Dosimetric evaluation and beam characterization of pair production enhanced radiotherapy (PPER) with the use of organometallics.

The current study evaluates dosimetric and spectral effects when platinum (Pt)-based chemotherapeutics and less toxic tungstophosphoric-acid (TPA) organometallics are present during x-ray radiotherapy. We hypothesize that the use of high energy photon beams (i.e. 18 MV) will increase absorbed dose due to increased pair production from high-Z elements and thus result in additional tumor cell kill. EGSnrc code was used to examine the contribution of pair production to dose in the presence of the high-Z material (TPA, Pt mixtures and tungsten, W) as a function of beam energy. Variables included different concentrations (100 µmolar, 1 mmolar), depths (5 mm, 10 cm), thicknesses (5 mm, 5 cm) and energies (6, 18 MV). Overall, for the deeper depth, the 511 keV photon fluence increase was up 31% (18 MV-1 mmolar) while at 6 MV it was between 10%-11% depending on the concentration. For the shallower depth, 18 MV fluence increase was up 14.6% (1 mmolar) and 18.6% (1 mmolar) for the 6 MV. The dose enhancement effect due to pair production was up 25%-30% and a total 33%-58% depending on the depth. The benefit related to pair production was more for 18 MV and under conditions that simulated a realistic clinical setup. While part of the effect could be attributed to photoabsorption, a significant contribution of dose could result from pair production. Experimental clonogenic survival assay was consistent with the theory in that the low dose shoulder region of a cell survival curve was reduced using TPA and 18 MV compared with TPA and 6 MV or compared with no TPA and 18 MV; RBE was approximately 2 at the dose commonly used in conventional fractionated clinical radiotherapy. This suggests a potential new strategy for dose enhancement based on pair production using higher energy beamlines.

Technical Note: Evaluation of the systematic accuracy of a frameless, multiple image modality guided, linear accelerator based stereotactic radiosurgery system.

PURPOSE: To evaluate the total systematic accuracy of a frameless, image guided stereotactic radiosurgery system. METHODS: The localization accuracy and intermodality difference was determined by delivering radiation to an end-to-end prototype phantom, in which the targets were localized using optical surface monitoring system (OSMS), electromagnetic beacon-based tracking (Calypso®), cone-beam CT, "snap-shot" planar x-ray imaging, and a robotic couch. Six IMRT plans with jaw tracking and a flattening filter free beam were used to study the dosimetric accuracy for intracranial and spinal stereotactic radiosurgery treatment. RESULTS: End-to-end localization accuracy of the system evaluated with the end-to-end phantom was 0.5 ± 0.2 mm with a maximum deviation of 0.9 mm over 90 measurements (including jaw, MLC, and cone measurements for both auto and manual fusion) for single isocenter, single target treatment, 0.6 ± 0.4 mm for multitarget treatment with shared isocenter. Residual setup errors were within 0.1 mm for OSMS, and 0.3 mm for Calypso. Dosimetric evaluation based on absolute film dosimetry showed greater than 90% pass rate for all cases using a gamma criteria of 3%/1 mm. CONCLUSIONS: The authors' experience demonstrates that the localization accuracy of the frameless image-guided system is comparable to robotic or invasive frame based radiosurgery systems.

Extracting the normal lung dose-response curve from clinical DVH data: a possible role for low dose hyper-radiosensitivity, increased radioresistance.

In conventionally fractionated radiation therapy for lung cancer, radiation pneumonitis' (RP) dependence on the normal lung dose-volume histogram (DVH) is not well understood. Complication models alternatively make RP a function of a summary statistic, such as mean lung dose (MLD). This work searches over damage profiles, which quantify sub-volume damage as a function of dose. Profiles that achieve best RP predictive accuracy on a clinical dataset are hypothesized to approximate DVH dependence.Step function damage rate profiles R(D) are generated, having discrete steps at several dose points. A range of profiles is sampled by varying the step heights and dose point locations. Normal lung damage is the integral of R(D) with the cumulative DVH. Each profile is used in conjunction with a damage cutoff to predict grade 2 plus (G2+) RP for DVHs from a University of Michigan clinical trial dataset consisting of 89 CFRT patients, of which 17 were diagnosed with G2+ RP.Optimal profiles achieve a modest increase in predictive accuracy--erroneous RP predictions are reduced from 11 (using MLD) to 8. A novel result is that optimal profiles have a similar distinctive shape: enhanced damage contribution from low doses (<20 Gy), a flat contribution from doses in the range ~20-40 Gy, then a further enhanced contribution from doses above 40 Gy. These features resemble the hyper-radiosensitivity / increased radioresistance (HRS/IRR) observed in some cell survival curves, which can be modeled using Joiner's induced repair model.A novel search strategy is employed, which has the potential to estimate RP dependence on the normal lung DVH. When applied to a clinical dataset, identified profiles share a characteristic shape, which resembles HRS/IRR. This suggests that normal lung may have enhanced sensitivity to low doses, and that this sensitivity can affect RP risk.

