First Year Exam: Instrumentation (no TG-51) Flashcards
1
Q
How do you calibrate NanoDots?
A
- Make sure you know an exact dose to some depth for your LINAC
- Place Nanodots ontop of atleast 5 cm of backscatter solid water
- Arrange 4 Nanodots in a square, so that the average reading corresponds to dose at central axis
- Place bolus on top of the nanodots
- Place more solid water on top of bolus, in order to get to a depth that you know the exact dose for
- Irradiate Nanodots and plot your curve
2
Q
What types of dosimeters are typically used as surface dosimeters? (4 common types)
A
OSLDs
TLDs
Diodes
MOSFETS
3
Q
Why would you prefer not to use OSLDs for out of field measurements for high energy beams?
A
They do not measure neutron component
4
Q
What are the common applications of a farmer chamber?
A
Absolute dosimetry in water, solid water phantoms, and air
5
Q
What do farmer chambers measure?
A
Charge (which is then corrected to absorbed dose in water)
Exposure
Air kerma
6
Q
What is the minimum field size that a farmer chamber can measure dosimetry for?
A
5 x 5 cm2
7
Q
What is a typical range of operating voltage for a farmer chamber?
A
100 to 400 V
8
Q
What is the sensitive volume of a PTW farmer chamber?
A
0.6 cc
9
Q
What is the typical radius of a farmer chamber?
A
3.05 mm
10
Q
What is the nominal response of a farmer chamber (that is, typical nC/Gy)?
A
20 nC/Gy
11
Q
What is the approximate maximum instability of a farmer chamber?
A
<= 0.5% change in sensitivity per year
12
Q
Approximately what magnitude is leakage charge in a ion chamber + electrometer system? What causes the majority of this leakage?
A
pico Coulombs
(most of it is due to the wire)
13
Q
What is a farmer chamber’s collection electrode made out of?
A
Aluminum
14
Q
In general, what are the walls of most cylindrical ion chambers made out of?
A
Graphite and PMMA
(graphited acrylic wall)
15
Q
What is the markus (parallel plate) chamber sensitive volume?
A
0.055 cc
16
Q
What is the nominal response (typical nC/Gy) of a parallel plate chamber?
A
0.67 nC/Gy
17
Q
What is the sensitive volume of the PTW semiflex 31013 chamber that we use?
A
0.3 cc
18
Q
What is the radius of the sensitive volume of the semiflex 0.3 cc chamber?
A
2.75 mm
NOTE: IT’S ACTUALLY NOT EXACTLY THE SAME AS THE FARMER CHAMBER. BUT THE DIFFERENCE IS SO SMALL, THAT FOR THE PURPOSE OF A TG-51 EPOM CORRECTION, THERE IS BARELY ANY DIFFERENCE
(also for absolute dosimetry (with solid water for example), the kQ factor for it already takes into account any shifts specific to that chamber)
19
Q
What is the nominal response of a semiflex 0.3 cc chamber?
A
10 nC/Gy
20
Q
What is the PTW TN 31013 semiflex chamber used for?
A
Absolute dosimetry (alternative for the farmer chamber)
21
Q
In general, what does any ion chamber measure?
A
Charge (to be converted to dose in water)
Exposure, air kerma
22
Q
What is the unit of radiation exposure?
A
Roentgen
Or Charge/mass (C/kg)
23
Q
What is the value of 1 R?
A
2.58 x 10-4 C/kg
24
Q
What is the sensitive volume of a PTW TN31014 pinpoint ion chamber?
A
0.015 cc
25
What is a PTW TN31014 pinpoint ion chamber used for?
Dosimetry of high energy photons with high spatial resolution
26
What is the smallest field size that a pinpoitn detector can measure dosimetry for?
2 x 2 cm2
27
What is the A16 chamber used for?
SRS and some IMRT dosimetry application
(due to its excellent spatial resolution and exact pinpoint beam profile characterization)
28
What is the collection volume of a A16 chamber?
0.007 cc
29
What is the smallest field size that a A16 detector can be used for?
What is the biggest field size it could be used for?
3.4 x 3.4 cm2
5 x 5 cm2
30
What are some main advantages of ion chambers?
Good uniform response
Accurate dose reading
Capable of measuring high dose rates
Designed with different sizes, shapes, and volumes (variety)
31
What are some disadvantages to ion chambers?
