Planning 4D CT Research Articles

Proton Beam Therapy without Fiducial Markers Using Four-Dimensional CT Planning for Large Hepatocellular Carcinomas.

We evaluated the effectiveness and toxicity of proton beam therapy (PBT) for hepatocellular carcinomas (HCC) >5 cm without fiducial markers using four-dimensional CT (4D-CT) planning. The subjects were 29 patients treated at our hospital between March 2011 and March 2015. The median total dose was 76 Cobalt Gray Equivalents (CGE) in 20 fractions (range; 66–80.5 CGE in 10–32 fractions). Therapy was delivered with end-expiratory phase gating. An internal target volume (ITV) margin was added through the analysis of respiratory movement with 4D-CT. Patient age ranged from 38 to 87 years (median, 71 years). Twenty-four patients were Child–Pugh class A and five patients were class B. Tumor size ranged from 5.0 to 13.9 cm (median, 6.9 cm). The follow-up period ranged from 2 to 72 months (median; 27 months). All patients completed PBT according to the treatment protocol without grade 4 (CTCAE v4.03 (draft v5.0)) or higher adverse effects. The two-year local tumor control (LTC), progression-free survival (PFS), and overall survival (OS) rates were 95%, 22%, and 61%, respectively. The LTC was not inferior to that of previous reports using fiducial markers. Respiratory-gated PBT with 4D-CT planning without fiducial markers is a less invasive and equally effective treatment for large HCCs as PBT with fiducial markers.

4. Application of a novel retrospective method for liver 4D-MRI

Introduction Liver imaging is challenging due to its constant movement with breathing. In Magnetic Resonance Imaging (MRI), different strategies have been developed for organ movement consideration [1] , [2] , [3] . These methods are limited by the temporal resolution, as well as image quality. A novel retrospective gating method for dynamic 3D MR imaging during free breathing was developed [1] and further improved by Celicanin et al. [4] . This work aims of assessing the method and the reconstruction technique for liver cases. The final clinical application is to improve liver lesion delineation for Stereotactic Body Radiation Therapy (SBRT). Methods The innovative 4D MRI approach has been evaluated for liver motion estimation during entire respiratory cycle on a healthy volunteer. It is based on simultaneous acquisition of 2D images and a navigator set using acceleration CAIPIRINHA technique [4] providing no temporal divergence between them. During the entire acquisition time, the navigator is fixed in the same position, while the image position is changing, in order to cover the entire organ during free breathing [1] . The entire respiratory cycle is acquired, using a modified balanced Steady State Free Precession sequence (bSSFP) and is reconstructed in a retrospective manner ( Fig. 1 ). Images are acquired with a 1.5T MRI (MAGNETOM Aera; Siemens Medical Solutions, Erlangen, Germany), using an 18-channel design body flex coil. The 3.5 mm 2D axial slices are obtained, using the following sequence parameters: TE = 2.27 ms, TR = 591.97 ms, FA = 70 °, FOV = 37 × 37 cm2, matrix = 256 × 246. Total acquisition time is 33.175 s (0.83 s per slice). Results The preliminary results obtained on a healthy volunteer are shown in Fig. 1 . Images are characterized by high resolution and great contrast, making this method promising for liver lesion examination. The reconstruction of 4D MRI and the sequence optimization to cover the entire respiration cycle with the appropriate time sampling is ongoing work. Conclusions The benefit of MRI acquisition in treatment position to improve lesion contouring for liver SBRT has already been demonstrated [5] . The presented 4D MRI method represents a highly time efficient technique for the free breathing liver motion registration, giving the possibility to sort and reconstruct the images according to the respiratory phases. It will allow registration with 4D CT planning, likely to improve delineation accuracy. The same approach can be applied to study moving organs others than liver.

Effect of density heterogeneity on absorbed dose with CyberKnife Synchrony Respiratory Tracking System

