Divergence – the tendency for radiation rays to spread out over the edges of a field

Cobalt machine

• All calibrations are gauged at 80cm from the source

Linear Accelerators (LINAC’s)

• All calibrations are gauged at 100cm from the source

SSD – source to skin distance

• Cobalt machine = 80cm SSD
• LINAC = 100cm SSD

Gantry – the movable arm of a treatment machine.  The therapist moves the gantry to treat multiple fields around the patient.

• This limits the amount of radiation that is given to each field
• Instead of one large dose you give several small doses

SSD ® SAD

Instead of treating to SSD they improved upon that and now treat SAD (the tumor)

SAD – source axis distance

Isocenter – center of the treatment room (is where the machine treats and the gantry rotates around)

• In SAD treatment the tumor has to be at the isocenter

ODI scale -  Optical Distance Indicator

Triangulation – using 3 set points to line up a patient (lasers)

ODI scale and triangulation are both used to line up the field on the patient

Patient Separation – is the patients thickness

• Measured with Calipers
• Also CT scans can measure the patient separation

Examples :

Treat to :

Midline – treatment (tumor) is in the middle

Depth of # - treat to the number that is given D = #