Review of Recent CT Accidents:Dosimetry, Risk Analysis, and Lessons Learned

Terry Yoshizumi, PhD*, David Enterline, MD Greta Toncheva, MS

Duke University Medical CenterDurham, NC

1

NC HPS SPRING MEETINGMARCH 4-5, 2010

RALEIGH, NC

ACKNOWLEDGEMENTS

• Robert Reiman, MD • Don Frush, MD• Ehsan Samei, PhD• James Colsher, PhD

2

TOPICS

1. Review of recent CT accidents2. What is CT perfusion (CTP)?3. Why is the dose so large?4. How do we measure organ doses?5. Results of Cedars-Sinai’s protocols6. Risks identified7. Scrutiny began at federal level8. Lessons learned9. Concluding remarks

3

Review of Recent CT Accidents

4

Eur Radiol (2005)15:41-46Hair loss: 3 casesCTP + angiography study

2005 2008 2009

2 1/2 yr old151 scans in period of 65 minutes(CBS13.com)

FDA Warning 10-8-09Cedars-Sinai, CTP206 pts over 18-mo.3-4 Gy (normal 0.5 Gy)80 hair loss56 direct exposure to lens of the eye

+ 50 identifiedfrom other states

(FDA Report)

Brain CT Perfusion ImagingClinical Applications

• Stroke and Ischemia• Diamox Challenge• Vasospasm• Tumor evaluation

– CTP and CT permeability

Tracer Kinetic Theory

Area under concentration curve

Flow =Volume

Mean transit time-0.02

0.0

0.02

0.04

0.06

0.08

0.10

0.12

0.14

MTT

-0.02

0.0

0.02

0.04

0.06

0.08

0.10

0.12

0.14

AreaVolume =

Analysis of time-concentration curves

Perfusion Imaging

• CT: Conc = k (density)

-0.02

0.0

0.02

0.04

0.06

0.08

0.10

0.12

0.14

CBVCBV

CBFCBF

MTTMTTTime Concentration Curve

Choosing Arterial Input Funct (AIF) & Venous Output F (VOF)

VOF

AIF

Compensated ischemia of LICA

MTT CBF CBV

Prolonged MTT (red) Low CBF (blue)

Increased CBV (green) due to autoregulation

Compensated ischemia of LICA

MTT CBF CBVNormal: 3-5 sec > 40 mL/min/100 g brain ~3

Borderline CBF With Low CBV Predicts Infarction (No

Intervention)

CBV - 5 hours

T2WI - 24 hours (MRI)

infarct

CT Perfusion- Absolute and Relative CBF Analysis

• Absolute CBF of 10-20 ml/100g/min represents ischemic tissue but viability depends on duration of ischemia

• When ischemia is associated with an area of core infarct, this represents the penumbra

• Decreased CBV with compromised CBF implies infarction

• CINE mode:• 80 kVp, 200 mA, 1 sec• Repeat 45 sec• Table fixed

13

CT Perfusion

WHEN THE DOSE BECOMES AN ISSUE IN CTP

When human errors introduced– scan parameters altered, i.e., the energy changed from 80 kVp to 120 kVp, or tube current (mA) modulation used without understanding consequences

Why:Display constantly updated images by continuous rotation of the tube at the same location; potential for high dose at the level of scan

14

How do we measure organ doses?

A. Manual Look-up tables (Outdated)B. Organ dose from CTDI (No value, but CTDI is

useful for monitoring dose)C. Monte Carlo based dose calculator (complex,

time consuming)D. Effective Dose from DLP (No value)E. Anthropomorphic phantom with TLDs (Labor

intensive)F. Anthropomorphic phantom with

MOSFET(metal oxide semiconductor field effect transistor) detectors (Best value)

15

Materials and Methods• Patient Dose Verification System AutoSense, Model TN-RD-60

•Radiation detectors: 20 Metal Oxide Field Effect Semiconductor Transistors (MOSFET), Model TN-1002RD, High sensitivityReader, Model TN-RD-15Bias supply, Model TN-RD -22AutoSense PC Software, TN-RD-45