Objective To measure the actual absorbed dose of the target in the QUASAR Respiratory Motion Phantom using the CyberKnife Synchrony Respiratory Tracking System, and to evaluate the effect of density heterogeneity on the absorbed dose of tumor gross target volume (GTV). Methods Nine groups were obtained by making different patterns of QUASAR phantom: rib thickness of 0, 20, and 50 mm, and motion amplitudes of 0, 10, and 15 mm. The nine groups were treated with static computed tomography (CT) in different time phases of four-dimensional CT (4DCT) plan, with the same beam and number of monitor units, and the 4D accumulated dose was calculated. The doses of static and 4D plans were calculated using Ray-tracing and Monte Carlo algorithms, and the absorbed doses of GTV in the nine groups were measured at the same time. Results There were a decrease in calculated absorbed dose of GTV and an increase in deviation between the planned and actual dose, with the increases in simulated rib thickness and motion amplitude. Conclusions The density heterogeneity has an impact on the absorbed dose of GTV. Both static CT and 4DCT plan can evaluate the absorbed dose of GTV in case of small rib thickness and motion amplitude, and 4DCT plan with Monte Carlo algorithm may be the optimal method for evaluation of the absorbed dose of GTV in case of large rib thickness and motion amplitude (deviation<3%) Key words: CyberKnife; Synchrony system; Tissue heterogeneous; Absorbed dose

Respiratory-gated (4D) contrast-enhanced FDG PET-CT for radiotherapy planning of lower oesophageal carcinoma: feasibility and impact on planning target volume

BackgroundTo assess the feasibility and potential impact on target delineation of respiratory-gated (4D) contrast-enhanced 18Fluorine fluorodeoxyglucose (FDG) positron emission tomography - computed tomography (PET-CT), in the treatment planning position, for a prospective cohort of patients with lower third oesophageal cancer.MethodsFifteen patients were recruited into the study. Imaging included 4D PET-CT, 3D PET-CT, endoscopic ultrasound and planning 4D CT. Target volume delineation was performed on 4D CT, 4D CT with co-registered 3D PET and 4D PET-CT. Planning target volumes (PTV) generated with 4D CT (PTV4DCT), 4D CT co-registered with 3D PET-CT (PTV3DPET4DCT) and 4D PET-CT (PTV4DPETCT) were compared with multiple positional metrics.ResultsMean PTV4DCT, PTV3DPET4DCT and PTV4DPETCT were 582.4 ± 275.1 cm3, 472.5 ± 193.1 cm3 and 480.6 ± 236.9 cm3 respectively (no significant difference). Median DICE similarity coefficients comparing PTV4DCT with PTV3DPET4DCT, PTV4DCT with PTV4DPETCT and PTV3DPET4DCT with PTV4DPETCT were 0.85 (range 0.65–0.9), 0.85 (range 0.69–0.9) and 0.88 (range 0.79–0.9) respectively. The median sensitivity index for overlap comparing PTV4DCT with PTV3DPET4DCT, PTV4DCT with PTV4DPETCT and PTV3DPET4DCT with PTV4DPETCT were 0.78 (range 0.65–0.9), 0.79 (range 0.65–0.9) and 0.89 (range 0.68–0.94) respectively.ConclusionsPlanning 4D PET-CT is feasible with careful patient selection. PTV generated using 4D CT, 3D PET-CT and 4D PET-CT were of similar volume, however, overlap analysis demonstrated that approximately 20% of PTV3DPETCT and PTV4DPETCT are not included in PTV4DCT, leading to under-coverage of target volume and a potential geometric miss. Additionally, differences between PTV3DPET4DCT and PTV4DPETCT suggest a potential benefit for 4D PET-CT.Trial registrationClinicalTrials.gov Identifier – NCT02285660 (Registered 21/10/2014).

Impact of 4D CT for treatment planning of moving targets: a computer-based biological evaluation

Abstract This study deals with comparison of conventional and 4D CT (GE Lightspeed) planning on the tumour control probability (TCP), using the TCP model of the AAPM-Report Task Group 166. In the first step a VMAT treatment plan was calculated (Varian Eclipse 13.7) on basis of conventional CT data. This treatment plan was transferred to the complete 4D CT, which represents the tumour volume in motion. Due to the increased volume and the resulting decrease of tumour coverage the TCP went down from 97,6% to 91,2%. After adding an internal target volume (ITV, ICRU 62) to the conventional CT according to our clinical protocols (1,0 cm cc and 0,3 cm axial plane) the TCP increased to 98,0% when applying the conventional plan to the 4D CT. This finding demonstrates the need of 4D CT for moving tumours in chest and abdomen region. Average IPs with increasing width have been created to evaluate the impact on the TCP and the non-malignant tissue. Our observations had shown that heart, lung and spinal cord radiation exposure did not correlate to chosen respiration segment. This could be explained by the extremely slight ratio of the planning target volume and the irradiated normal tissue. This procedure enables us to evaluate the efficacy of treatment plans. Furthermore, optimizing trials like the influence of respiration-gated RT, setting individual margins and fitting planning objectives and parameters are still under investigation.

