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DOE OSTI2013

Determination of transit dose profile for a {sup 192}Ir HDR source

Purpose: Several studies have reported methodologies to calculate and correct the transit dose component of the moving radiation source for high dose rate (HDR) brachytherapy planning systems. However, most of these works employ the average source speed, which varies significantly with the measurement technique used, and does not represent a realistic speed profile, therefore, providing an inaccurate dose determination. In this work, the authors quantified the transit dose component of a HDR unit based on the measurement of the instantaneous source speed to produce more accurate dose values. Methods: The Nucletron microSelectron-HDR Ir-192 source was characterized considering the Task Group 43 (TG-43U1) specifications. The transit dose component was considered through the calculation of the dose distribution using a Monte Carlo particle transport code, MCNP5, for each source position and correcting it by the source speed. The instantaneous source speed measurements were performed in a previous work using two optical fibers connected to a photomultiplier and an oscilloscope. Calculated doses were validated by comparing relative dose profiles with those obtained experimentally using radiochromic films. Results: TG-43U1 source parameters were calculated to validate the Monte Carlo simulations. These agreed with the literature, with differences below 1% for the majority of the points. Calculated dose profiles without transit dose were also validated by comparison with ONCENTRA{sup Registered-Sign} Brachy v. 3.3 dose values, yielding differences within 1.5%. Dose profiles obtained with MCNP5 corrected using the instantaneous source speed profile showed differences near dwell positions of up to 800% in comparison to values corrected using the average source speed, but they are in good agreement with the experimental data, showing a maximum discrepancy of approximately 3% of the maximum dose. Near a dwell position the transit dose is about 22% of the dwell dose delivered by the source dwelling 1 s and reached 104.0 cGy per irradiation in a hypothetical clinical case studied in this work. Conclusions: The present work demonstrated that the transit dose correction based on average source speed fails to accurately correct the dose, indicating that the correct speed profile should be considered. The impact on total dose due to the transit dose correction near the dwell positions is significant and should be considered more carefully in treatments with high dose rate, several catheters, multiple dwell positions, small dwell times, and several fractions.

60 APPLIED LIFE SCIENCES↗

DOE OSTI2014

Comparison of internal dose estimates obtained using organ-level, voxel S value, and Monte Carlo techniques

Purpose: The authors’ objective was to compare internal dose estimates obtained using the Organ Level Dose Assessment with Exponential Modeling (OLINDA/EXM) software, the voxel S value technique, and Monte Carlo simulation. Monte Carlo dose estimates were used as the reference standard to assess the impact of patient-specific anatomy on the final dose estimate. Methods: Six patients injected with{sup 99m}Tc-hydrazinonicotinamide-Tyr{sup 3}-octreotide were included in this study. A hybrid planar/SPECT imaging protocol was used to estimate {sup 99m}Tc time-integrated activity coefficients (TIACs) for kidneys, liver, spleen, and tumors. Additionally, TIACs were predicted for {sup 131}I, {sup 177}Lu, and {sup 90}Y assuming the same biological half-lives as the {sup 99m}Tc labeled tracer. The TIACs were used as input for OLINDA/EXM for organ-level dose calculation and voxel level dosimetry was performed using the voxel S value method and Monte Carlo simulation. Dose estimates for {sup 99m}Tc, {sup 131}I, {sup 177}Lu, and {sup 90}Y distributions were evaluated by comparing (i) organ-level S values corresponding to each method, (ii) total tumor and organ doses, (iii) differences in right and left kidney doses, and (iv) voxelized dose distributions calculated by Monte Carlo and the voxel S value technique. Results: The S values for all investigated radionuclides used by OLINDA/EXM and the corresponding patient-specific S values calculated by Monte Carlo agreed within 2.3% on average for self-irradiation, and differed by as much as 105% for cross-organ irradiation. Total organ doses calculated by OLINDA/EXM and the voxel S value technique agreed with Monte Carlo results within approximately ±7%. Differences between right and left kidney doses determined by Monte Carlo were as high as 73%. Comparison of the Monte Carlo and voxel S value dose distributions showed that each method produced similar dose volume histograms with a minimum dose covering 90% of the volume (D90) agreeing within ±3%, on average. Conclusions: Several aspects of OLINDA/EXM dose calculation were compared with patient-specific dose estimates obtained using Monte Carlo. Differences in patient anatomy led to large differences in cross-organ doses. However, total organ doses were still in good agreement since most of the deposited dose is due to self-irradiation. Comparison of voxelized doses calculated by Monte Carlo and the voxel S value technique showed that the 3D dose distributions produced by the respective methods are nearly identical.

