Overall Flashcards
(272 cards)
1
Q
Is energy deposition stochastic for very small masses?
A
Yes (microdosimetry)
2
Q
Is energy deposition stochastic for the mass elements that we are concerned with in this module?
A
No (stochastic fluctuations are negligible for the masses considered here)
3
Q
The equation for energy imparted (epsilon) by ionising radiation to the matter in a given volume is equal to what?
A
R_in (radiant energy in) - R_out (radiant energy out) + sum of Q (sum of all changes in rest mass energy)
4
Q
For the energy imparted equation, does the terms for radiant energy in and out include rest mass energy?
A
No
5
Q
The absorbed dose (D) is the quotient of d(epsilon with dash on top) by dm, what do the terms mean?
A
epsilon dash is the mean energy imparted by ionising radiation to matter of mass dm
6
Q
Sometimes we put D subscript med for for absorbed dose, why? (equivalent for kerma as well)
A
The absorbed dose generally depends on the medium (med) doing the absorbing
7
Q
What is the units of absorbed dose?
A
Gray (Gy) or J kg^-1
8
Q
What is a typical lethal whole body dose from ionising radiation?
A
5 J/kg (Gy)
9
Q
What does Kerma stand for?
A
Kinetic energy released in matter (or per unit matter)
10
Q
What is the equation for kerma?
A
Energy transferred (dE_tr) divided by dm (mass)
11
Q
What is the energy transferred term in the kerma equation?
A
The sum of the initial kinetic energies of all the charged ionising particles liberated by uncharged ionising particles in matter of mass dm
12
Q
What type of particles is kerma only defined for?
A
Indirectly ionising particles, which is usually photons
13
Q
What is the units of kerma?
A
Gray or J/kg (same as absorbed dose)
14
Q
Kerma is sometimes partitioned into two terms, what are these terms?
A
Collision kerma (K_c) and radiative kerma (K_r)
15
Q
What does the radiative kerma refer to?
A
The part of kerma that includes the energy the charged particles will eventually re-radiate via bremsstrahlung photons
16
Q
Collision kerma is related to total kerma by what equation that involves g and what does g stand for?
A
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
17
Q
What is cema (C)?
A
Converted energy per unit mass. 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)
18
Q
Particle fluence is a fundamental term to relate kerma or dose to the radiation field, what are these types of terms called in general?
A
Field quantities
19
Q
What is the equation for the particle fluence?
A
dN (number of particles striking a finite sphere) divided by dA (cross sectional area)
20
Q
What are the units of particle fluence?
A
m^-2 or cm^-2 (i.e number of particles per unit area)
21
Q
Is fluence a scalar quantity and what does this mean?
A
Yes, so the direction of the radiation is irrelevant
22
Q
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?
A
The volume of the sphere (dV)
23
Q
What is the equation for fluence differential in energy?
A
D fluence divided by d energy
24
Q
What is the energy fluence?
A
The product of fluence with energy
25
What is planar fluence?
The number of particles crossing a plane surface in either direction per unit area of the surface
26
What is the letter than symbolises fluence?
phi
27
What is the letter that symbolises energy fluence?
psi
28
What is the mass attenuation coefficient?
linear attenuation coefficient divided by the density (related to the probability of an interaction per unit effective length)
29
What is the mass energy transfer coefficient?
The probability of an energy transfer per unit effective length
30
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)
31
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
32
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
33
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
34
The charged particle equilibrium condition is responsible for what region of the dose curve?
The build up region (skin sparing effect)
35
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
36
Unlike stochastic photon behaviour, how do electrons (and other charged particles) lose energy?
Quasi-continuously through large numbers of Coulomb interactions
37
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
38
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
39
What is the mass collision stopping power?
Collision stopping power divided by density
40
What is the analogue of charged particle equilibrium (CPE) which is for photons, for electrons?
Delta-ray equilibrium
41
What is the cavity referred to in cavity theory?
The detector
42
The detector signal is proportional to what?
The energy absorbed in its sensitive material
43
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
44
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
45
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
46
When are large detectors considered?