Subpopulation-based correspondence modelling for improved respiratory motion estimation in the presence of inter-fraction motion variations

Correspondence modelling between low-dimensional breathing signals and internal organ motion is a prerequisite for application of advanced techniques in radiotherapy of moving targets. Patient-specific correspondence models can, for example, be built prior to treatment based on a planning 4D CT and simultaneously acquired breathing signals. Reliability of pre-treatment-built models depends, however, on the degree of patient-specific inter-fraction motion variations. This study investigates whether motion estimation accuracy in the presence of inter-fraction motion variations can be improved using correspondence models that incorporate motion information from different patients. The underlying assumption is that inter-patient motion variations resemble patient-specific inter-fraction motion variations for subpopulations of patients with similar breathing characteristics. The hypothesis is tested by integrating a sparse manifold clustering approach into a regression-based correspondence modelling framework that allows for automated identification of patient subpopulations. The evaluation is based on a total of 73 lung 4D CT data sets, including two cohorts of patients with repeat 4D CT scans (cohort 1: 14 patients; cohort 2: ten patients). The results are consistent for both cohorts: The subpopulation-based modelling approach outperforms general population modelling (models built on all data sets available) as well as pre-treatment-built models trained on only the patient-specific motion information. The results thereby support the hypothesis and illustrate the potential of subpopulation-based correspondence modelling.

Does Motion Assessment With 4-Dimensional Computed Tomographic Imaging for Non-Small Cell Lung Cancer Radiotherapy Improve Target Volume Coverage?

Introduction:Modern radiotherapy with 4-dimensional computed tomographic (4D-CT) image acquisition for non–small cell lung cancer (NSCLC) captures respiratory-mediated tumor motion to provide more accurate target delineation. This study compares conventional 3-dimensional (3D) conformal radiotherapy (3DCRT) plans generated with standard helical free-breathing CT (FBCT) with plans generated on 4D-CT contoured volumes to determine whether target volume coverage is affected.Materials and methods:Fifteen patients with stage I to IV NSCLC were enrolled in the study. Free-breathing CT and 4D-CT data sets were acquired at the same simulation session and with the same immobilization. Gross tumor volume (GTV) for primary and/or nodal disease was contoured on FBCT (GTV_3D). The 3DCRT plans were obtained, and the patients were treated according to our institution’s standard protocol using FBCT imaging. Gross tumor volume was contoured on 4D-CT for primary and/or nodal disease on all 10 respiratory phases and merged to create internal gross tumor volume (IGTV)_4D. Clinical target volume margin was 5 mm in both plans, whereas planning tumor volume (PTV) expansion was 1 cm axially and 1.5 cm superior/inferior for FBCT-based plans to incorporate setup errors and an estimate of respiratory-mediated tumor motion vs 8 mm isotropic margin for setup error only in all 4D-CT plans. The 3DCRT plans generated from the FBCT scan were copied on the 4D-CT data set with the same beam parameters. GTV_3D, IGTV_4D, PTV, and dose volume histogram from both data sets were analyzed and compared. Dice coefficient evaluated PTV similarity between FBCT and 4D-CT data sets.Results:In total, 14 of the 15 patients were analyzed. One patient was excluded as there was no measurable GTV. Mean GTV_3D was 115.3 cm3 and mean IGTV_4D was 152.5 cm3 (P = .001). Mean PTV_3D was 530.0 cm3 and PTV_4D was 499.8 cm3 (P = .40). Both gross primary and nodal disease analyzed separately were larger on 4D compared with FBCT. D95 (95% isodose line) covered 98% of PTV_3D and 88% of PTV_4D (P = .003). Mean dice coefficient of PTV_3D and PTV_4D was 84%. Mean lung V20 was 24.0% for the 3D-based plans and 22.7% for the 4D-based plans (P = .057). Mean heart V40 was 12.1% for the 3D-based plans and 12.7% for the 4D-based plans (P = .53). Mean spinal cord Dmax was 2517 and 2435 cGy for 3D-based and 4D-based plans, respectively (P = .019). Mean esophageal dose was 1580 and 1435 cGy for 3D and 4D plans, respectively (P = .13).Conclusions:IGTV_4D was significantly larger than GTV_3D for both primary and nodal disease combined or separately. Mean PTV_3D was larger than PTV_4D, but the difference was not statistically significant. The PTV_4D coverage with 95% isodose line was inferior, indicating the importance of incorporating the true size and shape of the target volume. Relatively less dose was delivered to spinal cord and esophagus with plans based on 4D data set. Dice coefficient analysis for degree of similarity revealed that 16% of PTVs from both data sets did not overlap, indicating different anatomical positions of the PTV due to tumor/nodal motion during a respiratory cycle. All patients with lung cancer planned for radical radiotherapy should have 4D-CT simulation to ensure accurate coverage of the target volumes.

