Phases Of 4DCT Images Research Articles

A preliminary investigation of re-evaluating the irradiation dose in hepatocellular carcinoma radiotherapy applying 4D CT and deformable registration.

PurposeTo investigate the effect of breathing motion on dose distribution for hepatocellular carcinoma (HCC) patients using four‐dimensional (4D) CT and deformable registration.MethodsFifty HCC patients who were going to receive radiotherapy were enrolled in this study. All patients had been treated with transarterial chemoembolization beforehand. Three‐dimensional (3D) and 4D CT scans in free breathing were acquired sequentially. Volumetric modulated arc therapy (VMAT) was planned on the 3D CT images and maximum intensity projection (MIP) images. Thus, the 3D dose (Dose‐3D) and MIP dose (Dose‐MIP) were obtained, respectively. Then, the Dose‐3D and Dose‐MIP were recalculated on 10 phases of 4D CT images, respectively, in which the end‐inhale and end‐exhale phase doses were defined as Dose‐3D‐EI, Dose‐3D‐EE, Dose‐MIP‐EI, and Dose‐MIP‐EE. The 4D dose (Dose‐4D‐3D and Dose‐4D‐MIP) were obtained by deforming 10 phase doses to the end‐exhale CT to accumulate. The dosimetric difference in Dose‐3D, Dose‐EI3D, Dose‐EE3D, Dose‐4D‐3D, Dose‐MIP, Dose‐EIMIP, Dose‐EEMIP, and Dose‐4D‐MIP were compared to evaluate the motion effect on dose delivery to the planning target volume (PTV) and normal liver.ResultsCompared with Dose‐3D, PTV D99 in Dose‐EI3D, Dose‐EE3D and Dose‐4D‐3D decreased by an average of 6.02%, 1.32%, 2.43%, respectively (P < 0.05); while PTV D95 decreased by an average of 3.34%, 1.51%, 1.93%, respectively (P < 0.05). However, CI and HI of the PTV in Dose‐3D was superior to the other three distributions (P < 0.05). There was no significant differences for the PTV between Dose‐EI and Dose‐EE, and between the two extreme phase doses and Dose‐4D (P> 0.05). Negligible difference was observed for normal liver in all dose distributions (P> 0.05).ConclusionsFour‐dimensional dose calculations potentially ensure target volume coverage when breathing motion may affect the dose distribution. Dose escalation can be considered to improve the local control of HCC on the basis of accurately predicting the probability of radiation‐induced liver disease.

Comparison of the dose on specific 3DCT images and the accumulated dose for cardiac structures in esophageal tumors radiotherapy: whether specific 3DCT images can be used for dose assessment?

BackgroundCardiac activity could impact the accuracy of dose assessment for the heart, pericardium and left ventricular myocardium (LVM). The purpose of this study was to explore whether it is possible to perform dose assessment by contouring the cardiac structures on specific three-dimensional computed tomography (3DCT) images to reduce the impact of cardiac activity.MethodsElectrocardiograph-gated 4DCT (ECG-gated 4DCT) images of 22 patients in breath-hold were collected. MIM Maestro 6.8.2 (MIM) was used to reconstruct specific 3DCT images to obtain the Maximal intensity projection (MIP) image, Average intensity projection (AIP) image and Minimum intensity projection (Min-IP) image. The heart, pericardium and LVM were contoured in 20 phases of 4DCT images (0, 5%... 95%) and the MIP, AIP and Min-IP images. Then, a radiotherapy plan was designed at the 0% phase of the 4DCT images, and the dose was transplanted to all phases of 4DCT to acquire the dose on all phases, the accumulated dose of all phases was calculated using MIM. The dose on MIP, AIP and Min-IP images were also obtained by deformable registration of the dose. The mean dose (Dmean), V5, V10, V20, V30 and V40 for the heart, pericardium and LVM in MIP, AIP and Min-IP images were compared with the corresponding parameters after dose accumulation.ResultsThe mean values of the difference between the Dmean in the MIP image and the Dmean after accumulation for the heart, pericardium and LVM were all less than 1.50 Gy, and the dose difference for the pericardium and LVM was not statistically significant (p > 0.05). For dose-volume parameters, there was no statistically significant difference between V5, V10, and V20 of the heart and pericardium in MIP, AIP, and Min-IP images and those after accumulation (p > 0.05). For the LVM, only in the MIP image, the differences of V5, V10, V20, V30 and V40 were not significant compared to those after dose accumulation (p > 0.05).ConclusionsThere was a smallest difference for the dosimetry parameters of cardiac structures on MIP image compared to corresponding parameters after dose accumulation. Therefore, it is recommended to use the MIP image for the delineation and dose assessment of cardiac structures in clinical practice.