Changes realized from extended bit-depth and metal artifact reduction in CT.

PURPOSE: High-Z material in computed tomography (CT) yields metal artifacts that degrade image quality and may cause substantial errors in dose calculation. This study couples a metal artifact reduction (MAR) algorithm with enhanced 16-bit depth (vs standard 12-bit) to quantify potential gains in image quality and dosimetry. METHODS: Extended CT to electron density (CT-ED) curves were derived from a tissue characterization phantom with titanium and stainless steel inserts scanned at 90-140 kVp for 12- and 16-bit reconstructions. MAR was applied to sinogram data (Brilliance BigBore CT scanner, Philips Healthcare, v.3.5). Monte Carlo simulation (MC-SIM) was performed on a simulated double hip prostheses case (Cerrobend rods embedded in a pelvic phantom) using BEAMnrc∕Dosxyz (400,000,0000 histories, 6X, 10 × 10 cm(2) beam traversing Cerrobend rod). A phantom study was also conducted using a stainless steel rod embedded in solid water, and dosimetric verification was performed with Gafchromic film analysis (absolute difference and gamma analysis, 2% dose and 2 mm distance to agreement) for plans calculated with Anisotropic Analytic Algorithm (AAA, Eclipse v11.0) to elucidate changes between 12- and 16-bit data. Three patients (bony metastases to the femur and humerus, and a prostate cancer case) with metal implants were reconstructed using both bit depths, with dose calculated using AAA and derived CT-ED curves. Planar dose distributions were assessed via matrix analyses and using gamma criteria of 2%∕2 mm. RESULTS: For 12-bit images, CT numbers for titanium and stainless steel saturated at 3071 Hounsfield units (HU), whereas for 16-bit depth, mean CT numbers were much larger (e.g., titanium and stainless steel yielded HU of 8066.5 ± 56.6 and 13,588.5 ± 198.8 for 16-bit uncorrected scans at 120 kVp, respectively). MC-SIM was well-matched between 12- and 16-bit images except downstream of the Cerrobend rod, where 16-bit dose was ∼6.4% greater than 12-bit. Absolute film dosimetry in a region downstream of a stainless steel rod revealed that 16-bit calculated dose, with and without MAR, agreed more closely with film results (1%-2% less than film) as compared to 12-bit reconstructions (5.6%-6.5% less than film measurements). Gamma analysis revealed that 16-bit dose calculations were better matched to film results than 12-bit (∼10% higher pass rates for 16-bit). Similar results were observed in two patient cases; the largest discrepancy was observed for a femur case where 12-bit doses, both with and without MAR correction, were 6-7 Gy lower (∼17%-20% of the prescription dose) as compared to 16-bit dose calculations. However, when beams are not directly traversing metal, such as a prostate cancer case with bilateral hip prostheses; the impact of 16-bit reconstruction was diminished. CONCLUSIONS: These results suggest that it may be desirable to implement 16-bit MAR-corrected images for treatment planning purposes, which can provide a more accurate dosimetric approach coupled with improved visualization by suppression of CT artifacts.

Commissioning of the Varian TrueBeam linear accelerator: a multi-institutional study.

