Dosimetry 1 Final Exam

Grenz rays (AKA, discoverer, origin of name, energy, uses)

• AKA Bucky rays

  • Gustav Bucky 1923

• Grenz = “border” in german

  • thought to border between UV rays and X-rays

• energy of <20 KV

• effective for treating inflammatory disorders, mycosis fungoides, and herpes simplex

bottom end of klystron where everything starts

electron gun

contact therapy (energy range, common energy, typical SSD, uses, example)

• energy range: 20-50 KV

• most common energy: 50 KV

• typical SSD: 2 cm

• used for endocavitary treatment (oral, anal, GYN)

• Phillips RT-50

superficial therapy (energy range, most common energy, typical SSD range, uses, example)

• energy range: 50-150 KV

• most common energy: 100 KV

• typical SSD range: 5-25 cm

• used for superficial tumors no deeper than .5 cm, endocavitary, and intra-oral treatment

• GE Maximar 100

cancer sites effectively treated with neutron beams

• GBM

• H&N

• N/V

• salivary glands

• lungs

• prostate

• soft tissue sarcoma

cancer sites effectively treated with proton beams

• ocular melanoma

• sarcomas

• prostate

• H&N

• tumors that lie near normal sensitive structures

• ***eyes, breast, brain***

cyclotron (person and date, characteristics, method of neutron production, function, uses, disadvantages, principle of operation)

• developed by Ernest Lawrence in late 1928

• produces neutrons via stripping

• 2 flat semicircular boxes called “dees” (size = direct relationship to energy of the beam)

• accelerates protons instead of electrons

• produces proton or neutron beam rather than x-ray

• used mostly for research and producing radionuclides used in nuclear medicine

• long set-up times

• non-isocentric

• principle of operation:

  • filament boils of electrons via thermionic emission

  • electrons ionize a gas in a chamber

  • the gas emits particles, usually protons

  • protons accelerate to a target via alternating magnetic fields in a spiral fashion

orthovoltage (AKA, dates used, pioneer, energy range, most common energy, SSD, filtration, uses, example)

• AKA deep/conventional therapy

• predominantly used in the 1930’s and 1940’s

• first to use multiple field techniques

• energy range: 150-500 KV

• most common energy: 250 KV

• typical SSD: 25-50 cm

• filtration: 1-4mm Copper (Cu)

• used for:

  • tumors no deeper than 2-3 cm

  • skin cancer

  • intra-oral cancers

  • endocavitary (cervix)

• GE Maximar 250

most common filtration materials in order of increasing Z#

• aluminum

• copper

• brass

• stainless steel

• lead

bending magnet (AKA, function)

• beam transport system

• circles the pencil electron beam in a 270 degree arc

linac historical people and dates

• Dr Henry Kaplan late 1940s - initiated investigation of use of linacs for medical purposes

• first unit commissioned in 1952 in London

• first treatment in 1953 with 8 MV

• first medical unit in US commissioned in 1956 at Stanford University

• became predominant machine for external radiotherapy in 1970’s

primary interaction in tissue of MV vs KV units

MV - Compton scattering

KV - photoelectric effect

advantages of MV over KV

• skin sparing effect

• increased PDD

• sharp edge gradient

• Compton’s scattering as primary interaction

Cobalt 60 (dates used, first to, HVL, energies, half-life)

• 1951: first medical use

• first to provide skin sparing

• predominantly used in the 1950’s and 1960’s

• HVL: 1.2 cm lead

• energy: 1.17 MV & 1.33 MV

  • average: 1.25 MV

• T1/2: 5.26 years, 1.1% per month

modulator

• provides DC power pulses from main electrical supply

• energizes the klystron and electron gun

• provides tiing for all phase of x-ray production

components in treatment head of a linac

• bending magnet (AKA beam transport system)

• target

• primary collimator

• flattening filter/scattering foil (carousel)

• dual ionization chamber

• field defining light

• secondary collimators

• accessory mount/slots for wedges, blocks, etc

components in gantry of a linac

• electron gun

• accelerator waveguide/structure

• vacuum pump

• treatment/collimator head

components in gantry/drive stand of a linac

• klystron

• waveguide

• circulator

• cooling system

PSA (material and advantages)

• patient support assembly

• treatment tabletop

• made of carbon fiber (pure carbon with a resin)

  • low Z # - doesn’t attenuate very much of the beam

  • high tensile strength - can support up to 450 pounds (200 kg)

What % of klystron beam comes out as microwave energy?