OA014] 4D lung tumor motion modelling using dynamic MLC tracking and EPID feedback

Purpose Accurate delivery of the prescribed dose to moving lung tumors is a key challenge in radiotherapy. Tumor tracking involves real-time specifying the target and correcting the geometry in order to compensate for the respiratory motion, that’s why tracking the tumor requires caution. This study proposes an approach to generate a model to track lung tumor motion by controlling dynamic multileaf collimators without any marker. Methods For this purpose, 4D-CT images of 10 patients were used and all the slices which contained the tumor were contoured for all patients. The first four phases of 4D-CT images which contained tumors were selected as input of the software, and the next six phases were considered as the output. A hybrid intelligent system, ANFIS (Adaptive neuro fuzzy inference system), is used to predict motion of lung tumor. The root mean square error (RMSE) was used to investigate the accuracy of ANFIS performance in tumor motion prediction. For modeling of respiratory motion the end-exhale phases of these images have been considered as the reference and were analyzed using the neuro-fuzzy method to predict the magnitude of displacement of the lung tumor. Then, the predicted data were used to determine the leaf motion in MLC. Finally, the trained algorithm was figured out using Shaper software to show how the MLCs can track the moving tumor and then imported on the Varian linac equipped with EPID. Results The results showed that the RMSE did not have a major variation. Also, there was a good agreement between the images obtained by EPID and Shaper for a respiratory cycle. Conclusions The data in the 4DCT images were used for motion tracking instead of using markers that leads to more information of tumor motion with respect to methods based on marker location. This developed approach can track the moving tumor with MLC based on the 4D modelling and so it can improve treatment accuracy, dose conformity and sparing of healthy tissues because of low error in margins that can be ignored. Therefore, this method can work more accurate compared with the gating and invasive approaches using markers.

Markerless Respiratory Tumor Motion Prediction Using an Adaptive Neuro-fuzzy Approach

Accurate delivery of the prescribed dose to moving lung tumors is a key challenge in radiation therapy. Tumor tracking involves real-time specifying the target and correcting the geometry to compensate for the respiratory motion, that's why tracking the tumor requires caution. This study aims to develop a markerless lung tumor tracking method with a high accuracy. In this study, four-dimensional computed tomography (4D-CT) images of 10 patients were used, and all the slices which contained the tumor were contoured for all patients. The first four phases of 4D-CT images which contained tumors were selected as input of the software, and the next six phases were considered as the output. A hybrid intelligent method, adaptive neuro-fuzzy inference system (ANFIS), was used to evaluate motion of lung tumor. The root mean square error (RMSE) was used to investigate the accuracy of ANFIS performance for tumor motion prediction. For predicting the positions of contoured tumors, the averages of RMSE for each patient were calculated for all the patients. The results showed that the RMSE did not have a major variation. The data in the 4D-CT images were used for motion tracking instead of using markers that lead to more information of tumor motion with respect to methods based on marker location.

A comparison of dosimetric variance for external-beam partial breast irradiation using three-dimensional and four-dimensional computed tomography.

