Anthony Doemer

Implementation of a Novel Algorithm For Generating Synthetic CT Images From Magnetic Resonance Imaging Data Sets for Prostate Cancer Radiation Therapy

PURPOSE To describe and evaluate a method for generating synthetic computed tomography (synCT) images from magnetic resonance simulation (MR-SIM) data for accurate digitally reconstructed radiograph (DRR) generation and dose calculations in prostate cancer radiation therapy. METHODS AND MATERIALS A retrospective evaluation was performed in 9 prostate cancer patients who had undergone MR-SIM in addition to CT simulation (CT-SIM). MR-SIM data were used to generate synCT images by using a novel, voxel-based weighted summation approach. A subset of patients was used for weight optimization, and the number of patients to use during optimization was determined. Hounsfield unit (HU) differences between CT-SIM and synCT images were analyzed via mean absolute error (MAE). Original, CT-based treatment plans were mapped onto synCTs. DRRs were generated, and agreement between CT and synCT-generated DRRs was evaluated via Dice similarity coefficient (DSC). Dose was recalculated, and dose-volume metrics and gamma analysis were used to evaluate resulting treatment plans. RESULTS Full field-of-view synCT MAE across all patients was 74.3 ± 10.9 HU with differences from CTs of 2.0 ± 8.1 HU and 11.9 ± 46.7 HU for soft tissue structures (prostate, bladder, and rectum) and femoral bones, respectively. Calculated DSCs for anterior-posterior and lateral DRRs were 0.90 ± 0.04 and 0.92 ± 0.05, respectively. Differences in D99%, mean dose, and maximum dose to the clinical target volume from CT-SIM dose calculations were 0.75% ± 0.35%, 0.63% ± 0.34%, and 0.54% ± 0.33%, respectively, for synCT-generated plans. Gamma analysis (2%/2 mm dose difference/distance to agreement) revealed pass rates of 99.9% ± 0.1% (range, 99.7%-100%). CONCLUSION Generated synCTs enabled accurate DRR generation and dose computation for prostate MR-only simulation. Dose recalculated on synCTs agreed well with original planning distributions. Further validation using a larger patient cohort is warranted.

Clinical commissioning and use of the Novalis Tx linear accelerator for SRS and SBRT

The purpose of this study was to perform comprehensive measurements and testing of a Novalis Tx linear accelerator, and to develop technical guidelines for commissioning from the time of acceptance testing to the first clinical treatment. The Novalis Tx (NTX) linear accelerator is equipped with, among other features, a high‐definition MLC (HD120 MLC) with 2.5 mm central leaves, a 6D robotic couch, an optical guidance positioning system, as well as X‐ray‐based image guidance tools to provide high accuracy radiation delivery for stereotactic radiosurgery and stereotactic body radiation therapy procedures. We have performed extensive tests for each of the components, and analyzed the clinical data collected in our clinic. We present technical guidelines in this report focusing on methods for: (1) efficient and accurate beam data collection for commissioning treatment planning systems, including small field output measurements conducted using a wide range of detectors; (2) commissioning tests for the HD120 MLC; (3) data collection for the baseline characteristics of the on‐board imager (OBI) and ExacTrac X‐ray (ETX) image guidance systems in conjunction with the 6D robotic couch; and (4) end‐to‐end testing of the entire clinical process. Established from our clinical experience thus far, recommendations are provided for accurate and efficient use of the OBI and ETX localization systems for intra‐ and extracranial treatment sites. Four results are presented. (1) Basic beam data measurements: Our measurements confirmed the necessity of using small detectors for small fields. Total scatter factors varied significantly (30% to approximately 62%) for small field measurements among detectors. Unshielded stereotactic field diode (SFD) overestimated dose by ~ 2% for large field sizes. Ion chambers with active diameters of 6 mm suffered from significant volume averaging. The sharpest profile penumbra was observed for the SFD because of its small active diameter (0.6 mm). (2) MLC commissioning: Winston Lutz test, light/radiation field congruence, and Picket Fence tests were performed and were within criteria established by the relevant task group reports. The measured mean MLC transmission and dynamic leaf gap of 6 MV SRS beam were 1.17% and 0.36 mm, respectively. (3) Baseline characteristics of OBI and ETX: The isocenter localization errors in the left/right, posterior/anterior, and superior/inferior directions were, respectively, −0.2±0.2 mm, −0.8±0.2 mm, and −0.8±0.4 mm for ETX, and 0.5±0.7 mm, 0.6±0.5 mm, and 0.0±0.5 mm for OBI cone‐beam computed tomography. The registration angular discrepancy was 0.1±0.2°, and the maximum robotic couch error was 0.2°. (4) End‐to‐end tests: The measured isocenter dose differences from the planned values were 0.8% and 0.4%, measured respectively by an ion chamber and film. The gamma pass rate, measured by EBT2 film, was 95% (3% DD and 1 mm DTA). Through a systematic series of quantitative commissioning experiments and end‐to‐end tests and our initial clinical experience, described in this report, we demonstrate that the NTX is a robust system, with the image guidance and MLC requirements to treat a wide variety of sites — in particular for highly accurate delivery of SRS and SBRT‐based treatments. PACS numbers: 87.55.Qr, 87.53.Ly, 87.59.‐e