Značaj 4D CT simulacije u preciznom definisanju ciljnog volumena intenzitetom modulisanog zračnog tretmana karcinoma pluća

Implementation of IMRT with 4DCT simulation offers possibility to correlate the respiratory movements and position of tumor volume during radiotherapy treatment. The aim of this study is to quantify movements of primary tumor during IMRT treatment in three axes-Z-superoinferior SI, X-mediolateral ML, Y-anteroposterior AP. This study included 15 patients with lung carcinoma referred to radical radiotherapy. Contouring of GTV was done in the phases FB- GTV FB, phase 0-GTV0, phase 50-GTV 50, and phase MIP-GTV MIP. Comparison was done with GTV FB as basic value. Evaluation was done in three axis-SI, AP, ML. The movements of GTV in relation to X axis were in the range 0.02-0.47 cm, in relation to Y axis in the range from 0.01-0.56 cm, and according to the Z axis in the range of 0.00-0.79 cm. Based on the results of t-test there was a statistically significant difference (p<0.05) related to X axis between the upper and lower lobe. There was no statistically significant difference (p>0.05) between the upper, lower and middle lobes related to Y and Z axis. The test results which were obtained after the 4D CT simulation and planning of IMRT treatment show the importance of high-quality and precise radiation therapy treatment.

ED08.02 The Role of Radiotherapy in Early-Stage NSCLC

Radiotherapy is a curative treatment for early-stage NSCLC. Following hypofractionated radiotherapy in 15 once-daily fractions of 4 Gy to biopsy-proven tumors, a prospective multicenter study reported a 3-year local control rate of 82.7% (95% CI = 69.7% to 90.5%) [Cheung PC, 2014]. In the past decade, stereotactic ablative radiotherapy (SABR or SBRT) has become established as the guideline-recommended standard of care for medically inoperable patients with a peripheral early-stage NSCLC, as 5-year local control rates of 90% have been reported [Louie AV, 2015]. SABR is usually delivered in 3-8 fractions, utilizes small margins for positional uncertainty, 4-dimensional computed tomography (4DCT) for treatment planning, multiple conformal beams or arcs for delivery, and cone-bean CT scans for daily setup. Where facilities for SABR are unavailable, hypofractionated radiotherapy delivered using 4DCT planning remains an acceptable curative treatment. Population studies reveal that a significant proportion of elderly patients, as well as those with severe co-morbidities, do not receive any treatment. Guidelines recommend that a tissue diagnosis be obtained before initiating treatment for early-stage NSCLC, but also permit the use of SABR following review by an expert tumor board, in tumors where the calculated probability of malignancy is high [Vansteenkiste J, 2014; Callister ME, 2015]. However, any decision to proceed to a FDG-PET directed SABR approach in less fit patients must take into account the likelihood of benign disease. Given a high incidence of pulmonary tuberculosis, guidelines for Asia have recommended performing early non-surgical biopsies in Asian patients [Bai C, 2016]. Treatment-related grades 3-4 toxicity are uncommon following SABR to peripheral lung tumors, while local control rates are approximately 90% [Louie AV 2015]. Commonly reported toxicities are chest wall pain, rib fractures, and except in patients who have pre-existing interstitial lung disease (ILD), the incidence of high-grade radiation pneumonitis is low. A systematic literature review of SABR in patients with ILD reported a treatment-related mortality in 15% [Chen H, Proc ASTRO 2016]. Guidelines recommend 6-monthly CT scans for up to 3 years following SABR, followed by annual scans thereafter. The assessment of radiological changes can be challenging in a sub-group of patients during long-term follow-op, and the so-called high-risk radiological features [HRF] can identify patients in whom a biopsy is warranted [Figure 1, Huang K, 2014]. The HRF’s identified in the literature are an enlarging opacity at primary site, a sequential enlarging opacity, enlarging opacity after 12-months, a bulging margin, loss of linear margin, loss of air bronchogram and cranio-caudal growth [Huang K, 2014]. Initial reports on surgery for local failures following SABR indicate that this salvage procedure can be performed safely [Allibhai Z, 2012; Hamaji M, 2015; Verstegen N, Proc ELCC 2015]. The observed rates for a second primary lung cancer following SABR appear similar to those following surgery [Verstegen N, 2015]. In this situation, a subsequent course of SABR can generally be performed safely. The role of SABR in fit patients remains a topic of active debate. Indirect comparisons of outcomes following the two modalities have revealed conflicting results. The role of SABR in surgical patients is currrently being investigted in 3 prospective randomized studies (NCT02468024, NCT02629458, NCT01753414), with a fourth study (VALOR) scheduled to open shortly. stereotactic radiotherapy, Early-stage lung cancer, follow-up, toxicity

