Retrospective 4D CT Research Articles

SU‐E‐T‐651: Quantification of Dosimetric Accuracy of Respiratory Gated Stereotactic Body Radiation Therapy

Purpose:To quantify the dosimetric accuracy of respiratory gated stereotactic body radiation therapy delivery using dynamic thorax phantom.Methods:Three patients with mobile target (2 lung, 1liver) were chosen. Retrospective 4DCT image sets were acquired for using Varian RPM system. An in‐house MATLAB program was designed for MIP, MinIP and AvgIP generation. ITV was contoured on MIP image set for lung patients and on MinIP for liver patient. Dynamic IMRT plans were generated on selected phase bin image set in Eclipse (v10.0) planning system. CIRS dynamic thorax phantom was used to perform the dosimetric quality assurance. Patient breathing pattern file from RPM system was converted to phantom compatible file by an in‐house MATLAB program. This respiratory pattern fed to the CIRS dynamic thorax phantom. 4DCT image set was acquired for this phantom using patient breathing pattern. Verification plans were generated using patient gating window and delivered on the phantom. Measurements were carried out using with ion chamber and EBT2 film. Exposed films were analyzed and evaluated in FilmQA software.Results:The stability of gated output in comparison with un‐gated output was within 0.5%. The Ion chamber measured and TPS calculated dose compared for all the patients. The difference observed was 0.45%, −0.52% and −0.54 for Patient 1, Patient2 and Patient 3 respectively.Gamma value evaluated from EBT film shows pass rates from 92.41% to 99.93% for 3% dose difference and 3mm distance to agreement criteria.Conclusion:Dosimetric accuracy of respiratory gated SBRT delivery for lung and liver was dosimetrically acceptable. The Ion chamber measured dose was within 0.203±0.5659% of the expected dose. Gamma pass rates were within 96.63±3.84% of the expected dose.

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.

Determining a Dosimetric Correlate for Acute Esophagitis in Patients with Advanced Non-small Cell Lung Cancer Treated with Intensity Modulated Radiation Therapy (IMRT)

Acute radiation esophagitis is a common complication in the treatment of lung malignancies which can lead to treatment interruption and undesirable morbidity. We report our experience treating locally-advanced non-small cell lung cancers using chemoradiotherapy via IMRT and the associated acute treatment-related toxicities, and we determined potential dosimetric parameters predictive for acute esophagitis. This retrospective, multi-site study assessed 37 Stage IA-IV lung cancer patients who received IMRT. All patients were scanned during free breathing using coached retrospective 4D-CT and 21 had phase-based gated treatment delivery. A median dose of 70.0 Gy (range: 50 - 81.9 Gy) over 35 fractions (range 25 - 39 fractions) was delivered to the tumor using inhomogeneity correction. The esophagus was contoured and the mean dose and the volume of esophagus receiving 30, 50, 60 and 70 Gy (V30, V50, V60, V70, respectively) was calculated for each patient. Patients were graded for acute and chronic toxicities using the CTCAE 3.0 and logistic regression was performed to test associations between dosimetric parameters and esophageal toxicity. All simulated patients completed their course of chemoradiotherapy treatment. Twenty-eight patients (76%) received radiation treatment with concurrent chemotherapy. The most common acute toxicities were nausea, esophagitis, and fatigue observed in 11, 26, and 18 patients respectively. Only 3 patients (8%) developed Grade 3 radiation pneumonitis confirmed clinical and radiographically and required treatment with oral steroids. Acute Grade 1, 2 and 3 nausea was observed in 5, 4 and 2 patients; acute Grade 1, 2 and 3 esophagitis was observed in 8, 17 and 1 patients and acute Grade 1, 2 and 3 fatigue was observed in 9, 7 and 2 patients, respectively. Patients who experienced any esophagitis had a mean dose in excess of 28.7 Gy to the esophagus (Odds ratio: 1.09, 95% confidence interval: 1.02 - 1.18, p = 0.01). Univariate logistic regression correlated acute Grade ≥2 esophagitis with V60 (p = 0.045) and mean esophageal dose (p = 0.049). We did not identify significant differences in acute esophagitis between patients receiving concurrent chemotherapy and radiation versus those receiving radiation alone (p = 0.058). We were not able to correlate any parameters with late/chronic esophagitis. The mean dose to the esophagus and the volume of esophagus receiving higher doses of irradiation, but not chemotherapy, are associated with the development of esophagitis. Maintaining a mean doses to the esophagus below 28.7 Gy and restricting the volume of esophagus receiving 60 Gy or more can help to minimize treatment associated esophagitis.

