bluuebunny/crossref_metadata_2025 huggingface dataset:
SU‐GG‐J‐103: Image‐Guided Adaptive Radiation Therapy for Head and Neck Cancer Using Adaptive Planning CT Images
Authors: Chan M , Sim S , Li J , Burman C
Published: July 2008

Purpose: The purpose of this study is to develop a practical methodology for near real‐time patient positioning and dosimetric re‐planning using CBCT. This study reports our investigation into the potential of using adaptive planning CT (ApCT) which combines the planning CT (pCT) and CBCT for dosimetric evaluation in the radiotherapy of head and neck cancers. Method and Materials: A Varian Clinac‐iX linear accelerator with the on‐board imager (OBI) was used to obtain CBCT images. The CBCT and pCT images of head and neck patients were registered weekly to generate ApCT images using our in‐house image registration software. The registration similarity measure was intensity based “mutual information” operated in the “neighborhood” region. The search was optimized using the “downhill simplex” algorithm. The registration registered internal anatomies on the pCT and CBCT, while the external contours were extracted from the CBCT and superimposed onto the pCT to form the ApCT. Dose distributions and dose‐volume histograms were re‐calculated based on ApCT. Depending on the results, a decision could be made whether re‐optimization was necessary — if needed it was performed using the ApCT. Results: It was found that the localized rigid image registration was satisfactory since most structures in head and neck patients were quite rigid. The doses to the respective PTVs and critical structures were compared between the initial treatment plan and the adaptive plans based on ApCT images. For those that failed to meet clinical criteria, the IMRT beams were re‐optimized using the ApCT images. Conclusion: Re‐planning with ApCT images was accurate, time efficient, and cost effective. The actual dose delivered could be accurately assessed based on the ApCT, which eliminates the potential dosimetric errors resulting from the uncertainty in inhomogeneity correction in CBCT‐based planning.
SU‐GG‐J‐111: Initial Dosimetric Validation of An Atlas‐Based Method for Automatic Intracranial Segmentation
Authors: Deeley M , Chen A , D'Haese P , Duggan D , Gensheimer M , Coffey C , Dawant B , Ding G
Published: July 2008

Purpose: We have developed a novel method for automatic segmentation of critical structures in the brain. The purpose of this study is to test the feasibility of this method as an alternative to manually‐derived physician contours. We test feasibility by evaluating the dosimetric consequences of auto‐segmentation versus physician‐drawn contours. Method and Materials: Brainstem, eyes, optic nerves and chiasm were segmented through non‐rigid registration of CT and MR‐based atlases to two patients. Patient A presents a challenging case in which a base of skull chondrosarcoma distorts normal brainstem anatomy. Patient B suffers from parotid disease and presents normal critical structure anatomy. Intensity‐modulated radiosurgery treatment plans were derived from physician contours and applied to the automatic contours. Results: For patient B (tumor far from critical structures) calculated doses for manual and automatic contours were within 2% of tumor dose for a given volume. Dose to the eyes, optic nerves, and chiasm of patient A were similar in agreement to those of patient B. The maximum dose to the brainstem of patient A, however, was 13% higher for the automatic contour. These dose differences were clinically negligible for all structures except the brainstem of patient A, in which case the difference was significant but acceptable. Conclusion: Clinical incorporation of our automated method is shown to be feasible dosimetrically. For the tumor lying far from the critical structures, the dose differences between automatically and manually‐derived contours were insignificant. The differences increased for the case in which a critical structure lay directly adjacent to a large tumor. These cases illustrate the system is accurate for critical structures far from the lesion but sensitive to local disturbance and inherently steeper dose gradients when the critical structures lie near the lesion.
SU‐GG‐J‐115: Investigating the Utility of Dose‐Functional Histograms in Risk Assessment of Thoracic Radiotherapy
Authors: Trofimov A , Gansemer C , Sharp G , Bortfeld T , Choi N
Published: July 2008

Purpose: Dose‐volume histograms (DVH) are commonly used for quality appraisal of treatment plans. However, since each volume of interest is treated as functionally and histologically uniform, the DVH reflects no information about the underlying anatomy. This simplification, while often reasonable, is excessive when applied to the lung. It has been frequently suggested that weighting dose distributions in thorax according to the relative functional integrity of the irradiated regions may provide better information for risk assessment of pneumonitis. We investigate the utility of dose‐density‐corrected histograms, and other potential dose‐functional metrics, in treatment plan evaluation, risk assessment of the treatment complications, and clinical decision‐making. Method and Materials: In the first approximation, the tissue density, estimated from CT data, can serve as a surrogate measure of lung perfusion, ventilation, or both. Potentially, the dose‐function histograms may be created based on more direct measurements of lung perfusion: e.g., with SPECT, helical CT with IV‐contrast, while ventilation may be estimated from point density changes between the extreme phases in respiratory‐correlated CT. For this study, clinical lung IMRT plans were analyzed using in‐house software, and DVH were compared to histograms corrected for tissue density (standard CT) and tissue‐density variations (respiratory‐correlated CT). Results: Generally, the DVH and density‐corrected histograms differ wherever density variations are present within the volumes of interest: e.g., in radiotherapy of tumors in the lung, paranasal sinuses. Substantial changes in certain histogram metrics were observed, e.g., the relative difference of over 20% between the dose to 60% of volume (D60) and that to 60% of the lung mass. Conclusion: The adjustments to the shape of the histogram, resulting from density‐correlated weighting of the dose, will result in a different data binning in the outcome analysis, which may, in its turn, lead to refinements in the risk assessment models.

