4D computed tomography scans for conformal thoracic treatment planning: is a single scan sufficient to capture thoracic tumor motion?

lower lobe lung tumors move with amplitudes of up to 2 cm due to respiration. To reduce respiration imaging artifacts in planning CT scans, 4D imaging techniques are used. Currently, we use a single (midventilation) frame of the 4D data set for clinical delineation of structures and radiotherapy planning. A single frame, however, often contains artifacts due to breathing irregularities, and is noisier than a conventional CT scan since the exposure per frame is lower. Moreover, the tumor may be displaced from the mean tumor position due to hysteresis. The aim of this work is to develop a framework for the acquisition of a good quality scan representing all scanned anatomy in the mean position by averaging transformed (deformed) CT frames, i.e., canceling out motion. A nonrigid registration method is necessary since motion varies over the lung. 4D and inspiration breath-hold (BH) CT scans were acquired for 13 patients. An iterative multiscale motion estimation technique was applied to the 4D CT scan, similar to optical flow but using image phase (gray-value transitions from bright to dark and vice versa) instead. From the (4D) deformation vector field (DVF) derived, the local mean position in the respiratory cycle was computed and the 4D DVF was modified to deform all structures of the original 4D CT scan to this mean position. A 3D midposition (MidP) CT scan was then obtained by (arithmetic or median) averaging of the deformed 4D CT scan. Image registration accuracy, tumor shape deviation with respect to the BH CT scan, and noise were determined to evaluate the image fidelity of the MidP CT scan and the performance of the technique. Accuracy of the used deformable image registration method was comparable to established automated locally rigid registration and to manual landmark registration (average difference to both methods < 0.5 mm for all directions) for the tumor region. From visual assessment, the registration was good for the clearly visible features (e.g., tumor and diaphragm). The shape of the tumor, with respect to that of the BH CT scan, was better represented by the MidP reconstructions than any of the 4D CT frames (including MidV; reduction of "shape differences" was 66%). The MidP scans contained about one-third the noise of individual 4D CT scan frames. We implemented an accurate method to estimate the motion of structures in a 4D CT scan. Subsequently, a novel method to create a midposition CT scan (time-weighted average of the anatomy) for treatment planning with reduced noise and artifacts was introduced. Tumor shape and position in the MidP CT scan represents that of the BH CT scan better than MidV CT scan and, therefore, was found to be appropriate for treatment planning.

The question remains regarding the dosimetric impact of intrafraction motion in 3D breast treatment. This study was conducted to investigate this issue utilizing the 4DCT scan. The 4D and helical CT scan sets were acquired for 12 breast cancer patients. For each of these patients, based on the helical CT scan, a conventional 3D conformal plan was generated. The breast treatment was then simulated based on the 4DCT scan. In each phase of the 4DCT scan, dose distribution was generated with the same beam parameters as the conventional plan. A software package was developed to compute the cumulative dose distribution from all the phases. Since the intrafraction organ motion is reflected by the 4DCT images, the cumulative dose computed based on the 4DCT images should be closer to what the patient received during treatment. Various dosimetric parameters were obtained from the plan and 4D cumulative dose distribution for the target volume and heart, and were compared to deduce the motion-induced impacts. The studies were performed for both whole breast and partial breast treatment. In the whole breast treatment, the average intrafraction motion induced changes in D95, D90, V100, V95, and V90 of the target volume were -5.4%, -3.1%, -13.4%, -5.1%, and -3.2%, respectively, with the largest values at -26.2%, -14.1%, -91.0%, -15.1%, and -9.0%, respectively. Motion had little impact on the Dmax of the target volume, but its impact on the Dmin of the target volume was significant. For left breast treatment, the motion-induced Dmax change to the heart could be negative or positive, with the largest increase at about 6 Gy. In partial breast treatment, the only non-insignificant impact was in the Dmin of the CTV (ranging from -15.2% to 11.7%). The results showed that the intrafraction motion may compromise target dose coverage in breast treatments and the degree of that compromise was correlated with motion magnitude. However, the dosimetric impact of the motion on the heart dose may be limited.

Standard deformity correction is based on full-length true anteroposterior (AP) and lateral radiographs. However, a three-dimensional computed tomography (3D-CT) scan may provide a more intricate and detailed understanding of the deformities, to plan for the accurate correction. We present a retrospective evaluation of the deformities in three-dimensional projection with 3D-CT scans and its application on the correction of malalignment of the lower limbs. Planning of 107 cases of deformities in the lower limb was done using a 3D-CT scan over a 5-year period from September 2017. Evaluation was done by measuring angles and lines from true AP and lateral image sections of 3D-CT scans instead of standing full length lower limb radiographs (Orthoscanogram). Osteotomies were stabilized with Ilizarov fixation, followed by correction with Hexapod system (ORTHO-SUV, Pitkar, India). The use of 3D-CT scan AP and lateral images provided accurate planning for deformity correction. Deformities can be understood in all three planes including rotational malalignment in a single scan. The precise information of the deformity in sagittal, frontal, and rotational planes is obtained by doing a 3D-CT scan, which allows for further precise planning and correction of lower limb deformities.

