RPM Gating System Research Articles

A simple method to measure the gating latencies in photon and proton based radiotherapy using a scintillating crystal.

In respiratory gated radiotherapy,low latency between target motion into and out of the gating window and actual beam-on and beam-off is crucial for the treatment accuracy. However, there is presently a lack of guidelines and accurate methods for gating latency measurements. To develop a simple and reliable method for gating latency measurements that work across different radiotherapy platforms. Gating latencies were measured at a Varian ProBeam (protons, RPM gating system) and TrueBeam (photons, TrueBeam gating system) accelerator. A motion-stage performed 1cm vertical sinusoidal motion of a marker block that was optically tracked by the gating system. An amplitude gating window was set to cover the posterior half of the motion (0-0.5cm). Gated beams were delivered to a 5mm cubic scintillating ZnSe:O crystal that emitted visible light when irradiated, thereby directly showing when the beam was on. During gated beam delivery, a video camera acquired images at 120Hz of the moving marker block and light-emitting crystal. After treatment, the block position and crystal light intensity were determined in all video frames. Two methods were used to determine the gate-on (τon ) and gate-off (τoff ) latencies. By method 1, the video was synchronized with gating log files by temporal alignment of the same block motion recorded in both the video and the log files. τon was defined as the time from the block entered the gating window (from gating log files) to the actual beam-on as detected by the crystal light. Similarly, τoff was the time from the block exited the gating window to beam-off. By method 2, τon and τoff were found from the videos alone using motion of different sine periods (1-10s). In each video, a sinusoidal fit of the block motion provided the times Tmin of the lowest block position. The mid-time, Tmid-light , of each beam-on period was determined as the time halfway between crystal light signal start and end. It can be shown that the directly measurable quantity Tmid-light - Tmin =(τoff +τon )/2, which provided the sum (τoff +τon ) of the two latencies. It can also be shown that the beam-on (i.e., crystal light) duration ΔTlight increases linearly with the sine period and depends on τoff - τon : ΔTlight =constant•period+(τoff - τon ). Hence, a linear fit of ΔTlight as a function of the period provided the difference of the two latencies. From the sum (τoff +τon ) and difference (τoff - τon ), the individual latencies were determined. Method 1 resulted in mean (±SD) latencies of τon =255±33ms, τoff =82±15ms for the ProBeam and τon =84±13ms, τoff =44±11ms for the TrueBeam. Method 2 resulted in latencies of τon =255±23ms, τoff =95±23ms for the ProBeam and τon =83±8ms, τoff =46±8ms for the TrueBeam. Hence, the mean latencies determined by the two methods agreed within 13ms for the ProBeam and within 2ms for the TrueBeam. A novel, simple and low-cost method for gating latency measurements that work across different radiotherapy platforms was demonstrated. Only the TrueBeam fully fulfilled the AAPM TG-142 recommendation of maximum 100ms latencies.

The dose delivery effect of the different Beam ON interval in FFF SBRT: TrueBEAM

The purpose of this study is to determine the dose delivery effect of the different Beam ON interval in Flattening Filter Free Stereotactic Body Radiation Therapy (FFF-SBRT). The three 10MV-FFF SBRT plans (2 half rotating Rapid Arc, 9 to10 Gray/Fraction) were selected and irradiated in three different intervals (100%, 50% and 25%) using the RPM gating system. The plan verification was performed by the ArcCHECK for gamma analysis and the ionization chamber for point dose measurement. The dose delivery time of each interval were observed. For gamma analysis (2%&2mm criteria), the average percent pass of all plans for 100%, 50% and 25% intervals were 86.1±3.3%, 86.0±3.0% and 86.1±3.3%, respectively. For point dose measurement, the average ratios of each interval to the treatment planning were 1.012±0.015, 1.011±0.014 and 1.011±0.013 for 100%, 50% and 25% interval, respectively. The average dose delivery time was increasing from 74.3±5.0 second for 100% interval to 154.3±12.6 and 347.9±20.3 second for 50% and 25% interval, respectively. The same quality of the dose delivery from different Beam ON intervals in FFF-SBRT by TrueBEAM was illustrated. While the 100% interval represents the breath-hold treatment technique, the differences for the free-breathing using RPM gating system can be treated confidently.

