Imagination at work. Imagination at work.
March 2015 William J. O’Connel, Dr. Ph, Senior Medical Physicist
State of the art and future development for standardized estimation of organ doses in CT
Agenda
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• Introduction
• Duke
• Florida
• UCLA / MD Anderson
• RPI
• AAPM TG 246
• Conclusion
Introduction
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Introduction
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What is the radiation dose?
Is the procedure safe? (risk)
Multi-faceted calculation requiring knowledge of energy deposited in defined mass (organ / tissue)
Growing interest in role of organ dose as descriptor of risk
National Radiological Protection Board
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• Monte Carlo simulations of calculated x-ray spectra in an adult, hermaphrodite, mathematical model (MIRD)
• 75 scanners (out of ≈ 200) operating in the UK at the time.
• 23 data sets produced for surveyed scanner models
• Original data from contiguous axial scans
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National Radiological Protection Board
ImPACT
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Estimating patient dose on current CT scanners: Results of the ImPACT* CT dose survey
M.A. Lewis, S. Edyvean, S.A. Sassi, H. Kiremidjian, N. Keat and A.J. Britten. ImPACT, Medical Physics, St. George's Hospital, London
Introduction
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• Estimated effective doses are not patient specific • DLP / k-factor method (k is not scanner specific) • Effective Dose in obese patients is problematic
Overview
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Introduction
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Limitations to existing patient dose metrics
CTDIVOL is a useful benchmarking tool but is not ideal indicator of organ dose and radiation risk
• Broader beam widths
• Variable Pitch
• Bow-tie filters
• Non-contiguous slices
Introduction
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Researchers are looking at many aspects of organ dose estimation
• Obese Patients
• Validated Monte Carlo modeling
• Computational Phantoms
• Tube Current Modulation
Roadmap to Organ Dose in computed tomography
Florida
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Florida – WE Bolch – AAPM Imaging Symposium – July 2014
Florida – WE Bolch – AAPM Imaging Symposium – July 2014
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Stylized (Mathematical) Phantom
Anatomically unrealistic
ORNL stylized adult phantom
Flexible
Errors
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Voxel (Tomographic) Phantom
Not flexible
Anatomically realistic
Florida – WE Bolch – AAPM Imaging Symposium – July 2014
Constructed from patient acquisitions
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Hybrid Phantom
Nurbs (non-uniform rational B-spline)
Florida – WE Bolch – AAPM Imaging Symposium – July 2014
Flexible
Realistic
mathematical model used for generating / representing curves and surfaces.
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Hierarchy of phantom morphometric categories
Reference 50th percentile individual, patient matching by age only
Patient-Dependent patient match: nearest height and weight
Patient-Sculpted patient match: height, weight, body contour
Patient-Specific patient match: individual patient morphology
Florida – WE Bolch – AAPM Imaging Symposium – July 2014
Florida - Long et al, Med. Phys. 40 (1), January 2013
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Benchmark Monte Carlo simulations against anthropomorphic phantoms SOMATOM Sensation 16
multidetector CT scanner Multiple axial and helical acquisitions
UF computational adult male
reference hybrid phantom
Florida - Long et al, Med. Phys. 40 (1), January 2013
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Monte Carlo radiation transport code, MCNPX version 2.6. SPEC78 spectrum generation program Bow-tie filter and over-ranging
fiber-optic coupled plastic scintillator dosimetry (PSD) system UF Series-B 9-month-old
voxel phantom
Florida - Long et al, Med. Phys. 40 (1), January 2013
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Florida - Long et al, Med. Phys. 40 (1), January 2013
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Florida - Long et al, Med. Phys. 40 (1), January 2013
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Florida - Long et al, Med. Phys. 40 (1), January 2013
On average, organ doses from the Monte Carlo simulations agreed with physically measured doses within 8%-9% for axial and helical imaging of the reference adult phantom
Agreement is within 6%-7% for the 9-month old child
Individual organ doses were found to be within 15% of measurements of organ dose for both phantoms
Duke
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Duke – Zhang et al, Med. Phys. 39 (6), June 2012
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• How are dose results affected by choice of computational anthropomorphic phantom?