Supported by NIH grants R01‐CA103904, RO1‐CA118200.
SU‐GG‐J‐127: Micro CT Imaging Evaluation for the Scaffold‐Guided Calvarial Bone Defects Repair in Small Animal Models
Authors: Hsieh H , Lee M , Chuang C , Hu Y , Tu S
Published: July 2008

Purpose: In recent years, micro CT imaging systems have been emerged as a promising imaging technique for the tissue engineering applications. We report our research results in using a commercial micro CT imager to evaluate the bone defect repair in small animals. Method and Materials: In this study, the animal model included Fischer 344 female rats. The surgery was performed at the top calvarial area and a circular‐shaped hole with diameter of 8.0 mm was cut out. Human mesenchymal stem cells with a baculovirus expressing BMP‐2 were engineered and implanted in the biodegradable scaffolds. These scaffolds were fabricated from polymers and then attached to the critical‐size area in the rats. In the experiments, there were three imaging groups including a control group for comparisons. A period of 12 weeks was scheduled to complete the micro CT imaging experiments. The imaging modality was a commercial SkyScan 1076 micro CT system. Mathematical methods, including density slice technique, were applied to quantify the bone regeneration progress. Results: Based on our preliminary analysis from the volumetric images reconstructed from the micro CT projections, significant mineralization can be detected at week four and full bony bridging in the critical‐size areas can be observed at week 12. The images were acquired at the settings of 80 kVp for the x‐ray energy, 124 μA for the current, 316 ms for the exposure time, and 360 projections for one full circular rotation. Conclusion: We have succeeded in using the micro CT imaging technique to quantify the scaffold‐guided bone repair progress in a small animal model. Our images showed that the full bony bridging can be observed at the week of 12. The results indicated that the feasibility of future potential clinical applications of baculovirus‐engineered mesenchymal stem cells in human bone regeneration.
SU‐GG‐J‐129: Monte Carlo Calculations of Secondary Electron Spectra for Various Mixtures of Gold and Water Mimicking Tumors Loaded with Gold Nanoparticles
Authors: Jones B , Cho S
Published: July 2008

Purpose: To determine the change in secondary electron spectrum due to gold nanoparticles present in a tumor under a treatment scenario of gold nanoparticle‐aided radiation therapy (GNRT). Method and Materials: The Monte Carlo code, EGSnrc, was modified to obtain the spectrum of secondary electrons. Simulations were performed with a water phantom containing a small gold‐loaded region at various gold concentration levels (0.1–3%). The atom of origin and energy of each Compton and photoelectron was tracked, yielding the secondary electron spectrum of gold and water for each simulation. Results: The presence of gold caused a significant increase in the number of photoelectrons generated. With 0.1%, 1%, and 3% gold in the region of interest, the total energy transferred to electrons through the photoelectric and Compton processes increased by 1.5%, 15%, and 49%, respectively. Conclusion: The spectra generated in this study are helpful for a better understanding of the physical mechanisms responsible for dose enhancement within a gold‐loaded tumor during GNRT. They can also be used as inputs for detailed history Monte Carlo calculations to perform a nanodosimetric estimation of tumor dose enhancement in the vicinity of a gold nanoparticle.
SU‐GG‐J‐136: Nanoshell‐Mediated Laser Induced Thermal Therapy in An In Vivo Model
Authors: Springer A , Shetty A , Elliott A , Paquet M , Wang J , Scwartz J , Hazle J , Stafford R
Published: July 2008

Purpose: To evaluate the thermal response of intravenously‐injected laser‐activated gold‐silica nanoshells in an in vivo tumor xenograft model using MRTI and histopathology. Method and Materials: PC3 cancer cells were inoculated subcutaneously on the backs of SCID mice and allowed to grow to ⩽1 cm diameter before treatment. Gold nanoshells (Nanospectra Biosciences Inc., Houston, TX) were injected intravenously in experimental mice. Saline was injected in control mice. Twenty‐four hours post‐injection the tumors were irradiated extracorporeally for 3 minutes by a coherent, near‐infrared diode laser (808 nm). Laser output was 4 W cm−2 to all tumors. Spatiotemporal temperature distributions in the tumors were monitored in real‐time during procedures by Magnetic Resonance Temperature Imaging (MRTI). Maximum temperatures and temperature profiles with respect to depth were measured from the MRTI data and the depths at which thermal ablation occurred were determined. Depths to which necrosis was induced and areas of necrosis relative to tumor areas were measured from histological (H&E) images. Results: There was a statistically significant difference in maximum temperature changes achieved (40.9±2.6°C with nanoshells versus 19.2±2.7°C without) (p<0.001). The average temperature increase in the nanoshell mice caused greater amounts of necrosis in tumors, in terms of depth (2.7±1.7 mm versus 0.1±0.2 mm) (p<0.01) and relative area (67%±37% versus 0%) (p<0.001). The depths to which ablative temperatures could be achieved for standard body temperature were also greater (p<0.05) in nanoshell mice (2.8±0.7 mm with nanoshells versus 0.4±0.6 mm without.) Conclusion: Laser‐induced ablation can be induced in nanoshell‐containing tumors at otherwise sub‐lethal laser powers.
SU‐GG‐J‐140: Online Re‐Planning Using Direct Machine Parameter Optimization: A Non‐Human Primate Lung Irradiation Study
Authors: Lerma F , Liu B , Wang Z , Yu C , Yi B , Koshy M , Farese A , Macvittie T
Published: July 2008

