Mm In Lateral Direction Research Articles

Retrospective study of random and systematic errors in Head and Neck cancer Patients with Image guided Helical Intensity Modulated Radiation Therapy

The Precision of radiotherapy is checked based on the matching of pre-treatment 2D portal imaging/CBCT to the reference image. The objective of this study is to find inter-fractional systematic and random setup errors (mm) for head and Neck cancer patients and also find setup margin for Planning Target Volume (PTV) at Mohan Dai Oswal Cancer Hospital Ludhiana Punjab. Inter-fractional motion errors were quantified for 10 Head and neck cancer patients who underwent image guided helical intensity modulated radiation therapy with Radixact X9 machine. One hundred fan beam computed tomography scans of 2mm slice thickness, 3.5 MV average energy and flattening filter free beam has been used to collect the data for the calculation of systematic and random errors. After patient immobilization, patient accuracy is checked with registration of pre-treatment image with reference planning image with the help of Accuray Precision image guidance protocol. For every patient 10 fan beam computed tomography images has been taken. Translational errors have been calculated in X, Y and Z direction to find systematic error (∑) and random error (σ). The final PTV margin is calculated by van Herk’s equation (2.5∑ + 0.7σ). The results of this study shows that mean translational errors varies from -2.3 mm to 3.3 mm in lateral direction (X), -3.6 mm to 1.7 mm in longitudinal direction (Y), -2.7 mm to 1.5 mm in vertical direction (Z). The mean and standard deviation (SD) for systematic errors are 1.467, 0.8, 0.923 and random error 0.0595, 0.02266, 0.03824 in X, Y and Z direction has been calculated. The Total Margin for CTV to PTV which include setup margin (mm) in X, Y and Z direction are 3.7 mm, 2.02 mm and 2.33 mm. In addition to that, a PTV margin of 5.00 mm is the appropriate margin for Mohan Dai Oswal Cancer Hospital’s patients. This work conclude that CTV to PTV margin of 5.00 mm is suitable for image guided helical intensity modulated radiation therapy for Head and neck patients to ensure the minimum 95% coverage of dose to target

Faster and more accurate patient positioning with surface guided radiotherapy for ultra-hypofractionated prostate cancer patients.

The aim of this study was to evaluate if surface guided radiotherapy (SGRT) can decrease patient positioning time for localized prostate cancer patients compared to the conventional 3-point localization setup method. The patient setup accuracy was also compared between the two setup methods. A total of 40 localized prostate cancer patients were enrolled in this study, where 20 patients were positioned with surface imaging (SI) and 20 patients were positioned with 3-point localization. The setup time was obtained from the system log files of the linear accelerator and compared between the two methods. The patient setup was verified with daily orthogonal kV images which were matched based on the implanted gold fiducial markers. Resulting setup deviations between planned and online positions were compared between SI and 3-point localization. Median setup time was 2:50min and 3:28min for SI and 3-point localization, respectively (p<0.001). The median vector offset was 4.7mm (range: 0-10.4mm) for SI and 5.2mm for 3-point localization (range: 0.41-17.3mm) (p=0.01). Median setup deviation in the individual translations for SI and 3-point localization respectively was: 1.1mm and 1.9mm in lateral direction (p=0.02), 1.8 and 1.6mm in the longitudinal direction (p=0.41) and 2.2mm and 2.6mm in the vertical direction (p=0.04). Using SGRT for positioning of prostate cancer patients provided a faster and more accurate patient positioning compared to the conventional 3-point localization setup.

