Physics Quiz 3

Radiation measurement quantities, Radiation detection & measurement, External beam radiation therapy units

SED

skin erythema dose

Exposure

Amount of ionizations in air, produced by photons

Traditional unit - roentgen

SI - C/kg

valid up to 3 MeV

Exposure formula

X=Q/m

X - exposure (total charge of ions)

Q - charge (either + or -)

m - mass

Free Air ion chamber

Standard to define and calibrate radiation measurements

absolute dosimeter

Dmax

depth of the maximum dose

where scatter in = scatter out

increase E = increase Dmax depth

Fluence

total number of particles entering a sphere of small cross sectional area

Φ = N/a

units are cm^-2, m^-2

KERMA

Kinetic energy release per unit mass in a medium

units are Gy or J/kg

Absorbed dose

amount of ionizing radiation depositied per unit mass of material

units are Gy or J/kg

kerma vs dose

kerma - energy transferred from photon entering body to electron that it knocked out

dose - the energy that the knocked out electron absorbs/deposits in tissue as it travels

Skin sparing

max dose occurs below skin surface, protecting outer skin from severe damage

Build up region

starts at skin surface and extends to dmax

(once photon hits skin, secondary electrons are produced, and more energy is deposited, hence dose increases)

Dose buildup and Skin sparing

Mean energy to produce ion pair

33.97 eV/ion pair

Roentgen to Rad conversion

converts exposure to absorbed dose

1 Roentgen = 0.876 Rad (IN AIR!!)

then must convert to find dose within another medium

exposure to absorbed dose formula

to get dose in the medium

Fomula to convert chamber reading to exposure

X = M (N_x)(C_TP)(P_ST)(P_ION)

X = exposure

M = electrometer reading

N_x = Calibration factor (given by calibration lab)

C_TP = correction for temp and pressure

P_ST = correction for stem leakage

P_ION = correction for ion recombination - 1-2% lost

Gold standard computational method in rad therapy

Monte carlo algorithm

tracks primary and all other particles created by the primary interaction

used in treatment planning systems

Downside of monte carlo calculations

large computational requirements

(time, processing power, data storage)

E relationship with ionization rate

Increase E = increase ionization rate

Ionization chamber

collects the charge

radiation knocks electrons off gas atoms, which travel through and are attracted to opposing sides (anions - and cations +)

Electrometer

measures the charge

LET (Linear energy transfer)

energy deposited per unit path length

units - keV/um

Quality factor

used to account for differences in LET between radiation types

Equivalent dose

attempt to account for biological effects of different types of radiations as they interact with tissue

units - sievert or rem

1 Sv = 100 rem

Radiation machine calibration

accurate measurement of radiation

not sensitive due to high radiation level

Survey work

detect & provide rough measure of radiation levels in environment

sensitive but not very accurate

personnel monitoring

needs to be sensitive and measure cumulative radiation exposure

In vivo patient measurements

monitor amount of radiation patients recieve during treatment

types of radiation detectors

1. Gas ionization detectors

• Ion Chambers

• Proportional Counters

• Geiger-Muller (GM) counters (survey meter)

2. Solid state detectors

• TLDs

• Film

• Diodes

• MOSFETs (Metal oxide semiconductor-field effect transistors) (in vivo dosimetry)

• Polymer gel

• Scintillation (survey meters

3. Liquid dosimeters

• Calorimeters (standardization labs)

• Chemical

phantom

material for radiation measurement that can’t be tested on patient

mimics patient scattering and absorption

2 types of phantoms

Geometric - in a simple geometrical shape that doesn’t imitate patient shape

ex. cubes, “virtual water” (slabs of epoxy resin), water tanks

Anthropomorphic - designed to mimic shape of average patient

why is the free air ion chamber impractical for field use

its size

Parts of free air ion chamber

1. Beam from Source (S) is defined/collimated by a Diaphragm and passes between two parallel plates

2. High voltage is applied between the plates so that ions are collected

3. Length (L) are the limit for collection of ions to collecting electrode

4. Guard Wires – provide a uniform electric field

5. Collecting volume - where charge is collected and measured

Gas ionization detectors

• Ion Chambers

Thimble chamber

• calibrated every 2 years

• collecting volume 0.1 to 1.0 cm³

Plane parallel chamber

• pancake chamber

Extrapolation chamber (type of plane parallel chamber)

• good for measuring surface dose

Extrapolation

using facts from a starting point to estimate a value that falls outside of given range

C_TP (Correction for temperature and pressure)

Convert F to C

What happens to particles when the temp drops

particles condense

higher remp = particles condense = more ionizations

Stem effect

radiation hits the stem/cable and creates extra ionization/charge = falsely increases reading

1-10%

Stem leakage

electrical charge leaks through the stem/cable or insulation when it shouldn’t = causes inaccurate electrometer reading

Ion recombination

loss of charge occurs when ions recombine with each other and never reach the collecting electrodes

Geiger Muller Counter

sensitive (detection) but not accurate

Annealing

heated to release residual signs as well as condition sensitivity

reset process

TLDs (Thermoluminescent dosimeters)

use lithium fluoride

• reusable

• wide dose range

  • not instaneous reading

OSL (Optically stimulated luminescence)

uses aluminum oxide

• more sensitive than TLD

• good long term stability (reusable)

  • wide dynamic range

2 part of film

Adhesive layer - connects base with emulsion layer

Emulsion - made of silver halide (silver bromide)

