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SU-F-T-537: Prone Breast Accelerated Partial Breast Irradiation Using Non-Coplanar Volumetric Arc Therapy

PURPOSE To demonstrate that Volumetric Modulated Arc Therapy (VMAT) can be an alternative technique to Brachytherapy Accelerated Partial Breast Irradiation (APBI) for treating large breasted women. The non-coplanar VMAT technique uses a commercially available couch and a small number of angles. This technique with the patient in the prone position can reduce high skin and critical structure doses in large breasted women, which are usually associated with Brachytherapy APBI. METHODS Philips Pinnacle treatment planning system with Smart Arc was used to plan a left sided laterally located excision cavity on a standard prone breast patient setup. Three thirty-degree arcs entered from the lateral side at respective couch angles of 345, 0, and 15 degrees. A fourth thirty degree arc beam entered from the medial side at a couch angle of 0 degrees. The arcs were selected to avoid critical structures as much as possible. A test run was then performed to verify that the beams did not collide with the patient nor support structures. NSABP B-39/RTOG 0413 protocol guidelines were used for dose prescription, normal tissue, and target definition. RESULTS Dose Volume Histogram analysis indicated that all parameters were equal or better than RTOG recommendations. Of particular note regarding the plan quality:1.(a) For a prescribed dose of 3850cGy the PTV-EVAL target volume receiving 100 percent of the dose(V100) was 93; protocol recommendation is V90 > 90 percent. (b) Maximum dose was 110 percent versus the allowed 120 percent .2. Uninvolved percentage of normal breast V100 and V50 were 17 and 47 versus allowed 35 and 60 percent respectively.3. For the skin, V100 was 5.7cc and the max dose to 0.1 cc was 4190cGy. CONCLUSION Prone Breast non-coplanar VMAT APBI can achieve better skin cosmesis and lower critical structure doses than Brachytherapy APBI.

SU-F-T-400: Clinical Comparison of An IMRT Plan Computed with Four Algorithms

PURPOSE This work demonstrates clinical differences in a prostate IMRT plan DVH depending on which of four common algorithms is used for computation. Previous comparisons were often based on simple static or re-optimized IMRT plans which may not be clearly indicative of the clinical end result due solely to the computation algorithm. METHODS A step and shoot prostate multiple field IMRT plan was created in Pinnacle with a Collapsed Cone Convolution Superposition (CCCS) algorithm. The computed patient plan with corresponding dose and the control points without dose were both independently exported to Eclipse. Using the same patient image and structure data sets the control points were recomputed with the Anisotropic Analytical Algorithm (AAA) and Acuros XB for dose-to-water in medium (AXBwater) and dose-to-medium in medium (AXBmedium). All DVH values were measured in Eclipse to eliminated known differences between various DVH systems. A 0.3cm3 volume was created at the isocenter for point dose (PD) comparison. RESULTS The prostate target Acuros dose-to-medium in medium is clearly different than the others with a mean value of 101.7 cGy versus 103.8, 103.5 and 103.1 cGy for AAA, CCCS and AXBwater respectively. The difference in other clinical parameters such as volume at 105% dose (V105) is also unexpectedly different with values of 1.6, 23.9, 16.0 and 11.6 % respectively. The mean value in a small volume around the isocenter was 99.6, 102.4, 102.2 and 101.5 respectively which though not a great difference could affect the plan normalization and add to other effects in all regions of interest. The V102 is 25% larger for AAA compared to AXBwater. The variation in V50 for the bladder and rectum was approximately 5%. CONCLUSION This work demonstrates how an individual plan can be recomputed and compared directly between various algorithms and that clinical parameters can indeed vary according to which algorithm is used for computation.

SU‐E‐T‐356: Efficient Segmentation of Flattening Filter Free Photon Beamsfor 3D‐Conformal SBRT Treatment Planning