Purpose. To investigate the stability of target motion amplitude and motion directionality throughout full stereotactic body radiotherapy (SBRT) treatments of tumors in the liver. Material and methods. Ten patients with gold markers implanted in the liver received 11 courses of 3-fraction SBRT on a conventional linear accelerator. A four-dimensional computed tomography (4DCT) scan was obtained for treatment planning. The time-resolved marker motion was determined throughout full treatment field delivery using the kV and MV imagers of the accelerator. The motion amplitude and motion directionality of all individual respiratory cycles were determined using principal component analysis (PCA). The variations in motion amplitude and directionality within the treatment courses and the difference from the motion in the 4DCT scan were determined. Results. The patient mean (± 1 standard deviation) peak-to-peak 3D motion amplitude of individual respiratory cycles during a treatment course was 7.9 ± 4.1 mm and its difference from the 4DCT scan was −0.8 ± 2.5 mm (max, 6.6 mm). The mean standard deviation of 3D respiratory cycle amplitude within a treatment course was 2.0 ± 1.6 mm. The motion directionality of individual respiratory cycles on average deviated 4.6 ± 1.6° from the treatment course mean directionality. The treatment course mean motion directionality on average deviated 7.6 ± 6.5° from the directionality in the 4DCT scan. A single patient-specific oblique direction in space explained 97.7 ± 1.7% and 88.3 ± 10.1% of all positional variance (motion) throughout the treatment courses, excluding and including baseline shifts between treatment fields, respectively. Conclusion. Due to variable breathing amplitudes a single 4DCT scan was not always representative of the mean motion amplitude during treatment. However, the motion was highly directional with a fairly stable direction throughout treatment, indicating a potential for more optimal individualized motion margins aligned to the preferred direction of motion.

Artifacts in Conventional Computed Tomography (CT) and Free Breathing Four-Dimensional CT Induce Uncertainty in Gross Tumor Volume Determination

Defining target volumes for stage I NSCLC using a single 4D CT scan (versus a single standard CT and 6 standard CT scans)

Introduction Odontoid fractures (OF) are frequent in the trauma population, and there is no universally accepted single method of management. The objective of this study was to evaluate the epidemiology and management of type II OF in Latin America treated either with rigid cervical orthosis or surgery. Patients and Methods A total of 83 patients treated conservatively or by surgery were enrolled in this retrospective study. Medical charts, imaging studies, and outcomes of patients were analyzed in the pretreatment period and at the last medical evaluation. The fractures were assessed using conventional radiographs, three-dimensional computed tomographic (3D-CT) scans, and magnetic resonance images. Fracture gaps, the direction and the degree of displacement of the odontoid process, the fracture line anatomy, the degrees of atlantoaxial instability, the comminuted fracture, and the surface contact area were analyzed. The decision for operative or nonoperative treatment was based on anesthesia risk, and patient's choice of the nonoperative treatment. The nonoperative management generally consisted of a rigid cervical orthosis for 3 months. The type of surgery to be performed was chosen by the surgeon. The solid bony union was defined as the presence of bony bridges and the definite continuity of cortical bone. Fibrous union was considered present when no degree of motion was evident in dynamic radiographs despite persistent cortical bone discontinuity within a fracture gap on 3D CT scans. Nonunion was defined as a definite fracture gap with abnormal motion of the fractured dens on dynamic radiographs and on a 3D CT scan. Results A total of 83 patients were included in this study. The patients were 78.3% men, the mean age = 44.98 ( ± 23.20 years) years. Traffic accidents (66.3%) were the most common cause of trauma. The main symptom was pain (85.5%) in the posterior cervical region. The median time elapsed from accident to surgery was 7 days (P25: 2/P75: 27.5). Median follow-up was 23.66 ( ± 25.43 months) months. Conservative treatment with cervical orthosis, for example, Miami J collar or halo-vest was used in 20.5% of the cases. Odontoid screw technics (57.6%) were the most common surgical treatment adopted as primary surgical treatment. Symptomatic nonunion was observed in two cases with conservative treatment and three cases after odontoid screw fixation. All the patients were referred to posterior C1–C2 fixation. The posterior fixation tended to be used after conservative failed therapy, after nonunion anterior screw surgery, and in fractures with greater displacement. The most common radiological feature was no displacement of the odontoid process in relation to the body of C2, horizontal fracture line, gap fracture &lt; 2 mm, no subluxation across each C1–C2 facet joint and no comminuted fracture. Conclusion The patients treated nonoperatively with a rigid collar may have an overall favorable outcome compared with surgical treatment. A well-designed prospective study is needed to better elucidate optimal treatment algorithms from both an outcomes and cost-effectiveness perspective.

