bluuebunny/crossref_metadata_2025 huggingface dataset:
SU‐E‐J‐161: Quality Assurance of 4DCT Process Using ET Gating Phantom System
Authors: Chen H , Chen Z , Nath R , Guo F
Published: June 2015

Purpose:

To develop a robust quality assurance procedure for 4DCT process using ET gating phantom (ET‐GPS), to optimize 4DCT scan protocols, and to investigate practical solutions for patients who have difficulties to be scanned with 4DCT.

Methods:

ET‐GPS (BrainLab AB) in conjunction with the marker block is used to test the motion tracking of 4DCT imaging and Real‐time Position Management (RPM) system (Varian). Several standard sinusoidal motion profiles as respiratory simulation were used as reference. A maximal motion range of 2.5mm to 16.5mm for horizontal motion (HM) and 6.4mm to 38.0mm for vertical motion (VM), with breathing period time (T) of 3s, 5s, and 7s, were applied to ET‐GPS, respectively. 4DCT images were acquired by LightSpeed 16‐slice CT simulator (GE Medical System), using our general 4DCT chest imaging protocol (cine, 120kVp, 350mA, 0.8s) with scan thicknesses of 1.25mm and 2.5mm, respectively. Maximal motion ranges obtained from 4DCT imaging were analyzed and compared to the references. Furthermore, acquired trajectory profiles from RPM system were played back to the ETGPS, agreement between 2nd acquired trajectories and reference sinusoidal profiles were analyzed quantitatively.

Results:

Test results of maximal horizontal motion ranges from 4DCT image agree well with the physical measurement (<3%) when the scan thickness is 1.25mm and with T = 5s or 7s. Motion range discrepancy is up to 13.5% for HM when 2.5mm thick is applied. The VM differences are less than 7%. ET‐GPS can reproduce the patient motion profiles very well.

Conclusion and discussion:

The developed method using ET‐GPS is a feasible and efficient way for quality assurance of 4DCT process. Imaging protocols can be further optimized when agreement is out of the expected. Further 4DCT QA procedure with different HU inserts and customized patient breathing profile will be investigated.
SU‐E‐J‐163: A Biomechanical Lung Model for Respiratory Motion Study
Authors: Liu X , Belcher AH , Grelewicz Z , Wiersma RD
Published: June 2015

Purpose:

This work presents a biomechanical model to investigate the complex respiratory motion for the lung tumor tracking in radiosurgery by computer simulation.

Methods:

The models include networked massspring‐dampers to describe the tumor motion, different types of surrogate signals, and the force generated by the diaphragm. Each mass‐springdamper has the same mechanical structure and each model can have different numbers of mass‐spring‐dampers. Both linear and nonlinear stiffness parameters were considered, and the damping ratio was tuned in a range so that the tumor motion was over‐damped (no natural tumor oscillation occurs without force from the diaphragm). The simulation was run by using ODE45 (ordinary differential equations by Runge‐Kutta method) in MATLAB, and all time courses of motions and inputs (force) were generated and compared.

Results:

The curvature of the motion time courses around their peaks was sensitive to the damping ratio. Therefore, the damping ratio can be determined based on the clinical data of a high sampling rate. The peak values of different signals and the time the peaks occurred were compared, and it was found that the diaphragm force had a time lead over the tumor motion, and the lead time (0.1–0.4 seconds) depended on the distance between the tumor and the diaphragm.

Conclusion:

We reported a model based analysis approach for the spatial and temporal relation between the motion of the lung tumor and the surrogate signals. Due to the phase lead of the diaphragm in comparing with the lung tumor motion, the measurement of diaphragm motion (or its electromyography signal) can be used as a beam gating signal in radiosurgery, and it can also be an additional surrogate signal for better tumor motion tracking.

The research is funded by the American Cancer Society (ACS) grant. The grant name is: Frameless SRS Based on Robotic Head Motion Cancellation. The grant number is: RSG‐13‐313‐01‐CCE
SU‐E‐J‐165: Dosimetric Impact of Liver Rotations in Stereotactic Body Radiation Therapy
Authors: Pinnaduwage D , Paulsson A , Sudhyadhom A , Chen J , Chang A , Anwar M , Gottschalk A , Yom S S. , Descovich M
Published: June 2015

Purpose:

Often in liver stereotactic body radiotherapy a single fiducial is implanted near the tumor for image‐guided treatment delivery. In such cases, rotational corrections are calculated based on the spine. This study quantifies rotational differences between the spine and liver, and investigates the corresponding dosimetric impact.

Methods:

Seven patients with 3 intrahepatic fiducials and 4DCT scans were identified. The planning CT was separately co‐registered with 4 phases of the 4DCT (0%, 50%, 100% inhale and 50% exhale) by 1) rigid registration of the spine, and 2) point‐based registration of the 3 fiducials. Rotation vectors were calculated for each registration. Translational differences in fiducial positions between the 2 registrations methods were investigated. Dosimetric impact due to liver rotations and deformations was assessed using critical structures delineated on the 4DCT phases. For dose comparisons, a single fiducial was translationally aligned following spine alignment to represent what is typically done in the clinic.

Results:

On average, differences between spine and liver rotations during the 0%, 50%, 100% inhale, and 50% exhale phases were 3.23°, 3.27°, 2.26° and 3.11° (pitch), 3.00°, 2.24°, 3.12° and 1.73° (roll), and 1.57°, 1.98°, 2.09° and 1.36° (yaw), respectively. The maximum difference in rotations was 12°, with differences of >3° seen in 14/28 (pitch), 10/28 (roll), and 6/28 (yaw) cases. Average fiducial displacements of 2.73 (craniocaudal), 1.04 (lateral) and 1.82 mm (vertical) were seen. Evaluating percent dose differences for 5 patients at the peaks of the respiratory cycle, the maximum dose to the duodenum, stomach, bowel and esophagus differed on average by 11.4%, 5.3%, 11.2% and 49.1% between the 2 registration methods.