60 APPLIED LIFE SCIENCES↗

DOE OSTI2015

Accumulated Delivered Dose Response of Stereotactic Body Radiation Therapy for Liver Metastases

Purpose: To determine whether the accumulated dose using image guided radiation therapy is a stronger predictor of clinical outcomes than the planned dose in stereotactic body radiation therapy (SBRT) for liver metastases. Methods and Materials: From 2003 to 2009, 81 patients with 142 metastases were treated in institutional review board–approved SBRT studies (5-10 fractions). Patients were treated during free breathing (with or without abdominal compression) or with controlled exhale breath-holding. SBRT was planned on a static exhale computed tomography (CT) scan, and the minimum planning target volume dose to 0.5 cm{sup 3} (minPTV) was recorded. The accumulated minimum dose to the 0.5 cm{sup 3} gross tumor volume (accGTV) was calculated after performing dose accumulation from exported image guided radiation therapy data sets registered to the planning CT using rigid (2-dimensional MV/kV orthogonal) or deformable (3-dimensional/4-dimensional cone beam CT) image registration. Univariate and multivariate Cox regression models assessed the factors influencing the time to local progression (TTLP). Hazard ratios for accGTV and minPTV were compared using model goodness-of-fit and bootstrapping. Results: Overall, the accGTV dose exceeded the minPTV dose in 98% of the lesions. For 5 to 6 fractions, accGTV doses of >45 Gy were associated with 1-year local control of 86%. On univariate analysis, the cancer subtype (breast), smaller tumor volume, and increased dose were significant predictors for improved TTLP. The dose and volume were uncorrelated; the accGTV dose and minPTV dose were correlated and were tested separately on multivariate models. Breast cancer subtype, accGTV dose (P<.001), and minPTV dose (P=.02) retained significance in the multivariate models. The univariate hazard ratio for TTLP for 5-Gy increases in accGTV versus minPTV was 0.67 versus 0.74 (all patients; 95% confidence interval of difference 0.03-0.14). Goodness-of-fit testing confirmed the accGTV dose as a stronger dose–response predictor than the minPTV dose. Conclusions: The accGTV dose is a better predictor of TTLP than the minPTV dose for liver metastasis SBRT. The use of modern image guided radiation therapy in future analyses of dose–response outcomes should increase the concordance between the planned and delivered doses.

62 RADIOLOGY AND NUCLEAR MEDICINE↗

DOE OSTI2014

MO-E-17A-05: Individualized Patient Dosimetry in CT Using the Patient Dose (PATDOSE) Algorithm

Purpose: Radiation dose to the patient undergoing a CT examination has been the focus of many recent studies. While CTDIvol and SSDE-based methods are important tools for patient dose management, the CT image data provides important information with respect to CT dose and its distribution. Coupled with the known geometry and output factors (kV, mAs, pitch, etc.) of the CT scanner, the CT dataset can be used directly for computing absorbed dose. Methods: The HU numbers in a patient's CT data set can be converted to linear attenuation coefficients (LACs) with some assumptions. With this (PAT-DOSE) method, which is not Monte Carlo-based, the primary and scatter dose are computed separately. The primary dose is computed directly from the geometry of the scanner, x-ray spectrum, and the known patient LACs. Once the primary dose has been computed to all voxels in the patient, the scatter dose algorithm redistributes a fraction of the absorbed primary dose (based on the HU number of each source voxel), and the methods here invoke both tissue attenuation and absorption and solid angle geometry. The scatter dose algorithm can be run N times to include Nth-scatter redistribution. PAT-DOSE was deployed using simple PMMA phantoms, to validate its performance against Monte Carlo-derived dose distributions. Results: Comparison between PAT-DOSE and MCNPX primary dose distributions showed excellent agreement for several scan lengths. The 1st-scatter dose distributions showed relatively higher-amplitude, long-range scatter tails for the PAT-DOSE algorithm then for MCNPX simulations. Conclusion: The PAT-DOSE algorithm provides a fast, deterministic assessment of the 3-D dose distribution in CT, making use of scanner geometry and the patient image data set. The preliminary implementation of the algorithm produces accurate primary dose distributions however achieving scatter distribution agreement is more challenging. Addressing the polyenergetic x-ray spectrum and spatially dependent scatter fractions will improve algorithm accuracy. Funding Support: UC Davis Clinical and Translational Science Center (CTSC) Award; NIH IMSD Student Fellowship; Saxon Student Fellowship Disclosure: PATENT Royalties from Samsung for dosimetry algorithm.

60 APPLIED LIFE SCIENCES↗

DOE OSTI2015

Semiautomated head-and-neck IMRT planning using dose warping and scaling to robustly adapt plans in a knowledge database containing potentially suboptimal plans

Purpose: Prior work by the authors and other groups has studied the creation of automated intensity modulated radiotherapy (IMRT) plans of equivalent quality to those in a patient database of manually created clinical plans; those database plans provided guidance on the achievable sparing to organs-at-risk (OARs). However, in certain sites, such as head-and-neck, the clinical plans may not be sufficiently optimized because of anatomical complexity and clinical time constraints. This could lead to automated plans that suboptimally exploit OAR sparing. This work investigates a novel dose warping and scaling scheme that attempts to reduce effects of suboptimal sparing in clinical database plans, thus improving the quality of semiautomated head-and-neck cancer (HNC) plans. Methods: Knowledge-based radiotherapy (KBRT) plans for each of ten “query” patients were semiautomatically generated by identifying the most similar “match” patient in a database of 103 clinical manually created patient plans. The match patient’s plans were adapted to the query case by: (1) deforming the match beam fluences to suit the query target volume and (2) warping the match primary/boost dose distribution to suit the query geometry and using the warped distribution to generate query primary/boost optimization dose-volume constraints. Item (2) included a distance scaling factor to improve query OAR dose sparing with respect to the possibly suboptimal clinical match plan. To further compensate for a component plan of the match case (primary/boost) not optimally sparing OARs, the query dose volume constraints were reduced using a dose scaling factor to be the minimum from either (a) the warped component plan (primary or boost) dose distribution or (b) the warped total plan dose distribution (primary + boost) scaled in proportion to the ratio of component prescription dose to total prescription dose. The dose-volume constraints were used to plan the query case with no human intervention to adjust constraints during plan optimization. Results: KBRT and original clinical plans were dosimetrically equivalent for parotid glands (mean/median doses), spinal cord, and brainstem (maximum doses). KBRT plans significantly reduced larynx median doses (21.5 ± 6.6 Gy to 17.9 ± 3.9 Gy), and oral cavity mean (32.3 ± 6.2 Gy to 28.9 ± 5.4 Gy) and median (28.7 ± 5.7 Gy to 23.2 ± 5.3 Gy) doses. Doses to ipsilateral parotid gland, larynx, oral cavity, and brainstem were lower or equivalent in the KBRT plans for the majority of cases. By contrast, KBRT plans generated without the dose warping and dose scaling steps were not significantly different from the clinical plans. Conclusions: Fast, semiautomatically generated HNC IMRT plans adapted from existing plans in a clinical database can be of equivalent or better quality than manually created plans. The reductions in OAR doses in the semiautomated plans, compared to the clinical plans, indicate that the proposed dose warping and scaling method shows promise in mitigating the impact of suboptimal clinical plans.

