Respiration-correlated CT Scans Research Articles

MO‐F‐WAB‐02: Constancy of Time Lag Relationship Between External and Internal Signal in Abdominal Tumor Sites

Purpose: To investigate constancy, within a treatment session, of the time lag relationship between implanted markers in abdominal tumors and an external motion surrogate. Methods: Six gastroesophageal junction and three pancreatic cancer patients (IRB‐approved protocol) received a respiration‐correlated CT (RCCT), and two cone‐beam CTs (CBCT), one before and one after the treatment. Time between initial and final scan varied from about 1 to 3 h. Each patient had at least one implanted fiducial marker near the tumor. In all scans, abdominal displacement (Varian RPM) was recorded as the external motion signal. In‐house software tracked fiducials, representing internal signal, in CBCT projection images and amplitude‐sorted RCCT. For each set of projection images, time lag between internal and external signals was found by maximizing the correlation coefficient in each breathing cycle and averaging over all cycles. For the RCCT, these quantities were calculated for the 1 –2 cycles during which the fiducial was imaged. Results: Mean ± standard deviation in time lag, over all scans and patients, was 0.10 ± 0.07 s (range 0.01 – 0.36 s). External signal lagged the internal in 20/27 scans. Change in time lag between pre‐and post‐RT CBCT was 0.06 ± 0.07s (range 0.01 – 0.22 s), corresponding to 3.1 ± 3.7% (range 0.6 – 10.8%) of the gate width (range 1.6 – 3.1 s). Including the RCCT scan, change in time lag increased to 6.5 ± 5.4% (range 0.2 – 16.1%) of the gate width, and in three patients, change in time lag exceeded 10% of the gate width. Conclusion: Time lag between internal and external signal is small compared to the treatment gate width in all patients. Change in time lag within a treatment session, inferred from pre to post‐RT measurements is also small, suggesting that a single measurement at the session start is adequate.NIH/NCI award R01 CA126993 and a research grant from Varian.

SU-E-J-119: Comparative Evaluation of Respiratory Motion-Corrected Cone-Beam CT Images Derived from Treatment-Day Vs. Simulation-Day Respiration-Correlated CT Scans.

Respiration-induced motion artifacts in cone-beam CT (CBCT) can be corrected using a model of patient motion obtained from respiration-correlated CT (RCCT). This approach assumes that respiration-induced organ deformations at simulation, when RCCT scans are normally acquired, are still valid at treatment. The purpose of this study is to compare lung tumor image quality in motion-corrected CBCT images derived from treatment-day RCCT(tx) to simulation-day RCCT(sim) patient images. In an IRB-approved study, lung cancer patients receive an RCCT at simulation, and an RCCT, gated CBCT and 1-minute CBCT at one treatment session. CBCT projections from the 1-minute scan are sorted according to breathing amplitude from an external monitor and reconstructed and warped to obtain a motion-corrected MC-CBCT at end expiration. Motion correction uses a model adapted from either RCCT(tx) or RCCT(sim), thus obtaining MC-CBCT(tx) and MC-CBCT(sim) images respectively. A gated CBCT, in which gantry rotation and projection acquisition occur within a gate at end expiration, serves as ground truth for comparison. Quality of MC-CBCT images is evaluated from tumor-to-background contrast ratio (TBCR) values measured by delineating the tumor and annular volume around it on the gated CBCT then transferring the contours and aligning them to each MC-CBCT. TBCR is found tobe lower in MC-CBCT(sim) images, relative to MC-CBCT(tx), in four out of five patients with mean 21% reduction in a range 9-39%. In the remaining case, where there was no change in TBCR, tumor motion observed in the RCCT was small (2mm). Tumor motion extent relative to diaphragm is observed to change between RCCT(tx) and RCCT(sim) scans. Preliminary results indicate that deformation patterns in lung do change between simulation and treatment. Such variations may reduce the validity of using simulation data for motion-corrected CBCT at treatment. The findings require confirmation with larger numbers of patients. NIH/NCI award R01 CA126993, research grant from Varian Medical Systems.