Purpose: It has been argued that a 3D-conformal technique (3DCRT) is suitable for SBRT due to its simplicity for non-coplanar planning and delivery. It has also been hypothesized that a high dose delivered in a short time can enhance indirect cell death due to vascular damage as well as limiting intrafraction motion. Flattening Filter Free (FFF) photon beams are ideal for high dose rate treatment but their conical profiles are not ideal for 3DCRT. The purpose of our work is to present a method to efficiently segment an FFF beam for standard 3DCRT planning. Methods: A 10×10 cm Varian True Beam 6X FFF beam profile was analyzed using segmentation theory to determine the optimum segmentation intensity required to create an 8 cm uniform dose profile. Two segments were automatically created in sequence with a Varian Eclipse treatment planning system by converting isodoses corresponding to the calculated segmentation intensity to contours and applying the “fit and shield” tool. All segments were then added to the FFF beam to create a single merged field. Field blocking can be incorporated but was not used for clarity. Results: Calculation of the segmentation intensity using an algorithm originally proposed by Xia and Verhey indicated that each segment should extend to the 92% isodose. The original FFF beam with 100% at the isocenter at a depth of 10 cm was reduced to 80% at 4cm from the isocenter; the segmented beam had +/−2.5 % uniformity up to 4.4cm from the isocenter. An additional benefit of our method is a 50% decrease in the 80%-20% penumbra of 0.6cm compared to 1.2cm in the original FFF beam. Conclusion: Creation of two optimum segments can flatten a FFF beam and also reduce its penumbra for clinical 3DCRT SBRT treatment.

SU-E-T-149: Brachytherapy Patient Specific Quality Assurance for a HDR Vaginal Cylinder Case

Purpose: Commonly Ir-192 HDR treatment planning system commissioning is only based on a single absolute measurement of source activity supplemented by tabulated parameters for multiple factors without independent verification that the planned distribution corresponds to the actual delivered dose. The purpose on this work is to present a methodology using Gafchromic film with a statistically valid calibration curve that can be used to validate clinical HDR vaginal cylinder cases by comparing the calculated plan dose distribution in a plane with the corresponding measured planar dose. Methods: A vaginal cylinder plan was created with Oncentra treatment planning system. The 3D dose matrix was exported to a Varian Eclipse work station for convenient extraction of a 2D coronal dose plane corresponding to the film position. The plan was delivered with a sheet of Gafchromic EBT3 film positioned 1mm from the catheter using an Ir-192 Nucletron HDR source. The film was then digitized with an Epson 10000 XL color scanner. Film analysis is performed with MatLab imaging toolbox. A density to dose calibration curve was created using TG43 formalism for a single dwell position exposure at over 100 points for statistical accuracy. The plan and measured film dose planes were registered using a known dwell position relative to four film marks. The plan delivered 500 cGy to points 2 cm from the sources. Results: The distance to agreement of the 500 cGy isodose between the plan and film measurement laterally was 0.5 mm but can be as much as 1.5 mm superior and inferior. The difference between the computed plan dose and film measurement was calculated per pixel. The greatest errors up to 50 cGy are near the apex. Conclusion: The methodology presented will be useful to implement more comprehensive quality assurance to verify patient-specific dose distributions

SU-C-BRD-03: Analysis of Accelerator Generated Text Logs for Preemptive Maintenance

PURPOSE To develop a model to analyze medical accelerator generated parameter and performance data that will provide an early warning of performance degradation and impending component failure. METHODS A robust 6 MV VMAT quality assurance treatment delivery was used to test the constancy of accelerator performance. The generated text log files were decoded and analyzed using statistical process control (SPC) methodology. The text file data is a single snapshot of energy specific and overall systems parameters. A total of 36 system parameters were monitored which include RF generation, electron gun control, energy control, beam uniformity control, DC voltage generation, and cooling systems. The parameters were analyzed using Individual and Moving Range (I/MR) charts. The chart limits were calculated using a hybrid technique that included the use of the standard 3σ limits and the parameter/system specification. Synthetic errors/changes were introduced to determine the initial effectiveness of I/MR charts in detecting relevant changes in operating parameters. The magnitude of the synthetic errors/changes was based on: the value of 1 standard deviation from the mean operating parameter of 483 TB systems, a small fraction (≤ 5%) of the operating range, or a fraction of the minor fault deviation. RESULTS There were 34 parameters in which synthetic errors were introduced. There were 2 parameters (radial position steering coil, and positive 24V DC) in which the errors did not exceed the limit of the I/MR chart. The I chart limit was exceeded for all of the remaining parameters (94.2%). The MR chart limit was exceeded in 29 of the 32 parameters (85.3%) in which the I chart limit was exceeded. CONCLUSION Statistical process control I/MR evaluation of text log file parameters may be effective in providing an early warning of performance degradation or component failure for digital medical accelerator systems. Research is Supported by Varian Medical Systems, Inc.

