Radiation Doses and Safety Considerations

Medical CollegeMedical Collegeof Georgiaof Georgia

G. David, M.S., DABR

Associate Professor of Radiology

Radiation SafetyWhom are we protecting?

Patient

Physicians & Staff

General Public

*

Patient Dose Factors / Considerations

Fluoroscopic exposure timeor

# of radiographic exposuresBeam parameters

IntensityPenetration

Distance from x-ray tubeBeam sizeSensitivity of exposed organs

damage threshold

Courtesy FDA Web Site: http://www.fda.gov/cdrh/rsnaii.html

16-21 weeks post fluoroscopic procedure

18-21 months post procedure

Close-up

*

Prolonged fluoroscopy with cumulative dose >1500 rads to a single field

Each accredited health care organization is encouraged, but not required, to report to The Joint Commission any sentinel event meeting these criteria.

Patient Dose Depends Upon

patientthicknessbody part in beam

Operator-controlled factorsTechnique settingsmagnification modeoperational mode

normal / high doseCollimation (beam

size)

*****

Patient Dose -Exposure Time / # exposures

Fluoroscopypatient exposure proportional

to beam-on timeRadiography

# studies ordered# of films / study

Cine / angioLong fluoro timesMany images recorded

*

Beam Size (Collimation)Reduces volume of tissue irradiated

IITube

X-RayTube

IITube

X-RayTube

Minimizing Patient ExposureConsistent with clinical goals minimize

fluoroscopic beam-on time# of exposures

cine / angio fluoro times & imagesBeam size (as small as

clinically feasible)

Operator Protection Considerations

TimeDistanceShieldingCollimation

Operator Protection - TimeMinimize “beam-on” time

Your exposure is directly proportional to beam time

Operator Protection – Distance(“Inverse Square Law”)

Exposure rate falls off quickly with distanceIf distance doubles, exposure rate drops by 4

Exposure Rate Fall-off with Distance

0

10

20

30

40

50

60

70

80

90

100

0 1 2 3 4 5 6 7 8 9 10

Distance

Exp

osu

re r

ate

Radiation Protection of Operator - ShieldingSources of radiation for operator

PrimaryScatterLeakage

Primary X-Ray BeamBeam coming from

x-ray tubeOperator should

avoid primary beamkeep hands, etc.

out of primary beam area

Source of most patient exposure

IITube

X-RayTube

X

Primary Beam (High Intensity)

Scatter (Indirect) RadiationArises mostly from

patientEmitted in all directions

intensity varies Much lower intensity

than primarySource of virtually all

operator exposure

IITube

Table

Patient

X-RayTube

TV Camera

Leakage RadiationSome radiation leaks

through x-ray tube housing

Intensity much lower than scatterNegligible

contribution

IITube

Table

Patient

X-RayTube

TV Camera

Operator Protection - ShieldingShield between patient &

operator significantly reduces exposure to operator

Operator Protection - Shielding

ApronGlovesLead DrapesFace ShieldThyroid ShieldCeiling-mounted shield

CollimationReducing field size significantly reduces scatter

radiation Smaller scattering volume More shielding from patient

Image Receptor

X-RayTube

Image Receptor

X-RayTube

Minimizing Operator ExposureConsistent with clinical goals minimize time

fluoroscopic exposure timescine run lengths & frame rates

Use available lead protective apparel whenever possible.

Collimate as tightly as feasibleEducation

Protecting the General Public:Lead Shielding for x-ray Rooms

Physicist calculates shielding for each wall or barrier

Shielding requirement depends onWorkloadDistancesExam TypesUse of adjacent space

Radiation Risk CategoriesDeterministic (non-stochastic)Stochastic

Deterministic (non-stochastic) Radiation Risks

Effect has known threshold radiation dose

ExamplesErythemaCataract formation

Clearly addressed by regulations

Stochastic Radiation RisksRadiation affects probability of condition

which also occurs naturallyCause of condition cannot be determined

Severity of condition independent of doseExamples

Genetic effectsFetal abnormalitiesCancer

Stochastic EffectsPublished data based primarily

on high dosesRegulations based on a linear

model1/10,000 of the dose produces

1/10,000 the frequency of the effect

Linear model is controversial!!!

Background Radiation

EarthAirCosmicPeople

Threshold for Skin Effects from Radiation

300 radtemporary epilation

600 radmain erythema

1500-2000 radmoist desquamationdermal necrosissecondary ulceration

Reference: Triumf Safety Group

Threshold for Other Biological Effects from Radiation

Cataract induction200 rads

Acute radiation syndrome100-200 rads whole body irradiation

Permanent Sterility300-400 rads to gonads

females500-600 rads to gonads

males

Reference: Huda

Threshold for Other Biological Effects from Radiation

Fetal doses below 1 rad result in negligible congenital abnormalities

Risk from acute doses below 10 rads considered “small”Abortion not commonly

considered

Reference: Huda

Diagnostic Radiology Exposures Generally very low compared to

previous valuesGreatest concerns

Fetal doses

Angiography / cardiac cath / interventional studies

CT

Exposure Measurement ProtocolsStandardized methodology for

determining how much radiation patient receives

Different protocol for each modalityUsually provided for “average” or

“typical” patient

Exposure Measurement Protocols

RadiograpyEntrance Skin Exposure (ESE)