• What uncertainties exist in the estimation of dose with different types of phantoms?
Duke – Zhang et al, Med. Phys. 39 (6), June 2012
• Organ doses, effective doses, risk indices, and conversion coefficients to effective dose and risk index were estimated
for ten body and three neurological examination categories
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1. Male and Female Extended Cardiac-Torso (XCAT)
2. ICRP No. 110 reference male and female phantoms
3. Impact Group phantoms
4. CT-Expo
Duke – Zhang et al, Med. Phys. 39 (6), June 2012
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Duke – Zhang et al, Med. Phys. 39 (6), June 2012
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Duke – Zhang et al, Med. Phys. 39 (6), June 2012
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XCAT Hybrid Phantoms
Visible Human anatomical data – National Library of Medicine
NURBS based phantoms modified to match ICRP 89 reference values
Duke – Zhang et al, Med. Phys. 39 (6), June 2012
Brains modeled separately
on MRI models
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ICRP 110 Voxelized Phantoms
Tomographic data of individuals whose body height matched reference values in ICRP Publication 89 Radiosensitive organs were directly segmented from tomographic data
Duke – Zhang et al, Med. Phys. 39 (6), June 2012
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ImPACT Phantoms
Stylized mathematical phantom
208 contiguous 5 cm slabs extending from upper legs to head
NRPB-R186
Duke – Zhang et al, Med. Phys. 39 (6), June 2012
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CT-Expo Phantoms
Stylized mathematical phantom
Design characteristics of MIRD-5 phantom
ADAM and EVA – GSF – ICRP Publication 23
Duke – Zhang et al, Med. Phys. 39 (6), June 2012
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Duke – Zhang et al, Med. Phys. 39 (6), June 2012
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Duke – Zhang et al, Med. Phys. 39 (6), June 2012
Duke - Tian et al, Radiology 270 (2), February 2013
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Validated Monte Carlo simulations performed on 42 pediatric patient models (normals)
Organ dose estimates for routine chest and abdominopelvic examinations
Feasible to estimate patient-specific organ dose with knowledge of patient size and CTDIVOL
Duke - Tian et al, Radiology 270 (2), February 2013
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Duke - Tian et al, Radiology 270 (2), February 2013
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Multi scanner study – Lightspeed VCT and SOMATOM Definition Flash CTDIVOL is used as an index of scanner radiation output
Calculate CTDIVOL conversion factor (hO, S, P ) – specific to each organ, scanner and patient model CTDIVOL determined with 100 mm chamber and 16-cm phantom CTDIVOL conversion factor showed exponential relationship with average patient diameter
Duke - Tian et al, Radiology 270 (2), February 2013
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Duke - Tian et al, Radiology 270 (2), February 2013
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Duke - Tian et al, Radiology 270 (2), February 2013
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Duke - Tian et al, Radiology 270 (2), February 2013
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Duke - Tian et al, Radiology 270 (2), February 2013
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Duke - Tian et al, Radiology 270 (2), February 2013
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Duke - Tian et al, Radiology 270 (2), February 2013
UCLA / MD Anderson
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UCLA - Khatonabadi et al, Med. Phys. 39 (8), August 2012
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• Most methods estimating patient dose from computed tomography are based on fixed tube current scans
• A growing number of CT scans are performed with tube current modulation (TCM)
• Detailed TCM data is difficult to obtain
• What is accuracy of organ dose estimates obtained using methods that approximate detailed TCM function?