When lower energies are used, like orthovoltage/superficial x-rays, HDR brachytherapy, diagnostic imaging
47
When are small detectors considered?
When higher energies are used and so the electrons travel further
48
What is the cavity factor also called for large detectors?
Mass-energy absorption coefficient ratio
49
Are TLDs (made of Lithium Fluroide) considered large or small detectors?
Large
50
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
51
What is the ideal voltage range for ionisation chambers (e.g. farmer chambers)?
200-400 V
52
What is the correction factor f_ion for?
To correct for ion recombination and whether all the ions have been detected
53
For ion chambers, what is the mean energy required to produce an ion pair in air per unit charge e (W_air/e) in dry air for a wide range of energies?
33.97 eV / ion pair (or J/C)
54
Why are perturbation factor added to the equation for ion chamber dosimetry?
Departures from Bragg-Gray assumptions
55
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) (not air equivalent), and the in-scattering of electrons caused by the air cavity (p_cav)
56
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)
57
Which perturbation effects are included in the perturbation factor for electron beams?
p_dis, p_cav
58
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)
59
Are the perturbation effects all above or below one?
Below
60
What is the sensitive volume of ion chambers usually filled with?
Ordinary air
61
In ion chamber dosimetry, what is the dose related measured quantity?
Charge, Q
62
In ion chamber dosimetry, what is the dose rate related measured quantity?
Current, I
63
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
64
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
65
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
66
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)
67
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 (sum of Q = + E (m->R) - E (R->m)
68
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.
69
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
70
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
71
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
72
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
73
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
74
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
75
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
76
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
77
All codes of practice have standard nomenclature, what does k_s (or P_ion) mean?
Factor to correct the response of the ionisation chamber for the lack of complete charge collection due to ion recombination
78
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
79
What are the methods for the standard lab calibration for air kerma?
Ionisation chamber, free air chamber
80
What are the methods for the standard lab calibration for absorbed dose to water?
Graphite calorimeter, water calorimeter, ionisation chamber, Fricke dosimeter
81
Where the reference quality is cobalt-60, what is omitted from the standard nomenclature terms?
Q_0
82
How is energy (quality) for MV photons defined in the UK and IAEA?
TPR 20/10
83
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
84
How is energy (quality) for lower energy photons defined?
Half value layer (HVL)
85
What are the three levels of dose measurement?
Quick check, calibration (traceable to a national standard) and definitive calibration
86
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
87
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. Traceable to the national standard
88
What are the action levels for calibration of dose measurements?
2% to consider recalibration when possible and 3% to suspend treatments
89
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
90
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
91
Other than external beam radiotherapy treatment machines, what other equipment requires definitive calibration?
Radiation dose measuring equipment
92
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
93
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
94
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
95
What is mostly used for in vivo dosimetry?
TLDs and diodes (sometimes also MOSFETs, OSL, gafchromic film and portal imager)
96
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
97
Roughly what percentage differences from the expected doses does in vivo dosimetry identify?
5%
98
Where can we measure the dose for in vivo dosimetry?
Surface, exit or intracavity
99
What factors are in vivo dosimeters dependent on and could affect the readings?
Dose rate, energy, temperature, incident angle, detector degradation
100
How are in vivo dosimeters calibrated?
Replicate treatment position (usually on surface). Measure at ref depth using calibrated dosemeter and correct dose reading to in-vivo dosimeter position and calibrate dosimeter reading to dose
101
What are some of the clinical uses of in vivo dosimetry?
Independent check of MU calculations, TBI calcs, intracavitary doses, OAR doses, errors in patient setup, non-standard cases where doses may be difficult to predict
102
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
103
What are the pros of TLDs?
Dose rate independent, read out temp is high compared to room/patient temp (so temp independent), no directional dependence, tissue equivalent properties, no cable
104
What are the cons of TLDs?
Readings aren't real time (couple hours of processing), destructive read out, one dimensional readings
105
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
106
Why do diodes need calibrating regularly?
They lose sensitivity due to radiation damage
107
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
108
Do diodes have angular dependence?