Conclusion:

Lack of accounting for liver rotation during treatment might Result in clinically significant dose differences to critical structures. Both rotational and translational deviations should be considered in planning margins when using spine alignment for liver treatments.
SU‐E‐J‐166: Sensitivity of Clinically Relevant Dosimetric Parameters to Contouring Uncertainty During Post Implant Dosimetry of Prostate Permanent Seed Implants
Authors: Mashouf S , Ravi A , Morton G , Song W
Published: June 2015

Purpose:

There is a strong evidence relating post‐implant dosimetry for permanent seed prostate brachytherpy to local control rates. The delineation of the prostate on CT images, however, represents a challenge as it is difficult to confidently identify the prostate borders from soft tissue surrounding it. This study aims at quantifying the sensitivity of clinically relevant dosimetric parameters to prostate contouring uncertainty.

Methods:

The post‐implant CT images and plans for a cohort of 43 patients, who have received I–125 permanent prostate seed implant in our centre, were exported to MIM Symphony LDR brachytherapy treatment planning system (MIM Software Inc., Cleveland, OH). The prostate contours in post‐implant CT images were expanded/contracted uniformly for margins of ±1.00mm, ±2.00mm, ±3.00mm, ±4.00mm and ±5.00mm (±0.01mm). The values for V100 and D90 were extracted from Dose Volume Histograms for each contour and compared.

Results:

The mean value of V100 and D90 was obtained as 92.3±8.4% and 108.4±12.3% respectively (Rx=145Gy). V100 was reduced by −3.2±1.5%, −7.2±3.0%, −12.8±4.0%, −19.0±4.8%, − 25.5±5.4% for expanded contours of prostate with margins of +1mm, +2mm, +3mm, +4mm, and +5mm, respectively, while it was increased by 1.6±1.2%, 2.4±2.4%, 2.7±3.2%, 2.9±4.2%, 2.9±5.1% for the contracted contours. D90 was reduced by −6.9±3.5%, −14.5±6.1%, −23.8±7.1%, − 33.6±8.5%, −40.6±8.7% and increased by 4.1±2.6%, 6.1±5.0%, 7.2±5.7%, 8.1±7.3% and 8.1±7.3% for the same set of contours.

Conclusion:

Systematic expansion errors of more than 1mm may likely render a plan sub‐optimal. Conversely contraction errors may Result in labeling a plan likely as optimal. The use of MRI images to contour the prostate should results in better delineation of prostate organ which increases the predictive value of post‐op plans. Since observers tend to overestimate the prostate volume on CT, compared with MRI, the impact of the contouring uncertainty on V100 and D90 fortunately, has a conservative effect of underestimating the prostate coverage.
SU‐E‐J‐171: The Dosimetric Consequence of a Failed Breath Hold for Left‐Sided DIBH Breast Treatment
Authors: Sintay B , Wentworth S , Vanderstraeten C , Maurer J , Liu H , Hayes L , Wiant D
Published: June 2015

Purpose:

The purpose of this work is to determine the dosimetric consequence of a failed breath hold which may arise out of failure to prompt the patient before initiating treatment or lack of compliance from the patient in taking or maintaining the breath hold. In this context, the patient is assumed to be aligned properly for treatment to the breath hold surface.

Methods:

Ten left‐sided breast cancer patients were simulated for treatment in the free‐breathing (FB‐CT) and deep‐inspiration breath hold (DIBH‐CT) positions using a bellows device to monitor respiration. The CT scans were fused using a common rigid coordinate system in the underlying couch allowing the analysis of the identical setup position for either FB‐CT or DIBH‐CT. The breast tissue, lumpectomy cavity, heart, and lungs were contoured on each CT. Patients were planned for treatment to 50Gy using a tangent field‐in‐field technique on the DIBH CT scan. The treatment plan was subsequently copied to the free‐breathing scan in the same geometric position and recalculated.

Results:

No statistically significant dose difference was found for the heart (V30, V20, mean, and maximum) or lumpectomy cavity (mean, maximum, minimum, and V95). However, in two cases, the lumpectomy cavity V95 was 20% less for FB‐CT. The lung V20 decreased by an average of 3.7±1.0% (p=0.004) for FB‐CT. No statistically significant dose difference was found for the mean and maximum breast dose. The minimum breast dose (16.9±3.4 for DIBH) decreased by an average of 11±3.4% (p=0.0001), and the breast V95 (84.7±2.0% for DIBH) decreased by an average of 13.9±7.2% (p=0.003).

Conclusion:

Our results show that a failed breath hold would not negatively affect the surrounding organs and increased lung sparing. In our study, the lumpectomy cavity remained satisfactorily targeted in 8 out of 10 cases. However, the minimum and V95 breast doses decreased considerably.
SU‐E‐J‐172: Bio‐Physical Effects of Patients Set‐Up Errors According to Whole Breast Irradiation Techniques
Authors: Lee S , Suh T , Park S , Kim M , Park J , Lee M
Published: June 2015

Purpose:

The dose‐related effects of patient setup errors on biophysical indices were evaluated for conventional wedge (CW) and field‐in‐field (FIF) whole breast irradiation techniques.

