Physics Flashcards
4.1
Definition of an element
Matter that cannot be decomposed
Definition of a compound
Matter that can be decomposed, a combination of 2 or more elements
Definition of an atom
Smallest particles of an element that can exist without losing chemical properties of the element
Definition of a molecule
Smallest particles of a compound that can exist without losing chemical properties of the compound e.g. a water molecule
Name the 3 subatomic particles
Proton, Neutron, Electron
Charge of a Proton
Positive
Charge of a Neutron
Neutral
Charge of an Electron
Negative
What is the atomic number
Number of protons in a nucleus
What is the mass number
Total number of protons and neutrons
What happens if you change the number of protons in an atom
The atom changes, atomic number changes
What happens if you change the number of neutrons in an atom
Atom has different characteristics (isotype)
Electrons reside in what around a nucleus
Shells
Number of electrons in each shell
K-2, L-8, M-18, N-32
How do electrons escape an atom
Gaining enough energy to overthrow the binding energy
What increases binding energy of electrons
Closer to nucleus or more protons
How does an electron move to a lower binding energy shell
Gaining energy
How does an electron move to a higher binding energy shell
Losing energy
What is electromagnetism
The force of a magnetic field on a moving charged particle
Electromagnetic radiation behaves as particles called
Photons
Moving magnetic fields and electrical fields travel in
Waves
The energy of a photon is proportional to
The frequency of the wave it is linked to
Wave frequency is
Number of crests that pass a given point in 1 second
Wavelength is
Distance between crests
High energy photons have what wavelength
Short wavelength
Low energy photons have what wavelength
Long wavelength
What does strong nuclear force do
Holds protons and neutrons together in nucleus
Subatomic particles existing in unstable arrangements are knowns as what
Radioactive materials
In decay what is made up of 2 protons and 1 neutron
Alpha particle
In decay what is made up of an electron
Beta particle
In decay what is made up of a photon
Gamma Ray
Continuous spectra results from and is dependent on
Results from Bremsstrahlung
Dependent on incoming electron energy and atomic number of target
Discrete Spectra is linear because
Spikes of particular energy intensities to materials
What is fluence
Intensity of photon beam leaving linear accelerator (through sphere)
What is attenuation
Beam intensity reduced passing through a material
What causes attenuation
Absorption (photon gives energy to material)
Scatter (photon collides and changes direction)
What is attenuation coefficient?
Loss of intensity of beam entering the material, proportional to the starting intensity of the beam and the thickness of the absorber
Half Value Layer (HVL)
Thickness of a material required to cut the intensity of a beam by half
More penetrating beams change half value layer by
Increasing it
What is the Compton effect
X-ray photon hits loosely bound outer shell electron, absorbs photon energy and is deflected, photon loses energy and is scattered.
Compton scatter changes with
1)mass density
2)electron density
3)x ray energy
4)atomic number
1)density- increased with increased density
2)electron density- increased with increased electron density
3)x ray energy- increased with lower x-ray energy
4)atomic number-no change
What is photoelectric effect
Xray or photon interacts with inner electron which is bound to shell, energy of photon transferred to electron which is ejected from shell, outer shell electron replaces which releases energy as radiation
Photoelectric effect changes with
1)mass density
2)electron density
3)x ray energy
4)atomic number
1)mass density- increase mass density- increased photoelectric effect
2)electron density - increased
3)x ray energy- higher when closer to electron binding energy
4)atomic number- higher atomic number increased photoelectric effect
What is pair production
Photon passes by nucleus which creates electron and positron, energy of at least 1.022 MeV required
Photoelectric effect changes with
1)mass density
2)electron density
3)x ray energy
4)atomic number
1)mass density- increased
2)electron density- increased
3)x ray energy- at least 10.22MeV
4)atomic number- increased
What is scattered radiation
Photon bounces off electron bound to parent atom,
What is secondary electron effect
Produced after EM interacts with matter via PE or Compton and produces secondary electrons. These secondary electrons deposit the dose in tissue via electron matter interactions
What is linear energy transfer
The rate at which energy is deposited along a particle track.
Collision interactions only
What is excitation
Electron gaining energy to move within particle to higher energy shell
What is ionisation
Electron gaining enough energy to leave atom resulting in ion pair (free electron -ve and rest of atom +ve)
What is collision loss
The process by which electrons lose energy when they interact with orbital electrons in a medium. This interaction can cause the atom to become excited or ionized
What is radiative loss
When radiation interacts with matter, electrons can be removed from atoms through a process called ionization
What is stopping power
Combination of collision loss and radiative loss. (collision with ionisation and excitation plus bremstrahlung)
What is particle range
Distance of a particle travelling through medium before stopping
What affects particle range
Starting energy and density of material
What is bremstraahlung
Electron passes nucleus, atrrcatuon makes change of course and slow. This releases energy as bremstraahlung xray photon
What is elastic interaction
When electron passes nucles with no change in energy
What is inelastic interaction
When electron passes nucles with energy release (bremstraahlung)
How to get protons to cover target if the Bragg peak is very narrow:
- Increase Scatter or spread out the bragg peak
- Spot scanning - multiple beams of different energy with different bragg peak
distances
What is absorbed dose?
Dose absorbed per unit mass
Gy= J/Kg
In what unit is absorbed dose measured
Gy
What is calorimetry?
