test 9/30 Flashcards
Skin:
KvP Exposure is limited by Skin.
-Depth first 5mm.
- Higher energy Doses MV spare skin.
-Fractionations effect biological dose.
Exposure =
Amount of ionization in Air.
-Limitation 3MeV
- Mass charge per/mass of air.
- photons only
- X = Sum of charge / mass of air collected in.
X = Q/m
Exposure Units
R - roentgen.
SI - Coulomb /kg of air
-1R = 2.58x10^-4/kg of air.
Exposure steps in free air chamber
- Photons pass through Air & give off Electrons.
- The electrons ionize.
- Voltage is applied + - .
- The charge then creates a current for reading by the electrometer.
D-max & equalibrium
D-max = scatter in meets scatter out.
Higher energy = deeper D-max.
Fluence
total number of particles entering a sphere of small cross sectional area.
- How many particles passing through and area.
- Fluence is maxed out @surface & declines as depth increases.
KERMA
Kinetic-Energy-Released per unit Mass in a medium.
Unit= J/Kg. - 1j/kg=1Gy
KERMA = greatest at surface / shallow depth.
Absorbed Dose formula
D = Eab/m
(Eab = total energy absorbed in mass of material.
R Roentgen (air) > RAD (body) > REM (limits of structures)
Quantities = Absorbed dose
photons - Kerma liberate charged particles - Dose - local energy deposit.
- Most damage from indirect - KERMA = forward moving energy = hasn’t reached back scatter yet.
-kerma = different then depth dose.
dose build & skin sparing
Old machines = limited by skin dose.
MV beams = skin sparing = forward moving energy = penetrates deeper then lower energy beams.
Skin sparing
Photon beam
D-max =
max dose as a % of beam attenuation.
- scatter in scatter out.
-electron equilibrium here.
-As photons move into a medium, they set electrons in motion
Electrons then deposit dose along their tracks
- Increase energy = increased DMAX depth.
- Surface dose (before Dmax) occurs from backscatter electrons & contamination.
- increased energy = Dmax
Mean energy to produce iron pair:
33.97eV/ion pair
1 R = .876 RAD IN AIR.
Used in converting R to RAD
dose in air - rad
Fmed = (.876) x (medium / air)
convert exposure to radiation dose , once you have (X)exposure
Dose in medium formula.
Convert chamber reading to exposure;
X = M (Nx) (Ctp) (Pst) (Pion)
X -exposure.
M- electrometer reading (from clinic).
Nx - calibration reading ( from lab).
Ctp - temp pressure correction.
Pst - Stem leakage.
Pion - ion recombination.
Fmed
If you take the exposure and multiply it by the FMED, you get the dose in the medium
Temp pressure effect on the ion reading
- Cold air is more dense = higher reading.
- Higher temp = more pressure = lower reading.
Farmer chamber
- 300v
- 0.1cGy
- 1-2% loss charge w/ recombination.
Dose in free space =
Measuring in an ION chamber (w/o water) free of obstruction.
-Dose in Free Space (Dfs): dose deliver @ center of a sphere of a medium which is just large enough to have electronic equilibrium at its center
Goal of dosimetry is to compute energy deposition in matter
- Monte Carlo Algorithm –
- most accurate method
- “gold standard.
- Tracks a particle history for each interaction.
- Amount of ionization in air is dependent upon photon energy
- Increase energy = Increase ionization rate
- Ionization Chamber – collects the charge
- Electrometer - measures the charge
LET – linear energy transfer
- larger mass & charge transfer energy easier.
- Alpha deposit all energy.
Measurement instruments categorized into type
- Radiation machine calibration
- Survey work
- Personnel monitoring
- In vivo patient measurements
Survey work instruments =
- Verify tray source.
- Sensitive but not accurate. enviroment
Radiation machine calibration =
- Data comparison to annual calibration.
- Very accurate, not sensitive.
- Acceptance test = after installation.
Personal Monitoring
- track workers.
- Needs to be sensitive = measures small amounts.
- Cumulative exposure.
OSL
In VIVO patient measurements
- monitors PT during Tx.
3 Categories of radiation detectors, according to medium.
1) GAS Ionization.
2) Solid State.
3) liquid
Gas Ionization Detectors:
- Ion chambers
- proportional counters.
- GM survey meters
(they air open AIR)
Solid state detectors :
- TLD, Film, Diodes, MOSETS, polymer gel, Scintillation ( survey meter).
Liquid Dosimeters:
Calorimeters ( Standarization lab)
Chemical
(very rare to use)
Phantom types
1) Geometric : virtual (epoxy slabs), Water tank (annual calibration),
2) Anthropomorphic – designed to mimic shape of (average) patient.
Gas ionization chambers -Detectors
- How we get exposure (X)
- Measured w/ electrometer.
- Ions attracted to opposite charged electrodes.
- High temp = air expands = lower air density = lower readings.
- Cold air = dense air = higher readings.
used mainly in National Standards Labs (NIST)
- Gas Ionization Detectors Free – Air Ion Chamber.
- X-rays enter camber & ionize along beam path.
- The standard of measurement.