Dosimetric Analysis of Respiratory-Gated RapidArc with Varying Gating Window Times

The gated RapidArc may produce a dosimetric error due to the stop-and-go motion of heavy gantry which can misalign the gantry restart position and reduce the accuracy of important factors in RapidArc delivery such as MLC movement and gantry speed. In this study, the effect of stop-and-go motion in gated RapidArc was analyzed with varying gating window time, which determines the total number of stop-and-go motions. Total 10 RapidArc plans for treatment of liver cancer were prepared. The RPM gating system and the moving phantom were used to set up the accurate gating window time. Two different delivery quality assurance (DQA) plans were created for each RapidArc plan. One is the portal dosimetry plan and the other is MapCHECK2 plan. The respiratory cycle was set to 4 sec and DQA plans were delivered with three different gating conditions: no gating, 1-sec gating window, and 2-sec gating window. The error between calculated dose and measured dose was evaluated based on the pass rate calculated using the gamma evaluation method with 3%/3 mm criteria. The average pass rates in the portal dosimetry plans were 98.72±0.82%, 94.91±1.64%, and 98.23±0.97% for no gating, 1-sec gating, and 2-sec gating, respectively. The average pass rates in MapCHECK2 plans were 97.80±0.91%, 95.38±1.31%, and 97.50±0.96% for no gating, 1-sec gating, and 2 -sec gating, respectively. We verified that the dosimetric accuracy of gated RapidArc increases as gating window time increases and efforts should be made to increase gating window time during the RapidArc treatment process.

SU-E-J-168: 4D-PET for Target Volume Delineation in Respiratory-Gated Radiation Therapy.

To find an optimized workflow for the use of respiratory-gated PET (4D-PET) in target volume delineation of tumors subject to respiratory-gated radiation therapy. 15 patients with lung (11) and pancreas (4) tumors who had FDG-PET-CT for target delineation prior to EBRT were studied. Patients were selected among the group that showed respiratory-induced tumor motion 〉5mm. 4D-PET was performed by means of a Philips Gemini BigBore scanner, using the Varian RPM gating system. An identical system was available at the linac for treatment. The breathing cycle was equally divided in 4 phases, according to a previous study. Since planning was made on a single CT-phase, no ITV was explicitly built from the set of phases. The BTV was identified with SUV=2.2 threshold and the PTV was obtained expanding the BTV by 8mm(S-I), 5mm(A-P) and 3mm(L-R) to account for residual motion and setup errors. The most advantageous CT-phase for treatment planning was then identified by simulating plans on each phase and analyzing the resulting DVHs of OARs (lung, trachea, oesophagus, spinal cord, left ventricle). The observed maximum range of motion was 5.5mm(L-R), 12.3mm(A-P) and 19.2mm(S-I). The standard deviation of the BTV volume in the 4 phases ranged from 6% to 13.7%. V20 (lung) ranged 7.1%-15.2% in inspiration and 7.8%-18.6% in expiration. The mean dose to the oesophagus ranged 0.1-2.2Gy in inspiration and 1.4-2.0Gy in expiration. In general, the dose to OARs was smaller when planning on a single phase than on the overall, respiratory-uncontrolled volume (p-value〈0.05). The BTV volume was almost constant between phases, confirming that the motion might be described by 4 phases. There was no obvious choice of the optimal phase for treatment planning, suggesting patient-by-patient studies. However, planning and delivery on one phase consistently allowed dose sparing to be obtained compared to non-gated techniques.

MO‐G‐213AB‐02: A Gated IMRT Study Based on Comparative 2D and 3D Dose Measurements