PURPOSE: Latest generation linear accelerators (linacs), i.e., TrueBeam (Varian Medical Systems, Palo Alto, CA) and its stereotactic counterpart, TrueBeam STx, have several unique features, including high-dose-rate flattening-filter-free (FFF) photon modes, reengineered electron modes with new scattering foil geometries, updated imaging hardware/software, and a novel control system. An evaluation of five TrueBeam linacs at three different institutions has been performed and this work reports on the commissioning experience. METHODS: Acceptance and commissioning data were analyzed for five TrueBeam linacs equipped with 120 leaf (5 mm width) MLCs at three different institutions. Dosimetric data and mechanical parameters were compared. These included measurements of photon beam profiles (6X, 6XFFF, 10X, 10XFFF, 15X), photon and electron percent depth dose (PDD) curves (6, 9, 12 MeV), relative photon output factors (Scp), electron cone factors, mechanical isocenter accuracy, MLC transmission, and dosimetric leaf gap (DLG). End-to-end testing and IMRT commissioning were also conducted. RESULTS: Gantry/collimator isocentricity measurements were similar (0.27-0.28 mm), with overall couch/gantry/collimator values of 0.46-0.68 mm across the three institutions. Dosimetric data showed good agreement between machines. The average MLC DLGs for 6, 10, and 15 MV photons were 1.33 ± 0.23, 1.57 ± 0.24, and 1.61 ± 0.26 mm, respectively. 6XFFF and 10XFFF modes had average DLGs of 1.16 ± 0.22 and 1.44 ± 0.30 mm, respectively. MLC transmission showed minimal variation across the three institutions, with the standard deviation <0.2% for all linacs. Photon and electron PDDs were comparable for all energies. 6, 10, and 15 MV photon beam quality, %dd(10)x varied less than 0.3% for all linacs. Output factors (Scp) and electron cone factors agreed within 0.27%, on average; largest variations were observed for small field sizes (1.2% coefficient of variation, 10 MV, 2 × 2 cm(2)) and small cone sizes (<1% coefficient of variation, 6 × 6 cm(2) cone), respectively. CONCLUSIONS: Overall, excellent agreement was observed in TrueBeam commissioning data. This set of multi-institutional data can provide comparison data to others embarking on TrueBeam commissioning, ultimately improving the safety and quality of beam commissioning.

TU-E-BRB-03: Biological Dose Optimization for SBRT of Lung Cancer: One Size Does Not Fit All.

PURPOSE: Given the differences in tumor size and location, encountered in lung SBRT, we hypothesize that 'one dose fractionation regimen does not fit all', i.e. that there is a role for patient-specific dose prescription based on optimization of biological models. METHODS: Sixty one NSCLC patients (tumor volume 46.5+/-47.3 cc) treated with stereotactic body radiotherapy (48 Gy in 4fx) were retrospectively studied. Clinically treated plans were generated using Brainlab's Pencil Beam (PB-BL), and then recalculated with fixed MUs using Anisotropic Analytic Algorithm (AAA), Pencil Beam (PB-EC), Monte Carlo (MC) and Collapsed-Cone-Convolution (CCC). DVHs were exported to calculate TCP (Poisson) and NTCP (Lyman-Kutcher-Burman). TCP/NTCP model parameters were utilized from published data. For each dose distribution two dose response curves were generated by scaling the prescription dose and assuming a linear relationship between the prescription dose and entire 3D dose distribution. In addition, associations were assessed between changes in each algorithm's TCP relative to PB-BL, target diameter, and local density (density of the 70% isodose covering the PTV). RESULTS: For PB-BL, mean TCP was 99.6%±0.9%, whereas for same MUs, mean TCP for PB-EC, AAA, CC and MC plans were 96.5±14.3%, 74.6±31.6%, 74.4±32.4% and 76.8±32.0%, respectively. With the same prescription dose for all plans, TCP values changed to 98.1±8.7%, 96.5±15.3%, 77.5±28.6%, 85.4±25.8% and 92.9±20.1% for PB-BL, PB-EC, AAA, and CCC, MC, respectively, indicating that AAA and CCC dose distributions are likely less homogeneous relative to MC. The TCP improvement was 12.3%, 8.9% and 4.4% for AAA, CCC and MC-based plans when the average NTCP before optimization was set as the upper limit for lung toxicity. CONCLUSIONS: This work supports patient-specific dose prescription strategies, based on biological optimization, for lung SBRT. However, further investigation is warranted. Acknowledgement: supported in part by a grant from Varian Medical Systems.

TU-E-BRB-10: Dosimetric Consequences of Setup Errors Using CBCT for SBRT Localization.

