bluuebunny/crossref_metadata_2025 huggingface dataset:
SU‐E‐J‐205: Dose Distribution Differences Caused by System Related Geometric Distortion in MRI‐Guided Radiation Treatment System
Authors: Wang J , Yang J , Marshall S , Wen Z , Court L , Ibbott G
Published: June 2015

Purpose:

MRI has superb soft tissue contrast but is also known for geometric distortions. The concerns and uncertainty about MRI's geometric distortion have contributed to the hesitation of using only MRI for simulation in radiation therapy. There are two major categories of geometric distortion in MRI; system related and patient related. In this presentation, we studied the impact of system‐related geometric distortion on dose distribution in a digital body phantom under an MR‐Linac environment.

Methods:

Residual geometric distortion (after built‐in geometric correction) was modeled based on phantom measurements of the system‐related geometric distortions of a MRI scanner of a combined MR guided Radiation Therapy (MRgRT) system. A digital oval shaped phantom (40×25 cm) as well as one ellipsoid shaped tumor volume was created to simulate a simplified human body. The simulated tumor volume was positioned at several locations between the isocenter and the body surface. CT numbers in HUs that approximate soft tissue and tumor were assigned to the respective regions in the digital phantom. To study the effect of geometric distortion caused by system imperfections, an IMRT plan was optimized with the distorted image set with the B field. Dose distributions were re‐calculated on the undistorted image set with the B field (as in MR‐Linac).

Results:

The maximum discrepancies in both body contour and tumor boundary was less than 2 mm, which leads to small dose distribution change. For the target in the center, coverage was reduced from 98.8% (with distortion) to 98.2%; for the other peripheral target coverage was reduced from 98.4% to 95.9%.

Conclusion:

System related geometric distortions over the 40×25 area were within 2mm and the resulted dosimetric effects were minor for the two tumor locations in the phantom. Patient study will be needed for further investigation.

The authors received a corporate research grant from Elekta
SU‐E‐J‐206: Adaptive Radiotherapy for Gynecological Malignancies with MRIGuided Cobolt‐60 Radiotherapy
Authors: Lamb J , Kamrava M , Agazaryan N , Cao M , Low D , Thomas D , Yang Y
Published: June 2015

Purpose:

Even in the IMRT era, bowel toxicity and bone marrow irradiation remain concerns with pelvic irradiation. We examine the potential gain from an adaptive radiotherapy workflow for post‐operative gynecological patients treated to pelvic targets including lymph nodes using MRI‐guided Co‐60 radiation therapy.

Methods:

An adaptive workflow was developed with the intent of minimizing time overhead of adaptive planning. A pilot study was performed using retrospectively analyzed images from one patient's treatment. The patient's treated plan was created using conventional PTV margins. Adaptive treatment was simulated on the patient's first three fractions. The daily PTV was created by removing non‐target tissue, including bone, muscle and bowel, from the initial PTV based on the daily MRI. The number of beams, beam angles, and optimization parameters were kept constant, and the plan was re‐optimized. Normal tissue contours were not adjusted for the re‐optimization, but were adjusted for evaluation of plan quality. Plan quality was evaluated based on PTV coverage and normal tissue DVH points per treatment protocol. Bowel was contoured as the entire bowel bag per protocol at our institution. Pelvic bone marrow was contoured per RTOG protocol 1203.

Results:

For the clinically treated plan, the volume of bowel receiving 45 Gy was 380 cc, 53% of the rectum received 30 Gy, 35% of the bladder received 45 Gy, and 28% of the pelvic bone marrow received 40 Gy. For the adaptive plans, the volume of bowel receiving 45 Gy was 175–201 cc, 55–62% of the rectum received 30 Gy, 21– 27% of the bladder received 45 Gy, and 13–17% of the pelvic bone marrow received 40 Gy.

Conclusion:

Adaptive planning led to a large reduction of bowel and bone marrow dose in this pilot study. Further study of on‐line adaptive techniques for the radiotherapy of pelvic lymph nodes is warranted.

Dr. Low is a member of the scientific advisory board of ViewRay, Inc.
SU‐E‐J‐214: MR Protocol Development to Visualize Sirius MRI Markers in Prostate Brachytherapy Patients for MR‐Based Post‐Implant Dosimetry
Authors: Lim T , Wang J , Frank S , Stafford R , Bruno T , Bathala T , Mahmood U , Pugh T , Ibbott G , Kudchadker R
Published: June 2015

Purpose:

The current CT‐based post‐implant dosimetry allows precise seed localization but limited anatomical delineation. Switching to MR‐based post‐implant dosimetry is confounded by imprecise seed localization. One approach is to place positive‐contrast markers (Sirius) adjacent to the negative‐contrast seeds. This patient study aims to assess the utility of a 3D fast spoiled gradient‐recalled echo (FSPGR) sequence to visualize Sirius markers for post‐implant dosimetry.

Methods:

MRI images were acquired in prostate implant patients (n=10) on Day 0 (day‐of‐implant) and Day 30. The post‐implant MR protocol consisted of 3D T2‐weighted fast‐spin‐echo (FSE), T2‐weighted 2D‐FSE (axial) and T1‐weighted 2D‐FSE (axial/sagittal/coronal). We incorporated a 3D‐FSPGR sequence into the post‐implant MR protocol to visualize the Sirius markers. Patients were scanned with different number‐of‐excitations (6, 8, 10), field‐of‐view (10cm, 14cm, 18cm), slice thickness (1mm, 0.8mm), flip angle (14 degrees, 20 degrees), bandwidth (122.070 Hz/pixel, 325.508 Hz/pixel, 390.625 Hz/pixel), phase encoding steps (160, 192, 224, 256), frequency‐encoding direction (right/left, anterior/posterior), echo‐time type (minimum‐full, out‐of‐phase), field strength (1.5T, 3T), contrast (with, without), scanner vendor (Siemens, GE), coil (endorectal‐coil only, endorectal‐and‐torso‐coil, torsocoil only), endorectal‐coil filling (30cc, 50cc) and endorectal‐coil filling type (air, perfluorocarbon [PFC]). For post‐implant dosimetric evaluation with greater anatomical detail, 3D‐FSE images were fused with 3D‐FSPGR images. For comparison with CT‐based post‐implant dosimetry, CT images were fused with 3D‐FSPGR images.