Purpose: Establish the feasibility of CBCT‐based, online re‐planning using direct machine parameter optimization (DMPO) to irradiate the total lung parenchyma in non‐human primates (NHPs). A study of acute‐radiation‐exposure‐ induced pneumonitis in NHPs involves total lung irradiation (TLI) to 12.5 Gy in a single session, where acute radiation exposure may also lead to the lethal hematopoeitic and gastrointestinal subsyndromes. The latter syndromes occur at 7–8 Gy and at 12–14 Gy, respectively, 1 and 2 weeks after irradiation. Hence, high dose conformity is essential to spare bone marrow, cord and bowel. Method and Materials: An offline DMPO plan is created on a CBCT scan of the NHP several days prior to the TLI session; using seven, 6MV photon beams. TLI targets a PTV from a 1 cm expansion of lung contours, sparing tissues outside the PTV. On the day of the irradiation a new CBCT is acquired, loaded into the planning system, and contours and DMPO dose objectives are propagated from the offline plan. DMPO is executed online using the original off‐line plan as starting points and the resultant on‐line plan is compared to the offline plan, on a Philips Pinnacle 8.1v workstation. Results: Online‐ and offline‐planned DMPO plans achieve V95 > 95% coverage of the PTV, and excellent bowel sparing. However, offline‐planned DMPO delivers > 7 Gy to more than 50% of the cord volume, while online‐planned DMPO delivers < 6 Gy to more than 50 % of the cord. Online DMPO re‐optimization from scanning to the start of irradiation takes less than 45 minutes. Conclusion: Online DMPO‐based re‐planning of NHP total lung irradiations is established in support of a study of radiation‐induced pneumonitis in non‐human primates. Online‐planning on NHPs is valuable experience prior to clinical implementation in human patients.
SU‐GG‐J‐144: Photon Detection Efficiency of TomoTherapy® Array Detectors and Impact On MVCT Image Quality
Authors: Chao E , Fang G , Beneke M , Ruchala K , Olivera G
Published: July 2008

Purpose: To quantify the photon detection efficiency of the TomoTherapy® X‐ray detector and assess its impact on megavolt CT image quality. Method and Materials: The signal to noise ratio of two similar X‐ray detector arrays available on TomoTherapy® Hi⋅Art® systems was measured during exposure to the megavolt X‐ray imaging beam. These measurements are compared to calculations of the ideal signal‐to‐noise ratio if all incident photons were detected to determine the detective quantum efficiency (DQE). Similarly, the signal‐to‐noise ratio is measured in CT images reconstructed from data collected by these detector arrays and compared to theoretical predictions. Results: The detection efficiency of the out‐of‐focus detector cells are 11% and 18% for the two types of detector arrays studied. The efficiency drops for the central detector cells that are aligned toward the source. The signal‐to‐noise ratio in the resulting CT images is 19.7 and 28.4 using a dose of 1 cGy to the center of the 30 cm diameter solid‐water phantom. These image signal‐to‐noise ratios are in good agreement with previous theoretical predictions at this dose level, beam energy, and detector efficiency. Conclusion: Compared with conventional flat‐panel detectors that can have detection efficiencies of only 1 – 2% at megavolt energies, the TomoTherapy® detector arrays are an order of magnitude more efficient, resulting in better signal‐to‐noise ratios in the resulting CT images or a lower dose (or a combination of both).
SU‐GG‐J‐147: Pitfalls of the ITV and PTV Dose in Thoracic Anatomy and Composite 4D Dose Calculation
Authors: Wu J , Fuss M , Shekhar R , D'Souza W
Published: July 2008

Propose: To point out conceptual errors in the ITV and PTV dose of thoracic anatomy with composite 4D dose calculation. Method and materials: Organs in the thorax and upper abdomen are susceptible to respiration induced internal organ motion. Due to organ motion, the dose delivered to tissues may differ from the planned dose. Four‐dimensional CT imaging and elastic image registration of the 4DCT images can be used in the delivered dose estimation. First, a treatment plan was generated on one of the CT image sets in the 4DCT. The plan parameters were copied to the remaining 3DCT images and the dose was recalculated on each individual image; next, the recalculated doses were warped to the reference CT to form a composite 4D dose using transformation fields obtained from elastic image registration of the 4DCT. The composite 4D dose was treated as an accurate estimation of the dose delivered to the patient. In the literature, variation of the ITV and PTV doses corresponding to the different respiratory phases has been reported. In this study, using a stereotactic body radiation therapy of lung tumor as an example, we point out conceptual errors in the delivered dose estimation. Results: It was shown that the ITV‐based SBRT plans can delivery the prescribed dose to the tumor (GTV). However, the tissues constituting the ITV, and hence the PTV, in the context of mobile anatomy depend on the particular phase of the respiration cycle. Conclusion: The comparison of the ITV and PTV dose at different respiratory phases is erroneous. Its usage can easily lead to the incorrect calculations of the delivered dose to the patient. We therefore believe that while the ITV and PTV can be used to guide the addition of treatment margins, 4D dose calculations are only relevant to the GTV.
SU‐GG‐J‐148: Planned Target Dose Vs. Delivered Target Dose for Prostate IMRT Treatment
Authors: Pawlowski J , Ding G , Coffey C , Yang E , Malcolm A
Published: July 2008