Adaptive step size algorithm to increase efficiency of proton macro Monte Carlo dose calculation

PurposeTo provide fast and accurate dose calculation in voxelized geometries for proton radiation therapy by implementing an adaptive step size algorithm in the proton macro Monte Carlo (pMMC) method.MethodsThe in-house developed local-to-global MMC method for proton dose calculation is extended with an adaptive step size algorithm for efficient proton transport through a voxelized geometry by sampling transport parameters from a pre-simulated database. Adaptive choice of an adequate slab size in dependence of material interfaces in the proton’s longitudinal and lateral vicinity is investigated. The dose calculation algorithm is validated against the non-adaptive pMMC and full MC simulation for pencil and broad beams with various energies impinging on academic phantoms as well as a head and neck patient CT.ResultsFor material interfaces perpendicular to a proton’s direction, choice of nearest neighbor slab thickness shows best trade-off between dosimetric accuracy and calculation efficiency. Adaptive reduction of chosen slab size is shown to be required for material interfaces closer than 0.5 mm in lateral direction. For the academic phantoms, dose differences of within 1% or 1 mm compared to full Geant4 MC simulation are found, while achieving an efficiency gain of up to a factor of 5.6 compared to the non-adaptive algorithm and 284 compared to Geant4. For the head and neck patient CT, dose differences are within 1% or 1 mm with an efficiency gain factor of up to 3.4 compared to the non-adaptive algorithm and 145 compared to Geant4.ConclusionAn adaptive step size algorithm for proton macro Monte Carlo was implemented and evaluated. The dose calculation provides the accuracy of full MC simulations, while achieving an efficiency gain factor of three compared to the non-adaptive algorithm and two orders of magnitude compared to full MC for a complex patient CT.

Kilovoltage cone-beam computed tomography imaging dose estimation and optimization: Need of daily cone-beam computed tomography.

The aim of the present study was to access the need of daily cone-beam computed tomography (CBCT) and the requirement of in-house protocols of image acquisition frequency to reduce unnecessary exposure to the patients undergoing radiotherapy treatment. The dose delivered during CBCT procedure (On-Board Imager, Trilogy, Varian medical system, Inc., Palo Alto, California) was assessed for pelvic and head and neck region. For dose estimation, cylindrical polymethyl methacrylate phantoms of 15 cm length, 16 cm, and 32 cm diameter were used to simulate the patient's head and neck and pelvic region thickness, respectively. More than 10 cm scatterer was added on either end of this phantom. Calibrated Ionization chamber DCT10 LEMO SN 1685 iba, dosimetry, Germany (10 cm active length) was used to measure the dose Index. The doses known as cone-beam dose index (CBDI100) were estimated for all the scanning protocols (kV and mAs setting) available on the machine. In this study, image acquisition frequency to correct the setup error was optimized. In-house protocol for image acquisition frequency during treatment has been suggested to reduce the dose. It was based on the principle of as low as reasonable achievable. Optimized dose protocol observed was the "standard dose head" for which the CBDI100 was 2.43 mGy. Whereas for pelvic imaging, single protocol of 125 kV, 80 mA was available by which a dose of 7.61 mGy is likely to be received by the patient during scan. Maximum shift of 6 mm in lateral direction was observed to the patient of Pelvis region and 5 mm was observed in the longitudinal direction for the H and N patients. Angular shift measured in patient position was 3.8° and 3.1° for H and N and pelvic region, respectively. Three consecutive-day CBCT-imaging at the beginning of the treatment followed by once weekly CBCT and two-dimensional (2D) imaging in remaining days of treatment can be an optimized way of imaging for the patient having malignancy in the region of pelvic and abdomen. For H and N, once in a week, CBCT with standard dose head protocol, followed by 2D-imaging in remaining days can be an optimized way of imaging.

Daily Positioning Variations in Image Guided Radiation Therapy of Pancreatic Cancer