Results:

The 3D‐FSPGR sequence facilitated visualization of markers in patients. Marker visualization helped distinguish signal voids as seeds versus needle tracks for more definitive MR‐based post‐implant dosimetry. On the CT‐MR fused images, the distance between the seed on CT to MR images was 3.2±1.6mm in patients with no endorectal coil, 2.3±0.8mm in patients with 30cc‐PFC‐filled endorectal‐coil and 5.0±1.8mm in patients with 50cc‐PFC‐filled endorectal‐coil.

Conclusion:

An MR protocol to visualize positive‐contrast Sirius markers to assist in the identification of negative‐contrast seeds was demonstrated.

S Frank is a co‐founder of C4 Imaging LLC, the manufacturer of the MRI markers.
SU‐E‐J‐215: Towards MR‐Only Image Guided Identification of Calcifications and Brachytherapy Seeds: Application to Prostate and Breast LDR Implant Dosimetry
Authors: Elzibak A , Fatemi‐Ardekani A , Soliman A , Mashouf S , Safigholi H , Han D , Ravi A , Morton G , Song WY
Published: June 2015

Purpose:

To identify and analyze the appearance of calcifications and brachytherapy seeds on magnitude and phase MRI images and to investigate whether they can be distinguished from each other on corrected phase images for application to prostate and breast low dose rate (LDR) implant dosimetry.

Methods:

An agar‐based gel phantom containing two LDR brachytherapy seeds (Advantage Pd‐103, IsoAid, 0.8mm diameter, 4.5mm length) and two spherical calcifications (large: 7mm diameter and small: 4mm diameter) was constructed and imaged on a 3T Philips MR scanner using a 16‐channel head coil and a susceptibility weighted imaging (SWI) sequence (2mm slices, 320mm FOV, TR/ TE= 26.5/5.3ms, 15 degree flip angle). The phase images were unwrapped and corrected using a 32×32, 2D Hanning high pass filter to remove background phase noise. Appearance of the seeds and calcifications was assessed visually and quantitatively using Osirix (http://www.osirix‐viewer.com/).

Results:

As expected, calcifications and brachytherapy seeds appeared dark (hypointense) relative to the surrounding gel on the magnitude MRI images. The diameter of each seed without the surrounding artifact was measured to be 0.1 cm on the magnitude image, while diameters of 0.79 and 0.37 cm were measured for the larger and smaller calcifications, respectively. On the corrected phase images, the brachytherapy seeds and the calcifications appeared bright (hyperintense). The diameter of the seeds was larger on the phase images (0.17 cm) likely due to the dipole effect.

Conclusion:

MRI has the best soft tissue contrast for accurate organ delineation leading to most accurate implant dosimetry. This work demonstrated that phase images can potentially be useful in identifying brachytherapy seeds and calcifications in the prostate and breast due to their bright appearance, which helps in their visualization and quantification for accurate dosimetry using MR‐only. Future work includes optimizing phase filters to best identify and delineate seeds and calcifications.
SU‐E‐J‐217: Multiparametric MR Imaging of Cranial Tumors On a Dedicated 1.0T MR Simulator Prior to Stereotactic Radiosurgery
Authors: Wen N , Glide‐Hurst C , Liu M , Hearshen D , Brown S , Siddiqui S , Chetty I
Published: June 2015

Purpose:

Quantitative magnetic resonance imaging (MRI) of cranial lesions prior to stereotactic radiosurgery (SRS) may improve treatment planning and provide potential prognostic value. The practicality and logistics of acquiring advanced multiparametric MRI sequences to measure vascular and cellular properties of cerebral tumors are explored on a 1.0 Tesla MR Simulator.

Methods:

MR simulation was performed immediately following routine CT simulation on a 1T MR Simulator. MR sequences used were in the order they were performed: T2‐Weighted Turbo Spin Echo (T2W‐TSE), T2 FLAIR, Diffusion‐weighted (DWI, b = 0, 800 to generate an apparent diffusion coefficient (ADC) map), 3D T1‐Weighted Fast Field Echo (T1W‐FFE), Dynamic Contrast Enhanced (DCE) and Post Gadolinium Contrast Enhanced 3D T1W‐FFE images. T1 pre‐contrast values was generated by acquiring six different flip angles. The arterial input function was derived from arterial pixels in the perfusion images selected manually. The extended Tofts model was used to generate the permeability maps. Routine MRI scans took about 30 minutes to complete; the additional scans added 12 minutes.

Results:

To date, seven patients with cerebral tumors have been imaged and tumor physiology characterized. For example, on a glioblastoma patient, the volume contoured on T1 Gd images, ADC map and the pharmacokinetic map (Ktrans) were 1.9, 1.4, and 1.5 cc respectively with strong spatial correlation. The mean ADC value of the entire volume was 1141 μm2/s while the value in the white matter was 811 μm2/s. The mean value of Ktrans was 0.02 min‐1 in the tumor volume and 0.00 in the normal white matter.

Conclusion:

Our initial results suggest that multiparametric MRI sequences may provide a more quantitative evaluation of vascular and tumor properties. Implementing functional imaging during MR‐SIM may be particularly beneficial in assessing tumor extent, differentiating radiation necrosis from tumor recurrence, and establishing reliable bio‐markers for treatment response evaluation.