Purpose: Inter‐fraction motion and deformation of a tumor target may result in an altered dose distribution delivered during radiotherapy than planned. On‐board cone‐beam CT (CBCT) imaging was used to study the dose delivered throughout the treatment course to a prostate cancer patient treated with IMRT. Method and Materials: IMRT planning was performed using conventional planning CT. During treatment the patient was positioned using reference skin markers and subsequently adjusted using image guidance based on implanted gold seeds in the prostate. CBCT images were taken weekly from a Varian Trilogy throughout a seven‐week treatment period and the targets and organs at risk were re‐contoured by oncologists. The original IMRT plan was applied to volumetric CBCT using both the treatment position with and without image guidance. Dose‐volume histograms (DVHs) were calculated. Results: Analysis of DVHs indicates that there is significant variability in the dose delivered to the target region due to inaccuracy of patient positioning and deformation of target contours. The minimum dose to 98% of the prostate PTV calculated without image guidance ranges from 49 to 84% (average 67%), whereas with image guidance the minimum dose ranges from 84 to 99% (average 92%). Bladder and rectal doses with and without image guidance depend on variation in bowel distention and bladder filling. Conclusion: This study provides information of the actual integrated dose that the patient received throughout the entire course. It also suggests that deformations of the PTV due to changing bladder and rectum content are dosimetrically significant such that patient setup achieved using fiducial markers may not be sufficient to ensure adequate dose coverage of the PTV. Additionally, an increased dose to organs at risk such as rectum and bladder may be prevented with this emerging technique. Conflict of Interest: The research is partially supported by a Varian research grant.
SU‐GG‐J‐173: Study of CT Number Dependence On Hardware Settings and Post Image Processing in Micro CT Systems
Authors: Tu S , Hsieh H , Lee M , Low C , Hsiao Y
Published: July 2008

Purpose: In recent years, micro CT systems have been emerged as a promising imaging modality for small animal model study. We report our efforts in the research investigation on the variations of CT numbers at various hardware and software settings in a commercial micro CT scanner as CT numbers are crucial to some imaging applications such as the bone mineral density evaluation. Method and Materials: In this work, a commercial micro CT imaging system (SkyScan 1076) was used to acquire the projection images. A modified Feldkamp reconstruction method was integrated in the scanner. Four different phantoms including lung, adipose, muscle, and bone were imaged at various energy values and then CT numbers were calculated at the mid plane of the phantoms. Different metallic filters in the x‐ray tube unit were used in the study. We also implemented two major image artifact reduction techniques (beam hardening and ring artifacts) and studied how those post image processing would effect the CT numbers. Results: The variability in CT numbers for different filters was −800 to −600 HU in lung phantom, −300 to −150 HU in adipose phantom, +20 to +70 HU in muscle phantom, and +1000 to +3000 in bone phantom. Meanwhile, the CT number variations ranged from +1000 to +4000 for the bone phantom when beam hardening reduction algorithm was applied. Conclusion: We have conducted a series of study for the variations in CT numbers computed from images acquired from a commercial micro CT imager at various hardware and software settings. Four uniform phantoms designed for the micro CT systems with known materials and densities were used. Imaging parameters included the x‐ray imaging energies, filters, and software settings. Accurate CT numbers are important and critical to the quantification applications such as the bones density calculations and tissue characterization.
SU‐GG‐J‐180: The Role of MRS in Radiation Therapy: Correlation Between T2‐Weighted MRI, Biopsy and MRS
Authors: Hossain M , Chen L , Buyyounouski M , Milestone B , Richardson T , Ma C
Published: July 2008

Purpose: To investigate the possibility of using objective parameters in MRS of the prostate to identify areas of viable cancer that might need higher radiation therapy dose. Clinically, MRI and biopsy results may not always agree. MRS data can be very helpful in objectively defining a set of parameters that can positively identify cancerous regions. Method and Materials: Patients who are treated for prostate cancer using radiation therapy in our department are analyzed retrospectively. These patients are imaged using a GE 1.5T MRI scanner. T2‐weighted MRI images and MR spectroscopy are performed on these patients. MR spectroscopic data, in particular the ratio of Choline and Creatine to Citrate can be used to identify areas of the prostate, which could be boosted with additional dose since those areas are more prone to radiotherapy failure. In this study an attempt is made to define a set of parameters that will enable identifying areas of the prostate needing higher dose. As a first step, the correlation of biopsy‐positive data, MRS data, and expert reading of the T2‐weighted MR images by a radiologist is investigated. The prostate is divided into six parts, namely Right and Left parts of the Base, Mid, and Apex regions for this study. MRS data is gathered using the GE prostate protocol PROSE. An endorectal coil with the balloon filled with perflubrom is used in the MRS data acquisition. Results: Preliminary results from the analyses of four patients show a varied degree of correlation between the biopsy, MRS data, and the radiologist's readings although a minimum MRS ratio for the cutoff of cancer‐positive identification can be found. Conclusion: Objective cutoff parameters can be found for a population of prostate cancer patients to identify regions of the prostate, which are candidates for additional higher radiation dose.
SU‐GG‐J‐183: The Use of Video Feedback for the Improvement of Respiratory Gating Radiation Treatments
Authors: Zatwarnicki C , Wang Z , Yin F
Published: July 2008

