Intrafractional Target Motion Research Articles

Dosimetric consequences of intrafraction prostate motion in scanned ion beam radiotherapy

Background and purposeScanned-beam interplay with the intrafraction target motion may result in dose deterioration in particle therapy. The magnitude of this effect and the possibilities to mitigate it were investigated for carbon ion prostate treatments. Methods and materialsFor 12 prostate cases, 9 carbon ion treatment plans were prepared using 3 scanned-beam settings (spot sizes of 6, 7 and 9mm and, respectively, raster pitches of 2, 2 and 3mm) for 3 planning margins (3, 6 and 9mm). Plans were recomputed in presence of 5 intrafraction prostate motion scenarios with and without intra-beam motion compensation. ResultsFor 6mm margin and 7mm spot, the median (max) CTV D95% change was −0.2 (−2.6) pp (percentual points) with pure drift motion, −3.8 (−6.0) pp and −2.8 (−3.1) pp in transient motion scenarios and −4.8 (−7.7) pp and −1.8 (−5.7) pp in mixed motion scenarios. No particular raster setting brought distinct advantage, while planning margin expansion showed statistically significant effects for drift-dominated scenarios. Intra-beam motion compensation yielded improved CTV coverage. ConclusionIntrafraction prostate motion can lead to marked target coverage deterioration, dependent on individual motion patterns, which can be only partially avoided through planning-time countermeasures. Among possible delivery-time countermeasures, intra-beam motion compensation is capable of improving target coverage.

Extreme Hypofractionated Image-Guided Radiotherapy for Prostate Cancer

An emerging body of data suggests that hypofractionated radiation schedules, where a higher dose per fraction is delivered in a smaller number of sessions, may be superior to conventional fractionation schemes in terms of both tumour control and toxicity profile in the management of adenocarcinoma of the prostate. However, the optimal hypofractionation scheme is still the subject of scientific debate. Modern computer-driven technology enables the safe implementation of extreme hypofractionation (often referred to as stereotactic body radiation therapy [SBRT]). Several studies are currently being conducted to clarify the yet unresolved issues regarding treatment techniques and fractionation regimens. Recently, the American Society for Radiation Oncology (ASTRO) issued a model policy indicating that data supporting the use of SBRT for prostate cancer have matured to a point where SBRT could be considered an appropriate alternative for select patients with low-to-intermediate risk disease. The present article reviews some of the currently available data and examines the impact of tracking technology to mitigate intra-fraction target motion, thus, potentially further improving the clinical outcomes of extreme hypofractionated radiation therapy in appropriately selected prostate cancer patients. The Champalimaud Centre for the Unknown (CCU)’s currently ongoing Phase I feasibility study is described; it delivers 45 Gy in five fractions using prostate fixation via a rectal balloon, and urethral sparing via catheter placement with on-line intra-fractional motion tracking through beacon transponder technology.

Time‐resolved dose distributions to moving targets during volumetric modulated arc therapy with and without dynamic MLC tracking

The highly conformal doses delivered by volumetric modulated arc therapy (VMAT) may be compromised by intrafraction target motion. Although dynamic multileaf collimator (DMLC) tracking can mitigate the dosimetric impact of motion on the accumulated dose, residual errors still exist. The purpose of this study was to investigate the temporal evolution of dose errors throughout VMAT treatments delivered with and without DMLC tracking. Tracking experiments were performed on a linear accelerator connected to prototype DMLC tracking software. A three-axis motion stage reproduced representative clinical trajectories of four lung tumors and four prostates. For each trajectory, two VMAT treatment plans (low and high modulation) were delivered with and without DMLC tracking as well as to a static phantom for reference. Dose distributions were measured continuously at 72 Hz using a dosimeter with biplanar diode arrays. During tracking, the MLC leaves were continuously refitted to the 3D target position measured by an electromagnetic transponder at 30 Hz. The dosimetric errors caused in the 32 motion experiments were quantified by a time-resolved 3%/3 mm γ-test. The erroneously exposed areas in treatment beam's eye view (BEV) caused by inadequate real-time MLC adaptation were calculated and compared with the time-resolved γ failure rates. The transient γ failure rate was on average 16.8% without tracking and 5.3% with tracking. The γ failure rate correlated well with the erroneously exposed areas in BEV (mean of Pearson r = 0.83, p < 0.001). For the final accumulated doses, the mean γ failure rate was 17.9% without tracking and 1.0% with tracking. With tracking the transient dose errors tended to cancel out resulting in the low mean γ failure rate for the accumulated doses. Time-resolved measurements allow pinpointing of transient errors in dose during VMAT delivery as well as monitoring of erroneous dose evolution in key target positions. The erroneously exposed area in BEV was shown to be a good indicator of errors in the dose distribution during treatment delivery.