Easily susceptible to moisture, temperature and pressure
Since air is not very dense, the typical nominal response is low. So you need larger volumes to get enough reading. Size limited
32
Fill in the diagram
33
What is the main role of a guard ring?
Reducing leakage of extraneous charge to the collecting electrode
34
Fill in the diagram
35
Fill in the diagram
36
Briefly describe the region 1
Response is voltage dependent and energy dependent
Large number of ions recombine prior to collection (not enough voltage to separate)
37
Briefly describe the region 6
Clinically useless
Too many secondary ionizations just burn through the gas in the chamber
38
Briefly describe the region 5
Townsend avalanche creates a very large number of secondary avalanches
No more information exists from the original radiation dose
Horrible for dosimetry. Great for detection
Secondary electrons begin to neutralize the local electric field, preventing further recombinations (hence why it levels)
39
Briefly describe the region 4
**This voltage region is not used clinically**
Response to collected energy diminishes due to dilution of counts due to secondary ionizations
40
Briefly describe the region 3
Response is proportional to energy collected and applied voltage
Some secondary ionization begins to occur
41
Briefly describe the region 2
Sufficient voltage to prevent recombination
Insufficient to produce secondary ionizations
Measured signal is directly proportional to number of ionizations produced by incident radiation
Very little increase in response vs voltage
42
How do diodes work?
1. ionizing radiation produces electron-hole pair
2. Electrons are attracted towards N side due to positive bias
3. Holes attracted to P side due to negative bias
4. This diffusion is quantified with an electrometer
5. No external bias needed since the electric field between the P and N junction is already enough for charge pair separation
43
What is measured with a diode by an electrometer?
Accumulated charge or induced current
44
What is a depletion zone of a diode?
Sensitive volume between the P and N junction
The large magnetic field separates all ions to either the P or N zones, further strengthening the field
45
What are the advantages to diodes? (6)
Immediate readout
No bias voltage required
High sensitivity
Very small
Good stability
Small energy dependence for mass stopping power ratios between silicon and water (can measure %DD curve directly)
46
What are diodes typically used for?
Small field dosimetry
Array devices
Electron PDD
In-vivo dosimetry
47
What are some disadvantages to diodes? (6)
Tempearture dependence (0.5% / C)
Dose per pulse dependence
Energy dependence (over response to low energy photons)
Angular dependence
Changes in sensitivity over time due to radiation damage
Need to be hooked up via wires/electronics during time of irradiation for you to record a measurement
48
How do OSLDs work?
1. Small crystals mounted on discs tightly encased in a very thin case
2. Ionizing radiation enter through the medium, excite and release electrons from valence band
3. Electrons from valence band enter conduction band
4. Due to crystal impurities, some of the electrons, as they drop from conduction to valence, are trapped in the in-between energy range
5. Holes are also trapped
6. These trapped electrons are freed during reading via light energy or heat
7. The freed electrons and holes escape their trap, allowing them to recombine at the luminescent center and release light
8. Emitted light is measured using either a PMT or a camera (CCD or CMOS)
9. Light is correlated to absorbed dose
49
How long after OSLD exposure can you get a readout?
10 mins after irradiation
50
What light is used to excite the trapped electrons in OSLDs?
Green light
51
What is one major benefit of OSLDs vs TLDs?
OSLD readout is non-destruction. Meaning you get enough of a readout to get a dose, but you can continue to keep measuring over and over again
TLDs are destructive. You can really only get the readout once
52
What is the dose range that OSLDs can measure?
1 uGy to 15 Gy
53
What are some advantages to OSLDs?
Small
Reproducible
Passive/active detector
No angular dependence
Can be re-read
No dose rate dependence below 100 keV
54
What are some disadvantages to OSLDs?
Delayed readout (10 mins)
Not water equivalent
Cerenkov stem effect
Sensitivity to light
Small temperature dependence
Sensitivity change vs accumulated dose (due to traps getting filled)
Energy dependent
55
What are OSLDs used for?
in-vivo dosimetry (nanodots)
Personnel dosimeters
End-to-end testing
56
What detectors are used in the chest badge? What about finger badge?
Chest badge - OSLD
Finger badge - TLD
57
What is a band gap?
Difference in energy between valuence and conduction bands
(minimum energy required to an excited valence electron to enter conduction band)
58
In terms of bandgaps, what are conductors, semiconductors and insulators?
