S Siddiqui

Practical methods for improving dose distributions in Monte Carlo-based IMRT planning of lung wall-seated tumors treated with SBRT

Current commercially available planning systems with Monte Carlo (MC)‐based final dose calculation in IMRT planning employ pencil‐beam (PB) algorithms in the optimization process. Consequently, dose coverage for SBRT lung plans can feature cold‐spots at the interface between lung and tumor tissue. For lung wall (LW)‐seated tumors, there can also be hot spots within nearby normal organs (example: ribs). This study evaluated two different practical approaches to limiting cold spots within the target and reducing high doses to surrounding normal organs in MC‐based IMRT planning of LW‐seated tumors. First, “iterative reoptimization”, where the MC calculation (with PB‐based optimization) is initially performed. The resultant cold spot is then contoured and used as a simultaneous boost volume. The MC‐based dose is then recomputed. The second technique uses noncoplanar beam angles with limited path through lung tissue. Both techniques were evaluated against a conventional coplanar beam approach with a single MC calculation. In all techniques the prescription dose was normalized to cover 95% of the PTV. Fifteen SBRT lung cases with LW‐seated tumors were planned. The results from iterative reoptimization showed that conformity index (CI) and/or PTV dose uniformity (UPTV) improved in 12/15 plans. Average improvement was 13%, and 24%, respectively. Nonimproved plans had PTVs near the skin, trachea, and/or very small lung involvement. The maximum dose to 1cc volume (D1cc) of surrounding OARs decreased in 14/15 plans (average 10%). Using noncoplanar beams showed an average improvement of 7% in 10/15 cases and 11% in 5/15 cases for CI and UPTV, respectively. The D1cc was reduced by an average of 6% in 10/15 cases to surrounding OARs. Choice of treatment planning technique did not statistically significantly change lung V5. The results showed that the proposed practical approaches enhance dose conformity in MC‐based IMRT planning of lung tumors treated with SBRT, improving target dose coverage and potentially reducing toxicities to surrounding normal organs. PACS numbers: 87.55.de, 87.55.kh

American Association of Physicists in Medicine Task Group 263: Standardizing Nomenclatures in Radiation Oncology

A substantial barrier to the single- and multi-institutional aggregation of data to supporting clinical trials, practice quality improvement efforts, and development of big data analytics resource systems is the lack of standardized nomenclatures for expressing dosimetric data. To address this issue, the American Association of Physicists in Medicine (AAPM) Task Group 263 was charged with providing nomenclature guidelines and values in radiation oncology for use in clinical trials, data-pooling initiatives, population-based studies, and routine clinical care by standardizing: (1) structure names across image processing and treatment planning system platforms; (2) nomenclature for dosimetric data (eg, dose–volume histogram [DVH]-based metrics); (3) templates for clinical trial groups and users of an initial subset of software platforms to facilitate adoption of the standards; (4) formalism for nomenclature schema, which can accommodate the addition of other structures defined in the future. A multisociety, multidisciplinary, multinational group of 57 members representing stake holders ranging from large academic centers to community clinics and vendors was assembled, including physicists, physicians, dosimetrists, and vendors. The stakeholder groups represented in the membership included the AAPM, American Society for Radiation Oncology (ASTRO), NRG Oncology, European Society for Radiation Oncology (ESTRO), Radiation Therapy Oncology Group (RTOG), Children’s Oncology Group (COG), Integrating Healthcare Enterprise in Radiation Oncology (IHE-RO), and Digital Imaging and Communications in Medicine working group (DICOM WG); A nomenclature system for target and organ at risk volumes and DVH nomenclature was developed and piloted to demonstrate viability across a range of clinics and within the framework of clinical trials. The final report was approved by AAPM in October 2017. The approval process included review by 8 AAPM committees, with additional review by ASTRO, European Society for Radiation Oncology (ESTRO), and American Association of Medical Dosimetrists (AAMD). This Executive Summary of the report highlights the key recommendations for clinical practice, research, and trials.

