Tumor Position Variability Research Articles

Investigation of interfractional variation in lung tumor position under expiratory-phase breath hold using cone-beam computed tomography in stereotactic body radiation therapy.

We investigated the interfractional variation in the tumor position during lung stereotactic body radiotherapy (SBRT) under expiratory-phase breath hold (BH) using cone-beam computed tomography (CBCT). A total of 79 patients with lung cancer were treated with lung SBRT, wherein the Abches system under expiratory-phase BH was used to study interfractional variation. The tumors were located in the upper lobe in 31 cases, in the middle lobe in 11 cases, and in the lower lobe in 37 cases. Planning CTs were scanned under expiratory-phase BH with the Abches system. The 3-degrees-of-freedom (DOF) tumor-based setup using CBCT images under expiratory-phase BH was performed after a 6-DOF bony vertebrae-based setup using an ExacTrac X-ray system. Interfractional variation in the lung tumor position was defined as the difference in the position of the lung tumor relative to the bone anatomy in the left-right (LR), antero-posterior (AP), and craniocaudal (CC) directions represented as absolute values. The interfractional variation in the lung tumor position was very similar in all the lung regions, and its mean ± standard deviation values in all patients were 1.0 ± 1.1, 1.6 ± 1.9, and 1.6 ± 1.9 mm in the LR, AP, and CC directions, respectively. Further, 99.1%, 92.4%, and 92.7% of all the fractions for the interfractional tumor positional variation in the LR, AP, and CC directions were less than 5 mm, respectively. The interfractional variation in the tumor position was small for lung cancer patients treated with the Abches system under expiratory-phase BH.

Visually guided inspiration breath-hold facilitated with nasal high flow therapy in locally advanced lung cancer

Background and purpose Reducing breathing motion in radiotherapy (RT) is an attractive strategy to reduce margins and better spare normal tissues. The objective of this prospective study (NCT03729661) was to investigate the feasibility of irradiation of non-small cell lung cancer (NSCLC) with visually guided moderate deep inspiration breath-hold (IBH) using nasal high-flow therapy (NHFT). Material and methods Locally advanced NSCLC patients undergoing photon RT were given NHFT with heated humidified air (flow: 40 L/min with 80% oxygen) through a nasal cannula. IBH was monitored by optical surface tracking (OST) with visual feedback. At a training session, patients had to hold their breath as long as possible, without and with NHFT. For the daily cone beam CT (CBCT) and RT treatment in IBH, patients were instructed to keep their BH as long as it felt comfortable. OST was used to analyze stability and reproducibility of the BH, and CBCT to analyze daily tumor position. Subjective tolerance was measured with a questionnaire at 3 time points. Results Of 10 included patients, 9 were treated with RT. Seven (78%) completed the treatment with NHFT as planned. At the training session, the mean BH length without NHFT was 39 s (range 15–86 s), and with NHFT 78 s (range 29–223 s) (p = .005). NHFT prolonged the BH duration by a mean factor of 2.1 (range 1.1–3.9s). The mean overall stability and reproducibility were within 1 mm. Subjective tolerance was very good with the majority of patients having no or minor discomfort caused by the devices. The mean inter-fraction tumor position variability was 1.8 mm (-1.1–8.1 mm;SD 2.4 mm). Conclusion NHFT for RT treatment of NSCLC in BH is feasible, well tolerated and significantly increases the breath-hold duration. Visually guided BH with OST is stable and reproducible. We therefore consider this an attractive patient-friendly approach to treat lung cancer patients with RT in BH.

