Linacs

Question 1

What is the photon target in an accelerator made out of?

Answer 1

E <15 MV High Z (tungsten or tantalum)
E >15 MV low Z (aluminum or copper)

Question 2

What is the electron scattering foil made out of?

Answer 2

Dual scattering foil:
primary = high Z (tantalum)
secondary = low Z (aluminum)

Question 3

What is the on-time for a pulsed linac?

Answer 3

6-7 microseconds

Question 4

What is the deadtime for a pulsed linac?

Answer 4

1.5 ms

Question 5

What are flattening filters made out of?

Answer 5

Often low Z, like aluminum

Question 6

How much leakage does the primary collimator have?

Answer 6

~0.1%

Question 7

How much leakage is there through the jaws?

Answer 7

2% by IEC spec, in practice <0.5%

Question 8

How much leakage is there through MLCs?

Answer 8

5% per IEC spec, in practice intraleaf <1%, interleaf 1-2%

Question 9

How efficient is photon production in a linac?

Answer 9

in the MV range, ~30-95%. In the kV range, only about 1%.

Question 10

What is the typical resonating frequency of a linac accelerating waveguide?

Answer 10

3 GHz

Question 11

How are wavelength and frequency related?

Answer 11

c = lambda* f

Question 12

How does the Linac modulate its output?

Answer 12

The monitor chamber tracks output and feeds back to the gun heater or grid voltage to modulate output as needed

Question 13

How does the linac modulate symmetry?

Answer 13

the monitor chamber compares the charge collected in the various sections of the chamber and modulates the steering magnets to control symmetry (servos)

Question 14

How much can the 2 monitor chambers differ before the interlock is thrown?

Answer 14

5%

Question 15

What is flatness?

Answer 15

measure of the difference between the maximum and minimum dose over the central 80% of the beam profile

Question 16

What is symmetry?

Answer 16

measure of the maximum difference between points at equal distance from the central axis within the central 80% of the beam profile

Question 17

Where (in a phantom) are flatness and symmetry measured?

Answer 17

10 cm for photons, dmax for electrons

Question 18

Where would you find the exact equation used to calculate flatness and symmetry?

Answer 18

usually in your machine’s purchase agreement

Question 19

What are some typical flatness magnitudes?

Answer 19

3-5%

Question 20

What are some typical symmetry magnitudes?

Answer 20

0-5% (mostly 0-4%)

Question 21

What was the SAD of most Co-60 units and why?

Answer 21

80 cm due to dose-rate limitations

Question 22

What was a typical spot size for a Co-60 beam?

Answer 22

1-2 cm

Question 23

How much of the usable beam does the flattening filter absorb?

Answer 23

50-90%

Question 24

What is the SAD for helical tomotherapy?

Answer 24

85 cm

Question 25

what is the standard msr for tomotherapy?

Answer 25

5x10 cm field

Question 26

How does an EPID work?

Answer 26

Most common has:
1) copper plate (~1 mm) to attenuate beam, thereby producing electrons
2) a phosphor screen to convert electrons into scintillation photons
3) amorphous silicon panel to convert scintillation photons to electrical signal to be read out as an image

Question 27

What is a thyratron?

Answer 27

high voltage switch (switches the high voltage on and off)
It is a tube filled with an insulating gas.

Question 28

Why do linacs produce pulsed beams?

Answer 28

mainly to manage heat dissipation
The pulsed nature due to the wave acceleration leads to nanosecond-scale pulses. The pulses we care about are on micro/millisecond scale

Question 29

What is the beam current for a photon beam?

Answer 29

100 mA/pulse

Question 30

What is the beam current for an electron beam?

Answer 30

1 mA/pulse

Question 31

What are some differences between flat and FFF beams?

Answer 31

FFF are:
- softer energy spectrum (no hardening in the FF)
- more constant E spectrum across field
- higher dose rate (don’t lose usable beam in FF)
- less electron contamination (most comes from FF)
- less head scatter

Question 32

How does dmax for a FFF beam compare to a flattened beam?

Answer 32

about the same - softer beam pulls it shallower but reduced electrons from the head push it deeper