Moving Lung Phantom Research Articles

Implementation of free breathing respiratory amplitude‐gated treatments

PurposeThe purpose of this study was to provide guidance in developing and implementing a process for the accurate delivery of free breathing respiratory amplitude‐gated treatments.MethodsA phase‐based 4DCT scan is acquired at time of simulation and motion is evaluated to determine the exhale phases that minimize respiratory motion to an acceptable level. A phase subset average CT is then generated for treatment planning and a tracking structure is contoured to indicate the location of the target or a suitable surrogate over the planning phases. Prior to treatment delivery, a 4DCBCT is acquired and a phase subset average is created to coincide with the planning phases for an initial match to the planning CT. Fluoroscopic imaging is then used to set amplitude gate thresholds corresponding to when the target or surrogate is in the tracking structure. The final imaging prior to treatment is an amplitude‐gated CBCT to verify both the amplitude gate thresholds and patient positioning. An amplitude‐gated treatment is then delivered. This technique was commissioned using an in‐house lung motion phantom and film measurements of a simple two‐field 3D plan.ResultsThe accuracy of 4DCBCT motion and target position measurements were validated relative to 4DCT imaging. End to end testing showed strong agreement between planned and film measured dose distributions. Robustness to interuser variability and changes in respiratory motion were demonstrated through film measurements.ConclusionsThe developed workflow utilizes 4DCBCT, respiratory‐correlated fluoroscopy, and gated CBCT imaging in an efficient and sequential process to ensure the accurate delivery of free breathing respiratory‐gated treatments.

Dose calculation errors as a component of failing IROC lung and spine phantom irradiations.

Between July 2013 and August 2019, 22% of the imaging and radiation oncology core (IROC) spine, and 15% of the moving lung phantom irradiations have failed to meet established acceptability criteria. The spine phantom simulates a highly modulated stereotactic body radiation therapy (SBRT) case, whereas the lung phantom represents a low-to-none modulation moving target case. In this study, we assessed the contribution of dose calculation errors to these phantom results and evaluated their effects on failure rates. We evaluated dose calculation errors by comparing the calculation accuracy of various institutions' treatment planning systems (TPSs) vs IROC-Houston's previously established independent dose recalculation system (DRS). Each calculation was compared with the measured dose actually delivered to the phantom; cases in which the recalculation was more accurate were interpreted as a deficiency in the institution's TPS. A total of 258 phantom irradiation plans (172 lung and 86 spine) were recomputed. Overall, the DRS performed better than the TPSs in 47% of the spine phantom cases. However, the DRS was more accurate in 93% of failing spine phantom cases (with an average improvement of 2.35%), indicating a deficiency in the institution's treatment planning system. Deficiencies in dose calculation accounted for 60% of the overall discrepancy between measured and planned doses among spine phantoms. In contrast, lung phantom DRS calculations were more accurate in only 35% and 42% of all and failing lung phantom cases respectively, indicating that dose calculation errors were not substantially present. These errors accounted for only 30% of the overall discrepancy between measured and planned doses. Dose calculation errors are common and substantial in IROC spine phantom irradiations, highlighting a major failure mode in this phantom and in clinical treatment management of these cases. In contrast, dose calculation accuracy had only a minimal contribution to failing lung phantom results, indicating that other failure modes drive problems with this phantom and similar clinical treatments.

Reducing 4D CT imaging artifacts at the source: first experimental results from the respiratory adaptive computed tomography (REACT) system

Breathing variations during 4D CT imaging often manifest as geometric irregularities known as respiratory-induced image artifacts and ultimately effect radiotherapy treatment efficacy. To reduce such image artifacts we developed Respiratory Adaptive Computed Tomography (REACT) to trigger CT acquisition during periods of regular breathing. For the first time, we integrate REACT with clinical hardware and hypothesize that REACT will reduce respiratory-induced image artifacts ≥ 4 mm compared to conventional 4D CT.4D image sets were acquired using REACT and conventional 4D CT on a Siemens Somatom scanner. Scans were taken for 13 respiratory traces (12 patients) that were reproduced on a lung-motion phantom. Motion was observed by the Varian RPM system and sent to the REACT software where breathing irregularity was evaluated in real-time and used to trigger the imaging beam. REACT and conventional 4D CT images were compared to a ground truth static-phantom image and compared for absolute geometric differences within the region-of-interest. Breathing irregularity during imaging was retrospectively assessed using the root-mean-square error of the RPM measured respiratory signal during beam on (RMSE_Beam_on) for each phase of the respiratory cycle.REACT significantly reduced the average frequency of respiratory-induced image artifacts ≥ 4 mm by 70% for the tumor (p = 0.003) and 76% for the lung (p = 0.0002) compared to conventional 4D CT. Volume reductions of 10% to 6% of the tumor and 2% to 1% of the lung compared to conventional 4D CT were seen. Breathing irregularity during imaging (RMSE_Beam_on) was significantly reduced by 27% (p = 0.013) using the REACT method.For the first time, REACT was successfully integrated with clinical hardware. Our findings support the hypothesis that REACT significantly reduced respiratory-induced image artifacts compared to conventional 4D CT. These experimental results provide compelling evidence for further REACT investigation, potentially providing clearer images for clinical use.

