Programmable Motion Platform Research Articles

SU‐E‐T‐351: Dosimetric Impact of Periodic and Random Motion on Proton Double‐Scattering and Uniform‐Scanning Deliveries

Purpose: To experimentally investigate target motion impact on proton dosimetry and the spatiotemporal interplay in double scattering (DS) and uniform scanning (US) deliveries using 3D polymer gel dosimeters and a programmable motion platform. Methods: A one beam proton plan with 16cm range and 6cm modulation (total 13 energy layers) in both DS and US modes was generated in the Eclipse treatment planning system (TPS) to be used for gel irradiation in static, periodic and random motion. The periodic motion was a single sinusoidal trace along the SI direction with 2‐cm peak‐to‐peak amplitude and 0.25Hz frequency. The random motion trace was an actual patient prostate trajectory from a treatment fraction. All irradiated gel dosimeters were setup on the platform in an identical fashion. The dose distribution of static gel dosimeter was compared to those of treatment plan. Dose distributions were compared between DS and US in both periodic and prostate motion conditions. 3D gamma analysis (4%, 4mm criterion) was calculated against planned dose distribution for all the gel dosimeters. Results: There was an overall good agreement in dose distributions between stationary DS delivery and TPS plan. 93% of voxels has gamma less than 1. 3D gamma failure maps were significantly different between DS and US periodic motion gels. There were 88% and 91% of voxels has gamma less than 1 for DS and US, respectively. In prostate motion cases, even though about 70% voxels has gamma larger than 1 for both DS and US deliveries, significant gamma failure map difference was still observed. Conclusion: Good dose agreement between TPS plan and stationary gel dosimeter demonstrated the validity of using gel dosimeter study target motion effect and its spatiotemporal interplay with proton irradiation. Isodose and 3D gamma comparisons between DS and US for both periodic and prostate motion confirmed their dosimetric impact.

SU‐E‐T‐413: Optimizing the Delivery of Intensity‐Modulated Stereotactic Body Radiation Therapy to Lung Tumours Influenced by Respiratory Motion

Purpose: To quantify the interplay effect for various IMRT techniques used to deliver Stereotactic‐Body Radiation Therapy (SBRT) to early‐stage lung cancer. Methods: Five lung cancer patients who received SBRT were retrospectively planned (54Gy/3fx) on the average 4D‐CT dataset with eight different IMRT techniques: three fixed‐beam IMRT plans (simple step‐and‐shoot (S/S), complex S/S, and sliding‐window (S/W)), Tomotherapy plans using three different beam sizes (1cm, 2.5cm, and 5cm), and two Volumetric‐Modulated Arc Therapy (VMAT) plans (one planned with Rapid Arc (Eclipse v8.9, Varian Medical Systems, Palo Alto, USA) and one planned with Smart Arc (Pinnacle v9.0, Philips Medical Systems, Cleveland, USA)). All treatment plans were calculated on a CT of the ArcCHECK phantom by Sun Nuclear (Gland, Switzerland) that was mounted on the QUASAR™ Programmable Respiratory Motion Platform (Modus Medical Devices Inc., London, Ontario). Each plan was delivered under the following motion conditions: 1) Static mode; 2) Sinusoidal mode (4s period) with S/I motion ranging from 5mm–20mm peak‐to‐peak in 5 mm increments; 3) Real‐patient waveforms ranging from 5mm–30mm peak‐to‐peak. A standard 3%/3mm gamma analysis compared each delivery to their corresponding calculated plan. Results: All methods were acceptable on average (>90% points pass) up to 15mm peak‐to‐peak motion except for the Rapid Arc plan, which had a significantly less pass rate (75.6+/−3.9%) than all other techniques (p<.02). Tomotherapy plans using either a 2.5cm beam (98.7+/−2.1%) or 5cm beam (96.8+/−3.5%) were still acceptable for motion up to 20 mm peak‐to‐peak. For irregular motion greater than 15mm, only the VMAT techniques failed ((74.9+/−16.6)% for Rapid Arc and (89.3+/−8.9)% for Smart Arc on average. Conclusions: Interplay effect in SBRT delivery was dependent on the IMRT technique for peak‐to‐peak motions greater than 15 mm. Tomotherapy was the least sensitive for motion up to 20mm. Rapid Arc was the most sensitive for motion greater than 15mm peak‐to‐peak.