Intrafractional dose variation and beam configuration in carbon ion radiotherapy for esophageal cancer.

BackgroundIn carbon ion radiotherapy (CIR) for esophageal cancer, organ and target motion is a major challenge for treatment planning due to potential range deviations. This study intends to analyze the impact of intrafractional variations on dosimetric parameters and to identify favourable settings for robust treatment plans.MethodsWe contoured esophageal boost volumes in different organ localizations for four patients and calculated CIR-plans with 13 different beam geometries on a free-breathing CT. Forward calculation of these plans was performed on 4D-CT datasets representing seven different phases of the breathing cycle. Plan quality was assessed for each patient and beam configuration.ResultsTarget volume coverage was adequate for all settings in the baseline CIR-plans (V95 > 98% for two-beam geometries, > 94% for one-beam geometries), but reduced on 4D-CT plans (V95 range 50–95%). Sparing of the organs at risk (OAR) was adequate, but range deviations during the breathing cycle partly caused critical, maximum doses to spinal cord up to 3.5x higher than expected. There was at least one beam configuration for each patient with appropriate plan quality.ConclusionsDespite intrafractional motion, CIR for esophageal cancer is possible with robust treatment plans when an individually optimized beam setup is selected depending on tumor size and localization.

SU‐C‐207A‐07: Cumulative 18F‐FDG Uptake Histogram Relative to Radiation Dose Volume Histogram of Lung After IMRT Or PSPT and Their Association with Radiation Pneumonitis

Purpose:To determine whether the difference in cumulative 18F‐FDG uptake histogram of lung treated with either IMRT or PSPT is associated with radiation pneumonitis (RP) in patients with inoperable stage II and III NSCLC.Methods:We analyzed 24 patients from a prospective randomized trial to compare IMRT (n=12) with vs. PSPT (n=12) for inoperable NSCLC. All patients underwent PET‐CT imaging between 35 and 88 days post‐therapy. Post‐treatment PET‐CT was aligned with planning 4D CT to establish a voxel‐to‐voxel correspondence between post‐treatment PET and planning dose images. 18F‐FDG uptake as a function of radiation dose to normal lung was obtained for each patient. Distribution of the standard uptake value (SUV) was analyzed using a volume histogram method. The image quantitative characteristics and DVH measures were correlated with clinical symptoms of pneumonitis.Results:Patients with RP were present in both groups: 5 in the IMRT and 6 in the PSPT. The analysis of cumulative SUV histograms showed significantly higher relative volumes of the normal lung having higher SUV uptake in the PSPT patients for both symptomatic and asymptomatic cases (VSUV=2: 10% for IMRT vs 16% for proton RT and VSUV=1: 10% for IMRT vs 23% for proton RT). In addition, the SUV histograms for symptomatic cases in PSPT patients exhibited a significantly longer tail at the highest SUV. The absolute volume of the lung receiving the dose &gt;70 Gy was larger in the PSPT patients.Conclusion:18F‐FDG uptake – radiation dose response correlates with RP in both groups of patients by means of the linear regression slope. SUV is higher for the PSPT patients for both symptomatic and asymptomatic cases. Higher uptake after PSPT patients is explained by larger volumes of the lung receiving high radiation dose.