Yes (corrections applied for incidence angles more than 30 degrees) unless encapsulated
109
What in vivo dosimetry type are MOSFETs used for? (rarely used)
Cavity readings
110
What does OSL stand for?
Optically Stimulated Luminescence
111
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
112
What can automatic segmentation be used for?
Outlining OARs automatically during treatment planning
113
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, high kV
114
What is the outer material and central electrode of Farmer chambers made of?
Outer material - graphite
Central electrode - aluminium
115
Are Farmer chambers used for relative or absolute dosimetry typically?
Absolute
116
What are PinPoint chambers used for?
Water tank measurements
117
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
118
What type of ionisation chamber is the NACP chamber?
Parallel plate
119
Why are Roos chambers often used for daily use as a parallel plate chamber compared to NACP or Markus chambers?
They are more robust
120
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
121
Why are water phantoms used?
Similar properties to tissue, directly relates to the chain of calibration and matches treatment planning system (TPS) reference conditions
122
What is the purpose of build-up caps for ion chambers?
To create electronic equilibrium (energy dependent), only measure primary radiation (avoid phantom scatter)
123
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)
124
What type of detector is used for CT scans?
Pencil ionisation chambers (central electrode like Farmer chambers)
125
What is measured for CT scanner output using pencil ionisation chambers?
CTDI
126
Which ion chambers are used for absolute dosimetry?
Large volume detectors, soft x-ray, pencil chambers, Farmer, Roos (can use Markus)
127
Which ion chambers are used for relative dosimetry?
Build-up caps, Markus, PinPoint, TLD (can use Farmer)
128
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
129
What equipment can be used for measuring dose distributions?
Plotting tanks (e.g. water tanks), arrays (ion chamber or diode), EPIDs
130
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
131
What other method of measuring dose distributions is similar to diode or ion-chamber arrays?
EPIDs
132
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, variable air-gap
133
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
134
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)
135
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
136
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
137
What are the names and energy ranges of different therapeutic kVs?
Grenz (10-20 kV), contact (40-50 kV), superficial (50-150 kV), orthovoltage (150-500 kV)
138
How does the x-ray distribution in space differ between low (50-150kv) and high energy x-rays?
Low energies are more or less equal in all directions, whereas MV is more forward directed
139
What is the typical target angle?
30 degrees
140
What is the Heel effect and what is it due to?
Intensity not constant in anode-cathode direction because of the inverse square law and absorption in the target
141
Why is the photoelectric absorption range avoided in therapy using x-rays?
Low kV may be hazardous to underlying bone
142
In the x-ray generator, how is the AC voltage increased and then changed to provide a constant polarity (DC)?
Transformer to increase and rectifier to make DC
143
Why are therapy beams filtered?
Removes low energy and characteristic xrays to increase the mean energy and reduce intensity (dose-rate)
144
Regarding safety interlocks, what is it called when there are dual monitors for a particular system?
A redundancy measure
145
Should safety interlocks fail safe or fail secure?
Fail safe
146
What are the categories (purposes) of interlocks?
Machine control (protects the machine itself), general safety (protects staff and the public) and treatment safety (radiation output consistent with the prescription - patient safety)
147
What does it mean that interlocks can be characterised as either binary or analogue?
Binary - the treatment is either allowing treatment or not.
Analogue - continuously variable with either predetermined limits or a feedback system
148
What are some examples of safety interlocks in the 'machine control' category and state if they are binary or analogue?
Binary: touch guards
Analogue: Couch position, temp/pressure/water level
149
What are some examples of safety interlocks in the 'general safety' category and state if they are binary or analogue?
Binary: door interlock and warning lights, emergency off buttons
150
What are some examples of safety interlocks in the 'treatment safety' category and state if they are binary or analogue?
Binary: flattening filter position, target position
Analogue: dose rate
151
What types of hazards are included in the general safety interlocks and safety systems?
Electrical (e.g. high voltage cabinet interlocks), mechanical (collision detectors) and radiation hazards
152
What do Record and Verify systems do?