Margin reduction and hypo-fractionated treatment of pancreatic cancer is increasing in interest and practice in radiation oncology, but pancreatic motion has the potential to limit clinical efficacy. This study details the magnitude of observed positional variations of pancreatic tumors treated with radiation therapy utilizing the results of daily image guided radiation therapy (IGRT). Daily IGRT shifts based on cone-beam CT (CBCT) and mega-voltage CT (MVCT) were recorded and evaluated in order to estimate positioning variations of pancreatic target volumes. A total of 72 patients and 1159 3D-images captured immediately prior to daily conventionally fractionated RT were considered. Overall positioning variations, inter-patient variations, and correlations between position variations and patient-specific factors including PTV volume, tumor location (head/body/tail of pancreas), and central or lateral positioning of the PTV were estimated. Statistical analysis of the positioning variations included Kolmogorov-Smirnov (KS) testing for normality of distributions. Estimation of appropriate margins was carried out based on M=2.5Σ+0.7σ assuming the measured inter-fraction shifts are representative of intra-fraction motion, and present a conservative estimate of spatial expansions to ensure coverage of the CTV. Mean and standard deviation of IGRT shifts for all patients was 0.2±5.2 mm in lateral directions, -0.3±4.3 mm in vertical directions, and -0.7±5.5 mm in longitudinal directions. The systematic variation (the standard deviation of inter-patient average shifts) was 2.6 mm, 2.6 mm, and 3.0 mm in lateral, vertical, and longitudinal directions. The random variation (the average of inter-patient standard deviations) was 3.9 mm, 3.0 mm, and 4.1 mm lateral, vertical, and longitudinal directions. Combined, these results suggest uniform margins about the CTV up to 10.4 mm are sufficient to account for observed positioning variations during IGRT. For each patient, KS testing implies the patient-specific shifts were representative of Gaussian distributions for 70/72 patients and 2/216 patient-specific directional distributions. The patient-specific distributions were not correlated with PTV volume, central or lateral location of the PTV, or whether the tumor was in the pancreatic head, tail, or body. Pancreas is a mobile organ with potential for large motion; our results suggest this motion is normally distributed and well-represented by Gaussian distributions. If intra- and inter- fraction motion are similar, which is plausible if pancreas motion is due to breathing and gastrointestinal motion, our data suggest margins of 10.4 mm will account for these spatial variations. These margins will surely overlap with surrounding bowel, duodenum, and/or stomach and imply dose escalation of margin-based PTVs may not be feasible.

SU‐G‐JeP1‐09: Evaluation of Transperineal Ultrasound Imaging as a Potential Solution for Target Tracking During Ablative Body Radiotherapy for Prostate Cancer

Purpose:Prostate SABR is emerging as a clinically viable, potentially cost effective alternative to prostate IMRT but its adoption is contingent on providing solutions for accurate tracking during beam delivery. Our goal is to evaluate the performance of the Clarity Autoscan ultrasound monitoring system for inter‐fractional prostate motion tracking in both phantoms and in‐vivo.Methods:In‐vivo evaluation was performed under IRB protocol to allow data collection in prostate patients treated with VMAT whereby prostate was imaged through the acoustic window of the perineum. The probe was placed before KV imaging and real‐time tracking was started and continued until the end of treatment. Initial absolute 3D positions of fiducials were estimated from KV images. Fiducial positions in MV images subsequently acquired during beam delivery were compared with predicted positions based on Clarity estimated motion.Results:Phantom studies with motion amplitudes of ±1.5, ±3, ±6 mm in lateral direction and ±2 mm in longitudinal direction resulted in tracking errors of −0.03 ± 0.3, −0.04 ± 0.6, −0.2 ± 0.9 mm, respectively, in lateral direction and −0.05 ± 0.30 mm in longitudinal direction. In phantom, measured and predicted fiducial positions in MV images were within 0.1 ± 0.6 mm. Four patients consented to participate in the study and data was acquired over a total of 140 fractions. MV imaging tracking was possible in about 75% of the time (due to occlusion of fiducials) compared to 100% with Clarity. Overall range of estimated motion by Clarity was 0 to 4.0 mm. In‐vivo fiducial localization error was 1.2 ± 1.0 mm compared to 1.8 ± 1.9 mm if not taking Clarity estimated motion into account.Conclusion:Real‐time transperineal ultrasound tracking reduces uncertainty in prostate position due to intrafractional motion.Research was supported by Elekta.

