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DOE OSTI2014

SU-F-18C-01: Minimum Detectability Analysis for Comprehensive Sized Based Optimization of Image Quality and Radiation Dose Across CT Protocols

Purpose: This work involved a comprehensive modeling of task-based performance of CT across a wide range of protocols. The approach was used for optimization and consistency of dose and image quality within a large multi-vendor clinical facility. Methods: 150 adult protocols from the Duke University Medical Center were grouped into sub-protocols with similar acquisition characteristics. A size based image quality phantom (Duke Mercury Phantom) was imaged using these sub-protocols for a range of clinically relevant doses on two CT manufacturer platforms (Siemens, GE). The images were analyzed to extract task-based image quality metrics such as the Task Transfer Function (TTF), Noise Power Spectrum, and Az based on designer nodule task functions. The data were analyzed in terms of the detectability of a lesion size/contrast as a function of dose, patient size, and protocol. A graphical user interface (GUI) was developed to predict image quality and dose to achieve a minimum level of detectability. Results: Image quality trends with variations in dose, patient size, and lesion contrast/size were evaluated and calculated data behaved as predicted. The GUI proved effective to predict the Az values representing radiologist confidence for a targeted lesion, patient size, and dose. As an example, an abdomen pelvis exam for the GE scanner, with a task size/contrast of 5-mm/50-HU, and an Az of 0.9 requires a dose of 4.0, 8.9, and 16.9 mGy for patient diameters of 25, 30, and 35 cm, respectively. For a constant patient diameter of 30 cm, the minimum detected lesion size at those dose levels would be 8.4, 5, and 3.9 mm, respectively. Conclusion: The designed CT protocol optimization platform can be used to evaluate minimum detectability across dose levels and patient diameters. The method can be used to improve individual protocols as well as to improve protocol consistency across CT scanners.

60 APPLIED LIFE SCIENCES↗
DOE OSTI2014

SU-F-18C-08: A Validation Study of a Commercially Available Software Package's Absorbed Dose Estimates in a Physical Phantom

Purpose: This study assesses the accuracy of the absorbed dose estimates from CT scans generated by Monte Carlo (MC) simulation using a commercially available radiation dose monitoring software program. Methods: Axial CT studies of an anthropomorphic abdomen phantom with dose bores at a central location and 4 peripheral locations were conducted using a fixed tube current at 120 kV. A 100 mm ion chamber and a 0.6 cc ion chamber calibrated at diagnostic energy levels were used to measure dose in the phantom at each of the 5 dose bore locations. Simulations using the software program's Monte Carlo engine were run using a mathematical model of the anthropomorphic phantom to determine conversion coefficients between the CTDIvol used for the study and the dose at the location of the dose bores. Simulations were conducted using both the software's generic CT beam model and a refined model generated using HVL and bow tie filter profile measurements made on the scanner used for the study. Results: Monte Carlo simulations completed using the generalized beam model differed from the measured conversion factors by an absolute value average of 13.0% and 13.8% for the 100 mm and 0.6 cc ion chamber studies, respectively. The MC simulations using the scanner specific beam model generated conversion coefficients that differed from the CTDIvol to measured dose conversion coefficients by an absolute value average of 7.3% and 7.8% for the 100 mm and 0.6 cc ion chamber cases, respectively. Conclusion: A scanner specific beam model used in MC simulations generates more accurate dose conversion coefficients in an anthropomorphic phantom than those generated with a generalized beam model. Agreement between measured conversion coefficients and simulated values were less than 20% for all positions using the universal beam model.

07 ISOTOPE AND RADIATION SOURCES↗
DOE OSTI2014

MO-C-17A-10: Comparison of Dose Deformable Accumulation by Using Parallel and Serial Approaches

Purpose: The uncertainty of dose accumulation over multiple CT datasets with deformable fusion may have significant impact on clinical decisions. In this study, we investigate the difference of two dose summation approaches involving deformable fusion. Methods: Five patients, four external beam and one brachytherapy(BT), were chosen for the study. The BT patient was treated with CT-based HDR. The CT image sets acquired in the imageguidance process (8-11 CTs/patient) were used to determine the dose delivered to the four external beam patients. (prostate, pelvis, lung and head and neck). For the HDR patient (cervix), five CT image sets and the corresponding BT plans were used. In total 44 CT datasets and RT dose/plans were imported into the image fusion software MiM (6.0.4) for analysis.For each of the five clinical cases, the dose from each fraction was accumulated into the primary CT dataset by using both Parallel and Serial approaches. The dose-volume histogram (DVH) for CTV and selected organs-at-risks (OAR) were generated. The D95(CTV), OAR(mean) and OAR(max) for the four external beam cases the D90(CTV), and the max dose to bladder and rectum for the BT case were compared. Results: For the four external beam patients, the difference in D95(CTV) were <1.2% PD between the parallel and the serial approaches. The differences of the OAR(mean) and the OAR(max ) range from 0 to 3.7% and <1% PD respectively. For the HDR patient, the dose difference for D90 is 11% PD while that of the max dose to bladder and rectum were 11.5% and 23.3% respectively. Conclusion: For external beam treatments, the parallel and serial approaches have <5% difference probably because tumor volume and OAR have less changes from fraction to fraction. For the brachytherapy case, >10% dose difference between the two approaches was observed as significant volume changes of tumor and OAR were observed among treatment fractions.