SU‐FF‐J‐39: The Impact of Improving Techniques for Phase Reassignment in 4D CT Reconstruction

Purpose: To develop the respiratory signal analysis program for a phase‐based retrospective 4D CT using RPM, spirometer, stereo‐camera and removed the irregular peaks of respiratory signals for accurate phase assignment in 4D CT. Methods: In this study, an in‐house respiratory signal analysis program was developed for the phase reassignment and the analysis of the irregular respiratory signals. Various irregular respiratory patterns were obtained from clinical experimental volunteers. After then, the in‐house program analyzed the factors affecting to phase assignment which is directly related to irradiate sector. The algorithms for separating individual respiratory signals are presented, which made use of baseline signals of input signal to identify the nodes between baseline signals and input signal. Therefore, we could achieve irregular peak reduction and noise elimination in phase assignment. Subsequently, accuracy of phase assignment was improved with removal of irregular signals by self‐developed algorithm. Furthermore, this program also has a function to detect period and amplitude automatically. Result: The standard deviation (SD) of breathing period representing the baseline method was shown stable compare with gradient method, but the difference was not statistically significant. Gradient method shows inaccurate phase assignment when all irregular peaks were detected and assigned. On the other hand, baseline method which was self‐developed algorithm confirmed accurate phase assignment with removal of error peak detection due to tussis. It was also verified superior peak detection with the suggested baseline method at respiratory signals which contained irregular period changes and drifts of amplitude. Conclusion: This study is considered to be useful for not only image reconstruction and elevation of irradiating accuracy through phase assignment of RPM system but also analysis of respiratory signals. Moreover, this respiratory analysis program can be applied to the 4D CT reconstruction using various respiratory sensors for the reduction of motion artifact.

TU-D-304A-06: Comparison of Retrospective Phase Sorting with Real-Time Sorting Methods for 4D CT Images

Purpose: To study if the magnitude and frequency of artifacts in 4D CTimages and the imaging dose for real‐time image sorting methods will decrease compared to retrospective 4D CT sorting methods. Method and Materials: 4D CTimage acquisition was simulated with current retrospective phase sorting (RPCT), and three real‐time sorting methods: direction dependent displacement sorting (DDCT), simultaneous displacement and phase sorting (DPCT), and simultaneous displacement and velocity based sorting (DVCT). Respiratory signals and corresponding tumor positions were acquired with the Cyberknife Synchrony system for 58 motion data sets of 4 minutes each. Results: The mean artifact magnitude in the reconstructed 4D CTimages for the real‐time methods was at least a factor of 2 smaller compared to RPCT. The dose delivered during image acquisition was at least a factor of 2 less for DDCT compared to RPCT. Image acquisition time per couch position for the real‐time methods increased by a factor of 2 to 3. With displacement tolerances of 0.5mm, artifacts > 2mm were reduced from 33% for RPCT to 8% for DDCT, while artifacts > 4mm were reduced from 14% for RPCT to 1% for DDCT. The maximum artifact magnitude was the smallest for DVCT and in most cases a factor of 2 smaller for DDCT compared to RPCT, up to a factor of 6 smaller in the extreme cases. Conclusion: This work showed that the frequency and magnitude of artifacts in reconstructed 4D CTimages and the dose delivered during image acquisition can be reduced markedly if a real‐time sorting method is used instead of RPCT. However, real‐time sorting methods lead to 2–3 fold increases in image acquisition time. It is concluded from this work that DDCT is the better sorting method to use for 4D CTimage reconstruction.

