Overall Flashcards
Is energy deposition stochastic for very small masses?
Yes (microdosimetry)
Is energy deposition stochastic for the mass elements that we are concerned with in this module?
No (stochastic fluctuations are negligible for the masses considered here)
The equation for energy imparted (epsilon) by ionising radiation to the matter in a given volume is equal to what?
R_in (radiant energy in) - R_out (radiant energy out) + sum of Q (sum of all changes in rest mass energy)
For the energy imparted equation, does the terms for radiant energy in and out include rest mass energy?
No
The absorbed dose (D) is the quotient of d(epsilon with dash on top) by dm, what do the terms mean?
epsilon dash is the mean energy imparted by ionising radiation to matter of mass dm
Sometimes we put D subscript med for for absorbed dose, why? (equivalent for kerma as well)
The absorbed dose generally depends on the medium (med) doing the absorbing
What is the units of absorbed dose?
Gray (Gy) or J kg^-1
What is a typical lethal whole body dose from ionising radiation?
5 J/kg (Gy)
What does Kerma stand for?
Kinetic energy released in matter
What is the equation for kerma?
Energy transferred (dE_tr) divided by dm (mass)
What is the energy transferred term in the kerma equation?
The sum of the initial kinetic energies of all the charged ionising particles liberated by uncharged ionising particles in matter of mass dm
What type of particles is kerma only defined for?
Indirectly ionising particles, which is usually photons
What is the units of kerma?
Gray or J/kg (same as absorbed dose)
Kerma is sometimes partitioned into two terms, what are these terms?
Collision kerma (K_c) and radiative kerma (K_r)
What does the radiative kerma refer to?
The part of kerma that includes the energy the charged particles will eventually re-radiate via bremsstrahlung photons
Collision kerma is related to total kerma by what equation that involves g and what does g stand for?
K_c = K (1-g), where g is the fraction of initial kinetic energy of the electrons that is re-radiated as bremsstrahlung in the particular medium of interest
What is cema (C)?
The charged particle equivalent of collision kerma (it stands for converted energy per unit mass) (i.e. instead of photons or uncharged particles, it is charged particles)
Particle fluence is a fundamental term to relate kerma or dose to the radiation field, what are these types of terms called in general?
Field quantities
What is the equation for the particle fluence?
dN (number of particles striking a finite sphere) divided by dA (cross sectional area)
What are the units of particle fluence?
m^-2 or cm^-2 (i.e number of particles per unit area)
Is fluence a scalar quantity and what does this mean?
Yes, so the direction of the radiation is irrelevant
An equivalent equation for fluence is the sum of the track lengths of the particles crossing the elementary sphere (sum of delta s) divided by what?
The volume of the sphere (dV)
What is the equation for fluence differential in energy?
D fluence divided by d energy
What is the energy fluence?
The product of fluence with energy
What is planar fluence?
The number of particles crossing a plane surface in either direction per unit area of the surface
What is the letter than symbolises fluence?
phi
What is the letter that symbolises energy fluence?
psi
What is the mass attenuation coefficient?
linear attenuation coefficient divided by the density (related to the probability of an interaction per unit effective length)
What is the mass energy transfer coefficient?
The probability of an energy transfer per unit effective length
What is the equation for the mass energy transfer coefficient?
The inelastic component linear attenuation coefficient divided by the density (interactions that result in energy transfer)
The mass energy absorption coefficient is the mass energy transfer but excluding which component of it?
The part of the initial kinetic energy of charged particles re-radiated as bremsstrahlung
When does the condition of charged particle equilibrium (CPE) apply?
The energy in equals the energy out and the energy imparted is just the net energy transferred
What is the bone dose enhancement?
The change in the absorbed dose when one goes from tissue to bone in a patient irradiated by a low-energy x-ray beam
The charged particle equilibrium condition is responsible for what region of the dose curve?
The build up region (skin sparing effect)
Is the dose build-up region longer or shorter for higher energy photons and why?
Longer because the secondary electrons are higher energy and travel further on average
Unlike stochastic photon behaviour, how do electrons lose energy?
Quasi-continuously through large numbers of Coulomb interactions
What is the total stopping power?
The rate of energy loss with distance and it is a sum of the electronic/collision stopping power and radiative stopping power
Why do we only consider collision stopping power within a dosimetry context?