Coupling surface cameras with on-board fluoroscopy: A feasibility study

To investigate the feasibility of using three-dimensional surface imaging cameras as an external surrogate of tumor motion through a temporal synchronization with kV imaging. To obtain an "x-ray on" signal from the on-board kV fluoroscopy system (XVI, Elekta), a hardware controller (Gate Controller) was interfaced between the kV fluoroscopy and Gate CT (VisionRT Ltd., London) computers. First, phantom experiments were performed using a programmable respiratory motion platform (sinusoidal motion, period = 3-5 s). The platform included a chest-wall component (A-P amplitude = 1 cm) tracked with the surface camera, while tumorlike objects translated in the superior-inferior direction were tracked using kV fluoroscopy (300 frames, frequency 5.5 fps). Accuracy of tracking the chest-wall platform was assessed, and the latency of the system was characterized by performing linear regression between the peak times obtained from Gate CT and fluoroscopy. Increasing the complexity of experiments, tumor displacement curves from three patients were simulated using synchronous tumor-abdomen data (RTRT). Our approach was further validated by imaging four free-breathing lung cancer patients with simultaneous Gate CT and kV fluoroscopy for approximately 55 s. Consideration was also given to varied sizes and locations of the tracked region of interest on the patient surface. For simple sinusoidal curves, measured amplitude (peak-to-peak) was 1.005 +/- 0.003 cm, 1.013 +/- 0.003 cm, and 1.003 +/- 0.005 cm for periods of 5, 4, and 3.3 s, respectively, demonstrating an excellent agreement with the actual chest platform amplitude of 1.0 cm. Period measurements were within 0.2% of actual using the surface cameras and within 0.9% of actual value using fluoroscopy. For the sinusoidal motion, the system latency was 0.64 +/- 0.02 s. This was further validated for the simulated tumor motion from three patients (latency = 0.65 +/- 0.03 s). Five of the nine patient fractions studied showed the associations between the abdomen and tumor were equivalent or better (Pearson r = 0.93-0.98) than those observed between the diaphragm and tumor (Pearson r = 0.89-0.95). A repeat analysis of five different tracked surfaces on the same patient further demonstrated strong agreement with the diaphragm and tumor, although no improvement in association strength was observed with increased size of region of interest. The feasibility of using surface imaging cameras to track the patient's abdomen as an external surrogate, while using kV imaging to track internal anatomy in synchrony, has been demonstrated. With further validation through additional patient studies to confirm these findings, gated radiation therapy treatments using surface imaging cameras as the external surrogate can be facilitated.

WE‐C‐204B‐03: 4D Treatment Delivery to Account for Motion, Rotation, and Deformation of Tumors and Normal Tissues

Purpose: To experimentally investigate 4D treatment delivery to account for motion, rotation, and deformation of tumors and normal tissues. Method and Materials: A 4D treatment plan was generated as a function of respiratory phase for an elliptical tumor that translates, rotates, deforms, and exhibits motion hysteresis throughout the respiratory cycle. A programmable motion platform with a marker block simulates the tumor motion. An independent patient position monitoring system monitors motion of marker block and provides 3D position and respiratory phase information in real time for the dynamic MLC motion‐tracking software. Using the position and phase information and the 4D treatment plan, the dynamic MLC motion‐tracking software repositions and/or reshapes the radiation beams to deliver treatment. An MV imager operating in a cine mode was used to acquire images to quantify tumor motion, rotation, and deformation. 4D treatment delivery was performed when the target motion during treatment delivery was the same as, smaller than, and larger than during imaging. For comparison, static delivery of treatment plans for each of the respiratory phases was performed. Results: The elliptical tumor segmented from MV images between 4D and static delivery showed the difference of centroid positions less than 5 mm, rotation less than 10 degrees, and major and minor axis lengths less than 2 mm each. 4D treatment delivery was comparable to static delivery of each phase treatment plan, demonstrating the ability of dynamic MLC to account for changes in anatomic motion, rotation, and deformation by varying MLC leaves as a function of phase. Conclusion: 4D treatment plans generated to account for motion, rotation, and deformation of tumors and normal tissues within a respiratory cycle at the planning stage can be delivered while still accounting for different target motion during delivery at the delivery stage. Research supported by NIH/NCIR01‐93626 and Varian Medical Systems

SU‐GG‐J‐41: Treatment Beam's‐Eye‐View Speed‐Modulated CBCT Imaging Used in a Triple Imaging Modality to Track Moving Tumors during Treatment Delivery