SU‐F‐J‐118: On‐Treatment 4D CT Reconstruction From Planning 4D CT Using Linear Amplitude Scaling

Purpose:To reconstruct on‐treatment 4D CT images from planning 4D CT images by adapting deformation vector field (DVF) of the planning CT to the on‐treatment condition, while the adaptation is based on the scaling of two amplitudes that are motion characteristics at the times of treatment and planning CT acquisition, respectively.Methods:An anthropomorphic digital phantom (XCAT) was used to generate 4D image sets with 1‐cm and 2‐cm tumor motions simulating conditions of planning CT and treatment, respectively. DVFs were acquired from the planning CT image set. The DRR images were acquired simulating setup kV images from the two CT image sets. On the DRR images, tumor positions and their motion amplitudes were quantified. The DVFs were scaled linearly by the amplitude ratio between the treatment and the planning CT times, assuming the elasticity of lung. The scaled DVFs were used to resample the planning 4D CT images generating on‐treatment 4D CT images. The on‐treatment 4D CT images thus acquired were compared with the reference on‐treatment images (2‐cm motion).Results:The resampled images showed good agreement within 1 mm residual errors with the reference images. The normalized cross correlation was 0.995.Conclusion:A linear model of amplitude scaling was developed to reconstruct on‐treatment 4D CT images from planning 4D CT images using the setup KV images acquired during treatment. The model was validated on a digital phantom. For the model to fully work, a further research needs to be followed, that aims at utilizing a phase‐specific CT image set that is geometrically identical between pretreatment and treatment conditions.

TH-CD-303-06: Deformable Registration-Based Image Estimation Method for 4D CBCT Using Region-Based PCA

Purpose: To investigate the feasibility of using region-based principal component analysis (PCA) for motion modeling to estimate on-board 4D-CBCT using prior information and limited angle projections for potential 4D target verification of lung radiotherapy. Methods: A technique for lung 4D-CBCT reconstruction has been previously developed using a deformation field map (DFM)-based strategy. In this method, each phase of the 4D-CBCT is generated by deforming a prior CT volume. The DFM is solved by a motion modeling based on global PCA and free-form deformation (MM-FD) technique, using data fidelity constraint and the deformation energy minimization. For this study, a new region-based PCA method was developed to optimize the motion-modeling part of the MM-FD technique in order to estimate target delineation more accurately using fewer on-board projection angles. First, the DFMs are estimated using the region-based PCA divides the motion model extracted from planning 4D-CT into two regions: tumor and everything else. The motion model parameters of both regions are optimized together based on data fidelity constraint with relative weighting added to the tumor PCA model. The 4D digital extended cardiac-torso phantom was used to evaluate the region-based PCA MM-FD technique. A lung patient with a 30 mm diameter lesion was simulated with various anatomical and respirational changes from planning 4D CT to onboard volume. The reconstruction accuracy was evaluated by the Volume-Percent-Difference(VPD)/Center-of-Mass-Shift(COMS) between lesions in the estimated and “ground-truth” on board 4D CBCT. Results: For patient scenarios with various shifts of tumor from planning CT to on-board acquisition with 15 degree orthogonal projections, the average VPD/COMS for the MM-FD technique was 5.04%/0.26 mm and 23.23%/2.4mm for region-based and global PCA MM-FD, respectively. Conclusion: The region-based PCA MM-FD technique can potentially further reduces the scan angle needed for onboard 4D CBCT reconstruction for ultra-fast 4D verification. National Institutes of Health Grant No. R01-CA184173 Varian Medical System

An accurate algorithm to match imperfectly matched images for lung tumor detection without markers.

In order to locate lung tumors on kV projection images without internal markers, digitally reconstructed radiographs (DRRs) are created and compared with projection images. However, lung tumors always move due to respiration and their locations change on projection images while they are static on DRRs. In addition, global image intensity discrepancies exist between DRRs and projections due to their different image orientations, scattering, and noises. This adversely affects comparison accuracy. A simple but efficient comparison algorithm is reported to match imperfectly matched projection images and DRRs. The kV projection images were matched with different DRRs in two steps. Preprocessing was performed in advance to generate two sets of DRRs. The tumors were removed from the planning 3D CT for a single phase of planning 4D CT images using planning contours of tumors. DRRs of background and DRRs of tumors were generated separately for every projection angle. The first step was to match projection images with DRRs of background signals. This method divided global images into a matrix of small tiles and similarities were evaluated by calculating normalized cross‐correlation (NCC) between corresponding tiles on projections and DRRs. The tile configuration (tile locations) was automatically optimized to keep the tumor within a single projection tile that had a bad matching with the corresponding DRR tile. A pixel‐based linear transformation was determined by linear interpolations of tile transformation results obtained during tile matching. The background DRRs were transformed to the projection image level and subtracted from it. The resulting subtracted image now contained only the tumor. The second step was to register DRRs of tumors to the subtracted image to locate the tumor. This method was successfully applied to kV fluoro images (about 1000 images) acquired on a Vero (BrainLAB) for dynamic tumor tracking on phantom studies. Radiation opaque markers were implanted and used as ground truth for tumor positions. Although other organs and bony structures introduced strong signals superimposed on tumors at some angles, this method accurately located tumors on every projection over 12 gantry angles. The maximum error was less than 2.2 mm, while the total average error was less than 0.9 mm. This algorithm was capable of detecting tumors without markers, despite strong background signals.PACS numbers: 87.57.cj, 87.57.cp87.57.nj, 87.57.np, 87.57.Q‐, 87.59.bf, 87.63.lm

Respiratory Motion, Anterior Heart Displacement and Heart Dosimetry: Comparison Between Prone (Pr) and Supine (Su) Whole Breast Irradiation.