SU‐F‐J‐34: Automatic Target‐Based Patient Positioning Framework for Image‐Guided Radiotherapy in Prostate Cancer Treatment

Purpose:Current image‐guided radiotherapy (IGRT) procedure is bonebased patient positioning, followed by subjective manual correction using cone beam computed tomography (CBCT). This procedure might cause the misalignment of the patient positioning. Automatic target‐based patient positioning systems achieve the better reproducibility of patient setup. Our aim of this study was to develop an automatic target‐based patient positioning framework for IGRT with CBCT images in prostate cancer treatment.Methods:Seventy‐three CBCT images of 10 patients and 24 planning CT images with digital imaging and communications in medicine for radiotherapy (DICOM‐RT) structures were used for this study. Our proposed framework started from the generation of probabilistic atlases of bone and prostate from 24 planning CT images and prostate contours, which were made in the treatment planning. Next, the gray‐scale histograms of CBCT values within CTV regions in the planning CT images were obtained as the occurrence probability of the CBCT values. Then, CBCT images were registered to the atlases using a rigid registration with mutual information. Finally, prostate regions were estimated by applying the Bayesian inference to CBCT images with the probabilistic atlases and CBCT value occurrence probability. The proposed framework was evaluated by calculating the Euclidean distance of errors between two centroids of prostate regions determined by our method and ground truths of manual delineations by a radiation oncologist and a medical physicist on CBCT images for 10 patients.Results:The average Euclidean distance between the centroids of extracted prostate regions determined by our proposed method and ground truths was 4.4 mm. The average errors for each direction were 1.8 mm in anteroposterior direction, 0.6 mm in lateral direction and 2.1 mm in craniocaudal direction.Conclusion:Our proposed framework based on probabilistic atlases and Bayesian inference might be feasible to automatically determine prostate regions on CBCT images.

Phase correction method based on interfacial signal in swept source optical coherence tomography

There are intrinsic phase errors in swept source optical coherence tomography (SS-OCT), which severely influences the functional imaging. To overcome this difficulty, a numerical correction method is presented in this paper to correct the phase artifacts due to wavenumber shift among the spectral interferograms, resulting from the random delay variance between the sampling trigger and the clock of the swept source laser. This correction method is based on the linear relationship of phase difference to the depth of the sample and the wavenumber shift. The detailed procedure to eliminate the phase artifacts is as follows. Firstly, we figure out the complex OCT signals through inverse Fourier transform of the initial interferograms. Then we fit the upper surface of the sample with the intensity information of the B-scan by setting a floating threshold. After that the wavenumber shifts of each A-line are determined by two steps with the phase information of the sample surface: the relative wavenumber shifts between adjacent A-lines are first obtained according to the phase difference between the adjacent A-lines, the signal depth, and the linear relationship mentioned above; then we figure out the absolute wavenumber shifts between each A-line and the first A-line of the B-scan by an iteration algorithm. With the information about the wavenumber shift, we align the initial interferograms, and obtain the corrected complex signal through re-inverse Fourier transform of the aligned interferograms. This method introduces no extra noise, realizing phase measurement limited by the signal-to-noise ratio. It is noted that we take the average phase information of several axial pixels near the sample surface to diminish the noise influence when calculating the wavenumber shifts. Besides, this corrected algorithm acquires oversampling along the scanning direction to ensure the signal correlation between adjacent A-lines. The SS-OCT system in the paper is set up with a vertical cavity surface emitting laser with a center wavelength of 1297 nm. The system measurement range is 12 mm in lateral direction, the axial resolution is 17 m, and the lateral resolution is 24 m. And the feasibility of this method is verified by Doppler imaging of a mirror, an infra-red detection card and the cerebral cortex of a mouse.

Determination of optimal planning target volume margins in patients with gynecological cancer

PurposeTo define optimal planning target volume (PTV) margins for intensity modulated radiotherapy (IMRT) ± knee-heel support (KHS) in patients treated with adjuvant radiotherapy. MethodsComputed tomography (CT) scans ± KHS of 10 patients were taken before and at 3rd and 5th week of treatment, fused and compared with initial IMRT plans. ResultsA PTV margin of 15 mm in anteroposterior (AP) and superoinferior (SI) directions and 5 mm in lateral directions were found to be adequate without any difference between ± KHS except for the SI shifts in CTV-primary at the 3rd week. Five mm margin for iliac CTV was found to be inadequate in 10–20% of patients in SI directions however when 7 mm margin was given for iliac PTV, it was found to be adequate. For presacral CTV, it was found that the most striking shift of the target volume was in the direction of AP. KHS caused significantly less volume of rectum and bladder in the treated volume. ConclusionsPTV margin of 15 mm in SI and AP, and 5 mm in lateral directions for CTV-primary were found to be adequate. A minimum of 7 mm PTV margin should be given to iliac CTV. The remarkable shifting in presacral CTV was believed to be due to the unforeseen hip malposition of obese patients. The KHS seems not to provide additional beneficial effect in decreasing the shifts both in CTV-primary and lymphatic, however it may have a beneficial effect of decreasing the OAR volume in PTV margins.