PurposeTo investigate the potential dosimetric benefits from four-dimensional computed tomography (4DCT) compared with three-dimensional computed tomography (3DCT) in radiotherapy treatment planning for external-beam partial breast irradiation (EB-PBI).Patients and methods3DCT and 4DCT scan sets were acquired for 20 patients who underwent EB-PBI. The volume of the tumor bed (TB) was determined based on seroma or surgical clips on 3DCT images (defined as TB3D) and the end inhalation (EI) and end exhalation (EE) phases of 4DCT images (defined as TBEI and TBEE, respectively). The clinical target volume (CTV) consisted of the TB plus a 1.0 cm margin. The planning target volume (PTV) was the CTV plus 0.5 cm (defined as PTV3D, PTVEI, and PTVEE). For each patient, a conventional 3D conformal plan (3D-CRT) was generated (defined as EB-PBI3D, EB-PBIEI, and EB-PBIEE).ResultsThe PTV3D, PTVEI, and PTVEE were similar (P=0.549), but the PTV coverage of EB-PBI3D was significantly less than that of EB-PBIEI or EB-PBIEE (P=0.001 and P=0.025, respectively). There were no significant differences in the homogeneity or conformity indexes between the three treatment plans (P=0.125 and P=0.536, respectively). The EB-PBI3D plan resulted in the largest organs at risk dose.ConclusionThere was a significant benefit for patients when using 3D-CRT based on 4DCT for EB-PBI with regard to reducing nontarget organ exposure. Respiratory motion did not affect the dosimetric distribution during free breathing, but might result in poor dose coverage when the PTV is determined using 3DCT.

A comparative study of planning target volumes based on three-dimensional computed tomography, four-dimensional computed tomography, and positron emission tomography-computed tomography in thoracic esophageal cancer

Objective To investigate the differences in position and volume between planning target volumes (PTV) based on positron emission tomography-computed tomography (PET-CT) images with an standardized uptake value (SUV) no less than 2.5, 20% of the maximum SUV (SUVmax), or 25% of SUVmax, three-dimensional (3D) CT, and four-dimensional (4D) CT in thoracic esophageal cancer. Methods Eighteen patients with thoracic esophageal cancer sequentially received chest 3DCT, 4DCT, and[18F]fluoro-2-deoxy-D-glucose (FDG) PET-CT scans. PTV3D was obtained by conventional expansion of 3DCT images; PTV4D was obtained by fusion of target volumes from 10 phases of 4DCT images. The internal gross tumor volumes (IGTV), IGTVPET2.5, IGTVPET20%, and IGTVPET25%, were generated based on PET-CT images with an SUV no less than 2.5, 20% of SUVmax, and 25% of SUVmax, respectively. These IGTVs were expanded longitudinally by 3.5 cm and radically by 1 cm to make PTVPET2.5, PTVPET20%, and PTVPET25%, respectively. Results PTV3D was significantly larger than both PTV4D and PTVPET(P=0.000-0.044), while there was no significant difference between PTV4D and PTVPET(P=0.216-0.633). The mutual degrees of inclusion (DIs) between PTV3D and PTV4D were 0.70 and 0.95, respectively, which were negatively correlated with 3D-Vector (P=0.039). The mutual DIs between PTVPET2.5, PTVPET20%, and PTVPET25% were 0.74, 0.72, 0.78, 0.73, 0.77, and 0.70, respectively, which showed no correlation with 3D-Vector (P=0.150-0.822). The mutual DIs between PTV3D and PTVPET were 0.86, 0.84, 0.88, 0.63, 0.67, and 0.59, respectively. Conclusions It is difficult to achieve complete volumetric overlap of PTVs based on 3DCT, 4DCT and PET-CT in thoracic esophageal cancer due to different target volume information. PET scan during free breathing should be used with caution to generate PTVs in thoracic esophageal cancer. Key words: Tomography, X-ray computed, three-dimensional; Tomography, X-ray computed, four-dimensional; Tomography, positron-emission, fluorodeoxyglucose; Planning target volume; Esophageal neoplasms/radiotherapy

