Measurement of Absorbed Dose Questions & Answers Ch 6

Question 1

The roentgen is not defined above what energy?

Answer 1

3 MeV

Question 2

The SI unit for absorbed dose is what unit and what is its definition?

Answer 2

Gray, Gy, 1 Gy = 1 Joule/kg = 1 J/kg

Question 3

What is the conversion between joule and ergs, and what is the definition of a rad? What SI unit is
used in place of the rad?

Answer 3

1 J = 107 ergs, 1 J/kg = 1 x 107 ergs/1,000 gm
1 J/kg = 1 x 104 ergs/gm
(1 x 104 ergs/gm-Gy) / (100 ergs/gm-rad) = 100 rads/Gy

Question 4

Why is absorbed dose used in radiation oncology rather than some other unit?

Answer 4

Because biological effect is a function of the absorbed dose

Question 5

In terms of charge and mass how is exposure defined?

Answer 5

Exposure = (charged collected in air) / (mass of air)
1 Roentgen, R = 2.58 x 10-4 C/kg

Question 6

Exposure can only be measured when what state of affairs exists in volume of matter dm?

Answer 6

Electronic Equilibrium

Question 7

What is the definition of exposure in terms of dQ, Charge, and dm, Mass?

Answer 7

X = dQ/dm

Question 8

What is the mean energy in eV required to create an ion pair in air, W/e?

Answer 8

33.97 eV per ion pair

Question 9

Define the mean energy de/dm in a mass of air using dQ, dm and W/e where dQ is the charge
liberated, dm is the mass of air, and W/e is the average energy in eV required to produce an ion pair in
air?

Answer 9

De/dm J/kg = (dQ/dm C/kg) (W/e J/C)

Question 10

Define Da the absorbed dose per unit mass of air using X the exposure.

Answer 10

Da = X x W/e

Question 11

Is the absorbed dose per unit exposure a constant for air at all energies and if so, why?

Answer 11

Yes, because W/e is a constant. This is probably not strictly true at very low photon energies
which are of little interest in radiation oncology measurements.

Question 12

. What is the absorbed dose per R for air in today’s units?

Answer 12

Da = J/kg = X (R) x 2.58 x 10-4 (C/kg)/R x 33.97 J/C
0.876 x 10-2 [(J/kg) /R] x R
1 rad = 0.01 J/kg
Da = 0.876 (rad/R) X(R)

Question 13

Given an exposure of 3 x 10-3 C/kg, what is the dose in rads to air, dose in cGy, and
dose in Gy?

Answer 13

3 x 10-3 (C/kg) / 2.58 x 10-4 C/kg-R = 11.63 R exposure
Da = 11.63 R x 0.876 rad/R = 10.19 rad or cGy
Da = 10.19 cGy x 0.01 Gy/cGy = 0.102 Gy

Question 14

What conditions must be present in order to calculate the absorbed dose in a medium
at a point from the energy fluence in Gy/m2 and the mass energy absorption coefficient in
m2/kg?

Answer 14

Charged particle or electronic equilibrium must exist

Question 15

Using the average energy to create an ion pair in air in J/C and the R in C/kg, what is
the average energy per kg of air absorbed for 1R? Under what conditions is this true?

Answer 15

2.58 x 10-4 C/kg x 33.97 J/C = 0.00876 (J/kg) /R

Question 16

In using Thimble chambers, what removes the perturbing effect of the chamber?

Answer 16

The calibration number derived by the calibration lab

Question 17

What is the purpose of the build-up cap on the thimble chamber?

Answer 17

To establish electronic equilibrium in the volume of the chamber

Question 18

In the megavoltage x-ray range, the roentgen to rad factor primarily varies with what
quantity?

Answer 18

Number of electrons per gram, primarily because of Compton effect

Question 19

The build-up cap for thimble chambers is usually made of what material and what are
its generic names?

Answer 19

Polymethylmethacrilate - Lucite

Question 20

The term exposure applies only to what radiations under what conditions?

Answer 20

Photon energies less than 3 MeV under conditions of electronic equilibrium

Question 21

What type of dosimetry method has been developed to circumvent the restrictions of
the roentgen definition for the dose in a medium?

Answer 21

Bragg-Gray cavity theory

Question 22

What is the principle advantage of the Bragg-Gray theory?

Answer 22

Methods involving the roentgen to rad conversion may not be used for photon
energies above 3 MeV where the roentgen is not defined and also cannot be used for
particle dosimetry. The Bragg-Gray theory may not be used at any photon energy and for
particles at any energy

Question 23

What is the difference between unrestricted and limited stopping power ratios?

Answer 23

Unrestricted stopping power accounts for all of the electrons that are releases into
the cavity. Limited stopping power begins at a certain electron cut-off energy which is
that electron at which the electron can just cross the cavity

Question 24

What is the cut-off energy for limited stopping power ratios? What does it mean in
physical terms related to the electrons originating in the wall of the chamber?

