dosi 2: isodose charts & typical field arrangements

what are 3 photon dose distribution can be represented

depth dose chart, beam profile, isodose chart

what is the depth dose chart

shows how dose differs at different depths

what is the depth dose chart along

central axis of the beam

what is the beam profile

shows how dose differs at points away from central axis

where is the beam profile

perpendicular to beam axis

what is the isodose chart

shows %DD and beam profile

what are parameters that influence isodose curves

beam quality, source size, SSD/SDD, the penumbra effect, collimation and flattening filter, field size, wedge filters, inhomogeneity

what will not influence the shape/tilt of an isodose curve

weighting

what will weighting impact with an isodose curve

dose distribution of a multifiled plan

what is visualized through the use of isodose curves

deposition of dose within a medium

a group of isodose curves =

isodose chart

what do isodose curves represent

how the dose is deposited within the medium

what do isodose charts that are obtained with beam directed perpendicular to a 3D water phantom have

flat surface and uniform density

what is one of the most common isodose charts

surface dose display

what is one popular way to display surface dose

dose cloud

what depth is the flattening filter meant to mostly flatten the field

10cm

where are horns at

dmax

what is dose normalization

basing dose around tumor than at dmax

what are 3 things that isodose charts must be adjusted to account for effects

irregular surface topography, oblique incidence, inhomogeneities encountered in the path of the beam

why is the effect of tissue inhomogeneity lower with higher energy photon beams

higher energy is less affected bc beam is more forward focused and more penetrating

why do drs use lower energy photon beams for bone mets

more energy will be absorbed in the bone

what does CT allow us to do with tissues

outline inhomogeneities in 3D and gain info about that volume

what is another way to evaluate dose to a volume of tissue

DVH

what is probably the most popular plan evaluation tool

DVH

what does a DVH graphically summarize

3D dose data into a single curve for each region of interest

what is the on the x-axis for the DVH

absolute dose

what is on the y-axis on the DVH

format of volume

what does a DVH allow

evaluation of target volumes and normal structures on one display

when is a DVH more accurate on CT slices

when they are relatively thin

what do thin CT slices allow for in a DVH

more accurate calculation of volume

what does a DVH represent

cumulative dose

what does a DVH not point to

a hot or cold spot in a volume

what kind of beams does inhomogeneities within tissue have a greater impact on

electron beams

what is the range of electrons be increased by a factor of in lung tissue

3

dmax for electrons _ at higher beam energies

increase

the dose to the skin surface _ with higher electron energy

increases

when does dose fall off very rapidly with depth with low energy electrons

after it has reached 100%

how is the peak in high energy electrons

less sharp

where does the dose stay close to with high energy electrons

close to 100% over larger range of depth

what dose line do electron curves tend to bulge out at

less than 50%

what does it mean when an electron curve less than 50% bulges out

a larger volume is given than desired

which electron curves tend to constrict

80 and 90

what two reasons does the 80 and 90% curves tend to constrict more

small fields and when higher energy electron beams are used

what beams do the tumor margins need to be more generous

for electrons

which isodose line is the therapeutic range of electrons

90%

what do you divide by when using therapeutic range of electrons`

4

what do you divide D80 by

3

what is practical range of electrons and what do you divide it by

maximum range of a particular energy; 2

what kind of tx can be used when considering and planning two fields near the spinal cord

adjacent fields

when will there be an overlap at if two fields abut on skin surface

at depth

what kind of penumbra do photons have

sharp

what are 4 reasons of what can happen when 2 photon fields abut

minimal cold spot at skin surface, gap bt fields increase, hot spot at depth, dr balancing size and location of hot spots

what does not eliminate overlap

applying a gap

where do you want to apply the gap

where there is no tumor

why do you never want to gap electrons

bc you are tx superficially

what kind of penumbra does electrons tend to have and how does that pronounce depth

larger; more pronounced at depth

when are 3 reasons you want to use adjacent electron beams

one energy will not be right for all of the port, one beam angle will not accommodate pt contour, area to be tx is too large for a single port

what will a small gap between abutting electron ports do

reduce size of hot spot but increase size of cold spot

where do you want to avoid small gaps on electrons

avoid near junction of the tumor, scar, or critical organ

what are 4 factors that we can adjust while planning to alter isodose distribution

energy, beam modifiers, weighting, beam arrangement/parameters

what are 3 examples of beam modifiers

wedge, bolus, compensators

what are factors that influence the shape/tilt of a single isodose curve

beam modifiers, inhomogeneity, oblique incidence

what is therapeutic window

range of doses which can treat disease effectively while staying within a safe range for normal tissue

what is the TI formula

NTTD/TLD

what do we want the TI to be greater than

1

when will the number be a larger therapeutic ratio

the further the normal tissue damage is to the right of the tumor control curve

what does TI below 1 mean

the toxicity exceeds the benefits of tx