Verify daily treatment setup against planned parameters, store data and images, record treatment parameters, assisting setup and verification imaging
153
What does the treatment console do in radiotherapy?
Retrieves patient plan and send info to linac, confirms linac parameters (to compare to plan and applies tolerances) and records treatment.
154
What are the two options for tracking MLC leaf position?
Encoder-based system or optical reflector and camera system
155
What is the main difference between a rotating anode and rotating envelope tube?
Only the anode rotates for the rotating anode, whereas in the rotating envelope tube it all rotates together around the axis of the anode
156
What are two features specific to a rotating envelope tube?
There is direct contact of the anode plate with the cooling oil. All rotating mechanical parts are outside the vacuum.
157
In what ways is the rotating envelope tube better than the rotating anode design?
Better reliability and increased heat capacity
158
What do the ion chambers in a linac measure?
Dose rate, flatness, beam energy and symmetry
159
For Hounsfield unit calculations, the equation is mu_med - mu_water / mu_water multiplied by what number?
1000
160
What is the calibration curve used when CT scans are used to plan radiotherapy treatment?
Convert between HU and electron density
161
In an electron beam, how does the water-to-air stopping power ratio vary with depth in a medium?
It increase as the depth increases
162
Why does the water-to-air stopping power ratio increase with depth for an electron beam?
There is a relativistic rise in stopping power for high energy electrons, which is more pronounced for les dense materials (e.g. air) and then the electrons lose energy as depth increases
163
Why does the stopping power ratio vary with depth for electron beams but not photon beams?
Electrons lose energy quasi-continuously, whereas photon attenuation is stochastic and creates electrons in its path
164
How can you measure the recombination correction for an ion chamber?
Change the voltage on the chamber and record the chamber readings with the same MU delivered
165
How do you calibrate a diode system to measure the dose at depth of dose maximum for in vivo dosimetry?
Place a diode on a phantom with ion chamber in the phantom. Correct the chamber reading to to the diode position and calculate a conversion factor between the measurements
166
What is the dE_tr (energy transferred) term in the kerma equation?
The sum of the initial KEs of all the charged ionising particles liberated by uncharged ionising particles in matter of mass dm
167
What do you highlight/circle in the volume for kerma calculations?
Circle all creations of charged particles from photons within the volume
168
What do you highlight/circle in the volume for absorbed dose calculations?
Highlight the particle tracks in the volume
169
How do you calculate the total fluence or energy fluence for a spectrum of energy?
Integral between 0 and E_max of phi (fluence) or psi (energy fluence) dE
170
What is the equation for photon attenuation through a medium that includes fluence (phi)?
Phi = phi_0 e^(-mu x)
171
What is the equation for kerma for a photon-irradiated medium that includes the mass energy transfer coefficient (mu_tr /rho)
Mass energy transfer coefficient multiplied by energy fluence (psi) (monoenergetic) or integral over energy range of mass energy transfer coefficient multiplied by energy fluence (psi_E) dE
172
What is the equation for collision kerma for a photon irradiated medium with the mass energy absorption coefficient (mu_en/rho)?
Mass energy absorption coefficient multiplied by energy fluence (psi) (monoenergetic) or integral over energy range of mass energy absorption coefficient multiplied by energy fluence (psi_E) dE
173
What is the energy imparted equation that includes net KE in and out of a layer?
Net KE entering the layer on charged particles - net KE leaving the layer on charged particles + net energy transferred
174
Under CPE, the absorbed dose to a medium is equal to what?
The collision kerma (so can use the collision kerma equation with the mass energy absorption coefficient for dose calcs)
175
What is CPE practically?
Each charged particles of a given type and energy leaving the volume is replaced by an identical particle of the same energy
176
When is CPE obtained and why?
After the build up region, as its longer than the maximum range of the secondary electrons generated
177
Why is true CPE often not achieved in practice?
Attenuation is always present. The photon fluence is not constant with depth, so the number of secondary particles (electrons) starting at different depths cannot be constant either.
