Matthew Mille

An analysis of dependency of counting efficiency on worker anatomy for in vivo measurements: whole-body counting

In vivo radiobioassay is integral to many health physics and radiological protection programs dealing with internal exposures. The Bottle Manikin Absorber (BOMAB) physical phantom has been widely used for whole-body counting calibrations. However, the shape of BOMAB phantoms-a collection of plastic, cylindrical shells which contain no bones or internal organs-does not represent realistic human anatomy. Furthermore, workers who come in contact with radioactive materials have rather different body shape and size. To date, there is a lack of understanding about how the counting efficiency would change when the calibrated counter is applied to a worker with complicated internal organs or tissues. This paper presents a study on various in vivo counting efficiencies obtained from Monte Carlo simulations of two BOMAB phantoms and three tomographic image-based models (VIP-Man, NORMAN and CNMAN) for a scenario involving homogeneous whole-body radioactivity contamination. The results reveal that a phantoms counting efficiency is strongly dependent on the shape and size of a phantom. Contrary to what was expected, it was found that only small differences in efficiency were observed when the density and material composition of all internal organs and tissues of the tomographic phantoms were changed to water. The results of this study indicate that BOMAB phantoms with appropriately adjusted size and shape can be sufficient for whole-body counting calibrations when the internal contamination is homogeneous.

Development of a calibration methodology for large-volume, solid 68Ge phantoms for traceable measurements in positron emission tomography

We have developed a methodology to calibrate the (68)Ge activity concentration in large (9L) cylindrical epoxy phantoms in a way that is traceable to national standards. The method was tested on two prototype cylindrical phantoms that are being used in a clinical trial and gave (68)Ge activity concentration values with combined standard uncertainties of about 1.1%. Imaging data from the phantoms using a calibrated PET-CT scanner gave values consistent with the calibrated activity concentrations within experimental uncertainties.

Calibration of Traceable Solid Mock (131)I Phantoms Used in an International SPECT Image Quantification Comparison.

The International Atomic Energy Agency (IAEA) has organized an international comparison to assess Single Photon Emission Computed Tomography (SPECT) image quantification capabilities in 12 countries. Iodine-131 was chosen as the radionuclide for the comparison because of its wide use around the world, but for logistical reasons solid 133Ba sources were used as a long-lived surrogate for 131I. For this study, we designed a set of solid cylindrical sources so that each site could have a set of phantoms (having nominal volumes of 2 mL, 4 mL, 6 mL, and 23 mL) with traceable activity calibrations so that the results could be properly compared. We also developed a technique using two different detection methods for individually calibrating the sources for 133Ba activity based on a National standard. This methodology allows for the activity calibration of each 133Ba source with a standard uncertainty on the activity of 1.4 % for the high-level 2-, 4-, and 6-mL sources and 1.7 % for the lower-level 23 mL cylinders. This level of uncertainty allows for these sources to be used for the intended comparison exercise, as well as in other SPECT image quantification studies.

TH‐A‐214‐04: Impact of Body Size of Obese Patients on PET/CT Dose Estimates: Monte Carlo Calculations Using a Set of BMI‐Adjustable Phantoms

Purpose: To assess the effect of obese patients body mass index (BMI) on organ doses from CT and PET exams. Methods: A set of 5 computational phantoms with different BMIs were created for PET and CT dose calculations using the Monte Carlo N‐Particle Extended (MCNPX) code. The phantoms represent patients having a BMI ranging from 23 (normal weight) to 44 (morbidly obese) with increasing amounts of subcutaneous and visceral fat. The external CT dose calculations assumed a whole‐body, diagnostic scan with a GE LightSpeed 16 scanner operating at a tube voltage of 120 kVp. The internal PET dose was based on S‐values for F‐18 and the ICRP‐106 FDG biokinetic model. Results: For the same tube current, ratios of the morbidly obese phantom CTorgan doses compared to the normal weight phantom ranged from 0.46∼1 with an average of 0.7 because of shielding by the extra fat. This effect was greatest for deep organs in the abdomen (e.g. colon, lungs, stomach, liver, urinary bladder). For the same injected radioactivity, the average PETorgan dose ratios were 0.95 and 0.65 for source organs (e.g. brain, heart, liver, urinary bladder) and organs in the remainder, respectively (e.g. colon, stomach, spleen, etc.). These PET dose differences are due primarily to the obese phantoms smaller remainder source S‐values which arise from the addition of the extra fat to the remainder compartment. Conclusions: After adjusting for the increased tube current and tube voltage, the results suggest that obese patients can receive higher absorbed dose to some organs, depending on the protocols. This work, as part of the VirtualDose software development, demonstrates the ability to improve the estimate of organ doses from PET and CT scans for obese patients and will be useful to on‐going efforts to optimize image quality and imaging doses.

