tx preparation: beam-modifying devices

when would standard hand blocks be used

palliative cases

what is a major disadvantage of a hand block

non divergent vertical edges

what is more efficient than MLC in attenuation of the beam

custom blocks

how many HVL do standard blocks need to have

5

what percent of the beam will be attenuated with a custom block

at least 95% of the beam

what are the 4 components of cerrobend

bismuth, lead, tin, cadium

what is the melting point of cerrobend

158F

what percent of the thickness of cerrobend attenuated the beam when compared to the same thickness of lead

85%

how much thicker should a cerrobend block be more than lead

20%

when are secondary electrons produced from the beam

when the photon beam interacts with materials in its path

how much should photon blocks be from the patient during treatment

15cm

why should photon blocks be 15cm from the pt

avoid electron contamination and maximize clearance around the pt

where is the lead width measured in MLCs

isocenter

which blocks will still provide the sharpest beam field edge

custom cerrobend blocks

how much leakage do you want no more transmitted through the beam

2%

how much leakage do you not want transmission between the leafs

4%

what is the goal of MLC in static mode

beam shaping

what is the goal of MLC in dynamic mode

deliver IMRT

what jaws only the dynamic capability

y jaw

what are some negatives of MLC shaping

limit field size, inter and intra leaf leakage, divergence at ends of leaf

why do we rotate the collimator with MLCs

shape field based on MLC motion leaf, get jaw in right orientation with dynamic wedge, avoid inter leaf leaks in same place

what percent of leakage between leafs

3-5%

where is primary collimator

head of gantry

what is the secondary collimation

jaws

what is the third collimation

leaves

which collimations are adjusted by user

2nd and 3rd

how is the beam shape when in primary collimation

circular

what are three ways of shielding critical structures

collimator (jaws), custom cerrobend block, MLC

where are lead shields place

directly on skin

what does wax coating do on lead shields

absorb secondary scatter

how do you find out the thickness of lead

E/2 + 1

where must cerrobend blocks be mounted on

cone

what percent of the beam needs to be attenuated with the thickness of cerrobend block

97% of the beam

what distance is needed for cerrobend blocks from the skin and why

5-10cm so scatter from block is minimal

what has allowed us to adjust for irregular surface topography and their results shifts in dose to underlying structure

IMRT

what are two reasons that compensators are used in electrons

accommodate surface irregularities and reduce depth of electron beam

what two things should be verified before compensators are added

they are placed correctly in the beam and that the desired and resulting isodose distributions agree

what material does bolus need to be equivalent too

tissue

what does bolus need to conform to

treatment surface

can bolus material be used as a tissue compensator

yes

does the addition of all tx accessories change the MUs required

yes

what are wedges used for

attenuate the beam progressively across the field

what are physical wedges made of

lead, brass, stell, or tungsten

where is the central axis on a wedge

central axis of the wedge

where is most of the physical wedge attenuate most of the beam

heel

where is no attenuation at in the dynamic wedge

toe

what is often required for dynamic wedges

collimator rotation

what is the field size limitations with dynamic wedges

40 × 30

what is the wedge angle

angle which specific isodose line is bent in relationship to central axis

what is hinge angle

angle between the central axis of two beams

what increases as hingle angle decreases

dose in overlapping areas

what isodose line is turned where it gives the wedge angle from

50%

when will you see more reason to add wedge

as the hinge angle decreases

what is the wedge angle formula

(180 - hinge)/2