bluuebunny/crossref_metadata_2025 huggingface dataset:
SU‐E‐J‐93: Perfusion CT and Tumor Response for Patients with Mesothelioma
Authors: Labby Z , Sensakovic W , Straus C , Shouldis J , Kindler H , Armato S
Published: November 2011

Purpose: A pilot study is underway to assess the viability of perfusion CT as an imaging modality for patients with mesothelioma, and to investigate correlations between changes in perfusion metrics and tumor burden measurements. Methods: Four patients were scanned with an IRB‐approved protocol using a 256‐slice CT scanner. Each perfusion scan consisted of time point data acquired for a 5.5cm axial extent at 120kVp and 100mAs every three seconds for one minute prior to the full‐thorax diagnostic scan, followed by five additional scans every five seconds after the diagnostic scan. The dose for each perfusion scan was estimated at 165mGy [CTDI], and dose‐length product was equivalent to approximately 1.5 chest CT scans. Perfusion CT scans were obtained at two sessions approximately six weeks apart. All timepoints from a single scan were co‐registered using a demons deformable registration algorithm to remove the effects of motion prior to the calculation of perfusion maps. Perfusion, blood volume, time‐to‐peak, peak enhancement, and mean transit time data maps were calculated using the slope method. Data map spatial statistics were calculated over manually defined regions of interest (ROIs) within the tumor and changes in data map values were correlated with tumor burden changes measured according to the Modified RECIST technique. Results: Tumor ROIs exhibited heterogeneous contrast uptake, ranging from no uptake to peak enhancement of 150HU. Median tumor perfusion and median tumor blood volume metrics were correlated with changes in tumor burden (rank correlation p=0.0054 and 0.0030, respectively), with “shrinking” tumors exhibiting decreased blood volume and perfusion metrics. Conclusions: We believe that perfusion CT has potential to become an imaging biomarker of tumor response for mesothelioma patients. Data presented in this study indicate that perfusion CT is a viable imaging modality for mesothelioma, and that changes in perfusion metrics correlate with changes in measured tumor burden.
SU‐E‐J‐98: PTV Margin Required for Head and Neck IGRT Derived from Deformable Image Registration
Authors: Yock A , Zhang L , Dong L
Published: November 2011

Purpose: To reconstruct the planning target volume margin necessary in head and neck image‐guided radiation therapy. Methods: For two patients, deformable image registration was used to identify voxels of corresponding anatomy in 35 daily in‐room CTs with reference to the planning CT. The resulting vector fields were applied to the original high‐, medium‐, and low‐risk clinical target volumes to devise daily CTVs relative to the isocenter. A volume equivalent to the union of the daily CTVs was considered an effective planning target volume (PTVeff) that retrospectively provided complete geometric coverage of the original CTV while minimizing the inclusion of normal tissue. 0mm, 1mm, 2mm, 3mm, and 4mm isotropic expansions to the original CTVs (CTVexp) were compared with each PTVeff, and evaluated based on the percent coverage of PTVeff provided by CTVexp as well as the volume of normal tissue included in CTVexp but external to PTVeff. An anisotropic expansion of 1mm in the transverse plane and 2.5mm in the superior‐inferior direction was also evaluated. Results: Both the percent coverage of PTVeff provided by CTVexp and the volume of normal tissue included in CTVexp were proportional to the expanded margin. The values from the anisotropic expansion lay between those from the 1mm and 2mm isotropic expansions. For one patient, an isotropic expansion of 2mm was required to cover at least 95% of PTVeff and actually covered at least 97% for the original CTVs. For the other patient, 3mm and 4mm expansions were necessary to provide 95% coverage of PTVeff. Conclusion: Deformable image registration may be utilized to retrospectively evaluate the efficacy of various CTV margin expansions. The two patients studied demonstrated different margin requirements for the desired coverage. Additional patients will be incorporated to establish empirically derived, clinically appropriate margins.

Funding provided by the University of Texas Graduate School of Biomedical Sciences at Houston.
SU‐E‐J‐99: Multi‐Atlas Based Auto‐Segmentation of Low‐Risk Clinical Target Volume (CTV) for Head‐and‐Neck Radiotherapy
Authors: Yang J , Williamson R , Garden A , Rosenthal D , Zhang Y , Zhang L , Ang K , Dong L
Published: November 2011

Purpose: To develop an auto‐segmentation approach using multiple atlases to improve the quality of target delineation for low‐risk CTV of unilateral tonsil cancers. Methods: Sixteen unilateral tonsil cancer patients received IMRT treatment for their left tonsil tumors. These patients were treated by a total of 8 oncologists, who delineated all contours manually on the planning CT. We chose 6 patients as atlases, and used deformable image registration to map each atlas CTV to the rest of 10 test patients. In each test patient, the final contour was produced by fusing 6 individually deformed contours using the Simultaneous Truth and Performance Level Estimation (STAPLE) algorithm. In addition, we also identified the best single atlas which best matched physicianˈs contour on each test case. The auto‐segmented contours were evaluated by comparing against the physician delineated contours using slice‐wise Hausdorff distance (HD) and Dice coefficient (DSC), and a total volume overlap index. Results: Evaluation results showed that not a single atlas could produce reasonably good results in all 10 testing cases. The multi‐atlas method achieved much better agreement than the single best atlas method. The multi‐atlas method produced the median slice‐wise HD of 7.35mm, the median slice‐wise DSC of 80.2%, and the total volume overlap of 77.8%. For radiation oncologists who contoured both the test case and one of the atlas cases, the best atlas almost always came from the same radiation oncologist, indicating that inter‐observer variation is dominated in target delineation. Our multi‐atlas result confirms that the use of multi‐atlas and STAPLE algorithm automatically reduces the inter‐observer variability. Conclusions: We developed and validated a multi‐atlas based auto‐segmentation method to improve the quality and usability of low‐risk target volume delineation for unilateral tonsil cancer radiotherapy.
SU‐E‐J‐102: Quantitative Analysis of CT Artifacts of Various Fiducial Markers in Image Guided Radiotherapy (IGRT) Application
Authors: Chen Q , Burns J , Hoke S , Herrick A , Parikh D , Rubin S
Published: November 2011