Conductors: allow for electron flow/current because they have no band gap (valence electrons can flow freely)
Semiconductors: have some intermedia band gap
Insulators: have a very large band gap. Make it very difficult for electrons to flow
59
What is a "trap" in luminescent dosimeters?
Energy levels between the valence band and conduction band caused by imperfections in crystal than can trap some electrons
The trap centers allow detectors to hold some of the absorbed dose energy until readout
60
What is a competitive center?
Trap charge carriers that don't contribute to luminescence other than removing charge from being able to recombine
61
What causes the supralinear response of TLDs vs accumulated dose?
Competitive centers start to fill up and aren't emptied
So electrons moving in the band gap are less likely to get trapped in a competitive center. So recombinations become more probable
62
What does a glow curve measure?
Thermoluminescence vs temperature
Note: the cumulative probability of emission is proportional to temperature AND time
This is why having a consistent heating protocol is so vital. Because your glow curve is directly dependent on the protocol used
63
What is the process used to reuse TLDs? How does it work?
Annealing
Trap centers are emptied and redistributed through a consistent heating protocol (ex. 400 celsius for 1 hour to reset lattice structures, then reduce heat to 80 celsius for 24 hours to rearrange traps that result in a certain peak)
64
What is the process used to reuse OSLDs called? How does it work?
Bleaching
Treat OSLD with light from halogen or fluorescent lamp, or green LED
Empties **most** trap centers
Deep trap centers will NOT be emptied during bleaching, thus changing OSLD sensitivity over time
You can actually avoid this, however, by annealing OSLD at 900 celsius to empty even the deep traps
65
Aluminum is the most common central electrode material for ion chambers. But what is the 2nd most common?
Graphite
66
What is the design difference between a farmer vs thimble chamber?
Farmer chambers have pointed ends
Thimble chambers have rounded ends
67
What are extrapolation chambers used for?
Measuring dose buildup region
68
How do extrapolation chambers work?
Very tiny collection volume
You can find tune the collection volume
You can extrapolate to depth zero (which is impossible to directly measure, because you would need 0 cc of collection volume. Thus dose would always read 0 which isn't always true)
Chamber is at water surface
69
What are the advantages to TLDs? (5)
No angular dependence
Wireless
Dose rate independent
Reusable
Energy independent above 100 keV
70
What are the disadvantages to TLDs? (6)
Can only be read out once
Requires significant time for readout
Supralinear response with reuse
Require special prep and calibration
Temperature dependent
Light sensitive
71
True or False
For diodes %DD scans, the EPOM is actually BELOW the point of measurement?
True
This is due to shielding in the diode
So EPOM shifts in %DD measurements give + something, instead of - as they would be in ion chambers
72
What type of detector is an edge detector?
Diode
73
When is the edge detector used?
Small field relative dosimetry and electrons
74
What are the uses of TLDs?
Secondary dose check
In-vivo dosimetry (ring badge)
75
What is the accuracy of a TLD?
What about OSLD?
3% for both
76
Why does solid water need a fudge factor vs actual water?
Because solid water has **a charge retaining effect** and liquid water doesn't. Literally, solid water an hold charge for a small amount of time.
Also the electron density of solid water is not exactly equal to that of actual water
And additional uncertainties in the construction of the solid water slabs and any inhomogeneities
77
Why are triaxial cables needed for ion chambers?
Because ion chambers have 3 electrodes...
1. Collector
2. Guard
3. HV bias
So you need 3 channels
78
What is the approximate leakage of a triaxial cable?
10-13 - 10-14 Amperes
79
What is the general construction of an IC Profiler?
251 parallel plate ion chambers
5 mm spacing on x and y axes
7.07 mm spacing on diagonal arrays
32x32 cm2 measurement range on x an y axes
45 x 45 cm2 measurement on diagonal axes
Trigger diodes within 5 mm of ion chambers to tell detectors when to start measuring
80
How do you measure a 40 x 40 cm2 field using ICP?
Just move the detector up
Flatness, symmetry and beam shift are not affected by SSD, so you can do this just find
81
equation for symmetry used in ICP?
Da - Db / DCAX \* 100%
Where a and b are two points oriented symmetric of one another across the central axis
82
True or False
ICP has temperature and pressure corrections?
True - recall the collecting chambers are unsealed ion chambers
But it only uses them for absolute dosimetry (which doesn't apply for us at all). For flatness and symmetry, it's not necessary at all
83
What can the ICP be used for?