The Department of Radiation Oncology at Henry Ford Health System has research agreement with Varian Medical System and Philips Health Care.
SU‐E‐J‐226: Efficient Use of Computed Tomography (CT) and Magnetic Resonance Imaging (MRI) for Cervical‐Cancer Brachytherapy
Authors: Damato A , Bhagwat M , Buzurovic I , Cormack R , Lee L , Viswanathan A
Published: June 2015

Purpose:

To investigate image modality selection in an environment with limited access to interventional MRI for image‐guided high‐dose‐rate cervical‐cancer brachytherapy.

Methods:

Records of all cervical‐cancer patients treated with brachytherapy between 1/2013 and 8/2014 were analyzed. Insertions were performed under CT guidance (CT group) or with >1 fraction under 3T MR guidance (MRI group; subMRI includes only patients who also had a CT‐guided insertion). Differences between groups in clinical target volume (CTV), disease stage (I/II or III/IV), number of patients with or without interstitial needles, and CTV D90 were investigated. Statistical significance was evaluated with the Student T test and Fisher test (p <0.05).

Results:

46 cervical‐cancer patients were included (16 MRI [3 subMRI], 30 CT). CTV: overall, 55±53 cm3; MRI, 81±61 cm3; CT, 42±44 cm3 (p = 0.017). Stage: overall, 24 I/II and 22 III/IV; MRI, 3 I/II and 13 III/IV; CT, 21 I/II and 9 III/IV (p = 0.002). Use of needles: overall, 26 without and 20 with; MRI, 5 without and 11 with; CT, 21 without and 9 with (p = 0.015). CTV D90: overall, 82±5 Gy; MRI, 81±6 Gy; CT, 82±5 Gy (p = 0.78). SubMRI: CTV and D90 (as % of nominal fraction dose) were 23±6 cm3 and 124±3% for MRI‐guided insertions and 21±5 cm3 (p = 0.83) and 106±12% (p = 0.15) for CT‐guided insertions.

Conclusion:

Statistically significant differences in patient population indicate preferential use of MRI for patients with high‐stage disease and large residual CTVs requiring the use of interstitial needles. CTV D90 was similar between groups, despite the difference in patient selection. For patients who underwent both CT and MRI insertions, a larger MR CTV D90 and similar CTVs between insertions were observed. While MRI is generally preferable to CT, MRI selection can be optimized in environments without a dedicated MRI brachytherapy suite.

This work was partially funded by the NIH R21 CA167800 (PI: Viswanathan; aviswanathan@partners.org)
SU‐E‐J‐227: Evaluation of Residual Geometric Distortion in MRI for Treatment Planning
Authors: Wang J , Yang J , Hwang K , Wen Z , Court L , Ibbott G
Published: June 2015

Purpose:

To determine an appropriate model to simulate system related geometric distortions in MR images in order to investigate the dosimetric impact on MRI‐LINAC treatment planning.

Methods:

Two sets of geometric distortion data in 3D volume were obtained independently from two different MRI vendors using different geometric distortion phantoms with visible markers. The distortion data were used to train two parametric models in X, Y, and Z directions separately: a second order polynomial model and a spherical harmonic model. The polynomial model was constructed in 2D slices with available measurements while the spherical harmonic model was built in 3D. After the models were created, they were used to generate simulated distortions at the marker positions to be compared with the measured data for evaluation.

Results:

The maximum measured distortions (after standard distortion corrections) were at the border of the scan, with 6.9 mm, 6.1 mm, and 3.6 mm in the X, Y, Z (slice) direction for first dataset, and 21.7 mm, 4.0 mm, and 12.0 mm for second dataset, respectively. For the first dataset, the polynomial model had average simulation errors in X, Y, and Z of 0.4±0.4 mm, 0.2±0.2 mm, and 0.1±0.2 mm, contrasting with those from the spherical harmonic model of 0.5±0.7 mm, 0.4±0.4 mm, and 0.6±0.4 mm. For the second dataset, the polynomial model had average simulation errors of 1.4±1.8 mm, 0.3±0.3 mm, and 0.4±0.5 mm, contrasting with those from the spherical harmonic model of 2.8±3.2 mm, 0.6±0.5 mm, and 2.5±2.1 mm. We found that using a maximum order of 2 in the spherical harmonic model gave the best estimation of the distortion.

Conclusion:

Polynomial model showed a better simulation of the residual geometric distortion than spherical harmonic model and may be a better choice to model the residual MR image distortion for MRI‐LINAC treatment planning purposes.

This work was partially supported by a research grant from Elekta.
SU‐E‐J‐231: Comparison of Delineation Variability of Soft Tissue Volume and Position in Head‐And‐Neck Between Two T1‐Weighted Pulse Sequences Using An MR‐Simulator with Immobilization
Authors: Wong O , Lo G , Yuan J , Law M , Ding A , Cheng K , Chan K , Cheung K , Yu S
Published: June 2015

Purpose:

There is growing interests in applying MR‐simulator(MR‐sim) in radiotherapy but MR images subject to hardware, patient and pulse sequence dependent geometric distortion that may potentially influence target definition. This study aimed to evaluate the influence on head‐and‐neck tissue delineation, in terms of positional and volumetric variability, of two T1‐weighted(T1w) MR sequences on a 1.5T MR‐sim

Methods:

Four healthy volunteers were scanned (4 scans for each on different days) using both spin‐echo (3DCUBE, TR/TE=500/14ms, TA=183s) and gradient‐echo sequences (3DFSPGR, TE/TR=7/4ms, TA=173s) with identical coverage, voxel‐size(0.8×0.8×1.0mm3), receiver‐bandwidth(62.5kHz/pix) and geometric correction on a 1.5T MR‐sim immobilized with personalized thermoplastic cast and head‐rest. Under this setting, similar T1w contrast and signal‐to‐noise ratio were obtained, and factors other than sequence that might bias image distortion and tissue delineation were minimized. VOIs of parotid gland(PGR, PGL), pituitary gland(PIT) and eyeballs(EyeL, EyeR) were carefully drawn, and inter‐scan coefficient‐of‐variation(CV) of VOI centroid position and volume were calculated for each subject. Mean and standard deviation(SD) of the CVs for four subjects were compared between sequences using Wilcoxon ranksum test.

