- 1.DME Bardo, MD1, KA Feinstein, MD2, D Pettersson, MD1, J
Wiegert, PhD3, JH Yanof, PhD4 Philips Research3, Philips
Healthcare4 Comparison of Patient-Specific & Reference-Phantom
Methods for CT Dose Estimation in the Pediatric Population 1 2
2. Purpose Compare the characteristics of a) age-based reference
phantoms used with Monte Carlo simulation to estimate organ doses
with b) patient-specific phantoms based on CT data sets. Compare
the use of reference-phantom and patient-specific dose distribution
maps to estimate organ doses. Describe how differences in organ
dose distribution affect the estimation of effective dose. 3.
Content Organization Reference phantom & Patient specific
phantoms Methods for estimating organ dose Effective dose
calculations Standard From geometrically defined organs in stylized
phantoms Patient Specific Using segmentation of organs in
patient-specific dose maps Standard CTDI and stylized phantoms
Patient Specific Voxelized models based on patient data sets
Standard Regression with DLP & E (k factors) Patient Specific
Weighted sum of dose map organ doses 4. Morphology of standard
reference phantoms used for CT dose estimation can differ greatly
from the anatomy of an individual patient A standard reference
(stylized, mathematical) phantom (ORNL, Cristy) is compared with CT
images of a 5-6 month old patient. Striking anatomical differences
between reference and the patient can effect the estimation of dose
by dosimetry simulation (Monte Carlo). 5. Limitations of standard
dose estimates k factor AAPM 95.6 Table 3 Volume CTDI & DLP are
based on PMMA cylindrical phantoms and are not intended to be
estimates of patient dose. They do not account for individual
patients body habitus, attenuation characteristics, or specific
scanner dosimetry. PMMA CTDIvol phantoms DLP conversion factors for
Effective dose (reviewed later) are based stylized phantoms with
fixed geometry (modified ORNL set for newborn, 1, 5, and 10 YO as
shown). [1,2,3] 6. The x-ray beam has more attenuation as it
traverses a larger patient (yellow to red to blue) in comparison
with a smaller patient (yellow to red). Thus, the average beam
intensity and dose, as the tube rotates, tends to increase with
decreasing size for the same scan parameters Dose (mGy) 10 20 30 40
10 mGy CtrLG 20 mGy, PrphryLG 40 mGy CtrSM 40 mGy Prphry,SM
Background Major factors that affect CT dose include size/diameter
& tissue/material absorption 7. Background Average patient dose
tends to increasewith decreasingpatient size/diameter The Size
Specific Dose estimates (SSDE) [6] also show that absorbed dose
increases with decreasing size. A regression model (above) relates
dose to effective diameter. The model data was based on a range of
dosimetry methods (measured and simulated), four sets of phantoms,
and four scanner vendors. All phantom sets included pediatric
sizes. Child Infant Child Anthropomorphic CTDI Voxelized (GSF)
Cylinders Adult 8. In contrast to a standard reference phantoms
(left), a patients CT data set can be used to create a
patient-specific (virtual) phantom (middle). In addition to
patient-specific dosimetry, this enables and individualized dose
maps (right). Standard reference vs. Patient-specific Phantoms and
Dosimetry Voxelized Phantom Patient Data setStandard reference
phantom Patient specific dose map 9. Dose maps can also be
displayed in units of CTDI normalized absorbed dose. In this case,
dose map pixels are divided by the simulated dose absorbed by the
CTDI phantom. The basic trend of CTDI-normalized average dose
increasing with decreasing patient size (relative to the CTDI
phantom) tends to agree with the SSDE correction factors. Monte
Carlo simulations with patient-specific phantoms: CTDI-Normalized
Dose Maps 15 y/o CTDIvol 32 = 6.3 mGy Average dose map value Dose
to 32 cm CTDI phantom 10. A Monte Carlo tool is used to simulate
the dose absorbed by the patient specific virtual phantom instead
of the CTDI phantom. This results in a patient specific dose map
(in units of mGy). This example shows that the scan parameters for
a 13 day old resulted in less absorbed relative to a 15 year old.
