KV beam and standard radiation and ionisation

Question 1

what are the features of a radiation beam?

Answer 1

• 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

Question 2

what is DMAX

Answer 2

it is the max dose which occurs at a certain depth (electron energy dependent)

Question 3

what only occurs in KV

Answer 3

filtration

Question 4

what is the primary component?

Answer 4

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

Question 5

what is the percentage of x-rays produced in kv

Answer 5

70%

Question 6

what is the direction of CS for KV

Answer 6

forward, backward and lateral
which results in very high doses to the patient surface, which adds additional radiation to the beam

Question 7

describe the isodose distribution for KV

Answer 7

• 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

Question 8

do electrons have a higher or lower skin dose than photons

Answer 8

higher

Question 9

what is build up?

Answer 9

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

Question 10

what shape fields can be used?

Answer 10

square, rectangle, circle, irregular

Question 11

describe the use of shapes within the field

Answer 11

• irregular shapes experience irregular scatter patterns
• impacts the primary and secondary components of the beam

Question 12

describe the use of the circle?

Answer 12

• 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

Question 13

describe the use of the rectangle?

Answer 13

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

Question 14

describe the use of the square?

Answer 14

• 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

Question 15

describe the effect of field size?

Answer 15

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

Question 16

what is included within the prescribed dose?

Answer 16

primary and secondary

PRESCRIBED DOSE REMAINS THE SAME

Question 17

what happens to the beam not in the target area

Answer 17

it is not included within the ABSORBED DOSE

Question 18

what has a greater proportion within the prescribed dose?

Answer 18

primary beam

Question 19

what are primary photons?

Answer 19

photons produced at the X-ray tube, which pass divergently through the patient

Question 20

what happens to the treatment time with larger distances

Answer 20

it increases, with a less intense beam

Question 21

what does the anode heel effect cause

Answer 21

a reduction in how intense beam is on the T-side, this results in the edges being less intense.

Question 22

is air a good conductor or insulator?

Answer 22

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.

Question 23

describe free air ionisation?

Answer 23

• 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

Question 24

describe the thimble ion chamber

Answer 24

• 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

Question 25

describe the cylindrical thimble chamber

Answer 25

• it is used in TBI
• build up cap is air equivalent
• placed on skin surface
• interaction with air, increases the volume of air collected

Question 26

what are thimble advantages?

Answer 26

• accurate and precise
• recommended for beam calibration
• instant read out
• used in QA

Question 27

what are thimble disadvantages?

Answer 27

• connection cable is required
• high supply voltage
• correction factors required for high energy beams

Question 28

describe parallel plate ionisation?

Answer 28

• 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

Question 29

describe ionisation air chamber

Answer 29

• 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

Question 30

what does collimetry measure?

Answer 30

absorbed dose

Question 31

what is the field size within kv

Answer 31

SXR = 15-20 cm (FSD)
DXR = 50 cm (MAX)