Patient Breathing Cycle Research Articles

Individualized breathing trace quality assurance for lung radiotherapy patients undergoing 4DCT simulation.

4DCT simulation is a popular solution for radiotherapy simulation of lung cancer patients as it allows the clinician to gain an appreciation for target motion during the patient breathing cycle. Resultant binning of images and production of the 4DCT dataset relies heavily on the recorded breathing trace; but quality assurance is not routinely performed on these and there lacks any substantial recommendations thereof. An application was created for Windows in C# that was able to analyze the VXP breathing trace files from Varian RPM/RGSC and quantify various metrics associated with the patient breathing cycle. This data was then used to consider errors in voluming of targets for several example cases in order to justify recommendations on quality assurance. For 281 real patient breathing traces from 4DCT simulation of lung targets, notable differences were found between RGSC and application calculations of phase data. For any new patient without individualized QA, the average marked phase calculation (which is used for 4DCT reconstruction) is only accurate to within 19% of the actual phases. The error in BPM within the scan due to breathing rate variation is 37%. The uncertainty in amplitude due to breathing variation is 34% in the mean. Phase uncertainty leads to misbinning which we have shown can lead to missing 66% of the target for gated treatment. Variation in inhalation/exhalation level leads to voluming errors which, without individualized QA, can be assumed to be 11% (PTV is smaller than actual). Without individualized quality assurance of patient breathing traces, large uncertainties have to be assumed for metrics of both phase and amplitude, leading to clinically significant uncertainties in treatment. It is recommended to perform individualized quality assurance as this provides the clinician with an accurate quantification of uncertainty for their patient.

Respiratory motion correction in prostate cancer positron emission tomography: A study on patients and phantom simulation

Introduction: To investigate the effects of breathing cycle and tree diaphragm motions on prostate cancer tumors standard uptake value (SUV) during positron emission tomography (PET) and to correct it. Materials and methods: Respiratory motion traces were simulated on the common patient breathing cycle and tree diaphragm motions using the 4D NCAT phantom. Lesions with different activities and diameters were simulated in prostate. This study was conducted in two phases. In the first phase; the generated PET sinograms taken from GATE (Geant4 Application Tomography Emission) Monte Carlo simulations corrected using computed tomography attenuation correction involving the cycle. In the next phase; on the obtained images, the motion-correction was carried out using software for Tomographic Image Reconstruction (STIR). By considering respiration- averaged computed as a true value, the SUVs were measured and analyzed for different motions. Results: In uncorrected measurements, the mean, minimum and maximum SUVs were 0.4%, 0.09% and 2% respectively, whilst in corrected measurements, these value were reported as 3.5%, 0.7% and 14% respectively. Conclusion: The respiratory motion-induced errors in tumor quantification and delineation. The SUVs depending on the phase of the raspatory. Motion correction could enhance the SUVs in prostate cancer PET.

Motion Management in PET/CT: Technological Solutions

Motion due to patient's breathing can introduce heavy bias in PET/CT, both in image quality and quantitation. This paper is a review of the main technical solutions available to manage movement in PET/CT studies: a) Respiratory Gated (RG), b) Motion Free (MF), c) End Expiration (EE), d) Banana Artefact Management (BAM) and e) Data Driven Gating (DDG). The most diffused solutions (RG, MF and EE) are based on LIST mode acquisition of a PET Field of View (4D FOV), centered on the anatomical region of interest; to link PET data not only to time and to spatial position but also to the corresponding breathing phase, the synchronized acquisition of the patient's breathing curve is performed by an external tracking device. Different commercial tools to track and to record patient breathing cycle are available to associate the internal organ motion with a measurable external parameter; for example these systems can measure the pressure on a chest elastic belt, the air flow trough patient nose, the breath-in and breath-out air temperature or the markers movement on the thorax/ abdominal region. Recently DDG techniques are developed to correct respiratory motion without the help of external motion tracking devices and to obtain a comparable result to that based on standard RG protocols. The final result of an RG or DDG protocol is a sequence of 3D images showing organs and lesions movement; using the other motion management options a single 3D motion-free image is obtained without motion artefacts and degradation. Compared to the previously described options the BAM solution is not a real motion management protocol but just a Banana Artefact correction technique obtained using an Attenuation Correction Map calculated merging the Whole Body Helical CT with a Cine CT on the diaphragm area. The motion management in PET/CT imaging shows benefits in terms of image quality, quantification and lesion detectability and it is useful both in diagnostic and radiotherapy planning.

