree of automation and manual over‐rides. Large differences were observed in the planning efficiency and flow of the planning process. From the user's perspective, the architecture of long legacy systems with large customer bases appeared to use a linear model with multiple workspaces. The system with the shortest legacy and smallest customer base appeared to use a task oriented model with a single workspace. Several potential factors that could contribute to these differences were identified. These factors may conspire to prevent deep integration of cohesive tools thus limiting planning efficiency. Conclusion: Before purchasing a system, customers should evaluate the cohesiveness of tools, integration of tools, degree of automation, and workflow. Occasionally, vendors need to “bite the bullet” and revise their systems to ensure the various tools are integrated together in a cohesive fashion that promotes efficiency of the planning process.
SU‐GG‐T‐477: Influence of Pitch and Jaw Width On Helical Tomotherapy Head and Neck Treatment Planning
Authors: Moldovan M , Lee T , Chu C , Hogstrom K , Fields R , Gibbons J
Published: July 2008

Purpose: To investigate the influence of pitch and jaw width selection on helical tomotherapy treatment plans for head and neck. Method and Materials: Helical tomotherapy plans using a combination of pitch and jaw width settings were developed for three patients previously treated for head and neck cancer. Three jaw widths (5, 2.5 and 1 cm) and six pitches (0.86, 0.7, 0.43, 0.34, 0.287 and 0.215) were used with a (maximum) modulation factor setting of 4. The 18 plans per patient were generated using an identical optimization procedure (e.g., number of iterations, objective weights and penalties, etc.), based on recommendations from TomoTherapy. The plans were compared using isodose plots, dose volume histograms, dose homogeneity indices, conformity indices, radiobiological models, and treatment times. Results: Smaller pitches and jaw widths showed better target dose homogeneity and sparing of normal tissue, as expected. The conformity index had a maximum for pitches between 0.287 and 0.43. The tumor control probabilities were greater than 99% for the 2.5‐ and 1‐cm jaw widths for all pitches except 0.86. The treatment time increased inversely proportional to the jaw width. Conclusion: Our study indicates that the smaller jaw widths (2.5 and 1 cm) and pitches near the range of 0.3–0.4 produce better plans for helical tomotherapy head and neck treatment. However, because of the large increase in treatment time with only slight improvement in sparing of the critical structures for the 1‐cm jaw width, the 2.5‐cm jaw width, currently used in our clinic, remains our preference.

Supported in part by a research agreement with TomoTherapy, Inc.
SU‐GG‐T‐480: Displacement of Implanted Gold Fiducials Due to Rectal Marker Placed for Delineating the Luminal Surface of Rectum
Authors: Jang S , Wu H , Sourivong P , Katz S , Rosen L
Published: July 2008

Purpose: To evaluate the displacement of implanted gold fiducials secondary to the placement of a rectal marker to delineate the luminal surface of rectum. Method and Materials: Five prostate‐cancer patients were scanned by using a CT scanner (Brilliance Big Bore, Philips) for planning purposes. CT images were acquired with a rectal marker (Intermark Inc., UK) and then followed by images without the marker. For all of the patients, two or three gold fiducials were implanted into the prostate for image‐guided radiation treatment. Both CT images were fused and registered based on the bony structure and then the gold fiducials were contoured for evaluating the displacement due to the rectal marker. Displacements of gold fiducials with and without the rectal marker were computed. Also, displacements of gold fiducials against the isocenter as well as femoral head were computed for comparison. Results: Mean displacement of gold fiducials was less than 2.5 mm in all directions (lateral:0.12±0.15cm, vertical:0.21±0.29cm, longitudinal:0.14±0.27cm). However, the maximum displacement of fiducials in anterior‐posterior and superior‐inferior directions was approximately 3.5mm and all cases were skewed toward the anterior direction, demonstrating that the movement of gold fiducials due to a rectal marker was strongly related. Maximum change in distance between gold fiducials with and without the rectal marker was 0.12±0.13cm. In addition, maximum displacement between the gold fiducials and insocenter/femoral head with and without the rectal marker was 0.17±0.09cm for insocenter and 0.15±0.08cm for femoral head, respectively. This finding confirmed that the treatment margin of 5mm was adequate to cover the deformation of prostate as well as rectum caused by rectal marker insertion, especially in the anterior‐posterior direction. Conclusions: Overall, the relative positions of gold fiducials were minimally changed with and without rectal marker. However, the rectal marker forced up the rectal wall and prostate anteriorly (mean:0.21±0.29cm) with maximum displacement of 3.5mm.
SU‐GG‐T‐482: Effects of Normal Lung Delineation Using PTV Vs. GTV On NTCP and Vx for Lung Cancer Cases Planned with and Without Tissue Inhomogeneity Correction
Authors: Podder T , Harrison A , Werner‐Wasik M , Xia Y , Galvin J , Yu Y
Published: July 2008

