Patient-specific Measurements Research Articles

WE-F-105-04: Evaluation of the Eclipse Electron Monte Carlo Algorithm Based On Patient Specific Dosimetry Measurement

Purpose: To evaluate the Eclipse electron Monte Carlo (eMC) dose calculation algorithm using patient specific dosimetry measurements, and to investigate possible patient specific QA guideline. Methods: Electron treatments were planned for 58 patients on Eclipse (Version 10.0) using the eMC algorithm for two Varian TrueBeam linear accelerators (Linac‐A and Linac‐B). The dose calculations were based on patient anatomy (patient plan), and a solid‐water phantom (QA plan). The calculation results were compared with physics measurement using a solid‐water phantom and a pinpoint ion chamber. Results: The monitor units as calculated on the QA plans were (2.1±2.0)% higher than the measurement on both accelerators, excluding outliers with small fields (3 cm or less) and extended SSD (above 105 cm). The difference of QA plan and measurement was consistent with the calculation uncertainty of the eMC algorithm (2%). The monitor units as calculated on the patient plans were (8.0±5.0)% (Linac‐A) and (5.5±5.0)% (Linac‐B) higher than the corresponding measurement. However, we observed that the eMC algorithm under‐estimated the dose for one patient treated on nose, by as much as 28.5% for the patient plan and 12.9% for the QA plan. For this patient, a calculation based on an output look‐up table was within 2% of the measurement. Conclusion: The measurements indicate that the eMC algorithm predicts dose reasonably well for QA plans using the same homogeneous phantom. However, for CT‐based patient plans, the eMC algorithm generates a larger number of monitor units versus the QA plan. It may reflect the impact of patient surface curvature and tissue inhomogeneity to the eMC algorithm and further investigation is needed. Caution should be taken for eMC patient plan with small field size and extended SSD. A lookup table of output factors based on measurement can be generated as a quick reference for electron dose calculation.

SU‐E‐T‐424: Improved Dosimetric Accuracy for Cyberknife Patient Plans Using a Dual‐Detector Measurement Method for Relative Output Factors

Purpose: The measurement of output factors (OFs) for small fields can lead to large dosimetric errors if detector effects are not accounted for. With its high spatial resolution and tissue equivalence, GAFCHROMIC film provides a correction free measure of OFs. We recently changed the OFs used in our Cyberknife planning system from uncorrected diode values to a dual detector method employing a diode with Monte‐Carlo corrections for the smallest collimators and a micro ion chamber for collimators >10 mm in diameter. Methods: We measured OFs for the CyberKnife G4 fixed collimators (5 to 60 mm) using an A16 microchamber and an Edge diode detector. The diode measured OFs for collimator sizes <10 mm were corrected using Monte‐Carlo correction factors. OFs were also measured using GAFCHROMIC film. We evaluated how this change in OFs influenced the dosimetric accuracy of patient specific QA measurements for 13 patient plans (9 before and 4 after the OF change) using film and the A16 chamber. Results: The OFs measured using the dual‐detector method agree with film to within two standard deviations for the full range of collimator sizes. When the dual detector method OFs are used, we achieve better dosimetric agreement (<2 sigma for pixels within the 80% isodose) than with uncorrected diode OFs for all patient specific QA plans measured using film. For patient specific QA using the microchamber, we get good agreement (<3%) for collimator sizes >5 mm, with differences observed for the 5 mm collimator consistent with volume averaging and a 1 mm setup uncertainty. Conclusions: OFs can be determined consistently using the dual‐detector method and verified using film. For patient specific QA, we achieve good agreement with microchambers for collimators >5 mm in diameter but film is the most appropriate detector for patient specific QA using the 5 mm collimator.