Quantification of Esophageal Tumor Motion and Investigation of Different Image-Guided Correction Strategies

PurposeTo accurately quantify esophageal tumor position variability and to optimize image guided correction strategies. Material and MethodsEsophageal cancer patients receiving chemoradiotherapy (41.4-50.4 Gy in 23-28 fractions combined with carboplatin plus paclitaxel) were included in a prospective cohort study (NCT02139488). Gold fiducial markers were inserted into the esophageal tumors during diagnostic endoscopic ultrasound. Four-dimensional (4D) planning computed tomography (CT) and daily 4D cone beam (CB) CT scans were acquired. Each CBCT was registered to the planning CT using different regions of interest (bone; 3D), and carina, diaphragm, clinical target volume (CTV), and fiducial markers (4D) for alignment and using the fiducial markers as the true tumor position. Subsequently, a planning target volume (PTV) margin accounting for residual uncertainties, including the average respiratory motion, was calculated for each of these registrations. ResultsFifty-six patients with tumors located in the proximal (n = 1), mid (n = 7), or distal esophagus (n = 25) or at the gastroesophageal junction (n = 23) were included. The average peak-to-peak respiratory tumor motion was 0.20, 0.92, and 0.34 cm on the planning CT in left–right (LR), cranial–caudal (CC), and anterior–posterior (AP) directions, respectively. The required PTV margin with average motion amplitude, depending on the correction strategy used for image guidance, ranged from 0.8 cm to 1.0 cm, 1.1 cm to 1.6 cm, and 0.7 cm to 0.9 cm in LR, CC, and AP direction, respectively. A registration based on the CTV resulted in the smallest PTV margins (0.8, 1.1, and 0.7 cm in LR, CC, and AP direction, respectively). For bone registration the calculated PTV margins were 1.0, 1.3, and 0.7 cm in LR, CC, and AP directions, respectively. The registration based on the diaphragm increased PTV margins. ConclusionsSubstantial and anisotropic position variability of esophageal tumors was observed during radiation therapy, and nonuniform margins should be considered. Cranial-caudal PTV margins need to be larger than those commonly used. Target positioning during image-guided radiotherapy could be improved with a CTV registration-based correction strategy.

EP-2176 Analysis of inter-fraction tumor position variability in lung SBRT

EP-2176 Analysis of inter-fraction tumor position variability in lung SBRT

Geometric Reproducibility of Fiducial Markers and Efficacy of a Patient-Specific Margin Design Using Active Breath Hold for Pancreas Stereotactic Body Radiation Therapy

In patients undergoing SBRT for pancreatic adenocarcinoma using an active breathing control (ABC), the reproducibility of fiducial positioning between breath-holds is unclear. We characterized this variation by acquiring multiple breath-hold CTs at time of simulation and analyzed whether a patient-specific margin based on this variation would help account for intra-fraction variation at time of treatment. We analyzed five pancreatic cancer patients who underwent SBRT for pancreatic adenocarcinoma and who were treated with ABC. At time of simulation, 3-additional breath-hold scans were acquired immediately after the primary contrast-enhanced planning CT with 2-mm slice thickness. To characterize the variability in tumor position between breath-holds, fiducial centroids from the planning CT were used as the reference and compared to the fiducial centroids from the additional breath-hold CTs. Both the average excursions in fiducial position (Sim Varavg) and the maximum excursions in fiducial position (Sim Varmax) were calculated. For treatment planning for each patient, the amount of variation in each direction between breath-holds was applied as a margin to the GTV, followed by an additional 2-mm margin to determine the final PTV. The difference in volume between this method versus directly expanding the GTV by a uniform 2-mm expansion was calculated. During 5-fraction treatment, 20-27 breath-hold CBCTs were acquired for each patient. The intrafraction variation in fiducial position was calculated using the same method as above, using the initial CBCT for that fraction as the reference CT and using similar metrics to characterize fiducial position variation (Tx Varavg and Tx Varmax). The adequacy of a direct 2-mm expansion on the GTV was analyzed by evaluating the percentage of fiducial locations within 2-mm of variability on CBCTs. Table 1 summarizes the breath-hold reproducibility averaged over all five patients at CT sim and treatment. By applying the variation seen during multiple breath-holds at time of simulation to the GTV volume before further expanding by 2 mm, the final PTV volume increased by 19.5 ± 7.0% as compared to a uniform 2-mm expansion off of the GTV. However, by using this patient-specific margin, 95% of the all fiducial locations during treatment were encompassed within this volume, while a uniform 2-mm expansion only covered the fiducials 80% in both LR and AP directions and 60% in the SI direction.Abstract SU_18_2175; Table 1Inter-BH reproducibilityLR (mm)AP (mm)SI (mm)Sim Varavg0.8 ± 0.51.0 ± 0.41.5 ± 0.7Sim Varmax1.2 ± 0.71.4 ± 0.52.3 ± 0.6Tx Varavg1.2 ± 0.51.4 ± 0.62.1 ± 0.4Tx Varmax4.7 ± 2.15.5 ± 3.06.4 ± 2.4 Open table in a new tab Variation in tumor position exists between breath-holds when treating with ABC. Acquiring multiple breath-hold CTs during simulation can help quantify inter-breath-hold reproducibility and can provide a patient specific margin design to compensate for the variation in tumor position between breath-holds at time of treatment.