Purpose: To track the target trajectory during the treatment based on the pre‐treatment CBCT projection images when the gantry is selectively slowed down at the treatment angles, the synchronized 3D surface imaging and the EPID in movie mode. Methods and Materials: A gantry‐speed device slows down the CBCT gantry speed (2min/360degrees) by half for a 6 degrees arc centered on the MV treatment beam, allowing us acquire twice as many frames within the treatment arc (22frames/5.5fps). The treatment arc is chosen to accommodate a typical full breathing period (4s). A CBCT kV beam‐on trigger is used to acquire 3D surface images using the AlignRT cameras. Phantom experiments were performed using a programmable respiratory motion platform. During the treatment delivery, MV imaging was performed in movie mode (3fps) while the surface cameras tracked the chest‐wall component. A mutual information algorithm was employed to compare the kV an MV images. Results: CBCT reconstruction was performed using every other frame when in the slow treatment arc for correct reconstruction. The longer scanning times were translated in ∼10–15% additional CBCT dose when 4 beams were used. For simple trajectory, the kV and MV images were phase correlated ∼98% of the time for the high contrast tumor and ∼92% for the low contrast tumor. Phase knowledge provided by the surface cameras increased the later kV‐MV correlation to ∼97%. Patient‐like trajectories were tested where the most irregular trace (3mm baseline‐shift) showed a moderate kV‐MV correlation, enabling one to interrupt the treatment for a second on‐line baseline correction. Conclusion: The selective beams‐eye‐view slow‐arc CBCT technique in conjunction with surface cameras and MV imaging has been demonstrated to be an effective tool for tumor tracking due to its treatment time immediacy and minimal impact on treatment flow.

WE‐C‐204B‐06: Tumor Specific Reconstruction of Motion Probability Density Function from 4DCT

Purpose: The actual delivered dose to a moving tumor can deviate from prescribed dose not only during each fraction, but during each IMRT field. This deviation can be anticipated and incorporated into a treatment plan if the tumor specific motion probability density function (PDF) is accurately identified. In this study, a novel technique in determining the actual, tumor specific PDF of a moving gross tumor volume (GTV) is described and confirmed through phantom experiments using a quantitative approach of 4DCT imaging. Methods and Materials: We hypothesize that a PDF of the GTV can be obtained through weighted sums of phase‐sorted 4DCT images. Experimental validation is performed using a lung phantom attached to a programmable motion platform. The ground truth motion PDF of the targets was calculated through convolution of the motion pattern and the GTV dimension along the direction of motion. These PDF convolutions are compared to the normalized, reconstructed CT numbers obtained from averaging ten phase sorted 4DCT images after background subtraction. Results: The PDF reconstructed from averaged, weighted sets of 4DCT images was almost identical to the ground truth PDF in all comparisons made. Through sums of phase sorted 4DCT images, the resulting CT number represents a relative probability of finding some portion of the GTV in each geometric location (or voxel). In physical phantom reconstructions, the Pearson correlation coefficient between CT‐based PDF and the ground truth PDF was greater than .97 in all cases considered. An actual patient PDF was also evaluated in frontal and sagittal planes. Conclusions: Combining phase sorted CT images is an effective method in obtaining the actual tumor PDF as seen by an IMRT field. With the patient specific PDF, treatment planning to ensure proper target coverage can be achieved through intra‐field modulation, dose rate, and monitoring initial beam‐on phase.

WE‐C‐204B‐02: Disparity between 4DCT and CBCT Reconstruction of Internal Target Volumes