Comparison of PTV based on FDG PET-CT versus 4DCT in non-small cell lung cancer

Objective To compare the positional and volumetric differences of planning target volumes(PTVs)based on fluorodeoxyglucose(FDG)positron emission tomography/computed tomography(PET-CT)and four-dimensional CT (4DCT)for the primary tumor of non-small cell lung cancer(NSCLC) . Methods Fifteen patients with NSCLC sequentially underwent three-dimensional CT (3DCT) , 4DCT, and FDG PET-CT scans of the thorax. IGTV10 was defined on 10 respiratory phases of 4DCT images; IGTVPET was defined on PET-CT images using a fixed threshold of 15% of the maximum standard uptake values. PTV4D and PTVPET were obtained by adding a 10 mm margin around IGTV10 and IGTVPET.The differences in volume, position, and degree of inclusion(DI)between PTVPET and PTV4D were evaluated. Results There was no significant difference in centroid position between PTVPET and PTV4D (P=0.589, 0.147, and 0.096). There was no significant difference in volume between PTVPET and PTV4D (P=0.156) ; however, PET-CT resulted in an >20% change in PTV in 5 of 15 patients, the PTVPET volume increased (average 30%) in 10 patients, and the PTVPET volume decreased (average 11%) in 5 patients. The mean DI of PTV4D in PTVPET was 85%, and 7%-46% of PTVPET was not included within the PTV4D. The DI between PTV4D and PTVPET showed no significant correlation with the 3D motion vector (P=0.134and 0.405) . Conclusions Although there are no significant differences in centroid position and volume between PTVPET and PTV4D for the primary tumor of NSCLC, the spatial mismatch between them is apparent and the mismatch shows no correlation with tumor displacement. Key words: Tomography, positron-emission, fluorodeoxyglucose; Tomography, X-ray computed, four-dimensional; Planning target volume; Degree of inclusion; Lung neoplasms/radiotherapy

Feasibility and potential benefits of defining the internal gross tumor volume of hepatocellular carcinoma using contrast-enhanced 4D CT images obtained by deformable registration.

ObjectiveTo study the feasibility and the potential benefits of defining the internal gross tumor volume (IGTV) of hepatocellular carcinoma (HCC) using contrast-enhanced 4D CT images obtained by combining arterial-phase (AP) contrast-enhanced (CE) 3D CT and non-contrast-enhanced (NCE) 4D CT images using deformable registration (DR).MethodsTen HCC patients who had received radiotherapy beforehand were selected for this study. The following CT simulation images were acquired sequentially: NCE 4D CT in free breathing, NCE 3D CT and APCE 3D CT in end-expiration breath holding. All 4D CT images were sorted into ten phases according to breath cycle (CT00 ~ CT90). Gross tumor volumes (GTVs) were contoured on all CT images and the IGTV-1 was obtained by merging the GTVs in each phase of 4D CT images. The GTV on the APCE 3D CT image was deformably registered to each 4D CT phase image according to liver shape using RayStationTM 3.99.0.7 version treatment planning system. The IGTV-DR was obtained by merging the GTVs after DR on the 4D CT images. Volume differences among the GTVs and between the IGTV-1 and the IGTV-DR were compared.ResultsThe edge of most lesions could be definitively identified using APCE 3D CT images compared to NCE 4D and 3D CT images. The GTV volume on APCE 3D CT images increased by an average of 34.79% (P < 0.05). There was no significant difference among the GTV volumes obtained using NCE 4D and 3D CT images (P > 0.05). The GTV volumes after DR on 4D CT different phase images increased by an average of 36.29% (P < 0.05), as was observed using the APCE 3D CT image (P > 0.05). Lastly, the volume of IGTV-DR increased by an average of 19.91% compared to that of IGTV-1 (P < 0.05).ConclusionNCE 4D CT imaging alone has the potential risk of missing a partial volume of the HCC. The combination of APCE 3D CT and NCE 4D CT images using the DR technique improved the accuracy of the definition of the IGTV in HCC.