Monte Carlo simulations with patient-specific phantoms:
Individualized dose maps in mGy 13 day old CTDIvol=1.5 mGy 15 year
old CTDIvol = 6.3 mGy 11. The CTDI of 2.5 mGy for a 13 day old and
6.3 mGy for a 15 year old would yield approximately the same
patient specific-dose map. Monte Carlo simulations of
patient-specific phantoms can be used to simulate new dose maps 13
day old CTDIvol= 2.5 mGy 15 year old CTDIvol = 6.3 mGy Simulated
dose maps 12. Morphological differences between patient-specific
& standard reference phantoms for dosimetry simulation (Monte
Carlo) CT data sets are used to form patient specific virtual
phantom (previous slide). The size and shape of organs and tissues
can have wide variation. Virtual phantoms do not extend beyond
reconstructed image volume. Organs are represented by patient
generic, fixed (stylized) geometric shapes. Anatomy extends
caudal-cranially, enabling dose simulation and scatter beyond scan
range. Oak Ridge National Lab Phantoms (Cristy)Virtual Patient
(Voxelized) Phantoms newborn 1 YO 5 YO 10 YO 10 YO 5 YO 1 YO
newborn 13. Acquisition of CT images 3 4 Monte Carlo simulations
were performed on voxelized image sets. Dose Maps displaying
CTDIvol normalized absorbed dose. Organ doses can be segmented from
dose maps CT image voxels were classified as 1 of 5 tissues based
on attenuation 5 21 Background Flowchartfor generation of dose
maps: CT data acquisition, creationof voxelized phantom,dose
simulation 3 14. Dose Map: Select radiosensitive organ or tissue
Average value of pixels segmented in the organ are used for organ
dose estimation (in mGy) Organ segmentation Background
Patient-specificorgan doses can be estimatedwith dose maps In
standard reference phantoms such as those used for the DLP
conversion factors, organs are defined with fixed geometry using
mathematical equations. 15. age 6 days 13 days 26 days 2 months
Lung dose 1.78 1.96 2.43 2.42 (mGy/mGy) show patient-specific
variability in organ doses that cannot be shown in the Oak Ridge
National Lab (ORNL) references phantoms Dose maps Estimated lung
dose (CTDI normalized) from patient specific dose maps varies from
1.78 to 2.43 mGy/mGy. CTDIvol normalized organ doses simulated in
the ORNL infant phantom (right) (new born) would not have any
variability. 16. NRPB reference phantom (center [3]) extended the
ORNL phantom concept (lower right) to include gender-based organs.
The NRPB phantoms were used by Jessen et al. (with IRCP 60 organ
weighing factors) to determine widely used DLP conversion factors
[5,11] Standard Reference Phantoms Can be modified to include
additionalradiosensitiveorgans 17. Patient-specific voxelized
phantoms have featuresnot included in standard reference phantoms
Breast dose can be measured (green contours) in dose maps -- this
tissue is represented in the NRCP phantoms (previous slide). 14 y/o
female (left) and 15 y/o male (right) with approximately the same
effective diameter (~27 mm). Bismuth shields (arrows) were used in
both exams (not represented in standard phantoms). Average lung
dose is higher for the female patient due to relatively larger lung
parenchyma (lower beam attenuation). 18. Comparison of
patient-specific & reference phantom methodsused for
effectivedose calculation Effective Dose, Reference Phantom LEGEND:
CTDI = Computed Tomography Dose Index DLP = Dose Length Product ED
= Effective Dose ODi = Organ Dose wi = tissue weighting factors,
ICRP 60 compare Effective Dose, Patient-Specific Phantom ODi From
Organ Segmentation Dose Map Monte Carlo Simulation (Scanner
Specific) Patient Data Set CTDI, DLPscan ED = k x DLPscan Reference
Phantom MonteCarlo Simulation ODi From Organ Compartments Dose Map
CTDI, DLP AAPM Report 96.5 k 19. Background Estimation of
effectivedose for organ weighting factors ED = wi x Odi where OD is
the individual organ dose measured from non-patient specific
mathematical phantom or patient specific dose map w is organ/tissue
weighting factor (ICRP 60 or 103) i = 13 (13 most radiosensitive
organs) The effective dose equivalent, therefore, represents a
total body dose. 20. Relative Organ Dose Sensitivities, Wi 0 0.05
0.1 0.15 0.2 0.25 Relative Organ Sensitivities ICRP (Used by both
Pt. Specific and Std. Ref. Phantoms) ICRP 60 ICRP 103 ICRP 60 and
NRPB phantoms were used for DLP conversion coefficients (Jessen).
Patient-specific phantoms Segmenting all the listed organs and
tissues for each individual voxelized phantom can pose a challenge.