Results:

The mean positional(<4%) and volumetric(<7%) CVs varied between tissues, majorly dependent on tissue inherent properties like volume, location, mobility and deformability. Smaller mean volumetric CV was found in 3DCUBE, probably due to its less proneness to tissue susceptibility, but only PGL showed significant difference(P<0.05). Positional CVs had no significant differences for all VOIs(P>0.05) between sequences, suggesting volumetric variation might be more sensitive to sequence‐dependent delineation difference.

Conclusion:

Although 3DCUBE is considered less prone to tissue susceptibility‐induced artifact and distortion, our preliminary data showed that both sequences had insignificant differences on positional and volumetric CV in most head‐and‐neck tissues except for PGL. This study is majorly limited in its small sample size. Influences of image contrasts(T1w v.s. T2w) and inter‐observer difference have to be further investigated.
SU‐E‐J‐236: Audiovisual Biofeedback Improves Breath‐Hold Lung Tumor Position Reproducibility Measured with 4D MRI
Authors: Lee D , Greer P , Lapuz C , Ludbrook J , Pollock S , Kim T , Keall P
Published: June 2015

Purpose:

Audiovisual biofeedback breath‐hold (AVBH) was employed to reproduce tumor position on inhale and exhale breath‐holds for 4D tumor information. We hypothesize that lung tumor position will be more consistent using AVBH compared with conventional breath‐hold (CBH).

Methods:

Lung tumor positions were determined for seven lung cancer patients (age: 25 – 74) during to two separate 3T MRI sessions. A breathhold training session was performed prior to the MRI sessions to allow patients to become comfortable with AVBH and their exhale and inhale target positions. CBH and AVBH 4D image datasets were obtained in the first MRI session (pre‐treatment) and the second MRI session (midtreatment) within six weeks of the first session. Audio‐instruction (MRI: Siemens Skyra) in CBH and verbal‐instruction (radiographer) in AVBH were used. A radiation oncologist contoured the lung tumor using Eclipse (Varian Medical Systems); tumor position was quantified as the centroid of the contoured tumor after rigid registration based on vertebral anatomy across two MRI sessions. CBH and AVBH were compared in terms of the reproducibility assessed via (1) the difference between the two exhale positions for the two sessions and the two inhale positions for the sessions. (2) The difference in amplitude (exhale to inhale) between the two sessions.

Results:

Compared to CBH, AVBH improved the reproducibility of two exhale (or inhale) lung tumor positions relative to each other by 33%, from 6.4±5.3 mm to 4.3±3.0 mm (p=0.005). Compared to CBH, AVBH improved the reproducibility of exhale and inhale amplitude by 66%, from 5.6±5.9 mm to 1.9±1.4 mm (p=0.005).

Conclusions:

This study demonstrated that audiovisual biofeedback can be utilized for improving the reproducibility of breath‐hold lung tumor position. These results are advantageous towards achieving more accurate emerging radiation treatment planning methods, in addition to imaging and treatment modalities utilizing breath‐hold procedures.
SU‐E‐J‐243: Reproducibility of Radiomics Features Through Different Voxel Discretization Levels in F18‐FDG PET Images of Cervical Cancer
Authors: Altazi B , Fernandez D , Zhang G , Biagioli M , Moros E , Moffitt H. Lee
Published: June 2015

Purpose:

Site‐specific investigations of the role of Radiomics in cancer diagnosis and therapy are needed. We report of the reproducibility of quantitative image features over different discrete voxel levels in PET/CT images of cervical cancer.

Methods:

Our dataset consisted of the pretreatment PET/CT scans from a cohort of 76 patients diagnosed with cervical cancer, FIGO stage IB‐IVA, age range 31–76 years, treated with external beam radiation therapy to a dose range between 45–50.4 Gy (median dose: 45 Gy), concurrent cisplatin chemotherapy and MRI‐based Brachytherapy to a dose of 20–30 Gy (median total dose: 28 Gy). Two board certified radiation oncologists delineated Metabolic Tumor volume (MTV) for each patient. Radiomics features were extracted based on 32, 64, 128 and 256 discretization levels (DL). The 64 level was chosen to be the reference DL. Features were calculated based on Co‐occurrence (COM), Gray Level Size Zone (GLSZM) and Run‐Length (RLM) matrices. Mean Percentage Differences (Δ) of features for discrete levels were determined. Normality distribution of Δ was tested using Kolomogorov ‐ Smirnov test. Bland‐Altman test was used to investigate differences between feature values measured on different DL. The mean, standard deviation and upper/lower value limits for each pair of DL were calculated. Interclass Correlation Coefficient (ICC) analysis was performed to examine the reliability of repeated measures within the context of the test re‐test format.

Results:

3 global and 5 regional features out of 48 features showed distribution not significantly different from a normal one. The reproducible features passed the normality test. Only 5 reproducible results were reliable, ICC range 0.7 – 0.99.