178
What is the equation for cema that includes the mass collision stopping power (S_col/rho)_med?
Mass collision stopping power multiplied by phi (fluence) (monoenergetic) or or integral over energy range of mass collision stopping power multiplied by fluence (phi_E) dE
179
Why is cema not equal to absorbed dose?
Delta rays can carry off energy
180
What is delta ray equilibrium?
Any KE leaving on delta rays is replaced by an exactly equal amount entering the layer (likely to happen as delta rays are rare and short ranged)
181
What is cema equal to under delta ray equilibrium?
Absorbed dose (so can use the cema equations for dose calcs e.g. the ones with S_col)
182
What is the Spencer-Attix breakthrough?
Energy losses below a cut off amount are assumed to be deposited entirely locally, whilst losses above this are assumed to escape entirely, which meant that restricted collision stopping power can be calculated
183
When are large and small detectors used in cavity theory?
Large for lower energy photons and small for higher energy photons or electrons
184
What is Fano's theorem?
Particle fluences are not altered by changing density under the conditions of equilibrium in a medium (i.e. fluence is the same in a detector as the medium surrounding it) (use for large detectors I think)
185
What is the mass-energy absorption coefficient ratio?
f_Q = D_med/avg D_det, so cavity factor for large detector. Short form: (avg mu_en /rho)_med,det
186
Out of CPE and DRE, which applies to large detectors and which applies to small detectors?
CPE to large and DRE to small
187
Under DRE, the dose equation includes fluence and not energy, does the dose equation under CPE include these terms?
It has energy fluence, so yes it includes energy multiplied by fluence
188
Can CPE be established in a small detector?
No (use DRE instead)
189
What is the Bragg-Gray cavity condition?
The cavity must not disturb the charged particle fluence exitsing in the medium in the absence of the cavity. In practice, this means cavity is small compared to electron ranges
190
What is the Bragg-Gray mass stopping power ratio?
f_Q = D_med/avg D_det = (S_col/rho)_med/(S_col/rho)_det. Cavity factor for small detectors with short hand form S^BG_med, det
191
What are the two options for the cavity factor for small detectors?
Bragg cavity assumption with unrestricted stopping power or Spencer-Attix with restricted stopping power
192
Is the Spencer-Attix assumption stronger or weaker than the Bragg-Gray theory?
Weaker as the fluences in the medium and detector are only equal when the energy is above the cut-off energy
193
Is Spencer-Attix or Bragg-Gray ratios typically used and why?
Spencer-Attix as it more closely predicts detector response (but values are the same in most situations)
194
What calculation do you use for the cavity factor for general cavities that are neither large of small?
Weighted average of small and large detectors cavity factors. So f_Q = d S_med,det + (1-d)(mu_en/rho)_med,det, where d = fraction of dose in cavity due to electrons
195
In ion chambers, dose is proportional to what measured quantity and dose rate is proportional to what quantity?
Dose with charge and dose rate with current
196
What is the ideal voltage range for ion chambers?
200-400 V
197
What is D_air equal to for ion chamber calcs?
Q/(V x rho_air) multiplied by (W_air/e)
198
Under Bragg-Gray conditions (small detector), what is D_water for ion chambers? Note: D_water = D_air S_water,air
Q/(V x rho_air) multiplied by (W_air/e) multiplied by S_water,air (Bragg-Gray mass stopping power ratio, or cavity factor for small cavities)
199
Under Bragg-Gray conditions, D_water is equal to Q multiplied by what factor?
N_D,w (this equals 1/(V x rho_air) multiplied by (W_air/e) multiplied by S_water,air and other perturbation factors)
200
What are perturbations?
Departures from Bragg-Gray assumptions. The perturbation factors explain why the fluence is not exactly the same in the medium as the detector phi_med = avg phi_det multiplied by p_q
201
How does the perturbation factor p_Q fit into the equation for the dose to the medium and average dose to detector?
D_med = avg D_det x S_med,det x P_Q
202
How can the displacement correction for ion chambers be done?
Either with the p_dis perturbation factor or move the chamber down to change the effective point of measurement
203
Are all 4 perturbation factors above or below 1 and why?