Quantification of Esophageal Tumor Motion and Recommendations on Setup Verification Strategy During Image Guided Radiation Therapy

In radiation therapy (RT) of esophageal cancer, CTV to PTV margins are generally isotropic and equal for all patients. However, detailed knowledge of the position variability and tumor motion caused by respiratory motion is lacking. The purpose of this study was to accurately quantify esophageal tumor position variability and respiratory motion and investigate possible surrogate structures for image guidance. The first 12 patients enrolled in a prospective cohort study (NCT02139488) were analyzed. Patients were treated with chemo-RT with a radiation dose of 23 to 28×1.8 Gy combined with weekly carboplatin/paclitaxel and daily 4D CBCT scans. As soft tissue contrast in CBCT is limited, gold fiducial markers (0.35 x 5 mm) were inserted during endoscopic ultrasonography preferably at the proximal border, in the middle and at the distal border of the tumor before the planning CT was made. The following regions of interest (ROI) were registered for each fraction: bony anatomy (vertebrae), carina, diaphragm, and fiducial markers using a rectangular ROI, and gross tumor volume (GTV), using a shaped ROI (“mask”). These different surrogates for setup and their implication on margins were calculated with statistics of the average residual marker displacement when using the different regions as reference (standard deviations of random [σ] and systematic [∑] errors). Breathing amplitudes also influence margins; therefore, their distribution within the cohort was determined by the fiducial ROI. Subsequently, a planning target volume margin including the average respiratory motion was determined for these scenarios according to the nonlinear van Herk formula. Since esophageal tumors border both to lung and mediastinal tissue, the parameters for this formula were conservatively chosen to be representative for water. A median of 3 fiducials was placed in 12 tumors located at the mid esophagus (n=3), lower esophagus (n=4), and gastroesophageal junction (n=5). The median (range) peak-to-peak respiratory tumor motion amplitude in the left-right (LR), craniocaudal (CC), and anteroposterior (AP) directions was 0.15 (0.07-0.73), 0.63 (0.39-0.95), and 0.30 (0.08-0.79) cm, respectively. The required margin, depending on surrogate used for setup correction, ranged from LR 0.69-0.88 cm and CC 0.76-1.14 cm to AP 0.56-0.7 cm (Table 1). Overall, the registration on mask results in the smallest margins. Substantial position variability of the GTV during RT of esophageal cancer was observed as well as interpatient variation in respiratory-induced motion. Tumor localization is considerably improved, compared to bony setup, when the GTV mask registration is used in CBCT guidance, despite low soft tissue contrast. Furthermore, patient-specific margins are required to mitigate breathing-induced motion.Abstract 3566; Table 1.Region of interestMargin LR (cm)Margin CC (cm)Margin AP (cm)Bone0.791.090.67Carina0.771.000.62Diaphragm0.881.140.70Mask0.690.760.56 Open table in a new tab