Noninvasive Real-Time Prostate Tracking Using a Transperineal Ultrasound: A Clinical Trial Comparison to RF Transponders With Visual Confirmation

Interfraction image guidance in the management of prostate cancer reduces uncertainty and has become standard of care. More recently, real-time fiducial/transponder tracking to further reduce uncertainty from intrafraction target motion has become available, but these techniques require an invasive procedure. A noninvasive transperineal ultrasound system using rigid structure registration (4DTPUS) has been developed to account for interfraction, as well as intrafraction motion. This technique has potential advantages over historic ultrasound-based interfraction adjustments. The 4DTPUS is less affected by user variability, abdominal pressure, approach, anatomy, bowel gas, and bladder filling. As older ultrasound techniques were transabdominal, they were not amenable to real-time tracking. Our objective was to validate the 4DTPUS accuracy of tracking using visual confirmation and a comparison of tracking to RF transponders. Seven patients were accrued to an IRB approved prospective clinical study where RF transponders were implanted and patients treated per institutional standard of care for biopsy proven prostate cancer. The 4DTPUS data was collected during each radiation treatment fraction for direct comparison of 3D motion of the prostate. In addition, visual tracking of the prostate by 4DTPUS was reviewed by 3 attending physicians who independently rated each ultrasound series on a score of 1-5 with 1 being the best. There was high concordance between 4DTPUS and RF transponder tracking of the prostate. Twenty-eight fractions were usable for the comparative analysis at the time of this report with a total of 188 minutes. The root mean square difference and standard deviation of the tracking for the 2 systems was 0.2 +/- 0.55 mm in the sup/inf, 0.1 +/- 0.4 mm in the left/right and 0.1 +/- 0.8 mm in the ant/post dimensions. The number of fractions that were analyzable was limited due to artifact in the ultrasound images caused by the RF transponder system. The 4DTPUS was visually reviewed for 364 minutes of patient treatment over 62 fractions. The average visual correlation was scored as 1.21. The majority of these fractions included some artifact from the transponders that did affect the 4DTPUS image quality. A fraction was only excluded if complete data for transponders and/or 4DTPUS was unavailable or if synchronization of the data could not be performed. A novel 4D transperineal ultrasound system accurately and reproducibly tracked prostate motion in patients. Visual confirmation of prostate tracking was excellent. Transperineal ultrasound tracking of the prostate offers a noninvasive alternative to currently available marker-based systems.

Quantifying variability of intrafractional target motion in stereotactic body radiotherapy for lung cancers

In lung stereotactic body radiotherapy (SBRT), variability of intrafractional target motion can negate the potential benefits of four‐dimensional (4D) treatment planning that aims to account for the dosimetric impacts of organ motion. This study used tumor motion data obtained from CyberKnife SBRT treatments to quantify the reproducibility of probability motion function (pmf) of 37 lung tumors. The reproducibility of pmf was analyzed with and without subtracting the intrafractional baseline drift from the original motion data. Statistics of intrafractional tumor motion including baseline drift, target motion amplitude and period, were also calculated. The target motion amplitude significantly correlates with variations (1 SD) of motion amplitude and baseline drift. Significant correlation between treatment time and variations (1 SD) of motion amplitude was observed in anterior‐posterior (AP) motion, but not in craniocaudal (CC) and left‐right (LR) motion. The magnitude of AP and LR baseline drifts significantly depend on the treatment time, while the CC baseline drift does not. The reproducibility of pmf as a function of time can be well described by a two‐exponential function with a fast and slow component. The reproducibility of pmf is over 60% for the CC motion and over 50% for the AP and LR motions when baseline variations were subtracted from the original motion data. It decreases to just over 30% for the CC motion and about 20% for the AP and LR motion, otherwise. 4D planning has obvious limitations due to variability of intrafractional target motion. To account for potential risks of overdosing critical organs, it is important to simulate the dosimetric impacts of intra‐ and interfractional baseline drift using population statistics obtained from SBRT treatments.PACS number: 87.55.‐x

