RPM System Research Articles

SU‐F‐T‐644: Reproducibility of Target Position Using Moderate Voluntary Breath‐ Hold During Liver Stereotactic Ablative Radiotherapy

Purpose:To evaluate the reproducibility of target position using moderate voluntary breath‐hold during liver stereotactic ablative radiotherapy (SABR).Methods:Two patients who underwent liver SABR on a Varian TrueBeam STx linac were used for this study. Fiducial markers were placed in and around the target in the liver as surrogates for the target position and motion. GTVs were contoured by assessing tumor extent on contrast enhanced CT. The PTV was created from the GTV by adding 2 mm margins to account for the residual motion during breath‐holds. A portable biofeedback system was used to facilitate the breath‐hold to a reproducible position. The Varian RPM system was used for gating the linac. Proceeding each treatment, orthogonal kV pairs were taken, and alignment to nearby bony anatomy was performed. Then the breath‐hold CBCT was acquired to align the fiducial markers. On‐line fluoroscopy was used to fine‐tune the breath‐hold gating thresholds to correlate with the positions of the fiducial markers. The inter‐fraction reproducibility of the target was evaluated by the offsets of the daily breath‐hold CBCTs from the paired kV matches as a direct measure of the target position relative to the bony anatomy. The intra‐fraction reproducibility of the target position was assessed by the gated window of the RPM marker block for each fraction.Results:The absolute mean offsets between the CBCT and paired kV matches in the vertical, longitudinal, and lateral directions were 0.06 cm, 0.10 cm, and 0.06 cm for patient 1, and 0.37 cm, 0.62 cm, and 0.09 cm for patient 2. The gated window of the RPM marker block for the breath‐hold for each fraction was within 0.63 ± 0.16 cm and 0.59 ± 0.12 cm for patients 1 and 2, respectively.Conclusion:Moderate voluntary breath‐hold showed good inter‐ and intra‐fraction reproducibility of target position during liver SABR.

SU‐F‐J‐119: Pilot Study On the Location‐Based Lung Motion Assessment

Purpose:In most of lung treatment cases with various radiotherapy beam modalities, 4DCT images are obtained in order to define ITV. ITV is defined with the signal from motion monitoring system, e.g. RPM. However, the signal is not consistent with tumor motion because it varies with location, its size, age, gender, etc. In the present study, the location‐based motion assessment is presented.Methods:4DCT images of 70 patients were reviewed: 28‐left‐lung and 42‐right‐lung patients; 36‐female and 34‐male patients; the age range of 51.2–89.9; tumor‐size range of 0.75–9.50cm with 25% of these adherent to bony‐anatomy. Philips Big‐Bore Simulation CT and RPM systems were used. The study was performed as follows. First, RPM signal and tumor motion in superior‐inferior direction was compared. Second, the tumor size and its motion amplitude in all directions were measured at multiple locations. Third, the average tumor motion was calculated to assess general motion amplitudes at various locations.Results:RPM amplitude is not consistent with lung tumor motion amplitude. The tumors of similar sizes at similar location present various motion amplitude up to 1.1cm difference, but in average, the standard deviation was <0.5cm. Almost regardless of tumor sizes, the tumor motion was greatest at lower lobe location (>=1.0cm), and the smallest at upper lobe location and when adherent to bony‐anatomy (<=0.5cm).Conclusion:The tumor size affects the motion amplitude less than does the tumor location. However, as the study results indicate that tumor motion has noticeable variation and so further study with more patient cases is needed. Also, for the same patient, the RPM signal presents instability of breathing, and clinically the patient with the instability of RPM breathing of <=10% is selected for respiratory‐gated radiotherapy and ∼25% of patients under current study was treated. Patient‐specific motion‐uncertainty margins are considered to be added following further study.