Purpose: There is no clear consensus as to how to define normal lung volume; clinicians subtract either PTV or GTV from total lung volume. This study quantifies the dosimetric and biological differences between these definitions for radiotherapy planning with and without inhomogeneity corrections. Method and Materials: In this study, ten lung cancer patient cases with prescription doses (PD) 50–70Gy (average 62Gy) were evaluated. Dosimetric plans were generated using XIO‐CMS software. Complex (4–6 fields) 3D conformal radiation therapy plans with and without tissue heterogeneity corrections were compared. NTCPs were computed for three scenarios; respiratory pneumonitis (RP) endpoint of SWOG grade ⩾2RP (TD50=30.8Gy, n=0.99, m=0.37; NTCP1), SWOG ⩾1RP (TD50=28Gy, n=0.87, m=0.18; NTCP2), and radiographic & symptomatic pneumonitis (TD50=21.9Gy, n=0.8, m=0.37; NTCP3). Normal lung for NTCP was defined in two ways: [(left lung + right lung) − PTV], and [(left lung + right lung) − GTV]. Volume (Vx) of normal lung receiving 5Gy (V5), 13Gy (V13), 20Gy (V20) and 30Gy (V30) were also investigated. Results: For inhomogeneity non‐corrected plans: average PTV dose coverage was 97% (92.1–101.7%) of PD; with −PTV definition, average V5=41.3% (24.3–70.2%), V13=26.4% (15.9–36.8%), V20=22.2% (13.1–32.4%), V30=18.3% (10.9–27.8%), NTCP1=6.5% (2–16.5%), NTCP2=1.3% (0.01–8.6%), NTCP3=23.3% (6.7–51.2%); and with −GTV definition, average V5=43.2% (25.6–71.3%), V13=28.7% (17.9–39.2%), V20=24.7% (15.1–34.9%), V30=21% (13–30.5%), NTCP1=8.8% (2.6–20.7%), NTCP2=3.5% (0.02–16.3%), NTCP3=31% (9.7–60.8%). For inhomogeneity corrected plans: average PTV dose coverage was 99.6% (93.1–107.6%); with −PTV definition, average V5=43.7% (25.3–74%), V13=27.9% (17–42.7%), V20=23.2% (14.7–33.7%) and V30=19% (11.2–28.5%), NTCP1=7.8% (2.2–19.9%), NTCP2=2.5% (0.01–14.1%), NTCP3=27.9% (7.4–58.2%); and with −GTV definition, average V5=45.5% (26.5–75%), V13=30.2% (19.5–44.5%), V20=25.7% (16.1–36.1%), V30=21.7% (13.6–31.1%), NTCP1=10.7% (2.8–24.9%), NTCP2=6% (0.03–24.9%), NTCP3=36.5% (10.7–67.8%). Discussion and Conclusion: Results revealed that for heterogeneity non‐corrected plans, relative increase (with respect to −PTV definition) of Vxs were 4.7%, 8.9%, 11.3%, 14.7%, and NTCPs were 32.6%, 193.4%, 32.8%; and similar increases were observed for heterogeneity corrected plans.
SU‐GG‐T‐485: Comparison of Rush University In‐House Dose Optimizer and Nucletron IPSA
Authors: Liao Y , Kirk M , Turian J , Bernard D , Zusag T , Chu J
Published: July 2008