Calculated deposited dose from temp rise in result of deposited energy and ioninsation
How does absorption of radiation vary in different tissues
Muscle, water air- Similar absorption with similar atomic number
In bone-Lower energy = higher absorbed dose (more photoelectric effect)
What is exposure
Historical quantity for measuring output form radiation material
Number of ionisation events in a medium
What is kerma
Kinetic energy released in a matter
Energy transfer within a volume of identified material
What is the difference between exposure and kerma
exposure= absorption of energy
Kerma = energy transfer
What is the difference between kerma and absorbed dose
Dose=damage
Kerma = potential
How do ionisation chambers work
Air chamber with eletrodes either side. Beam through air chamber, atoms and electrons measured either side therefore calculation dose by ionisation
What is calorimetry
Dose measure by temperature change as a result of photon activity
How does thermoluminescence measure radiation
Energy gain in atom from grounded state to conductive state, release of photons as light energy to go back to ground state, light output measured
What is primary standard for callibration
Naitionally maintained (middlesex) measure in air KERMA
How is national primary standard callibrated
air KERMA EBRT
Graphite calorimetry electrons and protons
How is local callibraton done
Thimble ionisation chambers, farmer chambers and geiger counter, nationally calibrated
What variating factors can affect radiation measurement
humidity, temperature, pressure, detector contamination, depth of measurement, dose rate, total dose delivered
What are isodose curves
lines joining points of equal depth percentage dose
What is a phantom
Solid water with equivalent atomic number for dose measurement
How does depth dose change in bone vs water
less depth in bone with more electron density
Xray energy range for superficial
-Treatment potential
-Depth
-Use
Xray energy range for superficial
-Treatment potential 50-160kV
-Depth <5mm
-Use skin
Xray energy range for orthovoltage
-Treatment potential
-Depth
-Use
Xray energy range for orthovoltage
-Treatment potential 160-300 kV
-Depth <6cm
-Use bone, ribs, skin shallow
Xray energy range for megavoltage
-Treatment potential
-Depth
-Use
Xray energy range for megavoltage
-Treatment potential- >1MV
-Depth <30cm
-Use internal organs
Effect on isodose curve- energy
Energy- Higher energy higher penetration
Effect on isodose curve- FSD
FSD- Increased FSD higher penetration, less skin dose
Effect on isodose curve- field size
Field size- Less steep fall off of curve with scatter and contamination. Higher skin dose higher field size.
Effect on isodose curve- surface obliquity
Surface obliquity- surface dose increases with oblique angle, more electron cross over
Effect on isodose curve- inhomogenous media
Inhomogenous media- Attenuation decreased when less dense
Effect on isodose curve- wedge
Modifys curve by increasing absoprtion and therefore protecting sensitive organs
What is a monitor unit
Measure of dose leaving linac head unit
How is monitor unit prescribed
Energy required to meet determined dose at isocentre.
Therefore increased density, depth, dose higher MU required
What is penumbra
Distance from 80-20% dose in beam in isocentric plan
What is geometric field
defined by front edge of collimator lines from source to projection, edge =50% isodose
What is dosimetric field
areas enclosed by specific isodose line
Dose distribution of electron beams effect by energy
Energy- range straggling means higher surface depth dose at higher energies with deeper dose
Dose distribution of electron beams effect by field size
Minimal change too smallfield size smaller dose
Dose distribution of electron beams effect by density
Increased density more scattering more oblique travels more dose
Dose distribution of electron beams effect by build up dose
Build up as electron travel needs obliquity to deposit dose, upside down tree
Dose distribution of electron beams effect by obliquity and surface inhomogeneity
more obliquity more dose deposition
Dose distribution of electron beams effect by bolus
brings dmax closer to surface
What is GTV
gross tumour volume, demonstratable extent of disease by imaging location
What is CTV
clinical target volume, volume treated to cover subclinical spread (cells)
What is PTV
planning target volume, movement or discrepancies in target
What is ITV
internal treatment volume, variation of internal anatomy
What is SM in radiotherapy
Set up margin, for set up error
What is TV in radiotherapy
Treatment volume of 95% isodose
What is OAR in radiotherapy
Organs at risk defined by normal tissue complication probabilities
What is PRV in radiotherapy planning
Planning organs at risk volume,OAR grown by margin to allow movement
What is IV in radiotherapy planning
Irradiated volume- volume of tissue receiving significant dose
What is FSD planning
Distance from source to head remains constant
What are pros and cons of FSD planning
-requires couch movement for each field
-lower scatter dose
-higher relative depth dose reduces entry dose
- treat larger fields
What is isocentre planning
Centre GTV at centre of treatment head
Pros and cons of isocentric planning
-Stable patient position
-Reduced treatment time
-More reliable field matching
-Option for rotational therapy
Characteristics of isodose curve in single field
-increased space with depth (flattened beam curve)
- widening penumbra depth
-increased rounding depth
Characterstics of isodose curve in parallel opposed beams
100% dose at isocentre but hot spots with exit/entrance cross over dose
What is IMRT
Linac using MLC and adjusting dose in arc
What is VMAT
Same as IMRT, using MLC and adjusting dose in arc but also changes speed of gantry, rate of dose and MLC position
What is ICRU reference point
A single point in percentage distribution is chosen (usually 100% isodose line) an assigned to prescribed dose
What is monte carlo modelling?
considers photon interaction probability for every interaction and then every interaction of resulting electrons and photons to calculate dose. Too large number but gold standard.
What is pencil beam modelling
Uses kernels based on monte carlo
What are kernels in modelling
Map of dose from energy of beam, added into required shape lateral scatter not considered so dose can be under/overestimated e.g breast gets less scatter from lung
Whats is superposition algorithm?
dose scaled in kernels in proportion to density of materials travelled through.
What is field matching
Aligning the edges of adjacent radiation beams to ensure a smooth transition in dose distribution between them, preventing gaps or overlaps.
Overlap=hotspot
What is quality assurance
standard of practice to maintain quality. Ie. procedures, records, documentation and definition of responsibility
what is quality control
test or monitoring to ensure criteria are met
According to PM77 what is reported as over dose in a course and one treatment
10% over a course
20% in one dose