To analyze respiratory motion of surgical clips, chest wall (CW) and the anterior displacement of the heart and its impact on heart dosimetry between prone (Pr) and supine (Su) positions during whole breast radiotherapy after breast conserving surgery. Sixteen patients underwent 4D-CT for radiotherapy planning in Pr and Su positions. Maximum inhale and maximum exhale phases were analyzed. Mean 3D vectorial displacements ± standard deviations (SD) of the surgical clips were measured. Volumetric changes of the CW were recorded and compared. Cardiac displacement was assessed by a volume between the inner surface of CW and the myocardium of the heart (CW/H-V). For left-sided cases, comparative dosimetry was performed in each position simulating no- (Pr-noC, Su-noC) versus daily correction protocols (Pr-C, Su-C). The movements of 81 surgical clips were analyzed. Prone positioning significantly reduced both the mean 3D vectorial displacements (1.1 ± 0.6 (Pr) vs. 2.0 ± 0.9 mm (Su), p < 0.01) and their variability (0.3 ± 0.2 vs. 0.5 ± 0.3 mm, p = 0.01). Respiration-induced volumetric changes of CW were also significantly lower in Pr (2.3 ± 4.9 vs. 9.6 ± 7.1 cm(3), p < 0.01). The CW/H-V was significantly smaller in Pr than in Su (39.9 ± 14.6 vs. 64.3 ± 28.2 cm(3), p < 0.01). Besides identical target coverage heart, left-anterior-descending coronary artery (LADCA) and ipsilateral lung dose parameters were lowered with Pr-C compared to Pr-noC, Su-C and Su-noC. Prone position significantly reduced respiration-related surgical clip movements, their variability as well as CW movements. Significant anterior heart displacement was observed in Pr. Prone position with daily online correction could maximize the heart and LADCA protection.

PO-1093 Comparison of IV contrast enhancement between 4DCT and helical radiotherapy planning scans for lung cancer

PO-1093 Comparison of IV contrast enhancement between 4DCT and helical radiotherapy planning scans for lung cancer

Implications of free breathing motion assessed by 4D-computed tomography on the delivered dose in radiotherapy for esophageal cancer

The aim of this study was to assess the effect of breathing motion on the delivered dose in esophageal cancer 3-dimensional (3D)-conformal radiotherapy (3D-CRT), intensity-modulated radiotherapy (IMRT), and volumetric modulated arc therapy (VMAT). We assessed 16 patients with esophageal cancer. All patients underwent 4D-computed tomography (4D-CT) for treatment planning. For each of the analyzed patients, 1 3D-CRT, 1 IMRT, and 1 VMAT (RapidArc—RA) plan were calculated. Each of the 3 initial plans was recalculated on the 4D-CT (for the maximum free inspiration and maximum free expiration) to assess the effect of breathing motion. We assessed the minimum dose (Dmin) and mean dose (Dmean) to the esophagus within the planning target volume, the volume changes of the lungs, the Dmean and the total lung volume receiving at least 40Gy (V40), and the V30, V20, V10, and V5. For the heart we assessed the Dmean and the V25. Over all techniques and all patients the change in Dmean as compared with the planned Dmean (planning CT [PCT]) to the esophagus was 0.48% in maximum free inspiration (CT_insp) and 0.55% in maximum free expiration (CT_exp). The Dmin CT_insp change was 0.86% and CT_exp change was 0.89%. The Dmean change of the lungs (heart) was in CT_insp 1.95% (2.89%) and 3.88% (2.38%) in CT_exp. In all, 4 patients had a clinically relevant change of the dose (≥ 5% Dmean to the heart and the lungs) between inspiration and expiration. These patients had a very cranially or caudally situated tumor. There are no relevant differences in the delivered dose to the regions of interest among the 3 techniques. Breathing motion management could be considered to achieve a better sparing of the lungs or heart in patients with cranially or caudally situated tumors.