SU‐E‐J‐133: Evaluation of Inter‐ and Intra‐Fractional Pancreas Tumor Residual Motions with Abdominal Compression

Purpose:Abdominal compression (AC) has been widely used to reduce pancreas motion due to respiration for pancreatic cancer patients undergoing stereotactic body radiotherapy (SBRT). However, the inter‐fractional and intra‐fractional patient motions may degrade the treatment. The purpose of this work is to study daily CBCT projections and 4DCT to evaluate the inter‐fractional and intra‐fractional pancreatic motions.Methods:As a standard of care at our institution, 4D CT scan was performed for treatment planning. At least two CBCT scans were performed for daily treatment. Retrospective studies were performed on patients with implanted internal fiducial markers or surgical clips. The initial motion pattern was obtained by extracting marker positions on every phase of 4D CT images. Daily motions were presented by marker positions on CBCT scan projection images. An adaptive threshold segmentation algorithm was used to extract maker positions. Both marker average positions and motion ranges were compared among three sets of scans, 4D CT, positioning CBCT, and conformal CBCT, for inter‐fractional and intra‐fractional motion variations.Results:Data from four pancreatic cancer patients were analyzed. These patients had three fiducial markers implanted. All patients were treated by an Elekta Synergy with single fraction SBRT. CBCT projections were acquired by XVI. Markers were successfully detected on most of the projection images. The inter‐fractional changes were determined by 4D CT and the first CBCT while the intra‐fractional changes were determined by multiple CBCT scans. It is found that the average motion range variations are within 2 mm, however, the average marker positions may drift by 6.5 mm.Conclusion:The patients respiratory motion variation for pancreas SBRT with AC was evaluated by detecting markers from CBCT projections and 4DCT, both the inter‐fraction and intra‐fraction motion range change is small but the drift of marker positions may be comparable to motion ranges.

SU-E-J-122: Quantification of Respiratory-Induced Pancreatic Head Tumor Rotation and Deformation Using 4DCT and Fiducial Markers.

To quantify pancreatic head tumor rotation and deformation due to the respiration using 4DCT and fiducial markers. This study included seventeen pancreatic head tumor patients who were treated with gated SBRT using Novalis system in our institution. Each patient had two 5-mm-long fiducial markers placed in pancreatic head approximately 2 cm apart for the gating treatment. All patients had 4DCT scans of 3mm-slice thickness using a CT scanner (SENSATION, SIEMENS) under free breathing condition to create the internal target volume (ITV). The respiratory curve was generated through a pressure sensor placed on the patient's abdomen. The 4DCT image data sets were binned into 8 phases: 0% inhale (end of exhale), 25% inhale, 50% inhale, 75% inhale, 100% inhale (end of inhale), 75% exhale, 50% exhale, and 25% exhale. The fiducial markers were contoured on each phase of 4DCT images, and the 3D coordinates (x,y,z) of centroid of the fiducial marker were recorded. The distance between two markers was calculated for each phase, and its variation on eight phases indicated the pancreatic head deformation. The orientation change of the two markers on eight phases indicated the pancreatic head rotation. Student t-test was performed for the statistical analysis. Pancreatic head rotation and deformation were observed through the breathing cycle. The largest rotation and deformation happened to the course from 0% inhale to 100% inahle. The rotation was 8.0±6.2 degree ranging from 2 to 20 degree. The deformation was 2.0±2.4 mm ranging from 0 to 6.6 mm. Both were statistically significant with p<10-4 for rotation and p=0.0033 for deformation. The 4DCT images showed significant pancreatic head rotation and deformation due to respiration, and both rotation and deformation were highly variable between patients. For patients receiving SBRT with tight margin, rotation and deformation need to be considered, and individualized margin may be necessary.