Measures of flatness, symmetry, field size, beam center, penumbra width, light radiation field
Can also be used to measure beam constancy, steering, collimator and rotational sag QA
84
How many diode detectors are in an ArcCheck? How does this compare to MapCheck?
ArcCheck: 1386
MapCheck: 1527
85
What is daisy chaining? (you don't need to know how to do it yet)
A method for measuring small field output factor using two different dosimeters
86
What are some main points of the Georgia State Regulations (111-8-90) for calibration and spot check QA?
(Don't spend too long on this card)
Keep records for everything as long as you can
Time between calibration should not exceed 12 months
Need to calibrate after change to machine performances
Need a "qualified expert" to do calibrations
Spot check calibrations which are our weekly/periodic QA's should be regularly performed
A qualified expert is a ABR certified medical physicist
87
What is the most common scintillation detector we may see in the clinic?
W1
88
Why do scintillation detectors need to be coupled with PMTs?
Because their light output is too low so it needs to be amplified
89
What symmetry equation does DailyQA3 use?
(Arealeft - Arearight) / (Arealeft + Arearight) \* 100%
90
How does DailyQA3 measure energy?
**Electrons:** has different materials of different densities and measures output at chambers under these materials for electron beams
**Photons:** measures flatness on the premise that changes in energy of beam will produce changes in beam flatness
91
What are some benefits to scintillator detectors? (9)
Water equivalent
Fast response
Temperature independent
Energy independent
Dose rate independent
Linear response to dose
Great spatial resolution
No angular dependence
Can come as organic, inorganic or plastic
Small volume
92
What are some disadvantages to scintillator detectors? (2)
Cerenkov noise
Change in sensitivity due to plastic yellowing
93
What are scintillation detectors used for?
Small field dosimetry
Electron measurements
Nuclear medicine
94
How do scintillation detectors account for Cerenkov signal? (3 ways)
1. Temporal avoidance (Cerenkov signal usually lasts 500 ns less than the actual scintillation signal. So you want to only measure the final 500 ns of a measurement)
2. Optical filtration (wavelength selection)
3. Dual light pipe design
95
What are three things that degrade SNR in scintillators?
1. Cherenkov
2. Dark current (electrical current that flows through photodetectors even when no photons enter) (can usually just be subtracted out)
3. Direct interactions between radiation and detector
96
How does a dual light pipe work for scintillation detectors?
One light pipe connects to the photodetector
The other is near scintillator but shielded from it. So it collects only background (Cherenkov) reading
97
What is the pro and con to GM counters?
**Pro:** Can easily detector contamination
**Con:** Cannot give a trustworthy exposure rate **unless measuring the source that it was calibrated with**
98
What is the general design of EPIDs? Give both the historical and current designs.
**Historical:** **camera** (radiation hits photophosphur, producing light, which bounces off a 45 deg mirror and hits a camera) or **ion chamber type** (array of liquid ion chambers)
**Current: Amorphus silicon photodiode arrays.** Metal plate on top, scintillator (phosphur) beneath, amorphus silicon later underneath those
99
How does an amorphus silicon EPID work?
Metal plate convers photons to compton electrons (4% interact with plate, 1% actually convert properly) (MASSIVE loss in signal)
Some photons that are missed by the copper plate may be caught by the phosphor
Phosphor scintillator converts photons and electrons into visible light
Photodiodes implanted on a amorphus silicon panel detect visible light and send signal to read-out electronics
100
What are EPIDs used for?
Patient setup verification
Assessment of target and organ motion
In-vivo dosimetry distributions
Patient specific QA
Machine QA (PF, HBB, WL, etc)
101
What is the resolution of amorphus silicon EPID?
\< 0.5 mm
102
what units do survey meters measure in?
Either exposure over time (mR/hr) or counts per minute (cpm)
103
Which survey meter is best for detection of beta radiation?
Scintillation probes (which measure in cpm)
104
Which survey meter is best for photon contamination surveying?
Geiger counters
105
Which survey meter is best for survey and measurement of exposure in air from neutron contamination?
REM balls
106
What does a REM ball measure?
It means both neutron AND photon exposure
107
How does a REM ball work?
Has a sphere filled with polythylene, which slows down neutrons, creating gamma rays in the process which are detected by a survey meter
108
If you want to measure only exposure from neutrons, how would you do it?