A novel deformable lung phantom with programably variable external and internal correlation.

Lung motion phantoms used to validate radiotherapy motion management strategies have fairly simplistic designs that do not adequately capture complex phenomena observed in human respiration such as external and internal deformation, variable hysteresis and variable correlation between different parts of the thoracic anatomy. These limitations make reliable evaluation of sophisticated motion management techniques quite challenging. In this work, we present the design and implementation of a programmable, externally and internally deformable lung motion phantom that allows for a reproducible change in external-internal and internal-internal correlation of embedded markers. An in-house-designed lung module, made from natural latex foam was inserted inside the outer shell of a commercially available lung phantom (RSD, Long Beach, CA, USA). Radiopaque markers were placed on the external surface and embedded into the lung module. Two independently programmable high-precision linear motion actuators were used to generate primarily anterior-posterior (AP) and primarily superior-inferior (SI) motion in a reproducible fashion in order to enable (a) variable correlation between the displacement of interior volume and the exterior surface, (b) independent changes in the amplitude of the AP and SI motions, and (c) variable hysteresis. The ability of the phantom to produce complex and variable motion accurately and reproducibly was evaluated by programming the two actuators with mathematical and patient-recorded lung tumor motion traces, and recording the trajectories of various markers using kV fluoroscopy. As an example application, the phantom was used to evaluate the performance of lung motion models constructed from kV fluoroscopy and 4DCT images. The phantom exhibited a high degree of reproducibility and marker motion ranges were reproducible to within 0.5mm. Variable correlation was observed between the displacements of internal-internal and internal-external markers. The SI and AP components of motion of a specific marker had a correlation parameter that varied from -11 to 17. Monitoring a region of interest on the phantom's surface to estimate internal marker motion led to considerably lower uncertainties than when a single point was monitored. We successfully designed and implemented a programmable, externally and internally deformable lung motion phantom that allows for a reproducible change in external-internal and internal-internal correlation of embedded markers.

Pencil Beam Algorithms Are Unsuitable for Proton Dose Calculations in Lung

To compare analytic and Monte Carlo-based algorithms for proton dose calculations in the lung, benchmarked against anthropomorphic lung phantom measurements. A heterogeneous anthropomorphic moving lung phantom has been irradiated at numerous proton therapy centers. At 5 centers the treatment plan could be calculated with both an analytic and Monte Carlo algorithm. The doses calculated in the treatment plans were compared with the doses delivered to the phantoms, which were measured using thermoluminescent dosimeters and film. Point doses were compared, as were planar doses using a gamma analysis. The analytic algorithms overestimated the dose to the center of the target by an average of 7.2%, whereas the Monte Carlo algorithms were within 1.6% of the physical measurements on average. In some regions of the target volume, the analytic algorithm calculations differed from the measurement by up to 31% in the internal gross target volume (iGTV) (46% in the planning target volume), over-predicting the dose. All comparisons showed a region of at least 15% dose discrepancy within the iGTV between the analytic calculation and the measured dose. The Monte Carlo algorithm recalculations showed dramatically improved agreement with the measured doses, showing mean agreement within 4% for all cases and a maximum difference of 12% within the iGTV. Analytic algorithms often do a poor job predicting proton dose in lung tumors, over-predicting the dose to the target by up to 46%, and should not be used unless extensive validation counters the consistent results of the present study. Monte Carlo algorithms showed dramatically improved agreement with physical measurements and should be implemented to better reflect actual delivered dose distributions.