Comparison of the planning target volume based on three-dimensional CT and four-dimensional CT images of non-small-cell lung cancer

Background and purpose To compare positional and volumetric differences of planning target volumes (PTVs) based on axial three-dimensional CT (3DCT) and four-dimensional CT (4DCT) for the primary tumor of non-small cell lung cancer (NSCLC). Materials and methods Twenty-eight patients with NSCLC underwent 3DCT and 4DCT scans of the thorax during normal free breathing. PTV vector was defined on 3DCT using the individual tumor motion vector measured by 4DCT accounting for tumor motion; PTV 4D was defined on all phases of 4DCT images. In addition, a 7 mm margin for microscopic disease and a 3 mm setup margin were used for above PTVs, respectively. The differences in target position, volume and coverage between PTV vector and PTV 4D were evaluated for tumors in different lobes, respectively. Results The median motion vector for tumors located in the upper lobe (group A) and in the middle lower lobe (group B) was 2.8 and 7 mm, respectively. The mean centroid shifts between PTV vector and PTV 4D in the LR, AP and CC directions for group A and B were close to zero. The median size ratio of PTV 4D to PTV vector was 0.75 and 0.52 for group A and B. The motion vector showed a significant correlation to the ratio of PTV 4D to PTV vector for group A and B ( p = 0.008 and 0.003). The median DI of PTV vector in PTV 4D was 69.19% for group A and 51.60% for group B. The median DI of PTV 4D in PTV vector was 98.99% for group A and 99.94% for group B. Conclusion It is necessary to expand the internal margin isotropically in a single direction for 3DCT treatment planning due to the uncertainty of the 3DCT-based target position. The 3DCT-based PTV using individual margins provides a good coverage of the 4DCT-based PTV, meanwhile encompasses relatively large normal tissues, especially for middle and lower lobe tumors. We should be cautious about the use of the individual PTV derived from 3DCT in treatment planning.

A multiple points method for 4D CT image sorting

Artifacts affect 4D CT images due to breathing irregularities or incorrect breathing phase identification. The purpose of this study is the reduction of artifacts in sorted 4D CT images. The assumption is that the use of multiple respiratory related signals may reduce uncertainties and increase robustness in breathing phase identification. Multiple respiratory related signals were provided by infrared 3D localization of a configuration of markers placed on the thoracoabdominal surface. Multidimensional K-means clustering was used for retrospective 4D CT image sorting, which was based on multiple marker variables, in order to identify clusters representing different breathing phases. The proposed technique was tested on computational simulations, phantom experimental acquisitions, and clinical data coming from two patients. Computational simulations provided a controlled and noise-free condition for testing the clustering technique on regular and irregular breathing signals, including baseline drift, time variant amplitude, time variant frequency, and end-expiration plateau. Specific attention was given to cluster initialization. Phantom experiments involved two moving phantoms fitted with multiple markers. Phantoms underwent 4D CT acquisition while performing controlled rigid motion patterns and featuring end-expiration plateau. Breathing cycle period and plateau duration were controlled by means of weights leaned upon the phantom during repeated 4D CT scans. The implemented sorting technique was applied to clinical 4D CT scans acquired on two patients and results were compared to conventional sorting methods. For computational simulations and phantom studies, the performance of the multidimensional clustering technique was evaluated by measuring the repeatability in identifying the breathing phase among adjacent couch positions and the uniformity in sampling the breathing cycle. When breathing irregularities were present, the clustering technique consistently improved breathing phase identification with respect to conventional sorting methods based on monodimensional signals. In patient studies, a qualitative comparison was performed between corresponding breathing phases of 4D CT images obtained by conventional sorting methods and by the described clustering technique. Artifact reduction was clearly observable on both data set especially in the lower lung region. The implemented multiple point method demonstrated the ability to reduce artifacts in 4D CT imaging. Further optimization and development are needed to make the most of the availability of multiple respiratory related variables and to extend the method to 4D CT-PET hybrid scan.