SU‐F‐J‐38: Dose Rates and Preliminary Evaluation of Contouring Similarity Metrics Using 4D Cone Beam CT

Purpose:4D imaging modalities require detailed characterization for clinical optimization. The On‐Board Imager mounted on the linear accelerator was used to investigate dose rates in a tissue mimicking phantom using 4D‐CBCT and assess variability of contouring similarity metrics between 4D‐CT and 4D‐CBCT retrospective reconstructions.Methods:A 125 kVp thoracic protocol was used. A phantom placed on a motion platform simulated a patient's breathing cycle. An ion chamber was affixed inside the phantom's tissue mimicking cavities (i.e. bone, lung, and soft tissue). A sinusoidal motion waveform was executed with a five second period and superior‐inferior motion. Dose rates were measured at six ion chamber positions. A preliminary workflow for contouring similarity between 4D‐CT and 4D‐CBCT was established using a single lung SBRT patient's historical data. Average intensity projection (Ave‐IP) and maximum intensity projection (MIP) reconstructions generated offline were compared between the 4D modalities. Similarity metrics included Dice similarity coefficient (DSC), Hausdorff distance, and center of mass (COM) deviation. Two isolated lesions were evaluated in the patient's scans: one located in the right lower lobe (ITVRLL) and one located in the left lower lobe (ITVLLL).Results:Dose rates ranged from 2.30 (lung) to 5.18 (bone) E‐3 cGy/mAs. For fixed acquisition parameters, cumulative dose is inversely proportional to gantry speed. For ITVRLL, DSC were 0.70 and 0.68, Hausdorff distances were 6.11 and 5.69 mm, and COM deviations were 1.24 and 4.77 mm, for Ave‐IP and MIP respectively. For ITVLLL, DSC were 0.64 and 0.75, Hausdorff distances were 10.74 and 8.00 mm, and COM deviations were 7.55 and 4.3 mm, for Ave‐IP and MIP respectively.Conclusion:While the dosimetric output of 4D‐CBCT is low, characterization is necessary to assure clinical optimization. A basic workflow for comparison of simulation and treatment 4D image‐based contours was established.This work was partially supported by a Research Scholar Grant (RSG‐15‐137‐01‐CCE) from the American Cancer Society.

Latency Characterization of Gated Radiotherapy Treatment Beams Using a PIN Diode Circuit

Background: Radiotherapy is based on the premise of accurate dose delivery to target volumes within a patient, while minimizing dose to surrounding tissues. Recent developments in the treatment of breast cancer have focused on “gating” the delivery of the treatment beams to minimize the effect of patient motion during treatment, and increasing separation between the target volume and organs at risk (OAR), such as lung, heart and left anterior descending coronary artery. The basic principle involves rapidly switching the treatment beam on or off depending on the patient breathing cycle. It is therefore important to know the characteristics of gated treatments such as latency.Methods: In this work an electrical PIN diode circuit (EPDC) was designed for quality assurance (QA) purposes to examine beam latency timing properties. Evaluation of the EPDC was performed on a TrueBeam™ (Varian, Palo Alto) linear accelerator and its internal gating system. The EPDC was coupled to a moving stage to simulate a binary pattern with fast beam triggering within predefined limits, the so called “gating window”. Pulses of radiation were measured with the PIN diode and the results were compared to measurements of current produced across the linac target. Processing of the beam pulses and calculation of the latency timings was performed by an Atmega328P microcontroller.Results: For beam-on latencies, 2.11 ms (6 MV) and 2.12 ms (10 MV) were measured using the PIN diode, compared to 2.13 ms (6 MV) and 2.15 ms (10 MV) using the target current signal. For beam-off latencies, 57.69 ms (6 MV) and 57.73 ms (10 MV) were measured using the PIN diode, compared to 57.33 ms (6 MV) and 56.01 ms (10 MV) using the target current.Conclusions: PIN diodes can be used for accurate determination of the beam-on and beam-off latency characteristics, which could potentially lead to improvements in gated radiotherapy treatments, for example optimizing the gating windows and in estimating dosimetric errors associated with treatment beam latencies.