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

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

TU‐AB‐202‐04: Development of a DeformableImage Registration (DIR) Error Correction Method Employing Kolmogorov‐Zurbenko(KZ) Filter

Purpose:This study aims to develop a DIR error correction method capable of utilizing sparse ground‐truth motion information and recovering missing data with the Kolmogorov‐Zurbenko (KZ) filter.Methods:The error correction method employs a two‐step approach. First, sparse ground‐truth displacement vectors are integrated into a pre‐correction deformable vector field (DVF) to estimate a post‐correction DVF with coarse resolution. Second, the coarse post‐correction DVF is boosted to a full‐resolution DVF through convolution with the KZ filter. To validate the use of the KZ filter for missing data recovery, recovery errors were determined by comparing a DVF recovered from down‐sampling with the original full‐resolution DVF. The entire error correction method was tested on an in‐house developed digital lung motion phantom consisting of a primary volume, a DVF, and a secondary volume synthesized by applying the DVF on the primary volume. Five pre‐correction DVFs were obtained by performing DIR between the two volumes using Velocity, MIM, ILK and OHS algorithms in DIRART toolbox, and Elastix, and then corrected. Primary volumes were synthesized with pre‐ and post‐correction DVFs, respectively. The error correction method was evaluated with pre‐ and post‐correction registration errors, and intensity errors in synthesized primary volumes.Results:Our test results for sparsely down‐sampled (<0.4%) DVFs showed that the KZ filter outperformed the cubic polynomial interpolation method for whole lung DVF map recovery in terms of median error (0.60mm vs 0.73mm) and mean error (1.18mm vs 1.29mm). Pre‐ and post‐correction 3D registration errors per voxel for Velocity, MIM, ILK, OHS, and Elastix are reduced by 2.39 mm on average. Pre‐ and post‐correction intensity errors are reduced by 0.37 in unit of image intensity on average.Conclusion:We have implemented a two‐step method capable of utilizing sparse ground‐truth displacement vectors for DIR error reduction, allowing DIR accuracy improvement utilizing clinically available motion data.This study is supported by NIH grant 1R21CA165384.

Development of deformable moving lung phantom to simulate respiratory motion in radiotherapy

Radiation treatment requires high accuracy to protect healthy organs and destroy the tumor. However, tumors located near the diaphragm constantly move during treatment. Respiration-gated radiotherapy has significant potential for the improvement of the irradiation of tumor sites affected by respiratory motion, such as lung and liver tumors. To measure and minimize the effects of respiratory motion, a realistic deformable phantom is required for use as a gold standard. The purpose of this study was to develop and study the characteristics of a deformable moving lung (DML) phantom, such as simulation, tissue equivalence, and rate of deformation. The rate of change of the lung volume, target deformation, and respiratory signals were measured in this study; they were accurately measured using a realistic deformable phantom. The measured volume difference was 31%, which closely corresponds to the average difference in human respiration, and the target movement was − 30 to + 32mm. The measured signals accurately described human respiratory signals. This DML phantom would be useful for the estimation of deformable image registration and in respiration-gated radiotherapy. This study shows that the developed DML phantom can exactly simulate the patient׳s respiratory signal and it acts as a deformable 4-dimensional simulation of a patient׳s lung with sufficient volume change.

Evaluation of the Elekta Symmetry™ 4D IGRT system by using a moving lung phantom

Purpose: 4D cone-beam computed tomography (CBCT) is a beneficial tool for the treatment of movable tumors because it can help us to understand where the tumors are actually located and it has a precise treatment plan. However, general CBCT images have a limitation in that they cannot perfectly perform a sophisticated registration. On the other hand, the Symmetry TM 4D image-guided radiation therapy (IGRT) system of Elekta offers a 4D CBCT registration option. In this study, we evaluated the usefulness of Symmetry TM . Method and Materials: Planning CT images of the CIRS moving lung phantom were acquired 4D multi-detector CT (MDCT), and the images were sorted as 10 phases from 0% phase to 90% phase. The thickness of the CT images was 1 mm. Acquired MDCT images were transferred to the contouring software, and a virtual target was generated. A one-arc volumetric-modulated arc therapy (VMAT) plan was performed by using the treatment planning system on the virtual target. Finally, the movement of the phantom was verified by using the XVI Symmetry TM system. Results: The physical movement of the CIRS moving lung phantom was ±10.0 mm in the superiorinferior direction, ±1.0 mm in the lateral direction, and ±2.5 mm in the anterior-posterior direction. The movement of the phantom was measured from the 4D MDCT registration as ±10.2 mm in the superior-inferior direction, ±0.9 mm in the lateral direction, and ±2.45 mm in the anterior-posterior direction. The movement of the phantom was measured from the SymmetryTM registration as ±10.1 mm in the superior-inferior direction, ±0.9 mm in the lateral direction, and ±2.4 mm in the anterior-posterior direction. Conclusion: We confirmed that 4D CBCT is a beneficial tool for the treatment of movable tumors, and that the 4D registration of SymmetryTM can increase the precision of the registration when a movable tumor is the target of radiation treatment.