You have to have a REM ball and a separate survey meter that doesn't have neutron capabilities
REM ball will measure photons and neutrons, so you want something to allow you to subtract out the photon component
109
What is the unit of measurement of most neutron detectors?
mrem/hr
(it gives dose equivalent using the known quality factor for the neutron energies it's designed to detect)
110
How does bonner sphere spectroscopy work?
Multiple spheres with thermal neutron detectors embedded between the moderating spheres
The moderating spheres will slow down the neutrons to thermal neutron range, which will be measured by the thermal neutron detector
Having multiple moderating thicknesses allows for spectroscopy (measuring of multiple energies)
111
Since a rem ball is just a single bonner sphere, it only measures a certain range of neutrons. How then does it account for all neutron dose?
By correcting sensitivity with an approximation for radiation weighting factor across a range of neutron energies
112
Explain the general design of a rem ball
A rem ball is an example of an activation foil neutron detector
Most outer layer is a cadmium coating (to filter out thermal neutrons)
In-between layer is a moderator (polythylene or BF3)
Inner most layer is a thermal activation foil to absorb the neutrons and produce photons
ontop of the REM ball is a survey meter that measures photon exposure
113
True or False
Unlike REM detectors, Bonner spheres are able to filter out photon exposure by themself
True
114
What are bubble detectors used for?
In-vault neutron dose monitoring
Personnel neutron monitoring
115
What is the useful range of TLDs and OSLDs?
mR to 100 Gy
116
What are MOSFETs useful for measuring?
In-vivo dosimetry
Penumbra
Very small field sizes
117
What is the quantity measured in a MOSFET detector?
Threshold voltage
118
Why does EPID dosimetry require many corrections?
Due to the high-Z materials found in the EPID
119
What is the lifetime of a MOSFET detector?
100 Gy
120
What are some cons to MOSFET detectors?
Not water-equivalent
Limited life-time
Temperature dependent
Response degrades with accumulated exposure
Energy dependent
Expensive
121
What are some pros to MOSFET detectors?
Small
Available in arrays
No angular dependence
Real-time in vivo capabilities
Dual bias eliminates temperature dependence
Permanent dose storage
122
For cross calibration of a PP chamber, what is the most appropriate energy to use (according to TG-21, TG-39 and TG-51)? Why?
Use the highst available electron energy
Therefore Prepl is no smaller than 0.98 (infact for energies \> 15 MeV it approaches unity)
123
How do you cross calibrate a PP chamber?
Remember: the only thing you don't have with cross calibration is the kecal, so that's included in your end result
124
What is the sensitivity reduction rate of diodes vs dose?
\< 1-2% reduction per 250 Gy
125
If you have two diodes, one for 6 MV and one for 18 MV, and you have a treatment that uses both 6 and 18 MV, but you can only use one diode, which should you use?
Use the 18 MV diode
That way the buildup is sufficient enough and reduces electron contamination still
126
What causes directional dependence of a diode?
Partly by detector construct (transmission through varying thicknesses or cable)
Partly by back scattering from patient or phantom
127
What phantom is used for QA of respiratory motion of CT and in-room EBRT tracking? How does it work?
Quasar phantom
It simulates a patient breathing by moving cylindrical inserts on the superior/inferior direction
The inserts can also have high density spheres that simulate tumors
128
How does a Las Vegas phantom work?
Varies contrast by increasing or decreasing hole depths, and varies resolution by increasing hole diameter
Allows you to measure EPID contrast and spatial resolution
129
What is the other version of a Las Vegas phantom, but circular orientation and uses line spacings in the center for the resolution test instead of varying hole thicknesses?
Leed Phantom
130
What are the planar imaging phantoms that we use for kV and MV OBI image quality?
the SNC kV-QA and MV-QA phantoms
131
What is the phantom that Charlotte uses to measure light vs radiation field coincidence and jaw positioning?
The SNC FS-QA phantom
132
What does a WL cube allow you to QA?
Imaging, Radiation and Mechanical isocenter coincidence
And the size of your isocenter
133
Approximately how much error exists between solid water and real water, on average?
0.5% error
134
How often do electrometers need to be calibrated?
Once every 2 years at ADCL
135
How is a electrometer calibrated at the ADCL?