Use of jaw tracking in intensity modulated and volumetric modulated arc radiation therapy for spine stereotactic radiosurgery

PURPOSE This study was conducted to evaluate the advantages of jaw tracking for intensity modulated radiation therapy (IMRT) and volumetric modulated arc therapy (VMAT) in spine radiosurgery. METHODS AND MATERIALS VMAT and IMRT plans were retrospectively generated for 10 RTOG 0631 spine radiosurgery protocol patients. A total of 8 plans for each patient were created for a Varian TrueBeam equipped with a Millennium 120 multileaf collimator. Plans were created to compare IMRT and VMAT plans with and without jaw tracking, as well as with different flattening-filter-free energies: 6 MV unflattened (6U) and 10 MV unflattened (10U). The plans were prescribed to the 90% isodose line to either 16 or 18 Gy in 1 fraction. Planning target volume coverage, conformity index, dose to the spinal cord, and distance to falloff from the 90% to 50% isodose line were evaluated. Ion chamber and film measurements were performed to verify calculated dose distributions. RESULTS Jaw tracking decreased spinal cord dose for both IMRT and VMAT plans, but a larger decrease was seen with the IMRT plans (P = .004 vs P = .04). The average D(10%) for the spinal cord (dose that covered 10% of the spinal cord) was least for the 6U IMRT plan with jaw tracking and was greatest for the 10U IMRT plan without jaw tracking. Measurements showed greater than 98.5% agreement for planar dose gamma analysis and less than 2.5% for point dose analysis. CONCLUSIONS The addition of jaw tracking to IMRT and VMAT can decrease spinal cord dose without a change in calculation accuracy. A lower dose to the spinal cord was achieved with 6U than with 10U, although in some cases, 10U may be justified.

Evaluation of adaptive treatment planning for patients with non-small cell lung cancer

The purpose of this study was to develop metrics to evaluate uncertainties in deformable dose accumulation for patients with non-small cell lung cancer (NSCLC). Initial treatment plans (primary) and cone-beam CT (CBCT) images were retrospectively processed for seven NSCLC patients, who showed significant tumor regression during the course of treatment. Each plan was developed with IMRT for 2 Gy  ×  33 fractions. A B-spline-based DIR algorithm was used to register weekly CBCT images to a reference image acquired at fraction 21 and the resultant displacement vector fields (DVFs) were then modified using a finite element method (FEM). The doses were calculated on each of these CBCT images and mapped to the reference image using a tri-linear dose interpolation method, based on the B-spline and FEM-generated DVFs. Contours propagated from the planning image were adjusted to the residual tumor and OARs on the reference image to develop a secondary plan. For iso-prescription adaptive plans (relative to initial plans), mean lung dose (MLD) was reduced, on average from 17.3 Gy (initial plan) to 15.2, 14.5 and 14.8 Gy for the plans adapted using the rigid, B-Spline and FEM-based registrations. Similarly, for iso-toxic adaptive plans (considering MLD relative to initial plans) using the rigid, B-Spline and FEM-based registrations, the average doses were 69.9  ±  6.8, 65.7  ±  5.1 and 67.2  ±  5.6 Gy in the initial volume (PTV1), and 81.5  ±  25.8, 77.7  ±  21.6, and 78.9  ±  22.5 Gy in the residual volume (PTV21), respectively. Tumor volume reduction was correlated with dose escalation (for isotoxic plans, correlation coefficient  =  0.92), and with MLD reduction (for iso-fractional plans, correlation coefficient  =  0.85). For the case of the iso-toxic dose escalation, plans adapted with the B-Spline and FEM DVFs differed from the primary plan adapted with rigid registration by 2.8  ±  1.0 Gy and 1.8  ±  0.9 Gy in PTV1, and the mean difference between doses accumulated using the B-spline and FEM DVFs was 1.1  ±  0.6 Gy. As a dose mapping-induced energy change, energy defect in the tumor volume was 20.8  ±  13.4% and 4.5  ±  2.4% for the B-spline and FEM-based dose accumulations, respectively. The energy defect of the B-Spline-based dose accumulation is significant in the tumor volume and highly correlated to the difference between the B-Spline and FEM-accumulated doses with their correlation coefficient equal to 0.79. Adaptive planning helps escalate target dose and spare normal tissue for patients with NSCLC, but deformable dose accumulation may have a significant loss of energy in regressed tumor volumes when using image intensity-based DIR algorithms. The metric of energy defect is a useful tool for evaluation of adaptive planning accuracy for lung cancer patients.

Characterization and evaluation of 2.5 MV electronic portal imaging for accurate localization of intra- and extracranial stereotactic radiosurgery

