aka dose area product - total amount of radiation incident on patient - product of entrance Kair and area of cross-sectional beam - independent of measurement location

Chapter 4 Patient Dosimetry Flashcards by Clara Fallone

is kV and mAs enough to characterize radiation on the patient?

No, also need tube design and filtration

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what is Kair

intensity of x-ray beam

number of x-ray photons per square millimeter

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how does Kair fall off with increasing distance

IS law

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how does Kair changed with mAs and tube voltage?

proportional to mAs

supralinearly with voltage

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how does Kair change with filtration

more filtration decreases Kair

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how does adding 3 mm of Al to a 80 kV beam reduce Kair?

by 50%

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2 things needed to characterize radiation incident on patients

quality and quantity of x-ray beam

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what is radiation intensity incident on a patient requird to generate a good image?

entrance Kair

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what does entrance Kair depend on

patient thickness, composition
patient size
beam quality

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entrance Kair for skull radiograph, PA chest x-ray, and lateral lumbar spine

skull: 1 mGy
chest: 0.1 mGy
spine: 10 mGy

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what is entrance Kair rate for for fluoro on 23 cm wide patient?

10 mGy/min

depends on FOV, frame rate, selected dose level

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kerma area product

aka dose area product

total amount of radiation incident on patient
product of entrance Kair and area of cross-sectional beam
independent of measurement location

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median KAP in radiographic imaging

1 Gy cm^2

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KAP for fluoro guided GI studies and urologic procedures

20 Gy cm^2

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KAP in interventional radiology

200 Gy cm^2

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what is meant by superficial doses

doses absorbed by skin, scalp, and eye lens

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why does an entrance Kair of 1 mGy result in superficial skin dose of up to 1.5 mGy?

tissues are higher Z than air (10% higher)

- backscatter can increase superficial tissue doses by up to 40 %

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usual radiography skin dose

< 10 mGy

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usual fluoro skin dose

< 500 mGy

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usual interventional radiology skin dose

> 500 mGy

-radiation burns, epilation, cataracts are possible

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how do organ doses change with Kair?

increase in proportion to Kair

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how are organ doses affected by beam quality for a given Kair?

-increase with beam quality due to more penetration

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what is embryo dose for abdominal radiograph

1/3 Kair for AP projection
about 1 mGy

for PA and lateral projections, 1/6 and 1/20 Kair

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are embryo doses cumulative

yes

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dose rate at embryo for fluoro

1.5 mGy/min

embryo doses in abdominal/pelvic CT

25 mGy

embryo doses in chest CT

0. 1 mGy - mostly from internal scatter - lead aprons don't provide benefit

what are genetically significant doses?

dose metrics that quantify potential genetic damage

GSD in US

0. 3 mGy | - takes into account dose received by gonads and how many offspring an individual is likely to produce

downside of use of gonad shields

have to repeat exam if poorly placed

what is integral dose

total energy imparted to a patient

what is integral dose for 1 Gy imparted to 70 kg patient?

70 J | i.e. 1 J/kg * 70 kg

chest x-ray integral dose

0.002 J

abdominal radiograph integral dose

0.02 J

head CT integral dose

0.15 J

body CT integral dose

0.5 J

what is plotted on cell survival curve

plots surviving fraction as function of radiation dose

is energy absorbed by cells sufficient to predict biologic damage?

No | for example, alpha particles cause more biologic damage than x-rays

why does some radiation cause more biologic damaga than others?

for example, alpha particles result in a more concentrated pattern of energy deposition than x-rays which produce a more diffuse pattern i.e. alpha particles have higher linear energy transfer (denser pattern of energy deposition)

unit of LET

keV/um

LET of alpha particles

100 keV/um

LET of x-rays, gamma rays, beta particles

1 keV/um

radiation weighting factor

higher LET radiation have higher weighing factors | higher Wr= more biologic damage at the same dose

Wr for x-rays, gamma rays, beta particles

Wr of protons

Wr of alpha particles

Wr of neutrons

1-20 depending on energy

equivalent dose

absorved dose X radiation weighting factor lets you compare different types of radiation -expressed in Sv

radiation detriment

defined by International Commission on Radiologic Protection (ICRP) -judges relative importance of fatal cancers, non fatal cancers, and genetic effects in future generations

tissue weighting factor Wt

fractional contribution of each organ to the total detriment (uniform whole-body radiation) Wt values are age and sex averages indicators of radiosensitivity of an organ

what are remainder organs

``` adrenals gall bladder heart kidneys pancreas prostate small intestine thymus uterus/cervix ```

Wt for red bone marrow, lung, stomach, breast, remainder organs

0.12 | detriment is cancer

Wt for gonads

0.08 | detriment is hereditary

Wt for bladder, liver, esophagus, thyroid

0.04 | detriment is cancer

Wt for skin, bone surfaces, salivary glands, brian

Wt is 0.01 | detriment is cancer

what is effective dose

accounts for equivalent dose to every organ as well as each organ's relative radiosensitivity - presented in mSv - multiply equivalent dose (H) to an organ by the organ weighting factor (Wt) summed for all irradiated organs - effective dose is uniform whole body dose that results in same patient detriment

effective dose for lateral skull radiograph

0.03 mSv

effective dose for PA chest radiograph

0.015 mSv

effective dose for AP Abdomen radiograph

0.5 mSv

effective dose for head CT

1.5 mSv

effective dose for chest CT

5 mSv

effective dose for abdomen/pelvic CT

6 mSv

very low vs low vs moderate vs high effective dose ranges

very low: < 0.1 mSv low: 0.1 - 1 mSv (most radiographic exams) moderate: 1 -10 mSv (GI studies, some CT exams) highL > 10 mSv : interventional studies, some CT exams

uniquitous natural background radiation

includes cosmic, internal, and terrestrial activity | -~ 1 mSv/yr in US

effective dose from cosmic radiation

0.4 mSv/y

effective dose from internal primordial radionuclides (40K and 14C)

0.4 mSv/y

effective dose from terrestrial activity

0.3 mSv/yr

dose from transcontinental US flight

0.03 mSv

additional dose to air crews

5 mSv/yr

space travel dose

0.01 mSv/yr

what does radon emit

alpha particles progeny of radon are radioactive and attach to aerosols that are inhaled and deposited in the lungs -15% of lung cancers due to exposure to radon

average annual effective dose from radon in US

2 mSv/yr | depends on location

total yearly natural background effective dose US

3 mSv

population-averaged incidence of fatal cancer from radiation

``` 4 %/Sv cancer incidence (fatal + non fatal) is 10%/Sv ```

nominal cancer detriment

5.5 %/Sv

total radiation detriment (cancer + hereditary effects)

6 %/Sv

what does effective dose not account for?

age sex | risk is higher for newborn than in retiree

mid organ radiation vs exit for abdomen

middle radiation about 10% of entrance Kair, , exit is 1 % of entrance Kair

are absorbed organ doses (mSv) always numerically equal to organ equivalent dose (mGy)?

yes

Chapter 4 Patient Dosimetry Flashcards

(79 cards)