60 APPLIED LIFE SCIENCES↗
DOE OSTI2014

MO-E-17A-08: Attenuation-Based Size Adjusted, Scanner-Independent Organ Dose Estimates for Head CT Exams: TG 204 for Head CT

Purpose: AAPM Task Group 204 described size specific dose estimates (SSDE) for body scans. The purpose of this work is to use a similar approach to develop patient-specific, scanner-independent organ dose estimates for head CT exams using an attenuation-based size metric. Methods: For eight patient models from the GSF family of voxelized phantoms, dose to brain and lens of the eye was estimated using Monte Carlo simulations of contiguous axial scans for 64-slice MDCT scanners from four major manufacturers. Organ doses were normalized by scannerspecific 16 cm CTDIvol values and averaged across all scanners to obtain scanner-independent CTDIvol-to-organ-dose conversion coefficients for each patient model. Head size was measured at the first slice superior to the eyes; patient perimeter and effective diameter (ED) were measured directly from the GSF data. Because the GSF models use organ identification codes instead of Hounsfield units, water equivalent diameter (WED) was estimated indirectly. Using the image data from 42 patients ranging from 2 weeks old to adult, the perimeter, ED and WED size metrics were obtained and correlations between each metric were established. Applying these correlations to the GSF perimeter and ED measurements, WED was calculated for each model. The relationship between the various patient size metrics and CTDIvol-to-organ-dose conversion coefficients was then described. Results: The analysis of patient images demonstrated the correlation between WED and ED across a wide range of patient sizes. When applied to the GSF patient models, an exponential relationship between CTDIvol-to-organ-dose conversion coefficients and the WED size metric was observed with correlation coefficients of 0.93 and 0.77 for the brain and lens of the eye, respectively. Conclusion: Strong correlation exists between CTDIvol normalized brain dose and WED. For the lens of the eye, a lower correlation is observed, primarily due to surface dose variations. Funding Support: Siemens-UCLA Radiology Master Research Agreement; Disclosures - Michael McNitt-Gray: Institutional Research Agreement, Siemens AG; Research Support, Siemens AG; Consultant, Flaherty Sensabaugh Bonasso PLLC; Consultant, Fulbright and Jaworski.

60 APPLIED LIFE SCIENCES↗
DOE OSTI2014

MO-F-16A-06: Implementation of a Radiation Exposure Monitoring System for Surveillance of Multi-Modality Radiation Dose Data

Purpose: We have implemented a commercially available Radiation Exposure Monitoring System (REMS) to enhance the processes of radiation dose data collection, analysis and alerting developed over the past decade at our sites of practice. REMS allows for consolidation of multiple radiation dose information sources and quicker alerting than previously developed processes. Methods: Thirty-nine x-ray producing imaging modalities were interfaced with the REMS: thirteen computed tomography scanners, sixteen angiography/interventional systems, nine digital radiography systems and one mammography system. A number of methodologies were used to provide dose data to the REMS: Modality Performed Procedure Step (MPPS) messages, DICOM Radiation Dose Structured Reports (RDSR), and DICOM header information. Once interfaced, the dosimetry information from each device underwent validation (first 15–20 exams) before release for viewing by end-users: physicians, medical physicists, technologists and administrators. Results: Before REMS, our diagnostic physics group pulled dosimetry data from seven disparate databases throughout the radiology, radiation oncology, cardiology, electrophysiology, anesthesiology/pain management and vascular surgery departments at two major medical centers and four associated outpatient clinics. With the REMS implementation, we now have one authoritative source of dose information for alerting, longitudinal analysis, dashboard/graphics generation and benchmarking. REMS provides immediate automatic dose alerts utilizing thresholds calculated through daily statistical analysis. This has streamlined our Closing the Loop process for estimated skin exposures in excess of our institutional specific substantial radiation dose level which relied on technologist notification of the diagnostic physics group and daily report from the radiology information system (RIS). REMS also automatically calculates the CT size-specific dose estimate (SSDE) as well as provides two-dimensional angulation dose maps for angiography/interventional procedures. Conclusion: REMS implementation has streamlined and consolidated the dosimetry data collection and analysis process at our institutions while eliminating manual entry error and providing immediate alerting and access to dosimetry data to both physicists and physicians. Brent Stewart has funded research through GE Healthcare.