Purpose: To perform a quantitative study of Varian RPM gating modes and parameters on a gated IMRT of esophagus cancer case using both 2D and 3D dose measurements. Methods: A 4DCT scan reconstructed into a 10‐phase series was acquired. For each of 7 IMRT fields, the target trajectory, due to respiratory and also cardiac motion, perpendicular to the beam axis was recorded. An alternative set of smoothed trajectories was also processed to form closed hysteresis‐like curves. Each trajectory was replicated using a MotionSim phantom in the plane perpendicular to beam axis. Motion was correlated to the position of a surrogate platform, monitored using a reflective marker block and RPM gating system. Dose planes were measured using a MapCheck2 detector placed upon the MotionSim phantom. Dynamic delivery was performed for non‐gated, narrow window (2mm or 40–60% in amplitude or phase of breathing cycle, respectively) and wide window (5mm or 30–70% in amplitude or phase of breathing cycle, respectively) gated delivery. Corresponding 3D dose distributions were obtained using 2D maps of dose difference between reference and measurement, backprojected in the direction of the given field onto the original reference 3D dose distribution. Results: Results based on 2D dose analysis do not always correlate with results based on reconstructed 3D doses, e.g. using narrow gating was found superior to wide gating based on conventional gamma 2D analysis. However, CTV DVH analysis did not show a significant difference. On other hand, both 2D and 3D approaches favour phase‐over amplitude‐based gating. Both methods also showed better results for adjusted smoother trajectories. Conclusions: Interpreting dose measurements using three‐dimensional reconstructed dose directly on patient model and volume structures provides more relevant information than conventional 2D dose‐to‐dose approaches and has potential to uncover different trends and the magnitude of a given effect as indicated in this study.

SU‐E‐T‐452: Impact of Abdominal Compression on the Tumor Motion in the Treatment of NSCLC Using Stereotactic Body Radiosurgery

Involuntary motion of the tumor during the treatment of NSCLC using stereotactic body radiosurgery presents its own unique challenges. In this study, we quantify the impact of abdominal compression technique to minimize tumor motion as a function of location of tumor in a patient. 25 patients (5 in each lung lobe viz. RUL, RML, RLL, LUL and LLL) were retrospectively analyzed. A 4DCT study encompassing the tumor was used on a 16 slice GE CT scanner along with Varian RPM gating system. The images were retrospectively binned in 10 phases. The motion of tumor was analyzed phase by phase in transverse, sagittal & coronal projections of the 3D image. This gave us 2 values for anterior-posterior, superior-inferior and lateral-medial motions each and was averaged out. Analysis of data reveals that the motion (mean ± 1SD) in the superior-inferior direction was 1.8 ± 0.9, 4.2±2.8, 7.4±2.3, 1.5±0.8, 3.1±2.8 mm for tumors located in RUL, RML, RLL, LUL and LLL, respectively. Along the anterior-posterior direction the respective values were 2.5 ± 1.9, 2.4 ± 1.1, 2.7 ± 1.3, 1.4 ± 0.2 and 1.8 ± 1 mm. Similarly, along the lateral direction, the respective values were 2.1 ± 1.1, 1.9 ± 1.1, 1.6 ± 1.3, 1.1 ± 0.3 and 1.8 ± 0.7 mm. When the data was analyzed removing the location of the tumor in the thorax, the median displacement along the superior-inferior, anterior-posterior, lateral direction was 2.3, 1.6 and 1.5 mm, respectively while the respective maximum value were 9.2, 5 & 3.8 mm. The use of abdominal compression provides a simple inexpensive yet easily tolerable device to control the motion of the tumor in the management of NSCLC for SBRT treatments. This will allows the treatment of the tumor without resorting to complex and time consuming gated treatments.

Video-coaching as biofeedback tool to improve gated treatments: Possibilities and limitations

For respiratory gated radiotherapy the manufacturers of linear accelerators offer dedicated gating technologies. The video-based Varian RPM Gating system (Varian Medical Systems, Palo Alto/CA, USA) includes in a standard configuration a support tool for regular breathing called audiocoaching. As this approach has limitations regarding direct control of the patient's breathing due to a missing feedback, we designed an additional tool offering videocoaching. In order to evaluate the impact of this additional functionality, we measured parameters defining the image quality of 4D-CT data as well as the treatment duration which is mainly influenced by the patient's limited ability to achieve a stable breathing pattern.

SU-FF-J-102: Determining Residual Target Motion and Gating Window Width From Gated CT for Routine Gated Delivery

