radiation risk and patient dosimetry

Question 1

what are the units of absorbed dose

Answer 1

Gy

Question 2

what are the units of equivalent dose

Answer 2

Sv

Question 3

eqn linking absorbed dose to equivalent dose

Answer 3

H_T=D x w_R

Question 4

what are stochastic effects

Answer 4

prob. proportional to dose, no thresh

Question 5

stochastic effect of interest

Answer 5

carcinogenesis

Question 6

why is there a reduced probability of bad effect after a certain dose

Answer 6

because of e.g. cell kill

Question 7

what are deterministic effects

Answer 7

tissue reactions
only happen above a threshold where the severity becomes increased with dose

Question 8

what does somatic effects mean

Answer 8

affect the irradiated individual

Question 9

what are two somatic effects

Answer 9

carcinogenesis and deterministic effects

Question 10

what radiation effect is there no convincing evidence for

Answer 10

chromosome damage

Question 11

what is chromosome damage mechanism

Answer 11

breakage followed by faulty repair

Question 12

what are some issues with a genetic risk assessment (3)

Answer 12

1. only gonad exposure is relevant
2. mutations may be recessive
3. mutations may be unstable

Question 13

problems estimating dose to an individual (2)

Answer 13

1. different organs and tissues have different radiosensitivies
2. doses to organs and tissues on uniformly distributed

Question 14

define absorbed dose and give the units

Answer 14

absorbed dose (D) = energy absorbed per unit mass (Gy)

Question 15

what does equivalent dose take into account

Answer 15

differing biological effects induced by the same absorbed dose from radiations with different LET

Question 16

what does effective dose take into account

Answer 16

differing radiosensitivities of tissues and organs

Question 17

what are the units of effective dose

Answer 17

sieverts

Question 18

evidence of stochastic effects

Answer 18

japanese survivors
early radiographers
uranium miners
radium dial painters

Question 19

definition of detriment

Answer 19

a measure of total harm arising form an exposure

Question 20

four components of detriment

Answer 20

1. prob. of fatal cancer
2. prob. of severe genetic effects
3. relative length of life lost
4. weighted prob. of non-fatal cancers

Question 21

average annual dose

Answer 21

2.7 mSv

Question 22

why are risk factors high for children (2)

Answer 22

greater opportunity for expression of induced effects

greater sensitivity of some forms of cancer

Question 23

why are there enhanced risks in utero

Answer 23

a small number of rapidly dividing cells may be radiosensitive

rel. small no. cells performing specific functions: possibility of abnormalities

Question 24

two types of stochastic effects

Answer 24

heritable effects (lack of data)
cancer induction

Question 25

examples of patient dose quantities (5)

Answer 25

1. surface dose 2. organ dose 3. dose area product (total energy imparted) 4. CT dose index 5. effective dose

Question 26

surface dose pros

Answer 26

easily measured easily calculated from radiographic factors

Question 27

surface dose cons (2)

Answer 27

no indication of volume irradiated non aditive if beam position changes

Question 28

how to calculate surface dose from radiographic factors

Answer 28

use ionisation chamber to measure tube output calculate surface dose = tube output x inv. square factor x backscatter factor x mAs (used for the examination)

Question 29

units for tube output in air

Answer 29

mGy/mAs

Question 30

how to calculate organ dose

Answer 30

calculate from measured/calculated surface dose if organ position known work out using the percentage depth dose and using normalised organ dose data available

Question 31

what is the percentage depth dose

Answer 31

the attenuation of the beam at the depth of the organ

Question 32

why use dose area product

Answer 32

the total energy imparted correlates reasonably well with risk

Question 33

when is DAP useful

Answer 33

when x ray beams vary in size and position

Question 34

what assumption is made for DAP

Answer 34

all the energy is absorbed

Question 35

DAP units

Answer 35

dose x area of the beam Gy cm2

Question 36

cons of direct dap measurement

Answer 36

confined to surface usually delay between measurement and readout not retrospective

Question 37

pros of indirect dap measurement

Answer 37

retrospective estimates possible not all factors may be routinely recorded calibration needed if AEC is used difficult in fluoro - use DAP meter

Question 38

how to perform CT scanner dosimetry

Answer 38

ion chamber measurements

Question 39

what are the ref dose quantities for CT called

Answer 39

CT dose index (CTDI)ho

Question 40

how to calculate CTDI

Answer 40

integrate D(x) (the dose profile across a slice) and divide by nominal slice width

Question 41

what is CTDI_w

Answer 41

weighted avg dose within the standard phantoms

Question 42

what is CTDI_vol

Answer 42

CTDI_w corrected for pitch or couch increment and the mAs used for the scan

Question 43

what is DLP

Answer 43

dose length product CTDI_vol x increment length