SU‐D‐207A‐05: Investigating Sparse‐Sampled MRI for Motion Management in Thoracic Radiotherapy

Purpose:Sparse sampling and reconstruction‐based MRI techniques represent an attractive strategy to achieve sufficiently high image acquisition speed while maintaining image quality for the task of radiotherapy guidance. In this study, we examine rapid dynamic MRI using a sparse sampling sequence k‐t BLAST in capturing motion‐induced, cycle‐to‐cycle variations in tumor position. We investigate the utility of long‐term MRI‐based motion monitoring as a means of better characterizing respiration‐induced tumor motion compared to a single‐cycle 4DCT.Methods:An MRI‐compatible, programmable, deformable lung motion phantom with eleven 1.5 ml water marker tubes was placed inside a 3.0 T whole‐body MR scanner (Philips Ingenia). The phantom was programmed with 10 lung tumor motion traces previously recorded using the Synchrony system. 2D+t image sequences of a coronal slice were acquired using a balanced‐SSFP sequence combined with k‐t BLAST (accn=3, resolution=0.66×0.66×5 mm3; acquisition time = 110 ms/slice). kV fluoroscopic (ground truth) and 4DCT imaging was performed with the same phantom setup and motion trajectories. Marker positions in all three modalities were segmented and tracked using an opensource deformable image registration package, NiftyReg.Results:Marker trajectories obtained from rapid MRI exhibited <1 mm error compared to kv Fluoro trajectories in the presence of complex motion including baseline shifts and changes in respiratory amplitude, indicating the ability of MRI to monitor motion with adequate geometric fidelity for the purpose of radiotherapy guidance. In contrast, the trajectory derived from 4DCT exhibited significant errors up to 6 mm due to cycle‐to‐cycle variations and baseline shifts. Consequently, 4DCT was found to underestimate the range of marker motion by as much as 50%.Conclusion:Dynamic MRI is a promising tool for radiotherapy motion management as it permits for longterm, dose‐free, soft‐tissue‐based monitoring of motion, yielding richer and more accurate information about tumor position and motion range compared to the current state‐of‐the‐art, 4DCT.This work was partially supported through research funding from National Institutes of Health (R01CA169102).

SU‐G‐JeP3‐15: Is the Reproducibility with Respect to Bone of Tumor Position at Simulation for Breath Hold CT Scans Correlated to the Reproducibility for Multiple Breath Hold CBCTs at Treatment in SBRT Thoracic Patients?

Purpose:To evaluate correlation between the reproducibility of tumor position under feedback guided voluntary deep inspiration breath hold gating at simulation and at treatment.Methods:All patients treated with breath hold (BH) have 3‐6 BH CTs taken at simulation (sim). In addition, if the relationship between the tumor and nearby bony anatomy on treatment BH CT(or CBCT) is found to be greater than 5 mm different at treatment than it was at sim, a repeat BH CT is taken before treatment. We retrospectively analyzed the sim CTs for 19 patients who received BH SBRT lung treatments and had repeat BH CT on treatment. We evaluated the reproducibility of the tumor position during the simulation CTs and compared this to the reproducibility of the tumor position on the repeat treatment CT with our in‐house CT alignment software (CT‐Assisted Targeting for Radiotherapy).Results:Comparing the tumor position for multiple simulation BH CTs, we calculated: maximum difference (max) = 0.69cm; average difference (x) = 0.28cm; standard deviation (σ) = 0.18cm. Comparing the repeat BH CBCTs on treatment days we calculated: max = 0.44cm; x = 0.16cm; σ = 0.22cm. We also found that for 95% of our BH cases, the absolute variation in tumor position within the same imaging day was within 5mm of the range at the time of simulation and treatment. We found that 75% of the BH cases had less residual tumor motion on treatment days than at simulation.Conclusion:This suggests that a GTV contour based upon the residual tumor motion in multiple BH datasets plus 2 mm margin should be sufficient to cover the full range of residual tumor motion on treatment days.