SU‐E‐T‐433: Evaluation of the ArcCheck Device for Patient‐Specific Quality Assurance in Step‐And‐Shoot IMRT for Single‐Fraction Spinal SBRT

Purpose: Current standard for patient specific spine SBRT IMRT QA in our institution is a combination of a point dose measurement in solid water under delivery of the beams at actual treatment angles, and 2D planar verification using an ion chamber (IC) array with all beams delivered at standard gantry angle of zero. The very small active area of ArcChecks diodes (∼0.6mm2) has the potential to improve measurement accuracy in high dose gradient regions present in spinal SBRT. Given high clinical workload we seek to improve ease of workflow and decrease time necessary for patient‐specific QA, by obtaining 2D verification and point dose measurement at actual treatment angles in one single measurement. Methods: The results of 10 patient specific QA measurements, as performed with single‐point and 2D IC array were compared to results obtained with ArcCheck. ArcCheck and 2D IC array pass rates are evaluated for 3%/3mm gamma criteria, single‐point measurement based on percent deviation from QA plan dose. The constancy of the device was assessed by re‐measuring the same treatment plan over the course of three weeks, and evaluating pass rate variationResults: The gamma pass rates are generally higher for ArcCheck (average 99.3%) than for Matrixx (average 93.1%). Average absolute deviation for the single‐point IC measurement was 3.4%. Maximum variation in pass rate between any two ArcCheck measurements within the three‐week evaluation period was 0.5%. Since ArcCheck allows for the use of an insert with an ion chamber in place, simultaneous redundant point measurement is possible with no time addedConclusion: Based on our results patient‐specific QA for spine SBRT treatments will be performed using ArcCheck with pass/fail criteria 98%. High sensitivity towards geometrical shifts necessitates careful initial alignment of the device.

SU‐E‐T‐226: SRT/SBRT Patient Specific QA with a New High Resolution 2D Detector Array

Purpose: To evaluate the performance of a new commercially available device for patient specific SBRT pretreatment QA.Materials and Methods: The PTW OCTAVIUS1000SRS was used in this study for patient specific QA measurements for patient undergoing SBRT treatments. Forty five (n=45) individual field and five (n=5) composite plans were measured. All plans were optimized and calculated using Eclipse v8.9. All measurements were performed with the gantry static at the upright orientation. The detector array was placed inside the PTW Octavius II phantom and the source to detector plane distance was set to 100cm. The gamma index was the metric of choice in order to quantify the agreement between measurements and calculations. The tolerance criteria used for the evaluation were 3% dose difference and 3mm distance to agreement and also 2%/2mm. Results: The results of the comparison using 3%/3mm were inconclusive. The number of detectors included in the comparison and the rather loose criteria used caused most comparisons to yield a gamma index of 100%. Comparing the measured and calculated planar dose distributions using 2%/2mm as gamma index tolerances, the average gamma index was 95.9% (min:90.9, max:99.2%). The agreement was also reflected while comparing profiles and isodose distributions. Conclusion: The new PTW Octavius 1000SRS device is able to provide a reliable solution for the patient specific QA of SBRT and SRS plans where small fields are commonly used. Project was partially funded by PTW, NY

SU‐E‐T‐199: A Software Tool for the Efficient Correction and Analysis of IMRT QA Experimental Data Using MatriXX

Purpose: The rationale for developing this software was to address shortcomings found while using OmniPro‐I' mRT software to evaluated patient specific IMRT QA measurements taken with MatriXXEvolution 2D ion chamber array. Our software addresses three major issues. It calculates average doses within each ion chamber of the array using 3D TPS DICOM field‐by‐field doses exported as input, corrects the resulting doses to account for gantry angle dependence of Matrixx, and automates the analysis of the measured data resulting in a concise report document ideal for patient electronic medical records (EMR).Materials/Methods: The software was developed in MATLAB to improve and streamline analysis of patient‐specific IMRT QA. It compares measured Matrixx doses to average doses within each ion chamber of the array, which are computed from 3D DICOM doses exported from TPS for each field. A correction factor is applied to the TPS doses to account for angular dependence. The Data is automatically evaluated using the 2D gamma analysis, evaluating a range of passing criteria (3%,4%,5%). This software allows for user‐defined passing criteria and automated reporting of quantitative results, eliminating potential user errors. A single‐sheet report is generated displaying patient information, planar dose distributions, x‐y‐profiles, and gamma plot noting the pixel based passing criteria. The effect of misalignment in direction perpendicular to detector plane is simulated by allowing a non‐zero shift along that direction. Results: This home‐built IMRT QA analysis tool gives a clear quantitative answer based on the user defined criteria. The implementation of an angular dependence correction and the averaging over the volume of the ion chambers improves the accuracy of the analysis accounting for ion chamber size (∼5mm x 5mm x5mm) and spacing (∼7mm × 7mm) Conclusion: Though this tool does not replace the commercial software completely, it improves the accuracy of the analysis and streamlines the process.