Conclusion:

Most of the radiomics features tested showed sensitivity to voxel level discretization between (32 – 256). Only 4 GLSZM, 3 COM and 1 RLM showed insensitivity towards mentioned discrete levels.
SU‐E‐J‐245: Sensitivity of FDG PET Feature Analysis in Multi‐Plane Vs. Single‐Plane Extraction
Authors: Harmon S , Galavis P , Jeraj R
Published: June 2015

Purpose:

Sensitivity of PET‐derived texture features to reconstruction methods has been reported for features extracted from axial planes; however, studies often utilize three dimensional techniques. This work aims to quantify the impact of multi‐plane (3D) vs. single‐plane (2D) feature extraction on radiomics‐based analysis, including sensitivity to reconstruction parameters and potential loss of spatial information.

Methods:

Twenty‐three patients with solid tumors underwent [18F]FDG PET/CT scans under identical protocols. PET data were reconstructed using five sets of reconstruction parameters. Tumors were segmented using an automatic, in‐house algorithm robust to reconstruction variations. 50 texture features were extracted using two Methods: 2D patches along axial planes and 3D patches. For each method, sensitivity of features to reconstruction parameters was calculated as percent difference relative to the average value across reconstructions. Correlations between feature values were compared when using 2D and 3D extraction.

Results:

21/50 features showed significantly different sensitivity to reconstruction parameters when extracted in 2D vs 3D (wilcoxon α<0.05), assessed by overall range of variation, Rangevar(%). Eleven showed greater sensitivity to reconstruction in 2D extraction, primarily first‐order and co‐occurrence features (average Rangevar increase 83%). The remaining ten showed higher variation in 3D extraction (average Rangevarincrease 27%), mainly co‐occurence and greylevel run‐length features. Correlation of feature value extracted in 2D and feature value extracted in 3D was poor (R<0.5) in 12/50 features, including eight co‐occurrence features. Feature‐to‐feature correlations in 2D were marginally higher than 3D, ∣R∣>0.8 in 16% and 13% of all feature combinations, respectively. Larger sensitivity to reconstruction parameters were seen for inter‐feature correlation in 2D(σ=6%) than 3D (σ<1%) extraction.

Conclusion:

Sensitivity and correlation of various texture features were shown to significantly differ between 2D and 3D extraction. Additionally, inter‐feature correlations were more sensitive to reconstruction variation using single‐plane extraction. This work highlights a need for standardized feature extraction/selection techniques in radiomics.
SU‐E‐J‐263: Repeatability of SUV and Texture Parameters in Serial PET Studies
Authors: Schwartz J , Humm J , Nehmeh S , Schoder H
Published: June 2015

Purpose:

Standardized uptake values (SUV) are standard quantitative PET measures of FDG tumor uptake used,and are used as a tool to monitor response to therapy. Textural analysis is emerging as a new tool for assessing intratumoral heterogeneity which may allow better tissue characterization and improved prediction of response and survival rate.Understanding what variations may be expected in these parameters is key in order to make decisions based on how the change throughout the course of treatment. The aim of this study was to assess repeatability in SUV measures and texture parameters,and establish criteria that differentiate changes associated with treatment rather than statistical variability.

Methods:

Eighty patients,167 random lesions total,were scanned in a GE Discovery STE PET/CT Scanner. One field‐of‐view was chosen centered on the largest lesion observed in a clinical whole‐body FDG PET.Immediately following,a gated 9 min scan was acquired in list mode,without changing the patient's position between any scans. Data was replayed into 3 time bins,3 min each,in order to insure equivalent noise characteristics in each replicate.Data was reconstructed into 128×128×47 square matrices.One VOI was drawn over each lesion for each patient and used to segment all 3 replicates. The mean.max and peak SUV were calculated for each VOI and replicate. First‐order textural features were also calculated (skewness and kurtosis). Repeatability was calculated as the average standard deviation over the mean for the 3 repeated measurements for each lesion.

Results:

The average percent error in the SUV max,peak and mean were 3.4%(0– 12.9%),1.9% (0–7.5%),2.8% (0–12.2%),respectively.For skewness and kurtosis they were 10.9% and 17.8%.

Conclusion:

We have shown that there is a large variation in %error in SUV measures across patients. SUVpeak is the least variable and kurtosis and skewness parameters are less reliable thatn SUVs.Higher order textures are be
SU‐E‐T‐05: 4D Measurement‐Guided Dose Reconstruction (4D‐MGDR) in End‐End Quality Assurance (E2E QA) for Assessing Safety Margin in Radiosurgery (SRS) From Clinical Perspectives
Authors: Chan M , Leung R , Wong M , Lee V , Law G , Lee K , Tung S , Blanck O
Published: June 2015

Purpose:

To assess the plan robustness and safety margin in SRS from 4DMGDR in E2E QA based on clinical objectives.

Methods:

OCTAVIUS SRS 1000 detector array and 4D phantom (PTW, Freiburg, Germany) were used to measure 5 coplanar SRS plans with 1 and 2 mm planning target volume (PTV). 3 targets were clinical, and 2 were virtual simulated to be 1mm from the brainstem (BS), and between chiasm (CS) and optic nerve (ON). Planning was done on Monaco v5.0 (Elekta, Maryland Heights, MO) to achieve 95–99% PTV and 100% gross tumor volume (GTV) prescription dose coverage. CBCT setup of the 4D phantom by 6D robotic couch was performed as for real patient. 4D‐MGDR in patient CT and dosimetric analysis were performed in PTW Verisoft v6.1. The safety margin that achieved 100% GTV coverage was determined, and doses to 2% (D2%) of BS, ON and CS were assessed from E2E QA.

Results:

100% GTV coverage was achieved with 1mm margin for 2 plans and 2mm margin for all plans. 98.3% and 99.4% GTV coverage were found in E2E QA for 1mm PTVs that either had sharp changing contour, or was nearby CS and ON or BS, and had either low planned minimum GTV dose (∼101% of the prescribed dose vs.∼106%) or compromised PTV coverage (95% vs. 99%). D2% to CS obtained with 4D‐MGDR for one virtual target were 18.8Gy for 1mm PTV and 19.2Gy for 2mm PTV, exceeding the planned tolerance of 18Gy/3 fractions for prescription dose of 24Gy.