Methods:

The treatment plans for 10 patients receiving whole left breast irradiation were retrospectively selected. Radiobiological and physical effects caused by dose variations were evaluated by shifting the isocenters and gantry angles of the treatment plans. Dose‐volume histograms of the planning target volume (PTV), heart, and lungs were generated, and conformity index (CI), homogeneity index (HI), tumor control probability (TCP), and normal tissue complication probability (NTCP) were determined.

Results:

For “isocenter shift plan” with posterior direction, the D95 of the PTV decreased by approximately 15% and the TCP of the PTV decreased by approximately 50% for the FIF technique and by 40% for the CW; however, the NTCPs of the lungs and heart increased by about 13% and 1%, respectively, for both techniques. Increasing the gantry angle decreased the TCPs of the PTV by 24.4% (CW) and by 34% (FIF). The NTCPs for the two techniques differed by only 3%. In case of CW, the CIs and HIs were much higher than that of the FIF in all cases. It had a significant difference between two techniques (p<0.01). According to our results, however, the FIF had more sensitive response by set up errors rather than CW in bio‐physical aspects.

Conclusions:

The radiobiological‐based analysis can detect significant dosimetric errors then, can provide a practical patient quality assurance method to guide the radiobiological and physical effects.
SU‐E‐J‐174: Volume Based Analysis of Several Doses Point and Their Relation to Changing Volume in a 4DCT
Authors: Nawaz AO , Schanne R , Ochsner G , Chang W
Published: June 2015

Purpose:

To evaluate several dose points in the low dose region of healthy lungs during XRT and their relationship to lung volume.

Methods:

10 lung cancer patients that had 4DCT simulations were selected for this study retrospectively. The lungs were countered in each of the 10 respiratory cycles. The clinical treatment plan was copied to each phase and the dose was recomputed. DVHs were tabulated for each phase and the V5, V20 and V30 dose points of the total lung, along with the volume of the lung for each phase was recorded. All these dose points and volumes were normalized to the free breathing scan and plan so as to establish if there was a trend, and what the nature of this trend was as a function of lung volume.

Results:

Of the first 4 patient analyzed, there was an established and reoccurring trend of the V5, V20, V30 dose normalized to the free breathing scan and plan. In the example of the two patients’ data(attached figure), normalized dose point and volumes are graphed as a function of the breathing phases. The precise variation of each point is not the same across each patient; however the trend of each variation seems predictive in nature. The next steps would be to run a statistical T‐test with all the patient data and see if there is a statistical significance and then try to establish a predictive model.

Conclusion:

Dose points for the lung such as V5, V20 and V30 are now better understood as toxicity end points for lung XRT. Although attention is paid to the low dose region of healthy lung tissue, this study shows that the volumetric affect on dose during breathing phases is measurable and could be statically significant. It therefore warrants further study to better understand this relationship.
SU‐E‐J‐179: Assessment of Tumor Volume Change and Movement During Stereotactic Body Radiotherapy (SBRT) for Lung Cancer: Is Adaptive Radiation Therapy (ART) Necessary?
Authors: Lee C , Lee C
Published: June 2015

Purpose:

Delineation of gross tumor volumes (GTVs) is important for stereotactic body radiotherapy (SBRT). However, tumor volume changes during treatment response. Here, we have investigated tumor volume changes and movement during SBRT for lung cancer, as a means of examining the need for adaptive radiation therapy (ART).

Methods:

Fifteen tumors in 15 patients with lung cancer were treated with SBRT (total dose: 60 Gy in 4 fractions). GTVs were obtained from cone‐beam computed tomography scans (CBCT1–4) taken before each of the 4 fractions was administered. GTVs were delineated and measured by radiation oncologists using a treatment planning system. Variance in the tumor position was assessed between the planning CT and the CBCT images. To investigate the dosimetric effects of tumor volume changes, planning CT and CBCT4 treatment plans were compared using the conformity index (CI), homogeneity index (HI), and Paddick's index (PCI).

Results:

The GTV on CBCT1 was employed as a baseline for comparisons. GTV had decreased by a mean of 20.4% (range: 0.7% to 47.2%) on CBCT4. Most patients had smaller GTVs on CBCT4 than on CBCT1. The interfractional shifts of the tumor position between the planning CT and CBCT1–4 were as follows: right‐left, −0.4 to 1.3 mm; anterior‐posterior, −0.8 to 0.5 mm; and superiorinferior, −0.9 to 1.1 mm. Indices for plans from the planning CT and CBCT4 were as follows: CI = 0.94±0.02 and 1.11±0.03; HI= 1.1±0.02 and 1.10±0.03; and PCI = 1.35±0.16 and 1.11±0.02, respectively.

Conclusion:

CI, HI, and PCI did not differ between the planning CT and CBCTs. However, daily CBCT revealed a significant decrease in the GTV during lung SBRT. Furthermore, there was an obvious interfractional shift in tumor position. Using ART could potentially lead to a reduced GTV margin and improved regional tumor control for lung cancer patients with significantly decreased GTV.
SU‐E‐J‐180: Imaging‐Based ITV May Provide Insufficient Internal Margin for Lung SBRT Patients with Tumor Misalignments Between 3D and 4D Planning CTs
Authors: Zheng D , Zhen W , Lei Y , Denniston K , Driewer J , Zhang Q , Zhu X , Wang S , Zhou S
Published: June 2015

Purpose:

To evaluate the target positional differences between 3D/4D planning CTs (pCTs) and daily CBCTs, and assess ITV coverage of daily GTVs, for lung SBRT patients with misaligned tumor positions between the 3D and 4D pCTs.