TH-AB-202-07: Radar Tracking of Respiratory Motion in Real Time

Purpose: To propose a method of real time tracking of respiratory motion in patients undergoing radiation therapy. Radar technology can be employed to detection the movement of diaphragm and thoracic anatomy. Methods: A radar transceiver was specially designed. During experiment, the radar device was securely attached to a fixed frame. Respiratory motion was simulated with: 1) Varian RPM phantom, 2) Standard Imaging Respiratory Gating Platform. Signals recorded with radar equipment were compared with those measured with Varian RPM system as a reference. Results: Motion generated by Varian RPM phantom was recorded by the radar device, and compared to the signals recorded by RPM camera. The results showed exact agreement between the two monitoring equipments. Motion was also generated by Standard Imaging Respiratory Motion Platform. The results showed the radar device was capable of measuring motion of various amplitudes and periods. Conclusion: The proposed radar device is able to measure movements such as respiratory motion. Compared to state-of-the-art respiratory detection instrument, the radar device is shown to be equally precise and effective for monitoring respiration in radiation oncology patients.

SU‐F‐T‐253: Volumetric Comparison Between 4D CT Amplitude and Phase Binning Mode

Purpose:Motion artifact in 4DCT images can affect radiation treatment quality. To identify the most robust and accurate binning method, we compare the volume difference between targets delineated on amplitude and phase binned 4DCT scans.Methods:Varian RPM system and CT scanner were used to acquire 4DCTs of a Quasar phantom with embedded cubic and spherical objects having superior‐inferior motion. Eight patients’ respiration waveforms were used to drive the phantom. The 4DCT scan was reconstructed into 10 phase and 10 amplitude bins (2 mm slices). A scan of the static phantom was also acquired. For each waveform, sphere and cube volumes were generated automatically on each phase using HU thresholding. Phase (amplitude) ITVs were the union of object volumes over all phase (amplitude) binned images. The sphere and cube volumes measured in the static phantom scan were Vsphere=4.19cc and Vcube=27.0cc. Volume difference (VD) and dice similarity coefficient (DSC) of the ITVs, and mean volume error (MVE) defined as the average target volume percentage difference between each phase image and the static image, were used to evaluate the performance of amplitude and phase binning.Results:Averaged over the eight breathing traces, the VD and DSC of the internal target volume (ITV) between amplitude and phase binning were 3.4%±3.2% (mean ± std) and 95.9%±2.1% for sphere; 2.1%±3.3% and 98.0% ±1.5% for cube, respectively.For all waveforms, the average sphere MVE of amplitude and phase binning was 6.5% ± 5.0% and 8.2%±6.3%, respectively; and the average cube MVE of amplitude and phase binning was 5.7%±3.5%and 12.9%±8.9%, respectively.Conclusion:ITV volume and spatial overlap as assessed by VD and DSC are similar between amplitude and phase binning. Compared to phase binning, amplitude binning results in lower MVE suggesting it is less susceptible to motion artifact.

SU‐G‐JeP4‐05: Effects of Irregular Respiratory Motion On the Positioning Accuracy of Moving Target with Free Breathing Cone‐Beam Computerized Tomography

Purpose:Average or maximum intensity projection (AIP or MIP) images derived from 4DCT images are often used as a reference image for target alignment when free breathing Cone‐beam CT (FBCBCT) is used for positioning a moving target at treatment. This method can be highly accurate if the patient has stable respiratory motion. However, a patient's breathing pattern often varies irregularly. The purpose of this study is to investigate the effect of irregular respiration on the positioning accuracy of a moving target with FBCBCT.Methods:Eight patients’ respiratory motion curves were selected to drive a Quasar phantom with embedded cubic and spherical targets. A 4DCT of the moving phantom was acquired on a CT scanner (Philips Brilliance 16) equipped with a Varian RPM system. The phase binned 4DCT images and the corresponding MIP and AIP images were transferred into Eclipse for analysis. CBCTs of the phantom driven by the same breathing curves were acquired on a Varian TrueBeam and fused such that the zero positions of moving targets are the same on both CBCT and AIP images. The sphere and cube volumes and centrioid differences (alignment error) determined by MIP, AIP and FBCBCT images were compared.Results:Compared to the volume determined by FBCBCT, the volumes of cube and sphere in MIP images were 22.4%±8.8% and 34.2%±6.2% larger while the volumes in AIP images were 7.1%±6.2% and 2.7%±15.3% larger, respectively. The alignment errors for the cube and sphere with center‐center matches between MIP and FBCBCT were 3.5±3.1mm and 3.2±2.3mm, and the alignment errors between AIP and FBCBCT were 2.1±2.6mm and 2.1±1.7mm, respectively.Conclusion:AIP images appear to be superior reference images than MIP images. However, irregular respiratory motions could compromise the positioning accuracy of a moving target if the target center‐center match is used to align FBCBCT and AIP images.