60 APPLIED LIFE SCIENCES↗

DOE OSTI2017

Is Dose Deformation–Invariance Hypothesis Verified in Prostate IGRT?

Purpose: To assess dose uncertainties resulting from the dose deformation–invariance hypothesis in prostate cone beam computed tomography (CT)–based image guided radiation therapy (IGRT), namely to evaluate whether rigidly propagated planned dose distribution enables good estimation of fraction dose distributions. Methods and Materials: Twenty patients underwent a CT scan for planning intensity modulated radiation therapy–IGRT delivering 80 Gy to the prostate, followed by weekly CT scans. Two methods were used to obtain the dose distributions on the weekly CT scans: (1) recalculating the dose using the original treatment plan; and (2) rigidly propagating the planned dose distribution. The cumulative doses were then estimated in the organs at risk for each dose distribution by deformable image registration. The differences between recalculated and propagated doses were finally calculated for the fraction and the cumulative dose distributions, by use of per-voxel and dose-volume histogram (DVH) metrics. Results: For the fraction dose, the mean per-voxel absolute dose difference was <1 Gy for 98% and 95% of the fractions for the rectum and bladder, respectively. The maximum dose difference within 1 voxel reached, however, 7.4 Gy in the bladder and 8.0 Gy in the rectum. The mean dose differences were correlated with gas volume for the rectum and patient external contour variations for the bladder. The mean absolute differences for the considered volume receiving greater than or equal to dose x (V{sub x}) of the DVH were between 0.37% and 0.70% for the rectum and between 0.53% and 1.22% for the bladder. For the cumulative dose, the mean differences in the DVH were between 0.23% and 1.11% for the rectum and between 0.55% and 1.66% for the bladder. The largest dose difference was 6.86%, for bladder V{sub 80Gy}. The mean dose differences were <1.1 Gy for the rectum and <1 Gy for the bladder. Conclusions: The deformation–invariance hypothesis was corroborated for the organs at risk in prostate IGRT except in cases of a large disappearance or appearance of rectal gas for the rectum and large external contour variations for the bladder.

62 RADIOLOGY AND NUCLEAR MEDICINE↗

DOE OSTI2013

Comparative dose evaluations between XVI and OBI cone beam CT systems using Gafchromic XRQA2 film and nanoDot optical stimulated luminescence dosimeters

Purpose: To investigate the effect of energy (kVp) and filters (no filter, half Bowtie, and full Bowtie) on the dose response curves of the Gafchromic XRQA2 film and nanoDot optical stimulated luminescence dosimeters (OSLDs) in CBCT dose fields. To measure surface and internal doses received during x-ray volume imager (XVI) (Version R4.5) and on board imager (OBI) (Version 1.5) CBCT imaging protocols using these two types of dosimeters. Methods: Gafchromic XRQA2 film and nanoDot OSLD dose response curves were generated at different kV imaging settings used by XVI (software version R4.5) and OBI (software version 1.5) CBCT systems. The settings for the XVI system were: 100 kVp/F0 (no filter), 120 kVp/F0, and 120 kVp/F1 (Bowtie filter), and for the OBI system were: 100 kVp/full fan, 125 kVp/full fan, and 125 kVp/half fan. XRQA2 film was calibrated in air to air kerma levels between 0 and 11 cGy and scanned using reflection scanning mode with the Epson Expression 10000 XL flat-bed document scanner. NanoDot OSLDs were calibrated on phantom to surface dose levels between 0 and 14 cGy and read using the inLight{sup TM} MicroStar reader. Both dosimeters were used to measure in field surface and internal doses in a male Alderson Rando Phantom. Results: Dose response curves of XRQA2 film and nanoDot OSLDs at different XVI and OBI CBCT settings were reported. For XVI system, the surface dose ranged between 0.02 cGy in head region during fast head and neck scan and 4.99 cGy in the chest region during symmetry scan. On the other hand, the internal dose ranged between 0.02 cGy in the head region during fast head and neck scan and 3.17 cGy in the chest region during chest M20 scan. The average (internal and external) dose ranged between 0.05 cGy in the head region during fast head and neck scan and 2.41 cGy in the chest region during chest M20 scan. For OBI system, the surface dose ranged between 0.19 cGy in head region during head scan and 4.55 cGy in the pelvis region during spot light scan. However, the internal dose ranged between 0.47 cGy in the head region during head scan and 5.55 cGy in the pelvis region during spot light scan. The average (internal and external) dose ranged between 0.45 cGy in the head region during head scan and 3.59 cGy in the pelvis region during spot light scan. Both Gafchromic XRQA2 film and nanoDot OSLDs gave close estimation of dose (within uncertainties) in many cases. Though, discrepancies of up to 20%-30% were observed in some cases. Conclusions: Dose response curves of Gafchromic XRQA2 film and nanoDot OSLDs indicated that the dose responses of these two dosimeters were different even at the same photon energy when different filters were used. Uncertainty levels of both dosimetry systems were below 6% at doses above 1 cGy. Both dosimetry systems gave almost similar estimation of doses (within uncertainties) in many cases, with exceptions of some cases when the discrepancy was around 20%-30%. New versions of the CBCT systems (investigated in this study) resulted in lower imaging doses compared with doses reported on earlier versions in previous studies.

