Dosimetry and radiation quality in fast- neutron radiation therapy
Radiation Dosimetry of the Patient Robert L. Metzger, Ph.D.
Transcript of Radiation Dosimetry of the Patient Robert L. Metzger, Ph.D.
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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1. Dosimetry1. Dosimetry
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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1. Dosimetry1. Dosimetry
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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1. Dosimetry1. Dosimetry
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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1. Dosimetry1. Dosimetry
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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1. Dosimetry1. Dosimetry
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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1. Dosimetry1. Dosimetry
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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1. Dosimetry1. Dosimetry
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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1. Dosimetry1. Dosimetry
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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1. Dosimetry1. Dosimetry
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
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. 798. ed., p. 798.
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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
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. 799.799.
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2. Radiographic Procedures2. Radiographic Procedures
Geometry for measuring the output free-in-air of a radiographic system
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. 801. ed., p. 801.
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2. Radiographic Procedures2. Radiographic Procedures
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. 802. ed., p. 802.
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2. Radiographic Procedures2. Radiographic Procedures
Geometry for measuring the output free-in-air of a radiographic system when phototiming is used
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. 804.804.
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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