Radiative stopping power is bremsstrahlung which carries the energy away and this only considers local energy deposition
What is the mass collision stopping power?
Collision stopping power divided by density
What is the analogue of charged particle equilibrium (CPE) which is for photons, for electrons?
Delta-ray equilibrium
What is the cavity referred to in cavity theory?
The detector
The detector signal is proportional to what?
The energy absorbed in its sensitive material
What is the goal of cavity theory?
Relate the average dose in the detector to that in the undisturbed medium for certain general classes of detector
What is the name and symbol for the coefficient that defines the ratio between the dose to the medium and the dose to the detector for a beam of quality Q?
Cavity factor and f_Q
Detectors can be split into large and small detectors, what is the size relative to?
Compared to the ranges of the dose-depositing charged particles
When are large detectors considered?
When lower energies are used, like orthovoltage/superficial x-rays, HDR brachytherapy, diagnostic imaging
When are small detectors considered?
When higher energies are used and so the electrons travel further
What is the cavity factor also called?
Mass-energy absorption coefficient ratio
Are TLDs (made of Lithium Fluroide) considered large or small detectors?
Large
For small detectors, the detector size is only a small fraction of the electron range, so what condition cannot be applied?
The charged particle equilibrium condition
What is the ideal voltage range for ionisation chambers (e.g. farmer chambers)?
200-400 V
What is the correction factor f_ion for?
To correct for ion recombination and whether all the ions have been detected
For ion chambers, what is the mean energy required to produce an ion pair in air per unit charge e in dry air for a wide range of energies?
33.97 eV / ion pair (or J/C)
Why are perturbation factor added to the equation for ion chamber dosimetry?
Departures from Bragg-Gray assumptions
What are the four effects of a real world ion chamber in water that cause changes to the electron fluence are corrected for by the perturbation factor?
The effect of displacing a volume of water with the chamber cavity (p_dis), the non-water equivalence of the chamber walls and any waterproofing material (p_wall), the effect of the central electrode (p_cel), and the in-scattering of electrons caused by the air cavity (p_cav)
Which perturbation effects are included in the perturbation factor for photon beams?
p_dis, p_wall, p_cel (significant for low energy photon beams - mostly diagnostic)
Which perturbation effects are included in the perturbation factor for electron beams?
p_dis, p_cav
Which perturbation effect means than Roos type chambers should be used for electron beams rather than farmer type chambers?
The in scattering effect (p_cav)
Are the perturbation effects all above or below one?
Below
What is the sensitive volume of ion chambers usually filled with?
Ordinary air
In ion chamber dosimetry, what is the dose related measured quantity?
Charge, Q
In ion chamber dosimetry, what is the dose rate related measured quantity?
Current, I
If the voltage across an ion chamber is low, what effect could occur? (think about charge vs voltage graph)
Ion recombination, so the collected charge is lower than expected for the ionisation chamber region
If the voltage across an ion chamber is high, what effect could occur? (think about charge vs voltage graph)
Charge multiplication, so the collected charge is higher than expected for the ionisation chamber region
In the energy imparted equation for epsilon, what is the radiant energy in and out?
The sum of the energies (excluding rest mass energies) of all charged and uncharged ionising particles either entering (in) or leaving (out) the volume
In the energy imparted equation for epsilon, what is the sum of Q term?
The sum of all the changes of the rest mass energy of nuclei and elementary particles that occur in the volume (Q>0 = decrease of rest energy)
For questions that use the energy imparted equation for epsilon, does the creation of photons (e.g. radioactive decay or pair production) mean a positive or negative Q in the equation?
Positive
Whilst kerma only considers the initial kinetic energies of charged particles created in the volume (if created outside it doesn’t count), how does this differ to dose?
Dose considers the energy deposited in the volume element, regardless of whether the particle was created inside or outside the volume element.
For absorbed dose calculations, highlight the charged particle tracks that are inside the volume element, what should be highlighted for kerma calculations?
Circle the charged particles that are generated in the volume element
To note the difference between absorbed dose and kerma: Absorption of energy does not take place in the same location as what?
The transfer of energy
What is the equation for cema?
dE_l / dm, where dE_l is the mean energy lost in electronic collisions by the primary charged particles
For charged particles, most of the energy loss is directly absorbed, but how is the energy of uncharged particles imparted to matter?