PURPOSE: Steep dose gradients and high dose per fraction in stereotactic ablative radiation therapy (SABR or SBRT) necessitate highly accurate tumor localization. This study evaluates inter-fraction shifts, as defined by couch correction analysis, and investigates the effect of tumor location and internal target volume (ITV) on these shifts. In addition, residual errors associated with post-CBCT correction and their dosimetric consequences were quantified. METHODS: Daily free-breathing (FB) CBCT images used for daily localization of 78 patients with non-small cell lung cancer were retrospectively evaluated. Among the population, 39 patients also received pre-treatment kV images after CBCT alignment. ITV inter-fraction displacement was evaluated by matching the CBCT and the FB helical CT images, and setup errors were quantified using orthogonal kV images. Associations between ITV location and inter-fraction motion were studied by categorizing tumors into the following locations: chest-wall seated (CWS) and island, peripheral, central, or upper, middle and lower. Dosimetric consequences for the patient with the largest setup error were explored. RESULTS: ITV inter-fraction motion included the mean of the systematic error, ?inter=(-1.4, 2.0, 1.6) mm, standard deviation (SD) of the systematic error, Σinter=(2.1, 4.2, 2.9) mm, and SD of random errors, sinter=(2.2, 3.2, 3.6) mm. No significant associations were observed between inter-fraction shifts and tumor location or volume. Using CBCT for image guidance reduced the observed errors to µsetup=(-0.3, 0.1, 0.0) mm, Σsetup=(0.6, 0.6, 0.4) mm and ssetup=(1.2, 0.7, 0.7) mm. Dosimetric consequences for the patient with the largest setup error were explored. It was shown that a 3.0 mm setup margin was sufficient to provide greater than 95% dose coverage to the ITV. CONCLUSION: CBCT image guidance reduced setup errors significantly such that 2-3 mm, population-based, setup margins provided proper dose coverage to the ITV. Further investigation of inter-and intrafraction error classification by tumor location is warranted.

WE-E-BRB-03: Development of a Deformable Dosimetric Phantom for Verification of 4D Dose Calculation Algorithms.

PURPOSE: To develop a deformable lung phantom to verify voxel mapping and dose accumulation in 4D dose calculation algorithms used under different scenarios of tissue compression. METHODS: The phantom consists primarily of a heterogeneous sponge with an embedded tissue-equivalent tumor. The sponge is wrapped in a latex balloon housed in a Lucite cylinder. The balloon is attached to a piston that compresses the sponge to mimic the human diaphragm. The phantom was programmed to simulate different breathing patterns. Radiochromic films and TLD were embedded in the sponge for 4D dosimetry algorithm verification. 37 anatomical landmarks were manually tracked to verify voxel mappings for four deformable image registration (DIR) algorithms: in-house developed Demons and finite element model algorithms, and two B-Spline based Velocity AI registration algorithms performed between end-inhale and end-exhale. A 6MV photon beam was simulated with BEAMnrc/DOSXYZnrc on the end-inhale image with the dose mapped to the end-exhale using voxel-based linear dose mapping (LDM) and particle-based energy-mass congruence mapping (EMCM) methods. RESULTS: The mean density of the artificial lung was increased by 10.2% as the sponge was compressed by 2.5cm. The reproducibility of the phantom deformation was within image resolution (1×1×3 mm3), and the accuracy of four DIR registrations of the extreme phases was within 3.0mm. With the same registration displacement vector field (DVF), EMCM and LDM had different doses mapped to the end- exhale image. Their difference at the center of a beam was up to 8.3% for a Demons DVF and 5.8% for a Velocity DVF. The maximum difference between EMCM and LDM was 13.2% at beam penumbra. CONCLUSIONS: The developed deformable dosimetric phantom readily demonstrated variations among different dose addition and image registration algorithms, and is appropriate to serve as a QA tool for verification of 4D dose calculation algorithms. The research was supported by NIH/NCI R01CA140341.

SU-E-T-107: Uncertainty Estimate of a Practical EBT-2 Film Dosimetry Approach: Transpose-And-Scan Technique.