Purpose: To search for a fiducial marker that generates fewer CT artifacts and displays reliably on online imaging system at treatment, and to search for a technique that minimizes artifacts. Methods: Five sets of different gold markers, 3 carbon markers, and 4 sets of different I‐125 dummy seeds in a 30x30x20 phantom were imaged on a Philips multi‐slice CT scanner for quantitative analysis. The phantom was also imaged on MVCT, kV‐CBCT, kV‐OBI, and MV‐EPID for visual evaluation. The CT artifacts of the markers were segmented using a program, based on mathematics operation and morphology operation, developed on MATLAB. Up to the 3rd order of moments of each segmented marker artifacts were then calculated. Quantities characterizing the artifacts, such as area, intensity, eigenvalues, etc., were derived and tabulated for comparison. Results: Eleven sets of CT images were acquired with slice thickness ranging from 0.7 mm to 3 mm and collimation ranging from 1.2 mm to 24 mm. The lower density markers generated almost no CT artifacts, but appeared poorly on online imaging display. All gold markers on the other hand exhibited significant CT artifacts. The severity is related to the mass and dimension of the markers, e.g., Visicoil markers showed the least artifacts. The imaging parameters also demonstrated an impact on the artifacts. For a given collimation, the thinner the slice thickness, the more severe are the artifacts for the same maker. For 2 mm slice thickness settings, 6 and 12 mm collimations produced the least artifacts. The differences diminished when the slice thickness increased to 3 mm. Conclusions: Lower density markers are unreliable in IGRT applications. All gold markers produced significant CT artifacts. With proper imaging parameters, the artifacts can be effectively reduced. Similar improvement was confirmed on patient CT images. Further investigations will focus on eliminating the CT artifacts.
SU‐E‐J‐105: Study on Liver Cancer Target Volume Variations between 4D‐CT and 3D‐CT Associated with Active Breathing Control Device
Authors: Chen J , Yin Y , Gong G
Published: November 2011

Purpose: the study aimed to observe and analyze the variations of liver cancer GTV between 4D‐CT and 3D‐CT associated with active breathing control device (ABC).Methods: 13 cases with primary liver cancer were selected and underwent CT simulation and localization. Each case underwent 4D‐CT scanning first, with ABC device working on to monitor and analyze the breath wave. Afterwards, 3D‐CT scanning were underwent, respectively when patient breathing freely, at the end of inspiration and expiration. GTVs were contoured, according the same criterion by one radiologist and one radiation oncologist jointly, respectively on 6 CT series: CT0 series (4D‐CT end‐exhale), CT50 series (4D‐CT end‐inhale), 4D‐CT MIP series, 3D‐CT free breathing CT series, 3D‐CT end‐exhale series, and 3D‐CT end‐inhale series, which were named GTV4D‐0, GTV4D‐50, GTVMIP, GTVFB, GTVEE, GTVEI. Afterwards, GTV4D‐M were obtained by merging GTV4D‐0 and GTV4D‐50, meanwhile GTV3D‐M were obtained by merging GTVEE and GTVEI. The volume of all GTVs were measured and analyzed using SPSS software. Paired Wilcoxson test was applied.Results: there was no significant difference between GTVEI and GTVEE (P=0.325), as well as GTVEI and GTV4D‐0 (P=0.125), GTVEE and GTV4D‐50 (P=0.325), GTV4D‐0 and GTV4D‐50 (P=0.125), GTV4D‐M and GTV3D‐M (P=0.125), GTVMIP and GTV3D‐M (P=0.325). GTVFB was smaller than GTV3D‐M and GTV4D‐M significantly (P=0.015 and P=0.016), and than GTVMIP without significant differenc (P=0.125). Notably, GTV4D‐M differed from GTVMIP (P=0.016) Conclusions: the margins from GTV to PTV should be noticed, when undergo CT simulation with patients breathing freely, due to the differences between GTVFB and GTV4D‐M and GTV3D‐M. To merge GTVEE and GTVEI could be an alternative to using 4D‐CT for simulation.
SU‐E‐J‐110: Structure Density Variations between Free Breathing CT and Respiration‐Gated 4D CT in Lung SBRT‐GATED 4D CT IN LUNG SBRT
Authors: Shang C , Vargas C
Published: November 2011