Experimental verification of a 4D MLEM reconstruction algorithm used for in-beam PET measurements in particle therapy

In-beam positron emission tomography (PET) has been proven to be a reliable technique in ion beam radiotherapy for the in situ and non-invasive evaluation of the correct dose deposition in static tumour entities. In the presence of intra-fractional target motion an appropriate time-resolved (four-dimensional, 4D) reconstruction algorithm has to be used to avoid reconstructed activity distributions suffering from motion-related blurring artefacts and to allow for a dedicated dose monitoring. Four-dimensional reconstruction algorithms from diagnostic PET imaging that can properly handle the typically low counting statistics of in-beam PET data have been adapted and optimized for the characteristics of the double-head PET scanner BASTEI installed at GSI Helmholtzzentrum Darmstadt, Germany (GSI). Systematic investigations with moving radioactive sources demonstrate the more effective reduction of motion artefacts by applying a 4D maximum likelihood expectation maximization (MLEM) algorithm instead of the retrospective co-registration of phasewise reconstructed quasi-static activity distributions. Further 4D MLEM results are presented from in-beam PET measurements of irradiated moving phantoms which verify the accessibility of relevant parameters for the dose monitoring of intra-fractionally moving targets. From in-beam PET listmode data sets acquired together with a motion surrogate signal, valuable images can be generated by the 4D MLEM reconstruction for different motion patterns and motion-compensated beam delivery techniques.

SU‐E‐J‐47: EPID Based Target Tracking During Intensity‐Modulated Radiation Therapy

Purpose: This work investigates the feasibility of using real‐time EPID images of the treatment fields acquired during IMRT for target localization and tracking. Methods: In this study, 35 patients treated with IMRT were retrospectively investigated. These patients were grouped as: prostate with lymph node (n=14), prostate without lymph node (n=17), and lung (n=4). For each patient, two to four fiducial markers were implanted inside the tumor. The DRR, which projects the patient anatomy and the fiducial marker at the EPID location, was reconstructed for each field. The MLC aperture of each control point was overlay on its corresponding DRR to evaluate the fractional time when the fiducial marker was seen on the EPID image. The probability of seeing at least one, two, three, or four fiducial markers during the treatment was recorded. Results: Our results show that for prostate IMRT patients without lymph nodes included in the target volume, the average probability of seeing at least one, two, three, and four fiducial markers during the treatment was 50% (35%–59%), 39% (23%– 51%), 24% (7%–38%), and 12% (4%–29%), respectively. For prostate IMRT patients with lymph nodes, the probability was 41% (24%–51%), 29% (12%– 42%), 15% (3%–24%), and 7% (4%–15%), respectively. For lung IMRT treatment, the average probability of seeing at least one fiducial marker was 34% (20%–52%). Conclusion: The continuous image acquisition from the EPID during IMRT treatment provides sufficient target movement information for real‐time target localization and intrafractional target motion correction for advanced radiotherapy treatments.

Verification of Respiratory Position Reproducibility With a Respiration Self-Monitoring Device: Results for 12 Patients With Lung Tumors

Respiratory movement of the tumors occurs during radiation therapy for patients with lung tumors. There are some techniques for decreasing intrafractional target motion derived from a patient's respiratory movement. A respiration self-monitoring device called Abches (APEX Medical, Inc., Tokyo, Japan) was developed for the purpose of decreasing respiratory target motion. Abches has two arms: one senses respiratory motion of the chest and the other senses that of the abdomen. When Abches is set on a patient, the indicator moves in accordance with the patient's respiration.