Standard reference phantoms The NRPB phantoms do not include all
organs and tissues listed. And they would need to be revised if the
ICRP adds new organs to the list. 21. Background Determinationof
DLPconversionfactors Each body and age specific DLP conversion
factor (k factor in units of mSv mGy-1 cm-1) was determined by
dosimetry simulations with varying DLP. They are based on linear
regression analysis of body-region specific effective dose (from
the simulations, y-axis) and DLP (x-axis). In this method,
effective dose is assumed to be linearly proportional to DLP, i.e.,
E = DLP x k . DLP is linearly proportional to irradiated scan
length (includes helical over-ranging). For pediatrics, DLP is
based on CTDI 16. Also, K-factors (i.e., DLP conversion) represent
an average over scanner types and are not gender specific. The
organ dose weighting factors were described on the previous slide.
DLP input to Dose Simulation Chest newborn Slope = k, for each age
Chest 1 YO Chest 5 YO Chest 10 YO EffectiveDose
asweightedsumoforgandoses 22. Effective dose using ICRP
weightingfactorscannot be patient-specific The organ sensitivity
(weighting) factors are based on population data from survivors of
the atomic blast, where the sum of the weighting factors is one. A
0.12 value for lung tissue implies that the relative likelihood of
developing lung cancer in the population of blast survivors, in
comparison with other listed organs, is 12%. Therefore, any
estimation of effective dose that uses these population based
weighting factors patient-specific dose maps or DLP conversion k
factors cannot be patient-specific. Although effective dose is not
patient specific, dose maps enable patient specific organ dose
estimates (next slide) and these increase the relative patient (as
well as scanner) specificity in comparison with DLP conversion
factors. 23. Partial irradiation of an organ tends to decrease the
organ dose estimate. This is because organ dose is defined as the
average over the entire organ. An advantage for the reference
phantoms is that the caudal- cranial range is not limited to a
reconstructed scan volume as with the voxelized phantom. This can
help estimate absorbed dose to partially irradiated organs. Basing
organ dose on only the fully irradiated voxels will tend of
overestimate the estimated organ dose. Organ doses partial
irradiation, ICRPweighting factors ICRP weighting factors are based
on full-body irradiation. Tissues that have wide distribution
throughout the body such as red bone marrow are almost certainly
partially irradiated in a CT examination. partial irradiation of
liver scanlength 24. Summary comparison dose maps and standard
reference phantoms Dose Map Method Standard Reference Phantom
Method Representation Voxelized Phantom Based on Data Set Four
pre-defined geometric representation of organs Morphology
Patient-specific Not Patient Specific Organs Organs must be
segmented. Organs pre-defined mathematically Caudal Cranial
End-effects Not modeled (easily) Extends beyond scan length to
model partial organ irradiation Computation Computed for each
patient and each examination Can be pre-tabulated for set of
examinations and stored for future use Material Models CT Numbers
are mapped to ICRU 44 ICRP Publication 89 Effective Dose Pt.
specific organ dose can be used to estimate eff. dose Generic organ
doses are used to determine DLP conversion coefficients. 25.
Estimation of CT dose is evolving 10 cm CTDI phantom Dose map
sequence (z-axis) based on Monte Carlo simulation with infant CT
data set 26. Summary Patient specific voxelized phantoms can
represent complex, patient specific anatomy and materials that are
not easily represented in standard reference phantom. Organ doses
estimated from patient specific dose maps ARE patient specific.
Patient-specific dose maps demonstrate the variability of organ
doses and highlight a key limitation of standard methods for
estimating effective dose. Use of more patient-specific methods to
estimate organ and effective doses could lead to better metrics and
reporting for CT dose management. Effective doses estimated from
ICRP wt. factors and NOT patient specific, but EDose Maps is more
patient- and scanner-specific than EDLP. 27. Clinical Relevance
Patient-specific doses estimated by applying dosimetry simulations
to voxelized phantoms may have advantages when patient morphology
significantly differs from the reference phantom. Quantitative
evaluation of patient-specific dose maps are underway. This will
lead to a better understanding how more accurate dose estimate
methods will impact CT radiation dose management. 28. References1.
Cristy M . Mathematical phantoms representing children of various
ages for use in estimates of internal dose. Report no. ORNL/
NUREG/TM-367. Oak Ridge, Tenn: Oak Ridge National Laboratory, 1980
. 2. Cristy M , Eckerman KF . Specifi c absorbed fractions of
energy at various ages from internal photon sources. I. Methods.