SU‐E‐J‐139: One Institution's Experience with Surface Imaging in Proton Therapy

Purpose:X‐ray system is commonly used for IGRT in proton therapy, however image acquisition not only increases treatment time but also adds imaging dose. We studied a 3D surface camera system (AlignRT) performance for proton therapy.Methods:System accuracy was evaluated with rigid phantom under two different camera location configurations. For initial clinical applications, post mastectomy chest wall and partial breast treatments were studied. X‐ray alignment was used as our ground truth. Our studies included: 1) comparison of daily patient setup shifts between X‐ray alignment and SI calculation; 2) interfractional breast surface position variation when aligning to bony landmark on X‐ray; 3) absolute positioning using planning CT DICOM data; 4) shifts for multi‐isocenter treatment plan; 5) couch isocentric rotation accuracy.Results:Camera locations affected the system performance. After camera relocation, the accuracy of the system for the rigid phantom was within 1 mm (fixed couch), and 1.5 mm (isocentric rotation). For intrafractional patient positioning, X‐ray and AlignRT shifts were highly correlated (r=0.99), with the largest difference (mean ± SD) in the longitudinal direction (2.14 ± 1.02 mm). For interfractional breast surface variation and absolute positioning, there were still larger disagreements between the two modalities due to different focus on anatomical landmarks, and 95% of the data lie within 5mm with some outliers at 7 mm–9 mm. For multi‐isocenter shifts, the difference was 1 ± 0.56 mm over an 11 cm shift in longitudinal direction. For couch rotation study, the differences was 1.36 ± 1.0 mm in vertical direction, 3.04 ± 2.11 mm in longitudinal direction, and 2.10 ± 1.66 mm in lateral direction, with all rotation differences &lt; 1.5 degree.Conclusion:Surface imaging is promising for intrafractional treatment application in proton therapy to reduce X‐ray frequency. However the interfractional discrepancy between the X‐ray and SI requires future validation.

Quantifying Vaginal Motion Associated With Daily Endorectal Balloon During Whole Pelvis Radiation Therapy for Gynecologic Cancers

30-60% of patients. Even 1 cm of PTV margin was inadequate in 20-30% of patients. The only beneficial effect of knee and feet support was found to reduce V45 and V50 doses of rectum and bladder. Conclusions: PTV margin of 15 mm in superior-inferior and posterioranterior, and 5 mm in lateral directions for CTV primary were found to be adequate in gynecological cancer patients. A minimum of 7 mm margin should be given to iliac CTV. The striking shifting in presacral CTV was believed to be due to the unforeseen malposition of the hip positions of obese patients. Author Disclosure: M. Gultekin: None. C. Yilmaz: None. G. Eren: None. F. Yildiz: None.

Line-field parallel spectral domain optical coherence tomography and its application in defect inspection

For the needs of online nondestructive testing method in glass industry, we have presented a large-range line-field parallel spectral domain optical coherence tomography system. Based on fast area scan CMOS camera, the whole cross-sectional image can be acquired by a single shot. Depth-resolved image at different lateral positions can be acquired simultaneously, without the lateral scanning mechanism. The axial resolution is 17.9 μm, the lateral resolutions in parallel direction and scanning direction are 55.7 μm and 24.8 μm, respectively. The system measurement range is 32 mm in lateral direction and 6 mm in axial direction. At 1 mm axial position, the system sensitivity can reach 62 dB at a rate of 128 000 A-scan/s. By using the present system, application in glass defect inspection has been investigated.