Patient‐specific QA for IMRT should be performed using software rather than hardware methods

Suggestions for topics suitable for these Point/Counterpoint debates should be addressed to Colin G. Orton, Professor Emeritus, Wayne State University, Detroit: ortonc@comcast.net. Persons participating in Point/Counterpoint discussions are selected for their knowledge and communicative skill. Their positions for or against a proposition may or may not reflect their personal opinions or the positions of their employers.

Direct Measurement of the Permeability of Human Cervical Tissue

The mechanical integrity of the uterine cervix is critical for a pregnancy to successfully reach full term. It must be strong to retain the fetus throughout gestation and then undergo a remodeling and softening process before labor for delivery of the fetus. It is believed that cervical insufficiency (CI), a condition in pregnancy resulting in preterm birth (PTB), is related to a cervix with compromised mechanical strength which cannot resist deformation caused by external forces generated by the growing fetus. Such PTBs are responsible for infant developmental problems and in severe cases infant mortality. To understand the etiologies of CI, our overall research goal is to investigate the mechanical behavior of the cervix. Permeability is a mechanical property of hydrated collagenous tissues that dictates the time-dependent response of the tissue to mechanical loading. The goal of this study was to design a novel soft tissue permeability testing device and to present direct hydraulic permeability measurements of excised nonpregnant (NP) and pregnant (PG) human cervical tissue from women with different obstetric histories. Results of hydraulic permeability testing indicate repeatability for specimens from single patients, with an order of magnitude separating the NP and PG group means (2.1 ± 1.4×10(-14) and 3.2 ± 4.8×10(-13)m(4)/N[middle dot]s, respectively), and large variability within the NP and PG sample groups. Differences were found between samples with similar obstetric histories, supporting the view that medical history may not be a good predictor of permeability (and therefore mechanical behavior) and highlighting the need for patient-specific measurements of cervical mechanical properties. The permeability measurements from this study will be used in future work to model the constitutive material behavior of cervical tissue and to develop in vivo diagnostic tools to stage the progression of labor.

Evaluation of Portal Dosimetry for VMAT Treatments

Evaluation of Portal Dosimetry for VMAT Treatments

A Novel Metric of Patient-specific Response to Treatment for Glioblastoma Discriminates Patients With Pseudo-progression

Purpose/Objective(s): A complete resolution of enhancing tumor on T1 weighted contrast MRI of patients with glioblastoma following resection and chemoradiation has been demonstrated to predict for longer survival. We have developed a novel method for discriminating time to progression following therapy for glioblastoma using computational models of the proliferative and diffusive tumor burden that account for the unique geometry and biology of each patient’s tumor. Using kinetic models informed by patient-specific measurements, we simulate the growth of an untreated tumor and estimate the number of “Days Gained” by the therapy. The goal of this study was to determine whether this metric of patient response would assist in evaluating patients who have residual T1-enhancing lesions following treatment with concurrent chemoradiation for glioblastoma. Materials/Methods: Twenty-eight patients with glioblastoma were prospectively enrolled prior to chemoradiation in our IRB approved image database study from 2003 to 2009. All patients underwent chemoradiation therapy at the University of Washington and had evidence of residual enhancing tumor on T1 weighted images in the first postradiation MRI study. Upon examination of subsequent images and clinical interventions we found evidence of pseudo-progression in 6 of these patients. We created individualized models of tumor growth for these 28 patients and computed their “Days Gained” score a measure of deflection of actual tumor growth from the model-predicted untreated tumor burden. Results: Based on a reporter-operator curve, we found that 104 Days Gained was an optimal discriminating cutoff (area under curve [AUC] Z 76.1%). We found that all patients with pseudo-progression (n Z 6) had Days Gained scores of 104 or above, with an odds ratio of 5.5 (p Z 0.03). None of the patients with Days Gained scores less than 104 (n Z 11) had pseudo-progression. There was no significant difference between the patients with greater than 104 Days Gained and no pseudo-progression (n Z 11) and those who had pseudo-progression. Additionally, patients with a higher Days Gained score had a longer predicted survival (median 610 vs. 376 days, p Z 0.04). Conclusions: Our novel method for measuring the predicted impact of treatment on both diffuse and proliferative tumor burden for glioblastoma discriminates those patients who are likely to have true progression on their first post-treatment scan. While we anticipated that patients with pseudo-progression would have the lowest estimate of Days Gained due to nonspecific imaging changes, we find instead that these patients are indistinguishable from the long-term responders. This difference in Days Gained may indicate that there is a robust biological response underlying the patients with an increased survival probability that is the subject of further investigation. Author Disclosure: A.D. Trister: None. M.L. Neal: None. T. Cloke: None. A.L. Baldock: None. S. Ahn: None. M.M. Mrugala: None. J.K. Rockhill: F. Honoraria; Elekta. R. Rockne: None. K.R. Swanson: None.