Characteristics of a novel treatment system for linear accelerator–based stereotactic radiosurgery

The purpose of this study is to characterize the dosimetric properties and accuracy of a novel treatment platform (Edge radiosurgery system) for localizing and treating patients with frameless, image‐guided stereotactic radiosurgery (SRS) and stereotactic body radiotherapy (SBRT). Initial measurements of various components of the system, such as a comprehensive assessment of the dosimetric properties of the flattening filter‐free (FFF) beams for both high definition (HD120) MLC and conical cone‐based treatment, positioning accuracy and beam attenuation of a six degree of freedom (6DoF) couch, treatment head leakage test, and integrated end‐to‐end accuracy tests, have been performed. The end‐to‐end test of the system was performed by CT imaging a phantom and registering hidden targets on the treatment couch to determine the localization accuracy of the optical surface monitoring system (OSMS), cone‐beam CT (CBCT), and MV imaging systems, as well as the radiation isocenter targeting accuracy. The deviations between the percent depth‐dose curves acquired on the new linac‐based system (Edge), and the previously published machine with FFF beams (TrueBeam) beyond Dmax were within 1.0% for both energies. The maximum deviation of output factors between the Edge and TrueBeam was 1.6%. The optimized dosimetric leaf gap values, which were fitted using Eclipse dose calculations and measurements based on representative spine radiosurgery plans, were 0.700 mm and 1.000 mm, respectively. For the conical cones, 6X FFF has sharper penumbra ranging from 1.2−1.8 mm (80%‐20%) and 1.9−3.8 mm (90%‐10%) relative to 10X FFF, which has 1.2−2.2 mm and 2.3−5.1 mm, respectively. The relative attenuation measurements of the couch for PA, PA (rails‐in), oblique, oblique (rails‐out), oblique (rails‐in) were: −2.0%, −2.5%, −15.6%, −2.5%, −5.0% for 6X FFF and −1.4%, −1.5%, −12.2%, −2.5%, −5.0% for 10X FFF, respectively, with a slight decrease in attenuation versus field size. The systematic deviation between the OSMS and CBCT was −0.4±0.2 mm, 0.1±0.3 mm, and 0.0±0.1 mm in the vertical, longitudinal, and lateral directions. The mean values and standard deviations of the average deviation and maximum deviation of the daily Winston‐Lutz tests over three months are 0.20±0.03 mm and 0.66±0.18 mm, respectively. Initial testing of this novel system demonstrates the technology to be highly accurate and suitable for frameless, linac‐based SRS and SBRT treatment. PACS number: 87.56.J‐

Evaluation of a magnetic resonance guided linear accelerator for stereotactic radiosurgery treatment

INTRODUCTION The purpose of this study was to investigate the systematic localization accuracy, treatment planning capability, and delivery accuracy of an integrated magnetic resonance imaging guided Linear Accelerator (MR-Linac) platform for stereotactic radiosurgery. MATERIALS AND METHODS The phantom for the end-to-end test comprises three different compartments: a rectangular MR/CT target phantom, a Winston-Lutz cube, and a rectangular MR/CT isocenter phantom. Hidden target tests were performed at gantry angles of 0, 90, 180, and 270 degrees to quantify the systematic accuracy. Five patient plans with a total of eleven lesions were used to evaluate the dosimetric accuracy. Single-isocenter IMRT treatment plans using 10-15 coplanar beams were generated to treat the multiple metastases. RESULTS The end-to-end localization accuracy of the system was 1.0 ± 0.1 mm. The conformity index, homogeneity index and gradient index of the plans were 1.26 ± 0.22, 1.22 ± 0.10, and 5.38 ± 1.44, respectively. The average absolute point dose difference between measured and calculated dose was 1.64 ± 1.90%, and the mean percentage of points passing the 3%/1 mm gamma criteria was 96.87%. CONCLUSIONS Our experience demonstrates that excellent plan quality and delivery accuracy was achievable on the MR-Linac for treating multiple brain metastases with a single isocenter.