Patterns of intrafractional motion and uncertainties of treatment setup reference systems in accelerated partial breast irradiation for right- and left-sided breast cancer

PurposeThis study investigated the patterns of intrafractional motion and accuracy of treatment setup strategies in 3-dimensional conformal radiation therapy of accelerated partial breast irradiation (APBI) for right- and left-sided breast cancers. Methods and MaterialsSixteen right-sided and 17 left-sided breast cancer patients were enrolled in an institutional APBI trial in which gold fiducial markers were strategically sutured to the surgical cavity walls. Daily pre- and postradiation therapy kV imaging were performed and were matched to digitally reconstructed radiographs based on bony anatomy and fiducial markers, respectively, to determine the intrafractional motion. The positioning differences of the laser-tattoo and the bony anatomy-based setups with respect to the marker-based setup (benchmark) were determined to evaluate their accuracy. ResultsStatistical differences were found between the right- and left-sided APBI treatments in vector directions of intrafractional motion and treatment setup errors in the reference systems, but less in their overall magnitudes. The directional difference was more pronounced in the lateral direction. It was found that the intrafractional motion and setup reference systems tended to deviate in the right direction for the right-sided breast treatments and in the left direction for the left-sided breast treatments. ConclusionsIt appears that the fiducial markers placed in the seroma cavity exhibit side dependent directional intrafractional motion, although additional data may be needed to further validate the conclusion. The bony anatomy-based treatment setup improves the accuracy over laser-tattoo. But it is inadequate to rely on bony anatomy to assess intrafractional target motion in both magnitude and direction.

Adaptive Radiation Therapy for Postprostatectomy Patients Using Real-Time Electromagnetic Target Motion Tracking During External Beam Radiation Therapy

Using real-time electromagnetic (EM) transponder tracking data recorded by the Calypso 4D Localization System, we report inter- and intrafractional target motion of the prostate bed, describe a strategy to evaluate treatment adequacy in postprostatectomy patients receiving intensity modulated radiation therapy (IMRT), and propose an adaptive workflow. Tracking data recorded by Calypso EM transponders was analyzed for postprostatectomy patients that underwent step-and-shoot IMRT. Rigid target motion parameters during beam delivery were calculated from recorded transponder positions in 16 patients with rigid transponder geometry. The delivered doses to the clinical target volume (CTV) were estimated from the planned dose matrix and the target motion for the first 3, 5, 10, and all fractions. Treatment adequacy was determined by comparing the delivered minimum dose (Dmin) with the planned Dmin to the CTV. Treatments were considered adequate if the delivered CTV Dmin is at least 95% of the planned CTV Dmin. Translational target motion was minimal for all 16 patients (mean: 0.02 cm; range: -0.12 cm to 0.07 cm). Rotational motion was patient-specific, and maximum pitch, yaw, and roll were 12.2, 4.1, and 10.5°, respectively. We observed inadequate treatments in 5 patients. In these treatments, we observed greater target rotations along with large distances between the CTV centroid and transponder centroid. The treatment adequacy from the initial 10 fractions successfully predicted the overall adequacy in 4 of 5 inadequate treatments and 10 of 11 adequate treatments. Target rotational motion could cause underdosage to partial volume of the postprostatectomy targets. Our adaptive treatment strategy is applicable to post-prostatectomy patients receiving IMRT to evaluate and improve radiation therapy delivery.

Four‐dimensional dose distributions of step‐and‐shoot IMRT delivered with real‐time tumor tracking for patients with irregular breathing: Constant dose rate vs dose rate regulation