SU‐E‐P‐13: Quantifying the Geometric Error Due to Irregular Motion in Four‐Dimensional Computed Tomography (4DCT)

Purpose:Respiratory correlated 4DCT images are generated under the assumption of a regular breathing cycle. This study evaluates the error in 4DCT‐based target position estimation in the presence of irregular respiratory motion.Methods:A custom‐made programmable externally‐and internally‐deformable lung motion phantom was placed inside the CT bore. An abdominal pressure belt was placed around the phantom to mimic clinical 4DCT acquisitio and the motion platform was programmed with a sinusoidal (±10mm, 10 cycles per minute) motion trace and 7 motion traces recorded from lung cancer patients. The same setup and motion trajectories were repeated in the linac room and kV fluoroscopic images were acquired using the on‐board imager. Positions of 4 internal markers segmented from the 4DCT volumes were overlaid upon the motion trajectories derived from the fluoroscopic time series to calculate the difference between estimated (4DCT) and “actual” (kV fluoro) positions.Results:With a sinusoidal trace, absolute errors of the 4DCT estimated markers positions vary between 0.78mm and 5.4mm and RMS errors are between 0.38mm to 1.7mm. With irregular patient traces, absolute errors of the 4DCT estimated markers positions increased significantly by 100 to 200 percent, while the corresponding RMS error values have much smaller changes. Significant mismatches were frequently found at peak‐inhale or peak‐exhale phase.Conclusion:As expected, under conditions of well‐behaved, periodic sinusoidal motion, the 4DCT yielded much better estimation of marker positions. When an actual patient trace is used 4DCT‐derived positions showed significant mismatches with the fluoroscopic trajectories, indicating the potential for geometric and therefore dosimetric errors in the presence of cycle‐to‐cycle respiratory variations.

WE-AB-BRA-03: Displacement Vector Field (DVF) Error Correction Using Sparsely Distributed Ground-Truth Displacement Vectors

Purpose: This study aims to develop a technique capable of reducing errors in deformable image registration (DIR) by integrating information of sparsely distributed ground-truth displacement vectors into a dense DVF. Methods: An in-house developed lung motion phantom consisting of an end-of-inhalation (EOI) MR image, a dense DVF derived from tagging MR data, and an end-of-exhalation (EOE) image obtained by deforming the EOI image with the DVF was used. The DVFs to be corrected were obtained by registration between the EOI and the calculated EOE images using Velocity, MIM, ILK and OHS algorithms in DIRART toolbox, and Elastix. A set of sparsely distributed ground-truth points were selected and displacement vectors at those points were treated as ground-truth displacement vectors. Displacement vectors in the DVFs to be corrected were converted into coefficient vectors using Wendland's compactly supported functions. The coefficient map was modified such that the displacement vectors at ground-truth points, determined by the modified map, match the ground-truth displacement vectors. DVFs before and after correction were compared with the ground-truth DVF (phantom DVF), respectively. Pre- and post-correction registration errors were calculated. Results: Reduced registration errors were found in all tested DVFs. Pre- and post-correction 3D errors per voxel for Velocity, MIM, ILK, OHS, and Elastix are 1.27mm vs 1.07mm, 1.56mm vs 1.55mm, 6.33mm vs 4.81mm, 3.53mm vs 2.94mm, 2mm vs 1.67mm. Reduced registration errors are also uniformly found in every direction (p<0.01). Conclusion: We have implemented a hybrid registration technique that can potentially reduce registration errors by introducing information of sparsely distributed ground-truth displacement vectors into a dense DVF. Simulation results show that ground-truth points less than 0.5% of total number of voxels can enhance the accuracy of DVFs. Further studies on integration of biomechanical models and fine-tuning of parameters are expected to provide further correction.