Respiratory Gating during Stereotactic Body Radiotherapy for Lung Cancer Reduces Tumor Position Variability

PurposeWe evaluated the effects of respiratory gating on treatment accuracy in lung cancer patients undergoing lung stereotactic body radiotherapy by using electronic portal imaging device (EPID) images.Materials and MethodsOur study population consisted of 30 lung cancer patients treated with stereotactic body radiotherapy (48 Gy/4 fractions/4 to 9 days). Of these, 14 were treated with- (group A) and 16 without gating (group B); typically the patients whose tumors showed three-dimensional respiratory motion ≧5 mm were selected for gating. Tumor respiratory motion was estimated using four-dimensional computed tomography images acquired during treatment simulation. Tumor position variability during all treatment sessions was assessed by measuring the standard deviation (SD) and range of tumor displacement on EPID images. The two groups were compared for tumor respiratory motion and position variability using the Mann-Whitney U test.ResultsThe median three-dimensional tumor motion during simulation was greater in group A than group B (9 mm, range 3–30 mm vs. 2 mm, range 0–4 mm; p<0.001). In groups A and B the median SD of the tumor position was 1.1 mm and 0.9 mm in the craniocaudal- (p = 0.24) and 0.7 mm and 0.6 mm in the mediolateral direction (p = 0.89), respectively. The median range of the tumor position was 4.0 mm and 3.0 mm in the craniocaudal- (p = 0.21) and 2.0 mm and 1.5 mm in the mediolateral direction (p = 0.20), respectively.ConclusionsAlthough patients treated with respiratory gating exhibited greater respiratory tumor motion during treatment simulation, tumor position variability in the EPID images was low and comparable to patients treated without gating. This demonstrates the benefit of respiratory gating.

Differential Motion Between Mediastinal Lymph Nodes and Primary Tumor in Radically Irradiated Lung Cancer Patients

In patients with locally advanced lung cancer, planning target volume margins for mediastinal lymph nodes and tumor after a correction protocol based on bony anatomy registration typically range from 1 to 1.5 cm. Detailed information about lymph node motion variability and differential motion with the primary tumor, however, is lacking from large series. In this study, lymph node and tumor position variability were analyzed in detail and correlated to the main carina to evaluate possible margin reduction. Small gold fiducial markers (0.35 × 5 mm) were placed in the mediastinal lymph nodes of 51 patients with non-small cell lung cancer during routine diagnostic esophageal or bronchial endoscopic ultrasonography. Four-dimensional (4D) planning computed tomographic (CT) and daily 4D cone beam (CB) CT scans were acquired before and during radical radiation therapy (66 Gy in 24 fractions). Each CBCT was registered in 3-dimensions (bony anatomy) and 4D (tumor, marker, and carina) to the planning CT scan. Subsequently, systematic and random residual misalignments of the time-averaged lymph node and tumor position relative to the bony anatomy and carina were determined. Additionally, tumor and lymph node respiratory amplitude variability was quantified. Finally, required margins were quantified by use of a recipe for dual targets. Relative to the bony anatomy, systematic and random errors ranged from 0.16 to 0.32 cm for the markers and from 0.15 to 0.33 cm for the tumor, but despite similar ranges there was limited correlation (0.17-0.71) owing to differential motion. A large variability in lymph node amplitude between patients was observed, with an average motion of 0.56 cm in the cranial-caudal direction. Margins could be reduced by 10% (left-right), 27% (cranial-caudal), and 10% (anteroposterior) for the lymph nodes and -2%, 15%, and 7% for the tumor if an online carina registration protocol replaced a protocol based on bony anatomy registration. Detailed analysis revealed considerable lymph node position variability, differential motion, and respiratory motion. Planning target volume margins can be reduced up to 27% in lung cancer patients when the carina registration replaces bony anatomy registration.