SU‐E‐E‐11: Novel Matching Module for Respiration‐Gated Motion Tumor of Cone‐Beam Computed Tomography (CBCT) to 4DCT

Purpose:Four dimensional computed tomography (4DCT) scans reliably record whole respiratory phase and generate internal target volumes (ITV) for radiotherapy planning. However, image guiding with cone‐beam computed tomography (CBCT) cannot acquire all or specific respiratory phases. This study was designed to investigate the correlation between average CT and Maximum Intensity Projection (MIP) from 4DCT and CBCT.Methods:Retrospective respiratory gating were performed by GE Discovery CT590 RT. 4DCT and CBCT data from CRIS Dynamic Thorax Phantom with simulated breathing mode were analyzed. The lung tissue equivalent material encompassed 3 cm sphere tissue equivalent material. Simulated breathing cycle period was set as 4 seconds, 5 seconds and 6 seconds for representing variation of patient breathing cycle time, and the sphere material moved toward inferior and superior direction with 1 cm amplitude simulating lung tumor motion during respiration.Results:Under lung window, the volume ratio of CBCT scans to ITVs derived from 10 phases average scans was 1.00 ± 0.02, and 1.03 ± 0.03 for ratio of CBCT scans to MIP scans. Under abdomen window, the ratio of CBCT scans to ITVs derived from 10 phases average scans was 0.39 ± 0.06, and 0.06 ± 0.00 for ratio of CBCT scans to MIP scans. There was a significant difference between lung window Result and abdomen window Result. For reducing image guiding uncertainty, CBCT window was set with width 500 and level‐250. The ratio of CBCT scans to ITVs derived from 4 phases average scans with abdomen window was 1.19 ± 0.02, and 1.06 ± 0.01 for ratio of CBCT to MIP scans.Conclusion:CBCT images with suitable window width and level can efficiently reduce image guiding uncertainty for patient with mobile tumor. By our setting, we can match motion tumor to gating tumor location on planning CT more accurately neglecting other motion artifacts during CBCT scans.

SU-D-18A-01: Tumor Motion Tracking with a Regional Deformable Registration Model for Four Dimensional Radiation Treatment of Lung Cancer

Purpose: To develop a tumor motion model from four-dimensional computed tomography (4DCT) of thoracic patients and demonstrate its impact on 4D radiation therapy simulation. Methods: A regional deformable image registration algorithm was introduced to extract tumor motion out of patient's breathing cycle. The gross target volume (GTV) was manually delineated on a selected phase of 4DCT and a subregion with 10mm margin supplemented to the GTV was created on the Eclipse treatment planning system (Varian Medical Systems, Palo Alto, CA). Together with 4DCT the structures were exported into an inhouse research platform. A free form B-Spline deformable registration was carried out to map the subregion to other respiratory phases. The displacement vector fields were employed to propagate GTV contours with which the center of mass (CoM) of the GTV was computed for each breathing phase of 4DCT. The resultant GTV motion and its volumetric shape are utilized to facilitate 4D treatment planning. Five lung cancer patients undergoing stereotactic body radiation therapy were enrolled and their 4DCT sets were included in the study. Results: Application of the algorithm to five thoracic patients indicates that clinically satisfactory outcomes were achievable with a spatial accuracy better than 2mm for GTV contour propagation betweenmore » adjacent phases, and 3mm between opposite phases. The GTV CoM was found to be in the range of 2.0mm through 2.5cm, depending upon the tumor location. Compared to the traditional whole image based registration, the computation of the regional model was found to be an order of magnitude more efficient. Conclusion: A regional deformable registration model was implemented to extract tumor motion. It will have widespread application in 4D radiation treatment planning in the future to maximally utilize the available spatial-tempo information.« less

Patient‐specific quantification of respiratory motion‐induced dose uncertainty for step‐and‐shoot IMRT of lung cancer

The objective of this study was to quantify respiratory motion-induced dose uncertainty at the planning stage for step-and-shoot intensity-modulated radiation therapy (IMRT) using an analytical technique. Ten patients with stage II∕III lung cancer who had undergone a planning four-dimensional (4D) computed tomographic scan and step-and-shoot IMRT planning were selected with a mix of motion and tumor size for this retrospective study. A step-and-shoot IMRT plan was generated for each patient. The maximum and minimum doses with respiratory motion were calculated for each plan, and the mean deviation from the 4D dose was calculated, taking delivery time, fractionation, and patient breathing cycle into consideration. For all patients evaluated in this study, the mean deviation from the 4D dose in the planning target volume (PTV) was <2.5%, with a standard deviation <1.2%, and maximum point dose variation from the 4D dose was <6.2% in the PTV assuming delivery dose rate of 200 MU∕min and patient breathing cycle of 8 s. The motion-induced dose uncertainty is a function of motion, fractionation, MU (plan modulation), dose rate, and patient breathing cycle. Respiratory motion-induced dose uncertainty varies from patient to patient. Therefore, it is important to evaluate the dose uncertainty on a patient-specific basis, which could be useful for plan evaluation and treatment strategy determination for selected patients.

