J Gu

Assessment of patient organ doses and effective doses using the VIP-Man adult male phantom for selected cone-beam CT imaging procedures during image guided radiation therapy

A Monte Carlo based computational procedure for determining organ doses and effective doses has been described for two procedures used in newly developed image-guided radiation treatment: kilovoltage cone-beam computed tomography (kV CBCT) and mega-voltage computed tomography (MV CBCT). A whole-body patient computational phantom, VIP-Man phantom, is employed for Monte Carlo dose calculations. Results indicate that the thyroid dose is always the highest in head and neck (H&N) scan for both kV and MV CBCT, and the bladder dose is the highest in prostate scan for both kV and MV CBCT. For the VIP-Man phantom, it has been found that the effective dose for kV CBCT (assuming a total exposure of 1350 mAs) is approximately 9.5 mSv for the two anatomical sites, whereas the effective dose for MV CBCT (assuming a total of 6 monitor unit) ranges from 5.10 mSv for the H&N case to 8.39 mSv for the prostate scan. The estimated whole-body effective doses allow different imaging procedures to be compared and evaluated.

Comparison of two types of adult phantoms in terms of organ doses from diagnostic CT procedures

The rapidly increasing number of diagnostic computed tomography (CT) procedures in the recent decades has spurred heightened concern over the potential risk to patients. Although an accurate organ dose assessment tool has now become highly desirable, existing software packages depend on stylized computational phantoms that were originally developed more than 40 years ago, exhibiting very large discrepancies when compared with phantoms that are anatomically realistic. However, past comparative studies did not focus on CT protocols for adult patients. This study was designed to quantitatively compare two types of phantoms, the stylized phantoms and a pair of recently developed RPI-adult male and adult female (RPI-AM and RPI-AF) phantoms, for various CT scanning protocols involving the chest, abdomen-pelvis and chest-abdomen-pelvis. Organ doses were based on Monte Carlo simulations using the MCNPX code and a detailed CT scanner model for the GE LightSpeed 16. Results are presented as ratios of organ doses from the stylized phantoms to those from the RPI phantoms. It is found that, for most organs contained in the scan volume, the ratios were within the range of 0.75-1.16. However, the stomach doses are significantly different and the ratio is found to be up to 1.86 in male phantoms and 2.29 in the female phantoms due to the anatomical differences between the two types of phantoms. Organs that lie near a scan boundary also exhibit a significant relative difference in organ doses between the two types of phantoms. This study concludes that, due to relatively low x-ray energies, CT doses are very sensitive to organ shape, size and position, and thus anatomically realistic phantoms should be used to avoid the dose uncertainties caused by the lack of anatomical realism. The new phantoms, such as the RPI-AM and AF phantoms that are designed using advanced surface meshes, are deformable and will make it possible to match the anatomy of a specific patient leading to further improvement in dose and risk assessments for patients undergoing CT examinations.

Fetal doses to pregnant patients from CT with tube current modulation calculated using Monte Carlo simulations and realistic phantoms

To investigate the radiation dose to the fetus using retrospective tube current modulation (TCM) data selected from archived clinical records. This paper describes the calculation of fetal doses using retrospective TCM data and Monte Carlo (MC) simulations. Three TCM schemes were adopted for use with three pregnant patient phantoms. MC simulations were used to model CT scanners, TCM schemes and pregnant patients. Comparisons between organ doses from TCM schemes and those from non-TCM schemes show that these three TCM schemes reduced fetal doses by 14, 18 and 25 %, respectively. These organ doses were also compared with those from ImPACT calculation. It is found that the difference between the calculated fetal dose and the ImPACT reported dose is as high as 46 %. This work demonstrates methods to study organ doses from various TCM protocols and potential ways to improve the accuracy of CT dose calculation for pregnant patients.

