TAR, TMR, extended factors, wedges (treatment dose and prescription)

what’s the ratio of an absorbed dose at a depth within the phantom at a given distance compared with the absorbed dose in free space at the same distance

tissue air ratio

relationship btw

energy

TAR

up

up

relationship btw

field size

TAR

up

up

relationship btw

TAR

depth ? past d max due to beam attenuation

down

increase

what’s the ratio of an absorbed dose at a specified depth in a phantom to the absorbed dose at d max at same distance from the radiation source

tissue maximum ratio TMR

the value of TMR is usually less than what?

1

TMR is used to set up MU ?

SAD aka isocenter setup

extended distances have SSD that are greater than what?

isocenter

when SSD is change and extended, the change of PDD is calculated by using what?

mayneord f factor

what changes the shape of the isodose curve and dose distribution within patient

wedges

The angle of the wedge is determined by what

The angle between an isodose curve and a line perpendicular to the central axis or a specific depth

(T/F) field sizes cannot be larger, than the wedge itself

T

(T/F) wedges “harden” the beam, meaning the low energy photons are absorbed by the wedge, so a lower number of photons are available for dose

T

ratio of the dose rate at a point off axis to the dose rate at a point on the central axis at the same distance from the source

off axis ratio

A change in dose rate is expected towards the field edge due to what

penumbra

of axis ratio can be used when what?

When the central axis is blocked by MLC, blocks, or jaws for half beam blocks

what are some factors that can change the isodose

beam energy

beam type

SSD

field weighting

beam modifiers

field size

why are higher energy beams isodose curves are more spread out than lower energy beams

because the higher energy beams are more penetrating

what shows the dose that normal tissue and tumor will receive during the treatment

dose volume histograms

if a tray is used what must be increased

MU

A ratio of a dose with a tray is placed to the dose without the tray in is found by a physicist

tray factor

if bolus brings the dose closer to the skin surface and decreases what

skin sparing

(T/F) bolus will attenuate some of the beam but does not require a factor to be addd into the calculation for MU

T

what factor shows how much of the beam is transmitted through the wedge or compensator

transmission factor

what factor compares the dose rate in a known field size to the dose rate in a reference field size

output factor

what is the output factor for the reference field size of 10×10 cm

1

for fields larger than 10×10 CM, the output factor will be ? then 1.0 due to what?

greater

increase in scatter