Conclusion:

1mm margin is generally sufficient for dose planning and machine delivery errors. Irregular GTV with just enough dose coverage to spare critical organs may need 2mm margin at the costs of possible higher organ doses. 4D MGDR in an E2E QA approach can put the treatment plan evaluation in clinical perspectives.
SU‐E‐T‐21: A D‐D Based Neutron Generator System for Boron Neutron Capture Therapy: A Feasibility Study
Authors: Hsieh M , Liu Y , Nie L
Published: June 2015

Purpose:

To investigate the feasibility of a deuterium‐deuterium (DD) neutron generator for application in boron neutron capture therapy (BNCT) of brain cancer

Methods:

MCNP simulations were performed using a head phantom and a monoenergetic neutron source, which resembles the point source in a DD generator that emits 2.45‐MeV neutrons. Source energies ranging from 5eV to 2.45MeV were simulated to determine the optimal treatment energy. The phantom consisted of soft tissue, brain tissue, skull, skin layer, and a brain tumor of 5 cm in diameter. Tumor depth was varied from 5–10 cm. Boron‐10 concentrations of 10 ppm, 15 ppm, and 30 ppm were used in the soft/brain tissues, skin, and tumor, respectively. The neutron flux required to deliver 60 Gy to the tumor as well as the normal tissue doses were determined.

Results:

Beam energies between 5eV and 10keV obtained doses with the highest dose ratios (3.3–25.9) between the tumor and the brain at various depths. The dose ratio with 2.45‐MeV neutrons ranged from 0.8–6.6. To achieve the desired tumor dose in 40 minutes, the required neutron flux for a DD generator was between 8.8E10 and 5.2E11 n/s and the resulting brain dose was between 2.3 and 18 Gy, depending on the tumor depth. The skin and soft tissue doses were within acceptable tolerances. The boron‐neutron interaction accounted for 54–58% of the total dose.

Conclusion:

This study shows that the DD neutron generator can be a feasible neutron source for BNCT. The required neutron flux for treatment is achievable with the current DD neutron technology. With a well‐designed beam shaping assembly and treatment geometry, the neutron flux can be further improved and a 60‐Gy prescription can be accurately delivered to the target while maintaining tolerable normal tissue doses. Further experimental studies will be developed and conducted to validate the simulation results.
SU‐E‐T‐32: A Feasibility Study of Independent Dose Verification for IMAT
Authors: Kamima T , Takahashi R , Baba H , Yamashita M , Sugawara Y , Sato Y , Tachibana H
Published: June 2015

Purpose:

To assess the feasibility of the independent dose verification (Indp) for intensity modulated arc therapy (IMAT).

Methods:

An independent dose calculation software program (Simple MU Analysis, Triangle Products, JP) was used in this study, which can compute the radiological path length from the surface to the reference point for each control point using patient's CT image dataset and the MLC aperture shape was simultaneously modeled in reference to the information of MLC from DICOM‐RT plan. Dose calculation was performed using a modified Clarkson method considering MLC transmission and dosimetric leaf gap. In this study, a retrospective analysis was conducted in which IMAT plans from 120 patients of the two sites (prostate / head and neck) from four institutes were retrospectively analyzed to compare the Indp to the TPS using patient CT images. In addition, an ion‐chamber measurement was performed to verify the accuracy of the TPS and the Indp in water‐equivalent phantom.

Results:

The agreements between the Indp and the TPS (mean±1SD) were −0.8±2.4% and −1.3±3.8% for the regions of prostate and head and neck, respectively. The measurement comparison showed similar results (−0.8±1.6% and 0.1±4.6% for prostate and head and neck). The variation was larger in the head and neck because the number of the segments was increased that the reference point was under the MLC and the modified Clarkson method cannot consider the smooth falloff of the leaf penumbra.

Conclusion:

The independent verification program would be practical and effective for secondary check for IMAT with the sufficient accuracy in the measurement and CT‐based calculation. The accuracy would be improved if considering the falloff of the leaf penumbra.
SU‐E‐T‐33: A Feasibility‐Seeking Algorithm Applied to Planning of Intensity Modulated Proton Therapy: A Proof of Principle Study
Authors: Penfold S , Casiraghi M , Dou T , Schulte R , Censor Y
Published: June 2015

Purpose:

To investigate the applicability of feasibility‐seeking cyclic orthogonal projections to the field of intensity modulated proton therapy (IMPT) inverse planning. Feasibility of constraints only, as opposed to optimization of a merit function, is less demanding algorithmically and holds a promise of parallel computations capability with non‐cyclic orthogonal projections algorithms such as string‐averaging or block‐iterative strategies.

Methods:

A virtual 2D geometry was designed containing a C‐shaped planning target volume (PTV) surrounding an organ at risk (OAR). The geometry was pixelized into 1 mm pixels. Four beams containing a subset of proton pencil beams were simulated in Geant4 to provide the system matrix A whose elements a_ij correspond to the dose delivered to pixel i by a unit intensity pencil beam j. A cyclic orthogonal projections algorithm was applied with the goal of finding a pencil beam intensity distribution that would meet the following dose requirements: D_OAR < 54 Gy and 57 Gy < D_PTV < 64.2 Gy. The cyclic algorithm was based on the concept of orthogonal projections onto half‐spaces according to the Agmon‐Motzkin‐Schoenberg algorithm, also known as ‘ART for inequalities’.

Results:

The cyclic orthogonal projections algorithm resulted in less than 5% of the PTV pixels and less than 1% of OAR pixels violating their dose constraints, respectively. Because of the abutting OAR‐PTV geometry and the realistic modelling of the pencil beam penumbra, complete satisfaction of the dose objectives was not achieved, although this would be a clinically acceptable plan for a meningioma abutting the brainstem, for example.