TH‐C‐201B‐10: Development and Testing of a CT Dose Software “VirtualDose” Using Anatomically Realistic Patient Phantoms: Preliminary Results for the Phase I of the Project

Purpose: To demonstrate the need and feasibility for developing a new software for reporting patient imaging dose who undergoing CT or PET/CT examinations. Method and Materials: Existing CT dose reporting software do not meet the need because of the simplified anatomical phantoms updated ICRP data and scanner information. A new software is being designed with original dose data derived from Monte Carlo simulations involving CTscannermodels and anatomically realistic phantoms. Specified scanning protocols and CT sources are modeled. Dosimetry capabilities for tube current modulation (TCM) and PET/CT protocols are currently under development. The RPI Pregnant Women series RPI Adult Male and Adult Female phantoms are used in the dose calculation. Organ doses and effective doses are computed using ICRP Publication 60 and 103. The software framework is developed using the Visual C#.NET. Results: VirtualDose offers a modern graphical user interface (GUI) designed to allow interactive 3D phantom display and user‐selectable scanning parameters. Standard scanning ranges can be selected from a pull‐down menu or manually specified on the displayed phantom. When compared with data reported by existing software using stylized MIRD‐type phantoms the organ dose estimates have been found to differ by a ratio ranging from 0.77 to 1.24 for organs or tissues covered in the scan range and a ratio as small as 0.13 for organs outside of the scan region. The TCM technique can reduce the dose by around 20% for pregnant patient phantoms. Conclusion: It is clear that existing software do not meet the need for accurate and state‐of‐the‐art CT dose reporting. The preliminary GUI design and reporting features of VirtualDose improve upon existing tools by considering the latest CTscanners new ICRP recommendations and anatomically realistic patient phantoms. VirtualDose is expected to improve both the accuracy and usability in reporting CT doses in the future.

SU‐EE‐A4‐06: Organ‐Specific Adjustment Factors for Calculating Dose from Any CT Scanner

Purpose: To demonstrate a method to estimate organ doses from CT examination of from multiple CT machine models. Method and Materials: A validated computational model of particular CT machine has been constructed and used to calculate organ doses from CT examinations. The dose distribution in space is derived by fitting a parabolic function to the CTDI‐center and CTDI‐peripheral values of each machine, which provides a dose ratio as a function of distance from isocenter. The distribution of organ location is also calculated in relation to the isocenter position. The dose to the organ is then adjusted based upon an average of the dose ratio weighted by the organ location distribution. This method is demonstrated using the RPI‐Adult Male and RPI‐Adult Female mesh‐based human phantoms but can be applied equally to any patient phantom.Results: If the centerline dose is held constant, the dose to the organs furthest from center of the body will vary the most. For example, using the RPI‐Adult Female phantom and a CT machine with the same CTDI‐center as the validated model, but a flatter dose distribution (lower CDTI‐peripheral), the resultant dose to the breast tissue is 36% lower, the dose to the lung is 20% lower, but the dose to the trachea is less than 1% lower. Conclusion: The ability to estimate organ doses from CT machines other than a fully validated model obviates the need to perform extensive Monte Carlo simulations for every new CT machine that enters the market. One or a few sets of complete Monte Carlo simulations can instead be used to estimate the dose from different CT machines. In conjunction with modern dose reporting software and a family of human phantoms, the dose from a virtually limitless combination of CT machines and CT scan protocols can be determined.

TH-C-201B-12: CT Dose Reduction with Tube Current Modulation: A Case Study of Fetal Doses to Pregnant Patients Using Monte Carlo Computational Phantoms

Purpose: To demonstrate CT fetal dose reduction with the use of tube current modulation (TCM) on pregnant patients using retrospective data. Method and Materials:Monte Carlo(MC) simulation techniques are used to model CT scanners the TCM schemes and the pregnant patients. Two MDCT scanners GE LightSpeed Pro 16 and GE LightSpeed™ 16 were modeled in MCNPX source code. The TCM data were selected from archived clinical records for pregnant patients who underwent CT procedures recently at Massachusetts General Hospital (MGH) in Boston. Three pregnant patient phantoms with different gestations were utilized. Results: According to the calculation results it is found that the fetal dose from TCM is always smaller than the fetal dose from non‐TCM. TCM schema can reduce fetal dose from 14% to 25%. Also fetal dose gets increased when fetus becomes larger. It is also found that results of ImPACT calculations are always larger than MC calculations performed in this research. The difference between the two sets of data can reach around 40%–50%. Conclusion: This work demonstrates that Monte Carlo method can be used to assess the organ doses and fetal doses of pregnant patients undergoing TCM CT examinations by modeling the CT scanner and TCM schema as well as patient phantoms. The comparison between TCM organ doses and non‐TCM organ doses indicates that the modulated current can effectively help reduce organ dose from CT scans from 14% to 25% according to the specific TCM schema.

