Photon Beams, Dose, and Kerma

Question 1

What quantity is related to the probability of interaction of the photon?

Answer 1

mass attenuation coefficient

Question 2

Write down the exponential attenuation equation using I as the intensity transmitted through some thickness x, if Io is the incident intensity.

linear attenuation coefficient = u or k

Answer 2

Question 3

Write the equation for HVL (thickness of material after which the beam intensity reduced to half) =

Answer 3

HVL = ln2/u

or

HVL = 0.693/u

Question 4

Write the equation for TVL (thickness of material after which the beam intensity reduced to the tenth) =

Answer 4

TVL = ln 10 / u

Question 5

Compton scattering occurs from the

Answer 5

electrons in the material. Therefore, materials with more electrons per unit gram produce more Compton scattering.

Question 6

What is the correlation between the number of electrons per gram and atomic number, Z?

Answer 6

The number of electrons per gram decrease with increasing atomic number, Z

Question 7

In high Z materials such as bone and metals, what is the chance that Compton scattering will happen?

Answer 7

Less likely because in higher Z elements there are more neutron rich which means that they have relatively fewer protons and fewer electrons

Question 8

Beam hardening results in a change in

Answer 8

the HVL as the beam penetrates through material.

Question 9

As the spectrum of the beam hardens, what happens to the average energy and the mass attenuation coefficient?

Answer 9

The average energy shifts to a higher energy and the average mass attenuation coefficient is reduced, since the mass attenuation coefficient is smaller at higher energy.

Question 10

The units of both dose and kerma are

Answer 10

the same, energy per unit mass

Question 11

What is the SI unit of dose and kerma?

Answer 11

J/kg = Gy

Question 12

What is Kerma?

Answer 12

Kinetic energy released when a photon enters materials

Question 13

What is Dose?

And how is it different from Kerma?

Answer 13

The dose is the energy absorbed at a particular location. It is similar to kerma, but different because the location where the energy is released (kerma) may not be the location where the energy is absorbed (dose).

This is because high-energy electrons wander some macroscopically large distance as they deposit dose.

Question 14

Describe the curve of Dose and Kerma vs. PDD (percent depth dose)

Answer 14

Question 15

Describe the relationship between Kerma and PDD.

Answer 15

Kerma is seen to be steadily decreaseing with depth. This is b/c photons are absorbed and removed as teh beam penetrates through materials.

Gradual fewere photons with depth means there is gradually lesss kerma curve with increasing depth.

Question 16

Why doesn’t the Dose curve exactly track the Kerma curve in PDD?

Answer 16

Because Kerma released at some depth, say 10 cm, is not abosrbed exactly at that depth.

Rather the electrons travel downstream and deposit their dose more distally in teh beam direction. Therefore, the dose adn kerma curves are offset, and the amount of offset depends on the range of the electrons in tissue.

Question 17

Higher energy beams result in higher energy electrons which have

Answer 17

a longer range and therefore the offset is more.

Question 18

What is the clinical significance of build up dose at superficial depth?

Answer 18

It results in skin-sparing effect which is one of the key characteristic advantages of therapy with high energy beam

Question 19

Why is the does buildup effect near the surface low?

Answer 19

B/c the photons themselves do not deposit dose and they have to first produce electrons which deposit dose.

Question 20

Define depth of maximum dose or dmax

Answer 20

The point where equilibrium is first reached and the dose is maximum. It is the depth where thd dose is reached maxmimum.

Question 21

What is the Dmax for 6MV and 12MV photon in the field size of 10 x 10 cm

Answer 21

Dmax for 6 MV is 1.62 cm and Dmax for 12 MV is 2.65 cm for field size of 10 x 10 cm.

Question 22

Describe the build up region, kerma, and absorbed dose in graph

Answer 22

Question 23

What does KERMA stand for?

Answer 23

Kinetic Energy Released per unit Mass.

It quantifies the average amount of energy transferred from indirectly ionizing radiation to directly ionizing radiation without concern as to what happens after this transfer.

Question 24

What is the definition of KERMA?

Answer 24

The KERMA is defined as the mean energy transferred from the indirectly ionizing radiation to charged particles (electrons) in the medium per unit mass dm.

Question 25

what is the unit of KERMA?

Answer 25

1 J/Kg = 1 Gy