Conclusion:

The cyclic orthogonal projections algorithm was demonstrated to be an effective tool for inverse IMPT planning in the 2D test geometry described. We plan to further develop this linear algorithm to be capable of incorporating dose‐volume constraints into the feasibility‐seeking algorithm.
SU‐E‐T‐34: A Feasible Action Level System for DVH‐Based IMRT QA
Authors: Redler G , Zhen H
Published: June 2015

Purpose:

Gamma‐based IMRT QA metrics have been shown to inadequately predict clinically relevant patient dose errors. In this study, we develop and evaluate an intuitive and feasible DVH‐based IMRT QA action level system, which incorporates both physical and clinical considerations.

Methods:

VMAT plans from previously treated prostate (n=11) and brain (n=13) patients are analyzed. Treatment plans were created using Eclipse TPS and delivered using Varian's Trilogy linac. Initial IMRT QA was performed using the ArcCHECK phantom. All plans passed gamma‐based criteria (3%/3mm) with at least a 97% passing rate. To design and test the feasibility of the DVH‐based IMRT QA metric, 3DVH software is used to process the QA data from the ArcCHECK measurement and predict the 3D distribution of dose delivered to the patient. This enables comparison of a standard set of clinically important DVH values between TPS calculations and that predicted by the QA process. A two‐level system is implemented to determine the IMRT QA Result. Level 1 requires that QA predicted DVH values for the PTV and OARs fall within a physical tolerance generated by scaling the original planned dose uniformly by ±5%. If level 1 tolerance is exceeded, level 2 requires that the QA‐predicted DVH values are within institutional normal tissue tolerances. OARs for prostate cases include bladder and rectum. OARs for brain cases include the lens, optic nerve, optic chiasm, and brainstem.

Results:

All analyzed QAs passed the proposed action levels, with prostate cases all satisfying physical tolerances. Ten brain cases had DVH values failing physical tolerances, but satisfying clinical tolerances. All DVHs failing physical tolerances are in low dose region.

Conclusion:

The proposed IMRT QA metrics represents a straightforward alternative to the gamma‐based metric. It has the potential to better address both physical and clinical concerns that are specific to individual patients.
SU‐E‐T‐52: A New Device for Quality Assurance of a Single Isocenter Technique for the Simultaneous Treatment of Multiple Brain Metastases
Authors: Maurer J , Sintay B , Varchena V
Published: June 2015

Purpose:

Comprehensive quality assurance (QA) of a single isocenter technique for the simultaneous treatment of multiple brain metastases is presently impractical due to the time consuming nature of measuring each lesion's dose on film or with a micro‐chamber. Three dimensional diode array and full field film measurements are sometimes used to evaluate these plans, but gamma analysis may not reveal local errors that have significant effects on one or a few of several targets. This work aimed to design, build and test a phantom to simplify comprehensive measurement and evaluation.

Methods:

A phantom was designed with 28 stackable slabs. The top and bottom slabs are 1.5 centimeters (cm) in thickness, and central 26 slabs are 0.5 cm thick. When assembled with radiochromic film in all 27 gaps, the phantom measures 16.5 × 15 × 19 cm. Etchings were designed to aide in identification of specific film planes on computed tomography (CT) images and correlation of individual PTVs with closest bisecting planes. Patient verification plans with a total of 16 PTVs were calculated on the phantom CT, and test deliveries both with and without couch kicks were performed to test the ability to identify correct film placements and subsequent PTV specific dose distributions on the films.

Results:

Bisecting planes corresponding to PTV locations were easily identified, and PTV specific dose distributions were clear for all 16 targets. For deliveries with couch kicks, the phantom PTV dose distributions closely approximated those calculated on the patient's CT. For deliveries without couch kicks, PTV specific dosimetry was also possible, although the distributions had ‘ghosts’ equaling the number of couch kicks, with distance between ghosts increasing with distance from the isocenter.

Conclusion:

A new phantom facilitates fast comprehensive commissioning validation and PTV specific dosimetry for a single isocenter technique for treating multiple brain metastases.

This work was partially funded by CIRS, Inc.
SU‐E‐T‐70: A Radiobiological Model of Reoxygenation and Fractionation Effects
Authors: Guerrero M , Carlson DJ
Published: June 2015

Purpose:

To develop a simple reoxygenation model that fulfills the following goals:1‐Quantify the reoxygenation effect in biologically effective dose (BED) and compare it to the repopulation effect.2‐Model the hypoxic fraction in tumors as a function of the number of fractions.3‐Develop a simple analytical expression for a reoxygenation term in BED calculations.

Methods:

The model considers tumor cells in two compartments: one normoxic population of cells and one hypoxic compartment including cells under a range of reduced oxygen concentrations. The surviving fraction is predicted using the linear‐quadratic (LQ) model. A hypoxia reduction factor (HRF) is used to quantify reductions in radiosensitivity parameters α_A and β_A as cellular oxygen concentration decreases. The HRF is defined as the ratio of the dose at a specific level of hypoxia to the dose under fully aerobic conditions to achieve equal cell killing. The model assumes that a fraction of the hypoxic cells ( ) moves from the hypoxic to the aerobic compartment after each daily fraction. As an example, we consider standard fractionation for NSCLC (d=2Gy,n=33) versus a SBRT (n=5, d=10Gy) fractionation and compare the loss in reoxygenation biological effect with the gain in repopulation biological effect.

Results:

An analytic expression for the surviving fraction after n daily treatments is derived and the reoxygenation term in the biological effect is calculated. Reoxygenation and repopulation effects are the same order of magnitude for potential doubling time Td values of 2 to 5 days. The hypoxic fraction increases or decreases with n depending on the reoxygenation rate Δ. For certain combinations of parameters, the biological effect of reoxygenation goes as ‐(n‐1)*ln(1‐Δ) providing a simple expression that can be introduced in BED calculations.