SU‐GG‐I‐68: Calculation and Evaluation of Internal and External Radiation Exposure to Adult and Pediatric Patients from PET/CT Examinations

Purpose: To assess organ and effective dose for patients undergoing whole‐body F‐18 FDG PET/CT examinations using available software. Materials and Methods: The OLINDA/EXM 1.1 code was used in conjunction with an F‐18 FDG biokinetic model to assess PET dose for adult male/female and pediatric patients of various ages. For the PET emission scan it was assumed that 555 MBq of F‐18 was administered to adults and that the activity for pediatric patients ranged from 18.5 to 370 MBq according to weight. The dose from the CT portion of the exam was assessed using the recently developed VirtualDose dose‐reporting software. The calculations assume a GE LightSpeed 16 scanner was used to perform a CTattenuation correction scan at 140 kVp and 25 mAs and a diagnoisic CT at 140 kVp and 200 mAs. Results: The effective doses for adult male and female patients undergoing PET/CT procedures were estimated to be ∼30 mSv and ∼40 mSv respectively with the CT portion contributing two‐thirds of the overall dose. A disadvantage of the OLINDA/EXM 1.1 code is that it utilizes stylistic phantoms which are not anatomically realistic and does not use the recently adopted ICRP 103 tissue weighting factors. VirtualDose does not have these shortcomings, but a potential weakness is that the current version does not calculate CT dose for pediatric phantoms. If F‐18 was administered by weight at 8 MBq/kg, the effective dose from the PET portion of the scan for the 1‐, 5‐, 10‐, and 15‐year‐old phantoms was 6.9, 8.3, 9.5, and 10.8 mSv respectively. Conclusions: This work has identified that there is a need for a single software package for assessing PET/CT dose which combines the strengths of OLINDA/EXM 1.1 and VirtualDose and utilizes state‐of‐the art voxel phantoms of various ages/sizes as well as up‐to‐date ICRP effective dose schemes.

TH‐C‐201B‐04: Methods to Account for Imaging Doses from Diagnostic MDCT or Kilovoltage CBCT in Prostate Treatment Planning: Monte Carlo Studies Using Scanner Models and Patient‐Specific Anatomy

Purpose: MDCT and kilovoltage CBCT are increasingly used in IGRT. AAPM TG‐75 has made a series of recommendations on imagingdoses. Recent studies have focused on an inclusion of imagingdoses using a radiation treatment planning (RTP) system. This paper describes the use of Monte Carlo methods to calculate imagingdose for a prostate IGRTtreatment case. Methods: An IMRT treatment plan per RTOG 0126 for a prostate carcinoma was used involving 28 initial fractions and a boost in 16 fractions. Constraints to rectum bladder and femoral heads were enforced. A total of 40 imaging procedures were considered involving a MDCT or a KVCBCT scanner that is operated at 250 mAs. A GE LightSpeed 16‐MDCT scanner and a Varian On‐Board Imager (OBI) were modeled with parameters reported in the literature. Planning CT images were used to construct a patient phantom within the MCNPX simulation environment. OARs and background voxels were categorixed into six tissue types. Results: For a total of 40 scans rectum received 69.7 cGy and 68.2 cGy from MDCT and KVCBCT respectively. The bladder received slightly greater doses 73.3 cGy and 69.3 cGy while the femoral heads received much higher doses 161.9 cGy and 125.7 cGy respectively. To investigate the impact MDCT imagingdoses are added to those from the original treatment plans and dose‐volume histograms evaluated. Among notable findings: the rectum dose after adding the imagingdoses may increase to or above the maximum acceptable level. Conclusion:Imagingdoses can reach the level that may require the adjustment of original planning in order to still satisfy the constraints. RTP systems may not be suitable for low‐energy X‐rays especially for bones and lungs. The data are expected to be useful to the newly formed AAPM TG‐180.