Below 1 because the effect males the chamber signal larger than if the effect wasn't there
204
Why is p_cav (in scattering of electrons caused by air cavity) required?
Electrons are more likely to scatter in to low density regions rather than out of, which increases the electron fluence
205
Why are parallel plate chambers used for electrons rather than Farmer chambers?
p_cav is more significant in Farmer chambers. Plane-parallel chambers are wide, flat and protected around the edges by a guard ring (most electrons enter through the top than side walls so no in-scattering)
206
What is the typical leaf width in MLCs?
5 mm
207
What are the MLCs for?
Beam shaping and used dynamically to modulate the fluence for IMRT or with dynamic gantry motion for VMAT
208
What limits the image quality of electronic portal imaging devices (EPIDs)?
Low subject contrast, large focal sport and long secondary particle range
209
What is the EPID for?
A dosimetric array for linac QC, plan QC or in-vivo dosimetry. Verify patient position
210
What does VMAT stand for?
Volumetric Modulated Arc Therapy
211
What are the pros and cons of a annulus (ring-shaped) gantry? (like halcyon)
Pros: Low collision risk, faster gantry rotation possible, no beam bending, fast, easier with other tech
Cons: single energy, limits treatment positions, less flexible, more claustrophobic
212
What is a gamma knife and what is it for?
192 Co-60 sources in a machine for cranial stereotactic radiosurgery (SRS)
213
What are the risks of alternative or novel devices in radiotherapy?
Different software and hardware systems. Additional training burden. Service resilience to faults. Technical support. Different QA. Limitations. Cost.
214
What are the benefits of alternative or novel devices in radiotherapy?
Improved treatment quality translating to improved clinical outcomes. New treatment options. Research and development opportunities. Recruitment and retention of staff
215
What are the parts of a linac (not in the head) to make X-rays?
RF power generator (produces microwaves), waveguide (transport the microwaves), electron gun, accelerating waveguide (uses microwaves to accelerate electrons), bending magnet, x-ray target
216
What is in the linac head for photons beams (in order)?
X-ray target, primary collimator, flattening filter, dual channel ionisation chamber, field defining light system, secondary collimators
217
What are scattering foils for?
Spread a pencil electron beam into a wide uniform beam (only in electron beam therapy)
218
What is on the carousel in a linac treatment head?
Scattering foil (electrons) or flattening filter (photons)
219
What features are more strict for a definitive calibration?
Independent checks and specific cross checks. Responsibility is a physicist, usually an MPE. Written procedures authorised by MPE. Distance and timers should be checked. All factors recorded.
220
What is the dosimetric leaf gap?
The effect of partial radiation transmission through the rounded ends of MLC leaves, interleaf leakage and the dynamic movement of the leaves (accounts in planning for dynamic MLCs )
221
What is the equation for instantaneous dose rate with a pulse (IDRP)?
Dose per pulse divided by pulse duration (answer in Gy/s)
222
How do reduce the mean dose rate?
Change the pulse repetition frequency (pulses/s) or dropping pulses
223
How does transmission still occur with MLCs?
Interleaf leakage (tongue and groove) and leaf edge (rounded)
224
What are tongue and grooves with MLCs?
The leaves are not flat sheets, instead they are jagged and fit together (like puzzle pieces)
225
How do you work out the dosimetric leaf gap?
Put an ion chamber in a field and measure dose for a series of sweeping windows of decreasing width and interpolated to a zero width (around 1-1.5 mm)
226
What is different with the move to IMRT and VMAT from simpler systems?
Dosimetry harder, delivery more challenging (new more servos to check), MU higher and beam on longer, low dose bath for VMAT, organ movement unpredictable, dose less uniform
227
How is a photon monitor unit defined at CCC?
1cGy at dmax at 100cm SSD
228
What attribute is related to the P_stem factor?
Field size (P_stem = 1 if field size for calibration is same as measurement)
229
What is the backscatter factor in radiotherapy?