SU-E-T-144: Effective Analysis of VMAT QA Generated Trajectory Log Files for Medical Accelerator Predictive Maintenance

PURPOSE To determine the effectiveness of SPC analysis for a model predictive maintenance process that uses accelerator generated parameter and performance data contained in trajectory log files. METHODS Each trajectory file is decoded and a total of 131 axes positions are recorded (collimator jaw position, gantry angle, each MLC, etc.). This raw data is processed and either axis positions are extracted at critical points during the delivery or positional change over time is used to determine axis velocity. The focus of our analysis is the accuracy, reproducibility and fidelity of each axis. A reference positional trace of the gantry and each MLC is used as a motion baseline for cross correlation (CC) analysis. A total of 494 parameters (482 MLC related) were analyzed using Individual and Moving Range (I/MR) charts. The chart limits were calculated using a hybrid technique that included the use of the standard 3σ limits and parameter/system specifications. Synthetic errors/changes were introduced to determine the initial effectiveness of I/MR charts in detecting relevant changes in operating parameters. The magnitude of the synthetic errors/changes was based on: TG-142 and published analysis of VMAT delivery accuracy. RESULTS All errors introduced were detected. Synthetic positional errors of 2mm for collimator jaw and MLC carriage exceeded the chart limits. Gantry speed and each MLC speed are analyzed at two different points in the delivery. Simulated Gantry speed error (0.2 deg/sec) and MLC speed error (0.1 cm/sec) exceeded the speed chart limits. Gantry position error of 0.2 deg was detected by the CC maximum value charts. The MLC position error of 0.1 cm was detected by the CC maximum value location charts for every MLC. CONCLUSION SPC I/MR evaluation of trajectory log file parameters may be effective in providing an early warning of performance degradation or component failure for medical accelerator systems.

SU‐E‐T‐175: ArcCheck Hypothesis Testing: Is Dose Error in a Phantom Applicable to the Patient?

PURPOSE The purpose of this work is to present a technique by which we are able to measure the error produced by computing the delivered dose to a patient based on the dose delivered to a uniform density cylindrical phantom. METHODS ArcCHECK by Sun Nuclear (3DVH, Sun Nuclear Corp., Melbourne, FL) has the desirable ability to perform VMAT QA and configure the results in the form of a DVH similar to the original plan evaluation. In this work we examine the hypothesis that measured errors in a phantom translate very closely to those in a 3D patient. It is possible to create Verification Plans (VP) whereby the treatment fields for a lung patient are applied to both the patient data set (VPpatient) and a predefined phantom data set (VPphantom). The VP 360 arcs are subdivided into 30 degree sectors. The most useful feature of the VP is that we now have the ability to make changes. Each of the individual sectors is essentially an independent control point with alterable treatment aperture and Monitor Units. We modified a control point in the phantom (VPphantomModified) to see the effect on the patient (VPpatientModified). We tested the hypothesis in the two dimensional isocenter plane by exporting the four relevant dose matrices to MATLAB and performing the calculations as described by Sun Nuclear. RESULTS The amount of error greater then the traditional acceptance vale of 3% can be significant. Most of this error is concentrated in the lung and rib regions as would be expected from their greatest deviation from the uniform density in the phantom. CONCLUSION This work shows a technique whereby the user of Sun Nuclear ArcCHECK can verify and quantify the hypothesis that measured dose error in a homogenous phantom is applicable to a heterogeneous patient.

SU‐E‐T‐209: Deconvolution of An Integral Film Exposure in a Cylindrical Phantom for VMAT QA

PURPOSE Present an algorithm that is capable of deconvoluting a single radiochromic film exposed in a cylindrical phantom into the basic control point components. Changes noted in the measured control points can be incorporated into a delivered treatment plan using a commercial treatment planning system to compare the delivered dose with the planned dose using DVH tools. METHODS A 180 degree VMAT verification plan was subdivided into control points every 5 degrees. Dose matrices for the individual control points were exported for analysis to a PC with MATLAB software. The dose in the center ring of the cylindrical phantom represents the simulated film measurement. The plan dose is equal to the sum of the control points. Mathematically, the control points are the basis vectors of thedelivered plan. The deconvolution algorithm utilizes linear optimization to first determine the coefficients of the vectors and then to check for changes in the individual control points. Changes in the control points were introduced to test the deconvolution algorithm. For development purposes only a single plane was investigated. Measurement of both the entrance and exit doses improves the accuracy and is also used to compute the control point angle. RESULTS (1) Measured film has superior resolution compared to electronic detectors, (2) the deconvolution algorithm can determine the individual delivered control point parameters, (3) the measured control points can be used to calculate the delivered dose distribution, and (4) the treatment planning system can be used to compare the plan and delivered dose distribution within the patient using its DVH tools. CONCLUSION Deconvolution of an integral film in a cylindrical phantom for a VMAT plan represents the delivered control points that can be entered into the treatment planning system for patient specific VMAT QA utilizing DVH tools.