Methods:

Simulation 3D and 4D pCTs, both under free breathing, were acquired on a Sensation Open (Siemens) with RPM (Varian). ITV_All is the union of the GTVs delineated on the 3D pCT (GTV_3D) and on all phases of the 4D pCT (ITV_4D). While for most patients GTV_3D is included in ITV_4D, for some patients GTV_3D shows a substantial misalignment in the axial plane from ITV_4D with spine‐based rigid registration. Under IRB approval, pCTs of 51 patients were retrospectively reviewed to assess the frequency of such misalignments. For one patient with daily kV CBCTs available, all planning and treatment CTs were rigidly registered. GTV_3D, ITV_4D, GTV_F1 through GTV_F5 (on daily CBCTs of Fractions 1–5) were delineated. ITV_All and ITV_5mm (5mm uniform expansion of GTV_3D) were generated. Volumes and centers‐of‐mass (COMs) of all structures were analyzed. The inclusion relations (percentage of Structure‐A included in Structure‐B) were studied between GTV_F1 to GTV_F5 (Structure‐As) and GTV_3D, ITV_4D, ITV_All, ITV_5mm (Structure‐Bs).

Results:

Three patients showed substantial target misalignments between the 3D and 4D pCTs. For the analyzed patient, daily GTV positions agreed better with GTV_3D than with ITV_4D, with a median (range) COM vector difference of 2.45mm (1.00–4.58) and 16.06mm (15.39–17.44), respectively. The average inclusion of daily GTVs was 99.47% and 69.53% in ITV_5mm and ITV_All, respectively; although ITV_5mm was ∼35% larger than ITV_All for the ∼2cc tumor (GTV_3D).

Conclusion:

Substantial lung tumor misalignments were observed between 3D and 4D pCTs for some patients. For one such patient, daily tumor positions agreed better with the 3D pCT. Using spinebased localization for SBRT, ITV_All may provide insufficient internal margin.
SU‐E‐J‐181: Effect of Prostate Motion On Combined Brachytherapy and External Beam Dose Based On Daily Motion of the Prostate
Authors: Narayana V , Ealbaj J , McLaughlin P
Published: June 2015

Purpose:

In this study, the adequacy of target expansions on the combined external beam and implant dose was examined based on the measured daily motion of the prostate.

Methods:

Thirty patients received an I–125 prostate implant prescribed to dose of 90Gy. This was followed by external beam to deliver a dose of 90Gyeq (external beam equivalent) to the prostate over 25 to 30 fractions. An ideal IMRT plan was developed by optimizing the external beam dose based on the delivered implant dose. The implant dose was converted to an equivalent external beam dose using the linear quadratic model. Patients were set up on the treatment table by daily orthogonal imaging and aligning the marker seeds in the prostate. Orthogonal images were obtained at the end of treatment to assess prostate intrafraction motion. Based on the observed motion of the markers between the initial and final images, 5 individual plans showing the actual dose delivered to the patient were calculated. A final true dose distribution was established based on summing the implant dose and the 5 external beam plans. Dose to the prostate, seminal vesicles, lymphnodes and normal tissues, rectal wall, urethra and lower sphincter were calculated and compared to ideal. On 18 patients who were sexually active, dose to the corpus cavernosum and internal pudendal artery was also calculated.

Results:

The average prostate motion in 3 orthogonal directions was less than 1 mm with a standard deviation of less than +2 mm. Dose and volume parameters showed that there was no decrease in dose to the targets and a marginal decrease in dose to in normal tissues.

Conclusion:

Dose delivered by seed implant moves with the prostate, decreasing the impact of intrafractions dose movement on actual dose delivered. Combined brachytherapy and external beam dose delivered to the prostate was not sensitive to prostate motion.
SU‐E‐J‐187: Management of Optic Organ Motion in Fractionated Stereotactic Radiotherapy
Authors: Manning M , Maurer J
Published: June 2015

Purpose:

Fractionated stereotactic radiotherapy (FSRT) for optic nerve tumors can potentially use planning target volume (PTV) expansions as small as 1–5 mm. However, the motion of the intraorbital segment of the optic nerve has not been studied.

Methods:

A subject with a right optic nerve sheath meningioma underwent CT simulation in three fixed gaze positions: right, left, and fixed forward at a marker. The gross tumor volume (GTV) and the organs‐at‐risk (OAR) were contoured on all three scans. An IMRT plan using 10 static non‐coplanar fields to 50.4 Gy in 28 fractions was designed to treat the fixed‐forward gazing GTV with a 1 mm PTV, then resulting coverage was evaluated for the GTV in the three positions. As an alternative, the composite structures were computed to generate the internal target volume (ITV), 1 mm expansion free‐gazing PTV, and planning organat‐risk volumes (PRVs) for free‐gazing treatment. A comparable IMRT plan was created for the free‐gazing PTV.

Results:

If the patient were treated using the fixed forward gaze plan looking straight, right, and left, the V100% for the GTV was 100.0%, 33.1%, and 0.1%, respectively. The volumes of the PTVs for fixed gaze and free‐gazing plans were 0.79 and 2.21 cc, respectively, increasing the PTV by a factor of 2.6. The V100% for the fixed gaze and free‐gazing plans were 0.85 cc and 2.8 cc, respectively increasing the treated volume by a factor of 3.3.