An externally and internally deformable, programmable lung motion phantom.

Most clinically deployed strategies for respiratory motion management in lung radiotherapy (e.g., gating and tracking) use external markers that serve as surrogates for tumor motion. However, typical lung phantoms used to validate these strategies are based on a rigid exterior and a rigid or a deformable-interior. Such designs do not adequately represent respiration because the thoracic anatomy deforms internally as well as externally. In order to create a closer approximation of respiratory motion, the authors describe the construction and experimental testing of an externally as well as internally deformable, programmable lung phantom. The outer shell of a commercially available lung phantom (RS-1500, RSD, Inc.) was used. The shell consists of a chest cavity with a flexible anterior surface, and embedded vertebrae, rib-cage and sternum. A custom-made insert was designed using a piece of natural latex foam block. A motion platform was programmed with sinusoidal and ten patient-recorded lung tumor trajectories. The platform was used to drive a rigid foam "diaphragm" that compressed/decompressed the phantom interior. Experimental characterization comprised of determining the reproducibility and the external-internal correlation of external and internal marker trajectories extracted from kV x-ray fluoroscopy. Experiments were conducted to illustrate three example applications of the phantom-(i) validating the geometric accuracy of the VisionRT surface photogrammetry system; (ii) validating an image registration tool, NiftyReg; and (iii) quantifying the geometric error due to irregular motion in four-dimensional computed tomography (4DCT). The phantom correctly reproduced sinusoidal and patient-derived motion, as well as realistic respiratory motion-related effects such as hysteresis. The reproducibility of marker trajectories over multiple runs for sinusoidal as well as patient traces, as characterized by fluoroscopy, was within 0.25 mm RMS error. The motion trajectories of internal and external radio-opaque markers as measured by fluoroscopy were found to be highly correlated (R > 0.95). Using the phantom, it was demonstrated that the motion trajectories of regions-of-interest on the surface as measured by VisionRT are highly consistent with corresponding fluoroscopically acquired surface marker trajectories, with RMS errors within 0.26 mm. Furthermore, it was shown that the trajectories of external and internal marker trajectories derived from NiftyReg deformation vector fields were within 1 mm root mean square errors comparing to trajectories obtained by segmenting markers from individual fluoro frames. Finally, it was shown that while 4DCT can be used to localize internal markers for sinusoidal motion with reasonable accuracy, the localization error increases significantly (by a factor of ∼ 2) in the presence of cycle-to-cycle variations that are observed in patient-derived respiratory motion. The authors have developed a realistic externally and internally deformable, programmable lung phantom that will serve as a valuable tool for clinical and investigational motion management studies in thoracic and abdominal radiation therapies.

SU-E-P-19: Patient-Specific Quality Control Workflow for Dedicated SBRT Vero System

Purpose/Objectives: To set up patient specific quality control (QC) process that ensures the integrity, deliverability and accurate dose for the SBRT lung plan. Materials/Methods: In 11/2012, SBRT Vero system (C‐band 6MV linac on gimbals, mounted on a rotating (+/− 60 degree) ring) was commissioned at our facility. The gimbals allow pan/tilt of the linac to mechanically account for gantry sag during gantry rotation, and track the motion of the target by moving the axis of the treatment beam. The linac is equipped with 0.5 mm (at isocenter) MLC. The treatment planning system (BrainLab iPlanTM) has the following algorithms available: Pencil Beam, Monte‐Carlo (Dose‐to‐water and Dose‐to‐Medium options). For the treated 37 targets (15 patients), 24 were located in lung. Monte‐Carlo (dose‐to‐water) algorithm was used, as it best corresponded to the film/ion chamber commissioning dosimetry. Quality assurance (QA) for lung targets utilizes 0.01cc ion chamber measurements in CIRSTM heterogeneous lung motion phantom with a range of small targets available. Per institutional policy, point dose from the patient plan mapped onto the phantom is compared to measured dose in the same heterogeneous phantom in static configuration. Additionally, the same QC process was repeated using 3D sinusoidally moving target with the amplitudes similar or exceeding the results of patient‐specific target motion evaluation performed for each 4DCT patient at UFPTI. A range of planning geometries was used and QC‐verified: ring angles within ±15degrees and fields 2.5‐5.0cm diameter. Results and Conclusions: The measured dose was within 2.5% from the Monte Carlo calculation, with 1.2 % average difference between plan and measurement in the static case, and within 3% with average 1.6% and 1% SD for dynamic case. The agreement was satisfactory and ensured the integrity of the treatment planning and delivery process