Purpose: Breath‐hold respiratory gating is currently being used to improve the quality of radiotherapy, especially for sites subject to internal motion. One technical challenge that exists is patient regulation of respiration. Typically, the radiation therapist verbally coaches the patient during treatment. This may not be the most efficient or accurate practice, since the patient is receiving delayed commands based on secondary cues. As such, a video feedback system has been implemented. During treatment, a live plot of breathing amplitude versus time is displayed to the patient for first‐hand adjustments. Method and Materials: Varian's RPM system was used to track the patient's respiratory motion. i‐O Display Systems' i‐Glasses, a headset displaying video feed, was worn by the patient for visual feedback. The RPM software interface was displayed in this headset. Patients with treatment sites in the abdomen and thorax, such as lung, liver, and breast cancer, were of interest in this study. Those who were candidates for breath‐hold gating treatment were selected for either video feedback or verbal coaching respiratory management techniques, based on the patient's physical and mental condition. Results: The accuracy and efficiency of the video feedback system, using i‐Glasses, was compared to the more conventional system of audio commands given by the therapist. Accuracy was based on the magnitude of motion within the threshold, while efficiency was based on beam‐on time during a particular field of treatment. Conclusion: Video feedback proved better accuracy and efficiency of treatment compared to audio coaching. However, not all patients were candidates for the video feedback system. Some patients did not understand what the plot of amplitude versus time meant. Some patients could not resolve the plot, due to poor eye sight. Other patients found the headset to be uncomfortable during treatment. These problems are currently being taken into consideration for future studies.
SU‐GG‐J‐199: Phase and Displacement Incorporated Model for Determination of Tumor Position with the Berathing Surrogate (RPM)
Authors: Gui M , Yi B , Han‐Oh Y , Lim S , Ahn S , Yu C
Published: July 2008

Purpose: Breathing surrogate, such as Real‐Time Position Monitor (RPM), is most commonly used to predict internal motion due to the difficulties in directly measuring internal motion. In this study, to account for both linear correlated displacement and possible phase shift between RPM signal and internal organ motion, a new model that incorporates both displacement and phase of RPM signal was proposed and its accuracy was estimated and compared with that of other models. Method and Materials: The diaphragm motion was traced from fluoroscopic lung procedures for 10 patients, with respiratory surrogate signals acquired simultaneously with RPM system which tracks motion of reflective markers mounted on the abdomen with an infrared‐sensitive camera. To estimate the diaphragm motion, a general linear function using RPM displacement as the input, a 6th order polynomial using RPM phase as the input, and a new model we proposed as the weighted sum of the previous two, using both RPM displacement and phase as parameters, were used. Least square fitting was applied for all the three models respectively to derive the corresponding parameters. Respiratory gating (30% duty cycle) was performed retrospectively based on the three diaphragm motion trajectories from the three models, and 95th percentile residual motion was evaluated for each patient. Also, deviations of the estimated trajectory from the true trajectory were calculated as 95th percentile tracking error for each model. Paired t‐test was performed and p<0.05 was determined significant. Results and Conclusion: All of the 10 patients demonstrated significantly smaller tracking error (∼1.5mm, p<0.05) and residual motion (∼1.2mm, p<0.05) when using the new model compared to the other two, indicating that this new model is more effective in estimating the diaphragm motion from breathing surrogate (RPM) signals compared to the models using either RPM displacement or phase as the only parameter.
SU‐GG‐T‐07: I‐125 Seed Dosimetry in the Near Field Using Gafchromic® EBT Film
Authors: Chiu‐Tsao S , Hanley J , Napoli J , Fan J
Published: July 2008

Purpose: To perform radiochromic EBT film dosimetry in the near field of model MED3631A/M I‐125 seed and to determine the TG‐43 dosimetric parameters. Method and Materials: Gafchromic® model EBT films (lot♯35076) were horizontally positioned, one at a time, above and in contact with a NAS model MED3631A/M I‐125 seed horizontally placed at the center of a solid water phantom (RMI457, 30×30×20cm3). A multiple‐film technique was employed. To cover the distance range of 0.06–5cm, 50 EBT films of different sizes (4×4–12×12cm2) were irradiated by 10 individual seeds (NIST traceable air kerma strengths ∼10U, based on current NAS calibration) using different exposure times (1–295hr). A separate set of 19 calibration films was exposed to 50kV x‐rays at 100 cm SSD for doses of 0.2–40Gy at the University of Wisconsin. All experimental, calibration and background films were scanned using a CCD100 densitometer using a green light source with fine spatial resolution (0.2mm). Based on the established calibration curve, dose conversion for the experimental films and generation of the TG‐43 dosimetric parameters (for L=4.2mm) were achieved using IDL v6.0. Results: The dose rate constant was 1.004 cGy/Uh. The radial dose function was obtained for the radial distances from 0.06 to 5cm. The 2D anisotropy functions for distances from 0.1 to 5cm were determined. General agreement with published values and those recommended by TG‐43U1 report for r⩾0.5cm was found. The near field data for 0.25cm is close to Rivard's Monte Carlo results for the “ideal capsule orientation”, but different from the TG43U1 consensus values based on Rivard's weighted “diagonal” and “vertical” capsule orientations. Conclusion: Using EBT film dosimetry with the multiple‐film technique, TG‐43 dosimetric parameters were determined for model MED3631A/M I‐125 seed in the near field and out to 5cm, allowing for accurate treatment planning calculations. Research supported partially by North American Scientific Inc.
SU‐GG‐T‐09: Dose Accumulation From Film‐Based Brachytherapy Planning and CT‐Based External Beam Radiotherapy Planning
Authors: Lian J , Cullip T , Deschesne K , Chang S
Published: July 2008