PURPOSE: To estimate the uncertainty of a practical EBT2 film dosimetry approach that has been established at our institution and used for routine patient-specific plan verifications, particularly for SBRT and RapidArc, as well as planning system commissioning. Our technique is unique from other common dosimetry protocols with respect to calibration, irradiation and scanning. METHODS: Film dosimetry for patient-specific quality assurance of 29 patient plans were retrospectively reviewed. For each case, four films were irradiated; two for calibration and two for treatment plan. Each pair of two films were irradiated together in a phantom with one film transposed (rotated 180 degrees relative to the other) to compensate for asymmetric film response. After a minimum of 12 hrs post-irradiation, each film was scanned in four different orientations to mitigate non-uniform response of the scanner light and detector elements. The scanned 8 calibration and 8 plan images were averaged into one calibration and one plan film image, respectively. Each color channel of the calibration film was correlated to the reference dose matrix to produce a 3rd order polynomial calibration curve. Finally, each color channel of the plan film was converted to a dose map using the corresponding calibration curve. Average dose maps of the red and green channels were correlated to the treatment planning dose matrix, and the mean dose differences at the center of dose distributions (5×5mm̂2 area) as well as a gamma analysis were evaluated. RESULTS: The absolute dose differences were -0.8±1.7% (range=-4.5-3.0%). The gamma pass-rates (3%/3mm) were 94±7% (min.=74%). The pass rate increased to 99±3%(min.=87%) with the film scaled relatively to the plan doses. CONCLUSIONS: Based on a large number of cases, our approach appears to be robust to non-uniform film and scanner responses, and is shown to have an uncertainty (1SD) of less than 2% for absolute film dosimetry.

SU-E-T-487: Spatial Assessment of Dose Distributions for Patients with Lung Cancer Treated with Stereotactic Ablative Radiotherapy (SABR).

PURPOSE: We hypothesize that PTV margin dose is an important factor for local tumor control. We evaluated dose distributions for patients originally treated with pencil-beam (PB)-based plans and retrospectively calculated with Monte Carlo (MC) method, with emphasis on the spatial region between the ITV and PTV (PTV-margin), where the largest dose differences were expected. METHODS: Forty-six stage I-II lung cancer patients with 51 lesions treated with SABR were retrospectively analyzed (23 central and 28 peripheral tumors). All patients received 4DCT imaging, and an ITV was generated from the maximum intensity projection and subsequent review of four 4DCT phases. An isotropic 3mm ITV-to-PTV margin was used. The iPlan TPS was used to generate the original treatment plans using PB-based heterogeneity correction. MC doses were recalculated using the same MUs as in the PB plan. Dose distributions for the ITV, PTV-margin, and PTV were analyzed using generalized equivalent uniform dose (gEUD) with a = - 20. Student's paired t-test elucidated differences between PB and MC-based gEUD and the two different tumor locations. RESULTS: Mean ITV and PTV volumes were 24.2 cc (range: 2.2 to 99.3 cc) and 50.4 cc (range: 6.4 to 229.7 cc), respectively. The mean gEUDs of ITV, PTV-margin and PTV, normalized to PB-based 100% isodose were 1.02+/-0.04, 1.01+/-0.04 and 1.01+/-0.04 for PB-based plans, compared to 0.94+/-0.06, 0.88+/-0.08 and 0.90+/-0.08 (all p<0.05) for MC-based plans. The maximum overestimations with the PB algorithm in the PTV-margin average dose were 10.4% and 19.6% (p < 0.05) for peripheral tumor cases and central tumor cases, respectively. CONCLUSIONS: PB-based dose distributions showed the highest dose overestimation (relative to MC) in the PTV-margin spatial region. Analysis of spatial dose differences is an important precursor toward assessment of patterns-of-local failure, to be investigated in future work to explore possible association between dose and regions of failure. Acknowledgement: supported in part by grants from NIH R01 CA106770 and from Varian Medical Systems.

SU-E-J-45: Validation of the ExacTrac Virtual Isocenter Based Target Localization Method.