Impact of Respiratory Gating on Tumor Position Variability During Stereotactic Body Radiation Therapy for Lung Cancer

Impact of Respiratory Gating on Tumor Position Variability During Stereotactic Body Radiation Therapy for Lung Cancer

Interfraction variation in lung tumor position with abdominal compression during stereotactic body radiotherapy

To assess the effect of abdominal compression on the interfraction variation in tumor position in lung stereotactic body radiotherapy (SBRT) using cone-beam computed tomography (CBCT) in a larger series of patients with large tumor motion amplitude. Thirty patients with lung tumor motion exceeding 8 mm who underwent SBRT were included in this study. After translational and rotational initial setup error was corrected based on bone anatomy, CBCT images were acquired for each fraction. The residual interfraction variation was defined as the difference between the centroid position of the visualized target in three dimensions derived from CBCT scans and those derived from averaged intensity projection images. The authors compared the magnitude of the interfraction variation in tumor position between patients treated with [n = 16 (76 fractions)] and without [n = 14 (76 fractions)] abdominal compression. The mean ± standard deviation (SD) of the motion amplitude in the longitudinal direction before abdominal compression was 19.9 ± 7.3 (range, 10-40) mm and was significantly (p < 0.01) reduced to 12.4 ± 5.8 (range, 5-30) mm with compression. The greatest variance of the interfraction variation with abdominal compression was observed in the longitudinal direction, with a mean ± SD of 0.79 ± 3.05 mm, compared to -0.60 ± 2.10 mm without abdominal compression. The absolute values of the 95th percentile of the interfraction variation for one side in each direction were 3.97∕6.21 mm (posterior∕anterior), 4.16∕3.76 mm (caudal∕cranial), and 2.90∕2.32 mm (right∕left) without abdominal compression, and 2.14∕5.03 mm (posterior∕anterior), 3.93∕9.23 mm (caudal∕cranial), and 2.37∕5.45 mm (right∕left) with abdominal compression. An absolute interfraction variation greater than 5 mm was observed in six (9.2%) fractions without and 13 (17.1%) fractions with abdominal compression. Abdominal compression was effective for reducing the amplitude of tumor motion. However, in most of the authors' patients, the use of abdominal compression seemed to increase the interfraction variation in tumor position, despite reducing lung tumor motion. The daily tumor position deviated more systematically from the tumor position in the planning CT scan in the lateral and longitudinal directions in patients treated with abdominal compression compared to those treated without compression. Therefore, target matching is required to correct or minimize the interfraction variation.

SU-E-J-10: Accuracy of Image Guidance Using Cone-Beam CT with Multiple Breath Hold Using Active Breathing Coordination.

To complete a CBCT for a treatment using ABC, multiple breath hold (BH) (>3) were used due to the slow gantry rotation and the short BH period. Inter-BH tumor position variability may introduce distortion in the reconstructed images. This study aims to determine a threshold of the inter-BH scan displacement so that the inconsistency can be identified from the CBCT images. A numerical phantom was constructed to represent the thorax region of a human body. To simulate the inter-BH displacements, known magnitudes of motion (s = 0, 1, 3, 5 mm) along the longitudinal direction were introduced for the 'tumor' and 'diaphragm' in the phantom. Two different irregular motion patterns (s1=s3=/=s2 and s1=/=s2=/=s3) during CBCT scans were tested. Furthermore, a physical phantom with a movable insert was scanned using a commercial CBCT system. The insert of the phantom was programmed to move in the longitudinal direction according to the same motion patterns as designed in the numerical simulations. Subsequently, nine CBCT's in 'half-fan' mode for the physical phantom were acquired with the insert in various positions. These CBCT images were then fused to the reference CT by aligning to either the body of the phantom or the 'tumor' inside the insert. Based on numerical simulation, position variation >1mm can be observed from the reconstructed CBCT images. Based on acquired CBCTs of the physical phantom, position variations of >3mm or 5mm were observed, depending on the motion pattern during the data acquisition. Because of the use of half-fan mode, we observed the order of position displacements of the tumor during CBCT acquisition drastically affected the outcome of imaging registration. Using ABC device, the inter-BH variability during a CBCT acquisition affects accuracy of tumor localization. A patient individualized planning margin might be necessary to account for this effect.