SU‐E‐T‐643: Optimum Beam Parameters for Lung SBRT Volumetric Arc Treatment

PURPOSE We present an analysis of the variation in acceptable SBRT lung plans with beam parameters. A figure of merit encompassing standard metrics is used for analytical comparison to determine the optimum plan quality. METHODS A set of optimization dose-volume constraints was formulated that consistently produced acceptable plans. Plans were normalized to deliver a prescription dose (PD) of 5000 cGy to 95% of the PTV volume. The Conformity Index (CI), Conformity Number (CN), and Gradient Index (GI), and mean GTV dose (MDgtv) were calculated. In SBRT hotspots near the target center are often deemed acceptable. The ratio MDgtv/PD is greater then 1.0 and larger values indicate that more dose is delivered where desired within the PTV. We combine the indices into a single figure of merit, FOM = (1/CI)*CN*(1/GI)*(MDgtv/PD), for which larger values indicate better plan quality dosimetrically. FOM values were normalized to 1.0 for the best plan. Twenty four plans were calculated for 6X, 6X flattening filter free (FFF), 10X, and 10X FFF photon beams. The gantry arc rotations were 0°-180° (180arc), 135°-30° (255arc), and 181°-179° (360arc). The couch angle was either 0° (coplanar) or +/- 15° (non-coplanar). RESULTS For the normal lung volume there was no significant variation in either mean dose or percent volume receiving 2000 cGy. However, the percent volume receiving 500 cGy varies significantly with energy and couch angle. Ninety six plan quality indices were tabulated. Overall, the 6X FFF non-coplanar beam with a 255 degree arc gave the best the figure of merit; it was 6.5% higher then nearest competitor largely due to superior conformality. CONCLUSIONS Individual plan quality indices were combined into a single figure of merit for various beam parameters that can be used to analytically select the optimum dosimetric plan.

SU‐GG‐T‐39: Optimized Planning of a High Dose Rate Prostate Brachytherapy Treatment Using Constrained Nonlinear Programming

Purpose: To describe an efficient prostate HDR optimization strategy using constrained nonlinear programming. Method and Materials: Calculation points, contours and source positions were exported from a clinical prostate HDR implant case to a PC running MatLab. A matrix Dij is computed for the dose rate to each calculation point i from each dwell position j. The dosed = Dij*t for dwell time t was calculated using the TG‐43 formalism. Both the prostate and urethra are planned for a uniform dosed0 with equal weights. The MatLab optimization toolbox is used to compute the minimum of an arbitrary nonlinear cost function ƒ( t ) = Σ abs ( d 0 − d )∧ n subject to linear inequality constraint Dij*t ⩾ d0 with t values between any lower and upper bound. We investigated the effects of performing an unbounded optimization and setting negative dwell times to zero; linear (n=1) versus quadratic (n=2) cost function with positive dwell times; and lowering the upper bound. These methods were compared to manual optimization performed by an expert user. Results: Utilizing a quadratic cost function, nearly perfect target coverage is obtained with acceptable urethral dose when negative dwell times are allowed. Setting negative dwell times to zero seriously compromises dose homogeneity. The effect of applying a linear cost function as well as that of lowering the upper dwell time bound is to significantly increase the hot spots. A quadratic cost function with positive dwell times and upper dwell time limit of nine times the mean provided a clinically optimal treatment plan. Compared to manual plans, optimized plans had superior target coverage, lower inhomogeneity and were calculated in minutes instead of hours. Conclusion: Constrained nonlinear programming is an effective tool to optimize prostate HDR treatment planning. A relatively high upper dwell time limit with a quadratic cost function yielded the best clinical results.