Forcing lateral electron disequilibrium to spare lung tissue: a novel technique for stereotactic body radiation therapy of lung cancer

Stereotactic body radiation therapy (SBRT) has quickly become a preferred treatment option for early-stage lung cancer patients who are ineligible for surgery. This technique uses tightly conformed megavoltage (MV) x-ray beams to irradiate a tumour with ablative doses in only a few treatment fractions. Small high energy x-ray fields can cause lateral electron disequilibrium (LED) to occur within low density media, which can reduce tumour dose. These dose effects may be challenging to predict using analytic dose calculation algorithms, especially at higher beam energies. As a result, previous authors have suggested using low energy photons (<10 MV) and larger fields (>5 × 5 cm2) for lung cancer patients to avoid the negative dosimetric effects of LED. In this work, we propose a new form of SBRT, described as LED-optimized SBRT (LED-SBRT), which utilizes radiotherapy (RT) parameters designed to cause LED to advantage. It will be shown that LED-SBRT creates enhanced dose gradients at the tumour/lung interface, which can be used to manipulate tumour dose, and/or normal lung dose. To demonstrate the potential benefits of LED-SBRT, the DOSXYZnrc (National Research Council of Canada, Ottawa, ON) Monte Carlo (MC) software was used to calculate dose within a cylindrical phantom and a typical lung patient. 6 MV or 18 MV x-ray fields were focused onto a small tumour volume (diameter ∼1 cm). For the phantom, square fields of 1 × 1 cm2, 3 × 3 cm2, or 5 × 5 cm2 were applied. However, in the patient, 3 × 1 cm2, 3 × 2 cm2, 3 × 2.5 cm2, or 3 × 3 cm2 field sizes were used in simulations to assure target coverage in the superior–inferior direction. To mimic a 180° SBRT arc in the (symmetric) phantom, a single beam profile was calculated, rotated, and beams were summed at 1° segments to accumulate an arc dose distribution. For the patient, a 360° arc was modelled with 36 equally weighted (and spaced) fields focused on the tumour centre. A planning target volume (PTV) was generated by considering the extent of tumour motion over the patient's breathing cycle and set-up uncertainties. All patient dose results were normalized such that at least 95% of the PTV received at least 54 Gy (i.e. D95 = 54 Gy). Further, we introduce ‘LED maps’ as a novel clinical tool to compare the magnitude of LED resulting from the various SBRT arc plans. Results from the phantom simulation suggest that the best lung sparing occurred for RT parameters that cause severe LED. For equal tumour dose coverage, normal lung dose (2 cm outside the target region) was reduced from 92% to 23%, comparing results between the 18 MV (5 × 5 cm2) and 18 MV (1 × 1 cm2) arc simulations. In addition to reduced lung dose for the 18 MV (1 × 1 cm2) arc, maximal tumour dose increased beyond 125%. Thus, LED can create steep dose gradients to spare normal lung, while increasing tumour dose levels (if desired). In the patient simulation, a LED-optimized arc plan was designed using either 18 MV (3 × 1 cm2) or 6 MV (3 × 3cm2) beams. Both plans met the D95 dose coverage requirement for the target. However, the LED-optimized plan increased the maximum, mean, and minimum dose within the PTV by as much as 80 Gy, 11 Gy, and 3 Gy, respectively. Despite increased tumour dose levels, the 18 MV (3 × 1 cm2) arc plan improved or maintained the V20, V5, and mean lung dose metrics compared to the 6 MV (3 × 3 cm2) simulation. We conclude that LED-SBRT has the potential to increase dose gradients, and dose levels within a small lung tumour. The magnitude of tumour dose increase or lung sparing can be optimized through manipulation of RT parameters (e.g. beam energy and field size).

