Dosimetry 1 Final Exam

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

• 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

orthovoltage

• energy range: 150-250KV

• most common energy: 250 KV

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

• energy range: 25-50 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

cyclotron (person and date, characteristics, function, uses, disadvantages)

• developed by Ernest Lawrence in late 1928

• produces neutrons via stripping

• 2 flat semicircular boxes called “dees”

• 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

orthrovoltage (AKA, dates used, pioneer, energy range, most common energy, 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

• 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

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

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

• 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

• patient support assembly

• treatment tabletop

• can support up to 450 pounds (200 kg)

• made of carbon fiber

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

• 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

• 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

What does the field light represent?

area of radiation exposure

Bragg peak

region of high dose at the end of a particles range

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

linear accelerator

• maximum field size: 40×40 cm

MLC

multi-leafed collimators

EPID

electronic portal imaging device

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

IMRT

intensity modulated radiation therapy

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

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

• typically 80-130 cm

GM counter

• Geiger-Muller Counter

• type of gas proportional counter

• most sensitive area monitor

  • can detect individual ionization events

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

Bragg-Gray cavity theory

corrects for energies above 3MV

film dosimetry

• affected by:

  • temperature

  • humidity

  • 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 (convenience)