SU-G-JeP1-14: Respiratory Motion Tracking Using Kinect V2

Purpose: Investigate capability and accuracy of Kinect v2 camera for tracking respiratory motion to use as a tool during 4DCT or in combination with motion management during radiotherapy treatments. Methods: Utilizing the depth sensor on the Kinect as well as code written in C#, the respiratory motion of a patient was tracked by recording the depth (distance) values obtained at several points on the patient. Respiratory traces were also obtained using Varian's RPM system, which traces the movement of a propriety marker placed on the patient's abdomen, as well as an Anzai belt, which utilizes a pressure sensor to track respiratory motion. With the Kinect mounted 60 cm above the patient and pointing straight down, 11 breathing cycles were recorded with each system simultaneously. Relative displacement values during this time period were saved to file. While RPM and the Kinect give displacement values in distance units, the Anzai system has arbitrary units. As such, displacement for all three are displayed relative to the maximum value for the time interval from that system. Additional analysis was performed between RPM and Kinect for absolute displacement values. Results: Analysis of the data from all three systems indicates the relative motion obtained from the Kinect is both accurate and in sync with the data from RPM and Anzai. The absolute displacement data from RPM and Kinect show similar displacement values throughout the acquisition except for the depth obtained from the Kinect during maximum exhalation (largest distance from Kinect). Conclusion: By simply utilizing the depth data of specific points on a patient obtained from the Kinect, respiratory motion can be tracked and visualized with accuracy comparable to that of the Varian RPM and Anzai belt.

SU‐F‐T‐636: Comprehensive Approach to Motion Assessment for Liver and Pancreas SBRT Patients

Purpose:Our past practice for liver and pancreas SBRT consisted of free breathing (FB) with gated treatment delivery using a 30‐70% phase window. We have recently adopted an assessment method leading to individualized motion management to minimize target motion. We present our results from 47 patients treated with this new approach.Methods:We perform an initial patient coaching and assessment session in our conventional simulator suite to observe the motion of the implanted fiducials with FB anterior and lateral 20‐second cine acquisitions. The physician decides whether to attempt inhale or exhale breath‐hold (BH). The patient is coached while observing with cine to ascertain their ability to achieve the desired BH mode for long periods as needed for treatment delivery. If the patient cannot comply, a FB approach is adopted using gating or simple ITV method (for patients without fiducials). After achieving a patient‐specific motion management mode, we perform CT‐simulation using the Varian RPM system to reproduce the chosen mode and record a reference session for treatment delivery. For pre‐treatment imaging, the fiducials are observed under fluoro while coaching the patient.Results:Of 47 SBRT cases analyzed, 32 were liver and 15 were pancreas. The chosen techniques were: 32 exhale BH (12 with abdominal compression), 7 FB gated, 4 inhale BH, and 4 FB ITV. Maximum fiducial motion amplitude was 5 mm for the FB gated patients, and less than 5 mm for all BH patients with most able to achieve a maximum amplitude of 3 mm.Conclusion:This study showed that an individualized motion management approach can reduce the target volume and, therefore, the volume of irradiated healthy tissue from liver or pancreas SBRT. Effective coaching is essential in achieving consistent BH with 3 mm amplitude. The fluoro/cine session is helpful in establishing the right coaching approach for each patient.