2.5 MV electronic portal imaging, available on Varian TrueBeam machines, was characterized using various phantoms in this study. Its low-contrast detectability, spatial resolution, and contrast-to-noise ratio (CNR) were compared with those of conventional 6 MV and kV planar imaging. Scatter effect in large patient body was simulated by adding solid water slabs along the beam path. The 2.5 MV imaging mode was also evaluated using clinically acquired images from 24 patients for the sites of brain, head and neck, lung, and abdomen. With respect to 6 MV, the 2.5 MV achieved higher contrast and preserved sharpness on bony structures with only half of the imaging dose. The quality of 2.5 MV imaging was comparable to that of kV imaging when the lateral separation of patient was greater than 38 cm, while the kV image quality degraded rapidly as patient separation increased. Based on the results of patient images, 2.5 MV imaging was better for cranial and extracranial SRS than the 6 MV imaging. PACS number(s): 87.57.C.2.5 MV electronic portal imaging, available on Varian TrueBeam machines, was characterized using various phantoms in this study. Its low‐contrast detectability, spatial resolution, and contrast‐to‐noise ratio (CNR) were compared with those of conventional 6 MV and kV planar imaging. Scatter effect in large patient body was simulated by adding solid water slabs along the beam path. The 2.5 MV imaging mode was also evaluated using clinically acquired images from 24 patients for the sites of brain, head and neck, lung, and abdomen. With respect to 6 MV, the 2.5 MV achieved higher contrast and preserved sharpness on bony structures with only half of the imaging dose. The quality of 2.5 MV imaging was comparable to that of kV imaging when the lateral separation of patient was greater than 38 cm, while the kV image quality degraded rapidly as patient separation increased. Based on the results of patient images, 2.5 MV imaging was better for cranial and extracranial SRS than the 6 MV imaging. PACS number(s): 87.57.C

SU‐C‐17A‐03: Evaluation of Deformable Image Registration Methods Between MRI and CT for Prostate Cancer Radiotherapy

PURPOSE We evaluated the performance of two commercially available and one open source B-Spline deformable image registration (DIR) algorithms between T2-weighted MRI and treatment planning CT using the DICE indices. METHODS CT simulation (CT-SIM) and MR simulation (MR-SIM) for four prostate cancer patients were conducted on the same day using the same setup and immobilization devices. CT images (120 kVp, 500 mAs, voxel size = 1.1×1.1×3.0 mm3) were acquired using an open-bore CT scanner. T2-weighted Turbo Spine Echo (T2W-TSE) images (TE/TR/α = 80/4560 ms/90°, voxel size = 0.7×0.7×2.5 mm3) were scanned on a 1.0T high field open MR-SIM. Prostates, seminal vesicles, rectum and bladders were delineated on both T2W-TSE and CT images by the attending physician. T2W-TSE images were registered to CT images using three DIR algorithms, SmartAdapt (Varian), Velocity AI (Velocity) and Elastix (Klein et al 2010) and contours were propagated. DIR results were evaluated quantitatively or qualitatively by image comparison and calculating organ DICE indices. RESULTS Significant differences in the contours of prostate and seminal vesicles were observed between MR and CT. On average, volume changes of the propagated contours were 5%, 2%, 160% and 8% for the prostate, seminal vesicles, bladder and rectum respectively. Corresponding mean DICE indices were 0.7, 0.5, 0.8, and 0.7. The intraclass correlation coefficient (ICC) was 0.9 among three algorithms for the Dice indices. CONCLUSION Three DIR algorithms for CT/MR registration yielded similar results for organ propagation. Due to the different soft tissue contrasts between MRI and CT, organ delineation of prostate and SVs varied significantly, thus efforts to develop other DIR evaluation metrics are warranted. CONFLICT OF INTEREST Submitting institution has research agreements with Varian Medical System and Philips Healthcare.

SU-F-R-49: A Novel Kinetic Model for Prediction of Tumor Local Control for Patients with Lung Cancer

PURPOSE Modeling tumor control probability (TCP) can help optimize treatment plans for better treatment outcomes. This study sought to quantify the radiobiological parameters of the TCP model for patients with lung cancer. METHODS A two-compartment kinetic model was developed to model tumor regression for five NSCLC patients. The model has three parameters: cell survival fraction, dead-cell-resolving time and tumor doubling time. The last one was extended to a function of tumor volume. Each of these patients was treated with 2 Gyx33 fractions. Daily CBCT images were acquired during the course of treatment. Gross tumor volume (GTV) was delineated on each of these CBCT images, and the visible tumor volumes were fitted to the kinetic model to optimize the parameters, where the Jacobian of this model was constrained by daily tumor volumetric changes. RESULTS Among the five patients, three had tumor recurrence: 455, 520 and 590 days after the completion of treatment. Recurrent tumor volumes were, respectively, 28.9, 13.7 and 4.8 cm3, measured at distant locations. With the assumption of cell density = 108 cells/cm3, tumor doubling times required for circulating tumor cells to progress to the recurrent volumes are 16.1, 19.2 and 23.1 days. By fitting the kinetic model with 33 measured tumor volumes, the average cell survival fraction for the five patients is 0.5±0.21, and tumor doubling times are in the range of 10∼35 days, comparable to the doubling time derived from tumor recurrence data. The doubling times modeled from invisible cells are different from the median (357 days) or mean (166 days) of the doubling time measured directly from visible tumor volumes. CONCLUSION A kinetic model has been developed to simulate the process of tumor progressing from invisible cells to visible tumor volumes. This model may be useful for TCP prediction for patients with locally advanced lung cancer.