Purpose: Moving structures cause uncertainness in dose calculation when different CT imaging techniques are employed. This study determines the variations of structure densities shown on free breathing simulation CT and respiration‐gated 4D CT. Methods: 16 consecutive cases of lung SBRT for non small cell lung cancer (NSCLC) with tumor motion range from 0.5–1.2 cm (mean, 0.7 cm) were selected for this study. Each case had a free breathing simulation CT immediately followed by a respiration‐gated 4D CT using a Siemens Sensation Open scanner. The mean CT Hounsfield units (HU) of ITV and both lungs outside PTV were measured separately using the simulation CT (SCT), Maximal Intensity Projection (MIP) and Mean Intensity Projection (AIP) of 4D CT. Results: The mean HU of lungs in SCT, MIP and AIP were measured as −707.8 ±47.9, −516.4 ± 49.7, and −682.9 ±40.7 respectively. Meanwhile, the corresponding mean HU of ITV were −329.5 ±145.1, −151.6 ±118.4, and −337.4 ±138.7. Comparatively, the mean HU of lungs and ITV based on SCT were −1.1 ±1.0 (rang, −3.4 to +0.4) % and +0.1 ±1.7 (rang, −3.2 to +2.1) % respectively, deviating from those measured with AIP. In the similar comparison using MIP against AIP, higher mean HU were detected for both lung (+8.3 ±1.0 %) and ITV (+9.3 ±1.8 %). Conclusions: In this study, MIP over‐estimated the HU for key structures by more than 8%. When free breathing SCT is used, the respiration‐induced HU deviations from measurements of AIP were as small as −1.1 ±1.0 % for the lungs and +0.1 ±1.7 % for ITV. The impact of the HU deviations on the calculated radiation dose is expected within tolerance, but shall be further investigated.
SU‐E‐J‐128: The Accuracy of Source Region Volume and Activity Estimates from SPECT/CT Imaging for Use in Internal Dose Calculations
Authors: Grimes J , Celler A , Birkenfeld B , Shcherbinin S , Piwowarska‐Bilska H , Listewnik M , Zorga P
Published: November 2011

Purpose: The aim of this study was to investigate the accuracy of volume and activity estimates derived from single photon emission computed tomography (SPECT) images using methods practical for clinical use in dose calculations for targeted radionuclide therapy. Methods: Five patients with suspected neuroendocrine tumours were injected with ∼900 MBq of Tc‐99m‐Tektrotyd and a SPECT/CT scan was acquired. A phantom experiment was performed to establish an adaptive threshold curve, allowing for the optimal choice of segmentation threshold based on the source‐to‐background ratio of activity. The SPECT images were reconstructed using a standard clinical reconstruction with attenuation correction (AC). Additionally, an advanced reconstruction method was applied that included AC, resolution recovery and scatter correction. Source regions were segmented in the SPECT images using three different thresholds: 1) 40%, as commonly used in the literature, 2) the adaptive threshold for true volume estimation (ThV), and 3) the adaptive threshold for true activity determination (ThA). Results: Using the fixed 40% threshold, organ volume estimates from the clinical reconstruction exceeded the advanced reconstruction estimates by 32.4% +/− 23.9% on average. When ThV was applied this difference in volume estimation improved to −2.5% +/− 9.1%. Visual inspection of the SPECT derived organ contours drawn on the CT slices confirmed that ThV organ segmentation corresponded to the true organ boundaries. Activity in regions segmented using ThA was on average 23% greater than activity inside the same regions delineated using ThV, highlighting the importance of using ThA to recover activity blurred out of the regionˈs true volume in the SPECT images. Conclusions: Adaptive thresholding can be used to accurately obtain source region volume and activity estimates from clinical reconstructions. This provides a clinically feasible alternative to obtaining these estimates from images reconstructed using time consuming, advanced methods or from manually delineating regions slice by slice on CT.
SU‐E‐J‐136: Clinical Evaluation of a Robotic 6‐Degree of Freedom Treatment Couch for Frameless Radiosurgery
Authors: Gevaert T , Verellen D , Engels B , Depuydt T , Heuninckx K , Tournel K , Duchateau M , Reynders T , Boussaer M , Poels K , DeRidder M
Published: November 2011

Purpose: To evaluate the added value of 6 degree‐of‐freedom (DOF) patient positioning with robotic couch compared to 4DOF positioning for intracranial lesions and to estimate the immobilization characteristics of the Brainlab frameless mask, more specifically the setup errors and the intrafraction motion. Methods: Forty patients with 66 brain metastases treated with frameless stereotactic radiosurgery and 6DOF robotic couch were enrolled. Patient positioning was performed with the Brainlab ExacTrac stereoscopic x‐ray system. Positioning results were collected before and after treatment to assess patient setup error and intrafraction motion. Existing treatment plannings were loaded and simulated for 4DOF positioning and compared to the 6DOF positioning. The clinical relevance was analyzed by means of the Paddick conformity index (CI) and the ratio of prescribed isodose volume covered with 4DOF to that obtained with the 6DOF positioning. Results: Results. The mean 3D setup error before 6DOF correction was 1.91 mm (SD1.25mm). The rotational errors were larger in the longitudinal (0.23° SD0.82°) direction compared to the lateral (−0.09° SD0.72°) and vertical (−0.10° SD1.03°) ones (p<0.05). The mean 3D intrafraction shift was 0.58 (SD0.42mm). The intrafractional rotational errors were comparable,0.01° (SD0.35°), 0.03° (SD0.31°), −0.03° (SD0.33°), for the vertical, longitudinal and lateral, respectively. The mean CI decreased from 0.68 (SD 0.08) (6DOF) to 0.59 (SD 0.12) (4DOF) (p<0.05). A loss of prescribed isodose coverage of 5% (SD0.08) was found with the 4DOF positioning (p<0.05). Half a degree for longitudinal and lateral rotations can be identified as a threshold for coverage loss. Conclusions: With a mask immobilization, patient setup error and intrafraction motions need to be evaluated and corrected for. The 6DOF patient positioning with 6DOF robotic couch to correct translational and rotational setup errors improves target positioning with respect to treatment isocenter, which is in direct relation with the clinical outcome, compared to 4DOF positioning.
SU‐E‐J‐139: Improving Effectiveness of an Online Adaptive Replanning Method for Pancreas Irradiation
Authors: Ahunbay E , Liu F , Li X
Published: November 2011