SU‐F‐T‐254: Dose Volume Histogram (DVH) Analysis of Breath Hold Vs Free Breathing Techniques for Esophageal Tumors

Purpose:Lung and heart doses and associated toxicity are of concern in radiotherapy for esophageal cancer. This study evaluates the dosimetry of deep‐inspiration‐breath‐hold (DIBH) technique as compared to freebreathing(FB) using 3D‐conformal treatment(3D‐CRT) of esophageal cancer.Methods:Eight patients were planned with FB and DIBH CT scans. DIBH scans were acquired using Varian RPM system. FB and DIBH CTs were contoured per RTOG‐1010 to create the planning target volume(PTV) as well as organs at risk volumes(OAR). Two sets of gross target volumes(GTV) with 5cm length were contoured for each patient: proximal at the level of the carina and distal at the level of gastroesophageal junction and were enlarged with appropriate margin to generate Clinical Target Volume and PTV. 3D‐CRT plans were created on Eclipse planning system for 45Gy to cover 95% of PTV in 25 fractions for both proximal and distal tumors on FB and DIBH scans. For distal tumors celiac nodes were covered electively. DVH parameters for lung and heart OARs were generated and analyzed.Results:All DIBH DVH parameters were normalized to FB plan values. Average of heart‐mean and heart‐V40 was 0.70 and 0.66 for proximal lesions. For distal lesions ratios were 1.21 and 2.22 respectively. For DIBH total lung volume increased by 2.43 times versus FB scan. Average of lung‐mean, V30, V20, V10, V5 are 0.82, 0.92, 0.76, 0.77 and 0.79 for proximal lesions and 1.17,0.66,0.87,0.93 and 1.03 for distal lesions. Heart doses were lower for breath‐hold proximal lesions but higher for distal lesions as compared to free‐breathing plans. Lung doses were lower for both proximal and distal breath‐hold lesions except mean lung dose and V5 for distal lesions.Conclusion:This study showed improvement of OAR doses for esophageal lesions at mid‐thoracic level utilizing DIBH vs FB technique but did not show consistent OAR sparing with DIBH for distal lesions.

Accuracy Verification of Respiratory-gated Radiotherapy that Combines the Respiration-Monitoring Device and Respiratory-gated System.

The purpose of this study is to evaluate the mechanical accuracy of a respiratory-gated radiation system that combines the Linear Indicator-equipped Abches respiration-monitoring device and the Varian Real-time Position Management system (LI-RPM system). This combined configuration, implemented for the first time in Japan, was compared with the stand-alone Varian RPM system (RPM system). The delay times, dose profiles, and output waveforms of the LI-RPM and RPM systems were evaluated using a self-produced dynamic phantom. The delay times for the LI-RPM and RPM systems were both 0.1 s for 4 s and 8 s test periods. The corresponding output waveform correlation factors (R2) for the 4 s and 8 s test periods were 0.9981 and 0.9975, respectively. No difference was observed in the dose profiles of the two systems. Thus, the present results indicate that the proposed LI-RPM combined respiratory-gated radiation system has similar properties to the RPM system. However, it offers several advantages in terms of its versatility, including its alignment assistance capabilities for non-coplanar treatments.

Respiratory surface motion measurement by Microsoft Kinect

Abstract In radiotherapy of abdominal and thoracic tumors, respiratory motion is a problem for an accurate treatment. Most current motion compensation techniques rely on externally acquired breathing signals of the patient. The systems in clinical use usually work with 1D surface motion signals to describe internal structure respiratory motion patterns. As a 1D signal is not able to describe complex motion patterns and breathing variations, in this work the Microsoft Kinect, which can record multidimensional respiratory surface motion signals, is proposed to be used instead. For the Kinect, a clinically acceptable measurement setup is designed and Kinect measurements are compared to the Varian RPM system (clinical standard). The results show that the signals are well aligned. An additional comparison of Kinect signals from different regions of interest on the chest further reveals variations between them. This illustrates that the use of a system that provides multidimensional signals is worthwhile; the knowledge about breathing variations could be applied for optimization of current clinical workflows.