60 APPLIED LIFE SCIENCES↗
DOE OSTI2014

TH-C-19A-03: Characterization of the Dose Per Pulse Dependence of Various Detectors Used in Quality Assurance of FFF Treatment Plans

Purpose: To present the dose per pulse dependence of various QA devices under Flattening Filter Free (FFF) conditions. Methods: Air and liquid filled ion chamber arrays, diode arrays, radiochromic film and optically stimulated luminescence detectors were investigated. All detectors were irradiated under similar conditions of varying dose per pulse on a TrueBeam linac. Dose per pulse was controlled by varying SSD from 70 to 160 cm providing a range from ~0.5 to ~3 mGy per pulse. MU rates of up to 2400 MU/min for 10X FFF and 1400 MU/min for the 6X FFF beam were used. Beam pulses were counted using the Profiler™ diode array and pulse timing was confirmed by examining linac node files. Delivered doses were calculated with the Eclipse™ treatment planning system. Results: The detectors show a range of behaviors depending on the detector type, as expected. Diode arrays show up to 4% change in sensitivity (sensitivity increases with increasing dose per pulse) over the range tested. Air and liquid ion chambers arrays show a change in sensitivity of up to 3% (air) and 6% (liquid) (sensitivity decreases with increasing dose per pulse) while film and OSLD do not demonstrate a dependence on dose per pulse. Conclusion: Dependence of detector response on dose per pulse varies considerably depending on detector design. Interplay between dose per pulse and MU rate also exists for some detectors. Due diligence is required to characterize detector response prior to implementation of a QA protocol for FFF treatment delivery. During VMAT delivery, the MU rate may also vary dramatically within a treatment fraction. We intend to further investigate the implications of this for VMAT FFF patient specific quality assurance. T Karan and F Viel have received partial funding through the Varian Research program.

60 APPLIED LIFE SCIENCES↗
DOE OSTI2014

TH-E-BRF-03: A Multivariate Interaction Model for Assessment of Hippocampal Vascular Dose-Response and Early Prediction of Radiation-Induced Neurocognitive Dysfunction

Purpose: Vascular injury could be a cause of hippocampal dysfunction leading to late neurocognitive decline in patients receiving brain radiotherapy (RT). Hence, our aim was to develop a multivariate interaction model for characterization of hippocampal vascular dose-response and early prediction of radiation-induced late neurocognitive impairments. Methods: 27 patients (17 males and 10 females, age 31–80 years) were enrolled in an IRB-approved prospective longitudinal study. All patients were diagnosed with a low-grade glioma or benign tumor and treated by 3-D conformal or intensity-modulated RT with a median dose of 54 Gy (50.4–59.4 Gy in 1.8− Gy fractions). Six DCE-MRI scans were performed from pre-RT to 18 months post-RT. DCE data were fitted to the modified Toft model to obtain the transfer constant of gadolinium influx from the intravascular space into the extravascular extracellular space, Ktrans, and the fraction of blood plasma volume, Vp. The hippocampus vascular property alterations after starting RT were characterized by changes in the hippocampal mean values of, μh(Ktrans)τ and μh(Vp)τ. The dose-response, Δμh(Ktrans/Vp)pre->τ, was modeled using a multivariate linear regression considering integrations of doses with age, sex, hippocampal laterality and presence of tumor/edema near a hippocampus. Finally, the early vascular dose-response in hippocampus was correlated with neurocognitive decline 6 and 18 months post-RT. Results: The μh(Ktrans) increased significantly from pre-RT to 1 month post-RT (p<0.0004). The multivariate model showed that the dose effect on Δμh(Ktrans)pre->1M post-RT was interacted with sex (p<0.0007) and age (p<0.00004), with the dose-response more pronounced in older females. Also, the vascular dose-response in the left hippocampus of females was significantly correlated with memory function decline at 6 (r = − 0.95, p<0.0006) and 18 (r = −0.88, p<0.02) months post-RT. Conclusion: The hippocampal vascular response to radiation could be sex and age dependent. The early hippocampal vascular dose-response could predict late neurocognitive dysfunction. (Support: NIH-RO1NS064973)