Conclusion:

Fixed gaze treatment appears to provide greater organ sparing than free‐gazing. However unanticipated intrafraction right or left gaze can produce a geometric miss. Further study of optic nerve motion appears to be warranted in areas such as intrafraction optical confirmation of fixed gaze and optimized gaze directions to minimize lens and other normal organ dose in cranial radiotherapy.
SU‐E‐J‐190: Development of Abdominal Compression & Respiratory Guiding System Using Gas Pressure Sensor
Authors: Kim T , Kim S , Kim D , Kang S , Cho M , Kim K , Shin D , Suh T
Published: June 2015

Purpose:

Abdominal compression is known to be effective but, often makes external‐marker‐based monitoring of breathing motion not feasible. In this study, we developed and evaluated a system that enables both abdominal compression and monitoring of residual abdominal motion simultaneously. The system can also provide visual‐biofeedback capability.

Methods:

The system developed consists of a compression belt, an abdominal motion monitoring sensor (gas pressure sensor) and a visual biofeedback device. The compression belt was designed to be able to compress the frontal side of the abdomen. The pressure level of the belt is controlled by air volume and monitored in real time using the gas pressure sensor. The system displays not only the real‐time monitoring curve but also a guiding respiration model (e.g., a breath hold or shallow breathing curve) simultaneously on the head mounted display to help patients keep their breathing pattern as consistent as possible. Three healthy volunteers were enrolled in this pilot study and respiratory signals (pressure variations) were obtained both with and without effective abdominal compression to investigate the feasibility of the developed system. Two guidance patterns, breath hold and shallow breathing, were tested.

Results:

All volunteers showed smaller abdominal motion with compression (about 40% amplitude reduction compared to without compression). However, the system was able to monitor residual abdominal motion for all volunteers. Even under abdominal compression, in addition, it was possible to make the subjects successfully follow the guide patterns using the visual biofeedback system.

Conclusion:

The developed abdominal compression & respiratory guiding system was feasible for residual abdominal motion management. It is considered that the system can be used for a respiratory motion involved radiation therapy while maintaining the merit of abdominal compression.

This work was supported by the Radiation Technology R&D program (No. 2013M2A2A7043498) and the Mid‐career Researcher Program (2014R1A2A1A10050270) through the National Research Foundation of Korea funded by the Ministry of Science, ICT&Future Planning.
SU‐E‐J‐200: Operation of An Electron Accelerator On An Integrated MR‐Linac System
Authors: Harasimowicz J , Kok J , Overweg J , Roberts D , Shinton I , Sund S , Wang H , Zhong M
Published: June 2015

Purpose:

An integrated MRI guided radiotherapy system poses a challenge of operating a linear accelerator in the presence of a magnetic field as the magnetic force acting on the electrons could Result in radiation source displacement and subsequent reduction of dose output. It was the purpose of this work to test the performance of a linac in the presence of a 1.5T MRI system.

Methods:

The first experimental MRI guided radiotherapy system at UMC‐Utrecht consisting of an Elekta linac rotating around a 1.5T Magnex magnet was examined. A passive magnetic shield was simulated, designed and installed to reduce the influence of the MRI magnet stray field on the electron beamline. The B field inside the shield was measured as a function of gantry angle and measurements of dose rate constancy upon gantry rotation were performed.

Results:

The magnitude of the magnetic field on the electron beam path without the shield was as high as 70G. It varied by up to 15G with gantry rotation due to the presence of metal beams in the bunker floor which resulted in dose output drop of up to 70% at certain gantry angles. With the prototype shield, field magnitude was reduced to well below 0.5G everywhere along the electron beam path. Field variation with gantry rotation was decreased to below 0.2G and enabled dose output of the linac to be recovered at all gantry angles. The homogeneity of the field inside the MRI magnet has not been compromised.

Conclusion:

It was demonstrated that the influence of the 1.5T magnet and the bunker design on the linac operation has been minimised. The performance will be further improved on the Elekta Atlantic system which incorporates a newly developed and optimised Philips magnet design and bunker construction.

J Harasimowicz, D Roberts, I Shinton and S Sund are employed by Elekta Limited Crawley, H Wang and M Zhong are employed by Elekta Beijing Medical Systems Co. Ltd., J Overweg is employed by Philips Technologie GmbH Forschungslaboratorien.
SU‐E‐J‐201: Investigation of MRI Guided Proton Therapy
Authors: Li JS
Published: June 2015

Purpose:

Image‐guided radiation therapy has been employed for cancer treatment to improve the tumor localization accuracy. Radiation therapy with proton beams requires more on this accuracy because the proton beam has larger uncertainty and dramatic dose variation along the beam direction. Among all the image modalities, magnetic‐resonance image (MRI) is the best for soft tissue delineation and real time motion monitoring. In this work, we investigated the behavior of the proton beam in magnetic field with Monte Carlo simulations.

Methods:

A proton Monte Carlo platform, TOPAS, was used for this investigation. Dose calculations were performed with this platform in a 30cmx30cmx30cm water phantom for both pencil and broad proton beams with different energies (120, 150 and 180MeV) in different magnetic fields (0.5T, 1T and 3T). The isodose distributions, dose profiles in lateral and beam direction were evaluated. The shifts of the Bragg peak in different magnetic fields for different proton energies were compared and the magnetic field effects on the characters of the dose distribution were analyzed.

Results:

Significant effects of magnetic field have been observed on the proton beam dose distributions, especially for magnetic field of 1T and up. The effects are more significant for higher energy proton beam because higher energy protons travel longer distance in the magnetic field. The Bragg peak shift in the lateral direction is about 38mm for 180MeV and 11mm for 120MeV proton beams in 3T magnetic field. The peak positions are retracted back for 6mm and 2mm, respectively. The effect on the beam penumbra and dose falloff at the distal edge of the Bragg peak is negligible.