Purpose: Internal target volumes (ITVs) generated from different imaging modalities may not be identical. These differences could be due to inherent limitations in image reconstruction, and in this study we investigate the differences between 4DCT‐MIP generated ITVs and CBCT reconstructions of moving targets. Methods/Materials: A cork lung phantom with three embedded targets was attached to a programmable motion platform to simulate lung and tumor motion. Ideal sinusoidal motion and other simulated patterns were used to compare ITV dimensions reconstructed from 4DCT and CBCT. Four 4DCT and ten CBCT generated ITVs from four lung cancer patients were also compared using a physician's contour as ground truth. Results: CBCT targets along the direction of motion were consistently smaller than the 4DCT equivalents (by 0.49 +/− 0.50 cm) in the phantom measurements. The amplitude was varied during a single scan from 1 to 4 cm, and both 4DCT and CBCT under‐represented the ground truth dimension by 2.13 (+/−0.64) cm and 2.69 (+/−0.50) cm. Patient lung tumors did not reveal a similar pattern of smaller ITVs from CBCT. Lateral motion due to proximity to the heart resulted in lateral discrepancies for two patients, with CBCT ITV 0.32 cm and 0.68 cm larger than the 4DCT ITV. For the two other patients, the CBCT ITV was larger along the tumor's primary direction of motion. Conclusions: Even under ideal periodic motion, CBCT generated ITVs can deviate from their 4DCT counterparts. These variations are often assumed to be due to physiological changes, but may actually occur due to limitations in CBCT reconstruction. If irregular motion in actual patient cases is observed, the discrepancy between the 4DCT ITV and the CBCT counterpart can be greater than 5mm and adaptive re‐optimization based on CBCT ITV may not be beneficial.

SU‐EE‐A3‐03: Audiovisual Biofeedback Significantly Reduces Motion Blurring Artifacts in Four‐Dimensional (4D) PET Images

Purpose: To investigate the hypothesis that audiovisual biofeedback reduces motion blurring artifacts in 4D PET images. Method and Materials: 4D PET scans were acquired on a GE PET/CT scanner using audiovisual biofeedback guided and free breathing traces. A static image was used as the reference. The physical phantom consisted of six hollow spheres (NEMA ICE body phantom) with 10, 13, 17, 22, 28 and 37 mm in diameter as targets and a cylinder (Hoffman 3D brain phantom) as background. The sphere targets were filled with 18F‐FDG at a fixed target‐to‐background ratio (8:1) based on 10 mCi/70 kg. The phantom was placed on a 3D programmable motion platform which produced the breathing patterns of four subjects with and without audiovisual biofeedback. 4D PET images with five respiratory bins were reconstructed for each breathing pattern. The segmented volumes, dice coefficients, 1D profiles, and 2D profiles were determined for evaluation and compared for each respiratory bin. The breathing patterns were analyzed for correlation between irregularity of breathing and motion blurring.Results: Audiovisual biofeedback significantly reduced motion blurring by 16.5% (1D profile, p=0.004), 22.6% (2D profile, p=0.003), 34.5% (volume, p=0.009) and 6.5% (dice coefficient, p=0.004) on average for the small spheres (≤17 mm), and by 4.3% (p=0.008), 2.7% (p=0.104), 0.7% (p=0.446) and 5.0% (p=0.014), respectively, for the large spheres (≥20 mm). In general, 4D PET motion blurring was dependent on the magnitude and irregularity of motion. Conclusion: The first investigation of the effect of breathing training on 4D PET images has been performed. The results indicate that audiovisual biofeedback can significantly reduce motion blurring in 4D PET images and may facilitate improved identification and localization of small lung tumors. Conflict of Interest: Supported by NIH/NCI R01 93626

Patient‐specific motion artifacts in 4DCT

Four-dimensional computed tomography (4DCT) has enhanced images of the thorax and upper abdomen during respiration, but intraphase residual motion artifacts will persist in cine-mode scanning. In this study, the source and magnitude of projection artifacts due to intraphase target motion is investigated. A theoretical model of geometric uncertainty due to partial projection artifacts in cine-mode 4DCT was derived based on ideal periodic motion. Predicted artifacts were compared to measured errors with a rigid lung phantom attached to a programmable motion platform. Ideal periodic motion and actual patient breathing patterns were used as input for phantom motion. Reconstructed target dimensions were measured along the direction of motion and compared to the actual, known dimensions. Artifacts due to intraphase residual motion in cine-mode 4DCT range from a few mm up to a few cm on a given scanner, and can be predicted based on target motion and CT gantry rotation time. Errors in ITV and GTV dimensions were accurately characterized by the theoretical uncertainty at all phases when sinusoidal motion was considered, and in 96% of 300 measurements when patient breathing patterns were used as motion input. When peak-to-peak motion of 1.5 cm is combined with a breathing period of 4 s and gantry rotation time of 1 s, errors due to partial projection artifacts can be greater than 1 cm near midventilation and are a few mm in the inhale and exhale phases. Incorporation of such uncertainty into margin design should be considered in addition to other uncertainties. Artifacts due to intraphase residual motion exist in 4DCT, even for ideal breathing motions (e.g., sine waves). It was determined that these motion artifacts depend on patient-specific tumor motion and CT gantry rotation speed. Thus, if the patient-specific motion parameters are known (i.e., amplitude and period), a patient-specific margin can and should be designed to compensate for this uncertainty.