60 APPLIED LIFE SCIENCES↗
DOE OSTI2015

Monte Carlo simulations of patient dose perturbations in rotational-type radiotherapy due to a transverse magnetic field: A tomotherapy investigation

Purpose: Several groups are exploring the integration of magnetic resonance (MR) image guidance with radiotherapy to reduce tumor position uncertainty during photon radiotherapy. The therapeutic gain from reducing tumor position uncertainty using intrafraction MR imaging during radiotherapy could be partially offset if the negative effects of magnetic field-induced dose perturbations are not appreciated or accounted for. The authors hypothesize that a more rotationally symmetric modality such as helical tomotherapy will permit a systematic mediation of these dose perturbations. This investigation offers a unique look at the dose perturbations due to homogeneous transverse magnetic field during the delivery of Tomotherapy{sup ®} Treatment System plans under varying degrees of rotational beamlet symmetry. Methods: The authors accurately reproduced treatment plan beamlet and patient configurations using the Monte Carlo code GEANT4. This code has a thoroughly benchmarked electromagnetic particle transport physics package well-suited for the radiotherapy energy regime. The three approved clinical treatment plans for this study were for a prostate, head and neck, and lung treatment. The dose heterogeneity index metric was used to quantify the effect of the dose perturbations to the target volumes. Results: The authors demonstrate the ability to reproduce the clinical dose–volume histograms (DVH) to within 4% dose agreement at each DVH point for the target volumes and most planning structures, and therefore, are able to confidently examine the effects of transverse magnetic fields on the plans. The authors investigated field strengths of 0.35, 0.7, 1, 1.5, and 3 T. Changes to the dose heterogeneity index of 0.1% were seen in the prostate and head and neck case, reflecting negligible dose perturbations to the target volumes, a change from 5.5% to 20.1% was observed with the lung case. Conclusions: This study demonstrated that the effect of external magnetic fields can be mitigated by exploiting a more rotationally symmetric treatment modality.

60 APPLIED LIFE SCIENCES↗
DOE OSTI2015

Attenuation-based size metric for estimating organ dose to patients undergoing tube current modulated CT exams

Purpose: Task Group 204 introduced effective diameter (ED) as the patient size metric used to correlate size-specific-dose-estimates. However, this size metric fails to account for patient attenuation properties and has been suggested to be replaced by an attenuation-based size metric, water equivalent diameter (D{sub W}). The purpose of this study is to investigate different size metrics, effective diameter, and water equivalent diameter, in combination with regional descriptions of scanner output to establish the most appropriate size metric to be used as a predictor for organ dose in tube current modulated CT exams. Methods: 101 thoracic and 82 abdomen/pelvis scans from clinically indicated CT exams were collected retrospectively from a multidetector row CT (Sensation 64, Siemens Healthcare) with Institutional Review Board approval to generate voxelized patient models. Fully irradiated organs (lung and breasts in thoracic scans and liver, kidneys, and spleen in abdominal scans) were segmented and used as tally regions in Monte Carlo simulations for reporting organ dose. Along with image data, raw projection data were collected to obtain tube current information for simulating tube current modulation scans using Monte Carlo methods. Additionally, previously described patient size metrics [ED, D{sub W}, and approximated water equivalent diameter (D{sub Wa})] were calculated for each patient and reported in three different ways: a single value averaged over the entire scan, a single value averaged over the region of interest, and a single value from a location in the middle of the scan volume. Organ doses were normalized by an appropriate mAs weighted CTDI{sub vol} to reflect regional variation of tube current. Linear regression analysis was used to evaluate the correlations between normalized organ doses and each size metric. Results: For the abdominal organs, the correlations between normalized organ dose and size metric were overall slightly higher for all three differently (global, regional, and middle slice) reported D{sub W} and D{sub Wa} than they were for ED, but the differences were not statistically significant. However, for lung dose, computed correlations using water equivalent diameter calculated in the middle of the image data (D{sub W,middle}) and averaged over the low attenuating region of lung (D{sub W,regional}) were statistically significantly higher than correlations of normalized lung dose with ED. Conclusions: To conclude, effective diameter and water equivalent diameter are very similar in abdominal regions; however, their difference becomes noticeable in lungs. Water equivalent diameter, specifically reported as a regional average and middle of scan volume, was shown to be better predictors of lung dose. Therefore, an attenuation-based size metric (water equivalent diameter) is recommended because it is more robust across different anatomic regions. Additionally, it was observed that the regional size metric reported as a single value averaged over a region of interest and the size metric calculated from a single slice/image chosen from the middle of the scan volume are highly correlated for these specific patient models and scan types.