Poster - Thur Eve - 21: ROC analysis in patient specific quality assurance.

Many institutions rely on a patient specific measurement for IMRT/VMAT patient QA. In diagnostic imaging, radiologists use Receiver Operator Curves (ROC) to help quantify the value of a diagnostic imaging test. The purpose of this work is to investigate the value or ROC methodology for patient specific IMRT QA. Beam fluences for 34 prostate IMRT patients were analyzed using gamma analysis. For half of these, measurements were done using the planned beam fluences. For the rest, perturbations to the MLC leaf positions were introduced. Gamma analysis was then used to measure fluence differences. Assuming that the unperturbed fluencies were positive measurements, distributions of true positive and false negatives were calculated. For poorly performing beam delivery systems the choice of γ-DTA criterion has little effect on test sensitivity and specificity. The AUC is increased by about 10% for high performance beam delivery systems. For a 3%/3mm γ-DTA condition, ideal cut off values are reasonably independent of MLC performance. At a tighter γ-DTA condition of 2%/2mm, then the optimal sensitivity and specificity of the test is more dependent on MLC performance. For a pass-fail test such as the γ-DTA map is, it is important to choose an optimal cut off value to maximize the sensitivity and specificity of the test. ROC methodology allows users to follow a prescriptive method to obtain ideal cut-off values for gamma analysis, and to assess improvements in sensitivity and specificity for higher performing beam delivery system.

TH‐C‐BRB‐01: Credentialing Results from a Spine Anthropomorphic Phantom

Purpose: To use an anthropomorphic spine phantom for dosimetric credentialing purposes in National Cancer Institute sponsored clinical trials. Methods: An anthropomorphic spine phantom that consists of lungs, a heart, an esophagus, a spinal cord, vertebrae and an abutting planning target volume (PTV) was sent to institutions interested in becoming credentialed for a radiosurgery protocol for spine metastasis. The phantom contained film in 2 planes and TLD in the PTV. Institutions were asked to fill the phantom with water, image the phantom, create an IMRT plan for the spinal SBRT, perform image‐guidance for the target localization, and to irradiate the phantom. They were also asked to perform the type of patient specific measurements that they would for an actual patient. TLD inside the PTV are required to be within ±7%. 85% of the analyzed area of film was required to pass a gamma analysis of ±5%/3 mm. Results: 125 phantom treatment plans from 88 institutions were analyzed since 2009. Only 83 (66%) of the phantom irradiations passed the acceptance criteria. Of the 42 failures, 14 of the irradiations failed both the TLD and film criteria, 27 failed only the film criteria and 1 failed only the TLD criteria. Six institutions passed the criteria after including the couch in the calculation. The most represented planning systems, machine manufacturers and algorithm types have the ability to pass the phantom. Both pencil beam and superposition/convolution type algorithms can adequately account for the bone heterogeneity present in this phantom. Conclusion: The phantom test is a critical credentialing tool to secure the high quality protocol study. Treatment couch attenuation could affect tumor dose delivery accuracy, especially in instances where the majority of the beams pass through the couch. Supported by PHS grants CA10953 and CA81647 (NCI, DHHS). The investigation was supported by PHS grants CA10953 and CA81647 (NCI, DHHS).

SU-E-T-226: Clinical Implementation of a Gravity-Oriented Wedge for Total Body Irradiation.