TU‐E‐BRC‐06: Rapid MRI for Personalized 4D Image‐Guided Lung Cancer Radiotherapy

Purpose: Respiratory motion causes thoracic anatomy to change continuously in all four dimensions (4D = 3D+time). However, current image‐guidance tools (e.g., 4DCT) provide only a reconstituted, single‐cycle snapshot of motion, thereby ignoring complex cycle‐to‐cycle spatial and temporal effects. In this work, we investigate the use of rapid MRI as a truly 4D, non‐invasive monitoring tool for lung SBRT Methods: Under an IRB‐approved protocol, two lung cancer patients were imaged under free breathing conditions, without extrinsic contrast, on a 1.5T MRI scanner using a 4‐channel cardiac coil. A balanced SSFP sequence (TE/TR: 1.68/3.16 ms; FOV: 240×240 mm2; half‐Fourier acquisition; voxel: 2.4×3x5 mm3) was used to acquire 2D+t and 4D (8 slices, parallel acceleration=4) time series for 20s and 60s respectively. For each 2D+t series, the tumor centroid, the diaphragm and ∼15 points on the tumor boundary were manually contoured on one image frame. An optical fluid‐flow‐based deformable image registration algorithm was applied to the time series to map the motion trajectory of each point on each contour. A temporal smoothness constraint was imposed so as to avoid non‐physical registration. Results: Acquisition times of ∼0.15s/frame and ∼1.5s/volume were achieved for 2D+t and 4D acquisition, respectively. In each case, image quality was adequate to clearly delineate and monitor the motion of the tumor, the diaphragm and the cardiac wall. Both patients showed significant cycle‐to‐cycle variation in tumor position. One of the patients with a left mid‐lobe lesion showed significant tumor deformation and rotation (&gt;4 degrees) due to the combined influence of cardiac and diaphragmatic motion. Conclusions: Rapid 2D+t and 4D MRI offers a highly attractive, non‐invasive tool for characterizing respiratory motion from a truly 4D perspective. Such image‐guidance will lay the groundwork for personalized motion management based on patient‐specific, long‐term monitoring, personalized 4D treatment planning and, if necessary, inter‐ and intra‐fraction treatment adaptations.

Intrafraction Variation in Tumor Position during Stereotactic Body Radiotherapy for Lung Cancer

Cone beam CT (CBCT) has demonstrated remarkable promise for pre-treatment target positioning. Questions remain, however, as to whether this pre-treatment position remains stable throughout the treatment fraction, particularly for the relatively long treatment times of stereotactic body radiotherapy (SBRT). The goal of this study was to quantify the change in tumor position from beginning to end of an SBRT fraction.

Is a Single Respiratory Correlated 4D-CT Study Sufficient for Evaluation of Breathing Motion?

Respiratory correlated computed tomography has been shown to be effective for evaluation of breathing-induced motion of pulmonary tumors. This study investigated whether a single four-dimensional CT study (4D-CT) is representative and sufficient for treatment planning in stereotactic body radiotherapy (SBRT). Four repeated helical 4D-CT studies were acquired every 10 min for 10 patients with 14 pulmonary metastases. Patients remained immobilized in a stereotactic body frame (SBF) for 30 min; abdominal compression was applied to seven patients. Using amplitude based sorting, eight phases equally distributed over the breathing cycle were reconstructed for each 4D-CT study. Tumor position was defined in a total of 406 CT series and variability of breathing motion and mean tumor position were evaluated. Peak-to-peak tumor motion was 9.9 mm +/- 6.8 mm (mean +/- standard deviation) and 9.0 mm +/- 7.4 mm at time point 0 min (t(0)) and t(30), respectively. In one patient with poor pulmonary function, continuous increase of breathing motion from 17.4 mm at t(0) to 28.3 mm at t(30) was seen. In five and two lesions, respectively, a drift of the mean tumor position greater than 3 mm and 5 mm was observed. A borderline significance was calculated for larger tumor position variability in midventilation phases compared with peak-ventilation phases of the breathing cycle (p = 0.08). Treatment planning based on a single 4D-CT study is reliable for the majority of patients. Increased intrafractional uncertainties were seen for patients with poor pulmonary function and with tumors located in the lower lobe.