60 APPLIED LIFE SCIENCES↗
DOE OSTI2015

Regional radiation dose susceptibility within the parotid gland: Effects on salivary loss and recovery

Purpose: Xerostomia is one of the most likely late toxic effects of radiotherapy treatment in patients with head-and-neck cancers. Modern treatment techniques can incorporate knowledge of complication risk into treatment plans. To this end, the authors attempt to quantify the regional radiotherapy dose-dependence of salivary output loss and recovery in a prospective study. Methods: Salivary output was collected from patients undergoing radiotherapy treatment for head-and-neck cancers at the BC Cancer Agency between February 2008 and May 2013. Regional dose-dependence (i.e., dose susceptibility) of loss and recovery is quantified using nonparametric (Spearman’s rank correlation coefficients, local linear regression) and parametric (least-sum of squares, least-median of squares) techniques. Results: Salivary flow recovery was seen in 79 of 102 patients considered (p < 0.0001, Wilcoxon sign rank test). Output loss was strongly correlated with left- and right parotid combined dose φ = min (D{sub L}, 45 Gy) + min (D{sub R}, 45 Gy), and can be accurately predicted. Median early loss (three months) was 72% of baseline, while median overall loss (1 yr) was 56% of baseline. Fitting an exponential model to whole parotid yields dose sensitivities A{sub 3m} = 0.0604 Gy{sup −1} and A{sub 1y} = 0.0379 Gy{sup −1}. Recovery was not significantly associated with dose. Hints of lateral organ sub-segment dose–response dimorphism were observed. Conclusions: Sub-segmentation appears to predict neither loss nor recovery with any greater precision than whole parotid mean dose, though it is not any worse. Sparing the parotid to a combined dose φ of <50 Gy is recommended for a patient to keep ≈40% of baseline function and thus avoid severe xerostomia at 12 months post-treatment. It seems unlikely that a population’s mean recovery will exceed 20%–30% of baseline output at 1 yr after radiotherapy treatment using current (whole-organ based) clinical guidelines.

60 APPLIED LIFE SCIENCES↗
DOE OSTI2015

Deterministic absorbed dose estimation in computed tomography using a discrete ordinates method

Purpose: Organ dose estimation for a patient undergoing computed tomography (CT) scanning is very important. Although Monte Carlo methods are considered gold-standard in patient dose estimation, the computation time required is formidable for routine clinical calculations. Here, the authors instigate a deterministic method for estimating an absorbed dose more efficiently. Methods: Compared with current Monte Carlo methods, a more efficient approach to estimating the absorbed dose is to solve the linear Boltzmann equation numerically. In this study, an axial CT scan was modeled with a software package, Denovo, which solved the linear Boltzmann equation using the discrete ordinates method. The CT scanning configuration included 16 x-ray source positions, beam collimators, flat filters, and bowtie filters. The phantom was the standard 32 cm CT dose index (CTDI) phantom. Four different Denovo simulations were performed with different simulation parameters, including the number of quadrature sets and the order of Legendre polynomial expansions. A Monte Carlo simulation was also performed for benchmarking the Denovo simulations. A quantitative comparison was made of the simulation results obtained by the Denovo and the Monte Carlo methods. Results: The difference in the simulation results of the discrete ordinates method and those of the Monte Carlo methods was found to be small, with a root-mean-square difference of around 2.4%. It was found that the discrete ordinates method, with a higher order of Legendre polynomial expansions, underestimated the absorbed dose near the center of the phantom (i.e., low dose region). Simulations of the quadrature set 8 and the first order of the Legendre polynomial expansions proved to be the most efficient computation method in the authors’ study. The single-thread computation time of the deterministic simulation of the quadrature set 8 and the first order of the Legendre polynomial expansions was 21 min on a personal computer. Conclusions: The simulation results showed that the deterministic method can be effectively used to estimate the absorbed dose in a CTDI phantom. The accuracy of the discrete ordinates method was close to that of a Monte Carlo simulation, and the primary benefit of the discrete ordinates method lies in its rapid computation speed. It is expected that further optimization of this method in routine clinical CT dose estimation will improve its accuracy and speed.

