Radiation Dosimetry of the Patient

Radiation Dosimetry of the PatientRadiation Dosimetry of the Patient

Robert L. Metzger, Ph.D.Robert L. Metzger, Ph.D.

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1. Dosimetry1. Dosimetry

Radiation dosimetry is primarily of interest because radiation Radiation dosimetry is primarily of interest because radiation dose quantities serve as indicators of the risk of biologic dose quantities serve as indicators of the risk of biologic damage to the patientdamage to the patient

The biologic effects of radiation can be classified as either The biologic effects of radiation can be classified as either deterministic (non-stochastic)deterministic (non-stochastic) or or stochasticstochastic

Deterministic or non-stochastic effects are believed to be Deterministic or non-stochastic effects are believed to be caused by cell killingcaused by cell killing if a sufficient number of cells in an organ or tissue are killed, if a sufficient number of cells in an organ or tissue are killed,

its function can be impairedits function can be impaired

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Deterministic or non-stochastic effects Deterministic or non-stochastic effects effects include terratogenic effects to the embryo or fetus, skin effects include terratogenic effects to the embryo or fetus, skin

damage and cataractsdamage and cataracts a threshold can be defined below which the effect will not a threshold can be defined below which the effect will not

occuroccur for doses greater than the threshold dose, the severity of the for doses greater than the threshold dose, the severity of the

effect increases with the doseeffect increases with the dose

to assess the likelihood of a deterministic effect on an organ to assess the likelihood of a deterministic effect on an organ from an imaging procedure, the dose to that organ is estimatedfrom an imaging procedure, the dose to that organ is estimated

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A stochastic effect is caused by damage to a cell that produces A stochastic effect is caused by damage to a cell that produces genetically transformed but reproductively viable descendantsgenetically transformed but reproductively viable descendants cancer and hereditary effects of radiationcancer and hereditary effects of radiation probability of a stochastic effect, instead of its severity increases probability of a stochastic effect, instead of its severity increases

with dosewith dose No dose thresholds below which the effects cannot occurNo dose thresholds below which the effects cannot occur

The NRC’s radiation dose limits described in Chapter 23 are intended The NRC’s radiation dose limits described in Chapter 23 are intended to limit the risks of stochastic effects and to prevent the non-stochastic to limit the risks of stochastic effects and to prevent the non-stochastic effectseffects

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Entrance Skin Exposure Entrance Skin Exposure The radiation exposure incident on a patient is the entrance skin The radiation exposure incident on a patient is the entrance skin

exposureexposure Skin doses are easy to measure but they are poor indicators of Skin doses are easy to measure but they are poor indicators of

patient riskpatient risk They do not take into account the exposed area, penetrating They do not take into account the exposed area, penetrating

power of the x-ray beam, or the radiosensitivity of the exposed power of the x-ray beam, or the radiosensitivity of the exposed regionregion

At diagnostic energies, the f-factor (roentgen-to-rad) conversion is At diagnostic energies, the f-factor (roentgen-to-rad) conversion is close to 1.0 so that dose is numerically equal to exposureclose to 1.0 so that dose is numerically equal to exposure

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Dose-Area ProductDose-Area Product (DAP) (DAP) Product of patient entrance skin exposure and cross-sectional Product of patient entrance skin exposure and cross-sectional

area of the x-ray beam (exposed area)area of the x-ray beam (exposed area) Units are in mGy-cmUnits are in mGy-cm22 or mrad-cm or mrad-cm22 Used in fluoroscopyUsed in fluoroscopy

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Radiation Dose Radiation Dose Radiation dose is defined as the absorbed energy per unit mass but Radiation dose is defined as the absorbed energy per unit mass but

this says nothing about the total mass of tissue exposed and the this says nothing about the total mass of tissue exposed and the distribution of the absorbed energydistribution of the absorbed energy

Would you prefer to receive a dose of 10 mGy to the whole body or Would you prefer to receive a dose of 10 mGy to the whole body or 20 mGy to the finger?20 mGy to the finger?