Report no. ORNL/TM-8381/V1. Oak Ridge, Tenn: Oak Ridge National
Laboratory, 1987 . 3. A Khursheed, Phd, M C Hillier, P C Shrimpton,
Phd And B F Wall, Bsc, Influence of patient age on normalized
effective doses calculated for CT examinations 4. Maria Zankl ,
Handbook of Anatomical Models for Radiation Dosimetry Edited by Xie
George Xu and Keith F Eckerman , 3] Taylor & Francis 2009 5.
American Association of Physicists in Medicine. The measurement,
reporting and management of radiation dose in CT. Report 96. AAPM
Task Group 23 of the Diagnostic Imaging Council CT Committee.
College Park, MD. American Association of Physicists in Medicine,
2008. 6. American Association of Physicists in Medicine.
Size-specific dose estimates (SSDE) in pediatric and adult body CT
examinations. Report 204. AAPM Task Group 204. College Park, MD.
American Association of Physicists in Medicine, 2011. 7. McCollough
CH, et al., CT Dose Index and Patient Dose: They Are Not the Same
Thing. Radiology: Volume 259:(2) 311-316. 8. Morgan HT., Dose
reduction for CT pediatric imaging, Pediatr Radiol. 2002
Oct;32(10):724-8; discussion 751-4. Epub 2002 Aug 29., 9. Adam C.
Turner1 and Michael McNitt-Gray, The feasibility of patient
size-corrected, scanner-independent organ dose estimates for
abdominal CT exams, Med Phys. 2011 Feb;38(2):820-9. 10. Boone JM,
Strauss KJ, Cody DD, McCollough CH, McNitt-Gray MF, Toth TL, Goske
MJ, Frush DP. Size-specific dose estimates (SSDE) in pediatric and
adult body CT examination. Report No. 204. 2011 11. Cynthia H.
McCollough et al. How Effective Is Effective Dose as a Predictor of
Radiation Risk?, AJR:194, April 2010 29. air adipose tissue lung
tissue soft tissue cortical bone A patient-specific (tomographic)
virtual phantom (i.e., model) is created by voxelizing and
automatically segmenting patients CT dataset. Each voxel is
assigned one of five material types based on an a priori, global HU
classification intervals (ICRU 44). These material types are also
assigned mass density to compute absorbed dose. The resulting
virtual patient phantoms are used for dose simulation (Monte Carlo)
and the results are referred to as Dose Maps (next slide). CT image
for 6 day old Virtual patient phantom Appendix I Creation of a
Patient-specificVoxelized Models 30. Limitations of CTDIvol The
CTDIvol reports scanner output based on a standard, fixed-sized
phantom (32 cm for body), not patient-specific dose. Therefore,
dose is over- and underestimated for patients significantly larger
or smaller (respectively) than the phantom. (see AAPM report 201)
EstimatedCTDIvol[mGy](120kV) 24 32 50 15 10 5 0 Patient diameter
[cm] Actual dose for 24 cm diameter patient Reported dose Actual
dose for 50 cm diameter patient 31. DLPand Effective Dose Effective
dose (ED) parameter shown here is also based on the plastic CTDI
phantoms. It is a risk-related quantity used to indicate equivalent
whole body exposure that includes DLP as well as other factors such
as the radiation sensitivities of the various organs in the body,
age, and gender. Notes: 1) Effective dose using DLP conversion
coefficients are estimated with averages over gender and age and
therefore do not estimate risk for an individual patient. 2)
Reference for ED are based on estimates for annual background
radiation (3 mSv). 3) Another method to compute ED is based on the
summation of organ dose estimates. CTDI (mGy) Dose Length Product
(mGy *cm) Effective Dose (mSv) Equation or dose calculation method
CTDIvol CTDIvol is presently measured with 16 and 32 cm phantoms
DLP = Irradiated Scan Length x CTDIvol Helical scan length: the
reconstructed scan length plus helical over-ranging Axial scan
length: the reconstructed scan length for one axial shot * number
of axial shots. (The CTDIvol accounts for over- beaming). k =
conversion coefficient for the DLP method of estimation 32. Dose
map reconstructions Dose maps show representations of: Absorbed
dose map [mGy] Typical range: 0-20 mGy Absorbed dose map/CTDIvol
[mGy / mGy] Typical range: 0-2.5 mGy/mGy Energy imparted map
[Joules] Typical range: 10-5 J/pixel