Conclusion:

Though significant magnetic effects on dose distribution have been observed for proton beams, MRI guided proton therapy is feasible because the magnetic effects on dose is predictable and can be considered in patient dose calculation.
SU‐E‐J‐210: Characterizing Tissue Equivalent Materials for the Development of a Dual MRI‐CT Heterogeneous Anthropomorphic Phantom Designed Specifically for MRI Guided Radiotherapy Systems
Authors: Steinmann A , Stafford R , Yung J , Followill D
Published: June 2015

Purpose:

MRI guided radiotherapy (MRIgRT) is an emerging technology which will eventually require a proficient quality auditing system. Due to different principles in which MR and CT acquire images, there is a need for a multi‐imaging‐modality, end‐to‐end QA phantom for MRIgRT. The purpose of this study is to identify lung, soft tissue, and tumor equivalent substitutes that share similar human‐like CT and MR properties (i.e. Hounsfield units and relaxation times).

Methods:

Materials of interested such as common CT QA phantom materials, and other proprietary gels/silicones from Polytek, SmoothOn, and CompositeOne were first scanned on a GE 1.5T Signa HDxT MR. Materials that could be seen on both T1‐weighted and T2‐weighted images were then scanned on a GE Lightspeed RT16 CT simulator and a GE Discovery 750HD CT scanner and their HU values were then measured. The materials with matching HU values of lung (−500 to −700HU), muscle (+40HU) and soft tissue (+100 to +300HU) were further scanned on GE 1.5T Signa HDx to measure their T1 and T2 relaxation times from varying parameters of TI and TE.

Results:

Materials that could be visualized on T1‐weighted and T2‐weighted images from a 1.5T MR unit and had an appropriate average CT number, −650, −685, 46,169, and 168 HUs were: compressed cork saturated with water, Polytek Platsil™ Gel‐00 combined with mini styrofoam balls, radiotherapy bolus material, SmoothOn Dragon‐Skin™ and SmoothOn Ecoflex™, respectively.

Conclusion:

Post processing analysis is currently being performed to accurately map T1 and T2 values for each material tested. From previous MR visualization and CT examinations it is expected that Dragon‐Skin™, Ecoflex™ and bolus will have values consistent with tissue and tumor substitutes. We also expect compressed cork statured with water, and Polytek™‐styrofoam combination to have approximate T1 and T2 values suitable for lung‐equivalent materials.
SU‐E‐J‐216: A Sequence Independent Approach for Quantification of MR Image Deformations From Brachytherapy Applicators
Authors: van Wieringen N , van Heerden L , Gurney‐Champion O , van Kesteren Z , Houweling A , Pieters B , Bel A
Published: June 2015

Purpose:

MRI is increasingly used as a single imaging modality for brachytherapy treatment planning. The presence of a brachytherapy applicator may cause distortions in the images, especially at higher field strengths. Our aim is to develop a procedure to quantify these distortions theoretically for any MR‐sequence and to verify the estimated deformations for clinical sequences.

Methods:

Image distortions due to perturbation of the B0‐field are proportional to the ratio of the induced frequency shift and the read‐out bandwidth of the applied sequence. By reconstructing a frequency‐shift map from the phase data from a multi‐echo sequence, distortions can be calculated for any MR‐sequence. Verification of this method for estimating distortions was performed by acquiring images with opposing read‐out directions and consequently opposing distortions. The applicator shift can be determined by rigidly matching these images. Clinically, T2W‐TSE‐images are used for this purpose. For pre‐clinical tests, EPI‐sequences with narrow read‐out bandwidth (19.5–47.5Hz), consequently large distortions, were added to the set of clinical MRsequences. To quantify deformations of the Utrecht Interstitial CT/MR applicator (Elekta Brachytherapy) on a Philips Ingenia 3T MRI, pre‐clinical tests were performed in a phantom with the applicator in water, followed by clinical validation.

Results:

Deformations observed in the narrow bandwidth EPI‐images were well predicted using the frequency‐shift, the latter giving an overestimation up to 30%/up to 1 voxel. For clinically applied MR‐sequences distortions were well below the voxel size. In patient setup distortions determined from the frequency‐shift map were at sub‐voxel level (<0.7mm). Using T2W‐images larger distortions were found (1–2mm). This discrepancy was caused by patient movement between/during acquisition of the T2W‐images with opposing read‐out directions.

Conclusion:

Phantom experiments demonstrated the feasibility of a clinical procedure for quantification of MR‐image distortions for any MR‐sequence. In a clinical set‐up the distortions from a Utrecht interstitial CT/MR applicator are sub‐voxel level.

This work was partially funded by Elekta Brachytherpy
SU‐E‐J‐220: Assessment of MRI Geometric Distortion in Head and Neck Cancer Patients Scanned in Immobilized Radiation Treatment Position
Authors: Hansen C , Mohamed A , Weygand J , Ding Y , Fuller C , Frank S , Wang J
Published: June 2015

Purpose:

Uncertainties about geometric distortion have somewhat hindered MRI simulation in radiation therapy. Most of the geometric distortion studies were performed with phantom measurements but another major aspect of MR distortion is patient related. We studied the geometric distortion in patient images by comparing their MRI scans with the corresponding CT, using CT as the non‐distorted gold standard.

Methods:

Ten H&N cancer patients were imaged with MRI as part of a prospective IRB approved study. All patients had their treatment planning CT done on the same day or within one week of the MRI. MR Images were acquired with a T2 SE sequence (1×1×2.5mm voxel size) in the same immobilization position as in the CT scans. MRI to CT rigid registration was then done and geometric distortion comparison was done by measuring the corresponding anatomical landmarks on both the MRI and the CT images by two observers. Several skin to skin (9 landmarks), bone to bone (8 landmarks), and soft tissue (3 landmarks) were measured at specific levels in horizontal and vertical planes of both scans.