Technical Note: Evaluation of plastic scintillator detector for small field stereotactic patient-specific quality assurance

Purpose: To evaluate the performance of a commercial plastic scintillator detector (PSD) for small‐field stereotactic patient‐specific quality assurance (QA) measurements using flattening‐filter‐free beam. Methods: A total of 10 spherical targets [volume range: (0.03 cc–2 cc)] were planned with two techniques: (a) dynamic conformal arc (DCA‐10 plans) and (b) volumetric modulated arc therapy (VMAT‐10 plans). All plans were generated using Varian Eclipse treatment planning system, and AcurosXB v.13 algorithm in 1.0 mm grid size. Additionally, 14 previously treated cranial and spine SRS plans were evaluated [6 DCA, 8 VMAT, volume range: (0.04 cc–119.02 cc)]. Plan modulation was quantified via two metrics: MU per prescription dose (MU/Rx) and Average Leaf Pair Opening (ALPO). QA was performed on the Varian Edge linear accelerator equipped with HDMLC. Three detectors were used: (a) PinPoint ion chamber (PTW; active volume 0.015 cc), (b) Exradin W1 PSD (Standard Imaging; active volume 0.002 cc), and (c) Gafchromic EBT3 film (Ashland). PinPoint chamber and PSD were positioned perpendicular to beam axis in a Lucy phantom (Standard Imaging); films were placed horizontally capturing the coronal plane. Results: PSD, film, and PinPoint chamber measured average differences of 1.00 ± 1.54%, 1.30 ± 1.69%, and −0.66 ± 2.36%, respectively, compared to AcurosXB dose calculation. As the target volume decreased, PinPoint chamber measured lower doses (maximum −5.07% at 0.07 cc target), while PSD and film measured higher doses (2.87% and 2.54% at 0.03 cc target) than AcurosXB. Film agreed with the benchmark detector PSD by an average difference of 0.31 ± 1.20%, but suffered from larger uncertainty; PinPoint chamber underestimated dose by more than 4% for targets smaller than 0.2 cc. Taking PSD as the measurement standard, DCA plans achieved good QA results across all volumes studied, with an average of −0.07 ± 0.89%; for VMAT plans, PSD measured consistently higher dose (1.95 ± 1.36%) than AcurosXB. Correlation study revealed that plan modulation quantified by both MU/Rx and ALPO correlated significantly with QA results. Conclusion: Among all three detectors, PSD demonstrated superior performances in plans with small fields and heavy modulation. High consistency and low uncertainty made PSD a suitable detector for clinical routine SRS QA. PinPoint chamber should be avoided for targets smaller than 0.2 cc; film dosimetry can be utilized with careful evaluation of its uncertainty bracket. Compared to PSD measurements, AcurosXB calculation demonstrated high accuracy for nonmodulated small fields. The positive correlation between plan modulation and QA discrepancy calls for our attention for clinical SRS plans with high modulation.

SU-G-BRC-07: Evaluation of AAA Focal Spot Size for SRS Planning Using End-To-End Dosimetric Data