Dose-rate-regulated tracking (DRRT) is a tumor tracking strategy that programs the MLC to track the tumor under regular breathing and adapts to breathing irregularities during delivery using dose rate regulation. Constant-dose-rate tracking (CDRT) is a strategy that dynamically repositions the beam to account for intrafractional 3D target motion according to real-time information of target location obtained from an independent position monitoring system. The purpose of this study is to illustrate the differences in the effectiveness and delivery accuracy between these two tracking methods in the presence of breathing irregularities. Step-and-shoot IMRT plans optimized at a reference phase were extended to remaining phases to generate 10-phased 4D-IMRT plans using segment aperture morphing (SAM) algorithm, where both tumor displacement and deformation were considered. A SAM-based 4D plan has been demonstrated to provide better plan quality than plans not considering target deformation. However, delivering such a plan requires preprogramming of the MLC aperture sequence. Deliveries of the 4D plans using DRRT and CDRT tracking approaches were simulated assuming the breathing period is either shorter or longer than the planning day, for 4 IMRT cases: two lung and two pancreatic cases with maximum GTV centroid motion greater than 1 cm were selected. In DRRT, dose rate was regulated to speed up or slow down delivery as needed such that each planned segment is delivered at the planned breathing phase. In CDRT, MLC is separately controlled to follow the tumor motion, but dose rate was kept constant. In addition to breathing period change, effect of breathing amplitude variation on target and critical tissue dose distribution is also evaluated. Delivery of preprogrammed 4D plans by the CDRT method resulted in an average of 5% increase in target dose and noticeable increase in organs at risk (OAR) dose when patient breathing is either 10% faster or slower than the planning day. In contrast, DRRT method showed less than 1% reduction in target dose and no noticeable change in OAR dose under the same breathing period irregularities. When ±20% variation of target motion amplitude was present as breathing irregularity, the two delivery methods show compatible plan quality if the dose distribution of CDRT delivery is renormalized. Delivery of 4D-IMRT treatment plans, stemmed from 3D step-and-shoot IMRT and preprogrammed using SAM algorithm, is simulated for two dynamic MLC-based real-time tumor tracking strategies: with and without dose-rate regulation. Comparison of cumulative dose distribution indicates that the preprogrammed 4D plan is more accurately and efficiently conformed using the DRRT strategy, as it compensates the interplay between patient breathing irregularity and tracking delivery without compromising the segment-weight modulation.

SU-E-J-139: Feasibility of Using EPID for Real-Time Target Localization during Treatment.

This study aims to investigate the feasibility of using the images of the treatment fields acquired by an electronic portal imaging device (EPID) for real-time target localization. Forty one patients treated with IMRT and RapidArc were recruited in this study including 37 prostate patients and 4 lung patients. These patients were grouped as: prostate IMRT with lymph node (n=14), prostate IMRT without lymph node (n=17), prostate RapidArc (n=6), and lung IMRT (n=4). For each patient, two to four fiducial markers were implanted inside the tumor. The DRR, which projects the patient anatomy and the fiducial marker at the EPID location, was reconstructed for each field. The MLC aperture of each control point was overlay on its corresponding DRR to evaluate the fractional time when the fiducial marker was seen on the EPID image. The probability of seeing at least one, two, three, and four fiducial markers during the treatment was recorded. For the prostate IMRT patients without lymph nodes included in the target volume, the average probability of seeing at least one, two, three, and four fiducial markers during the treatment was 50% (35%-59%), 39% (23%-51%), 24% (7%-38%), and 12% (4%-29%), respectively. For the prostate IMRT patients with lymph nodes, the probability was 41% (24%-51%), 29% (12%-42%), 15% (3%-24%), and 7% (4%-15%), respectively. For prostate RapidArc treatments using two arcs, the average probability of seeing at least one fiducial marker was 81% (58%-90%) for the full arc and 74% (53%-94%) for the partial arc. For the lung IMRT treatment, the average probability of seeing at least one fiducial marker was 34% (20%-52%). The continuous image acquisition from the EPID during the treatment provides sufficient target movement information for real-time target localization and intrafractional target motion correction for advanced radiotherapy treatments.

An experimental investigation into the effect of periodic motion on proton dosimetry using polymer gel dosimeters and a programmable motion platform

Organ motion in proton therapy affects treatment dose distribution during both double-scattering (DS) and uniform-scanning (US) deliveries. We investigated the dosimetric impact of target motion using three-dimensional polymer gel dosimeters and a programmable motion platform. A simple one-beam treatment plan with 16 cm range and 6 cm modulation was generated from the treatment planning system (TPS) in both the DS and US modes. One gel dosimeter was irradiated with a stationary DS beam. Two other gel dosimeters were irradiated with the DS and US beams while they moved in the same sinusoidal motion profile using a programmable motion platform. The dose distribution of the stationary DS delivery agreed with the TPS plan. Dosimetric comparisons between DS motion delivery and the MATLAB-based motion model showed insignificant differences. Dose–volume histograms of a cylindrical target volume inside the gel dosimeters showed target coverage degradation caused by motion. A three-dimensional gamma index calculation (3% and 3 mm) confirmed different dosimetric impacts from DS and US with the same target motion. This polymer-gel-dosimeter-based study confirmed the dosimetric impact of intrafraction target motion and its interplay with temporal delivery of different energy layers in US proton treatments.

