Superficial Flashcards
What are the room design requirements? (8)
- Adequate room
- console area
- interlocked door
- warning lights/sign
- shielding
- 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
- maze
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?
- the electron producing X ray beams in the x ray tube originate in the heated filament (cathode)
- 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?
- 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?
- translatinal
What directional movement does yaw refer too?
- coronal
What directional movement does pitch refer too?
- sagittal
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?
- 40-50kV
- 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?
- nodular
- infiltrating
- superficial
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
What are the treatment options for BCC/SCC?
- Mohs surgery
- standard surgical removal
- cutterage and cautery
- photodynamic therapy
- topical immunotherapy
- cryotherapy