TU‐C‐ValB‐07: Evaluation of 4DRT: CT Acquisition and Gated Delivery System

Introduction: Our objective was to characterize retrospectively acquired 4DCT data for prospective gated delivery, and the effects of gate length on beam energy stability, output constancy, and positional accuracy / inter‐device constancy. Materials and Methods: A barometric sensor gated the Siemens Oncor linac and Siemens Sensation CT scanner. Respiratory motion of 20 mm at 15 bpm over a stationary jig was used to assess radio‐opaque marker positions. Retrospective 4DCT reconstructions were obtained at 6 phases of inspiration and expiration, ranging from 0% to 100% by 20% intervals. The center of the pin was identified using 50% threshold values on the CT dataset. On the linac, gate windows of 1500, 850, 500, 350, 300, and 250 ms for the 12 phases were studied. Ion chambers were used to measure the beam energy and output stability at 10 cm and 20 cm in solid water simultaneously. Marker position during gated delivery was determined via film. Nine profiles, centered around the marker, were extracted for both the static and moving axes. The averages were smoothed, and the peak position and full‐width‐at‐half‐maximum (FWHM) were determined. The difference in FWHM along the static and moving axis is the intra‐gate motion. Results: Dosimetry for gates >= 500 ms was excellent. Although the average energy was constant, gate length reduction from 500ms to 250 ms resulted in an energy standard deviation increase of 1–14%, an output constancy increase from 1.6% to 4.6%, and a 50% dose rate decrease. Mean discrepancies between marker position measured on CT and linac were 3 mm, with 8 mm maximum. Conclusions: Dosimetric characteristics of the linac are reasonable for gating windows ⩾ 500 ms. Target position measured on retrospective 4DCT can introduce significant uncertainty for several phases of the respiratory cycle.

SU‐FF‐T‐419: The Impact of Gating On the Reduction of Heart Doses for Left Sided Breast Cancer Irradiation

Purpose: Because there are indications for an increased risk of cardiovascular diseases after radiotherapy of the left chest wall the dose to the heart should be reduced as far as possible. With respiratory gating technique irradiation can be restricted only to the inspiratory plateau phase. We investigated in all patients with left sided breast cancer dose reduction to the heart when treated only in the inspiration phase compared with not gated treatments. Materials and Methods: Between Sept 2004 and Feb 2006 107 patients with left sided breast cancer were treated with respiratory gating technique based on a retrospective 4D CT scan. We performed for all of these patients a normal and a respiratory gated planning CT. Planning was done with the same treatment parameters in both CT. DVH for the entire heart and the anterior left ventricle wall were calculated. All patients were treated with 2 Gy single dose to a total dose of 50 Gy to the entire left breast/chest wall. 68 patients received an additional boost of 10 Gy. Results: The mean dose to the entire heart was 0.7 Gy without and 0.6 Gy with respiratory gating (p=0.04) whereas the mean maximal dose was 40.2 Gy without and 11.7 Gy with respiratory gating (p=0.0003). The anterior heart wall receives 2.4 Gy without and 1.2 Gy with respiratory gating (p=0.0001) with a mean maximal dose of 39.6 Gy without and 10.1 Gy with respiratory gating (p=0.0004). Conclusion: 4D analysis has shown that the distance between the PTV and the heart is influenced by two separate parameters. Besides the movements of the chest wall the heart is pushed into the irradiated volume also by the diaphragm. Respiratory gating and irradiation only in the inspiratory phase significantly reduces radiation doses to the heart and especially to the anterior heart wall.

SU‐FF‐T‐118: Can the Use of Respiratory Gating Reduce Radiation Dose to Heart in Whole Left Breast Irradiation Treatment? — A Preliminary Study

Purpose: To investigate the potential benefit of using respiratory gating technique to reduce radiation doses to patient's heart in whole left breast irradiation treatment. Method and Materials: Conventional helical CT scan and retrospective 4D CT scans were acquired for 5 left sided breast patients. The 4DCT scans were sorted into 10 phases per respiratory cycle. Treatment plans, which consisted of two wedged tangential fields, were designed based on the helical CT scan. The PTV coverage, heart and lung doses in this plan were used as references for comparison. 10 separate optimal plans were generated from the 10 different phases obtained from the 4DCT images. All beam parameters, except for beam energy and wedges, were adjusted so that the PTV coverage in these plans was similar to or better than that in the corresponding reference plan. The heart and lung doses were then computed from these plans and compared to the corresponding doses in the reference plan. Results: The present results show that one patient would benefit from gated treatment at any given phase in terms of heart dose reduction. For four other patients respiratory gating would reduce dose to the heart. Gating at a given phase would reduce mean dose to the heart by 24%, 17%, 8% for 3 patients; for another patient a 17% reduction in maximum dose to the heart was found while the impact to the mean dose was insignificant. Conclusion: The use of respiratory gating in the irradiation of whole left breast treatment has the potential to reduce dose to the heart.

