Flashcards in Superficial Deck (53):
What are the room design requirements? (8)
- Adequate room
- console area
- interlocked door
- warning lights/sign
- machine mountings
- water supply
- storage space
What is the shielding requirements for <500kV?
- lead lined walls
- shielding doors
What is the shielding requirements for >500kV?
- concrete, high density concrete walls
What is a kilovolt?
- a unit of force equal to 1000 volts
What is a megavolt?
- a unit of force equal to 1,000,000 volts
- beams with the energy range of 4-25MV are used (linacs)
What is kilovoltage?
- the amount of electrical energy applied so that the electrons accelerate from the cathode towards the anode
What is electron voltage?
- a unit of energy equal to the energy acquired by an electron accelerated through a potential difference of 1 volt
What is 1 KeV?
- 1000 electron volts
What is 1 MeV?
- 1,000,000 electron volts
What is the conversion of electrons into x-rays in kilovoltage?
1. the electron producing X ray beams in the x ray tube originate in the heated filament (cathode)
2. these are accelerated in a vaccum towards the target (anode)
What is the effeicieny for x-ray production in the superficial and orthovoltage energy range?
- 1% or less
What happens to most of the electron kinetic energy deposited in the x-ray target?
- is transformed into heat and must be dissipated through an efficient target cooling system
What should the target material in a superificial and orthovoltage machine be?
- should have a high atomic number and high melting point
e.g. tungsten Z value = 74
What is an atomic number?
- the number of protons found in the nucleus of an atom
Why do you need to warm-up the kilovoltage machine slowly?
- heat dissipation in the target is very high and if this occurs in a cold target it will cause damage
- the tungsten will expand faster and larger than it copper surround and will crack
- morning warm-up of 20-30 minutes is needed
What information is required in the control panel?
- machine warmed up
- interlocks set
- patient ID checked (name, D.O.B., db no.)
- filter correct
- applicator correct
- exposure time set
What does a filter do?
- removes the softer photon beam components which would simply irradiate the most superficial layers and produce an undesirable rapid attenuation through those layers (HARDENS THE BEAM)
- prevents excessive photons with unwanted energies contributing to the dose (SMOOTHS THE BEAM)
- makes the beam more HOMOGENEOUS
What is the effect of filtration on the spectrum of a heterogeneous beam?
- though intensity of the beam is reduced when a filter is added, its quality (penetration) is improved as the low energy component is virtually removed from the beam
How is beam quality/penetration expressed?
- in terms of kV and Half-Value Layer (HVL)
What is the Half-Value Layer (HVL)
- is a measure of the penetration of a kv XRT beam
- the thickness of the material at which the intensity of radiation entering it is reduced by one half
- expressed in units of distance (mm)
What is a HVL 1mm Al suitable for?
- lesions 1-3mm in depth
What is a HVL 2mm Al suitable for?
- lesions 3-5mm in depth
What is a HVL 5mm Al suitable for?
- lesions 5-7mm in depth
What is a HVL 9mm Al suitable for?
- lesions 7-8mm in depth
What is a HVL 13mm Al suitable for?
- lesions 9-10mm in depth
What does increasing filtration cause?
- decrease in the number of x-ray photons, intensity
What is beam monitoring?
- tells us how much radiation is being produced
What are the 2 methods of controlling radiation output?
1. timer and backup timer
2. dose monitor
What beam monitoring is needed for <150kV (superficial)?
- a timer is sufficient
What beam monitoring is needed for >150kV (orthovoltalge)?
- need a timer and dose monitor
What are lead cutout (shields) castellated?
- ensures a reduction in the definition of the field edge on the skin
What is skin apposition?
- the applicator must be parallel to the skin
- ideally all the applicator edges should be the same distance away from the skin
What are difficulties with skin apposition?
- convex surfaces - "stand in"
- concave surfaces - "stand off"
How can stand-off/stand-in be corrected?
- if measurements are taken of actual SSD's across the area
What is the effect of SSD on depth dose?
- increased SSD causes increased effect on % DD in general due to the inverse square law
- at small SSD this effect is increased
What is the ISL?
- the intensity of radiation from a point source is inversely proportional to the distance from the source.
What is the dominant interaction process at superficial energies?
- photoelectric absorption
- photons have shallow penetration and backscatter accounts for a significant amount of dose delivery
What is the dominant interaction process at larger energies?
- comption scatter
- photons penetrate further, do not interact at shallow depths and back scatter reduces
- scatter that does occur goes in a forward direction
What is the beam characteristics at kilovoltage and why?
- As a result of the photoelectric effect
- less penetration
- more interactions at or just below skin due to back scatter (can be ask much as 50%)
- photoelectric absorption occurs, denser tissue will absorb more energy
What is backscatter factor?
- ratio of a quantity of radiation at the surface (of a patient) to the quantity of radiation at the same point but without a patient
- back scatter increases as field size increases and thus less MU needed
- more backscatter occurs from dense structures such as bone
What is photoelectric absorption?
- the higher atomic number of bone means that it absorbs more energy
- below 100kV, 3mm Al HVL, bone absorbs 4.5 times more energy than muscle
What directional movement does roll refer too?
What directional movement does yaw refer too?
What directional movement does pitch refer too?
What is grenz?
- grenz rays are produced at low kilovoltages (10-20kV) giving them a very low penetration power
- half their energy is absorbed within the first half millimeter of tissue (treat to 1mm)
- now use topical chemo or surgery to treat
What is contact/papillon technique?
- treat 1-2mm
- recommended for patient with rectal/anal tumours who are not fit enough for general anaesthesia or surgery
- cancer has to be small and superficial with no evidence of LN involvement
What is orthovoltage?
- treatment with 150-500kV
- more penetrative than superficial (2-3cm below skin)
- multuple beams used to decrease skin reaction
- not used when MV machines invented
What depth is superfical for?
- lesions within first 5mm of skin
What are indications for RT?
- patient medically unfit or refuse surgery
- cosmetic reason
- functional reason (to avoid nerve or function damage)
- patient at risk of microscopic disease or where small volume recurrence has occured
- older patients
- mutliple lesions
- patient prone to keloid scar formation
What are some features of basal cell carcinoma?
- starts with suble change to skin (small bump or flat red path)
- develop slowly
- may appear as non-healing sore
What are the three tyoes of BCC?
What are some features of squamous cell carcinoma?
- highly varibale clinical appearance
- raised scaly lump
- ulcer or reddish skin plaque
- crusted sore
- can appear as persistent small ulcer on lip
- slow growing
- can metastasise