Conclusion:

A novel radiobiological model was developed that can be used to evaluate the effect of reoxygenation in fractionated radiotherapy.
SU‐E‐T‐74: A Simple Model of Dose Falloff for Highly Convergent SBRT Beams
Authors: Hoban P , Niu Y , Yu C
Published: June 2015

Purpose:

The purpose of this study was to create a simple model of dose falloff outside the target for a SBRT system employing a highly convergent beam geometry. With an estimate of the rate of reduction in dose away from a target of a given size, dose to tissues at a given distance from the target can be estimated. Our hypothesis is that dose at a given distance away from the edge of a target can be approximated using an inverse square model.

Methods:

With the GammaPod™ system used for this study, thirty‐six rotating Co‐60 beams with circular cross section converge at an isocenter and then diverge distal to the isocenter. Dose falloff beyond the target is mainly due to reduction in beam overlap, which in turn depends on projected area of all beams and projected beam area swept out during rotation. Beams also decrease in intensity beyond the isocenter. The GammaPod™ TPS calculates dose via superposition of Monte Carlo‐generated kernels, each corresponding to the convergence of beams at an isocenter within a stereotactic breast cup.

Results:

The factors above yield a simple inverse square‐based model that has been compared with TPS predictions and measurements for a variety of targets. Good agreement is seen considering the simple nature of the model. Fitting of the treatment planning and measured dose falloff data with a power law expression yields a negative exponent close to 2, confirming the inverse square nature of dose falloff.

Conclusion:

It is possible to apply a simple inverse square dose falloff model for SBRT or SRS systems such as the GammaPod which employ a highly‐convergent beam geometry. This can be useful in estimating the “worst case scenario” for dose to critical organs at a known distance from the edge of the target volume.

Authors are employees of Xcision Medical Systems
SU‐E‐T‐77: A Statistical Approach to Manage Quality for Pre‐Treatment Verification in IMRT/VMAT
Authors: Jassal K , Sarkar B , Mohanti B , Roy S , Ganesh T , Munshi A , Chougule A , Sachdev K
Published: June 2015

Objective:

The study presents the application of a simple concept of statistical process control (SPC) for pre‐treatment quality assurance procedure analysis for planar dose measurements performed using 2D‐array and a‐Si electronic portal imaging device (a‐Si EPID).

Method:

A total of 195 patients of four different anatomical sites: brain (n1=45), head & neck (n2=45), thorax (n3=50) and pelvis (n4=55) were selected for the study. Pre‐treatment quality assurance for the clinically acceptable IMRT/VMAT plans was measured with 2D array and a‐Si EPID of the accelerator. After the γ‐analysis, control charts and the quality index Cpm was evaluated for each cohort.

Results:

Mean and σ of γ ( 3%/3 mm) were EPID γ %≤1= 99.9% ± 1.15% and array γ %<1 = 99.6% ± 1.06%. Among all plans γ max was consistently lower than for 2D array as compared to a‐Si EPID. Fig.1 presents the X‐bar control charts for every cohort. Cpm values for a‐Si EPID were found to be higher than array, detailed results are presented in table 1.

Conclusion:

Present study demonstrates the significance of control charts used for quality management purposes in newer radiotherapy clinics. Also, provides a pictorial overview of the clinic performance for the advanced radiotherapy techniques.Higher Cpm values for EPID indicate its higher efficiency than array based measurements.
SU‐E‐T‐80: A Study of the Effect of Dose Grid Resolution On Stereotactic Plan Evaluation as a Function of Target Size
Authors: Pappas E , Papanikolaou N , Gutierrez A , Kirby N , Mavroidis P , Efstathopoulos E , Makris D , Crownover R , Stathakis S
Published: June 2015

Purpose:

To investigate and quantify the impact of dose‐calculation grid‐size on dose volume histogram (DVH) for small targets, such as those in stereotactic radiosurgery (SRS).

Methods:

A cohort of ten patients with multiple metastasis was used in this planning study. All patients were treated with SRS and had target volumes ranging from 0.1 to 4.3cc. The Varian eclipse planning system was used for planning using a mono‐isocentric VMAT technique with four arcs. Each plan was calculated twice using a 1×1×1 mm3 and 3×3×3 mm3 dose grid respectively. Moreover, we used an open source DICOM‐RT viewer to read the raw‐RDose data (1×1×1 mm3 resolution) and were subsequently rescaled by linear interpolation to dose grids of 1.5×1.5×1.5, 2×2×2, 2.5×2.5×2.5 and 3×3×3 mm3. DVHs for all targets were recalculated and inter‐compared for the native and post‐processed resolutions to quantify the effect on DVH of dose grid resolution and correlate that with the PTV volume size.

Results:

DVHs shape and numerical values have a significant dependence on dose grid size resulting to a reduced Dmean, Dmax and Dmin with volume averaging effect being the main reason for these changes. Moreover, the magnitude of the DVH alteration is inversely proportional to tumor volume size. As the dose grid increases from 1×1×1 to 3×3×3 mm3, the DVH‐derived Dmean value is underestimated by ∼4, ∼7 and ∼10 % for tumors with volume sizes of 1.3, 0.5 and 0.1 cc respectively.

Conclusion:

The dose grid size that is used for the final dose calculation is affecting significantly the DVHs of small PTVs. The higher the dose grid size, the more noticeable is the DVH alteration, especially for tumors with a volume less than 1cc. For SRS planning, a dose grid of 1×1×1 mm3 should be used, especially for tumors with a volume < 1cc, to avoid erroneous DVHs.

UTHSCSA, division of medical physics, has a research grant with Brainlab