Respiratory-Induced Errors in Tumor Quantification and Delineation in CT Attenuation-Corrected PET Images: Effects of Tumor Size, Tumor Location, and Respiratory Trace: A Simulation Study Using the 4D XCAT Phantom

We investigated the magnitude of respiratory-induced errors in tumor maximum standardized uptake value (SUVmax), localization, and volume for different respiratory motion traces and various lesion sizes in different locations of the thorax and abdomen in positron emission tomography (PET) images. Respiratory motion traces were simulated based on the common patient breathing cycle and three diaphragm motions used to drive the 4D XCAT phantom. Lesions with different diameters were simulated in different locations of lungs and liver. The generated PET sinograms were subsequently corrected using computed tomography attenuation correction involving the end exhalation, end inhalation, and average of the respiratory cycle. By considering respiration-averaged computed tomography as a true value, the lesion volume, displacement, and SUVmax were measured and analyzed for different respiratory motions. Respiration with 35-mm diaphragm motion results in a mean lesion SUVmax error of 24%, a mean superior inferior displacement of 7.6mm and a mean lesion volume overestimation of 129% for a 9-mm lesion in the liver. Respiratory motion results in lesion volume overestimation of 50% for a 9-mm lower lung lesion near the liver with just 15-mm diaphragm motion. Although there are larger errors in lesion SUVmax and volume for 35-mm motion amplitudes, respiration-averaged computed tomography results in smaller errors than the other two phases, except for the lower lung region. The respiratory motion-induced errors in tumor quantification and delineation are highly dependent upon the motion amplitude, tumor location, tumor size, and choice of the attenuation map for PET image attenuation correction.

SU‐E‐T‐365: Respiratory Motion‐Induced Dose Uncertainty in Step‐And‐Shoot IMRT

Purpose: The objective of this study was to quantify respiratory motion induced dose uncertainty at the planning stage for step and shoot intensity modulated radiation therapy (IMRT) using a novel analytical technique. Methods: Ten patients with stage II/III lung cancer who had undergone a planning 4DCT scan and step and shoot IMRT planning were selected with a mix of motion and tumor size for this study. The maximum and minimum doses with respiratory motion were calculated for each plan, and the mean deviation from the 4D dose was calculated, taking delivery time, fractionation, and patient breathing cycle into consideration. Results: For all patients evaluated in this study, the mean difference between the total delivered dose and the 4D dose in the planning target volume (PTV) was &lt;2.5%, with a standard deviation &lt;1.2%, and maximum point dose variation from the 4D dose was &lt;6.2% in the PTV with delivery dose rate (DR) of 200 MU/min and patient breathing cycle of 8 seconds. The motion induced dose uncertainty is a function of fractionation, MU (plan modulation), dose rate, and patient breathing cycle. Conclusion: Respiratory motion induced dose uncertainty varies from patient to patient. Patient specific respiratory motion induced dose uncertainty quantification could be useful for plan evaluation and treatment strategy determination.

SU‐E‐J‐176: A Simple 4D MRI Technique for Radiation Therapy Planning

Purpose: 4D CT is routinely performed for radiation therapy planning when patient motion is a significant factor. As more benefits of using MRI become clearer in treatment planning, the feasibility of 4D MRI, however, has been questionable. The challenge of 4D MRI is the balance between satisfactory image quality and the speed to capture patient breathing cycle. So far, neither the image quality nor the speed has been satisfactory. Here, a simple MRI technique to do 4D MRI is introduced. Methods: 3T MRI (Philips Ingenia) was used for scanning. For monitoring breathing cycles, a regular respiratory belt was used. The breathing cycle was then divided into multiple phases. For each breathing phase, a predefined respiratory trigger time was set and the multi‐slice (30 slices) MRI scan was performed for each breathing phase. A single‐shot fast spin echo with partial k‐space filling was used with varied trigger time from 0 up to the length of breathing cycle. Results: As seen in Fig 1 (1 of 30 slices), the image quality for each breathing phase is quite good. The line profiles from the line ROI in Fig 1, which was placed over the liver dome, are shown in Fig 2 for a few different breathing phases. The position of the line was fixed across the breathing phases to see the displacement caused by breathing. This demonstrates the liver dome displacement (along the x axis) during breathing cycles. Conclusion: The 4D MRI for treatment planning can be done with satisfactory imaging quality and speed with a regular respiratory triggering technique, which can be directly implemented on most MRI devices without the need of extra device. However, since only a brief time‐window during each breathing cycle is used for acquisition, total acquisition time may be longer (∼30 min), and the improvement is undergoing.