Purpose: Radical radiation therapy that combines external beam therapy (EBRT) and brachytherapy (BRT) is effective in managing local‐regional confined cervical cancer. Although CT is widely used for EBRT treatment planning (TP), traditional 2D film is still commonly used today in many institutions for BRT TP. The incompatible image information between the BRT and EBRT leads to great difficulty in computation of the cumulative radiation dose from both treatments. To date, doses to target and critical structures are approximated by adding the point doses of BRT to the EBRT plan. In this project, we propose to register the orthogonal films of BRT and CT of EBRT so as to accumulate the doses in a more accurate way. Method and Materials: Five patients treated with both EBRT and low dose rate BRT were retrospectively used in the study. 3D dose grids from the BRT and EBRT treatment planning were merged by a film‐to‐CT registration, which was accomplished by creating DRRs from EBRT‐CT in the same projections of the BRT films. A landmark based image registration tool was developed to register films and DRRs of CT. The calculated shift, rotation and scaling were applied on the dose grid of BRT. Once the registration was completed, dose distributions from EBRT and BRT were merged. Results: A composite plan with accumulated doses of BRT and EBRT was created for each patient. The dose accuracy was verified at relevant points such as “A” points. The isodose curve and DVHs were analyzed, and appear reasonable. Conclusion: We developed a method to accumulate the dose distributions from film‐based brachytherapy and CT‐based external beam radiotherapy. This yielded a more realistic estimate of the cumulative dose received by the patient from both treatments.
SU‐GG‐T‐132: Evaluation of Hybrid Plan IMRT QA with a 2D Ionization Chamber Array
Authors: Depuydt T , Gomola I , Van den Heuvel F
Published: July 2008

Purpose: To evaluate suitability of a Helical Dosimetry® system (IBA), consisting from a 2D ion chamber array and a dedicated plastic phantom for hybrid plan IMRT QA. Method and Materials: For verification of IMRT and rotational dose delivery techniques the 2D array was placed horizontally in the phantom. A dose calculation of the hybrid treatment plan was performed on a CT scan of the phantom with unchanged treatment parameters. The measurements are compared with the calculated dose distribution in the active plane of the detector to verify the dose delivery. The angular dependence of the detector response was measured in 6 and 18 MV x‐ray beams. Due to the large amount of air cavities in the 2D array detector, the performance of the dose calculation algorithms was investigated both for pencil beam convolution (PBC) and the AAA algorithm of the Varian Eclipse TPS. In addition, the attenuation of the iBeamTM carbon couch top was quantified with 2D array measurements at different gantry angles. Results: Discrepancies with the calculation relative to the maximum field dose of up to 7% observed were due to the couch top attenuation and angular dependence of the detector. For IMRT treatment fields delivered to the backside of the detector and passing through the couch, the couch top attenuation contribution was approximately 4%. The angular dependence of the detector added another 3%. For beams with angles parallel to the detector plane, the AAA calculation showed better agreement with the measurement than the PBC. For the evaluation of the total dose of the hybrid plan the Δdose=3% and Δdistance=3mm criterion could be applied in the gamma evaluation. Conclusion: The Helical Dosimetry® system is a feasible solution for hybrid plan IMRT QA. It is advised to account for couch top attenuation and to apply advanced dose calculation algorithms.
SU‐GG‐T‐137: Patient Based IMRT QA Using Fluence Map Measurements
Authors: Webster G , Whitehurst P , Mackay RI , Rowbottom CG
Published: July 2008

Purpose: Traditionally IMRT QA involves measuring dose maps and comparing to the planning system using a gamma analysis. This methodology has been successful, but removes the link between the measured dose maps and DVHs relating to the patient used to define a clinically acceptable plan. A system containing a feedback loop for the treatment planning system and evaluating the dose distribution based on measured dose maps has been developed for H&N IMRT. Method and Materials: After an IMRT plan has been created, dose maps measured by film at Dmax and 0° Gantry are measured for each IMRT beam. The dose maps are converted to fluence maps by deconvolving the dose map with a Gaussian point spread function approximating phantom scatter present in the measurement geometry. The derived fluence maps are then re‐introduced into the Philips Pinnacle treatment planning system and the dose re‐calculated to provide a dose distribution based on the patient anatomy and the deliverable fluence from each beam. Once re‐calculated, DVH comparisons and dose statistics can be used to assess the deliverable plan. Results: The methodology is currently in the testing phase for H&N IMRT plans. The feedback system has provided a comparison of the planned and deliverable dose distributions for 5 IMRT plans so far. The methodology has shown some differences in doses delivered to the nodal regions, and small differences to organs‐at‐risk such as the spinal cord and contra‐lateral parotid. To date, no clinically significant differences in dose have been found by the system providing confidence in our H&N IMRT programme. Conclusion: The introduction of an IMRT QA system based on delivered fluence maps to establish the delivered dose distribution to the patient has been developed. The system allows the IMRT QA to be assessed based on clinical tolerances to the individual patient.
SU‐GG‐T‐139: Evaluation of a Biplanar Diode Array Detector for MLC‐ and Compensator‐Based IMRT QA
Authors: Feygelman V , Javedan K , Nilsson G , Forster K
Published: July 2008

