Week 3 - Rad Physics, Equipment & QA

attenuation equation

(fraction of x-rays that are stopped in the material) = 1 - (I/Io)

transmission equation

(fraction of x-rays that make it through the material) = I/Io

linear attenuation coefficient

(u) describes the probability that each photon in the beam will interact with the medium and lose its E, per cm of material that it passes through - this is dependent on the E of the beam and the material that is being interacted with

advantages of high E x-rays over low E-x rays

- D-max is under the skin - therefore, skin sparing - forward scatter - deeper penetration - almost equal absorption of bone and soft tissue

sources of radiation???

- beta decay - atomic nucleus emits an electron (and neutrino) - occurs in unstable nuclei that are neutron rich - neutron deficient nuclei undergo positron emission or electron capture

machine produced electrons

- have negative charge and mass - 2000x smaller than a proton - isidore curves have a lateral ballooning out effect under the surface due to scatter - as E increases - surface dose increases and PDD increases

quality of the image is based on...

- image protocols - window level selection - spatial resolution - noise

magnetron vs klystron

- klystron works for higher power machines, more expensive longer lifespan - magnetron for 6 MV machines (tomo or 6 MV linac), less expensive, shorter lifespan

thoreaus filter

- made of TIN (absorbs low E x-rays, nearest to target), - COPPER (absorbs characteristic x-rays from tin and x-ray Es below tin range), - & ALUMINUM (absorbs characteristic x-rays from copper and lowest E x-rays) remember: ACT (aluminum (lowest atomic #) - copper - tin (greatest atomic #))

sources of radionuclides

- natural - fission byproducts (Cs-137 & Sr-90) - neutron activation (I-125 & P-32) - particle accelerators

hardening the beam

- use correct filter to avoid under or overdosages - HVL of the beam is expressed in mm of Al - the lower the atomic # of the filter --> the less it will harden the beam, creating less skin sparing

Dmax 4 MV

1.0 cm

Dmax 6 MV

1.5 cm

Dmax 10 MV

2.5 cm

Dmax 15 MV

3.0 cm

Dmax 20 MV

3.5 cm

how many HVLs to reduce radiation to 3% of original intensity

5 HVLs

Dmax 25 MV

5.0 cm

range finder ("ODI") range (cm)

80-130 cm

homogeneity coefficient

= 1st HVL / 2nd HVL HC less than or equal to one

CT numbers ("hounsfield units")

CT images are represented by varying shades of gray - each pixel (2D) of an image is assigned an HU that represents the density of that tissue and is directly related to the x-ray linear attenuation coefficient for the tissue - HU are helpful for dose calc in tx planning HU values range from -1000 (air) to +1000 (bone) - water is zero

inverse square law

I2 = I1 (d1/d2)^2 the intensity of radiation in a non-absorbing medium decreases or increases as the inverse of the square of the distance (the intensity at twice the distance is one quarter of the original intensity)

isomer

Metastable state

electromagnetic decay

after alpha or beta decay, daughter is in excitable state - needs to get rid of extra energy in one of three ways: 1. nuclear de-excitation - gamma ray is emitted 2. internal conversion - electron is emitted 3. isomeric transition - nucleus remains metastable and emits gamma ray

flattening filter

after x-rays emerge from hitting the target - there are more in the center than at the edges flattening filter "flattens the beam" by absorbing more photons at the center of the beam if the linac is dual energy - there are separate flatting filters for each energy constructed of combination of metals - tungsten, steel, lead, uranium, aluminum

attenuation in mono energetic beams

all x-rays have the same E --> all will be attenuated at the same rate I = Ioe^-ut

reconstruction time

amount of time it takes to analyze and process the information to create an image

secondary collimator

are the field size jaws - X1, X2, Y1, Y2 asymmetric jaws in modern machines made of lead or depleted uranium

average energy of an MV beam

because photon beams are polyenergetic - average energy is 1/3 of the energy

components of the treatment head

bending magnet target (anode) primary collimator carousel dual ionization chamber secondary collimator field defining light range finder MLCs accessory mount

equation for wavelength, frequency and velocity

c = wavelength x frequency

x ray production

cathode (electron gun) - the negative side of the x-ray tube - consists of filament and focusing cup current (mA) is applied and heats the filament (small tungsten coil of wire), electrons are boiled off via thermionic emission - increase current = increases quantity of beam focusing cup is negatively charged and helps focus the electrons toward the anode anode (target) - positive side of the tube - receives electrons from cathode - houses the rotating tungsten target - electrons strike the target (focal spot) - photons are produced everything in a vacuum - when e strike anode, x-rays are created via Bremsstrahlung and characteristic interactions