Poster — Thur Eve — 36: Lung SBRT: Dosimetric Evaluation of 4DCT Based Treatment Planning in Presence of Respiratory Motion

The respiratory related dosimetric uncertainties in the 4DCT based lung stereotactic body radiotherapy (SBRT) treatment planning and treatment delivery are evaluated by performing density assignment correction (DAC) in the ITV. Siemens 40‐slice CT scanner was used to acquire 8 phases of 4DCT images and a free breathing CT scan using a CIRS Dynamic Thorax Phantom. The ITV was created from 4DCT scan and treatment planning was done in free breathing CT scan. The ITV dose coverage was measured by CIRS phantom with ionization chamber moving together with the tumor during the beam delivery under free breathing treatment, which considers the time and positional averaged distribution of tissue heterogeneities within the ITV. The measured results are compared with the treatment planning with and without DAC in the ITV. The ITV dose coverage from the measurement is 1.9% higher than the planned ITV mean dose. The ITV received adequate dose due to the presence of effective density of the tumor within the ITV when the treatment is delivered under free breathing. The plan accounting for respiratory motion by assigning an average tumor density into the treatment planning had better uniform dose than the plan without DAC. By incorporating the ITV tumor density override into treatment planning dose calculation, the ITV dose coverage becomes more uniform and the mean dose of ITV increases and more closely matches the measured values than the plan without DAC in the ITV.

Poster — Thur Eve — 35: Lung SBRT: 4DCT Based Treatment Planning in Presence of Respiratory Motion

The purpose of this study was to determine the impact of internal target volume (ITV) density on the 4DCT based lung stereotactic body radiotherapy (SBRT) treatment planning (TP) using the ITV density assignment correction (DAC). Siemens 40‐slice CT scanner was used to acquire 8 phases of 4DCT images and a free breathing CT scan of CIRS Dynamic Thorax Phantom with the tumor moving in three dimensions. The ITV was created from the merge of all the CTVs in all the phases in Eclipse and copied to the free breathing CT scan for planning dose calculation. The PTV was created by adding a 5mm margin around the ITV. Lung SBRT plans were created using the 0% inhale phase, the 50% inhale phase and the free breathing CT scan under the original CT scan and DAC in the ITV. Our data shows ITV coverage from the isodose line, DVH analysis and mean dose on the free breathing CT scan, 0% inhale phase and 50% inhale phase with and without ITV DAC. The high dose region follows the CTV without DAC in the ITV because of the higher build‐up of dose within the denser CTV volume than in the surrounding less dense lung tissue. The MU decreased 0.9%, 1.3% and 1.2% for free breathing scan, 0% inhale and 50% inhale respectively with DAC in the ITV. The DAC provided a clinical acceptable way to predict the ITV dose coverage in the treatment planning system.

SU‐GG‐T‐543: The Effect of Respiratory Motion on the Dose Distribution of Free‐Breathing Stereotactic Body Radiation Therapy for Spinal Tumors

Purpose: Stereotactic Body Radiation Therapy (SBRT) has been used increasingly for the management of spinal lesions. This study was performed to investigate the effect of respiratory motion on the dose distributions of spine SBRT patients. Method: 4D CT and treatment planning data for thoracic spinal lesions (T5 and T8) in two linac‐based SBRT patients was studied retrospectively. Bony structures of the spine were visually inspected in all 10 phases of the 4D CT images and there was no spinal motion observed. Tumors were contoured by radiation oncologists on the 50% phase of the 4D CT images, which corresponded to end exhalation. A single‐fraction dose of 20Gy and 14Gy was prescribed to CTV20 (gross tumor) and CTV14 (contiguous marrow cavity) respectively. Intensity modulated treatment plans were constructed on the 50% 4DCT phase and copied to all other phases of the 4D data set for comparative dose calculations. The DVHs of CTV20, CTV14, and the spinal cord were compared between actual dose, which is the sum of dose from all phases, and planned dose, which was done on 50% phase of 4D CT images. Results: The impacts of chest wall and diaphragm motion on the dose distribution were analyzed. Treatment plans that incorporated all respiratory phases resulted in improved CTV20 coverage less than 2% but the maximum dose within spinal cord was increased 3∼4%. Conclusion: Respiratory motion has a modest impact on the dose distribution of spine SBRT treatments including a 3–4% increase in cord dose. Motion control methods should be considered to reduce the uncertainties of dose delivery when motion amplitude is large.