Purpose: To evaluate a biplanar diode array detector, with two types of modulators used for IMRT — MLC (step‐and‐shoot) and brass compensators. All calibration procedures were evaluated. Method and Materials: An absolute calibration procedure in the rectangular PMMA phantom was evaluated and compared to water phantom measurements. IMRT plans were prepared on CMS XiO and modulated by either 120‐leaf MLC or brass compensators. 10 MLC based plans and 7 compensator plans were tested. Dose calculations were done on a homogeneous dataset and on an MVCT of the Delta4 phantom. Results: Effective point of measurement is at the midplane of the detector board. The PMMA to water conversion factor for absolute calibration with ion chamber is 0.988. An effective TMR calculation yields the same calibration dose. Angular response range is 0.977–1.033 (0.977–1.010 if the angles of ±5° around the detector planes are excluded, consistent with the intended use of the device). Point doses along the fan lines differ −0.6 to +0.3% from those calculated on a homogeneous dataset, and −0.9 to −2.2% from those on the MVCT dataset. For 7 compensator plans, the average percentage of detectors with γ(5%/3mm)⩽1.0 is 97% (range 89–100%). For 10 MLC plans the average is 93% (72–100%), but the statistics may be skewed because some detectors are frequently at the very edge of the beamlet aperture while others are always at least 5 mm away. Conclusion: For absolute dosimetry measurements the PMMA output to water conversion factor needs to be determined using an appropriate calibration technique. There was no advantage in utilizing an MVCT dataset. It is desirable to have a software modification (isocenter shift) to ensure that the reported dose statistics are always representative of the population. Conflict of Interest: G. Nilsson is President of ScandiDos, who loaned the unit for evaluation at no charge.
SU‐GG‐T‐144: IMRT Pre‐Treatment Plan and In‐Vivo Verification with 2D‐ARRAYs and Multi‐Wire Ionisation Chambers
Authors: Poppe B , Looe H , Ruehmann A , Harder D , Willborn K
Published: July 2008

Purpose: A concept for quality assurance and IMRT plan verifications with 2D‐ARRAYs and the DAVID‐ System is presented. Method and Materials: A 2D‐ARRAY (PTW‐Freiburg) and a multi‐wire ionization chamber (DAVID‐system, PTW Freiburg) are used in this work. The DAVID‐system is a translucent, multiwire transmission‐type ionisation chamber, placed in the accessory holder of the accelerator. Each detection wire is positioned in the projection line of a MLC leaf pair. The signal of each wire is proportional to the line integral of the ionisation density along this wire. The 2D‐ARRAY is used for daily dosimetrical checks (dose on central axis, MLC calibration, symmetry, flatness, energy) and for pretreatment IMRT plan verification. During the dosimetric verification of an IMRT plan reference values are measured with the DAVID‐system and stored in a patient specific database. During daily treatment the signals are re‐measured and compared to the reference values. Thus a direct connection to the IMRT plan verification is possible. A warning occurs if a deviation beyond a chosen threshold is detected. In an “expert” mode the physicist can analyse each single segment of the plan and detected errors are related to MLC pair. If necessary a re‐verification with 2D‐ARRAYs can be performed. Results: The application of the concept for standard IMRT cases and examples for error detection capabilities (e.g. de‐calibrated MLCs, neglected segments) are shown. For typical IMRT plans with field sizes beyond 10 cm × 10 cm the DAVID‐ system is able to detect positioning errors of MLC pairs in the sub‐millimeter region. Conclusion: The procedures can be used during daily routine with a minimum of additional time and are assuring a closed dosimetrical QA loop for IMRT.
SU‐GG‐T‐147: Automatic Detection of MLC Errors in VMAT Delivery
Authors: Usynin A , Ramsey C
Published: July 2008

Purpose: The purpose of this study was a) to characterize the effects of deviations in MLC leaf positions and velocity on Intensity Modulated Arc Treatment (IMAT) delivery, b) to detect MLC leaf pre‐collision events. Method and materials: MLC position errors were measured performing IMAT delivery sequences produced by a planning system. The MLC delivery files were converted into a single arc using “in‐house” software. Each test plan was delivered with three different beam‐on times. The dose rates for the plans were adjusted accordingly. MLC leaf positions were recorded using the MLC controller at a sampling rate of 20 sec−1. Results: In the test sequences the MLC leaves exhibited a number of positional errors greater than 0.3 cm when moved from or to the “under‐jaw” position. The MLC leaf position errors arose from the discrepancy between the planned and the maximum velocity the leaf was able to attain. Deviations in the leaves positions may cause the leaves to get closer than 0.5 cm to each other, which was considered as a potential collision. Performing the IMAT delivery we were able to detect leaves which had the interleaf distance of 0.15 cm. A possible cause of the detected pre‐collision events was the degraded performance of the leaf motor. Conclusion: Leaf position errors caused by the rapid leaf movements from or towards the “under jaw” position tend to induce errors in the segments where the leaf is expected to assume a certain position different from the “under jaw”. An algorithm has been developed to reveal segments that are prone to leaf position errors caused by “slow” leaves. A method to detect segments where there is a high risk of leaf collisions has been developed and applied to the performed delivery sequences. An analysis of relative positional errors has been performed.
SU‐GG‐T‐149: 4D IMRT Quality Assurance (QA) by Dynamic Log File Analysis
Authors: Xu J , Roland T , Shi C , Papanikolaou N
Published: July 2008