Answer 24

Usually 10 – 20 keV. The cut-off energy is that energy electron which can just
barely cross the cavity

Question 25

In determining general and approximate CE values, is the build-up cap used on the
chamber in the water phantom? What are the assumptions on the chamber?

Answer 25

Build-up cap is not used and chamber wall is assumed to be air equivalent for the
approximate CE. The perturbation factor and α are assumed to be 1 in the approximate
theory. Unrestricted stopping power ratios are also used in the approximate theory. The
general CE formula assumes a Co-60 build-up cap is on the chamber

Question 26

What happens to the stopping power ratio for a medium to air for photons as the
photon energy is increased?

Answer 26

The stopping power ratio decreases with increasing photon energ

Question 27

What happens to the stopping power ratio for a medium to air with increasing
incident electron energy and increasing depth in the medium?

Answer 27

The electron stopping power ratios increases with decreasing incident electron
energy and increases for increasing depth

Question 28

What happens to the electron chamber perturbation factor, pE with increasing
chamber radius and increasing electron energy?

Answer 28

It increases in both cases

Question 29

What is the definition of Ngas?

Answer 29

Dose absorbed in the gas divided by the charge produced in the chamber or dose
absorbed in cavity gas per unit charge

Question 30

As a general rule of thumb, what is the value for (μen/p)mw for most photon energies
and water to muscle?

Answer 30

0.99

Question 31

What is the final result of the absorption of ionizing radiation by an animate or
inanimate object?

Answer 31

A rise in temperature of the object. Some energy may disappear in the form of
altered electronic equilibrium traps which do not contribute to the heat rise. This is a
small fraction of the total energy absorbed.

Question 32

What is the principal reaction in a ferrous sulfate dosimeter when it is irradiated?

Answer 32

Fe2+ ions are oxidized to produce Fe3+ ions

Question 33

What are the names of the bands associated with electron energies in
Thermoluminescence dosimetry material, TLD?

Answer 33

a. conduction band
b. forbidden band
c. valence band

Question 34

Traps for electron energy exists in which on of the bands?

Answer 34

Forbidden

Question 35

How can an electron in an energy trap be released and what is the associated
phenomenon?

Answer 35

The TLD material is heated which dumps the electron from the trap with an
associated release of light radiation

Question 36

What phenomenon is quantitated to obtain a dose relationship in TLD?

Answer 36

The amount of light coming from the electron traps as they dump their electrons
during the heating is quantitated and is proportional to the original dose the material
received.

Question 37

What are the principal materials used in TLD?

Answer 37

LiF, Li2B4O7, CaF2 (doped and undoped), CaSO4

Question 38

Why is LiF used so much?

Answer 38

Its effective atomic number is close to that of tissue, thus making it an effective
dosimeter in “in vivo” measurements

Question 39

How do we determine the output of a TLD dosimeter?

Answer 39

By calibrating its light output against a known source of radiation. A curve is
constructed relating light output to the radiation exposure dose?

Question 40

Is the dose response curve of TLD linear or non-linear with increasing dose?

Answer 40

The material used most commonly are fairly linear up until about 1,000 cGy dose
and then become slightly non-linear beyond that dose. They read greater than the
absorbed dose.

Question 41

Are TLD’s energy dependent?

Answer 41

Yes; but it is well quantitated and is not a major problem in most situations.

Question 42

What would be the ideal characteristic for a TLD material?

Answer 42

Effective atomic number and density equal to tissue, linear response with dose and
equal response at all energies

Question 43

Why is it difficult to use film to perform absolute photon dosimetry?

Answer 43

The presence of silver, z=45, in the film means that the response is extremely
sensitive to photon energy because of the photoelectric effect

Question 44

How is the speed of film expressed in terms of its response to x-rays and gamma
rays?

Answer 44

The dose in rads required to reach an optical density of 1

Question 45

What is the name of the curve that relates film OD to dose and is this curve linear or
non-linear?

Answer 45

H-D or Hurter-Driffeld. The curve can be linear in some dose regions and non-
linear in others

Question 46

Consider the following methods for dose determination, ionization chambers,
calorimeters, chemical methods or ferrous sulfate, TLD, and film. Which of these
methods are absolute in that they do not require calibration to absolutely measure the
radiation dose?

Answer 46

Absolute: calorimeter and ferrous sulfate
A free air ionization chamber for certain energies below 3 MeV can also be considered an absolute measurement
Relative: most ionization chambers, TLD, and film
These methods require calibration against a known source output

Question 47

Using ionization chambers, calorimeters, and ferrous sulfate rank them in the order of
accuracy.

Answer 47

1. calorimeter – most accurate
2. ferrous sulfate
3. ionization chambers