* Set up a simple single capacitor circuit
* Capacitor is calibrated with known capacitance
* A known voltage is applied to the circuit
* Q = CV
* Vary capacitance from a range of pico to nano Coloumbs to measure linearity
* Charge reading should increase linearly with capacitance
* Unit of calibration: C/reading
* Estimated uncertainty to k=2, 95% confidence
136
What is typical electrometer leakage on low mode? What about high mode?
1 fA or 1 fC on low mode
1 pA or 1 pC for high mode
137
Which mode do we use on our electrometer? Low or High?
High mode
138
Briefly, how does radiochromic film work?
Film has an active layer of a radiation-sensitive polymers that changes chemical structure to a blue shade when exposed
Polymers sit on an inert polyster substrate
You measure optical density and plot that with dose
139
What are the advantages to radiochromic film? (7)
Large measuring area
Very good spatial resolution
Tissue equivalent
No chemical processing needed
Accurate within 2-3%
Energy independent
Not sensitive to visible light (only UV)
140
What are the disadvantages to radiochromic film? (5)
Need a calibration curve
Need a scanner
Need to wait 24 hours before reading due to continuing polymerization
High cost per film
Scanner readout is directional dependent
141
What is the useful range of radiochromic film?
(this is across all film, not just EBT3 or EBTXD)
0.1 Gy - 100 Gy
EBTXD is capped at 50 Gy
But some fi;m, particularly those used for Trigeminal neuralgia, will need to go up as high as possible
142
What are the advantages to radiographic film? (4)
Un-matched spatial resolution (micrometer)
Large measuring area
Cheaper than gafchromic film
Dose rate independent
143
What are the disadvantages to radiographic film? (7)
Require development
Accurate to only within 3-5%
Sensitive to visible light
Strong energy dependence below 400 keV (due to the silver)
Not tissue equivalent
Bulky readout system
Emulsion differences among films and batches
144
In general, what are some things that film good for measuring? (that we use in our QA)
Some IMRT QA
Light vs radiation field
Star shots
Profiles (although we don't really do it for anything other than Prp)
145
What type of radiation (and energy ranges), does the Fluke 451P detect?
Beta above 1 MeV
Gamma above 25 keV
146
What is the maximum reading you can get with a 451P?
5 R/hr (50 mSv/hr)
147
What is typical background for a 451P?
\< 10 uR/hr
148
What is the battery lifetime of a 451P?
200 hrs when using two batteries
You can also use only one baterry, and it operates for 100 hrs instead
149
What's approximate warmup time for a survey meter that has been off for 12 hours or more?
4 mins
150
In general, measuring from what area of the 451P is the most sensitive? Side, face or front?
Face
151
True or False
The 451P energy response is near unity for the entire detection range
False
It's low for lower energies, highest for 80-100 keV (1.2), and then levels out near unity for higher energies
152
What is the calibration energy range of the 451P?
20 keV to 2 MeV gamma and xrays
153
When a 451P "LOW BATTERY" indicator first appears. How many hours of operation are remaining?
6 hours
154
What batteries doe sa 451P take?
9V
155
Which is the positive side of a PN junction?
The N is the positive side
Remember: N-type is the electron **donors**. Meaning when the P and N junctions were initially placed together, the electrons of the N flowed to the P, giving the N a overall positive side
This electron flow happened for enough time until an electric field occured, retarding additional flow
156
When we say that a diode is losing sensitivity due to radiation damage, what is actually being damaged?
The crystal structure. Defects occur over time in a silicon diode due to radiation
The defects produce RG centers and carrier traps (defects which capture carriers but have a very small probability of recombination)
157
What are some correction factors that need to be applied when using diodes for in-vivo dosimetry? (8)
Entrance SSD
Exit SSD
Entrance field size
Exit field size
Accessories
Temperature
Angular
Patient thickness
158
What are the axes of a H&D curve?
Y: Optical density
X: log exposure or log dose
159
What types of film have better contrast, high speed or low speed?
High speed
160
What types of film have a larger linear region (or measurable region). High speed or low speed?
Low speed
161
In the following curve, what does "base" and what does "fog" mean?
Base is OD caused by the natural attenuation of the film
Fog is the OD caused by darkening of the film due to background radiation or, (for radiographic film only), light exposure
162
What is the dosimetric accuracy of radiographic film?
What about radiochromic?
Radiographic: 3-5%
Radiochromic: 2-3%
163
What is the approximate spatial resolution of radiochromic film?