Purpose: Online adaptive replanning can decrease doses to organs‐at‐risk (OARs) by reducing target margins. However, the practice of the online replanning can be challenging for a site with complex anatomy, e.g., pancreas. The purpose of this work was to develop an effective replanning strategy that uses only changes in target and/or duodenum (the most important dose‐limiting OAR), eliminating the need for delineating other OARs. Methods and Materials: We previously reported a replanning method composed of a segment aperture morphing (SAM). In this work, we applied the SAM algorithm based on only the anatomic changes in (1) the target (pancreatic head), (2) target and duodenum, and (3) the duodenum, with the final aperture shape formed by prioritizing individual aperture shapes. The importance of a structure imposes specific penalties on leaf position deviations from their SAM values. For the scenario considering only duodenum, the apertures morphed based on duodenum and those based on target translations as in the repositioning process were properly prioritized. Daily CTs acquired during IGRT for representative pancreatic cancer patients using a respiratory gated CT‐on‐Rails (CTVision, Siemens) were used to test each scenario and compare with the current IGRT practice of repositioning. Results: Application of SAM with all 3 scenarios resulted in better dosimetric results compared to the repositioning plans and did not result in increased doses to other OARs (liver, kidneys, stomach, small and large bowel, cord), while the SAM algorithm based on duodenum and target resulted in lowest duodenum doses (upto 7% reduction in the mean duodenum dose as compared to the repositioning plans). Conclusion: The approach of the online replanning based only on the changes in duodenum and/or target can yield improved dosimetry over the current IGRT repositioning for pancreas irradiation, and improves the practicality of the online replanning.
SU‐E‐J‐146: Gold Nanoparticles as a Contrast Agent for In‐Treatment Tumor Tracking
Authors: Berbeco R , Korideck H , Yue Y , Makrigiorgos M
Published: November 2011

Purpose: There has been recent interest in gold nanoparticles as vascular disruptive agents when combined with megavoltage x‐ray irradiation. In addition to the therapeutic benefits, the gold nanoparticles may also have a role as an alternative to implanted fiducials for improving localization for IGRT and real‐time tumor tracking. We demonstrate the feasibility of tracking a gold nanoparticle solution of moderate concentration with kV x‐ray fluoroscopy. Methods: A 2.0 mg/ml solution of polymer coated gold nanoparticles was injected into 0.5 cm clinical bolus material. Additional build up with solid water was used to create a more realistic radiological scenario. A cylindrical gold fiducial (3 mm × 1 mm) was placed nearby to act as a reference surrogate. The phantom was set in sinusoidal motion with 2 cm amplitude and 4.0 s period. Images were acquired at 30 frames/sec with the kV on‐board imaging system of a Varian IX Clinac. The ratio of the image intensity in each target versus background was measured. Target tracking was performed using a single template matching approach and non‐clinical research software. Tracking error was quantified as the difference in location of each track point from the input sine function. Results: The intensity ratio in a single image of a sequence was 1.3 for the gold fiducial and 1.2 for the 2.0 mg/ml gold nanoparticle solution. Tracking was maintained throughout the 25 seconds of target motion. RMS tracking error was 0.4 mm for the gold fiducial and 0.6 mm for the gold nanoparticles. The tracking score was smaller for the nanoparticles (mean: 0.63 +/− 0.02) than for the fiducial (mean: 0.89 +/− 0.6). Conclusions: Target tracking in kV fluoroscopy can be enabled by gold nanoparticles. A solution of 2.0 mg/ml can be tracked in a phantom with an accuracy similar to tracking a gold fiducial.
SU‐E‐J‐156: A Feasibility Study for Real‐Time Tumor Tracking Using Positron Emission Tomography (PET)
Authors: Yang J , Yamamoto T , Thielemans K , Mazin S , Graves E , Keall P
Published: November 2011

Purpose: To evaluate the feasibility of a real‐time position estimation algorithm using positron emission tomography (PET) and investigate the geometric accuracy of the proposed algorithm. Method and Materials:The algorithm was implemented using listmode PET and RPM respiratory tracking data acquired from a phantom programmed to move with measured respiratory traces from five human subjects, for which the ground truth target position was known as a function of time. The phantom consisted of fluid filled cylinder containing six hollow spheres of diameters 10, 13, 17, 22, 28, and 37 mm with a target background ratio of 8:1. Motion of the phantom was synchronized to respiratory motion during data acquisition. Images were continuously reconstructed with increasing acquisition time (steps of 100 msec) based on displacement gating. We evaluated the geometric accuracy of the algorithm by quantifying errors between the center of mass (COM) trajectory determined by the proposed algorithm and actual motion of the target. Results: The COM trajectory by the proposed algorithm converged to actual motion within the accuracy of 1.08±1.26 mm (average error), although shape distortion persisted due to the low number of coincidence events. The largest errors occurred 1) at the beginning of scans due to small number of coincidence events and 2) outside of gate boundaries. Better results were obtained for larger targets due to the partial volume effect. The average error over all subjects for the 10 and 37 mm targets was 1.68±1.88 mm and 0.78±0.74 mm, respectively. Conclusion: We investigated the potential to use PET for real‐time tumor tracking by estimating tumor positions. Even if a fast reconstruction algorithm is required, PET has the potential to enable real‐time tumor tracking for IGRT.