The purpose of CT simulation in radiotherapy is to acquire patient geometrical information and to build a patient geometrical model for treatment planning. Errors in patient model caused by motion artifacts will influence all treatment fractions and therefore should be handled carefully. Due to the tumor respiratory motion, the captured tumor position and shape can be heavily distorted. The distortions along the axis of motion could result in either a lengthening or shortening of the target. The center of the imaged target can be displaced by as much as the amplitude of the motion. A newly developed technique that can reduce motion artifacts and provide patient geometry throughout the whole breathing cycle is called respiration‐correlated or 4D CT scan. The basic idea for 4D CT scan is that, at every position of interest along patient's long axis, images are over‐sampled and each image is tagged with breathing phase information. After the scan is done, images are sorted based on the corresponding breathing phase signals. Thus, many 3D CT sets are obtained, each corresponding to a particular breathing phase, and together constitutes a 4D CT set that covers that the whole breathing cycle. 4D CT scan has been developed at various institutions with slightly different flavors. In this lecture, we will provide an overview of various implementations of 4D CT scan. 4D CT scan can be used to account for respiratory motion to generate images with less distortion than 3D CT scan. 4D images also contain respiratory motion information of tumor and organs that is not available in a 3D CTimage. This technology can be used for respiratory‐gated treatment to identify the patient‐specific phase of minimum tumor motion, determine residual tumor motion within the gate interval, and compare treatment plans at different phases. It can also be used for non‐gated treatment planning to define ITV by combining gross tumor volume at all breathing phases or using a method called maximum intensity projection. Of course 4D CT will also play a vital role in the futuristic 4D radiotherapy where the tumor is tracked dynamically during the treatment using multi‐leaf collimator. Existing problems for 4D CT scan include the increased imagingdose,CT tube heating, and data management. More importantly, one has to keep in mind that 4D CT scan is not really 4D. Temporal information is mapped into one breathing cycle. Irregular respiration will cause artifacts in 4D CTimages. Patient coaching can improve the regularity of breathing pattern and thus reduce the residual artifacts. However this issue still deserves further studies. Educational Objectives: 1. Understand the origin and magnitude of motion artifacts in free breathing helical CT scan. 2. Understand how 4D CT scan works. 3. Understand how 4D CT can be used in radiotherapy. 4. Understand the remaining artifacts in 4D CT scan and possible future improvements.

The purpose of CT simulation in radiotherapy is to acquire patient geometrical information and to build a patient geometrical model for treatment planning. Errors in patient model caused by motion artifacts will influence all treatment fractions and therefore should be handled carefully. Due to the tumor respiratory motion, the captured tumor position and shape can be heavily distorted. The distortions along the axis of motion could result in either a lengthening or shortening of the target. The center of the imaged target can be displaced by as much as the amplitude of the motion.A newly developed technique that can reduce motion artifacts and provide patient geometry throughout the whole breathing cycle is called respiration‐correlated or 4D CT scan. The basic idea for 4D CT scan is that, at every position of interest along patient's long axis, images are over‐sampled and each image is tagged with breathing phase information. After the scan is done, images are sorted based on the corresponding breathing phase signals. Thus, many 3D CT sets are obtained, each corresponding to a particular breathing phase, and together constitutes a 4D CT set that covers that the whole breathing cycle. 4D CT scan has been developed at various institutions with slightly different flavors. In this lecture, we will provide an overview of various implementations of 4D CT scan.4D CT scan can be used to account for respiratory motion to generate images with less distortion than 3D CT scan. 4D images also contain respiratory motion information of tumor and organs that is not available in a 3D CT image. This technology can be used for respiratory‐gated treatment to identify the patient‐specific phase of minimum tumor motion, determine residual tumor motion within the gate interval, and compare treatment plans at different phases. It can also be used for non‐gated treatment planning to define ITV by combining gross tumor volume at all breathing phases or using a method called maximum intensity projection. Of course 4D CT will also play a vital role in the futuristic 4D radiotherapy where the tumor is tracked dynamically during the treatment using multi‐leaf collimator.Existing problems for 4D CT scan include the increased imaging dose, CT tube heating, and data management. More importantly, one has to keep in mind that 4D CT scan is not really 4D. Temporal information is mapped into one breathing cycle. Irregular respiration will cause artifacts in 4D CT images. Patient coaching can improve the regularity of breathing pattern and thus reduce the residual artifacts. However this issue still deserves further studies.Educational Objectives:1. Understand the origin and magnitude of motion artifacts in free breathing helical CT scan.2. Understand how 4D CT scan works.3. Understand how 4D CT can be used in radiotherapy.4. Understand the remaining artifacts in 4D CT scan and possible future improvements.