SU‐E‐T‐402: Evaluation of the Accuracy of a Novel Open Mask System for Immobilization of Cranial Stereotactic Radiosurgery Patients

Purpose: To evaluate the accuracy of a novel open mask for intracranial SRS and to investigate the capability of monitoring intrafraction motion of the patient within this mask using the optical surface monitoring system (OSMS) on a linac-based platform. Methods: Efficiency was evaluated by measuring mask fabrication time during CT simulation and setup time in the treatment room. Mask shrinkage was assessed by comparing shim settings during treatment to simulation and also by measuring the distance between anterior and posterior sections of the mask on CBCT and Sim CT. Mask comfort was evaluated qualitatively post-treatment by surveying patients. Intrafraction motion was examined using 2D-3D autofusion of orthogonal kV images at mid-treatment. The intrafraction motion accuracy of the OSMS was analyzed by measuring the surface area of mask opening in Eclipse and pictures from the OSMS cameras. Results: The average preparation time for mask setup during simulation and treatment was 12.7 ± 2.3 min and 1.8 ± 0.9 min, respectively. Shims needed to be increased on average 0.8 mm, 0.8 mm, and 0.6 mm for right, left, and superior positions, respectively. CBCT measurements showed mask shrinkage, 0.8 mm both anteriorly and posteriorly. Greatest discomfort was reported on forehead followed by neck and chin. Intrafraction motion was less than 1 mm/1°. Accuracy of OSMS monitoring depended on the selected ROI. The OSMS monitored motion within 1 mm and 1° when the open surface area measured in Eclipse and captured by the camera > 147 cm2 and 33 cm2, respectively. When such conditions were not met, the accuracy somewhat degraded, an issue which can be mitigated by increasing the superior area or by angling the mask to expose greater surface to the camera. Conclusion: The new mask system rigidly immobilizes with sub-mm accuracy during treatment according to optical camera (OSMS) measurements. Research supported in part by a grant from QFix (Avondale, PA) and Varian Medical System (Palo Alto, CA)

SU-E-T-487: In VMAT of Spine Stereotactic Radiosurgery a 1 Mm Grid Size Increases Dose Gradient and Lowers Cord Dose Significantly Relative to a 2.5 Mm Grid Size

Purpose: Sharp dose gradients between the target and the spinal cord are critical to achieve dose constraints in spine stereotactic radiosurgery (SRS). In this study, the volume averaging effect of grid size (GS) on the dosimetric accuracy of volumetric modulated arc therapy (VMAT) spine SRS plans was investigated. Methods: The Eclipse v11.0 Anisotropic Analytical Algorithm (AAA) algorithm was used for dose calculation. Plan qualities of 10 treatment plans were evaluated with GS of 2.5mm (AAA’s default value) and 1mm. All plans were prescribed 18Gy to the 90% isodose line. Parameters used for comparison included the distance between 18Gy and 10Gy isodose levels in the axial plane, maximum cord dose (Dmax, defined as dose to 0.035cc), dose to 10% of the cord (D10%) and 0.35cc of the cord (D0.35cc), film gamma pass rate (3%, 1mm), film line profile through the cord, and calculation time. Paired t-test was used to investigate the statistical significance. Results: The 18Gy−10Gy distance was shorter for all plans with 1mm compared to 2.5mm GS (0.32±0.07mm vs. 0.38±0.07mm, p<0.001). In addition, 1mm GS plans showed lower cord Dmax (11.11±1.74Gy vs. 11.96±1.6Gy, p<0.001), D10% (8.26±1.08Gy vs. 9.11±1.18Gy, p<0.001) and D0.35cc (8.55±1.28Gy vs. 9.43±1.33Gy, p<0.001). Film analysis demonstrated better agreement in the high dose gradient near the cord. Calculation times for 1mm GS plans increased significantly (14:00 vs. 2:30, p<0.001). Conclusion: Due to more accurate dose calculations at the sharp dose gradient near the cord, we recommend the use of 1mm grid size for final dose calculation for VMAT SRS spine plans. This not only leads to more accurate dose gradient calculation near the cord, and lower spinal cord dose, but also, affords more room to balance between the needs of PTV coverage and cord sparing.

SU-E-J-217: Multiparametric MR Imaging of Cranial Tumors On a Dedicated 1.0T MR Simulator Prior to Stereotactic Radiosurgery

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.