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

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

SU‐E‐J‐153: Reconstructing 4D Cone Beam CT Images for Clinical QA of Lung SABR Treatments

Purpose:To verify that the planned Primary Target Volume (PTV) and Internal Gross Tumor Volume (IGTV) fully enclose a moving lung tumor volume as visualized on a pre‐SABR treatment verification 4D Cone Beam CT.Methods:Daily 3DCBCT image sets were acquired immediately prior to treatment for 10 SABR lung patients using the on‐board imaging system integrated into a Varian TrueBeam (v1.6: no 4DCBCT module available). Respiratory information was acquired during the scan using the Varian RPM system. The CBCT projections were sorted into 8 bins offline, both by breathing phase and amplitude, using in‐house software. An iterative algorithm based on total variation minimization, implemented in the open source reconstruction toolkit (RTK), was used to reconstruct the binned projections into 4DCBCT images. The relative tumor motion was quantified by tracking the centroid of the tumor volume from each 4DCBCT image. Following CT‐CBCT registration, the planning CT volumes were compared to the location of the CBCT tumor volume as it moves along its breathing trajectory. An overlap metric quantified the ability of the planned PTV and IGTV to contain the tumor volume at treatment.Results:The 4DCBCT reconstructed images visibly show the tumor motion. The mean overlap between the planned PTV (IGTV) and the 4DCBCT tumor volumes was 100% (94%), with an uncertainty of 5% from the 4DCBCT tumor volume contours. Examination of the tumor motion and overlap metric verify that the IGTV drawn at the planning stage is a good representation of the tumor location at treatment.Conclusion:It is difficult to compare GTV volumes from a 4DCBCT and a planning CT due to image quality differences. However, it was possible to conclude the GTV remained within the PTV 100% of the time thus giving the treatment staff confidence that SABR lung treatements are being delivered accurately.

SU-C-210-03: Impact of Breathing Irregularities On Gated Treatments

Purpose: To evaluate the effect of breathing irregularities on target location in gated treatments using amplitude and phase gating. Methods: 111 breathing patterns acquired using RPM system were categorized based on period and amplitude STD as regular (STD period ≤ 0.5 s, STD amplitude ≤ 1.5 mm), medium (0.5 s 1 s, STD amplitude > 3 mm). One pattern representative of the average defined population was selected per category and corresponding target motion reproduced using Quasar Respiratory Motion Phantom. Phantom in motion underwent 4D-CT scan with phase reconstruction. Gated window was defined at end of exhale and DRRs reconstructed in treatment planning at 40% (beam on) and 60% phase (beam off). Target location uncertainty was assessed by comparing gated kV triggered images continuously acquired at beam on/off on a True Beam 2.0 with corresponding DRRs. Results: Average target uncertainty with amplitude gating was in [0.4 – 1.9] mm range for the different scenarios with maximum STD of 1.2 mm for the irregular pattern. Average target uncertainty with phase gating was [1.1 – 2.2] mm for regular and medium patterns, while itmore » increased to [3.6 – 9.6] mm for the irregular pattern. Live gated motion was stable with amplitude gating, while increasing with phase gating for the irregular pattern. Treatment duration range was [68 – 160] s with amplitude and [70 – 74] s with phase gating. Conclusion: Breathing irregularities were found to affect gated treatments only when using phase gating. For regular and medium patterns no significant difference was found between the two gating strategies. Amplitude gating ensured stable gated motion within the different patterns, thus reducing intra-fraction target location variability for the irregular pattern and resulting in longer treatment duration.« less

SU-E-J-60: Evaluation of Temporal Lag in Radiotherapy Gating for Tumor Motion Trajectories