To describe a TBI technique designed within the limits of a small-room geometry and its clinical implementation. Following construction of the universal treatment devices, including the double-wedge, beam spoiler table, and patient support table, commissioning consists of measurements to determine the output, tissue-phantom ratio, effective source distance, and off-axis factor. Dose is calculated by applying these factors per patient-specific measurements to arbitrary point in the patient. Typically, ten calculation points are located at mid-separation along the mid-sagittal plane from the head to the ankles. When areas of unacceptably high dose are calculated, custom compensators are constructed from 5-mm sheets of PMMA and placed over the patient on top of the beam spoiler table. The typical dose homogeneity of the planning calculations is within 2% of the prescribed dose. To verify the accuracy of the technique, an anthropomorphic phantom was simulated and treated. In total, 128 thermoluminescent dosimeters (TLDs) were irradiated within the phantom. Concentrations of TLDs were located in the planes of selected calculation points, i.e. the head, neck, sternum, lung, umbilicus, and pelvis. Results showed the average dose to these locations differed from the intended dose by -3.5%, 3.4%, 2.6%, 9.5%, 2.8%, and 0.5%, respectively. Due to its heterogeneous material, a higher discrepancy in the lung dose was anticipated. To demonstrate the dosimetric size of the radiation field, ionization chamber measurements were taken on one lateral side of the treatment area at a constant depth of 5 cm. A few measurements on the contralateral side were within 1 %, verifying the field's lateral symmetry. The approximate treatment area for the current technique is approximately 180×50 cm. We have demonstrated a small-room technique capable of meeting the dosimetric goal of TBI. To improve the dosimetric characteristics, new universal treatment devices are currently being designed and constructed.

Validation of a virtual source model for Monte Carlo dose calculations of a flattening filter free linac

A linac delivering intensity-modulated radiotherapy (IMRT) can benefit from a flattening filter free (FFF) design which offers higher dose rates and reduced accelerator head scatter than for conventional (flattened) delivery. This reduction in scatter simplifies beam modeling, and combining a Monte Carlo dose engine with a FFF accelerator could potentially increase dose calculation accuracy. The objective of this work was to model a FFF machine using an adapted version of a previously published virtual source model (VSM) for Monte Carlo calculations and to verify its accuracy. An Elekta Synergy linear accelerator operating at 6 MV has been modified to enable irradiation both with and without the flattening filter (FF). The VSM has been incorporated into a commercially available treatment planning system (Monaco™ v 3.1) as VSM 1.6. Dosimetric data were measured to commission the treatment planning system (TPS) and the VSM adapted to account for the lack of angular differential absorption and general beam hardening. The model was then tested using standard water phantom measurements and also by creating IMRT plans for a range of clinical cases. The results show that the VSM implementation handles the FFF beams very well, with an uncertainty between measurement and calculation of <1% which is comparable to conventional flattened beams. All IMRT beams passed standard quality assurance tests with >95% of all points passing gamma analysis (γ < 1) using a 3%/3 mm tolerance. The virtual source model for flattened beams was successfully adapted to a flattening filter free beam production. Water phantom and patient specific QA measurements show excellent results, and comparisons of IMRT plans generated in conventional and FFF mode are underway to assess dosimetric uncertainties and possible improvements in dose calculation and delivery.

Dose monitoring and output correction for the effects of scanning field changes with uniform scanning proton beam

The output of a proton beam is affected by proton energy, Spread-Out Bragg Peak (SOBP) width, aperture size, dose rate, and the point of measurement. In a uniform scanning proton beam (USPB), the scanning field size is adjusted (including the vertical length and the horizontal width) according to the treatment field size with appropriate margins to reduce secondary neutron production. Different scanning field settings result in beam output variations that are investigated in this study. The measurements are performed with a parallel plate Markus chamber at the center of SOBP under the reference condition with 16 cm range, 10 cm SOBP, and 5 cm air gap. The effect of dose rate on field output is studied by varying proton beam current from 0.5 to 7 nA. The effects of scanning field settings are studied by varying independently the field width and length from 12 x 12 to 30 x 30 cm2. The results demonstrate that scanning field variations can produce output variation up to 3.80%. In addition, larger output variation is observed with scanning field changes along the stem direction of the patient dose monitor (PDM). By investigating the underlying physics of incomplete charge collection and the stem effects of the PDM, an analytical model is proposed to calculate USPB output with consideration of the scanning field area and the PDM stem length that is irradiated. The average absolute difference between the measured output and calculated output using our new correction model are within 0.13 and 0.08% for the 20 and 30 cm snouts, respectively. This study proposes a correction model for accurate USPB output calculation, which takes account of scanning field settings and the PDM stem effects. This model may be used to extend the existing output calculation model from one snout size to other snout sizes with customized scanning field settings. The study is especially useful for calculating field output for treatment without individualized patient specific measurements.

