Measurement, Dose Cal, Radiation Measurement

Question 1

What is Fluence Rate?

Answer 1

Beam intensity as a function of time.

Question 2

What is energy fluence?

Answer 2

Ratio of radiant energy passing through a sphere

Question 3

What is KERMA stand for?

Answer 3

Kinetic Energy Released in Matter

Question 4

What are the parts of KERMA?

Answer 4

*Radiative KERMA (KR)-Energy transfer via radiative photons (bremsstrahlung (g))
*Collisional KERMA (Kc)Collisions b/t photons and charged particles. Results in e- which dissipate energy from collisions, leading to ionization & excitation.

Question 5

Define KERMA

Answer 5

Kinetic energy released in the medium per unit mass.

Question 6

What is radiation equilibrium?

Answer 6

Radiation equilibrium- When the volume of radiation (photons/electrons) entering and leaving a system are the same in number, energy, and direction.

Question 7

What are the units of KERMA?

Answer 7

1Gy=1 J/kg

Question 8

What are units of exposure?

Answer 8

Roentgen (R); Coulomb per kilogram (C/kg)

Question 9

How do you convert R to C/kg?

Answer 9

1R=2.48x10-4 C/kg of air

Question 10

Define Exposure

Answer 10

“a measure of ionization produced in air by photons. (X=dQ/dm; Q-charge produced in mass of air (m)) (Charge/ Mass of air)

Question 11

What is Charged Particle Equilibrium used for?

Answer 11

Measure exposure-Ion Chamber

Question 12

Free air Ion Chambers

Answer 12

*Open system to outside air
*Big and Bulky/ delicate, not used in clinics
*Standard @ National labs to calibrate secondary/field equipment (ex thimble chamber)
*Low energy (equipment ex superficial/orthovoltage x-ray)

Question 13

What is Charged Particle Equilibrium?

Answer 13

Charged Particle Equilibrium-When the number, energy, and direction of charged particles are the same entering and leaving a measurement volume. (absorbed dose equal to collisional kerma)

Question 14

Purpose of Guard Electrons (Free Air Ion Chamber)

Answer 14

*Helps insulate collecting electrode
*reduce charge leakage to the collecting electrode
*Defines the active chamber volume

Question 15

Free Air Ion Chamber Corrections

Answer 15

i. Air Density: Temperature, pressure, and Humidity
ii. Ion recombination- charge created but not collected
iii. Air attenuation of photons
iv. Secondary chambers calibrated against fee air chambers use exposure calibration factors

Question 16

Optimal Chamber Characteristcs

Answer 16

*consistency in sensitivity
*non-dependent with direction of incident radiation
*minimal stem leakage
*minimal ion recombination
*Calibrated against primary standard

Question 17

What are the three electrodes in a well-guarded farmer chamber?

Answer 17

ii. Central electrode (collector)
iii. Thimble wall
iv. Guard electrode

USED IN THE CLINIC

Question 18

What is the role of the Guard electrode in a Farmer Chamber?

Answer 18

defines the collecting volume and prevents current leakage

Question 19

What does the Central electrode (aka collecting electrode) do?

Answer 19

Delivers current to the charge measuring device

(Bias Voltage 300 V)

Question 20

Why is a bias needed in an ion chamber?
What is the voltage?

Answer 20

Need an electric field b/t electrodes to pull the ions produced from radiation towards the electrodes. w/o the ions would recombine before reaching the electrodes. Thus reduced detection. Allows for measurement of the ions as a current.

300 V

Question 21

What is the stem effect?
How to minimize?

Answer 21

When radiating of the stem can cause arcing or false reading.

Minimize with fully guarded chamber and rectangular phantoms with stem rotated 90o

Question 22

AAPM stands for?
Purpose?

Answer 22

American Association of Physicists in Medicine

Advance patient care through education, research, and maintaining professional standards. (Task Groups)

Question 23

What does ADCL stand for?
What is it?

Answer 23

Accredited dosimetry calibration laboratory

Lab that test medical physics equipment. Sets standards.

Question 24

absorbed dose definition

Answer 24

Energy of ionizing radiation absorbed per unit mass (body).

Question 25

What are the unites for Absorbed dose?

Answer 25

Traditional -Rad
SI- Gy

100 rad= 1Gy

Question 26

What is dose equivalent?

Answer 26

biologic effect of radiation that is dependent on dose and type of radiation

Same Dose High LET (alpha) vs Low LET (gamma)

1 Sv= 1 J/kg
1rem= 10^-2 Sv

Question 27

Dose Equivalent Equation

Answer 27

H=D x Q
H-Dose Equivalent
D-dose
Q-Quality factor

Question 28

What happens with a deeper buildup region?

Answer 28

greater skin sparing

Question 28

What is relative biologic effectiveness?

Answer 28

Comparing biologic damage to a standard 250kVp.

The ability of a specific type of radiation to produce biologic damage.

Question 29

CPE stands for?

Answer 29

Charged Particle Equilibrium

Question 30

What is fmed factor?

Answer 30

Converts exposure to dose

Question 31

What are some fmed limitations?

Answer 31

relative to exposure (photons only)
up to 3MV

Question 32

What is Aeq?

Answer 32

???

Question 33

Describe Cavity Therory

Answer 33

Relates a measured dose in a cavity (ion chamber) to dose at same point in a medium w/out the cavity device.