carousel

contains the flattening filter and scattering foil

magnetron

creates and accelerates microwaves in circular motion used in machines that have a 6 MV photon max

particle interactions

directly ionizing particles with enough kinetic energy produce ionization by collision - either elastic or inelastic energy of the incident particle lost in multiple small increments if not sufficient to eject electron, then excitation of electrons

wave characteristics

electromagnetic radiation - characterized by oscillating electric and magnetic fields - wavelength and frequency are inversely related x and gamma rays have the shortest wavelengths and highest frequency

machine produced radiation

electron strikes the target of an x-ray tube or linac and interacts with the target resulting in: 1. characteristic x rays 2. bremsstrahlung emission

gamma rays

emitted from the nucleus of the radioactive element - do most of the damage through INDIRECT ACTION or reactions with WATER molecules

orthovoltage

energy range: 150-500 kV maximum dose on skin, 90% (tx range) at 2 cm 1-4 mm Cu HVL unsuitable for treating behind bone due to increased absorption and increased scattering in bone

contact therapy

energy range: 40-50 kV useful up to 2 mm depth, nearly all absorbed by 2 cm 0.5-1 mm Al filter short SSD - 2 cm or less

superficial therapy

energy range: 50-150 kV depth to 5 mm from surface typical 1-6 mm Al filter 15-20 cm SSD with applicator or cone

supervoltage

energy range: 500-1000 kV

grenz ray

energy range: <20 kV able to treat only on the surface plastic filter no longer used

field defining light

field light corresponding to the desired radiation field

scattering foil

for treating with electrons - scatters the pencil thin electron beam to obtain a flat field across the treatment area different scattering foil used for each energy

circulator

found between waveguide and klystron - a "one way street sign" prevents the back flow of microwaves into the klystron - prevents damage

klystron

generates and amplifies high power microwaves used to accelerate electrons in the accelerator guide - preferred as beam energy nears 20 MeV or greater - 10 cm in diameter and 1 m in length; cylinder shape

Ra-226

half life 1622 years alpha decay used historically, not used due long half life and toxic gas that the daughter produces

Pd-103

half life 17 days used for prostate seed implants

Au-198

half life 2.7 days used for prostate seed implants

Sr-90

half life 28.1 years beta decay used for eye treatment

Cs-137

half life 30 years gamma radiation used for LDR GYN implants

I-125

half life 60 days used for prostate seed implants

Ir-192

half life: 74 days gamma radiation used for ribbon form & HDR implants

primary collimator

helps shape the beam - sets maximum field size limit - also made of tungsten or other high Z materials

bending magnet

if accelerator structure is horizontal - need a bending magnet after electrons leave the accelerator structure they enter the bending magnet to be deflected in a loop of 270 degrees using magnetic fields

as the number of targets (electrons) increases, probability of compton scattering...

increases

electron interactions

interact with matter by collisions with other e's in the matter or via Bremsstrahlung interactions - electron paths are not straight, easily scattered due to low mass

image protocols

kVp - quality of the image, contrast high kVp = low contrast mA - quantity of the x-rays, density

components in the drive stand

klystron waveguide circulator cooling system

accelerator structure, "accelerator guide"

long, cylindrical structure with cavities - 30cm - 1 m in length, made of copper - mounted horizontally in linac due to high-e's used - longer distance needed to accelerate e's electron gun injects electrons into the accelerator structure microwaves from the klystron enter the accelerator structure to accelerate e's

x-ray beam quality

low energy x-rays in the beam contribute to unnecessary dose and scatter --> additional filtration can be added to "harden" the beam

practical range of e in tissue

mean energy / 2

80% e isodose line

mean energy / 3

90% e isodose line

mean energy / 4

dual ionization chamber

measures output of the machine - measures integrated dose, field symmetry, and dose rate shuts the machine off when the dose requested is reached called dual because there are two

target

needed to produce x-rays - anode made of tungsten if electrons are desired - target is removed from the beam path

atomic mass number

number of neutrons + protons - A

atomic number

number of protons - Z

alpha decay

parent emits daughter particle that contains 2 protons and 2 neutrons (helium) - deposits energy in a straight path in tissue - very densely ionizing - end in a BRAGG PEAK - stopped by paper or skin - damage is only done if alpha particles are inhaled or ingested