SU‐GG‐J‐36: Analysis of Artifacts in Four‐Dimensional CT Images of Phantoms with Regular Motions

Purpose: The aims of this study were to clarify the factors causes imaging artifacts and investigate the impact of partial volume and truncation artifacts on volume determination. Method and Materials: Moving cylindrical phantoms with 2cm of diameter with different heights, 0.3cm, 0.5cm, 1cm, 2cm, 3 cm, and 10cm were scanned (scan range 2cm/6cm0 by GE (General Electric Healthcare) LightSpeed. GE Advantage. 4D software was used to create 4D‐CT data sets. The artifacts were then visually and quantitatively analyzed. Scan. The target volume was determined by manual contouring and automatic volume detection using a cutoff value of 250 HU (Hounsfield Unit). The conformity index of the target volume was defined as the ratio of the manually contoured volume to the volume defined by HU cutoff. The ratio of volume from static CT to real volume was calculated in order to investigate the partial volume effect and truncation artifacts. The volumes measured in 10 phases of 4D CT images were also compared with true volume. Results: The ratio of static cylinder volume in CT images scanned 2cm length to the real volume was maximally twice by partial volume effect. The correlation between the velocity and target volume was not consistent potentially due to complicate interplay of different artifacts;,partial volume effect, truncation artifact and miss or different matching between adjacent beds . Conclusions: The static CT data volume can be quite different from true volume even in static CT images. If a lesion size is not enough to overcome the impact of the partial effects, significantly larger volume will be presented on static and 4DCT images. Any strong correlation between the velocity and the volume was not found.. The 4DCT images still presents partial volume and truncation artifact that requires the further improvement of imaging technology.

Definition of internal target volume and domestric study for hepatocellular carcinoma using four-dimensional CT

Objective To define individualized internal target volume (ITV) for hepatocellular car-cinoma using four-dimensional (4D) CT, and to compare the differences in target volume definition and dose distribution among 3D, 4D and respiratory-gated plans. Methods 4DCT scanning was obtained for 12 pa-tients with hepatoceUular. Gross tumor volume (GTV), clnical target volume (CTV) and normal tissues were contoured on all 10 respiratory phases of 4DCT images. The 3D, 4D and gated treatment plans were prepared for each patient using three different planning target volumes (PTVs): 1) PTV3D was derived from a single CTV plus conventional margins;2) PTV4D was derived from ITV4D, which encompassed all 10 CTVs plus setup margins (SMs);3) PTVGatingwas derived from ITVGating, which encompassed 3 CTVs within ga-ting-window at end-expiration plus SMs. The PTV volume and dose distribution were compared among differ-ent plans. Results The PTV3D was the largest in all 12 patients, but still missed partial target volume in 5 patients when comparing with PTV4D. Both the 4D plans and the gated plans spared more normal tissues than the 3D plans, especially the hver. Without increasing normal tissue dose, the 4D plans allowed for increas-ing the calculated dose from (50.8±2.0) Gy (3D plans) to (54.7±3.3) Gy, and the gated plans could further increase the dose to (58.0±3.9) Gy. Conclusions The 4DCT-based plans can ensure optimal tar-get coverage with less irradiation of normal tissues and allow dose escalation when compared with 3D plans.Respiratory gated radiotherapy can further reduce the target volumes to spare more surrounding tissues, espe-cially for patients with large extent of respiratory mobility. Key words: Liver neoplasm/radiotherapy; Radiotherapy,three-dimensional conformal; Tomo-graphy,X-ray computed, four-dimensional; Dosimetry