This work was supported by NIH/NCI R01 93626, Stanford Bio‐X and Kwanjeong Educational Foundation (KEF) in Korea
SU‐E‐J‐160: Intrafraction Motion of the Pancreas in Cyberknife Radiotherapy
Authors: Xie Y , Chao M , Xing L
Published: November 2011

Purpose: To report the characteristics of pancreas motion as tracked using implanted fiducials during radiotherapy treatments with CyberKnife. Methods: Twenty‐nine patients with pancreas cancer treated in Stanford University and Georgetown University using CyberKnife system were retrospectively selected for this study. The CyberKnife uses a Synchrony Respiratory Tracking system to obtain the position of the pancreas target. The system can monitor both the internal movement using implanted gold fiducial markers and the external movement using tracking markers. In pre‐treatment patient positioning, the deviation of the target relative to the planning position is corrected by moving the treatment couch. During the treatment, the deviation is examined every 3–4 nodes (∼45 s interval) and compensated by the robot. If there is significant deviation from the original step, the treatment is paused while the patient is repositioned by moving the couch. The pancreas displacement calculated from X‐ray images acquired within the time interval between two consecutive couch motions constitute a data set. Results: A total of 498 data sets and 4302 time stamps of X‐ray images were analyzed in this study. The average duration for each data set is 634 s. The location of the pancreas becomes more spreaded as the time elapses. The mean shift in each direction, averaged over all patients, was 5.38mm, 2.98mm, 3.27mm in the superior‐inferior (SI), lateral (RL), and anterior‐posterior (AP) dimensions, respectively. The average shift length is 7.06mm. Conclusions: The pancreas target moves significantly and unpredictably during treatment. The intra‐fractional movement of pancreas is 2∼3 times larger than prostate. Effective means of compensating the intra‐fractional movement is critical to ensure adequate dose coverage of the tumor target.
SU‐E‐J‐161: Dynamic Gating Window for Compensation of Baseline Shift in Respiratory‐Gated Radiation Therapy
Authors: Pepin E , Wu H , Shirato H
Published: November 2011

Purpose: To analyze and evaluate the necessity and use of dynamic gating techniques for compensation of baseline shift during respiratory‐gated radiation therapy of lung tumors. Methods: Motion tracking data from 30 lung tumors over 592 treatment fractions were analyzed for baseline shift. The finite state model (FSM) was used to identify the end‐of‐exhale (EOE) breathing phase throughout each treatment fraction. Using duty cycle as an evaluation metric, several methods of end‐of‐exhale dynamic gating were compared: An a posteriori ideal gating window, a predictive trend‐line‐ based gating window, and a predictive weighted point‐based gating window. These methods were evaluated for each of several gating window types: Superior/inferior (SI) gating, anterior/posterior beam, lateral beam, and 3D gating. Results: In the absence of dynamic gating techniques, SI gating gave a 39.6% duty cycle. The ideal SI gating window yielded a 41.5% duty cycle. The weight‐based method of dynamic SI gating yielded a duty cycle of 36.2%. The trend‐line‐based method yielded a duty cycle of 34.0%. Conclusions: Dynamic gating was not broadly beneficial due to a breakdown of the FSMˈs ability to identify the EOE phase. When the EOE phase was well defined, dynamic gating showed an improvement over static‐window gating.
SU‐E‐J‐164: Evaluation of Target Localization Accuracy of ExacTrac System for Spine SBRT
Authors: Venkataraman S , Sasaki D , Butler J , Schroeder G , West M , Fewer D
Published: November 2011

Purpose: To assess the positioning accuracy of ExacTrac image guidance system for spine SBRT patients and to quantify the residual errors of ExacTrac calibration in daily QA using an in‐house phantom.Methods: Twelve patients treated with spine SBRT in Siemens Artiste linac using ExacTrac system thus far are included in this study to assess the localization accuracy. The sequence of localization is as following. 1) Patient is aligned to the treatment isocenter using infrared markers. 2) Stereoscopic X‐ray images are taken for all fractions before and after treatment to calculate the setup errors and intrafraction motion. 3) In addition, X‐ray images are acquired whenever the markers showed a deviation of more than 2 mm in any direction. In order to quantitatively verify ExacTrac calibration, an in‐house phantom with high density inserts was developed and was used daily before treatments to acquire residual errors of the system.Results: The initial and final setup errors were calculated using A/P, R/L and S/I shifts of X‐ray localization before and after treatment. The average and standard deviation of initial and final setup errors were 1.00 ± 0.6 mm and 1.6 ± 0.9 mm respectively. Intrafraction motion was found to be 1.2 ± 0.8 mm. It was noted that the final setup error followed initial setup error in that it can be reduced if the initial setup error was kept well below the setup tolerance (2 mm). We observed no significant correlation between deviations shown by infrared markers and subsequent positional shifts of X‐ray images. The average residual error from phantom measurements after daily ExacTrac calibration was 0.2 ±0.11 mm. Conclusions: ExacTrac image guidance system is very useful for accurate target localization of spine SBRT patients. The daily QA checks using our in‐house phantom after ExacTrac calibration provides quantitative assessment of positional uncertainties.
SU‐E‐J‐171: The Possibility of Adaptive Breast Treatment on Tomotherapy Using a Patient‐Specific Density Calibration
Authors: Tournel K , Reynders T , Duchateau M , De Coninck P , Gevaert T , Depuydt T , Boussaer M , Poels K , Verellen D , Leysen K , De Ridder M
Published: November 2011