•MOSFET dosimeter:MOSFET dosimeter:Silicon chip 1mmSilicon chip 1mm22

Active area 0.2mm x 0.2mmActive area 0.2mm x 0.2mm

16

MOSFET AUTO-SENSE SYSTEM

MOSFET

Adult Male PhantomTissue Equivalent Anthropomorphic Phantom

Model 701-D StevenSN : 701-28539 slabsHeight -173 cm, Weight - 73 kgThorax Dimensions: 23 x 32 cmHead: AP 21cm, Lateral 17.1cmNeck at the thyroid: AP 13 cm, Lateral 14.6 cm

CIRSwww.cirsinc.com

A FEW WORDS ABOUT THE ED AND WEIGHTING FACTORS

18

CONCEPT OF EFFECTIVE DOSE EQUIVALENT OR EFFECTIVE DOSE

1-12-10 19

ICRP Report 26 (1977)Dose EquivalentQuality FactorWeighting FactorEffective Dose Equivalent

ICRP Report 60 (1990)Equivalent Dose Radiation Weighting FactorTissue Weighting FactorEffective Dose

•ICRP 103 (2007)•Equivalent Dose•Radiation Weighting Factor•Tissue Weighting Factor•Effective Dose

WT : ICRP 26 (1976), ICRP 60 (1990), ICRP 103 (2007)

ORGANORGAN ICRP26 ICRP26

(1977)(1977)

ICRP60ICRP60

(1990)(1990)

ICRP 103ICRP 103

(2007)(2007)

GonadsGonads 0.250.25 0.200.20 0.050.05

Bone marrowBone marrow 0.120.12 0.120.12 0.120.12

LungLung 0.120.12 0.120.12 0.120.12

StomachStomach 0.120.12 0.120.12

ColonColon 0.120.12 0.120.12

BreastBreast 0.150.15 0.050.05 0.120.12

BladderBladder 0.050.05 0.050.05

LiverLiver 0.050.05 0.050.05

EsophagusEsophagus 0.050.05 0.050.05

ThyroidThyroid 0.030.03 0.050.05 0.050.05

SkinSkin 0.010.01 0.010.01 0.010.01

Bone surfaceBone surface 0.030.03 0.010.01 0.010.01

KidneysKidneys 0.010.01

BrainBrain 0.010.01

Salivary glandsSalivary glands 0.010.01

Remainder tissuesRemainder tissues 0.300.30 0.050.05 0.100.10

1-12-10 20

ICRP 103 Weighting Factors

21

Organ ICRP 103 Remainder ICRP 103

Gonads 0.08 Adrenals 0.0086

Bone marrow (red) 0.12Extra-thoracic tissue 0.0086

Lung 0.12 Gall bladder 0.0086

Breast 0.12 Heart wall 0.0086

Thyroid 0.04 Kidneys 0.0086

Bone surface 0.01 Lymph nodes 0.0086

Colon 0.12 Muscle 0.0086

Stomach 0.12 Oral mucosa 0.0086

Bladder 0.04 Pancreas 0.0086

Liver 0.04 Prostate 0.0086

Esophagus 0.04 Small intestine 0.0086

Skin 0.01 Spleen 0.0086

Salivary glands 0.01 Thymus 0.0086

Brain 0.01 Uterus / cervix 0.0086

Remainder 0.12 Total 1 Total 0.12

Notes on Effective Dose Calculations

22

• Skin Dose: taken the highest of the anterior and posterior, the posterior skin dose is reduced due to attenuation by the table• Brain Dose: the dose is averaged from the dose of all brain locations• Bone marrow: the dose is a sum of the measured bone marrow dose at different locations multiplied by the % distribution • Lens of the eye: average of the two • Bone surface: the measured dose is adjusted with the dry bone f-factor (different for soft tissue and bone), f-factor is the conversion from R to cGy, at different tissues and energies used during the MOSFET calibrations • All protocols were measured three times and the averaged value was used• Misc.: in occasions where out of three measurements two are “0”, only the one number was used, or if there was one “0” and two numbers, the average of the only two was used • The new ICRP 103 was used with the new weight factors (listed in the next slide)

Red marrow

% Distributio

nSKull

(cranium + facial) 8.32

Scapulae 28.5Clavicles 0.79

Ribs 19Spine (upper

portion) 2.66Spine (middle

portion) 17

ADULT CTP

23

Scan # Protocol Images Scan typeTable speed

(mm/rot) kV mA

Total Exposure time (sec)

CTDIvol (mGy) DLP (mGy*cm)