PURPOSE To use end-to-end dosimetric measurements with Gafchromic film to evaluate the effects of focal spot size parameter for small-field dose calculations using AAA for SRS lesions. METHODS A total of 13 plans, corresponding to 7 patients previously treated with cranial SRS, were analyzed in this study (target volume range:[0.67cc,13.9cc]). The plans included DCA delivery (4 plans total) and VMAT delivery (9 plans total). All plans were mapped to a solid water phantom (15 cm thickness; isocenter and film plane at 7.5 depth). Dose calculation was performed with AAA v.11 (1.0mm grid size); three focal spot size settings were tested: 0mm, 0.5mm, and 1.5mm. For each plan, three calculated doses (corresponding to each focal spot size setting) were compared to measured film dose using quantitative methods [Gamma Analysis(1%,1mm,10% threshold criteria)] and qualitative methods (visual dose profile comparison). Film calibration and analysis were performed using in-house calibration methods and software package. RESULTS Gamma(1%,1mm) analysis passing rate results [mean(st.dev){%}] were as follows. For DCA plans: 98.74(0.54)-[0mm Focal Spot Size]; 98.24(1.26)-[0.5mm Focal Spot Size]; 95.42(2.29)-[1.5mm Focal Spot Size]. For VMAT plans: 98.75(0.54)-[0mm Focal Spot Size]; 98.89(0.73)-[0.5mm Focal Spot Size]; 97.43(1.30)-[1.5mm Focal Spot Size]. The majority of failing points (Gamma value>1.0) were found to be within the high dose region for all Focal Spot Size calculation models. Visual inspection of the dose profile, showed that the 1.5mm Focal Spot size calculation exhibited blurring in the high dose region (defined as >85% of the peak dose), resulting in a more gradual shoulder of the dose profile relative to measurements. CONCLUSION The dose calculation accuracy of DCA and VMAT plans is paramount for SRS treatment planning. Our results indicate similar behavior of the AAA model with focal spot sizes of 0mm and 0.5mm, while 1.5mm focal spot size tends to result in blurring of the high dose region. Henry Ford Health System has research agreements with Varian and Philips.

SU‐E‐T‐321: Dosimetric Evaluation of Non‐Coplanar Arcs in VMAT Planning for SBRT Lung Cases: A Planning Study

Purpose: Investigate use of standardized non-coplanar arcs to improve plan quality in lung Stereotactic Body Radiation Therapy(SBRT) VMAT planning. Methods: VMAT planning was performed for 9 patients previously treated with SBRT for peripheral lung tumors (tumor size:12.7cc to 32.5cc). For each patient, 7 VMAT plans (couch rotation values:0,5,10,15,20,25,and 30 deg) were generated; the coplanar plans were pushed to meet the RTOG0915 constraints and each non-coplanar plans utilized the same optimization constraints. The following plan dose metrics were used (taken from RTOG 0915): D-2cm: the maximum dose at 2 cm from the PTV, conformality index (CI), gradient index (GI), lung volume receiving 5 Gy (V5) and lung volume receiving 20 Gy (V20). The couch collision clearance was checked for each plan through a dry run using the couch position from the patient’s treatment. Results: Of the 9 cases, one coplanar plan failed to meet two protocol guidelines (both gradient index and D-2cm parameter), and an additional plan failed the D-2cm parameter. When introducing at least 5 degree couch rotation, all plans met the protocol guidelines. The largest feasible couch angle available was 15 to 20 degrees due to gantry collision issues. Non-coplanar plans resulted in the average (standard deviation) reduction of the following metrics: GI by 7.3% (3.7%); lung V20 by 11.1% (3.2%); D-2cm by 12.7% (3.9%). The CI was unchanged (−0.3%±0.6%), and lung V5 increased (3.8%±8.2%). Conclusion: The use of couch rotations as little as 5 degrees allows for plan quality that will meet RTOG0915 constraints while reducing D-2cm, GI, and lung V20. Using default couch rotations while planning SBRT cases will allow for more efficient planning with the stated goal of meeting RTOG0915 criteria for all clinical cases. Gantry clearance checks in the treatment room may be necessary to ensure safe treatments for larger couch rotation values.

TH‐A‐BRF‐03: Evaluation of Synthetic CTs Generated Using MR‐SIM Data

PURPOSE To describe and evaluate a novel algorithm for generating synthetic CT images from MR-SIM data for dose calculations in MR-only treatment planning. METHODS A voxel-based weighted summation method was implemented to generate synthetic CT (synCT) images. MR data were acquired using Philips 1.0T Panorama high-field open MR-SIM. Retrospective patient data from seven prostate patients and one brain patient (three lesions) enrolled in an IRB-approved study were used. 3D T1-weighted fast field echo and 3D T2-weighted turbo spin echo sequences were utilized for all patients. A 3D balanced turbo field echo sequence using spectral presaturation with inversion recovery was acquired for prostate patients, but 3D ultra-short echo time (UTE)-DIXON was instead acquired for the brain patient to amplify bone signal for semi-automatic bone segmentation. Weight optimization was performed using a training subset of patients. HU value differences between planning CT and synCTs were analyzed using mean absolute error (MAE). Original patient CT-based treatment plans were mapped onto synCTs, dose was recalculated using original leaf motion and MU values, and DRRs were generated. Dosevolume metrics and gamma analysis were used for dosimetric evaluation. RESULTS Average whole-body MAE of synCTs across all patients was 75+12 HU. In prostate cancer patients, average HU difference between planning and synCTs was 0.9±1.0% for soft tissue structures and 4.3±2.5% for bony structures. DRRs were generated from synCTs and qualitatively showed good geometric agreement with planning CT-generated DRRs. D99, mean dose, and maximum dose to CTV calculated using the synCT remained within 1.2% of planning CT-based dose calculations. All gamma analysis evaluated at 2%/2mm dose difference/distance to agreement) pass rates were greater than 95% with an average of 99.9±0.1% for prostate patients and 98.4±2.2% for three brain lesions. CONCLUSION SynCTs were generated with clinically acceptable accuracy comparable to planning CTs, enabling dose computations for MR-only simulation. Research supported in part by a grant from Philips HealthCare (Best, Netherlands).