WE-C-BRC-10: The Utility of Surrogates of the Distribution of Pulmonary Function in Individualizing Thoracic Radiotherapy

Purpose: Heterogeneity of lung tissue density and function is a hallmark of pathologic changes in chronic obstructive pulmonary disease (COPD). However, the standard risk assessment approaches, based on dose-volume histograms (DVH), do not currently use the functional information in evaluating pulmonary tolerance to radiation therapy. We investigated the utility of weighting of dose distributions by surrogate metrics of the lung function (parenchyma density, ventilation distribution) as complementary sources of information for assessing the risk of complications, and for further individualizing of thoracic radiation therapy. Methods: The tissue density and the distribution of ventilatory activity were estimated from respiration-correlated CT scans of 19 patients. Dose distributions from clinical IMRT (N=17) and proton (N=2) therapy plans were reevaluated by applying importance weighting to separate 1-cc volume elements in the lung, accordingly with their estimated relative functional load. The histogram metrics: mean lung dose (MLD), share of the 5-Gy isodose (V5), etc., were compared between various dose-weighting methods. Regions characterized by high ventilatory activity were segmented, and research IMRT plans were optimized with objectives aiming to maximize sparing of lung function. Results: Substantial differences between the metrics of DVH and ventilation-weighted histograms were observed, e.g., change of over 5% in MLD in 8 patients, relative increase of over 10% in V20 in 3 patients out of 19. The correlation between the density- and ventilation-weighted metrics was insignificant. The use of additional optimization constraints for highly-ventilated lung subvolumes allowed for a reduction in the ventilation-weighted MLD, without compromising the target coverage, or causing deterioration in the standard DVH. Conclusions: Elevated values of ventilation-weighted MLD may indicate the increased risk of complications in patients with COPD. The use of ventilation distribution maps for selection of beam angles, and definition of optimization constraints for functional avoidance, can potentially reduce the risk, without compromising the therapy efficiency.

PD5-1-4: Differences in pattern of practice in radiation therapy for patients with non-small cell lung cancer between physicians from the United States and Other Countries

Copyright © 2007 by the International Association for the Study of Lung Cancer S472 Conclusions: Serial respiration-correlated CT scans revealed important information on dosimetric consequences of anatomical changes during treatment. Preliminary data showed that variations in target and normal tissue volumes were small in average. Nevertheless, greater changes were observed in certain patients with larger tumor motion and in lower lobe lesions; therefore, effort to include image-guidance for setup and frequent imaging during the course of treatment, should be studied further.

SU-FF-J-104: Impact of Anatomical Changes On Dose Distributions During Three-Dimensional Radiotherapy (3D-CRT) of Lung Cancer: Preliminary Study Using Multiple 4-DCT

Purpose: To investigate the impact of anatomical changes on target coverage and dose to normal tissues during radiotherapy for lung cancer using repeat four-dimensional computed tomography (4-DCT) imaging. Methods and Materials: Four patients with approximately 5–10 (average: 8) 4-DCT image data sets acquired weekly during treatment were used for this study. Treatment portals were based on planning tumor volumes (PTV) from the initial data set. The beam arrangements from the initial data sets were transferred onto the 4-DCT data sets for the subsequent treatment weeks and the dose distributions recalculated. Changes in the dose distributions were evaluated by dose-volume histograms (DVH) for the internal target volumes (ITV) as well as for normal tissues including total lung, heart and spinal cord. Results: The overall average change in the percentage of volume of PTV and ITV covered by the 95% isodose (V95) between initial and during treatment was −5.2 ± 8.1% and −1.1 ± 1.3%, respectively. The overall average change in the dose to 95% of the PTV and ITV (D95) between initial and during treatment was −6.7 ± 10.1Gy and −0.9 ± 1.3Gy, respectively. Initial normal tissues volumes were comparable to the weekly datasets. Variation between initial vs. mean weekly percent total lung volume exceeding 20Gy (V20), mean total lung doses, and cord maximum doses were −1.4% ± 1.8%, −0.9Gy ± 1.3Gy and 6.1Gy ± 7.1Gy, respectively. Conclusions: Analysis of serial respiration-correlated CT scans provided useful information on dosimetric consequences of anatomical changes during treatment. Variation in target and normal tissue volumes were found to be small in average. However, changes were mainly observed in patients with mobile lower lobe lesions; therefore, effort to include image-guidance based setup or imaging during the course of treatment, should be considered.