Self‐adapting cyclic registration for motion‐compensated cone‐beam CT in image‐guided radiation therapy

In image-guided radiation therapy an additional kV imaging system next to the linear particle accelerator provides information for an accurate patient positioning. However, the acquisition time of the system is much longer than the patient's breathing cycle due to the low gantry rotation speed. Our purpose is a cyclic registration in the context of motion-compensated image reconstruction that provides high quality respiratory-correlated 4D volumes for on-board flat panel detector cone-beam CT scans. Based on the small motion assumption, widely used within registration algorithms, a strategy is developed for motion estimation. In this strategy temporal restrictions are incorporated, for example, the cyclic motion patterns of respiration. The resultant cyclic registration method is to show less sensitivity on image artifacts, in particular on artifacts due to projection data sparsification. Using a new cyclic registration method a motion estimation is performed on respiratory-correlated reconstructions, and the obtained motion vector fields are used for motion compensation. The proposed cyclic registration is evaluated in the context of motion-compensated image reconstruction using simulated data and patient data. Motion artifacts of 3D standard reconstructions can be significantly reduced by the resulting cyclic motion compensation. The method outperforms the respiratory-correlated reconstructions regarding sparse-view artifacts and maintains the high temporal resolution at the same time. Image artifacts show only minor to almost no effect on the motion estimation using the cyclic registration. The cyclic motion compensation approach provides respiratory-correlated volumes with high image quality. The cyclic motion estimation is of such low sensitivity to sparse-view artifacts, that it is capable to determine high quality motion vector fields based on reconstructions of low sampled data.

A real-time respiration position based passive breath gating equipment for gated radiotherapy: A preclinical evaluation

To develop a passive gating system incorporating with the real-time position management (RPM) system for the gated radiotherapy. Passive breath gating (PBG) equipment, which consists of a breath-hold valve, a controller mechanism, a mouthpiece kit, and a supporting frame, was designed. A commercial real-time positioning management system was implemented to synchronize the target motion and radiation delivery on a linear accelerator with the patient's breathing cycle. The respiratory related target motion was investigated by using the RPM system for correlating the external markers with the internal target motion while using PBG for passively blocking patient's breathing. Six patients were enrolled in the preclinical feasibility and efficiency study of the PBG system. PBG equipment was designed and fabricated. The PBG can be manually triggered or released to block or unblock patient's breathing. A clinical workflow was outlined to integrate the PBG with the RPM system. After implementing the RPM based PBG system, the breath-hold period can be prolonged to 15-25 s and the treatment delivery efficiency for each field can be improved by 200%-400%. The results from the six patients showed that the diaphragm motion caused by respiration was reduced to less than 3 mm and the position of the diaphragm was reproducible for difference gating periods. A RPM based PBG system was developed and implemented. With the new gating system, the patient's breath-hold time can be extended and a significant improvement in the treatment delivery efficiency can also be achieved.

The precision of respiratory-gated delivery of synchrotron-based pulsed beam proton therapy

A synchrotron-based proton therapy system operates in a low repetition rate pulsed beam delivery mode. Unlike cyclotron-based beam delivery, there is no guarantee that a synchrotron beam can be delivered effectively or precisely under the respiratory-gated mode. To evaluate the performance of gated synchrotron treatment, we simulated proton beam delivery in the synchrotron-based respiratory-gated mode using realistic patient breathing signals. Parameters used in the simulation were respiratory motion traces (70 traces from 24 patients), respiratory gate levels (10%, 20% and 30% duty cycles at the exhalation phase) and synchrotron magnet excitation cycles (Tcyc) (fixed Tcyc mode: 2.7, 3.0–6.0 s and each patient breathing cycle, and variable Tcyc mode). The simulations were computed according to the breathing trace in which the proton beams were delivered. In the shorter fixed Tcyc (<4 s), most of the proton beams were delivered uniformly to the target during the entire expiration phase of the respiratory cycle. In the longer fixed Tcyc (>4 s) and the variable Tcyc mode, the proton beams were not consistently delivered during the end-expiration phase of the respiratory cycle. However we found that the longer and variable Tcyc operation modes delivered proton beams more precisely during irregular breathing.