photons are attenuated when they interact by:

photoelectric compton pair production

3 types of interactions with matter

photon interactions electron interactions particle interactions

attenuation

photon interactions with matter cause the beam to be "used up" as it travels through matter expressed mathematically by LINEAR ATTENUATION COEFFICIENT

compton scattering

photon interacts with orbital electron and knocks it out of orbit - scatters photon & e at different angles varying amts of photon energy can be transferred to the e dominant interaction for photon energies between 20 keV and 25 MeV

pair production

photon interacts with the electric field of the nucleus and converts into 2 particles (photon must be 1.02 MeV initially) positron interacts with an electron and both are converted to energy in an annihilation reaction --> produces 2 photons of 0.511 MeV in opposite directions probability is directly proportional to E^2 and Z important in high E machines

photoelectric effect

photons interact with an orbital electron, knocking it out of its shell - all photon E is transferred to the electron (photon disappears) energy of the photoelectron = E(photon) - E(binding) occurs in tissue for photons up to 100 keV can be accompanied by characteristic x -rays if an inner shell electron is removed

cobalt 60

produced by bombarding co-59 in a reactor with neutrons - source is then housed in stainless steel in gantry head - emits 2 gamma energies - 1.17 MV and 1.33 MV (average of 1.25 MV) - Half life - 5.26 years - decay rate - 1% per month - disadvantage: large pneumbra

auxilary systems

pulsed power supply - found in modulator cabinet - high voltage, allows us to change the dose rate vacuum system - found in the accelerator structure, acts much like the vacuum in an x-ray tube pressure system - pressurizes waveguide with SHF, prevents electrical breakdown automatic frequency control - keeps microwave frequency in check between klystron and accelerator structure

klystron operation

pulsed power supply enters microwave amplifier tube and circles around microwave cavities; at the same time - e are formed from the cathode and meet the microwaves in the klystron both e and microwaves are accelerated - microwaves enter the waveguide & electrons enter electron beam collector and are dissipated as heat

half value layer

refers to the thickness of an absorber required to reduce the intensity of a beam to 1/2 its original strength HVL = .693/u refers to the quality or penetrating ability of the beam

electron gun (cathode)

responsible for producing electrons and injecting them into the accelerating structure - made of tungsten

bremsstrahlung radiation

results when a bombarding electron passes close to the nucleus in the target and is then accelerated

characteristic x-ray

results when a bombarding electrons ejects in an inner shell electron in the target medium

machine options

retractable beam stopper - made of steel and concrete - absorbs 99.9% of primary beam portal (MV) imager -made of amorphous silicon imaging technology, online or offline review on-baord imaging (for IGRT)

isotone

same number of Neutrons

isotope

same number of Protons

isobar

same number of nucleons

waveguide

series of hollow pipes that transports microwaves from klystron to accelerator guide - pressurized with sulfur hexafluoride (SHF)

cooling system

temperature controlled water - keeps drive stand and gantry at constant temp

E = hv

the amount of energy carried out by a photon E = energy in J h = plank's constant v = cycles/second as wavelength becomes shorter & frequency larger, the energy of the photon increases

inelastic collision

the collision causes a transfer of all kinetic energy

elastic collision

the total kinetic energy of all the particles is the same before and after the collision

2nd HVL

thickness of material to reduce beam intensity by an additional 1/2 in a mono energetic beam - 1st HVL = 2nd HVL in a heterogenous beam - 1st HVL < 2nd HVL homogeneity coefficient (HC) = 1st HVL/2nd HVL

for indirectly ionizing particles (neutrons)...

transfer E to charged particles, then charged particles deposit dose into the target material

megavoltage produced radiation

tungsten target and filament in the electron gun have a high Z - thus giving them the ability to endure the repeated byproducts of x-ray production without melting

CT scanner

uses slice-by-slice data acquisition approach to create an image - the x-ray tube and detectors rotate continuously around the pt with use of SLIP RING technology major components include the gantry, couch, computer control system

components of the gantry

works to direct the beam and rotate around iso electron gun (cathode) accelerator structure (guide) treatment head

gantry of CT scanner

x ray tube - can withstand great heat detector array - coverts radiation to light, photodiode assemblies convert light to electronic signal couch - low Z material (carbon fiber), with indexing capabilities

attenuation in a heterogeneous beam

x rays are different E's --> attenuated at different rates - lower energies absorbed easier

characteristic x-rays

x-rays created with an inner shell electron is removed