07 ISOTOPE AND RADIATION SOURCES↗
DOE OSTI2015

SU-E-J-109: Evaluation of Deformable Accumulated Parotid Doses Using Different Registration Algorithms in Adaptive Head and Neck Radiotherapy

Purpose: Three deformable image registration (DIR) algorithms are utilized to perform deformable dose accumulation for head and neck tomotherapy treatment, and the differences of the accumulated doses are evaluated. Methods: Daily MVCT data for 10 patients with pathologically proven nasopharyngeal cancers were analyzed. The data were acquired using tomotherapy (TomoTherapy, Accuray) at the PLA General Hospital. The prescription dose to the primary target was 70Gy in 33 fractions.Three DIR methods (B-spline, Diffeomorphic Demons and MIMvista) were used to propagate parotid structures from planning CTs to the daily CTs and accumulate fractionated dose on the planning CTs. The mean accumulated doses of parotids were quantitatively compared and the uncertainties of the propagated parotid contours were evaluated using Dice similarity index (DSI). Results: The planned mean dose of the ipsilateral parotids (32.42±3.13Gy) was slightly higher than those of the contralateral parotids (31.38±3.19Gy)in 10 patients. The difference between the accumulated mean doses of the ipsilateral parotids in the B-spline, Demons and MIMvista deformation algorithms (36.40±5.78Gy, 34.08±6.72Gy and 33.72±2.63Gy ) were statistically significant (B-spline vs Demons, P<0.0001, B-spline vs MIMvista, p =0.002). And The difference between those of the contralateral parotids in the B-spline, Demons and MIMvista deformation algorithms (34.08±4.82Gy, 32.42±4.80Gy and 33.92±4.65Gy ) were also significant (B-spline vs Demons, p =0.009, B-spline vs MIMvista, p =0.074). For the DSI analysis, the scores of B-spline, Demons and MIMvista DIRs were 0.90, 0.89 and 0.76. Conclusion: Shrinkage of parotid volumes results in the dose increase to the parotid glands in adaptive head and neck radiotherapy. The accumulated doses of parotids show significant difference using the different DIR algorithms between kVCT and MVCT. Therefore, the volume-based criterion (i.e. DSI) as a quantitative evaluation of registration accuracy is essential besides the visual assessment by the treating physician. This work was supported in part by the grant from Chinese Natural Science Foundation (Grant No. 11105225)

60 APPLIED LIFE SCIENCES↗
DOE OSTI2015

SU-E-T-44: A Micro-Raman Spectroscopy Study of the Dose-Dependence of EBT3 GafChromicTM Films for Quantifying the Degree of Molecular Polymerization

Purpose: Radiochromic films, such as the poly-diacetylene-based EBT3 GafChromic{sup TM} films (Ashland Specialty Ingredients, Wayne, NY, USA), are widely used for dosimetry applications because of their clear energy independence, high spatial resolution, near tissue equivalence and easy handling. The films undergo a slight color change by radiation-induced polymerization of diacetylene monomers. But more importantly, the film becomes optically less transparent with increasing radiation dose, with a saturation starting between 10 and 20 Gy, i.e. a common SBRT dose level. In contrast to the chromatic properties, less attention has been given to the underlying molecular mechanism that induces this partial color change and strongly reduces the transparency. Therefore, the current work investigates the variation of the molecular composition of the active layer of EBT3 films for an SBRT dose range. Method: Uncoated EBT3 films were irradiated with a 6 MV photon beam using dose levels between 0 and 20 Gy. The relative variation of the polymer content as a function of the applied radiation dose was studied using micro-Raman spectroscopy. Raman spectroscopy with a 633 nm probe laser incident on the active layer allowed to identify the film constituents and to estimate the amount of poly-diacetylenes from the intensities of the unique molecular vibrations of the molecule. Results: The normalized intensity of all polymeric vibrations, and most notably the polymeric triple and double carbon-carbon bonds at 2058 cm{sup −1} and 1446 cm{sup −1} respectively, increase with increasing dose up to a saturation level starting at about 10 Gy, indicating a corresponding increase and saturation of the amount of polymers. This molecular saturation process is the main cause of the non-linear dose response (i.e. a transparency reduction) and of the limited dose range of the studied films. Conclusion: Raman spectroscopy provides new and more fundamental insights in the mechanism of the dose-dependence of EBT3 films. This work was funded by the Research Foundation - Flanders (FWO)