Purpose: The tomotherapy MVCT images should in theory allow offline adaptive treatment monitoring by recalculation of the dose on each dayˈs image and summation over the entire treatment course. However, Instabilities in the MVCT‐beam can cause shifts in the calculated doses because of shifts in the HU‐calibration (Duchateau et al,2010 Phys. Med. Biol. 55). This work presents an approach with a patient specific HU‐calibration for breast patients. Methods: For 20 Breast patients irradiated on tomotherapy (15×2.8Gy) the daily delivered dose was calculated and summed over the entire treatment using a) the standard HU‐calibration of the MVCT (taken biweekly), b) a generic curve per patient generated by measuring the HU of breast, muscle, lung, bone and fat tissue, resulting in 15 different calibration curves/patient and c)an average for each patient of all “daily” curves. The total dose was calculated on the breast, contralateral breast, long and heart tissue and compared. Results: The generated patient‐specific HUcalibration curves show a large inter‐patient variation with shifts of up to 50HU for the breast. Also noise levels varied up to 30%. Using the standard HU‐calibration showed shifts of dose that were on average 1.3–1.5 times higher for the breast region and 1.2x higher for the organs‐at‐risk as opposed to a patient specific curve. No significant difference could be found between methods (b) and (c). Conclusions: Using the MVCT images to calculate the actual delivered dose for breast patients on tomotherapy the HU‐calibration for the MVCT is crucial to interpret the results. Using the standard calibration method can introduce large errors, even on phantoms (Duchateau et al,2010 Phys. Med. Biol. 55) This study shows that using a patient specific curve generated out of the MVCT data can solve this problem without introducing extra calibrations on phantom
SU‐E‐J‐173: In‐Vitro Investigations of the Urethral Warmer on Isotherms during Interstitial Cryoablations for Prostate Cancer
Authors: Gorny K , King D , Felmlee J , Rossman P , Woodrum D , Mynderse L
Published: November 2011

Purpose: Urethral warming system (Gaymar Industries, Orchard Park, NY) uses a peristaltic pump to circulate warming fluid through an urethral catheter and is essential in preventing injury to bladder neck and urethral tissues during interstitial cryoablations of prostate cancer. We present results of the urethral warmerˈs effect on isotherms during cryotherapy in a phantom model. Methods: Freeze‐thaw cycles using Presice Cryoablation SystemTM with 17‐gauge, 1.47mm IceRodTM cryoneedle (Galil Medical, Yoknean, Israel) were performed in a cylindrical gel phantom designed to simulate patient anatomy. The cryoneedle was inserted 5mm from the walls of the urethral catheter, which was positioned along the axis of the phantom. Temperatures were simultaneously measured at 17 locations using an arrangement of multipoint and single point thermal sensors. Three 10‐minute freeze cycles followed by 10‐minute thawing were performed for the following pump speed settings: 9 (10.3ml/sec), 3 (3.7ml/sec), and 0 (inactive warmer). Results: No statistically significant differences between temperatures corresponding to pump speed settings 9 and 3 were observed (p=0.1). For active warmer, average temperatures measured at the location of cryoneedle and along the warmer surface were −33±4 and −10±1°C, correspondingly. The temperatures corresponding to freeze cycle with the warmer inactive were −41±6 and 20±6°C (p=0.004). Conclusions: Active urethral warmer had a significant effect on the temperatures measured along the surface of the warmer and at the location of the cryoneedle. With the active warmer, temperatures at the warmer surface were elevated to levels inadequate for tissue ablation. At the same time, temperatures at the cryoneedle were also elevated beyond manufacturers specifications, which could affect efficiency of targeted tissues ablation and should be accounted for during treatment. These results support use of the urethral warming system as a protective measure during interstitial cryotherapies of prostate cancer and generate critical treatment planning information.
SU‐E‐J‐174: Quantitative Evaluation of Setup Error and Interfraction Prostate Motion Using Tomotherapy MVCT
Authors: Guo P , Beaumont L , Baldwin K , Jones S , Taylor B
Published: November 2011

Purpose: To evaluate daily setup error and interfraction prostate motion using tomotherapy MVCT. Methods: The evaluation was based on 400 daily shifts from 10 prostate patients. Prior to daily treatment, prostate patients were initially setup with the tattoos and then Tomotherapy MVCT scans were performed and registered with planning KVCT in 3D. The shifts based on the registration were applied before treatments. The shift information in LR, SI, and AP directions was analyzed to characterize the setup error and interfraction prostate motion. Statistic analysis was performed by using mean, standard deviation, maximum and minimum values. Results: The mean shifts of daily setup are 0.1mm, −0.5mm, and 1.5mm in LR, SI, and AP directions respectively and the standard deviations are 4.3mm, 2.0mm, and 4.0mm respectively. Maximum shifts of 18mm, 9mm, and 18.4mm in LR, SI, and AP directions were observed. The shifts are patient dependant. The mean shifts vary from 0.1mm to 4.0mm (LR), from 0.6mm to 2.8mm (SI), and from 0.1mm to 11.6mm (AP) among the 10 patients. Standard deviations range from 2.6mm to 7.2mm (LR), from 1.4mm to 2.6mm (SI), and from 2.8mm to 6.1mm (AP). However, in general, the LR and AP shifts showed larger standard deviation than the SI shifts. This is mostly due to filling status of bladder and rectum, which pushes or moves prostate in AP and LR directions more than in SI direction. The maximum shift value of 18mm was observed in the data set and was most likely caused by the initial setup error. Conclusions: The study shows larger interfraction prostate motion in AP and LR than in SI direction. The results suggest a smaller margin of prostate CTV in SI direction for treatment planning.
SU‐E‐J‐176: Further Investigation of Improved Reconstruction Algorithm for Tomosynthesis Plus Few Discrete Projections: Noisy Projections and TV Filtration
Authors: Wei Y , McKee M , Baydush A
Published: November 2011