Verification of four-dimensional photon dose calculations

Recent work in the area of thoracic treatment planning has been focused on trying to explicitly incorporate patient-specific organ motion in the calculation of dose. Four-dimensional (4D) dose calculation algorithms have been developed and incorporated in a research version of a commercial treatment planning system (Pinnacle3, Philips Medical Systems, Milpitas, CA). Before these 4D dose calculations can be used clinically, it is necessary to verify their accuracy with measurements. The primary purpose of this study therefore was to evaluate and validate the accuracy of a 4D dose calculation algorithm with phantom measurements. A secondary objective was to determine whether the performance of the 4D dose calculation algorithm varied between different motion patterns and treatment plans. Measurements were made using two phantoms: A rigid moving phantom and a deformable phantom. The rigid moving phantom consisted of an anthropomorphic thoracic phantom that rested on a programmable motion platform. The deformable phantom used the same anthropomorphic thoracic phantom with a deformable insert for one of the lungs. Two motion patterns were investigated for each phantom: A sinusoidal motion pattern and an irregular motion pattern extracted from a patient breathing profile. A single-beam plan, a multiple-beam plan, and an intensity-modulated radiation therapy plan were created. Doses were calculated in the treatment planning system using the 4D dose calculation algorithm. Then each plan was delivered to the phantoms and delivered doses were measured using thermoluminescent dosimeters (TLDs) and film. The measured doses were compared to the 4D-calculated doses using a measured-to-calculated TLD ratio and a gamma analysis. A relevant passing criteria (3% for the TLD and 5% /3 mm for the gamma metric) was applied to determine if the 4D dose calculations were accurate to within clinical standards. All the TLD measurements in both phantoms satisfied the passing criteria. Furthermore, 42 of the 48 evaluated films fulfilled the passing criteria. All films that did not pass the criteria were from the rigid phantom moving with irregular motion. The author concluded that if patient breathing is reproducible, the 4D dose calculations are accurate to within clinically acceptable standards. Furthermore, they found no statistically significant differences in the performance of the 4D dose calculation algorithm between treatment plans.

TU‐E‐BRC‐06: On the Synchronization of 3D Surface Cameras with Fluoroscopic Imaging

Purpose: To investigate the feasibility of using three‐dimensional surface imaging cameras as an external surrogate of target motion through a temporal synchronization with kV imaging. Method and Materials: A gate controller, connected between kV fluoroscopy and AlignRT (Vision RT Ltd, London) computers, was triggered with kV beam‐on and reflected in AlignRT output data. First, phantom experiments were performed using a programmable respiratory motion platform (sinusoidal curves, 12–18 BPM). The platform included a chest‐wall component (A‐P amplitude = 1 cm) tracked with the surface camera, while the object translated superior‐inferior was fluoroscopically tracked (300 frames, frequency ∼5.5 fps). Accuracy of tracking the chest‐wall platform was assessed. Increasing the complexity of experiments, tumor displacement curves from three patients were simulated. Our approach was further validated by imaging a free‐breathing lung cancer patient over 11 fractions with simultaneous AlignRT and kV (300 frames/session, ∼55 seconds). Results: For simple sinusoidal curves, measured amplitude (peak‐to‐peak) was 1.005 ± 0.003, 1.013 ± 0.003 cm, 1.003 ± 0.005 cm for 12, 15, and 18 BPM, respectively, demonstrating excellent agreement with the expected chest platform amplitude of 1.0 cm. Period measurements were within 0.2% of expected for the surface cameras, and within 0.9 % of expected for fluoroscopy. Using simulated patient data and the motion platform, the latency between the surface tracking system and fluoroscopy was determined by performing linear regression between the peak time from AlignRT and fluoroscopy. For the three patient cases studied, latency was found to be 0.65 ± 0.03 seconds. Moderate agreement was observed between the diaphragm, tumor, and abdomen tracked for the patient studied. Conclusion: The utility of surface imaging cameras for tracking while synchronized with fluoroscopy has been demonstrated. Additional patient studies are warranted for further validation, which may facilitate gated radiation therapy treatments using surface imaging cameras as the external surrogate.