61 RADIATION PROTECTION AND DOSIMETRY↗

DOE OSTI2016

SU-F-T-157: Physics Considerations Regarding Dosimetric Accuracy of Analytical Dose Calculations for Small Field Proton Therapy: A Monte Carlo Study

Purpose: To evaluate the accuracy of dose calculations by analytical dose calculation methods (ADC) for small field proton therapy in a gantry based passive scattering facility. Methods: 50 patients with intra-cranial disease were evaluated in the study. Treatment plans followed standard prescription and optimization procedures of proton stereotactic radiosurgery. Dose distributions calculated with the Monte Carlo (MC) toolkit TOPAS were used to represent delivered treatments. The MC dose was first adjusted using the output factor (OF) applied clinically. This factor is determined from the field size and the prescribed range. We then introduced a normalization factor to measure the difference in mean dose between the delivered dose (MC dose with OF) and the dose calculated by ADC for each beam. The normalization was determined by the mean dose of the center voxels of the target area. We compared delivered dose distributions and those calculated by ADC in terms of dose volume histogram parameters and beam range distributions. Results: The mean target dose for a whole treatment is generally within 5% comparing delivered dose (MC dose with OF) and ADC dose. However, the differences can be as great as 11% for shallow and small target treated with a thick range compensator. Applying the normalization factor to the MC dose with OF can reduce the mean dose difference to less than 3%. Considering range uncertainties, the generally applied margins (3.5% of the prescribed range + 1mm) to cover uncertainties in range might not be sufficient to guarantee tumor coverage. The range difference for R90 (90% distal dose falloff) is affected by multiple factors, such as the heterogeneity index. Conclusion: This study indicates insufficient accuracy calculating proton doses using ADC. Our results suggest that uncertainties of target doses are reduced using MC techniques, improving the dosimetric accuracy for proton stereotactic radiosurgery. The work was supported by NIH/NCI under CA U19 021239. CG was partially supported by the Chinese Scholarship Council (CSC) and the National Natural Science Foundation of China (Grant No. 11475087).

60 APPLIED LIFE SCIENCES↗

DOE OSTI2014

Oxidative Stress and Skeletal Health with Low-Dose, Low-LET (Linear Energy Transfer) Ionizing Radiation