Purpose: Evaluating timing differences between LINAC beam ON/OFF and the estimation of tumor positioning using gating systems is essential for establishing confidence when treating with gating during radiotherapy, and is an annual requirement of TG-142. Temporal discrepancies between the trajectories of external marker surrogates and beam delivery may vary depending upon the type of external marker motion, which is quantified in this work for several trajectories. Methods: A precise robotic 3D motion stage performed several trajectories typically used for gating phantoms, including sinusoidal and Lujan-type motion; a commercial respiratory motion simulator was also employed. The true motions were monitored using variable resistors. The beam ON/OFF was controlled separately by two RPM (Varian) systems, an integrated version delivered by a Varian Truebeam LINAC and version 1.6 delivered by a Varian Trilogy, and measured using a diode. The resistor and diode signals were read by a multichannel digital oscilloscope, and timing differences between beam ON/OFF as measured by the diode and the phantom motion were determined using a peak detection algorithm. Results: Timing differences between beam ON/OFF and 3D stage motion peaks (diode—true motion timing) were computed to be 79.4 & 57.7ms for sinusoidal motion and 109.1 & 63.6ms for Lujan-type motion on the Truebeam LINAC, for beam ON and OFF, respectively. Timing differences for the Trilogy LINAC were 34.4 & 55.2ms for the sinusoidal motion and 29.0 & 26.3ms for the Lujan-type motion, for beam ON and OFF, respectively. With the commercial motion simulator, the timing differences were found to be −9ms and −78ms for beam ON/OFF, respectively, with the Truebeam, and −97.6ms and −60.9ms for beam ON/OFF, respectively, with the Trilogy. Conclusion: Setup-dependent temporal lags were found using this methodology. These discrepancies have the potential to influence quality assurance on gating systems and ultimately the treatment dose during radiotherapy. NIH National Institute of Biomedical Imaging and Bioengineering grant T32 EB002103-21

Abstract P2-14-08: Immediate breast reconstruction with anatomical implants following mastectomy: The Radiation perspective

Abstract Purpose: Immediate implant-based breast reconstruction followed by postmastectomy radiation therapy (PMRT) is controversial because of the risk of compromised treatment plans, particularly left-sided plans including internal mammary nodes (IMNs) as target. Using direct-to-implant reconstruction with anatomical implants may improve plans due to their unique shape over the chest wall. This single-institution study evaluated the effects of immediate breast reconstruction with anatomical implants on the quality of PMRT delivered by 3-dimensional conformal radiation therapy (3D-CRT). Methods and Materials: Patients undergoing immediate direct-to-implant reconstruction with anatomic implant, performed by a single surgeon, were treated between 2008 and 2013. For each patient, 2 plans were created and calculated, including or excluding IMNs. No electron fields were used. The primary endpoint was the dose distribution among reconstructed breast (RB), heart, lungs and IMNs and between right and left breasts. Six patients were treated with the Varian RPM system for left sided BC due to anterior heart position. Results: Of 29 consecutive patients, 11 received right-sided and 18 received left-sided PMRT. For plans excluding IMNs, mean Dmean was 49.09 Gy (98.2% of the prescribed dose) for right and 48.51 Gy (97.0%) for left RBs. For plans including IMNs, mean Dmean was 49.15 Gy (98.3%) for right and 48.46 Gy (96.9%) for left RBs. Mean RB D95 ranged from 42.05 Gy (left side with IMNs) to 45.15 Gy (right side with IMNs). Mean IMN Dmean was almost identical for left- and right-sided treatment (47.89 Gy and 47.27 Gy, respectively; P=.340). Heart doses were very low, with mean Dmean values of 1.25 Gy (range, 0.83-1.46) and 1.56 Gy (range, 1.23-2.10) for left-sided plans excluding and including IMNs, respectively (P<.001). Mean lung V20 values ranged from 13.80% for left-sided treatment excluding IMNs to 19.47% for right-sided treatment including IMNs. Conclusion: PMRT can be delivered effectively and safely by 3D-CRT after direct-to-implant breast reconstruction with anatomical implants. The reconstructed breast and the IM chain coverage were excellent and the heart dose was very low, (probably due to the use of the RPM system). Lungs V20 was comparable with other publication. Our study support the hypothesis that immediate breast reconstruction per se is not an impediment to the delivery of high-quality PMRT by modern 3D-CRT technology. Based on our findings, reconstructive surgeons should consider the use of anatomical implants for immediate breast reconstruction in patients requiring subsequent PMRT. Citation Format: Merav Akiva Ben-David, Hila Granot, Ilana Gelernter, Michael Scheflan. Immediate breast reconstruction with anatomical implants following mastectomy: The Radiation perspective [abstract]. In: Proceedings of the Thirty-Seventh Annual CTRC-AACR San Antonio Breast Cancer Symposium: 2014 Dec 9-13; San Antonio, TX. Philadelphia (PA): AACR; Cancer Res 2015;75(9 Suppl):Abstract nr P2-14-08.