SU-GG-BRC-04: Electronic Versus HDR Ir-192 Brachytherapy: Organ Dose Comparisons for Breast Cancer Using a Monte Carlo Patient Phantom

Purpose: To quantify and compare the dose delivered to multiple organs‐at‐risk (OARs) in a female patient undergoing Xoft Axxent electronic (KVB) and high‐dose rate Ir‐192 (IBB) intracavitary balloon brachytherapy for breast cancer.Materials and Methods: A previous study has indicated that the dose to OARs such as the lungs and heart play a critical role in treatment planning. The anatomy of a female patient was represented by an adult female computational phantom which consists of over 140 organs. A balloon was inserted into a lumpectomy cavity in the left breast of the virtual patient. The Monte Carlo N‐Particle eXtended (MCNPX) code was used to simulate photon transport through the patient for hypothetical KVB and IBB scenarios. MCNPXs F6 tally was used to calculate the absorbed dose in organs distant from the treatment site. Results: In general, the KVB organdoses were more than a factor of 2 smaller than those of IBB because the low‐energy x‐rays are less penetrating. The distribution of organdoses shows a profound pattern depending on the distance, location, and organ shape. The largest doses were observed for organs such as the left lung and heart which are closest to the radiation source. For KVB, the doses received by the left lung and heart wall were 9.0% and 5.5% of that received by the planning target volume. These values were 11.0% and 11.3% for the IBB scenario. Conclusions: This paper reports, for the first time, a systematic comparison of multiple organdoses received from KVB and IBB. KVB may have safety advantages because its dose rate falls off faster than for IBB. As a previous clinical study found the target dose to be similar for these two methods, information on how healthy organs are irradiated will help decide when each modality is appropriate.

SU‐GG‐T‐239: Monte‐Carlo “Virtual” Calibration of Counting Efficiency for Assessing Patient Internal Radioactivity Burden Using Organ‐Adjustable RPI Adult Male Phantom

Purpose: To give better representation of a patient anatomy, the BREP methods was used to design virtual patients whose organ sizes and locations can be adjusted for virtual calibrations.Method and Materials: From the RPI Adult Male mesh model which has over 140 organs or tissues, a voxelized phantom containing over 22 million voxels of size of 3.2mm × 3.2mm × 3.2mm was created. With a Monet Carlo simulation interface, the mesh model was adjusted to match the patient whose body shape, weight and height were known. Geometrical descriptions of the RPI Adult Male phantom and a HPGe detector were ported into the MCNPX code to simulate typical whole‐body and lung counting scenarios. The counting efficiencies of the RPI Adult Male were calculated and compared to those of the NORMAN, BOMAB and LLNL. Results: The RPI Adult Males whole‐body counting efficiencies differed from NORMANs and BOMABs efficiencies by 1% to 8% over energies from 105 to 1460 keV. A comparison of the lung counting efficiencies of the RPI Adult Male and NORMAN found differences of 341% at 17.751 keV, 66% at 26.345 keV, 20% at 45.6 keV, 13% at 59.541 keV, and 2% ∼ 9% at energies from 80.997 to 1332 keV. The difference between the RPI Adult Males lung counting efficiency and LLNLs efficiency is much greater, from 24% at 1332 keV to 2290% at 17.751 keV. Conclusion: The whole‐body counting efficiencies of the RPI Adult Male are in good agreement with those of NORMAN and BOMAB. This suggests that the BOMAB may be acceptable to simulate workers with similar size, weight and height. Notably different lung counting efficiencies were observed of the RPI Adult Male, NORMAN and LLNL. For more accurate internal dosimetry, the deformable RPI phantoms will be very useful, especially when aided with partial‐body patient‐specific parameters.