KV beam and standard radiation and ionisation Flashcards
what are the features of a radiation beam?
- travel in straight lines
- exit dise
- affects vary dependent on the tissue
- divergent beam
- undergo attenuation (absorption and scatter)
- OAR should be avoided
- affects tissues infront and behind the volume
what is DMAX
it is the max dose which occurs at a certain depth (electron energy dependent)
what only occurs in KV
filtration
what is the primary component?
photons produced at the target, electrons are emitted from the linac. This is dependent/ influenced by the beam energy, x-ray tube design and filtration
what is the percentage of x-rays produced in kv
70%
what is the direction of CS for KV
forward, backward and lateral
which results in very high doses to the patient surface, which adds additional radiation to the beam
describe the isodose distribution for KV
- beam edges at 50%
- middle dashed line = central axis
- flat line = skin surface
- central axis is NOT divergent
- 100% = skin surface/ DMAX
- 90% = MAX energy deposited into the body
- 10% there is lateral scattering not our beam
do electrons have a higher or lower skin dose than photons
higher
what is build up?
it is when photons enter at the ss, the electrons emitted have high energy, which travel in the forward direction. As they travel deeper they deposit energy. The dose increases to its max at skin surface and just below
The photons which are emitted are added together which move deeper within the tissue, it has a slow start with minimal energy laid down
what shape fields can be used?
square, rectangle, circle, irregular
describe the use of shapes within the field
- irregular shapes experience irregular scatter patterns
- impacts the primary and secondary components of the beam
describe the use of the circle?
- most stable shape
- scatter which remains in the target is included within the dose, which can travel in all directions
- scatter can result in less dose remaining within the target, not included in the dose
- most remains inside
- higher primary proportion
describe the use of the rectangle?
if the scatter travels the width it will remain inside the volume (primary). secondary radiation is less likely to remain within the target, with more secondary photons leaving
- higher proportion of primary, less scatter
- more scatter is lost at the shorter distance
- high energy scatter will leave
- low energy scatter will remain within the field
describe the use of the square?
- it loses a high proportion of photons
- diagonal distance is greater than lateral, so photons can travel further within the square
- equidistant scatter will remain in the field so will be included within the PD
- higher proportion of secondary photons compared to primary
describe the effect of field size?
small fields have a larger proportion of primary dose compared to secondary, having an increase in primary components as more secondary photons are lost
a greater field will have a greater proportion of secondary photons
what is included within the prescribed dose?
primary and secondary
PRESCRIBED DOSE REMAINS THE SAME
what happens to the beam not in the target area
it is not included within the ABSORBED DOSE
what has a greater proportion within the prescribed dose?
primary beam
what are primary photons?
photons produced at the X-ray tube, which pass divergently through the patient
what happens to the treatment time with larger distances
it increases, with a less intense beam
what does the anode heel effect cause
a reduction in how intense beam is on the T-side, this results in the edges being less intense.
is air a good conductor or insulator?
air is a good electrical insulator, it doesn’t conduct. However once exposed to IR, which causes a release of electrons the air is then able to conduct. Therefore as more electrons are released the better conductor it becomes. The electrical current can then be measured.
describe free air ionisation?
- contains a cylinder with a known mass of air, standardised volume of air
- it is not a clinical measure
- demonstrates the principle of the ionisation chamber
- exposed to gamma/ x-rays
- more electrons are released when there is more radiation present
- the total charge is measured
describe the thimble ion chamber
- compact air chamber
- the wall is composed with an air equivalent material (low Z- aluminium)
- maintains the electron equilibrium
- number of protons is proportional to the exposure time
- electrons are created by the ionisation which moves towards the positive plate towards the air cavity
- current is measured
describe the cylindrical thimble chamber
- it is used in TBI
- build up cap is air equivalent
- placed on skin surface
- interaction with air, increases the volume of air collected
what are thimble advantages?
- accurate and precise
- recommended for beam calibration
- instant read out
- used in QA
what are thimble disadvantages?
- connection cable is required
- high supply voltage
- correction factors required for high energy beams
describe parallel plate ionisation?
- it is used for electron beam measurement, lower then 10 MeV
- surface dose measurement
- it has two collection plates with a 2mm gap, between circular electrodes
- very thin front window allows for electron penetration
- it is surrounded by a guard ring so that an electron equilibrium can be reached
describe ionisation air chamber
- collecting plated allow for electron interaction with air to increase the total charge
- electrons are attracted to the collecting plate
- magnifying the effect
- when the pd is high, it prevents the combining of the electrons
- electrons are released via ionisation
- air mass is known
- collimated beam
- electron equilibrium
what does collimetry measure?
absorbed dose
what is the field size within kv
SXR = 15-20 cm (FSD)
DXR = 50 cm (MAX)