Population and patient-specific target margins for 4D adaptive radiotherapy to account for intra- and inter-fraction variation in lung tumour position

In this work, five 4D image-guidance strategies (two population, an offline adaptive and two online strategies) were evaluated that compensated for both inter- and intra-fraction variability such as changes to the baseline tumour position and respiratory pattern. None of the strategies required active motion compensation such as gating or tracking; all strategies simulated a free-breathing-based treatment technique. Online kilovoltage fluoroscopy was acquired for eight patients with lung tumours, and used to construct inter- and intra-fraction tumour position variability models. Planning was performed on a mid-ventilation image acquired from a respiration-correlated CT scan. The blurring effect of tumour position variability was included in the dose calculation by convolution. CTV to PTV margins were calculated for variability in the cranio-caudal direction. A population margin of 9.0 ± 0.7 mm was required to account for setup error and respiration in the study population without the use of image-guidance. The greatest mean margin reduction was introduced by the offline adaptive strategy. A daily online correction strategy produced a small reduction (1.6 mm) in the mean margin from the offline strategy. Adaptively correcting for an inter-fraction change in the respiratory pattern had little effect on margin size due to most patients having only small daily changes in the respiratory pattern. A daily online correction strategy would be useful for patients who exhibit large variations in the daily mean tumour position, while an offline adaptive strategy is more applicable to patients with less variation.

Intra-patient variability of tumor volume and tumor motion during conventionally fractionated radiotherapy for locally advanced non-small-cell lung cancer: A prospective clinical study

The aim of this study was to investigate the change in tumor volume, motion, and breathing frequency during a course of radiotherapy, for locally advanced non-small-cell lung cancer. A total of 23 patients underwent computed tomography-positron emission tomography (CT-PET) and respiration correlated CT scans before treatment, which was repeated in the first and second weeks after the start of radiotherapy. Patients were treated with an accelerated fractionation schedule, 1.8 Gy twice a day, with a total tumor dose depending on preset dose constraints for the lungs and spinal cord. A striking heterogeneity of tumor volume changes was observed at all time points. In some patients the volume decreased >30% (3/23), whereas in others the volume increased >30% (4/24); but for the majority of patients (16/23), the tumor volume changed only slightly (<30%). No significant changes in average tumor motion or breathing frequencies were observed during treatment. Although a number of changes in individual tumor motion were seen, only in 1 patient would this have led to an increase of the internal margin >1 mm in 1 direction, 1 week after the start of treatment, and in 3 patients for 1 direction, 2 weeks after the start of the treatment. In this patients in this study, a large variability in changes in tumor volume was observed. This underscores the need for repeated imaging during the course of radiotherapy. However, the changes in tumor motion are small, which indicates that repeated respiration correlated CT does not appear to be necessary.

An “in silico” clinical trial comparing free breathing, slow and respiration correlated computed tomography in lung cancer patients

Background and purpose To determine which method of internal target volume (ITV) definition based on a respiration correlated CT (RCCT) allows optimal tumor coverage. Material and methods A free breathing CT (CT fb) and an RCCT scan were acquired in 41 lung cancer patients. For 12 patients with a motion >7 mm in any direction, a detailed analysis was made. The RCCT scan was used to measure tumor motion and to reconstruct a CT at 10 phases (CT 10ph), amongst which the half ventilation CT (CT hv). By averaging the CT 10ph, a slow CT (CT slow) was reconstructed. Based on those scans ITVs were delineated and treatments were planned, where for the ITV hv an internal margin of (motion amplitude)/4 was used. The treatment plans for the ITVs were projected on the 10 respiration phases. Doses were calculated and averaged over the 10 phases to estimate the actual CTV coverage. Results The 3D motion was on average 8.1 ± 1.0 mm (1 SD) for all patients; no statistical difference was found between lower and upper lobe tumors. The ITV slow was the smallest volume on average (142 ± 38 cm 3), followed by the ITV hv (160 ± 40 cm 3), the ITV 10ph (161 ± 41 cm 3) and the ITV fb (250 ± 63 cm 3). Mean CTV doses were between 95% and 107% of the prescribed dose for nearly all patients and treatment plans. Analysis of the CTV coverage suggested that underdosage may occur when the CT slow is used and a geographic miss occurred using the CT fb, due to uncorrect localization of the average tumor position. Conclusions The CT hv seems to be the optimal dataset for delineation, using an adequate anisotropic internal margin of (motion amplitude)/4.

Image-guided radiotherapy for lung cancer: Respiration-correlated (cone-beam) CT to verify tumor position and motion characteristics during treatment delivery

Image-guided radiotherapy for lung cancer: Respiration-correlated (cone-beam) CT to verify tumor position and motion characteristics during treatment delivery

Image-guided radiotherapy for lung cancer: Respiration-correlated (cone-beam) CT to verify tumor position and motion characteristics during treatment delivery

In radiotherapy of lung cancer it is essential that the average position and motion characteristics of the tumor during treatment delivery are identical to those during treatment preparation. In this work we present a method to verify the tumor position and motion characteristics by comparing respiration-correlated cone-beam CT scans (RC-CBCT) made on the treatment machine with respiration-correlated CT scans (RC-CT) made for treatment preparation.