TH‐E‐BRB‐03: Potential for a Monte Carlo Based Treatment Planning System to Replace Patient Specific QA Measurements in a Large University Hospital Setting

Purpose: To asses the feasibility and benefits of replacing patient specific QA measurements and back‐up MU calculations with Monte Carlo based dose calculations for improved plan verification in radiation therapy. Methods: An in‐house Monte Carlo based planning system (MMCTP) has been clinically implemented at McGill University. Over 50 patient plans have been recalculated on the system by both dosimetrists and physicists. Initial plans chosen for recalculation were head and neck IMRT and lung SBRT plans, where simple monitor unit calculators and measurements in water equivalent phantoms are often insufficient to show agreement of the planned and delivered doses within 5%. Results: MMCTP was found to be easy to use by both dosimetrists and physicists. Eighteen head and neck IMRT plans were recalculated by a dosimetrist and over 35 lung SBRT plans have been calculated by the physics staff. As no effort was spent in implementing fast MC engines, the calculation time is long but all mechanical operations, i.e., the transfer of the plans from the clinical treatment planning system to the MMCTP system and starting the calculation, take only a few minutes. Differences between MMCTP and the clinical TPS (Varian Eclipse, AAA) were small in terms of dose delivered to the PTV for the head and neck IMRT group but differences of greater the 5% were seen among the lung SBRT group. For both groups, the largest differences were seen for anatomy close to the skin and near air cavities. Conclusions: Verifying the delivery accuracy of complex treatment plans can be challenging. Current techniques requiring recalculation and measurement of plans in uniform phantoms are cumbersome and do not take into account the patient specific heterogeneity effects. The MMCTP treatment planning system requires minimal physics resources and likely introduces a more comprehensive method of evaluating calculation accuracy throughout the treatment volume.

SU‐E‐T‐113: Measurement of the Cutout Output Factor on Ortho‐Voltage Unit Using EBT‐2 Model GafchromicTM Film

Purpose: In this study, we evaluate feasibility of using EBT‐2 model GafchromicTM film to measure patient specific cutout output factors on an ortho‐voltage treatment machine. Methods: A Gulmay Orthovoltage D3225 unit was used to produce clinical beams in kV range from 50 kVp to 220 kVp. Previously established air kerma based radiochromic film dosimetry protocol for low energy x‐ray beams was used. A closed‐end applicator with 10 × 10 cm2 field size and SSD 50 cm was employed. Cutouts were made from a lead sheet of 1.8 mm in thickness. A 6‐cm thick slab of Solid WaterTM was used as a backscatter material for output factor measurement. In‐house made MatLab routines were used to analyze scanned images of exposed and unexposed films and a PTW 23342 parallel‐plate ion chamber (PP IC) was used to measure the cutout output factors for comparison. Results: Air‐Kerma based calibration curve was created in the 1–300 cGy range. Film dosimetic accuracy was verified for known doses to water of 1.00 and 2.00 Gy, respectively, with maximum observed difference of 0.4%. Two‐dimensional dose maps and dose profiles were obtained for various cutout sizes ranging from 0.8 cm diameter circle to 10 × 10 cm2 square. Comparisons of output factors between film and pp ion chamber measurements show that there are no differences for large cutouts but the chamber measurements are up to 11 % higher for small cutout size. Conclusions: We demonstrated that the air‐Kerma based radiochromic film dosimetry protocol using EBT‐2 model GafChromicTM film for low energy x‐ray beam is adequate for patient specific cutout output measurements on orthovoltage units, especially for small size cutout measurements, where IC measurements give higher output factor than films due to ICˈs over responses to electron contaminations from lead sheets.

