3. Beam Geometry Part 1

Question 1

Isocentre

Answer 1

Point in space where the central axis of radiation beam intersects from any of the 360 degrees gantry angles

The central axis must continue to pass through the isocentre even if parameters change (couch, collimator angles)

Question 2

SAD

Answer 2

Source to axis distance

The distance from the source of radiation to the axis of the radiation or isocentre

Question 3

SSD

Answer 3

Source to skin distance

Distance from the source of the radiation to the skin surface of the patient

Question 4

SAD treatment technique

Answer 4

All beams used have a common focus point- isocenter

Field size is defined at isocenter

Also called isocentric technique

Question 5

SSD treatment technique

Answer 5

Positions a fixed treatment distance (100cm for linacs or 80cm for Colbalt60) on patients skin for each field

Field size is defined on the skin

Beams do not have a common point

Also called non-isocentric technique

Question 6

Divergence

Answer 6

Refers to the ‘spreading’ of a radiation beam as it exits the treatment head

Photons move in straight but different directions (diverge) from a common point

The beam area grows larger the further distance it travels

Question 7

Field size (2)

Answer 7

Geometric

Physical

Question 8

Geometric field size

Answer 8

Or collimated field size

The rectangle collimator opening projected at isocenter

The dimensions set on the collimators that determine the size of the field at isocenter

Light field measured at isocenter distance represents geometric field size

Question 9

Physical field size

Answer 9

Or dosimetric

Dimensions refer to the distance perpendicular to the beams direction of incidence that corresponds to the 50% Isodose at the beams edge

Distance between 50% dose lines within the patient

Question 10

Geometric vs physical field size

Answer 10

For SAD and SSD setup at isocentre distance, the geometric and physical sizes are the same

Because of divergence physical field size changes with distance from the source so moving the patient down increases the field size

Question 11

Symmetric field size

Answer 11

Field is symmetric about the central axis of the beam

Question 12

Asymmetric field size

Answer 12

Field is asymmetric about the central axis beams

Question 13

Magnification Factor

Answer 13

Effect of divergence

Depends on

• source
• object
• medium which image is displayed

Important for

• simulator and port films
• creating poured blocks
• creating templates for loose blocks
• creating set up fields