60 APPLIED LIFE SCIENCES↗
DOE OSTI2015

SU-E-T-78: A Study of Dose Falloff Gradient in RapidArc Planning of Lung SBRT

Purpose: Rapid dose falloff beyond PTV is an important criterion for normal tissue sparing in SBRT. RTOG protocols use D2cm and R50% for plan quality evaluation. This study is aimed at analyzing the dose falloff gradient beyond the PTV extending into normal tissue structures and to ascertain the impact of PTV geometry and location on the dose falloff gradient in RapidArc planning of lung SBRT Methods: In this retrospective study, we analyzed 39 clinical RapidArc lung SBRT treatment plans that met RTOG-0915 criteria. Planning was done on Eclipse 8.9 for delivery on either Novalis NTx or TrueBeam STx equipped with HD MLCs. PTV volumes ranged between 5.3 and 113 cc (2.2 to 6 cm sphere equivalent diameter respectively) and their geographic locations were distributed in both lungs. 6X, 6X-FFF, 10X, and 10X-FFF energies were used for planning. All of these SBRT plans were planned using either 2 or 3 full or hemi arcs, with moderate couch kicks. Dose falloff gradients were obtained by generating 7 concentric 5 mm rings beyond PTV surface. Mean dose in each ring is used to evaluate percentage dose falloff gradient as a function of distance from the PTV surface. Results: The mean percentage dose falloff beyond PTV surface in all plans followed an exponential decay and the data was modeled with double exponential decay fit. Photon energy selection in the plan had a minimal impact on the mean percentage dose fall off beyond PTV surface. Conclusion: Dose falloff beyond PTV surface as a function of distance can be ascertained by the use of the double exponential decay fit coefficients in RapidArc planning of lung SBRT. This will help also in plan quality evaluation in addition to D2cm and R50% defined by RTOG.

60 APPLIED LIFE SCIENCES↗
DOE OSTI2015

SU-E-T-275: Dose Build Up and Bolusing Characteristics for Total Body Irradiation Dosimetry

Purpose: Total Body Irradiation (TBI) treatments are mainly used in a preparative regimen for haematopoietic stem cell (or bone marrow) transplantation. Our standard regimen is a 12 Gy / 6 fraction bi-daily technique. To evaluate the delivered dose homogeneity to the patient, EBT3 Gafchromic film is positioned at the head, neck, chest, pelvis and groin for all fractions. This work investigates and quantifies the build-up dose characteristics at TBI distances and requirements for in-vivo dosimetry bolusing. Methods: Percentage dose build up characteristics of photon beams have been investigated at large extended SSD’s using parallel plate ionisations chambers (Attix) and EBT3 Gafchromic film. Measurements were made to open fields at different field sizes as well as large 40cm × 40cm fields with differing scatter conditions such as the introduction of standard Perspex scattering plates at different distances to the measurement point. Results: Percentage surface dose measured values for open fields at 300 cm SSD were found to range from 20 % up to 65.5 % for fields of 5 cm × 5 cm to 40 cm × 40 cm. With the introduction of 1cm Perspex scattering plates used in TBI treatments the surface dose values increased up to 83% to 90%, depending on the position of the Perspex scattering plate compared to the measurement point. Our work showed that at least 3mm water equivalent bolus / scatter material should be placed over the EBT3 for accurate dose assessment for TBI treatments. Conclusion: Build up dose characteristics exist at long (300cm) SSD’s including treatments using Perspex scattering plates placed at various distances form the patient during TBI treatment. Top accurately assess the applied dose during treatment, in-vivo dosimeters such as Gafchromic EBT3 should have at least 3mm bolus / scatter material placed over them to measure actual applied doses.

60 APPLIED LIFE SCIENCES↗
DOE OSTI2015

SU-E-T-283: Dose Perturbations Near Heterogeneity Junctions for Modulated-Scanning Protons

Purpose: To compare calculated and measured doses near heterogeneity junctions of tissue-substitute materials for modulated-scanning protons. Methods: Three heterogeneous phantoms were configured using slabs of various plastics to simulate lung, fat, soft-tissue (polystyrene), and bone with known relative linear stopping powers (RLSPs). Each phantom consisted of soft-tissue and a single heterogeneity of a 5 or 10 cm thickness of a non-soft-tissue material. CT images were loaded into a Syngo treatment planning system and each material contoured and assigned its RLSP. Planning target volumes (PTVs) were drawn such that a beam would partially traverse the heterogeneity and partially only soft-tissue. Lateral profiles were measured using EDR2 films at a minimum of six depths between the phantom surface and the depth corresponding to the beam range. Absolute doses were measured inside and distal to the PTV in all phantoms using either a parallel plate or thimble chamber. Additional dose measurements were made between two lung slabs. Results: Profiles measured by film generally agreed with calculations except for depths distal to lung and fat junctions. Measured lateral penumbras for depths at the distal junction of lung were found to be wider than calculated ones. Compared with calculated doses, measured doses in the PTVs were 5.19% and 2.51% lower for lung and fat respectively but for bone were 0.2% higher. Measured doses for depths distal to the PTV were up to 29.65% and 10.58% higher for lung and fat, respectively but 6.30% lower for bone. Conclusion: The low measured doses in the PTVs for lung and fat might be due to underestimation of lateral scattering of protons. The higher measured doses distal to the PTV for the lung and fat are a Result of a shortened calculated beam range whereas the higher dose distal to the bone junction is within uncertainties.