Inverse planning for four‐dimensional (4D) volumetric modulated arc therapya)

To develop a 4D volumetric modulated arc therapy (VMAT) inverse planning framework. 4D VMAT inverse planning aims to derive an aperture and weight modulated arc therapy treatment plan that optimizes the accumulated dose distribution from all gantry angles and breathing phases. Under an assumption that the gantry rotation and patient breathing are synchronized (i.e., there is a functional relationship between the phase of the patient breathing cycle and the beam angle), the authors compute the contribution from different respiration phases through the registration of the phased CT images. The accumulative dose distribution is optimized by iteratively adjusting the aperture shape and weight of each beam through the minimization of the planning objective function. For comparison, traditional 3D VMAT plans are also performed for the two cases and the performance of the proposed technique is demonstrated. A framework for 4D VMAT inverse planning has been proposed. With the consideration of the extra dimension of time in VMAT, a tighter target margin can be achieved with a full duty cycle, which is otherwise not achievable simultaneously by either 3D VMAT optimization or gated VMAT. The 4D VMAT planning formulism proposed here provides useful insight on how the "time" dimension can be exploited in rotational arc therapy to maximally compensate for the intrafraction organ motion.

Measurement of time delay for a prospectively gated CT simulator

For the management of mobile tumors, respiratory gating is the ideal option, both during imaging and during therapy. The major advantage of respiratory gating during imaging is that it is possible to create a single artifact-free CT data-set during a selected phase of the patient's breathing cycle. The purpose of the present work is to present a simple technique to measure the time delay during acquisition of a prospectively gated CT. The time delay of a Philips Brilliance BigBore™ (Philips Medical Systems, Madison, WI) scanner attached to a Varian Real-Time Position Management™ (RPM) system (Varian Medical Systems, Palo Alto, CA) was measured. Two methods were used to measure the CT time delay: using a motion phantom and using a recorded data file from the RPM system. In the first technique, a rotating wheel phantom was altered by placing two plastic balls on its axis and rim, respectively. For a desired gate, the relative positions of the balls were measured from the acquired CT data and converted into corresponding phases. Phase difference was calculated between the measured phases and the desired phases. Using period of motion, the phase difference was converted into time delay. The Varian RPM system provides an external breathing signal; it also records transistor-transistor logic (TTL) ‘X-Ray ON’ status signal from the CT scanner in a text file. The TTL ‘X-Ray ON’ indicates the start of CT image acquisition. Thus, knowledge of the start time of CT acquisition, combined with the real-time phase and amplitude data from the external respiratory signal, provides time-stamping of all images in an axial CT scan. The TTL signal with time-stamp was used to calculate when (during the breathing cycle) a slice was recorded. Using the two approaches, the time delay between the prospective gating signal and CT simulator has been determined to be 367 ± 40 ms. The delay requires corrections both at image acquisition and while setting gates for the treatment delivery; otherwise the simulation and treatment may not be correlated with the patient's breathing.

SU-FF-J-117: Real-Time Characterization of Respiratory Motion Regularity During Radiation Therapy

Purpose: To develop a real‐time system to restrict gated imaging and treatmentdelivery to regular portions of a patient's breathing cycle thereby improving the accuracy of respiratory gated radiotherapy.Methods and materials: A real‐time system was developed based on a first order Fourier spectral analysis of respiratory patterns from 57 lungcancer patients. An initial training length (TL) of breathing was used to establish the reference spectral parameters (full width at half maximum, peak amplitude, and peak frequency). Deviations in breathing were characterized in real‐time by comparing reference spectral parameters to acquired parameters over a sliding window (signal history length SHL) and were characterized as either “regular” or irregular. The effect of different SHLs (1, 2 , 3 respiratory cycles) and TLs (30, 60, 120 seconds) on the accuracy of respiratory managed radiotherapy was determined by examining the standard deviation of the extrema of motion of an external fiducial in portions of the respiratory trace during “regular” and “irregular” breathing. Results: Standard deviations, in cm, of values of peak inhalation for “regular” respiration were (TL30 ≫ 0.15, 0.13, 0.14), (TL60 ≫ 0.16, 0.13, 0.14) and (TL120 ≫ 0.15, 0.21, 0.17) for SHL of 1, 2 and 3, respectively. Corresponding values for “irregular” portions were (TL30 ≫ 0.16, 0.16, 0.15), (TL60 ≫ 0.16, 0.16, 0.15) and (TL120 ≫ 0.18, 0.18, 0.16). For exhalation the values for “regular” respiration were (TL30 ≫ 0.05, 0.05, 0.06), (TL60 ≫ 0.05, 0.06, 0.06) and (TL120 ≫ 0.05, 0.05, 0.07) for SHL of 1, 2 and 3, respectively. Corresponding values for “irregular” portions were (TL30 ≫ 0.18, 0.19, 0.18), (TL60 ≫ 0.20, 0.19, 0.18) and (TL120 ≫ 0.25, 0.23, 0.22). Conclusion: TL of 60 seconds and a SHL of 2 cycles resulted in the lowest variation in external fiducial position during “regular” respiration.