Observation of Interfractional Variations in Lung Tumor Position Using Megavoltage Cone-Beam CT

Purpose/Objective: Several radiographic systems are now available to measure and correct for daily patient positioning errors. Most systems rely on bony landmark imaging and registration, ignoring possible variations in tumor position relative to bony anatomy. We report on interfractional variation in lung tumor position relative to bony anatomy as measured with megavoltage cone-beam CT (MVCBCT).

SU-FF-J-74: Tumor Motion and Tumor Shape Changes During Radiotherapy of Bladder Cancer

Purpose: First, to quantify bladder tumor motion in three dimensions. Secondly, to relate the motion to changes in bladder filling. Thirdly, to evaluate shape and volume changes of the GTV during the course of the radiotherapy. Method and Materials: CT images were obtained for 21 bladder cancer patients. Next to the planning CT scan, each patient underwent 8 to 11 follow-up CT scans. These scans were matched on the bony anatomy. Patients were instructed to empty their bladder and drink 250 cc water one hour prior to acquisition. Tumor, bladder and rectum were delineated manually. For each patient, the variation in tumor position (day-to-day tumor motion) was determined by calculating the shift of the center of gravity (CG) of each GTV relative to the mean CG. These shifts were compared with model results from a previous study, concerning patients with a healthy bladder. The correlation between bladder volume and GTV position was determined. To study GTV shape changes, we matched all GTVs and determined the residual shape errors in terms of SDs. Results: Tumor motion was largest in Cranial-Caudal and A-P direction and ranged from 0.1 to 0.9 cm. The movement was strongly correlated with the bladder filling and the tumor location on the bladder wall. The average SD of the GTV shape changes, excluding the 10% highest values, ranged from 0.1 to 0.35 cm. The SD of the GTV volume ranged from 0.5 to 13 cc and was not dependent on the GTV volume. We found no significant volume time trend. Conclusion: We demonstrated that, despite large differences in bladder filling, variations in GTV shape were small. We could not prove any GTV volume trend with time. Geometrical uncertainties of the GTV position were mainly caused by variation in bladder volume and depended strongly on the GTV location.

The correlation between internal and external markers for abdominal tumors: Implications for respiratory gating

The correlation of the respiratory motion of external patient markers and abdominal tumors was examined. Data of this type are important for image-guided therapy techniques, such as respiratory gating, that monitor the movement of external fiducials. Fluoroscopy sessions for 4 patients with internal, radiopaque tumor fiducial clips were analyzed by computer vision techniques. The motion of the internal clips and the external markers placed on the patient's abdominal skin surface were quantified and correlated. In general, the motion of the tumor and external markers were well correlated. The maximum amount of peak-to-peak craniocaudal tumor motion was 2.5 cm. The ratio of tumor motion to external-marker motion ranged from 0.85 to 7.1. The variation in tumor position for a given external-marker position ranged from 2 to 9 mm. The period of the breathing cycle ranged from 2.7 to 4.5 seconds, and the frequency patterns for both the tumor and the external markers were similar. Although tumor motion generally correlated well with external fiducial marker motion, relatively large underlying tumor motion can occur compared with external-marker motion and variations in the tumor position for a given marker position. Treatment margins should be determined on the basis of a detailed understanding of tumor motion, as opposed to relying only on external-marker information.