Through a two step process: energy transfer then energy absorption
What is the difference between kerma and cema?
Cema involves the energy lost in electronic collisions by the incoming charged particles, whereas kerma involves the energy imparted to outgoing charged particles
All codes of practice have standard nomenclature, what does N_D,w,Q0 mean?
The calibration factor in terms of absorbed dose to water for a dosimeter at a reference beam quality Q_0
All codes of practice have standard nomenclature, what does M_Q mean?
Reading of a dosimeter at quality Q, corrected for influence quantities other than beam quality
All codes of practice have standard nomenclature, what does k_pol mean?
Factor to correct the response of an ionisation chamber for the effect of a change in polarity of the polarising voltage applied
All codes of practice have standard nomenclature, what does k_s mean?
Factor to correct the response of the ionisation chamber for the lack of complete charge collection due to ion recombination
All codes of practice have standard nomenclature, what does k_Q,Q0 mean?
Factor to correct for the difference between the response of an ionisation chamber in the user beam quality compared to the reference beam quality
Which factor in the dose equation with the calibration factor do the perturbation factors contribute to?
k_Q,Q0
What are the methods for the standard lab calibration for air kerma?
Ionisation chamber, free air chamber
What are the methods for the standard lab calibration for absorbed dose to water?
Graphite calorimeter, water calorimeter, ionisation chamber, Fricke dosimeter
Where the reference quality is cobalt-60, what is omitted from the standard nomenclature terms?
Q_0
How is energy (quality) for MV photons defined in the UK and IAEA?
TPR 20/10
Why is the effective point of measurement for an ionisation chamber not at the centre? (closer to entrance region by 0.6 r)
Generally more electrons will come from the top
How is energy (quality) for lower energy photons defined?
Half value layer (HVL)
What are the three levels of dose measurement?
Quick check, calibration (traceable to a national standard) and definitive calibration
How can a quick check of dose measurement be performed and is the calibration directly traceable to the national standard?
Any dose measuring device and no
What should you do for calibrations of dose measurement (broad)?
There is a detailed protocol to follow and all factors should be recorded. Calibrations should be reviewed regularly
What are the action levels for calibration of dose measurements?
2% to consider recalibration when possible and 3% to suspend treatments
When is a definitive calibration of dose measurement used (general) and what is it used for?
Whenever there is a potential break in calibration history and it forms a baseline for subsequent confirmatory measurements
When are specific times that definitive calibrations should be done for linacs?
Commissioning, following major repair that may affects its calibration, following the replacement of radioactive sources and where the link with the previous calibration has been broken
Other than external beam radiotherapy treatment machines, what other equipment requires definitive calibration?
Radiation dose measuring equipment
Individual patient QA can be done for IMRT treatments with different centres having different frequencies, what are the different approaches to frequency of this?
Do individual patient QA for: all IMRT patients, no patients, representative sample or only for unusual cases
What are some benefits of audits?
Identifies necessary changes, ensures procedures of followed, conforms to ISO9000, avoids drifts to procedures, identifies hardware changes and ensures uniformity between centres
What are the different types/levels of audit?
Local (e.g. new machine), national (e.g. IPEM networks), international (e.g. IAEA) and clinical trials
What is mostly used for in vivo dosimetry?
TLDs and diodes (sometimes also MOSFETs, OSL, gafchromic film and portal imager)
What is in vivo dosimetry in radiotherapy?
Measurements made on the patient during treatment and it often involves placing detectors on the skin to measure surface dose
Roughly what percentage differences from the expected doses does in vivo dosimetry identify?
5%
Where can we measure the dose for in vivo dosimetry?
Surface, exit or intracavity
What factors are in vivo dosimeters dependent on and could affect the readings?
Dose rate, energy, temperature, incident angle, detector degradation
How are in vivo dosimeters calibrated?
What are some of the clinical uses of in vivo dosimetry?
Independent check of MU calculations, TBI/TSET calcs, intracavitary doses, OAR doses, errors in patient setup, non-standard cases where doses may be difficult to predict
How are TLDs calibrated?
A batch of 100-150 TLDs get uniformly irradiated and then the mean thermoluminescence reading of the batch is used generate correction factors
What are the pros of TLDs?
Dose rate independent, read out temp is high compared to room/patient temp, no directional dependence
What are the cons of TLDs?