Human CT Measurements of Structure/Electrode Position Changes During Respiration with Electrical Impedance Tomography

For pulmonary applications of Electrical Impedance Tomography (EIT) systems, the electrodes are placed around the chest in a 2D ring, and the images are reconstructed based on the assumptions that the object is rigid and the measured resistivity change in EIT images is only caused by the actual resistivity change of tissue. Structural changes are rarely considered. Previous studies have shown that structural changes which result in tissue/organ and electrode position changes tend to introduce artefacts to EIT images of the thorax. Since EIT reconstruction is an ill-posed inverse problem, any small inaccurate assumptions of object may cause large artefacts in reconstructed images. Accurate information on structure/electrode position changes is a need to understand factors contributing to the measured resistivity changes and to improve EIT reconstruction algorithm. Our previous study using MRI technique showed that chest expansion leads to electrode and tissue/organ movements but not significant as proposed. The accuracy of the measurements by MRI may be limited by its relatively low temporal and spatial resolution. In this study, structure/electrode position changes during respiration cycle in patients who underwent chest CT scans are further investigated. For each patient, sixteen fiduciary markers are equally spaced around the surface, the same as the electrode placement for EIT measurements. A CT scanner with respiration-gated ability is used to acquire images of the thorax. CT thoracic images are retrospectively reconstructed corresponding temporally to specific time periods within respiration cycle (from 0% to 90%, every 10%). The average chest expansions are 2 mm in anterior-posterior and -1.6 mm in lateral directions. Inside tissue/organ move down 9.0±2.5 mm with inspiration of tidal volume (0.54±0.14 liters), ranging from 6 mm to 12 mm. During normal quiet respiration, electrode position changes are smaller than expected. No general patterns of electrode position changes are observed. The results in this study provide guidelines for accommodating the motion that may introduce artefacts to EIT images.

Marges en radiothérapie du cancer de la vessie

Radical cystectomy is the gold standard treatment of invasive bladder carcinoma, but conservative treatment is a serious option for selected patients. It comprises a transurethral resection, as complete as possible, before a radiation therapy of the whole bladder and pelvis, with a concomitant chemotherapy. Bladder wall movements during the treatment course lead to the use of wide margins to cover the clinical target volume. Planning target volume margins must be anisotropic to correspond to the mobility of each bladder zone: 10mm in the inferior portion, 15mm in lateral directions, and 20 to 25mm in anterior and superior directions. The development of image-guided radiotherapy and adaptative radiotherapy should lead to a reduction of these margins. Besides, partial bladder radiotherapy is showing encouraging results, by reducing the clinical target volume in well-selected patients.

SU-E-J-28: Comparison of IGRT Shift Data Between Prostate Gland and Prostate Bed Obtained from Ct-On-Rails.

The daily shifts of prostate gland have been intensively reported in literatures. However, few papers reported daily shifts of prostate bed due to several practical difficulties (e.g. limited soft tissue contrast in MVCT and CBCT and significant deformation of prostate cavity). We have routinely performed IGRT for both prostate gland and bed with ct-on-rails, and the superior image quality allows us not only to differentiate both bony anatomy and soft tissue contrast of prostate gland and bed. In this study, we investigated if the shift of prostate bed is signifiant difference from that of prostate gland. we reviewed shift data of 50 prostate gland patients who underwent 43 fractions and 22 patients of prostatectomy underwent 37 fractions. In total 2150 CT scans were reviewed for prostate gland and 814 scans for prostate bed. Of the reviewed 814 CT images from 22 prostate bed patients, the standard deviation of shift was found to be 5.9 mm in AP direction (ranges from -22.4mm to 22mm), 3.2mm in SI direction (ranges from -14mm to 14mm), and 4.1mm in lateral direction (ranges from -15mm to 22mm). Of the 2150 CT images of prostate gland from 50 patients, the standard deviation of the shift was found to be 5.4 mm in AP direction (-20mm to 18 mm), 5.0mm in SI direction (-26mm to 20mm), and 4.3mm in lateral direction (range from-15 to 30mm). F tests of systematic /random shift distribution in three orthogonal directions between prostate gland and prostate bed were subsequently performed, it was found that the systematic shift in SI direction for prostate bed is smaller than for prostate gland (p=0.003). Our result suggests no significant difference existing in shift between prostate bed and gland. Therefore strategies for daily prostate gland motion can be directly applied to prostate bed.