Optical density

measure of light attentuated by film

Pros vs cons of solid state detectors

pros - high spatial resolution, permanent record, inexpensive

cons - requires developing, strong photon energy dependence, not tissue equivalent, sensitive to light

RadioChromic Film

• no silver halide

• roughly equal to all energies

• unexposed film is colorless, turns blue

• insensitive to light

  • no processing, self developing

Diode

converts AC to DC

Diode array (grid)

wiring grid where diodes are placed at intersections

MOSFET

metal oxide semiconductor field effect transistor

measures threshold voltage which is proportional to radiation dose

Pros and cons of MOSFET

Pros - small & lightweight, immediate dose reading, reusable

Cons - limited life of 20,000 mV

Calorimeter

insulated container used to measure small amout of heat energy

absolute dosimeter for measuring absorbed dose

used to calibrate ion chamber

Electrometer

measures current from ion chambers

Teletherapy

radiation delivered with an external beam

2 types of accelerators

Linear accelerators - accelerate charged particles in straight line

Circular accelerators - microtrons, cyclotrons, synchrotrons, betatrons

Linear accelerator

speeds electrons to almost speed of light

accelerated with microwaves

e- beam can treat pts, or can hit target and produce photons

Isocenter

point in space which the gantry, collimator, and couch rotate around

SAD

Source to axis distance

100 cm

SSD

Source to skin distance

in older systems would always be 100 cm but pt would have to move between fields

now not always 100 cm (pt doesn’t have to move) (always changing with gantry angles)

Patient support assembly

treatment couch

now made of carbon fiber

6 degree of freedom - X,Y,Z, movement & roll, pitch, tilt

3 manufacturers of electron linear accelerators

Varian, Siemens, Elekta

Electron vs Photon (mono or polyenergetic)

Electron beam = monoenergetic

Photon = polyenergetic

How to find photon energy from electrom beam energy

Take 1/3 of the max

Waveguide

copper “pipe” where electrons are accelerated in LINAC

2 types: traveling wave & standing wave

held under high vacuum

Traveling wave

electrons surf traveling wave, but have to travel at same speed as electromagnetic wave

Circulator

prevents microwaves from reflecting back

(for traveling wave)

Standing wave

traveling wave moving down tube is reflected at end and travels opposite

electrons don’t surf

2 devices used to produce/handle microwaves

Magnetron & Klystron

Magnetron

standalone generator (makes mircrowaves from scratch)

take DC and directly converts into high power microwave radiofrequencies

more common in low energy LINACS

shorter lifespan, less expensive

Klystron

takes existing weak microwave signal and amplifies (has “extra step”)

require low energy microwave source (RF driver)

more common in high energy LINACS

more stable, more expensive

Power input to electron output

Treatment head

1. Xray target - photon mode only

2. Primary collimator

3. Scattering foil - electron mode only

4. Flattening filter - photon mode only

5. Monitor ion chamber

6. Field defining light

7. Movable (adjustable) collimators

8. Optical distance indicator (ODI)

Flattening Filter

high energy beams are more forward peaked so makes intensity more uniform

Reduces overall dose (quantity) but hardens beam (increases quality)

sits on a carousel

1 flattening filter per E !!!!!

Lateral horns

when the flattening filter overcompensates at surface

at less than 10 cm depth

Beam profile at different depths (flattening filter)

below 10 cm = lateral horns

at 10 cm = flat

more than 10 cm = forward peaked

FFF

flattening filter free

beam naturally has more intensity in the center

Symmetry

a pair of points equidistant from the CA must be within 2%

Flatness

all points must be within 3% at 10 cm deep across 80% of the field width

Monitor unit

amount of time to deliver 1 cGy, with 10×10 cm FS, to a specified depth (usually Dmax) at a distance 100 cm away

Monitor Ion chamber 3 purposes

provide feedback to maintain dose rate

track total dose

measure flatness & symmetry

Max field size at 100cm

40 × 40 cm

Primary vs seconday collimators

Primary = fixed and set max field size

Secondary = movable, “jaws”, transmission usually 0.5% or less

Light localizing system

projects light down on patient congruent with radiation field

between monitor ion chamber and collimators

What parts of treatment head are not in electron mode

target and flattening filter

scattering foil must be used to spread out 3 mm electron beam

What parts of treatment head are only in electron mode

Scattering foil

Electron applicator

Electron block

Applicator

“electron cone”

allows beam to be collimated down

Bend magnets

change direction of horizontal beam to vertical

either 270 or 90 degrees

High vaccum serves 2 purposes

Prevents arcing

prevents collison or electrons & air molecules

Why are LINACs usually never completely powered down

would lose vacuum, which takes a while to re-establish

Circulating cooling water - 2 purposes

carry excess heat from critical components

keep accelerating waveguide at a constant temp

Sulfur hexafluoride (SF₆)

resides in waveguide to prevent arcing

checked each morning

How is the target moved out of beam path

compressed air

checked daily

Co-60

lower dose than LINACs

always on

half life - 5.26 yrs

How much of the beam do we usually measure

80%

Penumbra

edge of field that doesn’t receive whole treatment dose

Electron vs photon penumbra

Electrons have larger penumbra due to negative charges which repel each other

3 factors penumbra is caused by

Geometric penumbra

Transmission penumbra

Scattering of photons & secondary e-

What does geometric penumbra depend on

SSD, SDD, and source size