35%

characteristics of KV therapy units

• short SSD’s

• Dmax at skin surface

• field size set by cones and lead cut-outs

• reflection targets (usually 30° angle)

• used primarily for superficial lesions

• not well-suited for tumors underneath bone

• start/stop by timer

• photoelectric is primary interaction in tissue (except orthovoltage)

Dmax depths of commonly used energies

• KV = 0 cm or skin surface

• 1.25 MV (Cobalt) = 0.5 cm

• 4 MV = 1.0 cm

• 6 MV = 1.5 cm

• 8 MV = 2.0 cm

• 10 MV = 2.5 cm

• 15 MV = 3.0 cm

• 18 MV = 3.5 cm

• 24-25 MV = 4.0 cm

components moved out of the way when linac is set on electron mode

• target

• flattening filter (switched to scattering foil)

• mirror

circulator

component in gantry/drivestand of linac that keeps microwave power from reflecting backward to the klystron

scattering foil (description, made of, purpose)

• thin metallic foil

• usually made of lead

• scatters pencil electron beam (3 mm diameter)

Van De Graaff generator (person and date, disadvantages)

• electrostatic accelerator

• designed by R.J. Van de Graaff in 1937 at MIT

• no ODI

• not isocentrically-mounted

betatron (person and date, energy range, function, target size)

• induction type accelerator

• developed by Kerst in 1941

• energy range: 2-40 MV

• electrons accelerated around a “racetrack/donut” alternating magnetic fields

• target diameter = 0.2 mm

advantages and disadvantages of betatron

• advantages

  • multiple energies from a single machine

  • can produce electron beams

• disadvantages

  • low dose rate (3-5 min beam on time)

  • small field sizes

  • very loud

  • non-isocentric

factors affecting penumbra

• source size - direct relationship

• SSD - direct relationship

• SDD (diaphragm) - indirect relationship

machines that can produce electron beams

• linac

• betatron

• Van De Graaff generator

microtron

• combines principles of linac and cyclotron

  • accelerates electrons to a target in a spiral fashion

• one accelerator feeds multiple rooms

least practical type of radiation for therapy

alpha radiation

Where in the klystron do electrons end up in bunches?

the center/middle cavity

allows voltage to be stepped up to MV range in a very efficient manner

resonant transformer

beam stopper AKA beam interceptor (description, purposes, disadvantages)

• device attached to linac that reduces shielding needs of the room

• serves as a counterbalance to the weight of the treatment head

• can be cumbersome and hard to work around

• each subsequent linac purchase must have a beam stopper

magnetron

• serves the same basic function as a klystron but for lower energy linacs (<10MV)

• operates as its own RF driver

• still produces microwaves at 3000 MHz

waveguide (purpose, characteristics, pressurized by gas, separated by)

• carries microwave power from Klystron or magnetron to the accelerator structure

• system of hollow, usually rectangular, pipes

• pressurized by a gas, either freon or sulphur hexaflouride (SF6), which reduces arcing

• separated from Klystron and accelerator waveguide by ceramic windows that are transparent to microwaves

klystron (characteristics, purpose, frequency, developers)

• “horsepower” for the accelerator in a linac

• sits on top of an oil tank that insulates and cools it

• generates microwaves/RF power utilized in the accelerator structure to accelerate electrons

• frequency of microwaves: 3000 MHz

• developed by Russell and Sigurd Varian

electron gun

provides and pulses electrons into the accelerator waveguide/structure

flattening filter

• makes the naturally forward-peaked x-ray beam more uniform or flat under the skin’s surface

• evenly distributes the energy of the photon beam across a specified area

• usually made of lead

Bragg peak

region at the end of a particles range where doses rapidly peak and then fall off to near zero

vacuum system in linac

• provides low pressures needed for operation of other internal components

• keeps electron gun from “burning out”