Water distribution uniformity of the traveling rain gun

An experimental method combined with a computer simulation technique, specified for optimizing the water distribution of a traveling rain gun system is presented in the paper. For given operational pressure and flat terrain, this method enables accurate evaluation of track overlapping and carriage velocity of the traveling rain gun that guarantee uniform water distribution at desired water deposit level. The evaluation criteria for water deposit quality have been formulated: minimum standard deviation, skewness factor as close to zero as possible and the highest possible flatness factor. It is verified that tested rain gun system achieves the uniform water distribution of 20.36 mm in lateral direction at track overlapping of 58%. The simple accurate formula specified for carriage velocity correction is also presented. Key words: Traveling rain gun, water deposition, track overlapping, carriage velocity, descriptive statistics.

MO-F-BRC-01: 3D Target Trajectory Reconstruction Using CBCT Projection Images

Purpose: With wide use of CBCT in IGRT, extracting the 3D target trajectory from CBCT projection images would be useful to manage intra- fraction motion for motion inclusive, gated and tumor tracking delivery methods. The purpose of this work was to develop a target trajectory estimation method from CBCT by employing inter-dimensional correlation between each direction of target motion, and demonstrate its feasibility with a liver SBRT case. Methods: To estimate the target trajectory from CBCT projection images we utilize an inter-dimensional correlation between each direction of target motion: LR(t)=a_LR×SI(t)+b_LR, AP(t)=a_AP×SI(t)+b_AP. The parameters of the simple linear correlation model were determined with least-squares estimation by minimizing the discrepancy between the measured and model-estimated projection positions on the imager coordinates. The method was demonstrated using a CBCT projection images obtained during liver SBRT treatment for 3D/3D matching purpose using On-board imager of a Varian iX machine. From each projection image of the CBCT, positions of an implanted gold seed were extracted using Varian research tool (RPM-Fluoro). The estimated trajectory was compared with the marker trajectory obtained from 10-binned respiratory phase 4DCT. Results: In general the estimated trajectory followed well the reference trajectory from 4DCT in the case. Mean root- mean-square error of the model estimation was 0.67mm in lateral direction and 0.00mm in longitudinal direction on the imager coordinates. Conclusions: A simple 3D marker trajectory estimation method using CBCT projection images was developed. Preliminary results show that it could be used to accurately localize a moving tumor.

SU‐FF‐J‐02: 3D Target Localization Using Cone‐Beam CT for Head and Neck IMRT Patients

Purpose: To evaluate clinical effectiveness of target localization using cone‐beam CT (CBCT) for H&amp;N IMRT treatment. Method and Materials: All H&amp;N patients treated with an IRB approved protocol were imaged on the first day of treatment and weekly thereafter with Varian On‐Board Imager. After the patient was aligned with skin markers, 2D kilovoltage portal images were acquired and compared to the planning DRRs for set‐up corrections (2D‐based correction). Then, CBCT images were acquired and registered with the planning CT by matching both soft tissue and bony landmarks, which generated the 3D‐based corrections. The difference between two correction measurements yielded the treatment isocenter variation using the 3D/3D matching method. Dose distributions with different isocenter deviations were re‐constructed on the planning CT. The treatment DVHs of PTVs and critical structures were compared with those in the original plan. Results: A total of 12 CBCT were analyzed for 3 patients during their first 4‐weeks of treatment. Compared to set‐up variations, anatomical shrinkage/deformation was negligible. The mean isocenter deviations after 2D‐based corrections were 1.1mm in AP direction (max 3.0mm), 2.6mm in lateral direction (max 5.0mm), and 3.8mm in SI direction (max 6.0mm). For the worst isocenter deviation, the reconstructed mean PTV dose was reduced by 3.6% from the planned mean value. For two patients, the mean brainstem dose increased from 33% to 49.5% and from 36.9% to 66.8%, respectively, in worst cases after 2D‐based correction. Other critical structures that received significantly higher doses were cord (max dose changed from 41.8% to 64%), eye (mean dose changed from 37.5% to 83.9%) and parotid (mean dose changed from 58% to 72.5%). Conclusion: For H&amp;N IMRT treatment, CBCT image‐guidance improves localization accuracy compared to 2D‐based technique and should be the method of choice for target localization when tight margins are applied.Partially supported by Varian research grant.