The 10 mGy whole body dose represents about 1,000 times the The 10 mGy whole body dose represents about 1,000 times the ionizing energy absorbed for a 70-kg person with a 35 g fingerionizing energy absorbed for a 70-kg person with a 35 g finger

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Imparted energyImparted energy the total amount of energy deposited in matter is called the the total amount of energy deposited in matter is called the

imparted energy (Joules)imparted energy (Joules), is the product of the dose (Gray) , is the product of the dose (Gray) and the mass (Kg) over which the energy is impartedand the mass (Kg) over which the energy is imparted

assume each 1-cm slice of a head CT scan delivers a 30 assume each 1-cm slice of a head CT scan delivers a 30 mGy dose to the tissue in the slicemGy dose to the tissue in the slice

If the scan covers 15 cm, the dose is still the same, however If the scan covers 15 cm, the dose is still the same, however the imparted energy is approx. 15 times that of a single slice the imparted energy is approx. 15 times that of a single slice (you also have to consider scatter from adjacent slices, about (you also have to consider scatter from adjacent slices, about 10-25%)10-25%)

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The disadvantage of imparted energy is that it does not account The disadvantage of imparted energy is that it does not account for the different sensitivities of the exposed tissue to biologic for the different sensitivities of the exposed tissue to biologic damagedamage

Effective dose is used for comparing risk of stochastic effectsEffective dose is used for comparing risk of stochastic effects E (Sv) = E (Sv) = wwTT x H x HTT

has shortcomings, whas shortcomings, wTT were developed from epidemiologic were developed from epidemiologic

data and incorporate significant uncertaintiesdata and incorporate significant uncertainties

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Organ DosesOrgan Doses It is possible to estimate organ doses from a given entrance It is possible to estimate organ doses from a given entrance

skin exposure (ESE)skin exposure (ESE) Organ doses are substantially lower than skin doseOrgan doses are substantially lower than skin dose For For AP projectionsAP projections, the embryo dose will be between , the embryo dose will be between 1/31/3rdrd and and

1/41/4thth the ESE the ESE (in the direct beam) (in the direct beam) For For PA projectionsPA projections, the embryo dose will be about , the embryo dose will be about 1/61/6thth of the of the

ESEESE (in the direct beam) (in the direct beam) For For LAT projectionLAT projection, the embryo dose will be about , the embryo dose will be about 1/201/20thth of of

the ESEthe ESE (in the direct beam) (in the direct beam)

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c.f. Bushberg, et al. The Essential Physics of Medical c.f. Bushberg, et al. The Essential Physics of Medical Imaging, 2Imaging, 2ndnd ed., p. 59. ed., p. 59.

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Comparing ESE is useful for assessment of equipment performance and Comparing ESE is useful for assessment of equipment performance and calibration, when a comprehensive analysis of effective dose is unnecessarycalibration, when a comprehensive analysis of effective dose is unnecessary

c.f. Bushberg, et al. c.f. Bushberg, et al. The Essential The Essential Physics of Medical Physics of Medical Imaging, 2Imaging, 2ndnd ed., p. ed., p. 797.797.

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1. Risk1. Risk

The International Commission on Radiological Protection The International Commission on Radiological Protection (ICRP) estimates the risk of fatal cancer for exposures to adults (ICRP) estimates the risk of fatal cancer for exposures to adults of working age to be of working age to be 0.004 deaths per Sv or 0.0004 per rem0.004 deaths per Sv or 0.0004 per rem

this translates to 1 cancer death per 2,500 people receiving this translates to 1 cancer death per 2,500 people receiving an effective dose of 10 mSv (1 rem)an effective dose of 10 mSv (1 rem)

Because of the linear, no-threshold assumption used in risk Because of the linear, no-threshold assumption used in risk estimates, risk is presumed to be proportional to the estimates, risk is presumed to be proportional to the effective doseeffective dose

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1. Risk1. Risk

Risk is proportional to the effective doseRisk is proportional to the effective dose there would be a 1 in 25,000 chance that a fatal cancer there would be a 1 in 25,000 chance that a fatal cancer

would result from an effective dose of 1 mSv (0.1 rem), would result from an effective dose of 1 mSv (0.1 rem), oror

a 1 in 500 chance of a fatal cancer from an effective a 1 in 500 chance of a fatal cancer from an effective dose of 50 mSv (5 rem)dose of 50 mSv (5 rem)

The ICRP estimates the risk to be two or three times higher The ICRP estimates the risk to be two or three times higher for infants and children and substantially lower for adults for infants and children and substantially lower for adults older than 50 years of ageolder than 50 years of age

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1. Typical Absorbed and Effective doses1. Typical Absorbed and Effective doses


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1. Risks1. Risks

Procedure

Effective Dose (mSv)

Risk of Fatal Cancer

(per million)