Purpose: Investigations over the past few years have demonstrated some of the benefits and shortfalls of tomosynthesis in radiation therapy for patient positioning or dose tracking. Tomosynthesis generates excellent image quality in one reconstruction plane; however, there is a loss of edge and frequency information in the additional planes due to the spatial incompleteness of the projection data. In our previous study, we demonstrated improved results from a new discrete frequency interpolation technique (DFIT), which is used to incorporate tomosynthesis limited arc projection data and three additional “filling” projections. Here, we continue investigation into DFIT by studying noisy projections for the data and incorporation of total variation (TV) noise reduction into the reconstruction processing. Methods: For this investigation, we simulated a Shepp‐Logan phantom to generate 93 parallel beam projections with ±5% uniform random noise. 90 of these projections were over a 90‐degree tomosynthetic arc. The remaining three projections were evenly spaced (i.e. 22.5, 45, and 67.5 degrees from the end of the arc) to help “fill in” the incomplete data. DFIT+TV was used to reconstruct images by interpolating frequency values into the gaps of the Fourier spectrum, inverse Fourier transforming from frequency to image space, adding the resultant image to a filtered backprojection (FBP) image, and finally applying a TV noise reduction step. Results: The images by traditional FBP, DFIT, FBP+TV and DFIT+TV are compared visually and using horizontal and vertical line profile plots. Additionally, contrast results are examined. The DFIT+TV images show improved edge delineation in the usual tomosynthesis “blurred” direction and improved reconstructed contrast. Conclusions: The addition of limited “filling” discrete projections and DFIT+TV can be used to improve the reconstruction accuracy and quality of short arc imaging. Future work will study DFIT+TV in the cone beam geometry.

Research supported by Nucletron, B.V.
SU‐E‐J‐177: Improved Reconstruction Algorithm for Tomosynthesis Plus Few Discrete Projections
Authors: Wei Y , McKee M , Baydush A
Published: November 2011

Purpose: Investigations over the past few years have demonstrated some of the benefits and shortfalls of tomosynthesis in radiation therapy for patient positioning or dose tracking. Tomosynthesis generates excellent image quality in one reconstruction plane; however, there is a loss of edge and frequency information in the additional planes due to the spatial incompleteness of the projection data. In this study, we investigate the results of a new discrete frequency interpolation technique (DFIT), which is used to incorporate tomosynthesis limited arc projection data and three additional “filling” projections. Methods: For the initial investigations, we simulated a Shepp‐Logan phantom with parallel beam geometry to generate 93 projections. 90 of these projections were over a tomosynthetic arc at one projection per degree. The remaining three projections were generated to fill in the next 90 degrees evenly (i.e. 22.5, 45, and 67.5 degrees from the end of the arc) to help “fill in” the incomplete data. Simple filtered backprojection (FBP) results show no improvement as FBP is designed for uniform distribution of projections. Per the projection slice theorem, the tomosynthesis data fills in a portion of the frequency plane. The remaining sparsely sampled area of the frequency plane is filled using linear interpolation from the end arc projections and the 3 discrete projections. The DFIT image is the sum of the FBP image and the inverse Fourier transform of the interpolated frequencies Results: The images by traditional FBP and DFIT are compared visually and using line profile plots. Additionally, contrast results are examined. The DFIT images show improved edge delineation in the usual tomosynthesis “blurred” direction and improves reconstructed contrast. Conclusions: The addition of limited “filling” discrete projections and DFIT can be used to improve the reconstruction accuracy and quality of short arc imaging.

Research supported by Nucletron, B.V.
SU‐E‐J‐179: Clinical Application of 40o Very Limited Angle CBCT (VLA CBCT) for Target Localization of Lung Cancer Patient Who Is Treated with SBRT and ABC in Conventional Radiation Therapy
Authors: Huh S , Hoppe B , Zhao T , Li Z
Published: November 2011

Purpose: 40o VLA CBCT is used to verify the target position of lung cancer patient treated with x‐ray Stereotactic Body Radiation Therapy (SBRT) and Automatic Breath Coordinator (ABC) using Elekta SynergyTM linear accelerator, and XVITM CBCT. Methods: Limited Angle CBCT (LA CBCT) with 40o scanning angles is determined by (1) the maximum time the patient can hold breath (15–20 seconds) under ABC technique, and (2) the minimum scan angles to reconstruct the target with minimum artifacts due to incomplete projections, and (3) the start and stop angles of the LA CBCT, which is determined by the location of the tumor as well as geometry of the lung. Based upon the phantom study, 40o CBCT (with S20 collimator and H&N reconstruction option) starting from 160 to 200 is the optimum to reconstruct the target in coronal images. In the range of these angles, the target is minimally over‐shadowed by the vertebral bodies and mediastinum, thus the target can be determined in the CBCT images. Free‐breathing (FB) CBCT is taken to determine the maximum range of the target movements, and 4 sets of VLA CBCT are taken to investigate the reproducibility of the VLA CBCT procedure.Results: The target movements with FB CBFT are ∼1cm in lateral and AP direction, and 2.5cm in Sup/Inf direction. The average motions during 4 breadth‐hold procedures are less than 5mm, which is on the order of uncertainties in intra/inter user variation. Also, VLA CBCTˈs are taken every 3 fields of the treatment. Their movements are the same ranges. Conclusions: The VLA CBCT is very effective to evaluate the target movements in the middle of the lung and to acquire images (20 seconds for VLA CBCT vs 2minutes for conventional 360o CBCT). Also, patient dose is reduced from 3–4cGy with standard CBCTto<0.1cGy.
SU‐E‐T‐04: A Generalized Linear‐Quadratic Model Incorporating Reciprocal Time of Radiation Damage Repair
Authors: Huang Z , Mayr N , Yuh W , Lo S , Gao M
Published: November 2011