Dynamic System Theory for Lung Cancer Patient Breathing

Dynamic System Theory for Lung Cancer Patient Breathing

Setting Action Levels for Abdominal Stereotactic Body Radiation Therapy Monitored With Frequent kV Imaging Surveillance

We hypothesize that intra-fractional motion monitoring with frequent kV imaging surveillance together with an appropriate intervention action level ensures adequate tumor dose to abdominal cancer patients with breathing amplitude restricted by compression who are treated with stereotactic body radiation therapy on a Linac. We retrospectively studied the treatment plans and intra-fractional images of 10 upper-abdominal cancer patients treated on a Linac with hypofractionated IMRT. For all patients, one or more radio-opaque markers were implanted near the tumor. Abdominal compression reduced respiratory marker motion below 6 mm as evaluated by fluoroscopy before each treatment. The PTV was defined by a 5 mm 3D expansion of the GTV. Intra-fractional kV radiographs were automatically acquired at 5-6 second intervals during beam delivery, triggered by the RPM system’s response to a motion phantom mounted above the patient. Planned marker positions are overlaid on each radiograph and displayed on the treatment console; therapists check compression and correct setup if persistent deviation from planned position is seen. kV images were retrospectively analyzed using in-house software to determine marker shifts from their planned positions. To estimate dosimetric effects, the MLC position at each control point is shifted according to the superior-inferior component of the fiducial trajectory, and the resulting dose to GTV and surrounding organs at risk (OAR) is reconstructed. The relationship between the degradation of GTV D95, dose to OAR, mean and maximal fiducial displacement is explored. The allowable mean and maximal marker motion excursion is determined to obey objectives such as GTV D95 degrades less than 5% and no OAR constraints are violated for this patient population. The marker motion excursion ranged from 2 to 6 mm on pre-treatment fluoroscopy, but from 2 to 23 mm on intra-fractional kV projections. The correlation between the two is 0.44. The weak correlation and bigger amplitude argue for intra-fractional motion monitoring and management. The relative difference between the planned GTV D95 and that reconstructed accounting for intra-fractional motion ranges from -8.8% to 4.4%, and averages -1.4%. Without additional intra-fractional intervention, the 5 mm margin around GTV resulted in 10% of fractions violating the objectives. However, the objectives can be met with tools to enforce marker-based action levels that limit mean marker motion to [-2mm 2mm] and maximal excursion to <12 mm. The combination of compression belt, intra-fractional kV imaging surveillance, and advanced intervention methods can ensure patients receive improved dose coverage.

Measurement of time delays in gated radiotherapy for realistic respiratory motions.

Gated radiotherapy is used to reduce internal motion margins, escalate target dose, and limit normal tissue dose; however, its temporal accuracy is limited. Beam-on and beam-off time delays can lead to treatment inefficiencies and/or geographic misses; therefore, AAPM Task Group 142 recommends verifying the temporal accuracy of gating systems. Many groups use sinusoidal phantom motion for this, under the tacit assumption that use of sinusoidal motion for determining time delays produces negligible error. The authors test this assumption by measuring gating time delays for several realistic motion shapes with increasing degrees of irregularity. Time delays were measured on a linear accelerator with a real-time position management system (Varian TrueBeam with RPM system version 1.7.5) for seven motion shapes: regular sinusoidal; regular realistic-shape; large (40%) and small (10%) variations in amplitude; large (40%) variations in period; small (10%) variations in both amplitude and period; and baseline drift (30%). Film streaks of radiation exposure were generated for each motion shape using a programmable motion phantom. Beam-on and beam-off time delays were determined from the difference between the expected and observed streak length. For the system investigated, all sine, regular realistic-shape, and slightly irregular amplitude variation motions had beam-off and beam-on time delays within the AAPM recommended limit of less than 100 ms. In phase-based gating, even small variations in period resulted in some time delays greater than 100 ms. Considerable time delays over 1 s were observed with highly irregular motion. Sinusoidal motion shapes can be considered a reasonable approximation to the more complex and slightly irregular shapes of realistic motion. When using phase-based gating with predictive filters even small variations in period can result in time delays over 100 ms. Clinical use of these systems for patients with highly irregular patterns of motion is not advised due to large beam-on and beam-off time delays.

