Linacs Flashcards
target/flattening filter combos for MV linacs
For E < 15 MV, use High Z target and low Z filter (good bremstrahlung in target)
For E > 15 MV, use low Z target and low Z filter due to pair production at higher energy
Types of steering coils
position radial and transverse
angular radial and transverse
-angular radial bends electron beam towards target
-angular transverse adjusts position of beam on its way to the target
SEE DIAGRAM PG 23 OF STEREO NOTES
why does steering affect beam flatness and symmetry?
affects how electrons strike target and how resulting photon beam strikes the flattening filter
what do focusing coils do?c
correct for beam divergence, which results from a small radial component of the electric field in the accelerating waveguide, and from the repulsion among electrons in the pencil beam
what forms the epid?
- copper plate- yields compton
2.phosphor: gadolinium oxysulfide, to convert x-rays to light rays and enhance sensitivity by 10
3.photodiode with thin film transistor for readout
4.ADC (analog to digital converter)
how are pulses used in linacs?
o DC power supply provides DC power to pulsed modulator which includes pulse forming network, which produces flat topped pulses of ~5 microseconds, with ~5 ms interpulse duration. These pulses are delivered to the magnetron/klystron (Truebeams use klystron) and to the electron gun. Amplified microwaves from the klystron are then injected into the waveguide along with electrons from the electron gun.
o The C series linacs (ix, ex) modify the dose rate by dropping or including pulses, while pulse width remains fixed. On Trubeams, the pulse width is modified. In fact, for the 2.5x beam, in order to obtain adequate dose rate with such a low energy beam, the pulses must be very long; almost the same length as the pulse interval itself, such that the beam is almost continuous.
describe the EPID design
image detector unit = photodiodes (amorphous silicon) with scintillating layer above
image acuisition system reads and stores data from the IDU
Microwave frequency of linacs
3 GHz
cyber knife is X band (10GHz)
how does the linac accelerate electrons?
-the linac accelerates electrons by standing or moving EM waves
-in the standing wave design, the electrical component of the wave moves the electrons; microwave power is joined into the structure by side coupling cavities rather than through the beam aperture
-in the travelling waveguide, the electrons are moved on the accelerating wave
how to get higher energy beam?
use more accelerating waveguides
why is the couch made of carbon fiber?
reduces scatter radiation (higher image quality and better dose distribution)
why does HD120 have smaller DLG than millenium MLC
The increased radius of curvature
and leaf thickness in the HD120 MLC will cause a
decrease in the transmission through the rounded leaf
ends, resulting in a smaller DLG compared to the Millennium
MLC
what dos DLG not depend on?
-DLG measurements are independent of SSD and
depth, assuming correction for geometric magnification
of the gaps
DLG is independent of dose rate.
to measure DLG, plot dose vs gap and extrapolate to where dose = 0
millenium MLC leaf specs
40x0.5 cm inner
20x1 cm outer
40x40 field size
leaf thickness 6.7 cm
radius of curvature 8 cm
HD 120 leaf specs
32X 0.25 cm outer
28 x 0.5 cm inner
22Y X 32 X FS
6.9 cm thickness
16 cm radius of curvature
dosimetric advantages for HD 120 are likely only for tumours < 1 cm in diameter