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-EE-A4-02: An Iterative Method of Modeling Multidetector CT (MDCT) Source From Measured CTDI Values: A Feasibility Study

Purpose: To propose a method of using the Monet Carlo (MC) technique to model MDCT scanners from CTDI values so the assessment of organ doses can be performed more accurately and easily. Method and Materials: The MC code, MCNPX, was used to perform all the simulations. Several parameters influencing CTDI values were analyzed and prioritized. The modeling method starts with employing the preliminary parameters necessary to perform the single axial scan to obtain the calculated CTDI values. Along with a priority list, each parameter was adjusted by comparing MC calculated CTDI values with measured CTDI values. The iterative process is completed when the parameters yield results that match the calculated and measured CTDI values. The validated CT model was then integrated with patient phantoms to calculate the organ doses for a specific CT procedure. Results: It was found that, in the modeling procedure, there are actually only two main parameters that exhibit the greatest influence on the finally calculated CTDI values: the thickness of the bowtie filter (BTF) and the length of BTF semimajor axis. These two parameters were thus given the highest priority. The modeling algorithm involves a total of six parameters including anode angle, thickness of flat filter, width of BTF, thickness of BTF, length of BTF semimajor axis, and source number. Based on this method a MDCT was modeled and the calculated CTDI values within around 5% of the measured CTDI values. Conclusion: Results demonstrated the feasibility of this iterative method based on analysis and priorities of various parameters. To our knowledge, this work is among the first attempts to develop the modeling method only using CTDI values. The CT scanners therefore can be modeled without the common problem of lacking information of parameters, thus facilitating accurate and easy MDCT dose assessment.

TU‐C‐304A‐04: Monte Carlo Based Multidetector CT Modeling and Dose Calculations for Pregnant Patients

Purpose: To model and validate the multidector CT (MDCT) scanner and to assess radiation dose to the fetus and pregnant patient in three different gestational periods. Method and Materials:Monte Carlo code, MCNPX, was used to simulate the x‐ray source including the energy spectrum, filter, and scan trajectory. Detailed CTscanner components were specified using an iterative trial‐and‐error procedure for a GE LightSpeed CTscanner. The scannermodel was validated by comparing simulated results against measured CTDI values and dose profiles reported in the literature. The source movement along the helical trajectory was simulated using the pitch of 0.9375 and 1.375, respectively. The validated scannermodel was then integrated with phantoms of a pregnant patient in three different gestational periods to calculate organ doses and fetal doses. Results: Comparison between simulated results and reported results in literature shows good agreement in terms of CTDI values as well as dose profiles. It was found that the dose to the fetus of the 3‐month pregnant patient phantom was 0.13 mGy/100mAs and 0.57 mGy/100mAs from the chest and kidney scan, respectively. For the chest scan of the 6‐month patient phantom and the 9‐month patient phantom, the fetal doses were 0.21 mGy/100mAs and 0.26 mGy/100mAs, respectively. All these scans were performed with protocols that did not contain the fetus directly in the x‐ray beam. The paper also discusses how these fetal dose values can be used to evaluate imaging procedures and to assess risk using recommendations of the report from AAPM Task Group 36. Conclusion: This work demonstrates the ability of modeling and validating MDCT scanner by Monte Carlo method, as well as rapidly and accurately assessing fetal dose and organ doses by combining the MDCT scannermodel and pregnant patient phantom.