Purpose: Respiratory gating is widely used to reduce errors in target volume in planning CT and errors in treatment delivery. In selecting a patient-specific gating window width for gated treatment, the planner needs to correlate the magnitude of external motion signal to that of the 3D target motion in order to determine the magnitude of residual motion. We report our experience in determining patient-specific residual variation in routine clinics. Material and Methods: Amplitude-gated CT scans were performed on 96 patients with lung cancer and 32 patients with liver cancer using Varian RPM gating system. Two gated CT scans with gating windows at the end of expiration and end of inspiration were acquired. The two gated CTs were registered to determine the two extreme positions of the target. The amplitude of the external motion signals between end of expiration and inspiration was correlated to these two extreme positions. The magnitude of residual external motion during CT beam-on time was converted to 3D target residual motion. Results: In 96 lung cancer patients, significant target motions were found in inferior (84 patients, 3–18 mm, median 7), anterior (57 patients, 3–20 mm, median 5 mm), posterior (23 patients, 3–15 mm, median 6 mm), left (30 patients, 3–15 mm, median 5 mm), right (28 patients, 3–15 mm, median 5 mm) direction. In 32 liver cancer patients, significant target motion were found in inferior (32 patients, 5–20 mm, median 10), anterior (20 patients, 3–13 mm, median 7 mm), right (13 patients, 3–10 mm, median 7 mm) directions. Residual motion during CT beam-on time ranged 14–32% of the excursion. This information was used to determine gating window width for delivery and motion margin. Conclusion: Intrafractional residual target motion is not negligible and can be routinely addressed with this method.

SU‐FF‐J‐59: Adaptive Gating of Radiotherapy Treatment Based On Marker‐Less Tracking of Lung Tumors with KV Fluoroscopy

Purpose: We investigated using kV fluoroscopy for adapting the respiratory gating window prior to each radiation treatment fraction. The gate adaptation was based on marker‐less tracking of the target and correlating the internal motion trace with the external breathing signal. Method and Materials: A phantom study was performed with the on‐board kV imager of the Novalis Tx machine. A dynamic chest phantom with a water‐filled sphere placed inside a movable lung density insert was utilized. Breathing traces from patients that underwent lung SBRT were used to drive the target and external surrogate motion. The Varian RPM gating system was employed to obtain an external breathing signal, while acquiring kV fluoroscopy images. The moving target was tracked and an internal signal was generated. One kV acquisition was used for adapting the gating window based on the correlation of the internal and external signal and another one for verifying the gated position of the target. Gating at end‐exhale is the most commonly used technique in the clinic, while gating at mid‐ventilation makes it more likely that the tolerance window for residual motion includes the tumor during the entire breathing cycle. The two gating windows were investigated and the results were compared. Results: No significant difference in the gated target position errors was observed between the two gating windows. The target was in the expected position about 90% of the time for each one. The maximum deviation outside the tolerance window for residual motion was 1.9mm for the end‐exhale and 2.2mm for the mid‐motion gating windows. Conclusion: The adapt and verify procedure examined in this study, if applied in real‐time, has the potential to improve the accuracy of lung cancer radiotherapy by facilitating the reduction of safety margins used for moving tumors.Conflict of Interest: Varian Medical Systems, Inc.

SU‐GG‐T‐142: Organ Motional Effect On the Dose Distribution in Intensity Modulated Radiation Therapy

Purpose: The respiratory organ motional effect on the dosimetric accuracy in IMRT was analyzed and the function of RPM gating system in reducing the organ motional effect was evaluated with the respiration simulating phantom. Method and Materials: The phantom system which can simulate respiratory organ motion was manufactured. Total 4 IMRT plans for liver cancer were evaluated in the 3 cases of situation that were a static status, a no‐gated free‐breathing motion and a gated radiotherapy with RPM system. The motional range was set 1.6 cm considering the real movement of tumor in the fluoroscopic images and respiratory movement cycle was fixed to 5 sec. The dosimetric accuracy of all the fields of IMRT plan was analyzed with MapCHECK (SunNuclear, USA) device laid on the moving table of the phantom. The comparison analysis between a dose distribution in plan and a real one in treatment was done with a coronal dose distribution in the depth of diode detector arrays in the MapCHECK. The decision of pass in each measurement points was verified by a gamma evaluation in the criteria of 3 mm distance and 3% dose. Results: The average pass rates in each motional status are 98.4% at static case, 49.9% at motional case and 97.9% at gated IMRT case. These data shows that the RPM gated radiation therapy decreased a large amount of dosimetric error due to the organ motional effect in IMRT. Conclusion: The considerable dosimetric error due to organ motion was occurred even in the motional range (1.6 cm) of normal breathing condition. This organ motional error could be reduced with a RPM gating system and the gated radiation therapy with a proper gated phase should be performed for the IMRT of abdominal lesions which are moved by a respiration.