• keeps out foreign particles (air) which electrons can collide with and reduce efficiency

1st vs 2nd vs 3rd generation linacs

• 1st generation linacs

  • not isocentrically mounted

  • noisy and broke down often

• 2nd generation

  • first isocentrically mounted unit commissioned in 1961

  • not computerized

• 3rd generation

  • emergence of computerization

  • newest technologies such as asymmetric jaws, dual photon energy, MLCs, EPI

therapeutic gain

relates to having a greater effect on tumor cells than on normal cells

star formation

release of several other particles such as protons, neutrons, and alpha particles due to pion capture

VDT

video display terminal

basic components of a linac

• gantry

• drive/gantry stand

• patient support assembly (PSA)

• console

• modulator cabinet***

accelerator structure/waveguide (types, material, functions of cavities)

• can be either traveling or standing

  • traveling = “surfer” or “whipping a rope”, electrons pushed from back to front

  • standing = Coolidge’s cascading theory, “strumming a guitar string”, total energy field is a sum of the forward and backward waves

• made of copper

  • high heat and electrical conductivity

• electrons gain energy by initially going through non-uniform buncher cavities

  • bunch up electrons and accelerate them to near light speed

• uniform cavities allow them to maintain speed dan continue to gain both energy as mass (E=mc²)

penumbra

• the area at the edge of the radiation beam at which the dose rate changes rapidly as a function of distance from the beam axis

• represented by the 0-50% isodose lines on a plane as seen from BEV

• geometric penumbra: caused by diameter size (geometry) of the source; direct relationship

Cesium (Cs137) teletherapy energy

662 KV

ODI

• optical distance indicator

• SSD light

• usually a quartz-iodide light source

field light (consists of, range, represents)

• combination of mirror and light source between the chambers and secondary jaws

• typically 80-130 cm

• represents area of radiation exposure

GM counter (name, type, description, advantages, uses)

• Geiger-Muller Counter

• type of gas proportional counter

• consists of a cylindrical cathode with a fine wire stretched along the axis of the cylinder

• most sensitive area monitor

  • can detect individual ionization events

• good for measuring beta and gamma radiations but not pulsed radiations

• used for brachytherapy room surveys and preliminary studies after an accident

Bragg-Gray cavity theory

corrects for energies above 3MV

conditions that affect film dosimeters, film badges, and TLD’s

• temperature

• humidity

• barometric/mechanical pressure

nuclear reactor workers must wear a personnel monitoring device detecting what type of radiation?

neutron

characteristics of a good dosimeter

• measure independent of energy/dose rate

• linear response

• stability of calibration and precision

• small size and convenient to use

• preferably reusable

stem leakage

when a ionization chamber records ionization produced anywhere other than its sensitive volume area

normal atmospheric pressure to which chambers are calibrated

760 mmHg

electronic equilibrium

ionization loss is compensated by the ionization gained

Roentgen or “R” (date, mathematical equation, types of radiation/measurement)

• 1937

• 2.58 × 10^-4 Coulombs/KG of air

• only valid for gamma rays and x-rays below 3 MeV

• unit of measurement for ionizations in air AKA exposure/intensity

rad vs Gray

• both units of dose or energy absorbed in tissue

• rad (radiation absorbed dose)

  • 1953

  • standard unit

  • 100 ergs/gram

• Gray

  • SI unit

  • 1 joule/KG

• 1 rad = 1 cGy

biological method of dose measurement

skin erythema dose (SED)

absolute dosimeter

an instrument that can calibrate a beam of unknown intensity without having first been calibrated against another standard dosimeter and is inherently accurate

types of absolute dosimeters

• “free air” ionization chamber

• calorimeters

• chemical dosimeters

ionization chambers (characteristics, connected to, shape, accuracy, uses)

• chamber filled with gas (usually air) surrounded by a solid wall made of a material that nearly matches absorption properties of the gas (air)

• cable-connected to a device that collects and counts ions such as an electrometer