Motion Compensation in Radiotherapy

Image-guided radiotherapy (IGRT) has helped to dramatically reduce safety margins compensating for positioning uncertainties in radiotherapy. A remaining issue posing problems for photon radiotherapy (RT), but even more so for particle RT, is target motion during treatment delivery. This review outlines the various strategies currently being developed or already in clinical use to compensate for organ motion, predominantly breathing-induced motion of liver and lung targets. Several motion compensation strategies have recently been introduced clinically. Among these are optimized margins encompassing the individual range of target motion, treatment under breath hold, gated treatments, and tumor tracking with a dedicated treatment device. A variety of surveillance strategies for gating and tracking, such as indirect tracking with external fiducial markers and surface scanning devices, direct tracking with implanted electromagnetic markers, fiducial markers, and fluoroscopy, and ultrasound-based tracking are already in clinical use or are under development. Tracked treatment with linear accelerators based on tumor-synchronous MLC- or treatment-table adaptation are moving toward clinical use. A multitude of strategies to reduce the impact of intrafractional target motion in RT have been developed and are increasingly being used clinically. The clinical introduction of advanced strategies currently under development is imminent. After IGRT minimized treatment margins for static tumors, the implementation of motion compensation strategies will achieve the same for targets being subject to intrafractional breathing-induced motion.

Position detection accuracy of a novel linac-mounted intrafractional x-ray imaging system

The authors have developed a system that monitors intrafractional target motion perpendicular to the treatment beam with the aid of radioopaque markers by means of separating kV image and megavoltage (MV) treatment field on a single flat-panel detector. They equipped a research Siemens Artiste linear accelerator (linac) with a 41 × 41 cm(2) a-Si flat-panel detector underneath the treatment head. The in-line geometry allows kV (imaging) and MV (treatment) beams to share closely aligned beam axes. The kV source, usually mounted directly across from the flat-panel imager, was retracted toward the gantry by 13 cm to intentionally misalign kV and MV beams, resulting in a geometric separation of MV treatment field and kV image on the detector. Two consecutive images acquired within 140 ms (the first with MV-only and the second with kV and MV signal) were subtracted to generate a kV-only image. The images were then analyzed "online" with an automated threshold-based marker detection algorithm. They employed a 3D and a 4D phantom equipped with either a single radioopaque marker or three Calypso beacons to mimic respiratory motion. Measured room positions were either cross-referenced with a phantom voltage signal (single marker) or the Calypso system. The accuracy of the back-projection (from detected marker positions into room coordinates) was verified by a simulation study. A phantom study has demonstrated that the imaging framework is capable of automatically detecting marker positions and sending this information to the tracking tool at an update rate of 7.14 Hz. The system latency is 86.9 ± 1.0 ms for single marker detection in the absence of MV radiation. In the presence of a circular MV field of 5 cm diameter, the latency is 87.1 ± 0.9 ms. The total RMS position detection accuracy is 0.20 mm (without MV radiation) and 0.23 mm (with MV). Based on the evaluated motion patterns and MV field size, the positional accuracy and system latency indicate that this system is suitable for real-time adaptive applications.

Volumetric modulated arc planning for lung stereotactic body radiotherapy using conventional and unflattened photon beams: a dosimetric comparison with 3D technique