SU‐C‐141‐02: Evaluation of Dosimetric and Geometric Accuracy of Gated Lung SBRT with CIRS Lung Motion Phantom

Purpose: To evaluate the dosimetric and geometric accuracy of the gated SBRT delivery using film measurements and intra‐fractional kV images with predefined software‐markers. Methods: End‐to‐end tests of gated SBRT delivery were performed using a Varian TrueBeam™ linac and a CIRS lung motion phantom with targets of 1 and 2 cm in diameter and implanted fiducials. 4DCT images of the phantom were acquired with 1 mm slice thickness. RapidArc plans were created in Eclipse TPS with software‐markers placed on the fiducials at maximal expiration. SBRT plans were delivered to the phantom in stationary and in gating mode (2 mm and 5 mm‐gating window). Plans were also delivered with 5 mm targets shift with respect to the boney anatomy in stationary mode and 10% phase shift in 2 mm‐gating mode. To investigate the dosimetric accuracy, dose was measured by EBT3 film and compared with the TPS calculation (AcurosXB algorithm) using γ‐index criteria (3mm distance‐to‐agreement and 3% dose‐difference). The geometric accuracy was evaluated using kV intra‐fraction images of the fiducials relative to the predefined software‐markers. Results: Comparing with the stationary mode, the percentage passing γ‐index (%γ) for 1 cm target decreased significantly from 98.0% to 94.8% (2 mm‐gating window) and 84.3% (5 mm‐gating window) while for 2 cm target %y decreased from 99.5% to 99.2% (2 mm‐gating window) and 86.6% (5 mm‐gating window). The %γ for the conditions of target shift and phase shift for the 2 cm target were 89.0% and 89.9%, respectively. Geometric deviation was less than 0.05 mm, except for the phase shift condition (0.45 mm). Conclusion: Higher dosimetric impact from motion was found for the smaller target. With a proper gating window the planned dose can be accurately delivered with the use of kV intra‐fraction image and software‐maker to ensure the geometric accuracy during treatment.

Real‐time tumor tracking: Automatic compensation of target motion using the Siemens 160 MLC

Advanced high quality radiation therapy techniques such as IMRT require an accurate delivery of precisely modulated radiation fields to the target volume. Interfractional and intrafractional motion of the patient's anatomy, however, may considerably deteriorate the accuracy of the delivered dose to the planned dose distributions. In order to compensate for these potential errors, a dynamic real-time capable MLC control system was designed. The newly developed adaptive MLC control system contains specialized algorithms which are capable of continuous optimization and correction of the aperture of the MLC according to the motion of the target volume during the dose delivery. The algorithms calculate the new leaf positions based on target information provided online to the system. The algorithms were implemented in a dynamic target tracking control system designed for a Siemens 160 MLC. To assess the quality of the new target tracking system in terms of dosimetric accuracy, experiments with various types of motion patterns using different phantom setups were performed. The phantoms were equipped with radiochromic films placed between solid water slabs. Dosimetric results of exemplary deliveries to moving targets with and without dynamic MLC tracking applied were compared in terms of the gamma criterion to the reference dose delivered to a static phantom. Our measurements indicated that dose errors for clinically relevant two-dimensional target motion can be compensated by the new control system during the dose delivery of open fields. For a clinical IMRT dose distribution, the gamma success rate was increased from 19% to 77% using the new tracking system. Similar improvements were achieved for the delivery of a complete IMRT treatment fraction to a moving lung phantom. However, dosimetric accuracy was limited by the system's latency of 400 ms and the finite leaf width of 5 mm in the isocenter plane. Different experimental setups representing different target tracking scenarios proved that the tracking concept, the new algorithms and the dynamic control system make it possible to effectively compensate for dose errors due to target motion in real-time. These early results indicate that the method is suited to increasing the accuracy and the quality of the treatment delivery for the irradiation of moving tumors.