WE‐E‐BRB‐03: Precision of Respiratory‐Gated Delivery of Synchrotron‐Based Proton Therapy

Purpose: To investigate the precision of synchrotron based passively scattered respiratory gated delivery of proton irradiation. A simulation study was undertaken for the analysis of the residual target motion uncertainty during synchrotron based respiratory gated proton treatment. Method and Materials: In‐house software was developed to simulate a synchrotron based respiratory gated proton treatment. The interactions of respiratory motion traces (70 traces, 22 patients), respiratory gate threshold levels (10%, 20% and 30% duty cycle around peak exhalation) and synchrotron Tcyc patterns (fixed Tcyc =2.7, 3.0 ∼ 6.0 second, average patient breathing‐cycle and variable Tcyc) were plotted along the same time scale, similar to an oscilloscope display. Proton beam delivery within a gate threshold only occurred during a portion of each Tcyc pattern. A specific pattern of Tcyc acts as a “gate‐within‐a‐gate”, which produces a smaller effective gating window. Precision of respiratory gated proton delivery was analyzed by examining distribution of distance from the respiratory gate threshold where 95% of gated beam delivery (DGT95) occurred. Results: With shorter fixed Tcyc (&lt; 4 sec), average DGT95 values were 0.30 cm, 0.23 cm and 0.17 cm respectively for 30%, 20% and 10% respiratory gate duty cycles. With longer fixed Tcycs (&gt; 4 sec) average DGT95 values were 0.25 cm, 0.19 cm and 0.14 cm respectively for 30%, 20% and 10% respiratory gate duty cycles. With Tcyc ≈ average patient breathing cycle, average DGT95 values were 0.27 cm, 0.20 cm and 0.15 cm for 30%, 20% and 10% respiratory gate duty cycles. With variable Tcyc, DGT95 values were 0.21 cm, 0.17 cm and 0.14 cm for 30%, 20% and 10% respiratory gate duty cycles. Conclusion: Variable Tcyc mode offered the greatest precision of respiratory gated delivery for passively scattered synchrotron proton irradiation.

MO-D-I-609-06: Image Registration of Multi-Frame PET Data: A Simulation Study Comparing Individual Versus Collective Frame Registration Techniques

Purpose: Image registration of a multi-frame data set in PET imaging has traditionally been performed by registering individual frames to a reference data set (IND method). In this abstract we propose, test and compare the results of a new approach that collectively register all image sets together (COL method) using a simulation study. Method and Materials: A lung lesion was simulated as a hot sphere oscillating in a warm background. The oscillation represented a patient breathing cycle. Images of the sphere at different phases of the breathing cycle were generated by first forward projecting the true image of the sphere into sinogram space, adding Poisson noise, and reconstructing the result using ML-EM. The reconstructed images were then registered using IND and COL methods. The same registration optimization was used for both methods. The registration quality of both methods was evaluated by calculating the mean square error (MSE) between the estimated and true displacement of the sphere at each phase of the breathing cycle following registration as well as by visual inspection. The results were also compared to an image of the sphere at a single phase and to that without motion. The effect of several simulation parameters on the registration quality were considered: 1) number of bins/cycle (10, 20, 40, 80); 2) Sphere to background ratio (2, 4, 8); and 3) number of breathing cycles (1, 2, 4, 8). Results: The MSE of COL is smaller than IND for the different simulation parameters. The improvement of COL over IND decreases as the sphere to background ratio increases, the number of bins decreases, and number of cycles increases. Visual inspection of COL registration showed better quality when compared to IND. Conclusion: COL gives better registration results by making use of all of the individual frames rather than registering one image set to another.