TU‐FF‐A2‐06: The Verification of Respiratory‐Gated Radiation Treatments Using 3D Dosimetry Techniques

Purpose: This work investigates the feasibility of verifying respiratory gated treatments using a new 3D dosimetry system consisting of a radiochromic plastic (PRESAGE™) with optical‐CT readout. Method and Materials: Three identical PRESAGE™ dosimeters (5cm radius × 5cm length) were manufactured for insertion into a CIRS' Dynamic Thorax Phantom. CT scans of the phantom were acquired corresponding to 3 motion scenarios; (1) static CT (no motion), (2) moving non‐gated CT (free‐breathing), and (3) moving gated CT. In the moving scans, the PRESAGE™ insert sinusoidally oscillated in the sup‐inf direction with a 3cm amplitude and 4.5sec period. A simple plan with AP/PA parallel opposed beams was created in the Eclipse planning system to treat a simulated small lung nodule (1.5cm), central to the insert, for all 3 scenarios. The simple plan enabled accurate independent measurement using EBT films. Each plan was then delivered to the phantom twice, first containing the PRESAGE™ insert and second the EBT insert. The gated treatments were delivered using the Varian RPM gating system. The dose recorded in the PRESAGE™ dosimeters was determined by optical‐CT using a novel, fast, in‐house CCD based scanner, which acquired all data in ∼10min. The doses among PRESAGE™, EBT, and Eclipse were compared. Results: In the cases of static and moving gated treatments, excellent agreement (<1mm distance‐to‐agreement) was observed between the PRESAGE™ and EBT measurements. For the moving non‐gated treatment, good agreement was again observed between PRESAGE™ and EBT. Significant differences are seen between the PRESAGE™/EBT films and the Eclipse distribution due to the modeling effects of penumbral broadening at small fields. Conclusion: The combination of small field sizes and sophisticated gating techniques represents a challenging scenario for dosimetry verification. The PRESAGE™ /optical‐CT 3D dosimetry system is demonstrated to achieve high quality, fast, relative dosimetry verification of gating treatments.

SU‐FF‐I‐33: A Graphical and Quantitative Procedure for Amplitude‐Gated Treatment Planned Using Phase‐Based 4DCT Using the Varian RPM System

Purpose: A graphical and quantitative procedure for amplitude‐gated treatment planned using phase based 4DCT of free breathing patients using the Varian RPM gating system has been developed as a part of the study of phase and amplitude based imaging and treatment. There are several phase‐based CT scanning systems that produce phase‐based images for gated treatments, but for free breathing phase may not be a reliable predictor of tumor location. Good correlation has been shown between abdomen height and diaphragm position, making it a promising treatment modality. Method and Materials: A phase‐based retrospective CT scan from a Philips large bore 16 slice CT scanner is obtained and the phase markers are adjusted to delineate actual breaths. A gating window that is a subset of the whole CT scan is set and a visual representation of gated motion called a MIP(maximum intensity projection) is created to visualize any anatomical excursions. The Varian RPM data is then compared with the scan data to quantify the actual marker block motion(or chest height) during the gating window as well as for an entire phase. The scan data is then presented not only as a percentage of an average breath, but as a chest height range within the total. This information is checked on treatment days and adjusted if necessary. The gating signal and fluoroscopy are compared to a 4D DRR generated from the MIP to check for geographic as well as systematic misses. Before treatment a gated port film (gated over several breaths and dose limited) can be captured to verify gating of the treatment beam. Results: For phantom based studies of regular breathing the method is robust. The model converts phase to amplitude of abdomen height. Conclusion: This method can validate a gated treatment using different imaging modalities.

SU‐FF‐T‐174: A New Device for the Verification of Temporal Function of the 4DCT and Gating Delivery System

