Mobius Medical Systems Research Articles

SU-E-T-554: Dosimetric Impact of Implementing Kernel Hardening and Material-Specific Kernels in the Convolution/Superposition Method

Purpose: To investigate the dosimetric impact of implementing two improvements to the convolution/superposition (C/S) method that are generally ignored in traditional implementations of the algorithm. First, the impact of taking into account spectral changes of the incident photon beam in the energy deposition kernel (EDK) calculation (kernel hardening method), rather than using a single polyenergetic kernel that reflects the spectrum at a single location, was investigated. Second, the impact of implementing material‐specific kernels, rather than performing density scaling of water kernels, was investigated. Methods: To investigate the impact of kernel hardening, depth dose curves were calculated for a clinical 6MV photon beam incident on a water phantom using two different implementations of the collapsed cone C/S method (Mobius Medical Systems, Houston, TX), one using a single polyenergetic kernel and one that fully takes into account spectral changes in the kernel calculation. To investigate the impact of material‐specific kernels, depth dose curves were calculated for a simplified titanium implant geometry (4×4×4 cm3 titanium cavity inside a water phantom) using both a traditional C/S implementation that performs density scaling of water kernels and a novel implementation using titanium kernels (generated using the EGSnrc user code EDKnrc). Results: Implementation of kernel hardening increased the PDD value at 25 cm depth by 2.1% to 5.8% depending on the field size. Implementation of titanium kernels gave a 1.5% higher dose upstream of the metal cavity (i.e., higher backscatter dose) and a 5.9% lower dose downstream of the cavity. Conclusion: Implementation of kernel hardening affected the shape of the C/S‐calculated depth dose curves and thus has the potential to affect beam modeling parameters obtained in the commissioning process. For metal implants, the C/S algorithms generally underestimate the dose upstream and overestimate the dose downstream of the implant; implementation of material‐specific kernels mitigated both of these errors. This work was supported by Public Health Service grants CA010953, CA081647, and CA21661 awarded by the National Cancer Institute, United States Department of Health and Human Services.

SU-E-T-575: Independent Verification of VMAT Treatment Plans Using a DICOM-RT Framework

Purpose: To compare volumetric modulated arc therapy (VMAT) treatment plans calculated with Eclipse 10 (RapidArc, Varian Medical Systems) and Pinnacle 9.2 (SmartArc, Philips Healthcare) to those computed with an independent verification system utilizing a DICOM‐RT framework Mobius3D (M3D 1.1, Mobius Medical Systems, LP). Methods: Mobius3D (M3D) utilizes standardized machine data, supplemented by custom scaled measured depth dose curves, output factors and off‐axis ratios, to create institution specific beam models as an independent dose validation check. A randomly selected set of clinical radiotherapy plans generated by two treatment planning systems (TPS) consisting of 26 RapidArc prostate plans and 29 SmartArc head and neck plans were exported to M3D. The requisite DICOM‐RT files sent to M3D included CT images, contoured structure sets, RT plan and RT dose. This data was used to recalculate 3D dose distributions using a collapsed cone algorithm in M3D. Dose distributions were compared to those calculated by the TPS 3D dose using a 3D gamma analysis with 3% global dose difference and 3 mm isodose point distance criteria. Additionally, TPS's and M3D's target coverage and regions of interests sparing were compared by calculating the mean dose percent difference for each structure. DVH from each system were also compared. Results: On average, M3D plans showed excellent 3D gamma passing rates agreement of (99.1%±1.1%) and (95.7%±2.2%) relative to RapidArc and SmartArc plans, respectively. Similarly, the M3D recalculated and RapidArc and SmartArc calculated mean target dose percent differences demonstrated small differences of (−0.3%±1.2%) and (− 1.2%±0.9%), respectively. Conclusion: Mobius3D enables a paradigm shift in clinical quality assurance practice by moving beyond single point dose and MU verification to a full 3D treatment plan QA check. The VMAT results obtained utilizing fast and fully automated M3D software showed that M3D is suitable for an independent check of RapidArc and SmartArc plans.

SU-E-T-149: Couch and Gantry Rotation Isocenter QA Using Portal Imager

Purpose: Propose a method for extending digital image starshot analysis to gantry and couch rotation isocenters using and EPID. Methods: The portal imager can easily be used for creating spoke shots for the collimator rotation, but for the couch and gantry rotation typically films (traditional or GafChromics) are used. (a) For couch rotation a metal rod or a narrow slit in a metal sheet (e.g. an electron block cutout) is placed on the top of the couch. Exposures with several couch angles are obtained with the portal imager to create the spoke shot. (b) For gantry angle measurement a long rod is placed obliquely on the table top from the table axis and exposures are obtained with several gantry angles resulting in a spoke shot. The image summations and spoke shot evaluations were performed with DoseLab quality assurance/image analysis software (developed by Mobius Medical Systems). Results: The resulting spoke shots were all suitable for analysis (see supporting data). For couch rotation isocenter, the slit in an electron block worked better than a solid rod. For the gantry rotation isocenter the best Results was obtained with about ∼60 degree angle between the rod and the long axis of the table, and using 0, 70, 90, 110 and 180 degree gantry angles. The source‐portal imager distance was extended to 180cm, in order to decrease the differences in intensity between the proximal and distal parts of the rod. Conclusion: The method is easy to use and is faster than using films for isocenter analysis. This process also eliminates the need to scan films after exposure. All tests can be performed in one session with three entries into the treatment room. The DoseLab software performed the rotation isocenter tests for all three easily and efficiently.

SU-E-T-41: 2D Vs 3D Gamma Analysis: Establishment of Comparable Clinical Action Limits

Purpose: As clinics begin to use 3D quality assurance (QA) metrics, an investigation into appropriate action limits is required. This can be accomplished by comparing results of the 3D metric to the existing 2D metric. To investigate this, we compared 2D and 3D gamma analysis results to determine 3D gamma action limits that would Result in comparable 2D QA outcomes. Methods: 2D and 3D gamma QA analysis results for ten clinical IMRT patients using a variety of distance‐to‐agreements (DTA), dose‐thresholds, and interpolated slice thicknesses were chosen for this study. Clinical treatment planning system (TPS)‐calculated dose (evaluated) distribution and Monte Carlo‐calculated (reference) dose distribution were compared using DoseLab (Mobius Medical Systems) gamma analysis software that calculates 2‐ and 3‐dimensional gamma index for a given DTA. Each comparison evaluated the 2‐ and 3‐D average gamma and percent of pixels passing for the following acceptance criteria: ±5%/5mm, 3%/3mm, 2%/2mm, and 1%/1mm using 1 mm slice thicknesses (no dose threshold used). The effect of low‐dose thresholds and slice thickness was also evaluated. Results: The percent of pixels passing for each DTA was better for 3D as compared to the 2D analysis (2D vs 3D): 99.8% vs. 99.9% for 5%/5mm, 99.0% vs. 99.7% for 3%/3mm, 96.4% vs. 98.4% for 2%/2mm, and 79.3% vs. 83.4% for 1%/1mm. Using the low‐dose threshold resulted in worse gamma for both 2D and 3D gamma. However, similar to the no‐threshold results, use of dose threshold resulted in worse agreement for 2D than 3D for all DTA (4.1% decrease compared to 1.7% at 15% low‐dose threshold). No appreciable differences in gamma were observed as a function of slice thickness. Conclusions: Clinical use of 3D gamma analysis requires use of action limits (% of pixel passing and γ values) that are more stringent than 2D for comparable QA results.