Sub mm
164
What is the difference between un-symmetric and symmetric radiochromic film? What kind is EBT3?
Symetric film, as the name would suggest, is constructed symmetrically when an equal amount of upper and lower material sandwiching the active layer. Non-symmetric has an unequal amount sandwiching the active layer
EBT3 is symmetric
165
What is one consideration you need to have when working with non-symmetric film, that you don't need to have when working with symmetrical film?
For non-symmetric film, orientation during measurement and readout matter
(in reality, it's shown that this impact is very minimal, but worth accounting for)
166
Which detector did we use to measure cone output factors?
Edge detector
167
What are the 4 modules for the catphan 504 used for?
1. Uniformity
2. High resolution (21 aluminum line pair high resolution. Most systems can only distinguish 7-9)
3. Low contrast resolution
4. Slice geometry, contrast linearity, HU constancy, geometric distortion
168
What is the name of our parallel plate chamber?
The Markus Chamber
169
In terms of cost, what is cheaper, EPID or film for portal verification?
EPID is more expensive at first (initial installation and purchase), but over time the cost per scan for EPID is much lower than for film. So over time film cost outweighs EPID
170
What is the collection diameter of the A16 chamber?
0.33 mm
171
What kind of QA would you do on an electrometer? What would you check for? (5 things in mind)
Linearity checks
V=QC to get an exact expected charge and make sure your electrometer confirms it
Cross checking two electrometers
Expecting a nominal response of roughly 20 nC/Gy for a farmer chamber
Inherent leakage and dark current checks
172
Why would you not use a PP chamber for photons?
Small volume, not as sensitive, too much noise
You need to be extra careful that the chamber is flat and perpendicular to beam
Not isotropic
Less researched
It's one good use for photons, buildup dose, is usually given by varian
173
Where do you want to use a REM ball to measure neutron contamination from LINACS?
Outside vault door
At the shadow shields (extra shielding added to to some kind of hole or compromised shielding for architectural reasons)
174
What are the upper and lower limits of a GM counter?
mR/hr - 2 R/hr
175
What is the design of a hankins rem ball?
Outer layer of very thin cadmium foil
Inner moderator of 6 cm Polysthylene sphere
Ludlum 12-4 or Eberline NRD detectors in center (both of which are BF3 or He-3 detectors), and **both of which measure gamma**
176
What 4 detectors that are commonly known would you consider using for a small field measurement?
A16, W1, W2, Edge detector
177
For DailyQA3 dose calibration, are you getting a energy specific factor, or a machine specific factor?
It's machine specific
You only collect **one dose calibration factor** per machine, as opposed to ArcCheck and MapCheck where it's energy specific.
Note: ArcCheck and MapCheck should actually also be machine specific, but our beams are matched so we just do energy specific.
178
What is a typical value for a diode's sensitivity variation with temperature?
0.5% / celsius
179
How soon after irradiation are TLDs read? Why?
24 hours after irradiation
The lowest energy traps willr elease their electrons in the first 10 hours. After this, the accuracy of the TLD becomes maintained and stable to within 12 weeks after exposure. So 24 hours is a safe strategy to avoid the first 10 hour releases
180
When reading a OSLD, what percentage of trapped electrons are released per reading?
0.2% released per reading
181
Between TLDs and OSLDs, which is smaller? Which has a smaller angular dependence?
TLDs are both smaller AND have a lower angular dependence
182
What are the advantages and disadvantages of diodes vs ion chambers?
Diodes are much more sensitive, and can therefore be built much smaller than ion chambers (2 eV/ip on average, vs Ion Chambers which need 34 eV/ip)
BUT
Diodes have energy dependence, especially in the kV range, and lose sensitivity with accumulated dose
183
To what depth are diodes typically calibrated?
Either skin for skin in-vivo (0.5 mm deep) or to dmax of the beam energy
184
List all the detectors that can be used for absolute calibration of a linear accelerator
Ion Chamber...
Nothing else
185
True or False
Radiochromic film is not sensisitive to visible light
False
It's very slightly sensitive. Can be close to negligible
186
True or False
Radiographic film is not sensisitive to visible light
False
187
What device is used to measure OD of radiographic film?
What device is used to measure OD of radiochromic film?
Radiographic - densitometer
Radiochromic - Laser scanner, microdensitometer, spectrophotometer
188
What does ADCL stand for?