- Primary gold standard = sent to lab for calibration.
GAs ionizing detectors free- Cavity ion chamber
- Air cavity w/ air shell that surrounds.
- entire cavity irradiated.
- shell + cap = proper build up 5mm.
- thickness of cap and chamber wall 5mm
Provides ability to measure dose @ Dmax:
Free air cavity.
- X = M * Nx * the 3 correction factors .
- Air cavity of mass
Gas Ionization Detectors Free- The 3 types of Cavity Ion chambers
1) Thimble chambers = Farmer, mini.
2) Flat cavity chambers = plane parallel.
3) Well ion chambers
= Test radioactive sources. Brachy source calibration
Farmer Chambers
- thimble.
- exclusive photon calibration in RT.
- calibrated every 2 years.
- Inner wall serves as electrode.
- volume 0.1 to 1.0cm^3
Thimble chamber / farmer
Insulator , thimble wall, to electrometer, central electrode.
-
Plane parallel ion chamber
Pancake chamber.
- Changing depth interferes.
- Strength = measuring dose at shallow depth / at wild up region
- collection diameter 5mm - high spatial res in beam direction
Plane parallel ion chamber
Lieanear accelerator = has 2 parallel ionization chambers.
- to double check for in case of failure.
Gas free cavity - Extrapolation chamber
extrapolation = using tarting point to estimate values outside of given range.
- Good for measuring SURFACE DOSE.
extrapolating to ZERO = SURFACE dose .
Correction for temp & pressure:
Ctp=
(760/p) = (273+T/295)
22c is standard temp.
760mmHg standard pressure.
correction factor are unit-less.
F to C formula
C = (F - 32) *5/9
C to F formula
F = (C * 9/5) +32
electrodes in well guarded ion chamber:
1) central / collector -1mm AL rod.
2) Guard electrode 2-purposed: prevent leak & define vol.
3) Thimble wall - ground potential & kept same as collector electrode (300V)
4 variations within Free Air chamber
1) Air attenuation.
2) Recombination of ions - when a +&- cancel out.
3) temp, pressure, Humidity.
4) Ionization produced by scatter photons.
Free Air chamber - Stem Effect/ Stem Leakage:
- 1-10%
- Increase energy = increased stem effect.
- corrects for falsely created charges collected
Ion recombination :
A loss of charge occurs from + -
1-2% loss id charge is seen
GM counter
range of 0.01mR/h to 1.0R/h
Very sensitive not accurate
Solid State
TLD
- Used in vivo, phantom, personal.
-Crystal storage.
- heated, releases light = to RAD.
- 300-400cGy
- lithium fluoride Z 8.2.
glow curve
Annealing
Heated to release residual signs and condition sensitity.
400 at 1 hour before.
TLD pros & cons
Pro: small, reusability, wide range .001-1000cGy, tissue equivalence, no wire.
CON: not instant, only be read once
FILM
Degree of film blackening related to absorbed dose.
- PRO: spatial resolution, smaller crystals, permeant record, inexpensive.
-CON: developing, sensitive to light, strong photon energy dependance.
- OD optical density = measure of light attenuated by film.
Radiochromic film
- not based on silver halide, rather monomer.
- sensitive to all energies,, self developing.
- CON: ingestive to light - hight energy needed 10-50Gy
- expensive
Diodes solid state
- AC TO DC
- ELECTROSTATIC
- in vivo skin sensitive.
- daily QA
- instant readouts
con - connected to wire.
MOSFET
Measures threshold voltage shift which is proportional to radiation dose
- pro - reusable, instant read.
- con- limited lifetime
Liquid Dosimeters - Calorimeters
– insulated container used to measure small amount of heat energy
- direct absorbed dose measurement.
- Functions as absolute dosimeter for measuring absorbed dose.
Only found in standards laboratories.
All energy absorbed in a material by radiation appears as heat
Used to calibrate ion chambers
Liquid - Chemical
Fricke dosimeter.
- not common RT.
- Color change measured with spectrophotometer
-
Ideal Chamber Characteristics
- minimal variations in: Sensitivity or exposure calibration over wide energy range.
- minimal stem leakage.
- minimal ion recombination losses.
to convert chamber reading to exposure we use ?
X = M (Nx)(Ctp)(Pst)(Pion)
R to RAD
(X)(Fmed)
Fmed= 0.876 * medium/Air
REM conversion
RAD * QF
Teletherapy –
radiation delivered with an external beam
Two types of accelerators
- Linear accelerators:
accelerate charged particles in straight line.
- Circular accelerators:
microtrons, cyclotrons, synchrotrons & betatrons
electron linear accelerator
Accelerated with microwaves.
Electron beam can treat patients, or it can hit a target to produce photons
Collimation determines beam size
Has 2 photon energies and 5-6 electron beam energies
SAD – source to axis distance
-non-adjustableiso
- iso always 100cm
SSD – source to skin distance.
- All fields have an SSD.
- SSD changes as the gantry angle changes
Couch
- Mylar top.
- 6 degree of freedom.
- orthogonal 90 degrees.
- low z.
Electron gun:
Source of electrons.