Purpose: 4D IMRT was a technique developed to account for dosimetry errors from intra‐fraction motion, especially from anatomy transformation caused by respiration. To perform 4D IMRT dose validation some dynamic phantoms are used. Measurements with dynamic phantom involve the complex setting procedure. In order to simplify the QA procedure and quickly check the IMRT plan, a 4D IMRT verification tool was developed. The delivered fluence map to a moving target can be simulated from dynamic log (Dynalog) files using this tool. Method and Materials: A 4D CT was acquired for the patient. The breathing cycle was divided into 10 phases. An IMRT plan was generated for the exhale phase, and its leaf sequence was exported. The original leaf sequence was modified by superimposing the tumor trajectory onto the leaf trajectory in a two dimensional beam's eye view plane. The modified leaf sequence was delivered by Varian 2100 CD with Millenium 120 leaf MLC. After dose delivery Dynalog files were analyzed by an in‐house software to simulate the fluence map of a moving target. The fluence map measured by the dynamic phantom was compared with the simulation of delivered fluence map. Results: The comparison between the measured fluence map using the dynamic phantom and the simulated fluence map by Dynalog files shows that the difference of isodose lines was within 3mm and the difference of line profiles was within 3%. Conclusion: A 4D IMRT QA tool was developed to simulate the delivered fluence map to a moving target. The simulated fluence map shows a good agreement with the measured fluence map based on dynamic phantom measurements. When compared to phantom measurement, this tool can assist to quickly check the 4D IMRT plan and handle irregular tumor trajectories.
SU‐GG‐T‐160: Single Field IMRT QA Is Insensitive to Dosimetric Errors
Authors: Kruse J
Published: July 2008

Purpose: To assess the sensitivity of single field planar dosimetry in detecting IMRT dosimetric errors. Method and Materials: Based on ion chamber measurements of the total plan in a cylindrical phantom, three IMRT plans were found to be calculated accurately. Each plan was then reoptimized with aggressive dose constraints so that the new treatment fields were highly modulated. Highly modulated fields are calculated less accurately in our planning system, and ion chamber measurements confirmed these plans contained significant dosimetric errors. All six plans (three clinical, three faulty) were analyzed with two means of individual field planar dosimetry — portal dose imaging with an EPID and a commercial ion chamber array. Gamma analysis was performed with 2%/2mm and 3%/3mm criteria to determine a score which would detect the three flawed IMRT plans. Results: With the EPID and 2%/2mm criteria, the fraction of pixels passing gamma were between 0.94 and 0.96 for both the acceptable and flawed IMRT plans. (Standard deviation = 0.02 to 0.03) With the ion chamber array and 2%/2mm criteria, the acceptable plans had gamma scores from 0.92 to 0.95 (standard deviation = 0.04 to 0.05) and the flawed plans had scores from 0.87 to 0.98 (standard deviation = 0.01 to 0.07). The difference between acceptable and faulty plans was diminished further when gamma criteria were expanded to 3%/3mm. No threshold was found for any analysis which could predict dosimetric accuracy. Conclusion: Deconstruction of an IMRT plan for field‐by‐field QA requires complex analysis methods such as the gamma function. Distance to agreement, a component of the gamma function, has clinical meaning in a composite plan, but when applied to individual, highly modulated fields, it can mask real dosimetric errors. While planar dosimetry may comprise one facet of an effective QA protocol, it is insensitive to dosimetric inaccuracies.
SU‐GG‐T‐168: Measurement Errors Associated with Linear Accelerator Commissioning Data
Authors: Fox C , Sun J , Simon T , Simon W , Palta J , Liu C
Published: July 2008

Purpose: Linear accelerator (LINAC) commissioning employs 3‐D water phantoms (WP) to accurately assess LINAC dosimetric characteristics. The accuracy of the treatment planning system (TPS) modeling is based on the collected data. This investigation aims to assess the errors associated with the collection of commissioning data. Method and Materials: Possible sources of error within 3‐D WP measurements that were assessed are water tank mechanics, data acquisition, and chamber aspects. WP scanning software, mechanical measurements, and infra‐red motion tracking cameras were used to evaluate the mechanical performance of the WP. Continuous motion and point by point measurements were compared for a variety of standard detectors. Minor field size variations were scaled to the nominal field size for TPS import purposes using two methods, 1) geometric scaling of the whole profile or 2) geometric scaling of 80% of the field width. The detector orientation effects were evaluated for profiles and percentage depth dose. Results: WP positional accuracy and reproducibility was within 0.4mm and mechanical hysteresis within 0.46mm. The collection method impacted the results obtained for small detectors (<0.01cc) with the point measurement technique resulting in reduced noise level. However, the time scale increased considerably with the point by point technique. For field width scaling the 80% scaling method showed improved results compared to scaling the whole profile. The detector orientation showed variations in the build up region for small detectors and small variations were observed for penumbral widths of profile measurements. Conclusion: Error sources in data collection have been identified and quantified in this work. Detailed error analysis of experimental set up is presented.

This work supported in part with Federal funds from the National Cancer Institute, Contract No HHSN261200522014C, and by Sun Nuclear Corporation.