Readings aren’t real time (couple hours of processing), destructive read out, one dimensional readings
What are diodes made of and what does this mean?
Silicon and they have a higher Z than water, so the photoelectric effect will be dominant for lower energies, so they will be overly sensitive in the kV range
Why do diodes need calibrating regularly?
They lose sensitivity due to radiation damage
Are diodes temperature dependent and what does this mean for in vivo dosimetry using them?
Yes and the response will change if the temperature of the diode changes when placed on patient skin
Do diodes have angular dependence?
Yes
What in vivo dosimetry type are MOSFETs used for? (rarely used)
Cavity readings
What does OSL stand for?
Optically Stimulated Luminescence
Are OSLs similar to TLDs and how are they different?
Very similar but for OSLs, the stored energy is released by light irradiation rather than heating
What can automatic segmentation be used for?
Outlining OARs automatically during treatment planning
What is the difference between the CT scans between diagnostic and radiotherapy planning?
Flat-topped couch to attach immobilisation equipment to, wide-bore to accommodate different treatment positions, reference position lasers, indexing system (zero position from lasers), high kV
What is the outer material and central electrode of Farmer chambers made of?
Outer material - graphite
Central electrode - aluminium
Are Farmer chambers used for relative or absolute dosimetry typically?
Absolute
What are PinPoint chambers used for?
Water tank measurements
What are the pros and cons of pinpoint chambers when considering its size?
It is small so it has a high resolution (small volume averaging) but worse SNR as less ionisation events in sensitive volume so more intense beam required
What type of ionisation chamber is the NACP chamber?
Parallel plate
Why are Roos chambers often used for daily use as a parallel plate chamber compared to NACP or Markus chambers?
They are more robust
What are the purpose of guard rings in cylindrical ionisation chambers and parallel plate chambers?
In cylindrical ionisation chambers, to avoid leakage current in the insulator and in parallel plate chambers, it defines the effective collection volume
Why are water phantoms used?
Similar properties to tissue, directly relates to the chain of calibration and matches treatment planning system (TPS) reference conditions
What is the purpose of build-up cups?
To create electronic equilibrium (energy dependent), only measure primary radiation (avoid phantom scatter)
Why are brass build-up caps used instead of other materials of lower atomic number?
They can be smaller (thinner walls) to reach electronic equilibrium (electron path length smaller in higher Z material)
What type of detector is used for CT scans?
Pencil ionisation chambers (central electrode like Farmer chambers)
What is measured for CT scanner output using pencil ionisation chambers?
CTDI
Which ion chambers are used for absolute dosimetry?
Large volume detectors, soft x-ray, pencil chambers, Farmer, Roos (can use Markus)
Which ion chambers are used for relative dosimetry?
Build-up caps, Markus, PinPoint, TLD (can use Farmer)
What is the main application/motivations for measuring dose distributions?
Treatment planning system beam modelling (profiles, PDDs), machine performance characterisation (flatness, symmetry), patient specific QA, commissioning new techniques, in-vivo dosimetry
What equipment can be used for measuring dose distributions?
Plotting tanks (e.g. water tanks),
What are arrays and what are they for?
Lots of detectors in rows and columns (e.g. ion chambers or diodes) to measure multiple points in the field size rather than just one
What other method of measuring dose distributions is similar to diode or ion-chamber arrays?
EPIDs
What are the dosimetry issues with using EPIDs for measuring dose distributions?
Lack of build up, sensitive to contaminant electrons, not water equivalent material, scatter from detector assembly, over-response at low energies
What are the practical issues with using EPIDs for measuring dose distributions?
Image artefacts if poorly calibrated, ghosting (previous irradiation leaves a faint response), saturation, not independent from linacs or TPS for QA considerations
What dosimeters with high spatial resolution can be used for measuring dose distributions?
Film (e.g. radiochromic, radiographic), digital luminescent radiography (computed radiography), gel (3D)
Why are films, computed radiography and gel for measuring dose distributions less convenient compared to digital detector arrays?
Not real-time measurements and the dosimetry is sensitive to processing conditions
What applications is radiochromic film used for in radiation dosimetry?
When high dose gradients and small fields are used, penumbra and build up regions, IMRT/VMAT, stereotactic radiotherapy, brachy, protons, low energy electrons
