L 3: Clinical RT generators Flashcards
(33 cards)
What is removed for electron treatment
- Target
- Flattening filter
Continuos form of energy
Beta decay and bremmstrualong produce continous spectrum of energy. from 0-max
Bremsstrahlung production
Efficiency = 9 x 10^-10
1 mCi
37 MBq
Diagnostic X-ray tube
They have
* Rotating anodes
* Glass tube
* Metal target
* Heating cathode
Bowtie filters in CT Machine
- It absorbs the low energy x-rays
- It reduced the dose to the periphery of the patient
- It reduces scatter to periphery.
Output of X-ray tube increase with
- Tube current
- square of tube voltage
What happens if the ion chamber is replaced in a Linac?
Output can change so it needs to be monitered.
MLC transmission areas and leaks
- Through the rounded leaf end
- Through the center of the leaf (Intra-leaf leakage)
- Between the adjacent leaves (Inter-leaf leakage)
The tongue and groove pattern of the MLC s into prevent the leak in the interlock region of the adjacent leaf on the same bank.
Small MLC s are used for what?
Irregular shaped tumors at a shallow depth…
For deep seated tumors size of MLCs does not matter as there is lot of beam scattering so that precision does not matter.
Why do we need flattening filter
Higher energy electrons generate fewer X-rays angled away from the incident beam, leading to forward peaking of the beam - Horns
Filter is used to create a uniform photon beam
Accelarating waveguide
It accelerates electrons into narrow high energy beam using strong electromagnetic pulses of higher power microwaves
Klystron/Megnetron
Megnetrons: Sources of microwaves
Klystrons: Amplifies microwaves
Modulator
- It generates the high power and short duration pulses to run the electron gun and klystron
- It includes the pulse forming network and a switch controlling the modulator known as thyratron.
Diagnostic X-ray tube
needs only electric potential between cathode and anode to generate kVp X-rays
To get sharper images we need to smaller focal spot and tilt the target at an angle to decrease the apparent focal spot
X-ray output changes with
doubling the peak kilovoltge
Linac components
MU from linac head with Flattening Filter
400-600 Mu/Min
* If flattening filter is removed there will be more dose
MU with FFF
1400-2400 MU/Min
Ideal target material for x-ray/Linac
High melting point: able to withstand the heat
High Z material: increased efficiency
Geometric Penumbra
Formula
Use of FFF
- Shorter treatment time
- Less uniform PTV dose
- Increase in skin dose
Therapy Tubes
* Large focal spot
* No rotating anode
* Hooded anode
Diagnostic Tubes
* Smaller focal spot
* Rotating anode