Purpose: It is well known that repair of sublethal damage plays an important role in radiation therapy. The repair rate has been conventionally assumed to be constant during the entire radiation course. However, there is increasing evidence from animal modes showing that the repair process may slow down with time and the experimental data does not fit an exponential pattern. To address this enigma, we presented a generalized linear‐quadratic (gLQ) model incorporating a reciprocal time of damage repair and applied it to fit published experimental data. Methods: In the gLQ model, the parameter G in the surviving fraction represents the repair process of sublethal damage with Tr as the repair half‐time. When a reciprocal pattern of repair process was adopted, a generalized form of G was derived analytically for arbitrary radiation schemes. The published animal data was used to test the reciprocal formulas. Results: The gLQ model incorporating a reciprocal pattern to describe the repair process was presented. Subsequently, formulas for special cases including constant dose rate were derived from this general form. The reciprocal model showed better fit to the animal data than the exponential model, particularly for the ED50 data, resulting in the following parameters: alpha/beta=2.6–4.8 Gy, Tr= 3.2–3.9 h for rat feet skin, and alpha/beta=0.9 Gy, Tr =1.1 h for rat spinal cord. Conclusions: These modeling results suggest that the gLQ model incorporating the reciprocal time of sublethal damage repair facilitates the interpretation of complex experimental designs, especially for irregular dose delivery schedules. These formulas can be used to analyze experimental and clinical data, where a slowing‐down of the repair process occurs during the radiation therapy course.
SU‐E‐T‐05: Biophysical Modeling Intercomparison of Proton Radiation Effectiveness
Authors: Carabe‐Fernandez A , Grassberger C , Carlson D , Stewart R , Frese M , Gerweck L , Skarsgard L , Wouters B , Paganetti H
Published: November 2011

Purpose: Evaluate the power of three biophysical models to predict proton relative biological effectiveness (RBE) using cell survival data for 60CoMethods: The Repair‐Misrepair‐Fixation (RMF), Linear Scaling (LSM) and Total‐isoeffective Dose Ratio (TiDR) models were compared to experimental data for V79 cells irradiated in vitro using gamma‐rays from 60Co and at selected depths in a proton spread out Bragg peak (SOBP). Dose, Linear Energy Transfer (LET) and fluence were computed for this SOBP using Monte Carlo (GEANT4) simulation. Biological parameters for the LSM and TiDR were obtained from published survival data for protons, whereas parameters for the RMF were determined using only the V79 survival data for 60CoResults: The models predict similar trends in cell survival with dose and depth within the SOBP. Some small differences in model predictions for lower and higher doses are attributable to uncertainties in the experimental data and to assumptions for the LET dependence of beta. For 2 Gy, the predicted proton RBE ranges from 1.06 to 1.12 at a depth of 17.2 mm, and from 1.18 to 1.3 on the distal edge of the SOBP (depth=155.8 mm). In the dose fall‐off region of the SOBP, the RBE may be as large as 1.5 to 2.4. Estimates of RBE from all three models are in good agreement with the measured data for V79 cellsConclusions: The current study provides evidence for substantial variations in proton RBE within the SOBP. The three models considered in the study mainly differ in their use of physical parameters (e.g., dose and LET) and the way intrinsic radiation sensitivity is parameterized. Still, all three models appear surprisingly robust in their ability to predict proton RBE from data for other radiations in the same cell line (RMF) or from published data for other cell lines (TiDR and LSM)
SU‐E‐T‐11: Some Analytic Results of Cell Radiation Survival Fraction for Lethal and Potentially Lethal Model
Authors: Zhou S
Published: November 2011

Proposed by Curtis in 1989, lethal and potentially lethal (LPL) model is one of the most comprehensive mechanism‐based mathematical models for radiation cell survival fraction (SF) study. One important reason why it has not been widely used is due to its rich but complex mathematical structure. Even for simple continuous irradiation with constant dose rate case, there is no general analytic solution for radiation cell SF with arbitrary initial condition. In this paper a closed‐form analytical exact solution of cell radiation SF under continuous irradiation with constant dose rate with or without initial lethal and non‐lethal lesions is presented for LPL model. Also asymptotic behaviors of SF for cells obeying LPL model with low radiation doses at the beginning of irradiation, with high radiation doses after a long time of irradiation, and for radiation delivered at very high or very low dose rate were provided. Connections between LPL model and the conventional linear‐quadratic (LQ) model or traditional target theory were discussed with the help of our exact analytic solutions. We hope our new analytic results will renew interest of LPL model among scientists and facilitate its use, particularly for stereotactic radiosurgery/stereotactic body radiation therapy treatments and delivery time effect on intensity modulated radiotherapy, in radiobiology and radiotherapy communities.