PurposeFrequently, three-dimensional (3D) conformal beams are used in lung cancer stereotactic body radiotherapy (SBRT). Recently, volumetric modulated arc therapy (VMAT) was introduced as a new treatment modality. VMAT techniques shorten delivery time, reducing the possibility of intrafraction target motion. However dose distributions can be quite different from standard 3D therapy. This study quantifies those differences, with focus on VMAT plans using unflattened photon beams.MethodsA total of 15 lung cancer patients previously treated with 3D or VMAT SBRT were randomly selected. For each patient, non-coplanar 3D, coplanar and non-coplanar VMAT and flattening filter free VMAT (FFF-VMAT) plans were generated to meet the same objectives with 50 Gy covering 95% of the PTV. Two dynamic arcs were used in each VMAT plan. The couch was set at ± 5° to the 0° straight position for the two non-coplanar arcs. Pinnacle version 9.0 (Philips Radiation Oncology, Fitchburg WI) treatment planning system with VMAT capabilities was used. We analyzed the conformity index (CI), which is the ratio of the total volume receiving at least the prescription dose to the target volume receiving at least the prescription dose; the conformity number (CN) which is the ratio of the target coverage to CI; and the gradient index (GI) which is the ratio of the volume of 50% of the prescription isodose to the volume of the prescription isodose; as well as the V20, V5, and mean lung dose (MLD). Paired non-parametric analysis of variance tests with post-tests were performed to examine the statistical significance of the differences of the dosimetric indices.ResultsDosimetric indices CI, CN and MLD all show statistically significant improvement for all studied VMAT techniques compared with 3D plans (p < 0.05). V5 and V20 show statistically significant improvement for the FFF-VMAT plans compared with 3D (p < 0.001). GI is improved for the FFF-VMAT and the non-coplanar VMAT plans (p < 0.01 and p < 0.05 respectively) while the coplanar VMAT plans do not show significant difference compared to 3D plans. Dose to the target is typically more homogeneous in FFF-VMAT plans. FFF-VMAT plans require more monitor units than 3D or non-coplanar VMAT ones.ConclusionBesides the advantage of faster delivery times, VMAT plans demonstrated better conformity to target, sharper dose fall-off in normal tissues and lower dose to normal lung than the 3D plans for lung SBRT. More monitor units are often required for FFF-VMAT plans.

Image-guided radiation therapy for muscle-invasive bladder cancer

Organ preservation protocols that incorporate chemoradiotherapy have shown good efficacy in bladder cancer. Owing to changes in rectal filling, urinary inflow and subsequent bladder volume with bladder wall deformations, irradiation must take into account interfractional and intrafractional internal target motion. Growing evidence suggests that image guidance during irradiation is essential in order to appropriately treat bladder cancer in this way. We performed a literature search on the imaging techniques and margins used for radiation therapy planning in the context of whole-bladder and partial-bladder irradiation. The most common image-guided radiation therapy (IGRT) method was based on cone-beam CT using anisotropic margins. The role of cine-MRI for the prediction of intraindividual bladder changes, in association with cone-beam CT or ultrasonography, is promising. Drinking protocols, diet and laxatives were used in most cases to minimize large variations in bladder size and shape. IGRT is crucial for avoiding tumor undercoverage and undue toxicity during radiation therapy for bladder cancer. IGRT-based adaptive radiation therapy can be performed using cone-beam CT or ultrasonography: modeling of bladder changes with cine-MRI or other imaging techniques might also be useful for facilitating adaptive radiation therapy with personalized margins.

SU-E-J-25: Just-in-Time Tomography (JiTT): A New Imaging Technique for Monitoring Target Motion during Volumetric Modulated Arc Therapy (VMAT)

Purpose: Volumetric modulated arc therapy (VMAT) is a new efficient way of delivering intensity‐modulated radiation therapy (IMRT) on a conventional linear accelerator(Linac) in which the gantry rotates while the beam is on. Intrafraction target motion is still a concern for high precision VMAT treatments. In this work, we propose to apply a new concept — just‐in‐time tomography (JiTT) to monitor target motion during a VMAT delivery. Methods: Similar to the cone beam computed tomography(CBCT) approach, JiTT (Pang and Rowlands, Phys. Med. Biol. 50, N323 (2005)) uses a gantry mounted X‐ray source (e.g., a kV source on a Linac) and an X‐ray area detector to acquire projection data by simultaneously rotating them around the target. Differing from CBCT, JiTT can be performed during the treatment (i.e., imaging and treatment at the same time), and it takes less time to generate the needed tomographical, beam's‐eye‐view images of the treatment target in real time. A computer simulation using MATLAB has been conducted to investigate the feasibility of this new approach. Results: JiTT images of the Shepp‐Logan phantom with motion have been obtained. It has been shown that target motion can result in significant motion artifacts in JiTT images. These motion artifacts in the images could then be used to determine target motion and guide the VMAT treatment. Conclusions: We have proposed to apply a new concept — just‐in‐time tomography (JiTT) to image‐guided VMAT delivery on a conventional Linac equipped with a kilovoltage cone beam computed tomography (kV‐CBCT) system. A computer simulation has been conducted and it has been demonstrated that JiTT images are sensitive to target motion.