Purpose: A quality assurance device was developed for 4DCT (GE Medical System, Waukesha, Wisconsin) and the Real‐Time Position Management Respiratory Gating System (Varian Medical Systems, Palo Alto, CA). It verifies the temporal‐phase function of the 4DCT and the gated delivery function of the Linac. Method and Materials: The QA device is composed of a mobile phantom, static phantom, acrylic lobe, AC motor, power transformer and transmission mechanism. There are eight radio‐opaque lines embedded in the static phantom, which are used as reference indicators. A “Z” shape radio‐opaque marker is fixed on the top of the mobile. This phantom moves periodically with a maximum displacement of 3 cm in a horizontal direction. It was scanned under our 4DCT at a period of 5 seconds. The collected images were divided into 10 groups representing 10 phases of the moving cycle. The digitally reconstructed radiograph of each phase was generated for the verification of the temporal‐phase function of the 4DCT. To verify the gated delivery function of the Linac, a radiographic film was attached to the surface of the mobile phantom and a 0.5 cm by 10 cm X‐ray field was used to expose the film at certain phases. The expected position of 0% phase was marked by punching 2 small holes on the film. Results: The “Z” mark was correctly shown and aligned with the reference lines at each phase. The film exposed under the Linac has shown that the radiation had been delivered to the correct position. Conclusion: A reliable quality assurance device for 4DCT and respiratory gating system is necessary to ensure that the radiation dose is accurately delivered to the patient's target volume. This QA device is effective and convenient for checking temporal‐phase function on the 4DCT and RPM gating system.

SU-FF-J-05: A Novel 4-D CT Acquisition Protocol to Validate Respiratory Gating with the RPM System

Purpose: To introduce a method in acquiring a 4‐D CT data set without the use of external markers, to validate the use of the RPM Gating System, and to present the results from the first lungcancer patient treated with this method. Method and Materials: Patients were scanned on a GE Multi‐Slice CT Scanner. Registration of the images was based on internal correlation at tissue interfaces between successive respiratory phases. This resulted in a 4‐D CT data set acquired without the use of external markers. Regions of interest (ROI) were also constructed on an axial CT slice containing the chest wall, the tumor, and any other critical structure that was affected by respiratory motion. The mean CT number in each ROI was plotted as a function of mid‐scan time. The RPM signal was also plotted over the same mid‐scan time interval and the sample correlation coefficient, r, of each ROI and RPM curve was determined. Results: 4‐D CT improved the quality of images affected by respiratory motion. For our first patient treated, the sample correlation coefficient was calculated for all combinations of the tumor, chest wall, liver, and the signal acquired by the RPM and the magnitude ranged between 0.897 and 0.982. This suggested that the RPM could predict tumor/internal organ motion determined by the 4‐D CT data set. Electronic portal images were taken at the treatment phase and matched well with digitally reconstructed radiographs taken at the same phase. Portal images of the posterior field used in treatment were also taken in cine loop format during three different treatment fractions throughout treatment and showed no changes in anatomy at the treatment phase over the treatment period. Conclusion: The RPM system is an effective tool for respiratory gating if treatment planning and verification is based on 4‐D CT.

SU‐FF‐J‐52: Breathing Adapted Radiotherapy (BART) Using Modern Gating Technologies

Purpose: The aim of this study was to evaluate the impact of the Varian RPM‐gating™ System for gated treatment of breast cancer/thoracic wall cancer as well as Varian On‐Board Imager™ (OBI) Fluoroscopic Pretreatment Setup Verification for gated treatment of lung cancers/upper intestinal tract cancers. Method and Materials: A passive, infrared light reflecting marker is placed on the patient's chest wall over the xiphoid process. The vertical motion of the marker, i.e. the breathing excursion, is tracked by an infrared sensitive video‐camera‐based hardware. The system is used for acquisition of the CT‐Scan (4D‐CT) as well as the treatments; for the latter it manages the controlled switching of the radiation beam during a pre‐selected specific phase of the respiratory cycle. The additional Fluoroscopic Setup Verification allows a fluoroscopy analysis of inner target movement with regard to a chosen therapeutic window of the RPM Gating system just prior to treatment. Results: Varian RPM‐gating technology optimizes the dose delivery to regions with respiration‐induced movements of the target. Safety margins and thereby irradiated critical volumes can be significantly reduced for breast cancer with RPM‐gating as well as lung cancer with the OBI Fluoroscopic Pretreatment Setup Verification. Conclusion: Gating offers the possibility to apply doses with higher accuracy. As typical duty cycles are around 50% ore even 10%, the overall beam‐on time is prolonged by a factor of up to 4. In order to compensate for this effect, a dose rate change from 300 to 600 MU/min when using enhanced dynamic wedges was chosen; in case of electronic compensation a dose rate change is not needed. The additional fluoroscopy mode offers for the very first time the possibility to verify patient positioning with regard to the moving target, which ‐ for most cases — shows a phase shift relative to the RPM marker box movement.