We performed in vivo and in vitro experiments to accomplish the following specific aims of this project: 1) determine if low dose, low LET radiation affects skeletal remodeling at structural, cellular and molecular levels and 2) determine if low dose, low LET radiation modulates skeletal health during aging via oxidative mechanisms. A third aim is supported by NASA supplement to this DOE grant focusing on the influence of high LET radiation on bone. A series of experiments were conducted at the NASA Space Radiation Laboratory at Brookhaven, NSRL-BNL, using iron (56Fe) or a sequential exposure to protons / iron / protons, and separate experiments at NASA Ames Research Center (ARC) using 137Cs. The following provides a summary of key findings. (1) Exposure of nine-week old female mice to priming doses of gamma radiation (10cGy x 5) did not significantly affect bone volume/total volume (BV/TV) or microarchitecture as analyzed by 3D microcomputed tomography. As expected, exposure to the challenge dose of 2 Gy gamma irradiation resulted in significant decreases in BV/TV. The priming dose combined with the 2Gy challenge dose had no further effect on BV/TV compared to challenge dose alone, with the sole exception of the Structural Model Index (SMI). SMI reflects the ratio of rods-to-plates in cancellous bone tissue, such that higher SMI values indicate a tendency toward a weaker structure compared to lower SMI values. Mice treated with both priming and challenge dose had 25% higher SMI values compared to sham-irradiated controls and 7% higher values compared to mice treated with the challenge dose alone. Thus, although this priming regimen had relatively modest effects on cancellous tissue, the difference in SMI suggests this fractionated priming doses have adverse, rather than beneficial, effects on bone structure. (2) In 10-week old male mice, a single exposure to 100cGy of 137Cs reduces trabecular bone number and connectivity density by 20% and 36% respectively one month after irradiation (IR). At four months post-IR, these animals were comparable to sham-treated controls with regards to the abovementioned structural parameters. Irradation at 1 or 10 cGy did not result in any significant changes in bone structural parameters. (3) Irradiation of 16-wk old male mice with high doses of 56Fe or proton (50 or 200cGy), but not at low doses (5 or 10cGy), showed a similar loss of cancellous BV/TV and trabecular number at five weeks post-IR. (4) Age-related bone loss overtook acute radiation-induced decrements in bone structure within four months post-IR with 100 cGy gamma and 12 months post-IR with 200 cGy iron. Transgenic mice globally overexpressing human catalase gene in mitochondria did not exhibit cancellous bone loss as assessed at four month post-IR with 10 cGy proton, 50 cGy iron, or in combination. (5) The cellular and molecular mechanisms responsible for loss of bone with radiation are mediated primarily through increased osteoclastogenesis. Our data provide evidence that there are increases in gene expression of TNF alpha and MCP1 in the bone marrow cells 24 hours post-IR and of osteoclastogenic differentiation factor RANKL by day 3. These cytokines in the marrow may stimulate mature osteoclasts or drive osteoclastogenesis from precursors. (6) Osteoblastogenesis from marrow progenitors evaluated ex vivo decreased following whole body 56Fe irradiation at a dose threshold between 20 and 50 cGy whereas osteoclastogenesis ex vivo increased with doses as low as 10cGy two days post-IR of mice. However, the latter finding was not observed in more than a single experiment. (7) Gamma irradiation of cells in vitro requires relatively high doses (200cGy) to disturb normal osteoblastogenesis and osteoclastogenesis as evidenced by decrements in mineralized nodule formation, osteoclast counts, and expression of osteoblast related genes such as runx2, col1a1. (8) We also investigated the effect of antioxidants on osteoblastogenesis following low dose in vitro gamma irradiation (15cGy) on day four bone marrow stromal cell cultures. Superoxide dismutase (SOD) was added to the cell culture medium for 2 or 3 days post-irradiation and cell colonies were counted on days 7 and 10. SOD treatment increased cell growth as measured by DNA content and colony forming units (CFU) in both irradiated cells and 0 cGy control groups. However, low dose radiation of 15cGy abolished SOD stimulatory effects on cell growth and CFU number. These results suggest that exogenous SOD increases osteoblast cell growth and colony formation and that low-dose radiation (15cGy) can interfere with the antioxidant effects. In summary, our findings indicate that acute, whole body irradiation at high doses (50-200 cGy) results in prompt tissue degradation and bone loss. Lower doses (<50 cGy) do not cause bone structural deterioration but may deplete stem/progenitor cell pools in the bone marrow.

59 BASIC BIOLOGICAL SCIENCES↗

DOE OSTI2015

SU-E-J-181: Effect of Prostate Motion On Combined Brachytherapy and External Beam Dose Based On Daily Motion of the Prostate

Purpose: In this study, the adequacy of target expansions on the combined external beam and implant dose was examined based on the measured daily motion of the prostate. Methods: Thirty patients received an I–125 prostate implant prescribed to dose of 90Gy. This was followed by external beam to deliver a dose of 90Gyeq (external beam equivalent) to the prostate over 25 to 30 fractions. An ideal IMRT plan was developed by optimizing the external beam dose based on the delivered implant dose. The implant dose was converted to an equivalent external beam dose using the linear quadratic model. Patients were set up on the treatment table by daily orthogonal imaging and aligning the marker seeds in the prostate. Orthogonal images were obtained at the end of treatment to assess prostate intrafraction motion. Based on the observed motion of the markers between the initial and final images, 5 individual plans showing the actual dose delivered to the patient were calculated. A final true dose distribution was established based on summing the implant dose and the 5 external beam plans. Dose to the prostate, seminal vesicles, lymphnodes and normal tissues, rectal wall, urethra and lower sphincter were calculated and compared to ideal. On 18 patients who were sexually active, dose to the corpus cavernosum and internal pudendal artery was also calculated. Results: The average prostate motion in 3 orthogonal directions was less than 1 mm with a standard deviation of less than +2 mm. Dose and volume parameters showed that there was no decrease in dose to the targets and a marginal decrease in dose to in normal tissues. Conclusion: Dose delivered by seed implant moves with the prostate, decreasing the impact of intrafractions dose movement on actual dose delivered. Combined brachytherapy and external beam dose delivered to the prostate was not sensitive to prostate motion.

60 APPLIED LIFE SCIENCES↗

DOE OSTI2016

SU-F-P-56: On a New Approach to Reconstruct the Patient Dose From Phantom Measurements

Purpose: The development of complex radiation treatment schemes emphasizes the need for advanced QA analysis methods to ensure patient safety. One such tool is the Delta4 DVH Anatomy software, where the patient dose is reconstructed from phantom measurements. Deviations in the measured dose are transferred to the patient anatomy and their clinical impact is evaluated in situ. Results from the original algorithm revealed weaknesses that may introduce artefacts in the reconstructed dose. These can lead to false negatives or obscure the effects of minor dose deviations from delivery failures. Here, we will present results from a new patient dose reconstruction algorithm. Methods: The main steps of the new algorithm are: (1) the dose delivered to a phantom is measured in a number of detector positions. (2) The measured dose is compared to an internally calculated dose distribution evaluated in said positions. The so-obtained dose difference is (3) used to calculate an energy fluence difference. This entity is (4) used as input to a patient dose correction calculation routine. Finally, the patient dose is reconstructed by adding said patient dose correction to the planned patient dose. The internal dose calculation in step (2) and (4) is based on the Pencil Beam algorithm. Results: The new patient dose reconstruction algorithm have been tested on a number of patients and the standard metrics dose deviation (DDev), distance-to-agreement (DTA) and Gamma index are improved when compared to the original algorithm. In a certain case the Gamma index (3%/3mm) increases from 72.9% to 96.6%. Conclusion: The patient dose reconstruction algorithm is improved. This leads to a reduction in non-physical artefacts in the reconstructed patient dose. As a consequence, the possibility to detect deviations in the dose that is delivered to the patient is improved. An increase in Gamma index for the PTV can be seen. The corresponding author is an employee of ScandiDos.