SU-E-T-601: Patient Specific Margin Selection to Compensate for Intrafraction Motion during External Beam Radiation Therapy of the Lung

Purpose: Attaining a positive outcome from external beam radiation therapy (RT) is heavily dependent on delivering the prescribed radiationdose to the intended structures. RT of lungcancers is complicated by target motion caused by the patient's natural breathing during treatmentdelivery, commonly known as ‘intrafraction motion’. This target motion must be managed to help ensure successful treatment. Methods: In order to accommodate target motion on a patient specific basis, we have employed a convolution model to predict the ‘blurred’ dose distribution delivered in the presence of known target motion. The convolution model requires two inputs: the planned ‘static’ dose distribution and a probability distribution function (PDF) describing the position of the target over the course of a breathing cycle. The model was experimentally verified by Gafchromic EBT2 film measurements. In a simulation study, 502 unique patient breathing traces were used to generate corresponding blurred dose distributions. Results: Analysis of the differences between the static and blurred dose distributions with respect to characteristics of the PDFs revealed strong trends between dose coverage metrics (percent mean dose difference and penumbral width) and features of the PDFs (amplitude, standard deviation, magnitude of maximum gradient and location of maximum gradient). In particular the magnitude of the maximum PDF gradient showed a clear inverse relationship with penumbral width, while the location of the maximum PDF gradient (measured relative to the PDF's geometric center) has a clear positive correlation with penumbral width. Conclusion: The convolution model has been verified for intrafraction target motion and analysis of changes present in the blurred dose distributions highlighted trends which can be used by physicians to guide target margin selection on a patient specific basis.

Observations on prostate intrafraction motion and the effect of reduced treatment time using volumetric modulated arc therapy

PurposeIntensity modulated radiation therapy (IMRT) has allowed accurate delivery of prostate radiotherapy; Volumetric modulated arc therapy (VMAT) offers an advancement of this technique with possible dosimetric advantages and delivery in a shorter time than standard IMRT. We hypothesize that treatment duration is a controllable factor associated with intrafraction target motion. MethodsIncluded patients were treated for localized prostate cancer using IMRT or VMAT (RapidArc, Varian Medical Systems, Palo Alto, CA). Continuous motion data were monitored simultaneously using electromagnetic transponders (Calypso 4D Localization System, Calypso Medical Technologies, Inc, Seattle, WA). Displacements were recorded in the RL (right-left), SI (superior-inferior), and AP (anterior-posterior) directions at 10/second (Hz). Daily motion was reported as the mean (R̄̄) and 95th percentile (R95) displacement value for the entire session. Time effect was assessed by measuring daily displacement variables (R̄̄, R95) after each successive minute of treatment. ResultsThirty-seven patients were included, accounting for 1332 treatment sessions. Mean session time was 7.4 minutes (range, 0.5-37.2; interquartile range, 4.8-9.2). R̄̄ (0.06, 0.08, 0.11, 0.18) and R95 (0.14, 0.18, 0.23, 0.36) values (RL, SI, AP, 3-dimensional [3D], respectively) were evaluated for the entire cohort. Regression analysis showed treatment time to be the strongest predictor of observed displacements (P < .001 AP, SI, 3D; P < 0.05 RL). Ninety-five displacements increased continuously from 0.05 cm, 0.09 cm, 0.12 cm, and 0.16 cm after 1 minute to 0.21 cm, 0.20 cm, 0.29 cm, and 0.47 after 10 minutes (RL, SI, AP, and 3D). Mean session time for VMAT was 4.6 minutes compared to 8.4 minutes for IMRT (difference = 3.8 min, P < .0001); VMAT was associated with reduced motion for both (difference = 0.02, 0.03, 0.05, 0.07 cm) and (0.03, 0.04, 0.11, 0.12 cm) displacements. ConclusionOur study is unique in exploring the role of session duration on intrafraction motion in the setting of electromagnetic transponders as well as VMAT. Our main results demonstrate that observed intrafraction prostate motion during radiotherapy is greater with increasing session time. Additionally, VMAT, due to shorter treatment sessions, resulted in significant reduction (30%-40%) in intrafraction displacements.