Accredited Dosimetry Calibration Laboratory
189
What dependencies do MOSFETs have?
Angular dependence and energy dependence (not water equivalent)
190
What are three dosimeters that measure absolute dose without needing calibration?
Calorimeter, Fricke Dosimeter and Free Air Chamber
Below is an image of a fricke dosimeter
191
What is the most famous anthropomorphic phantom named?
RANDO phantom
192
True or False
All TLDs are able to measure neutron component
False
Some TLDs can measure neutron component, some cannot, and some even over-respond to neutrons
193
What causes the "SSD Dependence" in a diode, leading to the need to calibrate diodes at specific SSDs?
Diodes are dependent on instantaneous dose rates, and depending on the SSD, the instantaneous dose rate will differ
Thus, the SSD factor for diodes is actually a measure of dependence of instantaneous dose rate dependence
194
Why are diodes not sensitive to the time between pulses (dose rate set at console)?
Because the collection time of a diode is much shorter than the time between pulses. So no deadtime affect
195
What is the sensitivity variation of a diode from 80 - 120 cm SSD? What causes this sensitivity change?
1%
Caused by change in instantaneous dose rate per pulse
196
Up to what percent are diodes field size dependent?
Change in reading up to 5% with change in field size
197
Does diode sensitivity increase or decrease with increasing temperature?
Increases with increasing temperature
0.5% / celsius
198
For our truebeams, how many diodes, at minimum, should we have?
3 diodes at minimum
One for low energy photons (6 - 10 MV)
One for high energy photons (15 -18 MV)
One for all electrons
199
Why can't we use one diode for all energies?
A diode needs to have enough buildup to shield contaminate electrons, but not enough that the dose shadowing (shielding) below the diode becomes significant (this is especially pronounced when a photon diode is used in electron therapy)
200
Does diode sensitivity increase or decrease with increasing field size?
Increases
201
For diode in-vivo dosimetry for TBI, which of the following correction factors are of particular concern?
Temperature
Field Size
Leakage
Angular
SSD
For diode in-vivo dosimetry for TBI, which of the following correction factors are of particular concern?
**Temperature**
**Field Size**
**Leakage**
Angular
**SSD**
202
According to TG-235, approximately what dpi should you use to scan radiochromic film?
70 dpi
203
Why can you not use a GM counter for detection of LINAC fields?
Because the pulsed beam will produce excessive dead time accumulation
204
Give the general design of a triaxial cable
It has an inner cable/electrode and is surrounded by two outer layers, an insulator and a conducting sheath.
This protects from noise and leakage signal
205
206
What is the minimum 3D tank dimensions as required by TG-106 for LINAC commissioning?
Each dimension must be atleast 5 cm larger than the maximum field size at max depth, and the max depth itself.
So max depth is 40 cm, so depth of tank must be atleast 45 cm. Max field size is 40x40, but at 40 cm depth it's larger, so do the math for how large the tank needs to be. Conservative estimate is atleast 75x75 cm2
207
Why is the use of distilled water recommended for commissioning per TG-106?
Because the length of time of data collection is so long, algae may grow and mess up the setup/equipment.
208
Below what field size does TG-106 recommend using a micro-ion chamber or diode?
4 x 4 cm2
209
For diodes used for electron scans, what region would they fail in on the electron depth dose curve?
They fail in the Bremmstrahlung region
This is because diodes are designed for either photons or electrons. So when you use an electron diode, and you measure in the photon region of the electron curve, there's gonna be issues
210
Why can you not use a farmer chamber for beam profile commissioning?
Because of volume averaging in the penumbra
211
What device would you use to measure wedge profiles during beam commissioning?
Detector Arrays
212
In a cylindrical ion chamber, where is the electric field occuring?
Between the central collection electrode, and the outer electrode which is the inner layer of the chamber wall, and sometimes even in the wall itself is a electrode.
213
What is the most common material diodes are made out of?
Silicon
214
What materials are OSLDs?
Aluminum oxide doped with carbon
215
What materials are TLDs?
Lithium Fluoride doped with Magnesium and Titanium
216
Why can you not calibrate a surface dosimeter by having it on surface and delivering a corresponding known dose?
You can, if all you want to do is have the dosimeter as a form of secondary check on machine output. That's allowed
But if you want something for In-Vivo dosimetry, it makes no sense since in real life most of your dose is not coming from en-face beams