Results:

The mean distortion for all landmark measurements in all scans was 1.8±1.9mm. For each patient 11 measurements were done in the horizontal plane while 9 were done in the vertical plane. The measured geometric distortion were significantly lower in the horizontal axis compared to the vertical axis (1.3±0.16 mm vs 2.2±0.19 mm, respectively, P=0.003*). The magnitude of distortion was lower in the bone to bone landmarks compared to the combined soft tissue and skin to skin landmarks (1.2±0.19 mm vs 2.3±0.17 mm, P=0.0006*). The mean distortion measured by observer one was not significantly different compared toobserver 2 (2.3 vs 2.4 mm, P=0.4).

Conclusion:

MRI geometric distortions were quantified in H&N patients with mean error of less than 2 mm.

JW received a corporate sponsored research grant from Elekta
SU‐E‐J‐221: A Novel Expansion Method for MRI Based Target Delineation in Prostate Radiotherapy
Authors: Ruiz B , Feng Y , Shores R , Fung C
Published: June 2015

Purpose:

To compare a novel bladder/rectum carveout expansion method on MRI delineated prostate to standard CT and expansion based methods for maintaining prostate coverage while providing superior bladder and rectal sparing.

Methods:

Ten prostate cases were planned to include four trials: MRI vs CT delineated prostate/proximal seminal vesicles, and each image modality compared to both standard expansions (8mm 3D expansion and 5mm posterior, i.e. ∼8mm) and carveout method expansions (5mm 3D expansion, 4mm posterior for GTV‐CTV excluding expansion into bladder/rectum followed by additional 5mm 3D expansion to PTV, i.e. ∼1cm). All trials were planned to total dose 7920 cGy via IMRT. Evaluation and comparison was made using the following criteria: QUANTEC constraints for bladder/rectum including analysis of low dose regions, changes in PTV volume, total control points, and maximum hot spot.

Results:

∼8mm MRI expansion consistently produced the most optimal plan with lowest total control points and best bladder/rectum sparing. However, this scheme had the smallest prostate (average 22.9% reduction) and subsequent PTV volume, consistent with prior literature. ∼1cm MRI had an average PTV volume comparable to ∼8mm CT at 3.79% difference. Bladder QUANTEC constraints were on average less for the ∼1cm MRI as compared to the ∼8mm CT and observed as statistically significant with 2.64% reduction in V65. Rectal constraints appeared to follow the same trend. Case‐by‐case analysis showed variation in rectal V30 with MRI delineated prostate being most favorable regardless of expansion type. ∼1cm MRI and ∼8mm CT had comparable plan quality.

Conclusion:

MRI delineated prostate with standard expansions had the smallest PTV leading to margins that may be too tight. Bladder/rectum carveout expansion method on MRI delineated prostate was found to be superior to standard CT based methods in terms of bladder and rectal sparing while maintaining prostate coverage. Continued investigation is warranted for further validation.
SU‐E‐J‐222: Feasibility Study of MRI‐Only Proton Therapy Planning
Authors: Spadea M , Izquierdo D , Catana C , Collins‐Fekete C , Bortfeld T , Seco J
Published: June 2015

Purpose:

To assess the dosimetric equivalence of MRI based proton planning vs. single energy x‐ray CT.

Methods:

8 glioblastoma patients were imaged with CT and MRI after surgical resection. T1‐weighted 3DMPRAGE was used to delineate the GTV, which was subsequently rigidly registered to the CT volume. A pseudoCT was generated from the aligned MRI by combining segmentation and atlas‐based approaches. The spatial resolution both for pseudo‐ and real CT was 0.6×0.6×2.5mm. Three orthogonal proton beams were simulated on the pseudoCT. Two co‐planar beams were set on the axial plane. The third one was planned parallel to the cranio‐caudal (CC) direction. Each beam was set to cover the GTV at 98% of the nominal dose (18Gy). The proton plan was copied and transferred to the real CT, including aperture/compensator geometry. Dose comparison between pseudoCT and CT plan was performed beam‐by‐beam by quantifying the range shift of dose profile on each slice of the GTV. The GTV's V98 was computed for the CT.

Results:

For beams in axial plane the median absolute value of the range shift was 0.3mm, with 0.9mm and 1.4mm as 95th percentile and maximum, respectively. Worst scenarios were found for the CC beam, where we measured 1.1mm (median), 2.7mm (95thpercentile) and 5mm (maximum). Regardless the direction, beams passing through the surgical site, where metal (Titanium MRI‐compatible) staples were present, were mostly affected by range shift. GTV's V98 for CT was not lower than 99.3%.

Conclusion:

The study showed the clinical feasibility of an MRI‐alone proton plan. Advantages include the possibility to rely on better soft tissue contrast for target and organs at risk delineation without the need of further CT scan and image registration. Additional investigation is required in presence of metal implants along the beam path and to account for partial volume effects due to slice thickness.
SU‐E‐J‐224: Using UTE and T1 Weighted Spin Echo Pulse Sequences for MR‐Only Treatment Planning; Phantom Study
Authors: Yu H , Fatemi A , Sahgal A
Published: June 2015

Purpose:

Investigating a new approach in MRI based treatment planning using the combination of (Ultrashort Echo Time) UTE and T1 weighted spin echo pulse sequences to delineate air, bone and water (soft tissues) in generating pseudo CT images comparable with CT.

