Beam Planning 1

Question 1

isocentre

Answer 1

the point in space where the central axis of radiation beam intersects from and of the 360-degree gantry angles

Question 2

is isocentre allowed to change if collimator or bed angles change?

Answer 2

no, it must remain the same

Question 3

SAD (2)

Answer 3

source to axis distance

distance from source to isocentre

Question 4

what is SAD treatment technique (3)

Answer 4

all beams used have a common focus point, isocentre
field size defined at isocentre
aka isocentric technique

Question 5

what is a perk of using SAD technique

Answer 5

much easier to calc with POP however more complicated beam arrangements get more difficult

Question 6

SSD

Answer 6

source to skin distance

Question 7

what is SSD treatment technique (4)

Answer 7

positions a fixed treatment distance from source to skin
field size is defined on skin
beams do not have a common point
non-isocentric technique

Question 8

divergence (3)

Answer 8

refers to the spreading of a radiation beam as it exits the treatment head
photons move in straight but different directions from a common point
the beam grows larger the further the distance photons travel

Question 9

what is the calc to find divergence angle

Answer 9

tan(theta) = (1/2 field length)/distance from source

Question 10

why might we need to know the angle of divergence?

Answer 10

sometimes when we are matching fields

Question 11

what part of the machine determines the size of the treatment field?

Answer 11

collimators

Question 12

geometric field size (2 ways that mean the same)

Answer 12

rectangular collimator opening projected to isocentre

the dimensions set on the collimators that determine the size of the field at isocentre

Question 13

physical field size

Answer 13

• aka dosimetric field size
• dimensions refer to the distance perpendicular to the direction of the beam of incidence that corresponds to the 50% isodose at the edge of the beam
• distance between the 50% dose lines within the pt

Question 14

where is physical field size defined at SSD and SAD

Answer 14

SSD - defined at skin surface

SAD - defined at isocentre

Question 15

what is the dose delivered to the borders of a field immediately adjacent to the jaws and shielding?
why is this important?

Answer 15

50%

@ edge of beam field, it is only 50% this effects planning as we want a certain amount to hit the tumour volume

Question 16

in what case would physical field size and geometric field size be different

Answer 16

• full-body treatments
• field size is limited depending on how far the pt is from the source and also where the couch is
• therefore geometric field size (which is unmovable as its at isocentre) and physical field size (field on skin) are different

Question 17

symmetric field size

Answer 17

the field is symmetric about the central axis point

Question 18

asymmetric field size

Answer 18

the field is asymmetric about the central axis of the beam

Question 19

having fields that are too asymmetric might not be ideal what can we do to change that

Answer 19

shift the bed

Question 20

what is the equation for magnification factor

Answer 20

image size/object size

ssd/sad

Question 21

when might MF be needed

Answer 21

when creating templates for loose blocks
blocking edges on hard film
transparency used on linac