Reduction of irradiated small bowel volume and accurate patient positioning by use of a bellyboard device in pelvic radiotherapy of gynecological cancer patients

Purpose: To reduce the volume of small bowel within pelvic treatment fields for gynecological cancer using a bellyboard device and to determine the accuracy of the prone treatment position. Materials and methods: Fifteen consecutive patients with a gynecologic malignancy who were treated with postoperative pelvic radiotherapy were selected for this study. The volume of small bowel within the treatment fields was calculated for both the supine and prone treatment positions. The patients were treated in the prone position in a so-called bellyboard device. During treatment sessions electronic portal images were obtained. An off-line setup verification and correction protocol was used and the setup accuracy of the positioning in the bellyboard was determined. Results: The average volume of small bowel within the treatment fields was 229 cm 3 and 66 cm 3 in the supine and prone treatment, respectively, which means an average volume reduction in the prone position of 64% (95% CI 56–72%), as compared with the supine position. For the position of the patient in the field, the systematic error defined by the standard deviation (SD) of the mean difference per patient between simulation and treatment images was 1.7 mm in the lateral direction, 2.1 mm in the craniocaudal direction and 1.7 mm in the ventrodorsal direction. On average, only 0.4 setup correction per patient was required to achieve this accuracy. The random day-to-day variations were 1.9 (1SD), 2.6 and 2.3 mm, respectively. Standard deviations of the systematic differences between patient positioning relative to the bellyboard were 6.2 mm in lateral direction and 9.1 mm in craniocaudal direction. Conclusions: Treatment of gynecological cancer patients in the prone position using a bellyboard reduces the volume of irradiated small bowel. An off-line verification and correction protocol ensures accurate patient positioning. Daily setup variations using the bellyboard were small (1 SD<3 mm). Therefore for pelvic radiotherapy in patients with a gynecological malignancy, the use of a bellyboard is recommended.

Stereotactic radiotherapy of extracranial targets: CT-simulation and accuracy of treatment in the stereotactic body frame

Background and purpose: Evaluation of set-up accuracy and analysis of target reproducibility in the stereotactic body frame (SBF), designed by Blomgren and Lax from Karolinska Hospital, Stockholm. Different types of targets were analyzed for the risk of target deviation. The correlation of target deviation to bony structures was analyzed to evaluate the value of bones as reference structures for isocenter verification. Materials and methods: Thirty patients with 32 targets were treated in the SBF for primary or metastatic peripheral lung cancer, liver metastases, abdominal and pelvic tumor recurrences or bone metastases. Set-up accuracy and target mobility were evaluated by CT-simulation and port films. The contours of the target at isocenter level, bony structures and body outline were compared by matching the CT-slices for treatment planning and simulation using the stereotactic coordinates of the SBF as external reference system. The matching procedure was performed by using a 3D treatment planning program. Results: Set-up accuracy represented by bony structures revealed standard deviations (SD) of 3.5 mm in longitudinal, 2.2 mm in anterior–posterior and 3.9 mm in lateral directions. Target reproducibility showed a SD of 4.4 mm in longitudinal, 3.4 mm ap and 3.3 mm in lateral direction prior to correction. Correlation of target deviation to bones ranged from 33% (soft tissue targets) to 100% (bones). Conclusion: A security margin of 5 mm for PTV definition is sufficient, if CT simulation is performed prior to each treatment to correct larger target deviations or set-up errors. Isocenter verification relative to bony structures is only safe for bony targets but not for soft tissue targets.