TH-C-141-11: Evaluation of MR Images as the Planning and Reference Dataset for Daily CBCT-Based IGRT of the Prostate

PURPOSE An important question rarely discussed in the CT vs. MRI simulation debate is whether MRI reference images provide an adequate image-set to use with daily localization such as cone-beam CT (CBCT). This study compares clinical couch shifts based on daily CBCT images to shifts measured from MR images as the reference dataset for prostate IMRT treatment. METHODS Eight patients undergoing a pilot study had MR imaging along with CT simulation with the intent of evaluating a MR-simulation process. Patients had T1, T2 and bTFE (balanced Turbo Field Echo) sequences. The remainder of the treatment planning process continued using traditional procedures using CT. Therapists used only the CT scan as a reference for localization. Retrospectively, an observer measured shifts between daily CBCT images and MR reference images. RESULTS The differences in shift positions for the cohort between therapists and the observer are -0.16cm ± 0.25cm (AP), 0.04cm ± 0.19cm (SI), and - 0.01cm ± 0.14cm (LR). The mean group error for the therapists and the observer were less than 2 mm in all directions. Based on these shifts, the calculated margins for the therapists would be 0.87cm (AP), 0.65cm (SI), and 0.71cm (LR) and for the observer would be 1.1cm (AP), 0.66cm (SI), and 0.70cm (LR). For SI and LR directions both sets of margins are very close to one another. An outlier impacted the AP margin difference by 2.3mm and should be investigated further. This initial analysis suggests that each modality can be considered clinically sufficient for daily localization. CONCLUSION The results of this study suggest that MR reference image-sets can be used for daily image-guided localization of prostate cancers with at least the same accuracy as current methods. MR simulation provides substantial soft-tissue contrast and can improve tissue targeting in radiation oncology, as a Result further investigation is warranted.

SU‐E‐J‐19: A Feasility Study: Tube Current Modulation as a Dose Savings Technique for Cone‐Beam CT‐Based IGRT

Purpose: In image‐guidedradiotherapy(IGRT), cone‐beam CT(CBCT) scan parameters do not take into account patient size information, which can result in unnecessary dose. So‐called “smart scans” in radiology attempt to customize exposure parameters to reduce dose without sacrificing image quality. This work presents the feasibility of tube current modulation as a dose reduction technique in CBCT‐based IGRT. Methods: A CATPHAN® phantom with water‐equivalent annulus was used to evaluate noise. A CIRS phantom was used to evaluate dose characteristics. Gies et. al.[Med Phys, 26:1999], tells us that noise for the central pixel is minimal for tube current modulated by the square root of the given angular attenuation. This formulation is used to calculate modulation factors for each 10 degree imaging arc. Modulated image dataset was generated by compositing projection data from scans performed with each modulated current setting. Noise characterization was performed by analyzing the standard deviation of the uniformity section of the CATPHAN. Imagingdose was measured using an ionization chamber located in the phantom. For the modulated scan, each ten degree segment was measured separately and then accumulated to get a composite dose reading.Results: Central axis dose reduction of 31% was attained using this modulation scheme. Visual inspection of a modulated CBCT slices shows appreciable image quality relative to the conventional CBCT. The noise characteristics of the modulated scan were about 20 HU worse than a conventional scan, likely due to the presence of scatter in the enlarged CATPHAN phantom because theoretical predictions are based on primary beam attenuation alone Conclusions: Initial results demonstrate that the use of a modulated scanning technique, developed by taking into account patient anatomic variation, may be a feasible method to reduce dose while preserving image quality in CBCT‐based IGRT.