1.1 Linear Accelerator Components

flow of internal components of linac (MV)

1. modular (magnetron/klystron) — pulse into waveguide (microwaves)

2. electron gun injects electrons into waveguides (electrons come from heating tungsten filaments)

3. microwaves accelerate electrons through the accelerating waveguide (@ near SOL), using steering magnets used to focus beam

   1. theres a vacuum in the waveguide to avoid contamination with other particles

4. bending magnets deflect the beam to the tungsten xray target and when electrons interact, theyre converted into photons/xrays

5. photons/xrays focused thru primary collimator to rid of scatter

6. shape the beam thru the flattening filter — to uniformly distribute photon into cone shape of beam

7. beam pass thru 2 ion chambers (dual ionization chambers) — measuring dose thats delivered

8. pass thru treatment head (jaws: form x and y field size) and multi-leaf collimator (MLC: conforms beam to unique shapes thats specific to the treatment)

where do electrons come from in a linac

produced by heating a tungsten filament (↑ heat = ↑ electrons)

what prevents overheating of the linac

machine (accelerating waveguide areas) are bathed in water to cool down and prevent overheating

gantry

 large, rotating arm-like structure that houses and moves the radiation-producing components around the patient during radiation therapy

• Rotates 360 deg.

AKA of bed

bed, table, couch

how far does the gantry rotate vs collimator vs floor

gantry + collimator = both 360 deg.

floor = 180 deg.

how does the couch move?

• move laterally. longitudinally, verticaly, rotation (along floor) (some beds allow for pitch and roll tilting ⟹ “6-degrees of freedom”)

6 degrees of freedom

bed moving laterally, longitudinally, vertically, rotation, + pitch (head/feet rise), and roll (R/L tilt)

onboard kV (CBCT) imaging + location on the linac

Onboard kV (CBCT) imaging: source panel/detector 

• Kv produces diagnostic level xrays for pictures (orthogonal pictures)

• located — on the left and right side of the gantry (hands that come out as the source and/or detector

orthogonal pictures

two 2D X-ray projection images (can be taken in AP or lateral view, 90 deg. angle from each other) — moving machine

CBCT

cone beam computed tomography

• taking pictures while rotating kV image 360deg.

MV imaging panel + location

AKA “portal imager” taking pictures of the treatment itself

• located — south of the gantry

hand pendant

handheld control device to control linac/couch/positioning

mechanical isocentre

invisible point in the room where everything is centered and everything rotates relative to this point

• collimator, gantry, bed etc all move relative to the mechanical isocentre

radiation isocentre

point in space where the radiation beam intersect when the gantry is rotated and the beam is on

isocentre

basically the same place of where the mechanical and radiation isocentre is located

CAX

central axis

SSD

source-skin distance

SAD

source-axis distance (axis = isocentre in pt) — ū ~100cm

SID

source-imager distance

TTH

tabletop height (isocenter to flat top of table)

separation

total thickness of pt’s body from entrance of radiation beam to the exit point of radiation

laser

thin beams of light that intersect at the isocentre

• acting as visual reference for the isocentre

• used to position pt and accurately align the isocentre in the body

what color are the lasers typically

red or green

where are the lasers in the linac room

• Ceiling laser: used when gantry is rotated, bisects pt into R/L planes AND superior-inferior plans  

• Sagittal laser: shine from foot of bed and splits pt into R/L planes  

• right/left laser: bisect into SUP-INF AND ANT-POST

ceiling lasers

replaces the gantry’s crosshair when its rotated in different angles (thats not zero’d)

is the linac room light or dark

Ū light (esp during cleaning, marking tattoos entry/exit), but during treatment = dim lighting to see field and lasers for setup