UCLA - Khatonabadi et al, Med. Phys. 39 (8), August 2012
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MCNPX (Monte Carlo N-Particle eXtended v2.6.0)
Two MDCT scanners: Sensation 64 and LightSpeed 16
Twenty adult female chest voxelized models Twenty pediatric female models (whole body)
UCLA - Khatonabadi et al, Med. Phys. 39 (8), August 2012
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UCLA - Khatonabadi et al, Med. Phys. 39 (8), August 2012
For each patient model, detailed TCM function was extracted from the raw projection data Over-ranging region can be determined from start and end locations of the image data and locations of x-ray beam on and x-ray beam off
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UCLA - Khatonabadi et al, Med. Phys. 39 (8), August 2012
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UCLA - Khatonabadi et al, Med. Phys. 39 (8), August 2012
Longitudinal approximated TCM function obtained from the image data is reasonable surrogate to detailed TCM function for use in Monte Carlo dose simulations.
Longitudinal approximated TCM function only represents the z-axis modulation of the TCM algorithm and it does not
capture the over-ranging information that the detailed TCM function
Results suggest angular modulation has a stronger effect on
smaller peripheral organs (breasts) compared to larger and more central organs (lungs).
RPI
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RPI - Ding et al, Phys. Med. Biol. 57 (9), May 2012
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Study the effect of obesity on the calculated radiation dose to organs and tissues
Developed BMI-adjustable phantoms ( Range 23.5 – 46.4)
Subcutaneous adipose tissue (SAT)
Visceral adipose tissue (VAT)
RPI - Ding et al, Phys. Med. Biol. 57 (9), May 2012
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SAT layer is added to phantom in the space between the body surface and internal organs Thickness of adipose tissue is only considered for attenuation properties
Dose to adipose tissue is not estimated
RPI - Ding et al, Phys. Med. Biol. 57 (9), May 2012
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No data to estimate the
effect of VAT on internal organ placement / deformity Internal organ size and VAT volume held constant for all BMI
settings VAT density is corrected for obesity with waist circumference (WC)
RPI - Ding et al, Phys. Med. Biol. 57 (9), May 2012
RPI – 3D - BMI Adjustable Phantoms
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Data validated against anthropomorphic data from literature (Ogden 2004)
AP and LAT measurements performed at mid-chest and mid-abdomen
RPI - Ding et al, Phys. Med. Biol. 57 (9), May 2012
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Good correlation between AP and LAT measurements and Ogden data BMI-adjustable phantoms are realistic representation of overweight and obese patients for the purpose of estimating CT imaging doses
RPI - Ding et al, Phys. Med. Biol. 57 (9), May 2012
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RPI - Ding et al, Phys. Med. Biol. 57 (9), May 2012
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RPI - Ding et al, Phys. Med. Biol. 57 (9), May 2012
AAPM Task Group 246
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AAPM TG 246
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William Pavlicek – Chair Daniel Bednarek Wesley Bolch Dianna Cody Frank Dong Sue Edyvan Aaron Jones Cynthia McCollough Ed McDonagh Michael McNitt-Gray Donald Miller Donald Peck Madan Rehani Ehsan Samei Mark Supanich
AAPM TG 246
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CTDIVOL
DLP
Dose Page
SSDE
Enhanced SSDE
Phantom Organ Dose
True Patient Dose
AAPM TG 246
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• MC calculation tools have been validated numerous times
with physical measurements and are considered capable of accuracy equal to measured values
• Possible to assemble tables of dose coefficients to convert individual episodes of patient exposures to dose
• Approach would reduce need for long duration MC computations for each patient
• MC with a near matched scanning device and patient matched phantom computation
Conclusion
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Conclusion
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• Dose to radiosensitive organs is a useful basis for estimating metrics related to risk
• Organ dose is more informative than CTDIVOL, DLP or Effective Dose
• Better accounts for scanner differences
• Better accounts for variability in patient size
• Better accounts for changes in target region
• Better accounts for tube current modulation
Conclusion
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• Not quite ready for implementation
• How many computational phantoms are required?
• What level of accuracy is required? ± 20%, ± 35%
THANK YOU !