60 APPLIED LIFE SCIENCES↗

DOE OSTI2013

Reducing radiation dose to the female breast during CT coronary angiography: A simulation study comparing breast shielding, angular tube current modulation, reduced kV, and partial angle protocols using an unknown-location signal-detectability metric

Purpose: The authors compared the performance of five protocols intended to reduce dose to the breast during computed tomography (CT) coronary angiography scans using a model observer unknown-location signal-detectability metric.Methods: The authors simulated CT images of an anthropomorphic female thorax phantom for a 120 kV reference protocol and five “dose reduction” protocols intended to reduce dose to the breast: 120 kV partial angle (posteriorly centered), 120 kV tube-current modulated (TCM), 120 kV with shielded breasts, 80 kV, and 80 kV partial angle (posteriorly centered). Two image quality tasks were investigated: the detection and localization of 4-mm, 3.25 mg/ml and 1-mm, 6.0 mg/ml iodine contrast signals randomly located in the heart region. For each protocol, the authors plotted the signal detectability, as quantified by the area under the exponentially transformed free response characteristic curve estimator (A-caret{sub FE}), as well as noise and contrast-to-noise ratio (CNR) versus breast and lung dose. In addition, the authors quantified each protocol's dose performance as the percent difference in dose relative to the reference protocol achieved while maintaining equivalent A-caret{sub FE}.Results: For the 4-mm signal-size task, the 80 kV full scan and 80 kV partial angle protocols decreased dose to the breast (80.5% and 85.3%, respectively) and lung (80.5% and 76.7%, respectively) with A-caret{sub FE} = 0.96, but also resulted in an approximate three-fold increase in image noise. The 120 kV partial protocol reduced dose to the breast (17.6%) at the expense of increased lung dose (25.3%). The TCM algorithm decreased dose to the breast (6.0%) and lung (10.4%). Breast shielding increased breast dose (67.8%) and lung dose (103.4%). The 80 kV and 80 kV partial protocols demonstrated greater dose reductions for the 4-mm task than for the 1-mm task, and the shielded protocol showed a larger increase in dose for the 4-mm task than for the 1-mm task. In general, the CNR curves indicate a similar relative ranking of protocol performance as the corresponding A-caret{sub FE} curves, however, the CNR metric overestimated the performance of the shielded protocol for both tasks, leading to corresponding underestimates in the relative dose increases compared to those obtained when using the A-caret{sub FE} metric.Conclusions: The 80 kV and 80 kV partial angle protocols demonstrated the greatest reduction to breast and lung dose, however, the subsequent increase in image noise may be deemed clinically unacceptable. Tube output for these protocols can be adjusted to achieve a more desirable noise level with lesser breast dose savings. Breast shielding increased breast and lung dose when maintaining equivalent A-caret{sub FE}. The results demonstrated that comparisons of dose performance depend on both the image quality metric and the specific task, and that CNR may not be a reliable metric of signal detectability.

61 RADIATION PROTECTION AND DOSIMETRY↗

DOE OSTI2014

Radiochromic film calibration for low-energy seed brachytherapy dose measurement

Purpose: Radiochromic film dosimetry is typically performed for high energy photons and moderate doses characterizing external beam radiotherapy (XRT). The purpose of this study was to investigate the accuracy of previously established film calibration procedures used in XRT when applied to low-energy, seed-based brachytherapy at higher doses, and to determine necessary modifications to achieve similar accuracy in absolute dose measurements. Methods: Gafchromic EBT3 film was used to measure radiation doses upwards of 35 Gy from 75 kVp, 200 kVp, 6 MV, and (∼28 keV) I-125 photon sources. For the latter irradiations a custom phantom was built to hold a single I-125 seed. Film pieces were scanned with an Epson 10000XL flatbed scanner and the resulting 48-bit RGB TIFF images were analyzed using both FilmQA Pro software andMATLAB. Calibration curves relating dose and optical density via a rational functional form for all three color channels at each irradiation energy were determined with and without the inclusion of uncertainties in the measured optical densities and dose values. The accuracy of calibration curve variations obtained using piecewise fitting, a reduced film measurement area for I-125 irradiation, and a reduced number of dose levels was also investigated. The energy dependence of the film lot used was also analyzed by calculating normalized optical density values. Results: Slight differences were found in the resulting calibration curves for the various fitting methods used. The accuracy of the calibration curves was found to improve at low doses and worsen at high doses when including uncertainties in optical densities and doses, which may better represent the variability that could be seen in film optical density measurements. When exposing the films to doses > 8 Gy, two-segment piecewise fitting was found to be necessary to achieve similar accuracies in absolute dose measurements as when using smaller dose ranges. When reducing the film measurement area for the I-125 irradiations, the accuracy of the calibration curve was degraded due to the presence of localized film heterogeneities. No degradation in the calibration curves was found when reducing the number of calibration points down to only 4, but with piecewise fitting, 6 calibration points as well as a blank film are required. Variations due to photon energy in film optical density of up to 3% were found above doses of 2 Gy. Conclusions: A modified procedure for performing EBT3 film calibration was established for use with low-energy brachytherapy seeds and high dose exposures. The energy dependence between 6 MV and I-125 photons is significant such that film calibrations should be done with an appropriately low-energy source when performing low-energy brachytherapy dose measurements. Two-segment piecewise fitting with the inclusion of errors in measured optical density as well as dose was found to result in the most accurate calibration curves. Above doses of 1 Gy, absolute dose measurements can be made with an accuracy of 1.6% for 6 MV beams and 5.7% for I-125 seed exposures if using the I-125 source for calibration, or 2.3% if using the 75 kVp photon beam for calibration.