Use of maximum intensity projections (MIP) for target volume generation in 4DCT scans for lung cancer

50. Magnetic Resonance Imaging optimization for liver SBRT: Breath-triggered acquisition in treatment position to improve lesion contouring

Effect of contrast enhancement in delineating GTV and constructing IGTV of thoracic oesophageal cancer based on 4D-CT scans

Quality Assurance of 4D-CT Scan Techniques in Multicenter Phase III Trial of Surgery Versus Stereotactic Radiotherapy (Radiosurgery or Surgery for Operable Early Stage (Stage 1A) Non–Small-Cell Lung Cancer [ROSEL] Study)

Differences in gross target volume (GTV) and central point positions among moving lung cancer models constructed by CT scanning at different frequencies were compared, in order to explore the effect of different respiratory frequencies on the GTV constructions in moving lung tumors. Eight models in different shapes and sizes were established to stimulate lung tumors. The three-dimensional computed tomography (3DCT) and four-dimensional computed tomography (4DCT) scanning were performed at 10, 15 and 20 times/min in different models. Differences in GTV volumes and central point positions at different motion frequencies were compared by means of GTV3Ds (GTV3D-10, GTV3D-15, GTV3D-20) and IGTV4Ds (IGTV4D-10, IGTV4D-15, IGTV4D-20). Volumes of GTV3D-10, GTV3D-15, GTV3D-20 were 12.41±14.26, 10.38±11.18 and 12.50±15.23 cm3 respectively (P=0.687). Central point coordinates in the x-axis direction were −8.16±96.21, −8.57±96.08 and −8.56±95.73 respectively (P=0.968). Central point coordinates in the y-axis direction were 108.22±25.03, 110.41±22.47 and 109.04±24.24 (P=0.028). Central point coordinates in the z-axis direction were 65.19±13.68, 65.43±13.40 and 65.38±13.17 (P=0.902). The difference was significant in the y-axis direction (P=0.028). Volumes of IGTV4D-10, IGTV4D-15, IGTV4D-20 were 17.78±19.42, 17.43±19.56 and 17.44±18.80 cm3 (P=0.417). Central point coordinates in the x-axis direction were −7.73±95.93, −7.86±95.56 and −7.92±95.14 (P=0.325). Central point coordinates in the y-axis direction were 109.41±24.54, 109.60±24.13 and 109.16±24.28 (P=0.525). Central point coordinates in the z-axis direction were 65.52±13.31, 65.59±13.39 and 65.51±13.34 (P=0.093). However, the central point position of GTV in the head and foot direction by 3DCT scanning was severely affected by the respiratory frequency.

A unique capability of the CyberKnife system is dynamic target tracking. However, not all patients are eligible for this approach. Rather, their tumors are tracked statically using the vertebral column for alignment. When using static tracking, the internal target volume (ITV) is delineated on the four-dimensional (4D) CT scan and an additional margin is added to account for setup uncertainty [planning target volume (PTV)]. Treatment margins are difficult to estimate due to unpredictable variations in tumor motion and respiratory pattern during the course of treatment. The inability to track the target and detect changes in respiratory characteristics might result in geographic misses and local tumor recurrences. The purpose of this study is to develop a method to evaluate the adequacy of ITV-to-PTV margins for patients treated in this manner. Data from 24 patients with lesions in the upper lobe (n = 12), middle lobe (n = 3), and lower lobe (n = 9) were included in this study. Each patient was treated with dynamic tracking and underwent 4DCT scanning at the time of simulation. Data including the 3D coordinates of the target over the course of treatment were extracted from the treatment log files and used to determine actual target motion in the superior-inferior (S-I), anterior-posterior (A-P), and left-right (L-R) directions. Different approaches were used to calculate anisotropic and isotropic margins, assuming that the tumor moves as a rigid body. Anisotropic margins were calculated by separating target motion in the three anatomical directions, and a uniform margin was calculated by shifting the gross tumor volume contours in the 3D space and by computing the percentage of overlap with the PTV. The analysis was validated by means of a theoretical formulation. The three methods provided consistent results. A uniform margin of 4.5 mm around the ITV was necessary to assure 95% target coverage for 95% of the fractions included in the analysis. In the case of anisotropic margins, the expansion required in the S-I direction was larger (8.1 mm) than those in the L-R (4.9 mm) and A-P (4.5 mm) directions. This margin accounts for variations of target position within the same treatment fraction. The use of bony alignment for CyberKnife lung stereotactic body radiation therapy requires careful considerations, in terms of the potential for increased toxicity or local miss. Our method could be used by other centers to determine the adequacy of ITV-to-PTV margins for their patients.