  • Keithley ionization chamber has its own electrometer

• can be any shape but most commonly cylinidarical

• accuracy: ± 1%

• all used to calibrate machines except free air (calibrates other dosimeters) and pocket dosimeter (personnel monitoring)

materials and thickness of ionization chambers

• .5 mm thick

• nylon

• graphite

• bakelight

principles of operation of ionization chambers

• x-ray interacts with atoms in the ch amber and ionizes them

• electrons attach to the + side/electrode

• ionized atoms attach to the - side

• collected ions are counted and converted into an electrical charge and then into a digital reading

recombination

• when positive and negative charges become unstuck in an ionization chamber before being able to get an accurate reading

• minimized or eliminated by setting voltages to between 100 and 300 volts

atmospheric conditions that affect accuracy of ionization chamber readings

• temperature

• barometric pressure

• humidity

types of ionization chambers

• standard free air ion chamber

• parallel plate chamber

• Thimble/Farmer chamber

• condensor chamber

  • Victoreen R-meter

  • pocket dosimeter

• extrapolation chamber

• ***pulse mode detectors*** (gas other than air)

  • gas proportional counters

    • GM counter

    • Cutie Pie

  • scintillation detectors

standard free air ion chamber

• housed at National Standardizing Laboratories (NSL’s)

• most accurate dosimeter

• absolute dosimeter

• only used to calibrate other dosimeters

• large, expensive, and not practical to own

parallel plate chambers

• sealed

• air-tight

• not affected by atmospheric conditions

Thimble chamber

• AKA Farmer chamber

  • developed in 1955 by Farmer and Baldwin

• most commonly used chamber in hospitals

• cable connected to an electrometer

condenser chambers (characteristics, examples, disadvantages)

• thimble chamber directly hooked up to a condenser rather than cable-connected to an electrometer

  • chamber stores energy, then releases it into the condenser

• ex. Victoreen R-meter and pocket dosimeter

• less convenient and less accurate

extrapolation chamber (AKA, uses)

• AKA flat chambers

• extrapolates doses at various depths, especially at the surface

• used for measuring dose of electrons and beta particles

calorimeter (function, advantages, disadvantages)

• measures heat produced directly related to dose

• advantages

  • absolute dosimeter, accurate

  • durable

• disadvantages

  • can only measure large doses

  • dose rate must be high to prevent heat loss

  • bulky and difficult to transport

film dosimeter (function, advantages, disadvantages)

• radiation exposure creates optical densities on the film that can be measured with a densitometer

• advantages

  • can be used for electron beam isodose summations

  • single film can give an entire isodose summation by placing densitometer at various depths

  • provides a permanent record

  • commercial availability

• disadvantages

  • not as accurate, especially for photons/x-rays

  • dependence on chemical processing

  • shelf life

chemical dosimeter (function, example, advantages/disadvantages)

• dose is determined from chemical changes in a medium with irradiation

• ex. Fricke dosimeter

  • made from ferrous sulfite

  • works by oxidizing (rusting) iron compounds and the amount of rust is proportional to dose

• advantages

  • absolute

  • liquid dosimeters are more tissue equivalent

• disadvantages

  • lack of commercial availability

  • short shelf life and contamination of chemicals

pulse mode detectors (types, description, uses, calibration)

• gas proportional counters

  • ionization chambers but use gases other than air

  • used for area surveys

  • typically calibrated to read mR/hr

    • GM counter

    • Cutie Pie

• scintillation detectors

dead time

time in between pulses of x-rays

Cutie Pie

• detects pulsed radiations

• good for x-ray room surveys

• looks similar to a police radar gun

scintillation detectors (function, uses)

• utilize materials that emit flashes of visible light or “scintillate”

• photomultiplier tube (PM tube) picks up flashes of light

• used as nuclear medicine scanner (gamma camera)

solid state detectors/diode devices (function, example, advantages, disadvantages)

• utilize diodes instead of ion chambers to give a relative measurement of current ot dose

• ex. Monitrex

• advantages

  • quick

  • easy

  • good for quick spot checks

• disadvantages

  • not as accurate

  • not able to measure electron beams

Wilson cloud chamber

• can detect several types of radiations including neutrons (Rascal is another neutron detector)