Methods:

A gel phantom containing chicken bones, ping pang balls filled with distilled water and air bubbles, was made. It scanned with MRI using UTE and 2D T1W SE pulse sequences with (in plane resolution= 0.53mm, slice thickness= 2 mm) and CT with (in plane resolution= 0.5 mm and slice thickness= 0.75mm) as a ground truth for geometrical accuracy. The UTE and T1W SE images were registered with CT using mutual information registration algorithm provided by Philips Pinnacle treatment planning system. The phantom boundaries were detected using Canny edge detection algorithm for CT, and MR images. The bone, air bubbles and water in ping pong balls were segmented from CT images using threshold 300HU, ‐ 950HU and 0HU, respectively. These tissue inserts were automatically segmented from combined UTE and T1W SE images using edge detection and relative intensity histograms of the phantom. The obtained segmentations of air, bone and water inserts were evaluated with those obtained from CT.

Results:

Bone and air can be clearly differentiated in UTE images comparable to CT. Combining UTE and T1W SE images successfully segmented the air, bone and water. The maximum segmentation differences from combine MRI images (UTE and T1W SE) and CT are within 1.3 mm, 1.1mm for bone, air, respectively. The geometric distortion of UTE sequence is small less than 1 pixel (0.53 mm) of MR image resolution.

Conclusion:

Our approach indicates that MRI can be used solely for treatment planning and its quality is comparable with CT.
SU‐E‐J‐225: CEST Imaging in Head and Neck Cancer Patients
Authors: Wang J , Hwang K , Fuller C , Mohamed A , Ding Y , Frank S , Hazle J , Zhou J
Published: June 2015

Purpose:

Chemical Exchange Saturation Transfer (CEST) imaging is an MRI technique enables the detection and imaging of metabolically active compounds in vivo. It has been used to differentiate tumor types and metabolic characteristics. Unlike PET/CT,CEST imaging does not use isotopes so it can be used on patient repeatedly. This study is to report the preliminary results of CEST imaging in Head and Neck cancer (HNC) patients.

Methods:

A CEST imaging sequence and the post‐processing software was developed on a 3T clinical MRI scanner. Ten patients with Human papilloma virus positive oropharyngeal cancer were imaged in their immobilized treatment position. A 5 mm slice CEST image was acquired (128×128, FOV=20∼24cm) to encompass the maximum dimension of tumor. Twenty‐nine off‐set frequencies (from −7.8ppm to +7.8 ppm) were acquired to obtain the Z‐spectrum. Asymmetry analysis was used to extract the CEST contrasts. ROI at the tumor, node and surrounding tissues were measured.

Results:

CEST images were successfully acquired and Zspectrum asymmetry analysis demonstrated clear CEST contrasts in tumor as well as the surrounding tissues. 3∼5% CEST contrast in the range of 1 to 4 ppm was noted in tumor as well as grossly involved nodes. Injection of glucose produced a marked increase of CEST contrast in tumor region (∼10%). Motion and pulsation artifacts tend to smear the CEST contrast, making the interpretation of the image contrast difficult. Field nonuniformity, pulsation in blood vesicle and susceptibility artifacts caused by air cavities were also problematic for CEST imaging.

Conclusion:

We have demonstrated successful CEST acquisition and Z‐spectrum reconstruction on HNC patients with a clinical scanner. MRI acquisition in immobilized treatment position is critical for image quality as well as the success of CEST image acquisition. CEST images provide novel contrast of metabolites in HNC and present great potential in the pre‐ and post‐treatment assessment of patients undergoing radiation therapy.
SU‐E‐J‐232: Feasibility of MRI‐Based Preplan On Low Dose Rate Prostate Brachytherapy
Authors: Huang Y , Tward J , Rassiah‐Szegedi P , Zhao H , Sarkar V , Huang L , Szegedi M , Kokeny K , Salter B
Published: June 2015

Purpose:

To investigate the feasibility of using MRI‐based preplan for low dose rate prostate brachytherapy.

Methods:

12 patients who received transrectal ultrasound (TRUS) guided prostate brachytherapy with Pd‐103 were retrospectively studied. Our care‐standard of the TRUS‐based preplan served as the control. One or more prostate T2‐weighted wide and/or narrow‐field of view MRIs obtained within the 3 months prior to the implant were imported into the MIM Symphony software v6.3 (MIM Software Inc., Cleveland, OH) for each patient. In total, 37 MRI preplans (10 different image sequences with average thickness of 4.8mm) were generated. The contoured prostate volume and the seed counts required to achieve adequate dosimetric coverage from TRUS and MRI preplans were compared for each patient. The effects of different MRI sequences and image thicknesses were also investigated statistically using Student's t‐test. Lastly, the nomogram from the MRI preplan and TRUS preplan from our historical treatment data were compared.

Results:

The average prostate volume contoured on the TRUS and MRI were 26.6cc (range: 12.6∼41.3cc), and 27.4 cc (range: 14.3∼50.0cc), respectively. Axial MRI thicknesses (range: 3.5∼8.1mm) did not significantly affect the contoured volume or the number of seeds required on the preplan (R2 = 0.0002 and 0.0012, respectively). Four of the MRI sequences (AX‐T2, AX‐T2‐Whole‐Pelvis, AX‐T2‐FSE, and AXIALT2‐ Hi‐Res) showed statistically significant better prostate volume agreement with TRUS than the other seven sequences (P <0.01). Nomogram overlay between the MRI and TRUS preplans showed good agreement; indicating volumes contoured on MRI preplan scan reliably predict how many seeds are needed for implant.

Conclusion:

Although MRI does not allow for determination of the actual implant geometry, it can give reliable volumes for seed ordering purposes. Our future work will investigate if MRI is sufficient to reliably replace TRUS preplanning in patients where preplan TRUS may be technically challenging.