Digital reconstruction of high-quality daily 4D cone-beam CT images using prior knowledge of anatomy and respiratory motion

Conventional in-room cone-beam computed tomography (CBCT) lacks explicit representation of patient respiratory motion and usually has poor image quality and inaccurate CT numbers for target delineation and/or adaptive treatment planning. In-room four-dimensional (4D) CBCT image acquisition is still time consuming and suffers the same issue of poor image quality. To overcome this limitation, we developed a computational framework to digitally synthesize high-quality daily 4D CBCT images using the prior knowledge of motion and appearance learned from the planning 4D CT dataset. A patient-specific respiratory motion model was first constructed from the planning 4D CT images using principal component analysis of displacement vector fields across different respiratory phases. Subsequently, the respiratory motion model as well as the image content of the planning CT was spatially mapped onto the daily CBCT using deformable image registration. The synthesized 4D images possess explicit patient motion while maintaining the geometric accuracy of patient's anatomy at the time of treatment. We validated our model by quantitatively comparing the synthesized 4D CBCT against the 4D CT dataset acquired in the same day from protocol patients undergoing daily in-room CBCT setup and weekly 4D CT for treatment evaluation. Our preliminary results have demonstrated good agreement of contours in different motion phases between the synthesized and acquired scans. Various imaging artifacts were also suppressed and soft-tissue visibility was enhanced.

Spatial and volumetric changes of retroperitoneal sarcomas during pre-operative radiotherapy

PurposeTo determine the positional and volumetric changes of retroperitoneal sarcomas (RPS) during pre-operative external beam radiotherapy (PreRT). Material and methodsAfter excluding 2 patients who received chemotherapy prior to PreRT and 15 RPS that were larger than the field-of-view of cone-beam CT (CBCT), the positional and volumetric changes of RPS throughout PreRT were characterized in 19 patients treated with IMRT using CBCT image guidance. Analysis was performed on 118 CBCT images representing one image per week of those acquired daily during treatment. Intra-fraction breathing motions of the gross tumor volume (GTV) and kidneys were measured in 22 RPS patients simulated using 4D-CT. Fifteen other patients were excluded whose tumors were incompletely imaged on CBCT or who received pre-RT chemotherapy. ResultsA GTV volumetric increase (mean: 6.6%, p=0.035) during the first 2weeks (CBCT1 vs. CBCT2) of treatment was followed by GTV volumetric decrease (mean: 4%, p=0.009) by completion of radiotherapy (CBCT1 vs. CBCT6). Internal margins of 8.6, 15 and 15mm in the lateral, anterior/posterior and superior/inferior directions would be required to account for inter-fraction displacements. The extent of GTV respiratory motion was significantly (p<0.0001) correlated with more superiorly positioned tumors. ConclusionInter-fraction CBCT provides important volumetric and positional information of RPS which may improve PreRT quality and prompt re-planning. Planning target volume may be reduced using online soft-tissue matching to account for interfractional displacements of GTVs. Important breathing motion occurred in superiorly placed RPS supporting the utility of 4D-CT planning.

SU‐E‐J‐54: Dosimetric Evaluation of Amplitude‐Based Gating in Volumetric Modulated Arc Therapy (VMAT)

Purpose:The purpose of this work is to evaluate the dosimetric influence in the gated volumetric modulated arc therapy.Methods:This study is performed in three liver cancer patients. Execution of 4D CT simulation and plan deliveries on one‐dimensional respiratory motion phantom were based on patient's respiratory trace data sets. We chose the regular and irregular respiratory amplitude patterns among the patient respiratory‐gated treatment log file. Dose distributions were evaluated using EBT3 film by comparing the gated delivery with static delivery.Results:The presented results of static measured and calculated dose distributions showed good agreement with gamma‐index passing rates above 97% for 3%/3 m dose accuracy/distance‐to‐agreement criteria. In regular respiratory amplitude pattern, the results of two in three patients evaluated gamma‐index passing rates above 92% except for another patient who was evaluated of 87.16%. Patients whose plans had the largest percentage failing the gamma statistics exhibited irregular respiratory amplitude pattern that evaluated less than 70%. Minimum passing rate was 37.40%. This results showed that the correlation of between sorted 4D CT phase and irregular patient respiratory amplitude pattern may results in considerable dosimetric difference.Conclusion:The effort to maintain regular patient respiratory amplitude pattern between 4D CT simulation and treatment is the most important in gated volumetric modulated arc therapy.