TU-FF-A3-04: Empirical Investigation of 3D Intrafraction Motion Management Using a Generalized Methodology for Tracking Translating, Rotating and Deforming Targets

Purpose: Real‐time tracking of tumor motion is a highly promising approach for intrafraction motion management in thoracic and abdominal cancerradiotherapy. We investigate the geometric and dosimetric impact of a generalized methodology for conformal and IMRT‐based radiation delivery to translating, rotating and deforming targets. Materials and Methods: The methodology is based on the concept of “relocation vectors”, which correlate instantaneous target position to each point on the treatment aperture(s), throughout the entire respiratory cycle. These vectors are determined during 4D treatment planning by combining instantaneous target position information, obtained from a real‐time position‐monitoring (RPM) system, with concurrently acquired 4DCT. The resulting plan comprises multiple “control points”. For each point, a set of MLC leaf positions is defined so as to conform to the instantaneous shape of the target and deliver the desired fluence. The methodology is, therefore, independent of the nature of target motion. During dose delivery, real‐time RPM data are used to update the relocation vectors so as to dynamically account for changes target shape/position relative to the shape/position observed in the planning stage. Initial studies were performed to demonstrate tracking of linear and elliptical motion. A laboratory system was designed and optical measurements of tracking accuracy and system latency were obtained. The methodology was subsequently tested on a moving lung phantom placed under a clinical linac, and dosimetric measurements were performed using a 2D ion‐chamber array. Results: Tracking accuracy (without using any predictive algorithm) was observed to be ∼1.25 mm for motion parallel to MLC leaf travel. For motion perpendicular to leaf travel, accuracy was significantly lower. Dosimetric measurements indicate that tracking achieves efficient dose delivery to the target and, simultaneously, significant dose reduction in surrounding regions. Conclusions: We have developed a robust, universal tracking methodology to manage 3D intrafraction motion. This work was partially supported by Varian.

SU‐FF‐T‐197: Dosimetry in a Lung Moving Phantom

Purpose: Respiratory organ motion is known to be one of the largest intrafractional organ motions. In the lung phantom, secondary electron transport in addition to the photon transport tends to extend the dose disequilibrium region, resulting in a larger penumbra region and thus potentially different motion effect on dose distribution for IMRT fields. This study quantifies the effect of motion on dose distribution in lung medium. Method and Materials: Measurements were performed in a water‐equivalent made of solid water and a lung phantom composed of 15‐ cm thick 0.25 g/cm2 cork slabs at several depths a computerized motion phantom. Dose distributions for 6 MV and 15 MV photon beams are measured for a 10×10 cm2 open field and for IMRT fields. The motion is created using a ±1‐cm motion amplitude and 6 second per breathing cycle. EDR2 films were used to determine the dose distribution with or without motion at various depths in the solid and lung phantom. Results: At 10 cm depth, the penumbra of dose profile in the lung phantom is much broader than the solid phantom. The penumbra width increases with increasing photon energy. The effect of motion can be modeled with a motion kernel determined by the probability function of the breathing cycle and is independent of the phantom (solid or lung) types used. At 2.5 cm depth, the penumbra width between the lung and solid phantom is similar since the same solid water material was used above the point of measurement. Conclusion: We have shown that dose distribution in a moving lung phantom can be predicted from the measured dose distribution in the static lung phantom and a motion kernel. The introduction of secondary electron transport actually reduces the effect of motion on the dose distribution because the penumbra width is wider in lung material.

TU‐FF‐A1‐05: A Motion Phantom Study On Helical Tomotherapy: The Dosimetric Impacts of Delivery Technique and Motion

Purpose: To determine the optimal delivery technique for treating moving targets with Helical Tomotherapy (HT) we measured the dosimetric effects of various motion and Tomotherapy plan parameters using a lung motion phantom. Method and Materials: A motion phantom system was constructed using a programmable motor driving a moving platform. Tomotherapy plans were delivered to a phantom which mimicked lung heterogeneity by using wood inserts with the density of lung tissue and a 3.3cm spherical tissue density material as tumor. A cylindrical planning target volume (PTV) was used with a length and diameter of 3.3cm including the sphere. Treatment plans were created using jaw sizes of 1.04 and 2.47cm with incremental gantry rotation times ranging from (10s) to (60s). Treatments based on these plans were delivered to the phantom with motion periods of 3 and 5 seconds, and amplitudes of +/−6 and 10mm. All plans were normalized to 2.0 Gy fractional doses to 95% of the PTV and Kodak EDR‐2 film was used for 2‐D dose measurements. Axial dose profiles and cumulative dose volume histograms (DVH) of plans for moving and static phantom conditions were compared. Results: Target edge under‐dosing is less with a jaw size of 2.47cm than 1.04cm in this experimental system. Shorter motion periods (3s) and greater gantry rotation (&gt;5 times motion period) resulted in better PTV coverage referenced to static phantoms. Greater than 90% of the PTV received the prescription dose even when the phantom moves +/−10mm. When the gantry rotation time to motion period ratio approached 3 PTV coverage was compromised. Conclusions: PTV expansions smaller than motion excursion may be possible for lung tumor treatment by HT. Larger jaw width provides better target coverage because of the wider inferior/superior penumbra. Increasing the ratio of the gantry rotation time to the motion period improves target edge coverage.