MammographyMean glandular dose

CTCT dose index (CTDI)Dose length product (DLP)

Radiography / FluoroscopyEntrance Skin Exposure

Ionization measured where radiation enters patient

Does not address internal doses which depend uponBeam penetrabilityAbsorber

Tablet op

“Patient”R

Entrance Skin Exposures

PA Chest 10-20 mR Abdomen: ~300 mR

Entrance Skin Exposures

Elbow: ~20 mR

Hand ~ 20 mR

Femur ~ 200 mR

AP Skull ~ 150 mR

Comparison of Entrance Skin Exposure

0 50 100 150 200 250 300 350 400 450

Skull

C Spine

T Spine

L Spine

Pelvis

Abdomen (KUB)

Chest

Ribs (above diaph.)

Ribs (below diaph.)

Shoulder

Femur

Knee

Hand/Wrist

Elbow

Foot

Ankle

Finger/Toe

Humerus

mR

Entrance skin exposures.

Internal doses will be substantially less.

Typical Fluoroscopy Exposure Rates @ Tabletop

“Cruise control” varies exposure rate automatically

Varies greatly withPatientImaged anatomy

Typical Skin Exposure for “Average” patients2 - 5 R / minute Beam on time

Legal maximum table top exposure: 10 R/min (20 R/min in high dose mode)

Angiography / Interventional / Cardiology

Long fluoroscopic beam timesMultiple imaging exposures

Cine (cardiology)Subtraction images (Angiography)

Caution

Mammography Mean Glandular Dose (MGD)

Calculated from entrance skin exposure“Typical” breast assumptions

4.2 cm thick (accreditation phantom)Breast firmly compressedBreast composed of 50% adipose / 50%

glandular tissue average breast closer to 70% adipose / 30%

glandular tissue

Measuring Mean Glandular Dose (MGD)

Measure ESE with chamberCompression paddle

& accreditation phantom in place

MGD calculated from ESE

MammoTube

Compression Device

Breast Support

Image Receptor

Grid

Phantom

R

Mammography Mean Glandular Dose

LimitsACR

100 mrad w/o grid 300 mrad w/ grid

MQSA 300 mrad CC View FDA approved phantom

Typical~100 mrad (digital)

CT Patient DoseCT Patient Dose

Because tube rotates around patient, dose distribution different from radiography

Skull dose distributionFairly uniform

Body dose distributionDose to center of body ~ half of skin dose

CT Dose PhantomLucite5 holes

One centerFour in periphery

Comes in two flavors“Head”“Body”

CT Dose MeasurementChamber placed in one holeLucite plugs placed in remaining

4 holesSlice centered on phantomStandardize technique

kVpmAsscan timepitchbeam thickness

Chamber

Plugs

Measuring CT Dose“Pencil” ion chamber usedPencil pointed in “Z” direction

Dose Phantom

Chamber

Z

Beam

Typical CT Doses4 rads head2 rads bodySurface doses for

body scans may be 2X the dose at center

CT Usage

Annual growthU.S. Population: <1%CT Procedures: >10%

~ 67,000,000 procedures in 2006about 10% pediatric CT

Computed Tomography — An Increasing Source of Radiation ExposureDavid J. Brenner, Ph.D., D.Sc., and Eric J. Hall, D.Phil., D.Sc.

New England Journal of Medicine, 2007

Exposure Increase 1982-2006

60%

40%CT

Other

0

1

2

3

4

5

6

mSv

Total Medical

U.S. Per Capita Exposure

1982

2006

500% medical exposure increase

in 24 years

CT Usage16% of imaging

procedures

23% of total per capita exposure

49% of medical exposure

CT Causes Cancer?“On the basis of …data on CT use from 1991 through 1996, it has been estimated that about 0.4% of all cancers in the United States may be attributable to the radiation from CT studies…By adjusting this estimate for current CT use this estimate might now be in the range of 1.5 to 2.0%.”

Computed Tomography — An Increasing Source of Radiation ExposureDavid J. Brenner, Ph.D., D.Sc., and Eric J. Hall, D.Phil., D.Sc.

New England Journal of Medicine, 2007

CT Causes Cancer?In the United States, of approximately

600,000 abdominal and head CT examinations annually performed in children under the age of 15 years, a rough estimate is that 500 of these individuals might ultimately die from cancer attributable to the CT radiation.

Estimated Risks of Radiation-Induced Fatal Cancer from Pediatric CT;Brenner, Elliston, Hall, & Berdon; AJR-176 Feb. 2001

Other ModalitiesUltrasound

No known biological effects as used clinicallyGreatest concerns

Fetus Temperature elevation

MRINo known biological effects as used clinically