Physics and Beam Profile

Rontgen

Who was awarded a Nobel Prize in Physics for his discovery of xrays in 1901?

False, he believed they should belong in the public domain to benefit human kind

T/F: Rontgen profited from his discovery of xrays

Fluence

____________ is the total number of particles entering a sphere of small cross sectional area

phi = N (# of particles) / a (area)

What is the equation for fluence?

decreases

As the area increases, the fluence ________

decreases

As the number of particles ___________, the fluence decreases

kerma

___________ is an important quantity in radiological physics for indirectly ionizing radiation

Kinetic Energy Released per unit Mass in a medium at a specific point

What does the kerma acronym stand for?

absorbed dose (AD)

___________ is the total energy absorbed from indirectly or directly ionizing radiation inside a mass of material

AD=Eab/m

What is the equation for absorbed dose?

Gray (Gy)

What is the SI unit of dose?

rad

What is the old unit of dose that is still sometimes used?

1

100 rad = _____ Gy

-attenuation by matter

-attenuation due to the inverse square law

What two factors do we consider when talking about beam attenuation?

-divergence

-inverse square effect

As a beam traverses matter its intensity will decrease in what two ways?

absorption or scattering

When photons traverse matter, some re removed from the beam, either by _______________ or ____________________

intensity

When a photon is absorbed, it ceases to exist. When it is scattered, it is deflected, possibly out of the beam. Both of these mechanisms results in a loss of ____________

mass attenuation coefficient

for attenuation, the amount (density) and type (atomic composition) of matter traversed count the most

one

water has a mass attenuation coefficient of _______

False, different coefficient due to a difference in density

T/F: Ice and steam have the same coefficient as liquid water

true

T/F: Human tissue has an approximate mass attenuation coefficient to that of water

Half-Value Layer (HVL)

the thickness of a material that is necessary to reduce the intensity of a radiation beam to half its incident value

true

T/F: A beam with a higher HVL is more penetrating and is dependent on the beam energy and traversed material

shielding

We consider HVL with _________ regarding the materials used to build treatment vaults

inverse square law

Attenuation due to ________________ involves reduction in beam intensity due to beam divergence assuming radiation travels in straight lines and is not scattered or absorbed by the medium it traverses

source

Intensity is inversely proportional to the square of the distance from the ________

point, diverge

Radiation sources begin at a ________, The rays emitted draw apart or _________ with distance from the point source

I1/I2 = (D2^2/D1)^2

What is the equation for inverse square law?

indirectly

__________ ionizing radiation is a two step process in which charged particles are put into motion and can consist of the photoelectric, Compton, or pair production

directly

______________ ionizing radiation happens when charged particles cause ionization and excitation and form free radicals

-directly ionizing

-indirectly ionizing

What two categories can radiation be put into?

directly

_____________ ionizing radiation involves fast charged particles like electrons and protons; they deliver energy to matter directly by many small interactions, causing atomic excitation and ionization, leading to biological damage

indirectly

__________ ionizing radiation involves neutral particles like photons and neutrons; they first transfer energy to charged particles, then the charged particles deliver energy directly to matter, the energy transfer is indirect and the photons deliver energy to the electrons and the neutrons deliver energy mostly to the protons

more

Higher energy = _________ penetrating

-photon energy

-composition of matter

The interaction of photons with matter varies depending on:

-Compton Scattering

-Coherent or elastic scattering

-Photoelectric effect

When photons interact with electrons, there are 3 major types of interactions:

-pair production

-photonuclear reactions

When photons interact with nuclei, there are 2 major processes:

Compton Scattering

What is the most important interaction process in tissue at therapy energy?

25 keV to 10 MeV

What energy range does the Compton effect dominate in soft tissues?

bombarding photon interacts with an atomic electron, this electron then absorbs some of the photon energy and is then ejected from the atom, a photon loses some of its energy and is deflected in its path while the electron will deposit dose as it moves through material

How does the Compton effect work?

False, low energy

T/F: In the early days of RT, only high-energy (kV) superficial beams were available, limiting the dose delivered to the patient due to skin intolerance

True

T/F: With megavoltage (MV) xray beams, the relative dose to the skin is low

MV beams are more skin sparing

What is the major advantage of MV vs kV beam energies?

d-max

Beam dose initially rises with increasing depth, reaches a maximum value known as _______________, and then declines as the depth increases further

dose buildup region

The region between the surface and d-max is the __________________

before the photons are interacting with electrons, and goes down after DMAx because of scatter, attenuation, and inverse square law

What happens during the dose buildup region and after?

surface, source

For external beam of photon radiation incident upon matter, the fluence has its maximum value at the _________, and it declines with increasing depth of penetration and distance from the __________

True

T/F: Absorption of energy by indirectly ionizing photons is a two step process and that is the electrons set in motion by the photons that deliver dose

Monte Carlo method

What is considered the "gold standard" in radiation oncology for evaluating the detailed consequences of the interactions of radiation with matter; it is one way of using computer programs for simulating the interactions by following individual particles as they interact?

monoenergetic

The energy of electrons in a linac electron beam is near ______________, which is why electron beam energies are referred to in units of Megaelectron Voltage (MeV)

True

T/F: Electrons continuously lose energy in matter and the stated energy of electrons is at entrance

direct

Electron beams interact with matter via ________ ionization, the charged particles cause ionization and excitation, producing biological damage

direct

Protons interact with matter via _________ ionization, as protons traverse matter they gradually lose energy and deposit dose until they have almost come to rest, at which they lose energy precipitously