Linac-integrated 4D cone beam CT: first experimental results

A new online imaging approach, linac-integrated cone beam CT (CBCT), has been developed over the past few years. It has the advantage that a patient can be examined in their treatment position directly before or during a radiotherapy treatment. Unfortunately, respiratory organ motion, one of the largest intrafractional organ motions, often leads to artefacts in the reconstructed 3D images. One way to take this into account is to register the breathing phase during image acquisition for a phase-correlated image reconstruction. Therefore, the main focus of this work is to present a system which has the potential to investigate the correlation between internal (movement of the diaphragm) and external (data of a respiratory gating system) information about breathing phase and amplitude using an inline CBCT scanner. This also includes a feasibility study about using the acquired information for a respiratory-correlated 4D CBCT reconstruction. First, a moving lung phantom was used to develop and to specify the required methods which are based on an image reconstruction using only projections belonging to a certain moving phase. For that purpose, the corresponding phase has to be detected for each projection. In the case of the phantom, an electrical signal allows one to track the movement in real time. The number of projections available for the image reconstruction depends on the breathing phase and the size of the position range from which projections should be used for the reconstruction. The narrower this range is, the better the inner structures can be located, but also the noise of the images increases due to the limited number of projections. This correlation has also been analysed. In a second step, the methods were clinically applied using data sets of patients with lung tumours. In this case, the breathing phase was detected by an external gating system (AZ-733V, Anzai Medical Co.) based on a pressure sensor attached to the patient's abdominal region with a fixation belt. The comparison of the reconstructed 4D CBCT images and the corresponding 4D CT images used for the treatment planning provides the required information for the calculation of possible setup errors. So, a repositioning of the patient is feasible even though the patient moves due to respiration. In addition to the external signal, the position of the diaphragm in the cranial-caudal direction could be extracted from each projection. Both independent sources of information show a very good agreement of the phase and even the amplitude of the movement and the external signal respectively. This suggests the usability of such a system for a gated dose delivery approach. However, more studies involving patients with different incidences have to be carried out to confirm these first results.

Compensation for respiratory motion by gated radiotherapy: an experimental study

Respiratory organ motion is known to be one of the largest intrafractional organ motions. Therefore, it is important to investigate the potential benefit of gated dose delivery approaches which aim to account for the respective dose uncertainties. In this study respiration is simulated by a moving lung phantom; the movement is not restricted to a normal sinusoidal progression and simulates the one of the embedded lung tumour in the cranial–caudal direction. An IMRT plan with a total of 29 beam segments was designed for the treatment of this tumour. It was irradiated in its resting position—which is the position at exhalation—and during movement. Furthermore the irradiation was triggered using different amplitude thresholds, which means that the irradiation only proceeded if the deviation of the tumour's position from its resting position is smaller than the given threshold. We determined the gating-related increase of the treatment time for various gating procedures. We also measured the resulting dose distribution in specific slices of the phantom perpendicular to the direction of the movement using film dosimetry and compared it to the dose distribution of the static case. Since these film measurements cannot be done inside the whole tumour, additionally the movement and gating was simulated using the planning software to calculate the 3D dose distribution inside the tumour and to generate dose volume histograms for different treatment modalities. The total treatment time was observed to increase by 20%–100% depending on the individual gating threshold and can be calculated easily. The analysis of the films showed that irradiation without gating leads to significant underdosages up to 33%, especially at the edge of the tumour. With gating it is possible to considerably reduce this underdosage down to 9% depending on the trigger threshold. The calculation of the dose volume histograms makes it possible to find a reasonable compromise between the improvement of the dose distribution and the increase of the treatment time.