PURPOSE: With no stable landmarks available for localization, a 'virtual isocenter' "'or surrogate landmark near the target'" can be used for image guidance. However, using a virtual isocenter in ExacTrac has not been thoroughly validated. This study evaluates its target localization accuracy and investigates the impact of two different couch correction sequences. METHODS: A CT scan was acquired on an anthropomorphic thoracic phantom with a 2mm-diameter ball bearing (BB) marker implanted in thelung region. A treatment plan was created with isocenter placed at the BB center, and exported to ExacTrac. In ExacTrac, a virtual isocenter wasplaced on a spine vertebral body where three translational shifts (8.8cm laterally, 1.5cm longitudinally and 6cm vertically) were present. A series ofcouch rotations (+/-3 degrees, 1 degree increment) was intentionally applied to simulate angular setup variations. For each rotation, two stereoscopic x-rayimages were acquired and fused using the ExacTrac 6D registrationalgorithm. Calculated shifts were applied using two sequences: (1)automatic 5D corrections (three translations/two robotic couch rotations) followed by manual couch rotation; (2) manual couch rotation then automatic 5D corrections. After each ExacTrac localization, orthogonal (anterior-posterior and right-lateral) portal images were acquired to quantify BB center deviations from the radiation isocenter as an indicator of residual error. RESULTS: Minimal difference between investigated table correction sequences was observed. Average translational deviations between the BB and radiation isocenter (mean+/-1SD) were 0.3+/-0.3mm and 1.0+/-0.2mm for lateral and vertical axis respectively. Longitudinally, the deviations were 0.8+/-0.4mm from the anterior-posterior image and 0.1+/-0.3mm from the right-lateral image. The systematic difference (0.7+/-0.1 mm) between thetwo may have been attributed to gantry sagging during rotation. CONCLUSIONS: ExacTrac system successfully corrected angular shifts using the virtual isocenter method in a rigid phantom setup. The sequence ofcouch correction did not influence the localization accuracy. Further patient study is warranted.

SU-E-J-59: Dual Imaging Guided Localization System for Spine Radiosurgery.

PURPOSE: To compare localization accuracies between an ExacTrac and cone beam computed tomography (CBCT) systems for single fraction spine adiosurgery. The work also aimed to evaluate the inherent systematic deviation of both ExacTrac and CBCT systems to achieve highly accurate localization in the spine radiosurgery. METHODS: ExacTrac and CBCT imaging systems were evaluated using the linac isocenter as the mutual reference point. First, a BB was placed in an anthropomorphic pelvic phantom. The phantom was localized with both imaging systems and the procedure was repeated 12 times. These results were used to devise a localization protocol using both imaging systems in spine radiosurgery, and employed for 51 patients (81 isocenters) prescribed for single fraction treatment. The displacement discrepancy between the isocenter and two systems were quantified in four dimensions (three translations, one rotation). A Student's two-tailed t-test was used to test for significant differences between the two imaging systems. RESULTS: The phantom study showed 1.4±0.5, 0.6±0.5, and 0.1±0.5 mm differences between the two imaging systems in the anterior/posterior (A/P), superior/inferior (S/I) and left/right (L/R) directions, respectively. The angular difference was minimal along all three axes. The patient study revealed similar isocenter discrepancies between ExacTrac and CBCT of 1.1 ± 0.7 mm, 1.0±0.9 mm, and 0.2±0.9 mm in the A/P, S/I, and L/R directions, respectively, with the A/P and S/I directions showing statistical significance ((t(80) = 13.5 and 7.6 respectively, p = 0.000). The couch yaw discrepancy was 0 ± 0.3°. Overall, 1 mm systematic differences were observed in the A/P and S/I directions between ExacTrac and CBCT localization systems, both in phantom and patient. A procedure was developed to mitigate this systematic discrepancy. CONCLUSIONS: These findings have justified our patient localization tolerance levels of 2 mm translation and 1 degree rotation for spine SRS treatment.

SU-E-T-197: A Comprehensive Variance Reporting System and an Analysis of Variances Reported at Our Institution.

PURPOSE: It is essential for radiation oncology departments to have comprehensive patient safety and quality programs. Two years ago we undertook a systematic review of our safety/QA program. Existing policies were updated and new policies created where necessary. One crucial component of any safety/QA program is continually updating it based on current information, the 'check' and 'act' portions of the Deming Cycle. We accomplished this with a transparent variance reporting system and a safety/QA committee reviewing and acting on reported variances. METHODS: With 5 radiation oncology centers in our institution, we needed to devise a system that would allow anyone to report a variance and provide our QA committee the ability to review variances system-wide. We developed the system using web-based tools. The system allows individuals to report variances, anonymously or named, specify the nature of the variance and indicate the tools used to identify the variance. RESULTS: In 2011, 285 variances were reported, 102 were reported by physicists, 86 anonymously, 71 by therapists and 26 by dosimetrists. We realized the need to develop clear classifications for variances. We added a high priority category, defined as variances which resulted in or had the potential to result in harm to a patient or when a policy is purposely overridden. Of the 285 variances reported, 5 were high priority. We created a process variance category, defined as variances where a specific clinical process is not followed. Of the 285 reported variances 155 were process variances. CONCLUSIONS: Reporting of variances through a centralized database is central toward developing a robust patient safety/quality assurance program. Anonymous reporting fosters a non-punitive environment, and promotes the 'safety culture'. The goal of such a system is to review trends in clinical processes and ultimately to improve safety/quality by reducing variances associated with these processes.