• can be used for calibration but more commonly used for experiments performed with extremely high energy linacs

types of personnel monitoring devices

• film badges

• pocket dosimeter

• TLD

• OSL

film badge (characteristics, function, types of radiation measured, dose range, accuracy, worn at/for, disadvantages)

• most commonly used personnel monitoring device

• dental film with lead foil to absorb backscatter

• varying thicknesses and materials are used to measure the energy of radiation exposed to

• can measure x-rays, gamma, beta, electrons, and fast neutrons

• dose range: 10 milliREM (MR) to 2,000 REM

• accuracy: ±20%

• worn at the level of the collar typically for at least 4 weeks

• disadvantages:

  • length of time between readings

  • not as accurate

  • can be affected by atmospheric conditions

TEDE

• total effective dose equivalent

• sum of deep and shallow doses measured by film badge

specific types of film badges worn by different workers

• ring badge

  • brachytherapy works

  • nuclear medicine workers

• neutron badge

  • nuclear reactor workers

  • cyclotron workers

pocket dosimeter (type, advantages, disadvantages)

• type of condenser/ionization chamber

• advantages

  • immediate readings

  • can be cost effective over time

• disadvantages

  • accuracy depends on avoiding high humidity and mechanical shock

  • calibration problems

TLD (name, uses, advantages, disadvantages, crystals used)

• thermoluminescent dosimeter

• uses

  • personnel monitoring

  • machine dosimetry/calibration

  • TSI/TBI patients ot verify treatment dose

  • long term area survey functions (e.g., brachytherapy suite)

• advantages

  • reusable

  • economical

  • small

  • accurate (±5%)

• disadvantages

  • storage instability

  • fading

  • TLD readers can be unstable

• crystals used

  • lithium fluoride (most common)

  • lithium borate

  • calcium flouride

principle of operation of TLD

• based on imperfections in crystal lattice structures and their ability to capture electrons released by ionising radiation

• When the crystal is exposed to ionising radiation, electrons may be liberated from the structure. These electrons are 'trapped' by the first impurity (magnesium). The electron may remain trapped for a long period of time (years). The number of electrons trapped in this way is proportional to the amount of ionising radiation absorbed by the crystal.

• When the crystal is heated, the electrons are freed from the impurity but recaptured by the second impurity (titanium). As they are captured by the titanium, they release their excess energy as a light photon. This light photon is captured by a photoamplifying tube, which is able to amplify the energy in the light photon to readable levels (in an electrometer).

OSL (function, sensitivity, accuracy)

• optically stimulated luminescence

• use laser light in a “reader” to stimulate a rearrangement of electrons trapped in aluminum oxide (al2O3) when irradiated which give off light proportionate to dose

• use filters like film badges to determine radiation energy

• sensitivity: can read as low as 1 milliREM (mR)

• accuracy: ±1 mR

company that produces and reads most film badges and OSL’s

Landauer company

phantom (characteristics, materials)

• tissue equivalent material

• allows for convenient placement of dosimeters nearly anywhere in the phantom

• materials

  • inner filling: most commonly water

  • outer casing: water equivalent

    • lucite

    • lexan

    • perspex

    • plexiglass

At what energy do ionization chambers become ineffective without an added correction factor?

3 MV

stereo film technique AKA stereo shift technique

filming technique wherein the film and patient remain stationary while the tube rotates (films taken at 15 degree gantry intervals)

positive vs negative contrast

• positive

  • appears opaque or white on film

• negative

  • appears lucent or black/dark on film

  • ex. any gas including air

positive contrast agents (uses, contraindications, possible side effects, types)

• barium

  • used for imaging the bowels, esophagus, and stomach (GI tract)

  • contraindicated if possible bowel perforation or obstruction

    • gastrograffin used as an alternative

  • not water soluble

  • rectum = warm water, oral = cold water

• iodine contrast agents (ex. Conray)

  • used for kidney and bladder localization

  • requires kidney and liver evaluation

  • can produce a warm metallic taste in mouth when injected

  • can cause anaphylactic reaction

• solid metallic markers such as BB’s, solder wire, or coins

purpose of split/half beam block (asymmetric jaws) for breast fields

reduces divergence from the tangents into the lungs