- Beam width 3mm
- SF6 gas housed, to reduce arcing.
- primary collimator max FS 40x40.
- Secondary XY jaws.
Electron beam energy:
Photon beam energy:
electron: 6MV Mono
Photon: 0-6MeV, AVG 1/3max = 2MeV Poly
Linear Accelerators - Waveguide:
Evacuated copper pipe - electrons are accelerated.
2 types of microwaves used.
v=3000MHz pulsed radiation
Traveling Wave
has circulator: prevents microwaves from reflecting back
2 high power microwave devices ;
1) Magnetron. - produce own microwaves - lower energy linac.
2) Klystron. - Amplifies, more stable, expensive, high power, RF source needed.
Magnatron
cheaper
Own low power microwaves
Electrons exposed to mag field.
auto generates
Klystron
Higher power 8MW
requires RF driver
More stable.
more expensive
Amplifies
Power in put power output
1) micro power to waveguide in pulses.
2) power supply DC to modulator.
3) high power pulses to klystron or magnetron & electron gun.
4) Gun injects pulsed electrons to waveguide.
5) Electrons accelerate down waveguide in 3mm beam.
Modulator:
Controls pulses to microwave
- power regulator = PFN pulse forming network = sets the dose rate === helps have patient on the table shortest amout of time.
Treatment Head Function:
to shield leakage & house major components
Treatment Head components
1) X-ray target; photon mode only
2) Scattering foil; electron mode only
3) Flattening filter; photon mode only
4) Monitor ion chamber (2x)
5)Fixed (primary) & moveable (adjustable) collimators
6) Field defining light
7) Optical Distance
Indicator (ODI)
photon Beam - treatment head path :
1) Electron beam
2) X-ray target.
3) Primary.
4) Carousel.
5) Flattening.
6) Ion Chamber.
7) Secondary.
8) Mount locks
Electron Beam - Treatment head path
1) Electron beam.
2) Primary.
3) Carousel.
4) Scattering Foil.
5) Ion Chamber.
6) Secondary.
7) Mount locks.
8) Cone applicator.
Flattening Filter
as high energy beams are more forward peaked, a flattening filter which is thicker in middle attenuates more than the edges
- Makes intensity uniform across the beam
X-ray mode
transmission target used for Photon mode.
Flattening filter characteristics
mixed metal = uranium, AL, tungsten.
Attenuates the High energy photons from the forward peaked photon beam .
Reduces dose rate 4x (24Gy to 6Gy/min)
Makes beam profile more uniform.
SRT faster w/ removed
Diffrent fillter for diff energy
flat in terms of quantity
Flatness
Quantity
all points equal within 3% at 10cm depth.
- Only account for the central 80% edge penumbra
- 20% fall off from lateral sides
- past 10cm depth the profile is higher at central axis.
- Overcopensates at the surface, making lateral horns.
FFF
Flattening Filter Free
higher dose rates w/ 10x energy it is 4x higher - treat in 1/4 the time
Symmetry Vs Flatness
Symmetry – 2%
A pair of points equidistant from the CA must be within 2%
10x10FS
Flatness – 3%
Variation of dose in comparison to the CA dose (80% of field) at a depth of 10cm
MU
Monitoring Units :
Amount of time to deliver 1cGy, w/ 10x10FS to 10cm Dmax @100cm away
Monitor Ion Chamber 3 purposes
1) feedback to maintain dose rate.
2) track total dose Mu delivered.
3) Measure flatness & symmetry.
Primary collimators
fixed at max FS 40x40 @100cm
secondary collimators
movable
tungstan .5%
Aysymmetric jaw abillity
transmission penumbra avoided w/ jaws
electron mode
Target removed & scattering filter used
thin foil high Z to scatter electron beam , not to cause brems
Applicator
cone used with electron mode , can’t used w/o it .
interlocks prevent use w/o it
start at 6x6
Bend Magnets
change direction of horizontal to vertical
linac 270 instead of 90
Co 60 teletherpy unit
first particle MV therapy unit
very reliable , same decay rate
Co-60 half-life: 5.26 years; lose 1% of activity per month
Photon beam charteristic
Field Divergence – the radiation beam spreads with increased distance from the source
Annual QA measuring across central axis water tank
Penumbra – edge of the field that doesn’t receive the whole treatment dose
Geometric penumbra: non-point source
Transmission Penumbra: collimator or jaw transmission
Scattering of photons & secondary electrons
P = s (SSD + d -SDD) / SDD
Electrons always have larger / wider penumbra = they scatter more.
electron penumbra bulge and scatter laterally.
Grenz Ray therapy
20Kv
Contact Therapy
40-50Kv
2mA @2cm
Superficial therapy
50-150Kv
Orthovoltage
150-500Kv
Supervoltage
500-1000Kv
G C S O S
20,
40-50,
50-150,
150-500,
500-1000
Van de Graff
early treatment machine
Betatron
doughnut
6-40MeV
Microtron
Circular w/ linac head
tomotherapy
rotating
no flat filter no FS limitation single energy
Cyclotron
protons
particle accelerator
proton spread out Bragg peak