07 ISOTOPE AND RADIATION SOURCES↗

DOE OSTI2015

SU-E-T-430: Feasibility Study On Using Overlap Volume Histogram to Predict the Dose Difference by Respiratory Motion

Purpose: The dose difference between three-dimensional dose (3D dose) and 4D dose which considers motion due to respiratory can be varied according to geometrical relationship between planning target volume (PTV) and organ at risk (OAR). The purpose of the study is to investigate the dose difference between 3D and 4D dose using overlap volume histogram (OVH) which is an indicator that quantify geometrical relationship between a PTV and an OAR. Methods: Five liver cancer patients who previously treated stereotactic body radiotherapy (SBRT) were investigated. Four-dimensional computed tomography (4DCT) images were acquired for all patients. ITV-based treatment planning was performed. 3D dose was calculated on the end-exhale phase image as a reference phase image. 4D dose accumulation was implemented from all phase images using dose warping technique used deformable image registration (DIR) algorithm (Horn and Schunck optical flow) in DIRART. In this study OVH was used to quantify geometrical relationship between a PTV and an OAR. OVH between a PTV and a selected OAR was generated for each patient case and compared for all cases. The dose difference between 3D and 4D dose for normal organ was calculated and compared for all cases according to OVH. Results: The 3D and 4D dose difference for OAR was analyzed using dose-volume histogram (DVH). On the basis of a specific point which corresponds to 10% of OAR volume overlapped with expanded PTV, mean dose difference was 34.56% in minimum OVH distance case and 13.36% in maximum OVH distance case. As the OVH distance increased, mean dose difference between 4D and 3D dose was decreased. Conclusion: The tendency of dose difference variation was verified according to OVH. OVH is seems to be indicator that has a potential to predict the dose difference between 4D and 3D dose. This work was supported by the Radiation Technology R&D program (No. 2013M2A2A7043498) and the Mid-career Researcher Program (2014R1A2A1A10050270) through the National Research Foundation of Korea funded by the Ministry of Science, ICT&Future Planning.

60 APPLIED LIFE SCIENCES↗

DOE OSTI2015

Dose Calculation Methodology and Data for Solid Waste Performance Assessment and Composite Analysis at the Savannah River Site

This report provides a detailed description of the methodology developed to perform dose calculations for E-Area Low Level Waste Facility Performance Assessments and Savannah River Site Composite Analyses. Chapters 2 – 8 give a complete set of equations to be used to calculate dose for individual exposure pathways. The first set of dose scenarios assume that a member of the public establishes residence near the waste site and uses contaminated groundwater or contaminated surface water for personal consumption and to irrigate a garden and pasture where produce and farm animals are raised. Products from the garden and farm animals are used for personal consumption. This scenario applies to both E-Area Performance Assessments and Savannah River Site Composite Analyses. General dose exposure pathways for the resident farmer scenario are: 1. Ingestion pathways described in Chapter 2, 2. Inhalation pathways described in Chapter 3, 3. External exposure pathways described in Chapter 4, and 4. Recreational pathways described in Chapter 5 (Composite Analysis only). Doses from all of the above pathways are directly related to the contaminant concentration in the water. Additional dose scenarios considered for E-Area Performance Assessment assume that an inadvertent intruder encroaches on the waste disposal site after loss of institutional control. General dose exposure pathways considered for the inadvertent intruder are: 1. Ingestion pathways described in Chapter 6, 2. Inhalation pathways described in Chapter 7, and 3. External exposure pathways described in Chapter 8. Doses from all of the intruder pathways are directly related to the contaminant concentration in the buried waste. Special dose considerations related to human consumption of meat and dairy, and treatment of tritium in dose calculations are discussed in Chapter 9. Chapter 10 describes an Excel database named “Radionuclide, Element and Dose Parameter Data Package” that has been created to provide a system for maintaining the data required to perform radionuclide transport and dose calculations. This data includes: Isotope physical parameters and radioactive decay data needed for both transport and dose calculations, Isotope-specific dose coefficients and soil shielding factors associated with exposure pathways, Radionuclide drinking water concentration limits either published by the Environmental Protection Agency or derived from internal dose coefficients to meet the groundwater protection requirements in Department of Energy Order 435.1, Element-specific bio-transfer factors used in dose calculations, Dose equations and values of physical parameters and human factors used in dose calculations, and Key physical constants used in dose calculations.