Purpose: We quantitatively compared the Rush University In‐house Optimizer (RUIDO) with the Inverse Planning with Simulated Annealing (IPSA) used in Nucletron Plato Treatment Planning System. Method and Materials: RUIDO uses Adaptive Simulated Annealing (ASA) optimization algorithm, which is capable of optimizing a plan based on either physical dose, EUD, or TCP/NTCP. In this study, only physical dose optimization is compared. Three Syed implant HDR cases were planned using both RUIDO and IPSA. For each case, the number and positions of the catheters remain the same, and dwell time for each source position is optimized. Based on the Dose‐Volume‐Histogram (DVH) from both optimized plans, detailed comparison of coverage (CI), homogeneity (HI), and overdose (OI), etc. were performed. The source strengths in RUIDO plans were scaled to provide similar CI as the corresponding IPSA plan during plan comparison. Results: For all three cases, the normalized RUIDO plans show similar CI (within 0.1%) as the IPSA plans. In two cases, RUIDO plans appear better with higher HI (9% and 16%) and lower OI (16% and 45%). In the third case, two plans were tied, as RUIDO plan shows preferable HI (13% higher) but less preferable OI (12% lower). In addition, for all three cases, mean dose in both plans agree within 7%. It takes 10–15 seconds for IPSA to optimize the plan once. However, it might take several more trials for the user to obtain the optimal set of parameters depending on the user's experience. RUIDO plan, on the other hand, can generate the optimized plan within minutes (1–16 minutes), but it only takes one trial to do this hence needs possible shorter overall planning time. Conclusion: RUIDO is able to provide equivalent plan with possible shorter overall planning time compared with IPSA and is less user‐dependent.

Partially funded by Varian Medical Systems.
SU‐GG‐T‐487: Field‐In‐Field Technique to Reduce High Dose Regions in the Treatment of the Axillary and Supraclavicular Lymph Nodes
Authors: Stevenson R , Jozsef G , Hitchen C , De Wyngaert J
Published: July 2008

Purpose: To eliminate the 10–30% excess dose in the standard parallel opposed beam treatment of the axillary and supraclavicular lymph nodes to decrease related skin toxicity. Method and Materials: In a field‐in‐field technique a new field was added to the supraclavicular (SCLAV) field with the projection of the posterior axillary boost (PAB) field blocked out. With appropriate weighting of the 3 fields equal (prescription) doses can be delivered to 3 cm under both the blocked and unblocked part of the SCLAV and to the midplane under the PAB field. The weightings can be obtained by solving a simple linear equation system (e.g. in Excel). The field‐in‐field technique was planned with Eclipse treatment planning system on 12 patient CT scans and compared to the standard two beam arrangement. Film measurements were made on phantom to evaluate the match of the field‐in‐field with the PAB field at a depth of 3 cm and at the skin surface. In addition, plans were calculated in phantoms to study the relationship of the weighting and the separation. Results: The prescription requirements, similar coverage to the standard technique, and minimal excess dose were all achieved with the proposed technique. The average maximum dose for the 12 sample CT scans decreased from 119.8% as treated to 107.9%. The match between the proposed SCLAV field‐in‐field and PAB field used was good and would be clinically acceptable. Conclusion: The proposed field‐in‐field technique provides more uniform coverage for the treatment of the supraclavicular/axillary region in the treatment of breast cancer. Since the third field is only a rearrangement of the MLCs in the supraclavicular field, it does not add much to either the planning or the treatment time.
SU‐GG‐T‐489: Inhomogeneity Correction On the Proton Beam in Radiotherapy Planning Using Computed Tomography for Head‐And‐Neck Tumors
Authors: Kim J , Kwak J , Shin D , Yoon M , Park S , Shin J , Lim Y , Kim D , Lee S , Park S , Cho K
Published: July 2008

Purpose: The inhomogeneous stopping power in a treatment planning system is determined by the relation of the hounsfield units (HU) of CT images to stopping power. To quantify differences between the dose distributions calculated by the proton treatment planning system and the actual dose distribution, because the inaccuracy of calculations may arise due to inhomogeneity by the presence of surface curvature, air cavities, and bony structures in the head‐and‐neck region. Method and Materials: We compared the two planning strategies for head‐and‐neck patients: (i) original CT; and (ii) corrected CT in which the HU values inside air cavity were replaced by −1000 of air HU value. Proton beam plans for original CT images were performed and then verification plans, using the same proton beam settings as those for the uncorrected CT images, were applied to the corrected CT images. In the plan comparison, the range shift of the proton beam and change of dose distribution between original CT and corrected CT images were revealed. Results: In some clinical data, DVH of plan using original CT demonstrated was different from the result using corrected CT. Target coverage is unaffected by whether the CT images are corrected or not, the irradiated volume of normal organs such as brain stem, optic chiasm, optic lens in verification plans with corrected CT is higher than that in plans with uncorrected CT. Conclusion: In proton radiotherapy, the correct prediction of proton beam range in treatment planning is critical for precise delivery of radiation does to the target. It could be important for purpose of range calculation of proton beams in treatment planning that the HU value of air cavity is replaced by optimal HU value making an allowance for CT artifacts for head‐and‐neck tumors.