SU-E-J-265: Practical Issues and Solutions in Reconstructing and Using 4DCT for Radiotherapy Planning of Lung Cancer

Purpose: To report practical issues and solutions in reconstructing and using 4DCT to account for respiratory motion in radiotherapy planning. Methods: Quiet breathing 4DCT was used to account for respiratory motion for patients with lung or upper abdomen tumor. A planning CT and a 4DCT were acquired consecutively with a Philips Brilliance CT scanner and Varian RPM System. The projections were reconstructed into 10 phases. In Pinnacle RTP system, we contour a GTV in each phase and unite all 10 GTVs as ITV. The ITV is then mapped to the planning CT. We describe practical issues, their causes, our solutions and reasoning during this process. Results: In 6 months, 9 issues were reported for 8 patients with lung cancer. For two patients, part of the GTV (∼50% and 10%) in planning CT fell outside the ITV in 4DCT. There was a 7 mm variation in first patient back position because less restricted immobilization had to be used. The second discrepancy was due to moderate variation in breathing amplitude. We extended the ITV to include the GTV since both variations may likely happen during treatment. A LUL tumor showed no motion due to a 10-s long no-breathing period. An RLL tumor appeared double due to an abnormally deeper breath at the tumor region. We repeated 4DCT reiterating the importance of quiet, regular breathing. One patient breathed too light to generate RPM signal. Two issues (no motion in lung, incomplete images in 90% phase) were due to incorrect tag positions. Two unexplainable errors disappeared when repeating reconstruction. In summary, 5 issues were patient-related and 4 were technique issues. Conclusion: Improving breathing regularity avoided large artifacts in 4DCT. One needs to closely monitor patient breathing. For uncontrollable variations, larger PTVs are necessary which requires appropriate communication between physics and the treating physician.

SU-E-J-261: Statistical Analysis and Chaotic Dynamics of Respiratory Signal of Patients in BodyFix

Purpose: To quantify respiratory signal of patients in BodyFix undergoing 4DCT scan with and without immobilization cover. Methods: 20 pairs of respiratory tracks recorded with RPM system during 4DCT scan were analyzed. Descriptive statistic was applied to selected parameters of exhale-inhale decomposition. Standardized signals were used with the delay method to build orbits in embedded space. Nonlinear behavior was tested with surrogate data. Sample entropy SE, Lempel-Ziv complexity LZC and the largest Lyapunov exponents LLE were compared. Results: Statistical tests show difference between scans for inspiration time and its variability, which is bigger for scans without cover. The same is for variability of the end of exhalation and inhalation. Other parameters fail to show the difference. For both scans respiratory signals show determinism and nonlinear stationarity. Statistical test on surrogate data reveals their nonlinearity. LLEs show signals chaotic nature and its correlation with breathing period and its embedding delay time. SE, LZC and LLE measure respiratory signal complexity. Nonlinear characteristics do not differ between scans. Conclusion: Contrary to expectation cover applied to patients in BodyFix appears to have limited effect on signal parameters. Analysis based on trajectories of delay vectors shows respiratory system nonlinear character and its sensitive dependence on initial conditions. Reproducibility of respiratory signal can be evaluated with measures of signal complexity and its predictability window. Longer respiratory period is conducive for signal reproducibility as shown by these gauges. Statistical independence of the exhale and inhale times is also supported by the magnitude of LLE. The nonlinear parameters seem more appropriate to gauge respiratory signal complexity since its deterministic chaotic nature. It contrasts with measures based on harmonic analysis that are blind for nonlinear features. Dynamics of breathing, so crucial for 4D-based clinical technologies, can be better controlled if nonlinear-based methodology, which reflects respiration characteristic, is applied. Funding provided by Varian Medical Systems via Investigator Initiated Research Project