MO-F-BRA-04: Voxel-Based Statistical Analysis of Deformable Image Registration Error via a Finite Element Method.

Purpose Clinical implementation of adaptive treatment planning is limited by the lack of quantitative tools to assess deformable image registration errors (R-ERR). The purpose of this study was to develop a method, using finite element modeling (FEM), to estimate registration errors based on mechanical changes resulting from them. Methods An experimental platform to quantify the correlation between registration errors and their mechanical consequences was developed as follows: diaphragm deformation was simulated on the CT images in patients with lung cancer using a finite element method (FEM). The simulated displacement vector fields (F-DVF) were used to warp each CT image to generate a FEM image. B-Spline based (Elastix) registrations were performed from reference to FEM images to generate a registration DVF (R-DVF). The F- DVF was subtracted from R-DVF. The magnitude of the difference vector was defined as the registration error, which is a consequence of mechanically unbalanced energy (UE), computed using 'in-house-developed' FEM software. A nonlinear regression model was used based on imaging voxel data and the analysis considered clustered voxel data within images. Results A regression model analysis showed that UE was significantly correlated with registration error, DVF and the product of registration error and DVF respectively with R̂2=0.73 (R=0.854). The association was verified independently using 40 tracked landmarks. A linear function between the means of UE values and R- DVF*R-ERR has been established. The mean registration error (N=8) was 0.9 mm. 85.4% of voxels fit this model within one standard deviation. Conclusions An encouraging relationship between UE and registration error has been found. These experimental results suggest the feasibility of UE as a valuable tool for evaluating registration errors, thus supporting 4D and adaptive radiotherapy. The research was supported by NIH/NCI R01CA140341.

A Monte Carlo tutorial and the application for radiotherapy treatment planning.

Monte Carlo-based treatment planning algorithms are advancing rapidly and will certainly be implemented as part of conventional treatment planning systems in the near future. This paper was designed as a basic tutorial for using the Monte Carlo method as applied to radiotherapy treatment planning. The tutorial addresses the basic transport differences between photon and electron transport as well as the sampling distributions. The implementation of a virtual linac source model and the conversion from the Monte Carlo source modeling reference plane into the treatment reference plane is discussed. The implementation of a thresholding algorithm for converting CT electron density to patient specific materials is also presented. A 6-field prostate boost treatment is used to compare a conventional treatment planning algorithm (pencil beam model) with a Monte Carlo simulation algorithm. The agreement between the 2 calculation methods is good based upon the qualitative comparison of the isodose distribution and the dose-volume histograms for the prostate and the rectum. The effects of statistical uncertainty on the Monte Carlo calculation are also presented.

A virtual source model for Monte Carlo modeling of arbitrary intensity distributions.

A photon virtual source model was developed for simulating arbitrary, external beam, intensity distributions using the Monte Carlo method. The source model consists of a photon fluence grid composed of a matrix of square elements, located 25-cm downstream from the linear accelerator target. Each particle originating from the fluence map is characterized by the seven phase space parameters, position (x, y, z), direction (u, v, w), and energy. The map was reconstructed from fluence and energy spectra acquired by modeling components of the linear accelerator treatment head using the Monte Carlo code MCNP4B. The effect of contaminant electrons is accounted for by the use of a sub-source derived from a phase-space simulation of a 25-MV linac treatment head using the code BEAM. The BEAM sub-source was incorporated into the MCNP4B phase-space model and is sampled using a field-size dependent sampling ratio. A Gaussian blurring kernel is convolved with the photon fluence map to account for the finite focal spot size and scattering effects from structures such as the flattening filter and MLC leaves. Depth dose and profile source calculations for 6-MV and 25-MV photon beams, for 5 x 5 cm2, 10 x 10 cm2, and 15 x 15 cm2 field sizes, are in good agreement with measurement and are well within acceptability criteria suggested by the AAPM Task Group Report No. 53. Irregular field calculations compared with film measurement and with a 3-D pencil beam algorithm show that the source model is capable of accurately simulating arbitrary MLC fields.