54 ENVIRONMENTAL SCIENCES↗

DOE OSTI2019

Dose Calculation Methodology and Data for Solid Waste Performance Assessment and Composite Analysis at the Savannah River Site

54 ENVIRONMENTAL SCIENCES↗

DOE OSTI2021

Dose Rate Analysis of the WCS Consolidated Interim Storage Facility

This report describes a confirmatory calculation for a proposed consolidated interim storage facility (CISF) to support the US Nuclear Regulatory Commission’s review of the license application submitted by the Interim Storage Partners. The scope of this report is limited to an assessment of the annual dose from direct radiation associated with Phase 1 of the Waste Control Specialists (WCS) CISF and the determination of the minimum controlled area boundary based on the 10 CFR 72.104 limit of 25 mrem annual dose to the whole body. The confirmatory dose rate calculations used the source term and shielding calculation capabilities of the SCALE 6.2.3 computer code system. The calculation method employed in the confirmatory calculations uses a detailed Monte Carlo radiation transport simulation from source to dose rate. This method differs from the two-step method used in the safety analysis report (SAR), which requires determination of the photon and neutron energy and angular distributions on the cask external surface and then the use of this surface source in a new radiation transport calculation to determine dose rate as a function of distance from the storage facility. A simulation was made using the complete site geometry (all casks present) but with only one cask containing source, and a total of 467 independent calculations were performed to obtain the dose rate maps produced by each storage cask. Basic input data and assumptions used in the SAR (e.g., cask design parameters and design basis assembly characteristics) were used in this confirmatory calculation for consistency with the SAR. Local atmospheric conditions for skyshine calculations were simulated using US standard atmosphere data, which describe average air temperature, pressure, and density as a function of altitude. The determined minimum required distances from the WCS Phase 1 CISF to a contour defined by the 25 mrem annual dose limit were approximately 623 m (0.387 miles) in the N-NE direction, 594 m (0.369 miles) in the SSW direction, and 533 m (0.331 miles) in the E-SE direction and the W-NW direction. The annual dose results of the confirmatory calculation are in relatively good agreement with the annual dose values presented in the WCS SAR. The annual dose produced by the Phase 1 CISF at the nearest site boundary, 0.75 miles away from the CISF, was estimated by this study to be approximately 6.02E-02 ± 2.0E-04 mrem, which is significantly smaller than the 25 mrem annual dose limit provided in 10 CFR 72.104. The SAR annual dose rate value for the nearest site boundary was 7.52E-2 mrem, which included 7.73E-03 mrem due to postulated leakage of the FO-, FC-, and FF-canisters. A comparison with the dose rate values reported in the SAR for 16 detector locations, D1 through D16, on the protected area boundary is also included in this report. This comparison shows that the SAR determined higher annual dose values for detectors D1 through D14 by 12% to 45% and lower annual dose values for detectors D15 and D16 by 18% and 8%, respectively, as compared with this ORNL confirmatory calculation. Sensitivity studies have been performed to support a comprehensive review of the application. In these studies, the effects on dose rate of the modeled air volume, density, and humidity were analyzed. The sensitivity calculations showed that the air volume used in the Monte Carlo simulations was adequate and that a larger air volume would not further increase the dose rate due to skyshine contributions. The sensitivity analysis showed that simulation of local atmospheric conditions is important for the accuracy of dose rate estimates at large distances from the facility. Air humidity was demonstrated to have negligible effects on dose rate values for the simulated local atmospheric conditions.

61 RADIATION PROTECTION AND DOSIMETRY↗

DOE OSTI2014

SU-E-J-63: Estimating the Effects of Respiratory Motion On Dose Heterogeneity for ITV-Based Treatments

Purpose: To quantify the relationship between the amount of dose heterogeneity in a treatment plan that uses an internal target volume (ITV) to account for respiratory motion and the true amount of heterogeneity in the dose delivered to the tumor contained within that ITV. Methods: We develop a convolution-based framework for calculating dose delivered to a tumor moving inside an ITV according to a common sinusoid-based breathing model including asymmetry. We model the planned ITV dose distribution as a centrally peaked analytic function approximating the profile of clinical stereotactic body radiotherapy treatments. Expressions for the minimum and maximum dose received by the tumor are derived and evaluated for a range of clinically relevant parameters. Results of the model are validated with phantom measurements using an ion chamber array. Results: An analytic expression is presented for the maximum and minimum doses received by the tumor relative to the planned ITV dose. The tumor dose heterogeneity depends solely on the ratio of tumor size to ITV size, the peak dose in the planned ITV dose distribution, and the respiratory asymmetry parameter. Under the assumptions of this model, using a typical breathing asymmetry parameter and a dose distribution with a fixed size ITV covered by the 100% line and with a 130% hotspot, the maximum dose to the tumor varies between 113%–130%, and the minimum dose varies between 100%–116% depending on the amount of tumor motion. Conclusion: This modeling exercise demonstrates the interplay between motion and dose heterogeneity. Tumors that exhibit large amounts of respiratory motion relative to their size will receive a more homogeneous dose and a larger minimum dose than would be inferred from the ITV dose distribution. This effect is not captured in current clinical treatment planning methods unless 4D dose calculation techniques are used. This work was partially supported by a Varian Medical Systems research grant.

60 APPLIED LIFE SCIENCES↗