It is STILL necessary to validate each individual IMRT treatment plan with dosimetric measurements before delivery

It is STILL necessary to validate each individual IMRT treatment plan with dosimetric measurements before delivery

Computational Hemodynamics in Cerebral Aneurysms: The Effects of Modeled Versus Measured Boundary Conditions

Modeling of flow in intracranial aneurysms (IAs) requires flow information at the model boundaries. In absence of patient-specific measurements, typical or modeled boundary conditions (BCs) are often used. This study investigates the effects of modeled versus patient-specific BCs on modeled hemodynamics within IAs. Computational fluid dynamics (CFD) models of five IAs were reconstructed from three-dimensional rotational angiography (3DRA). BCs were applied using in turn patient-specific phase-contrast-MR (pc-MR) measurements, a 1D-circulation model, and a physiologically coherent method based on local WSS at inlets. The Navier-Stokes equations were solved using the Ansys®-CFX™ software. Wall shear stress (WSS), oscillatory shear index (OSI), and other hemodynamic indices were computed. Differences in the values obtained with the three methods were analyzed using boxplot diagrams. Qualitative similarities were observed in the flow fields obtained with the three approaches. The quantitative comparison showed smaller discrepancies between pc-MR and 1D-model data, than those observed between pc-MR and WSS-scaled data. Discrepancies were reduced when indices were normalized to mean hemodynamic aneurysmal data. The strong similarities observed for the three BCs models suggest that vessel and aneurysm geometry have the strongest influence on aneurysmal hemodynamics. In absence of patient-specific BCs, a distributed circulation model may represent the best option when CFD is used for large cohort studies.

Preoperative Evaluation of Patient Anatomy to Increase Success of Robotics-Assisted Bypass Surgery

Robotics-assisted endoscopic atraumatic coronary artery bypass has been shown to be effective in reducing surgical morbidity and length of hospital stay. Unfortunately, the criteria for selecting eligible patients for this procedure are still primitive. This has motivated the use of preoperative computed tomography scans to establish patient eligibility. The objective of this study is to establish which image measurements can be correlated to procedure success. A retrospective study was performed in 144 patients who underwent robotics-assisted coronary bypass surgery. After an initial set of 55 patients, preoperative computed tomography scans of the other patients were used to obtain patient specific measurements: the lateral distance between the midline of the sternum to the left anterior descending coronary artery and its depth from the skin surface, anteroposterior diameter of the thoracic cavity, and the transverse diameter of the thoracic cavity. The procedures were rated as successful if completed in a minimally invasive manner. Different combinations of the variables were evaluated and correlated with success. A strong correlation was found between success rate and the ratio of the lateral distance to the transverse diameter in the female patients only (0.532, P = 0.006). A ratio of less than 0.20 significantly increased the occurrence of conversion during this procedure in female cases. The lateral distance of the left anterior descending coronary artery from the midline divided by the transverse thoracic width of a female patient shows a significant correlation with procedure success. No significant correlations were found for male patients.

SU-GG-T-455: Experimental Evaluation of Irradiated Dose in Clinical Practice for Proton Therapy

Purpose: The first objective of this study is to experimentally evaluate accuracy and reliability of proton patient‐specific dose per monitor unit(DMU) determination method currently used in our institute. The second objective is to explore the possibility of improving the accuracy of the DMU determined as above by using each of the pencil beam algorithm(PBA) and the simple Monte Carlo algorithm(SMCA) in addition to the patient specific measurement. Material and method: In order to emulate clinical situations as closely as possible, we used prostate and head and neck phantoms, performing the same procedures as our practice. We planned proton treatments for these phantoms, and determined DMU by patient‐specific measurement at the calibration point in polyethylene block(DMUpoly). Normally, we assume the dose absorbed at this point is approximately the same as the dose absorbed at the reference point in patient. By comparing DMUpoly with DMU measured at the reference point in phantoms, the error in irradiated dose was estimated. Next, we additionally used each of the PBA and SMCA to include the effect of inhomogeneities in phantoms. The DMU errors remaining even after this correction were examined. Result: The dose error of the present method was 1.4% for prostate case, while it was above 3% for 2 out of 6 head and neck cases. For one case, it even amounted to 9.4%. By use of PBA, the error decreased in 3 out of 7 cases. On the other hand, by using the SMCA, the resulting dose error decreased for all but one prostate case, where the error increased only by 0.7%. Conclusion: We could show that the SMCA is indeed capable of improving the accuracy of irradiated dose even for the very complex geometry as head and neck cases.