60 APPLIED LIFE SCIENCES↗
DOE OSTI2015

SU-E-T-329: Dosimetric Impact of Implementing Metal Artifact Reduction Methods and Metal Energy Deposition Kernels for Photon Dose Calculations

Purpose: To investigate two strategies for reducing dose calculation errors near metal implants: use of CT metal artifact reduction methods and implementation of metal-based energy deposition kernels in the convolution/superposition (C/S) method. Methods: Radiochromic film was used to measure the dose upstream and downstream of titanium and Cerrobend implants. To assess the dosimetric impact of metal artifact reduction methods, dose calculations were performed using baseline, uncorrected images and metal artifact reduction Methods: Philips O-MAR, GE’s monochromatic gemstone spectral imaging (GSI) using dual-energy CT, and GSI imaging with metal artifact reduction software applied (MARs).To assess the impact of metal kernels, titanium and silver kernels were implemented into a commercial collapsed cone C/S algorithm. Results: The CT artifact reduction methods were more successful for titanium than Cerrobend. Interestingly, for beams traversing the metal implant, we found that errors in the dimensions of the metal in the CT images were more important for dose calculation accuracy than reduction of imaging artifacts. The MARs algorithm caused a distortion in the shape of the titanium implant that substantially worsened the calculation accuracy. In comparison to water kernel dose calculations, metal kernels resulted in better modeling of the increased backscatter dose at the upstream interface but decreased accuracy directly downstream of the metal. We also found that the success of metal kernels was dependent on dose grid size, with smaller calculation voxels giving better accuracy. Conclusion: Our study yielded mixed results, with neither the metal artifact reduction methods nor the metal kernels being globally effective at improving dose calculation accuracy. However, some successes were observed. The MARs algorithm decreased errors downstream of Cerrobend by a factor of two, and metal kernels resulted in more accurate backscatter dose upstream of metals. Thus, these two strategies do have the potential to improve accuracy for patients with metal implants in certain scenarios. This work was supported by Public Health Service grants CA 180803 and CA 10953 awarded by the National Cancer Institute, United States of Health and Human Services, and in part by Mobius Medical Systems.

60 APPLIED LIFE SCIENCES↗
DOE OSTI2015

SU-E-T-579: On the Relative Sensitivity of Monte Carlo and Pencil Beam Dose Calculation Algorithms to CT Metal Artifacts in Volumetric-Modulated Arc Spine Radiosurgery (RS)

Purpose: Investigating the relative sensitivity of Monte Carlo (MC) and Pencil Beam (PB) dose calculation algorithms to low-Z (titanium) metallic artifacts is important for accurate and consistent dose reporting in post¬operative spinal RS. Methods: Sensitivity analysis of MC and PB dose calculation algorithms on the Monaco v.3.3 treatment planning system (Elekta CMS, Maryland Heights, MO, USA) was performed using CT images reconstructed without (plain) and with Orthopedic Metal Artifact Reduction (OMAR; Philips Healthcare system, Cleveland, OH, USA). 6MV and 10MV volumetric-modulated arc (VMAT) RS plans were obtained for MC and PB on the plain and OMAR images (MC-plain/OMAR and PB-plain/OMAR). Results: Maximum differences in dose to 0.2cc (D0.2cc) of spinal cord and cord +2mm for 6MV and 10MV VMAT plans were 0.1Gy between MC-OMAR and MC-plain, and between PB-OMAR and PB-plain. Planning target volume (PTV) dose coverage changed by 0.1±0.7% and 0.2±0.3% for 6MV and 10MV from MC-OMAR to MC-plain, and by 0.1±0.1% for both 6MV and 10 MV from PB-OMAR to PB-plain, respectively. In no case for both MC and PB the D0.2cc to spinal cord was found to exceed the planned tolerance changing from OMAR to plain CT in dose calculations. Conclusion: Dosimetric impacts of metallic artifacts caused by low-Z metallic spinal hardware (mainly titanium alloy) are not clinically important in VMAT-based spine RS, without significant dependence on dose calculation methods (MC and PB) and photon energy ≥ 6MV. There is no need to use one algorithm instead of the other to reduce uncertainty for dose reporting. The dose calculation method that should be used in spine RS shall be consistent with the usual clinical practice.

61 RADIATION PROTECTION AND DOSIMETRY↗