It accounts for the increase in dose due to scattered radiation from the surrounding tissue (especially with low energy photons)
230
What is contoured on the treatment plan CT?
Target and OARs
231
What were simulators used for before CTs were used in planning?
To select appropriate beam angles and to add MLCs
232
At treatment energies, what is the predominant interaction for energy deposition?
Compton scatter
233
What are the various techniques to monitor breathing trace?
Reflective marker block on chest, direct optical tracking of chest wall, spirometer, expanding belt
234
What does a 4D CBCT?
It indicates if tumour/OAR motion is consistent. CBCT over respiratory cycle
235
What are the two types of CBCT fans?
Half fan or full fan
236
What CBCT QA is required?
Image quality, scaling, isocentre
For 4D: breathing amplitude, beam turns on and terminates correctly
237
What are the limitations of plotting tanks?
Not good for dynamic delivery and impractical for complex distributions
238
What types of arrays are there to measure dose distributions?
Check devices, simple 2D, rotating 2D, 3D
239
In what way are EPIDs different to diode or ion chamber arrays?
Smaller elements that are closer together and integrated with linac
240
What is an EPID made from?
A metal plate (converts x-ray photons to electrons), phosphor GadOx (scintillator to covert from electrons to light) and a detector array (photodiodes)
241
What is pre-treatment QA for patient dose measurements and what is the goal of it?
Delivered in air with no patient. Goal: check linac can accurately deliver the planned distribution
242
How do you do the readout for radiochromic film?
Densitometer or a flatbed scanner to measure optical density (software to convert to dose)
243
Is radiochromic film sensitive to visible light?
No (yes to UV light though so store in dark envelope)
244
What are the practical issues for accurate dosimetry with radiochromic film?
Storage, handling, time from exposure to scanning, scanner settings and temp, film orientation, calibration
245
Why do in vivo dosimetry?
Detects major errors, identifies planning system problems, independent check
246
How are TLD readings converted to dose?
Performing measurements with TLD simultaneously with absolute dosimeters (Farmer or Roos)
247
Why is it a good idea to measure the relationship between dose and TLD reader output for at least two points?
The response of thermoluminescence in linear up to a point then enters the super linear region
248
Why do diodes often have build up caps?
Reduces PE effect (diode too sensitive in kV range), and it reaches electronic equilibrium
249
What are diodes connected to?
Multichannel electrometers
250
Is there a linear relationship between charge generated and dose for diodes?
Yes (between 1-11 Gy)
251
Do diodes have a real time response?
Yes
252
Are N-type diodes dose rate dependent?
Yes. (P-type less so)
253
How can the temperature dependence of diodes be mitigated for?
Zeroing the current before measuring (need time for background measurements) or if you know the temperature response of the diode, corrections
254
What is the dose range of TLDs?
0.001 to 100 Gy (large)
255
What is the dose range for diodes?
Up to 10 Gy (a lot smaller than TLDs)
256
What is the readout time for diodes?
Immediate
257
Are diodes accumulated dose and dose rate dependent?
Yes
258
Why do MOSFETs have a finite lifetime?
It measures the radiation damage the MOSFET experiences
259
Do MOSFETs have a small or large dose range?
Large (1-180 Gy)
260
Are MOSFETs dose rate dependent?
No
261
Are MOSFETs temperature dependent?
Yes
262
How is radiochromic film sometimes used for in vivo dosimetry?
Placed underneath the patient to measure exit dose
263
Is radiochromic film readout destructive?
No
264
Is radiochromic film accumulated dose or dose rate dependent?
No
265
Is radiochromic film temperature dependent?
Yes
266
Is radiochromic film directionally dependent?
No
267
Is the dose range for EPIDs small or large?
Relatively small (1-20 cGy)
268
Are EPIDs dose rate dependent?
Yes
269
Are EPIDs accumulated dose dependent?
No
270
Do OSLs have a destructive read out?
No but it does have early fading
271
What are the cons of diodes?
Requires cumbersome calibration with corrections. Cables required during measurements
272
Which factor are the perturbation factors included in?
K_Q,Q0