Equivalent to Number of Cigarettes Smoked

Equivalent to Number of Highway

Miles Driven

Chest Radiograph 0.04 1.6 12 29

Skull Exam 0.1 4.0 29 71

Mammography 0.1 4.0 29 71

Thoracic Spine 1.0 40.0 292 714

Pelvis 1.1 44.0 321 786

Abdomen 1.2 48.0 350 857

CT Head 1.8 72.0 526 1286

Lumbar Spine 2.1 84.0 613 1500

Intravenous Urography 4.2 168.0 1226 3000

CT Pelvis 7.1 284.0 2073 5071

CT Abdomen 7.6 304.0 2219 5429

CT Chest 7.8 312.0 2277 5571

Barium Enema (with fluoro) 8.7 348.0 2540 6214

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1. Interventional Radiologic Procedures1. Interventional Radiologic Procedures


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2. Radiographic Procedures2. Radiographic Procedures

Geometry for measuring the output free-in-air of a radiographic system


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Geometry for measuring the output free-in-air of a radiographic system when phototiming is used


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ProceduresProcedures Eff. Dose (mSv)Eff. Dose (mSv) Equivalent no. Equivalent no. of chest x-raysof chest x-rays

Approx. period Approx. period of of background background

radiationradiation

Chest PAChest PA 0.020.02 11 3 days3 days

PelvisPelvis 0.70.7 3535 4 months4 months

AbdomenAbdomen 1.01.0 5050 6 months6 months

CT ChestCT Chest 88 400400 3.6 years3.6 years

CT Abdomen or CT Abdomen or PelvisPelvis 10-2010-20 500500 4.5 years4.5 years

3. Effective Dose Comparison with Chest PA Exam3. Effective Dose Comparison with Chest PA Exam

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QuestionQuestion

Assuming the skin entrance dose from a single slice CT study is 5 Assuming the skin entrance dose from a single slice CT study is 5 rad, the dose for a 10 slice examination would be approximately rad, the dose for a 10 slice examination would be approximately _____ rad and the imparted energy would be ____ rad (ignore _____ rad and the imparted energy would be ____ rad (ignore scatter).scatter).

A. 5, 15A. 5, 15

B. 15, 5B. 15, 5

D. 50, 5D. 50, 5

E. 5, 50E. 5, 50

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QuestionQuestion

The skin entrance exposure from a CT slice is 2.0 R. Ten The skin entrance exposure from a CT slice is 2.0 R. Ten contiguous slices are taken, then dye is injected and 10 slices are contiguous slices are taken, then dye is injected and 10 slices are repeated. The total entrance skin exposure is about _____ R.repeated. The total entrance skin exposure is about _____ R.

A. 2.0A. 2.0

B. 2.2B. 2.2

D. 5.0D. 5.0

E. 20.0E. 20.0

You have to consider scatter. 25% of 2 R = 0.5. So 2.5 per scan is You have to consider scatter. 25% of 2 R = 0.5. So 2.5 per scan is the rad exp. For two scans, 2.5*2 = 5.0the rad exp. For two scans, 2.5*2 = 5.0

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QuestionQuestion

The national average ESE for a normal 23 cm thick A/P abdomen The national average ESE for a normal 23 cm thick A/P abdomen film with a 400 speed screen-film system is:film with a 400 speed screen-film system is:

A. 13 mRA. 13 mR

B. 150 mRB. 150 mR

C. 300 mRC. 300 mR

D. 850 mRD. 850 mR

E. 3000 mRE. 3000 mR

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QuestionQuestion

Match the exposure or dose with the appropriate item:Match the exposure or dose with the appropriate item:

A. 15 mRA. 15 mR

B. 40 mRB. 40 mR

C. 5 RC. 5 R

D. 10 RD. 10 R

E. 50 mremE. 50 mrem

1. CT head scan ESE 1. CT head scan ESE

2. Lateral chest ESE 2. Lateral chest ESE

3. 10 min fluoro (thin patient)3. 10 min fluoro (thin patient)

4. Monthly limit for a pregnant technologist4. Monthly limit for a pregnant technologist

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QuestionQuestion

Match the exposure or dose with the appropriate item:Match the exposure or dose with the appropriate item:

A. 15 mRA. 15 mR

B. 40 mRB. 40 mR

C. 5 RC. 5 R

D. 10 RD. 10 R

E. 50 mremE. 50 mrem

1. CT head scan ESE – 4 to 6 R typical 1. CT head scan ESE – 4 to 6 R typical

2. Lateral chest ESE – 10-15 mR for PA. 2 to 3 times for Lateral2. Lateral chest ESE – 10-15 mR for PA. 2 to 3 times for Lateral

3. 10 min fluoro (thin patient) – 1-2 R/min for thin patient3. 10 min fluoro (thin patient) – 1-2 R/min for thin patient

4. Monthly limit for a pregnant technologist – 0.5 mSv or 50 mrem4. Monthly limit for a pregnant technologist – 0.5 mSv or 50 mrem

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Radiation Dosimetry of the Patient