SU-FF-J-32: Use of Internal Body-Area as a Metric for Retrospective 4D CT Gating

Purpose: Current 4D CT acquisition techniques require the use of an external breathing metric, which adds time, cost and complexity to the procedure. If the use of the metric is not required after the imaging session, an internal metric may be adequate. The purpose of this study was to evaluate the use of the cross-sectional body area as an imaging-based internal metric for breathing motion using CT images acquired during free breathing. Method and Materials: A 16-slice CT scanner (Philips Brilliance) was operated in cine mode to acquire 25 scans consecutively at each couch position while patients underwent simultaneous quantitative spirometry. The cross-sectional body area was computed by automated image segmentation. The body areas for the 16 slices within each 2.4 cm-thick couch position were summed and compared to the corresponding tidal volume. The correlation between tidal volume and body area were examined to evaluate the quality of body area as a metric. Results: Three patients were analyzed. The body area consistently show a high correlation with the tidal volume in the abdomen (correlation coefficients > 0.8, and residual error < 10% of the total tidal volume). In the upper lung region, the correlation coefficients varied in a range of 0.2 to 0.99, and the residual errors were 5% to 30%. For each patient there was a transition region near the mid sternum where the correlation degraded dramatically. Conclusion: For imaging of the upper abdomen, the body area appears to be a good breathing metric for generating 4D imaging studies. One method for improving this process may be to overlap successive couch positions by one slice location, thereby providing a CT slice with overlapping body-area measurements.

Lung tumor motion measured using retrospective 4D CT and correlated with tumor location and attachment

To measure lung tumor motion using retrospective 4D CT and correlate the motion with tumor position and attachment. Lung patients were scanned using retrospective 4D CT to account for respiratory motion of the tumor. For 4D CT, multiple images are acquired at each slice location and then sorted according to the phase of the respiratory cycle in which the image was acquired. In order to acquire the 4D CT, each patient was positioned on the CT couch with his/her arms in a wingboard and feet tied together. While acquiring a scout CT and a helical CT (not yet the 4D CT), the patient’s breathing cycle is observed using the “RPM computer” (Varian Medical Systems). The respiratory cycle is observed using a marker block taped to the patient’s abdomen about 5 cm below the xiphoid. The motion of the marker block is relayed to the RPM computer by an infrared camera. An audible breathing coach is programmed with the patient’s breathing cycle time and started. The patient is asked if the rhythm is comfortable and if not, slight adjustments are made to the time until a comfortable rhythm is achieved. The CT slice spacing is set to 2.5 mm so that motions as small as 2.5 mm in the superior-inferior (S-I) direction can be observed. The volume to be scanned is determined from the scout image. The number of images is limited to 1500. The user programs the scanner to remain at each slice location acquiring multiple images for at least one full breathing cycle of the patient. All of these images are synchronized with the breathing cycle and sorted according to the breathing phase at which they were acquired. A 3D image is created for each breathing phase. The location of the tumor was identified by the lung lobe in which the tumor resided. Any attachment observed superiorly, inferiorly, anteriorly, posteriorly, medially, or laterally was recorded even if the attachment was only via a distinguishable vessel. Tumor displacement was measured by locating each edge of the tumor on the phases representing its most superior and most inferior positions. If the Medial-Lateral (M-L) displacement and the Anterior-Posterior (A-P) displacements were less than 3 mm, the superior and the inferior edges of the tumor were recorded on the phases representing its most superior and most inferior positions. The average displacement of the inferior and superior edges was identified as the tumor displacement. Tumors with A-P or M-L displacements of more than 3 mm were contoured before identifying their displacements. To contour the tumors, the 3D images representing its most superior and most inferior positions were exported to the planning computer (Eclipse, Varian Medical Systems). The tumor was contoured on both 3D images. The planning computer was then used to identify the center of the tumor on each phase using a built in center of mass calculation. These centers at the superior and inferior positions were recorded and their S-I difference represents the S-I displacement of the tumor. Lung tumor motion has been measured in 10 patients and associated with tumor position and attachment. Tumors located in the superior and inferior lobes of the lungs have been observed. Superior, inferior, anterior, posterior, lateral, and medial attachments have all been observed. The tumors that moved more than 3 mm were all located in the inferior lobes. Of these tumors, those attached posteriorly were observed to move the most. Tumors with extensive attachment moved the least. Tumor S-I motions from 0 to 13.8 mm were observed.

Lung tumor motion measured using retrospective 4D CT and correlated with tumor location and attachment

Lung tumor motion measured using retrospective 4D CT and correlated with tumor location and attachment