Detector coverage

1

Adult Perfusion 1-360

CINE Full Axial 5.0 8i 1sec 80 200 45 531.43 2125.71 40 mm

2

Adult Perfusion 1-360 CINE Full 5.0 8i 1sec 120 200 45 1714.29 6857.13 40 mm

3

Adult PerfusionAuto mA 1-360 CINE Full 5.0 8i 1sec 120

min 100 max 520

NI=2.4 45 4457.14 17828.55

FDA 0.5 Gy

24

TUBE CURRENT MODULATION

Basic Concept:

Adjust the tube current to accommodate the patient contour and composition

25

TUBE CURRENT MODULATION

mA2

mA1mA1

mA2

modulation around the z-axis modulation around the cross-section

mA2mA1

modulation – axial and helical

26

How smart mA works• Projection data from a scout scan measures the

patient and determines how to modulate the mA for improve dose efficiency and consistent IQ

0

50

100

150

200

250

300

0.0 100.0 200.0 300.0 400.0 500.0

100% 100% 55%55%

40%40%

mA

sm

As

Fixed mAFixed mA Z Modulation - Auto mA Z Modulation - Auto mA

XYZ ModulationXYZ Modulation

Jim Colsher, GE Healthcare

CHEST ABDOMEN PELVIS

27

sd 25.4

sd 23.7 sd 22.6

An AutomA Example (Noise Index =24)

GE Healthcare

Results: Effective Dose

28

ED per scan (mSv)

Head 0.9 - 4

Chest 4 – 18

Abdomen-Pelvis

3 - 25

Angiography: head

1 – 10

Angiography: heart

5 - 32

ED (Adult), excerpt from NCRP 160, 2009

Results: Organ doses

29

Results: Lens of the Eye

30

Results: Skin Dose

31

Brain Dose

32

Risks identified from CTP

33

Threshold dose (Gy) Clinical CT dose (Gy)FDA: 0.5 Gy

Temporary Epilation 3 Gy (FDA) Quite possible

Main Erythema 6 Gy (FDA) Unlikely, but possible

Moist desquamation, dermal necrosis, secondary ulceration

15-20 Gy (FDA) Unlikely, but possible

Cataracts •2-5 Gy (acute or a few fractions)•0.6-0.8 Gy (chronic exposure to diag. x-rays over yrs, or involve radiations other than photons (Reiman)

Possible for both acute and lower threshold

FDA Report (10-7-09)http://www.accessdata.fda.gov/scripts/cdrh/cfdocs/cfMAUDE/Detail.CFM?

MDRFOI__ID=1495886

SUMMARY:• No malfunction on the scanner• Protocol altered by the site user

34

(GE Version)

Recommendations from FDA

• Check for excess radiation from CT Perfusion• Review radiation dosing protocols for all CT perfusion

studies• Implement quality control procedures• Check the CT scanner display panel before performing a

study to make sure the amount of radiation to be delivered is at the appropriate level for the individual patient.

• For multiple scans on a patient during one imaging session, practitioners should adjust the dose of radiation

35

Lessons learned• Need for quality assurance of protocol

development/modification including dosimetry oversight

• Need for team approach in dealing with patient safety

• Need for radiation safety education for all personnel involved

• Need for institutional oversight by the Radiation Safety Committee

36

Concluding remarks

Key questions to ask regarding Cedars-Sinai incident?

1.Was it machine failure?

2.Was it human failure? Wrong scan protocols implemented – no one

challenged changing 80 kVp to 120 kVp, or use of Auto mA mode with low noise factor

Failure by the technologists, radiologists and physicist

3.What signs were there? High CTDI values missed. You must look for it!

4.What was the protocol review process?37

Concluding Remarks

38

CONGRESSIONAL HEARING OCCURRED (F-2-26-2010). This may be the beginning of a new period for radiation protection.

Concluding remarksWHAT WAS LOST IN ALL THESE HYPE?• Cold Facts on Stroke

– The third leading cause of death in the US– The leading cause of adult disability– Every year about 750,000 Americans experience stroke and about 160,000

(21%) die from it

• Fundamental issue: Over-radiation without oversight– CTP is a valid protocol that uses 0.5-0.8 Gy , but should be < 1Gy

Live/die vs. Hair Loss

39WHICH WOULD YOU CHOOSE?

40

THANK YOU.