Raphex 14-17 Flashcards
How does the surface dose on an e- beam vary with energy?
It increases w/ energy, unlike for photons
How does e- beam obliquity change side scatter?
Increases it
How does the surface dose on an e- beam vary with energy?
It increases w/ energy, unlike photons
How does the range and Bragg peak of charged particle, like protons and e-, vary with particle mass?
- The heavier the particle, the less it scatters
- Less range uncertainty
- Sharper Bragg peak
How does obliquity affect e- dose and PDD?
- It increases the surface dose
- Leads to less sharp dose fall off
What happens to the 90% isodose line as you increase the energy of e-?
It constricts
How does the e- current required to produce photons 𝛥 w/ the photon beam energy?
- Photon production efficiency increases linearly w/ beam energy
- Higher current is required at lower than higher beam energies to produce the same dose rate
Which F isotope is used for PET scans? Which F isotope is stable?
- F-18 is used for PET scans
- F-19 is the stable isotope
Is 3H stable or unstable?
- Unstable and radioactive
- It’s used in research
Is 3H stable or unstable?
- Unstable and radioactive
- It’s used in biomedical research
How does the energy of positrons compare w/ PET spatial resolution?
- Positrons travel some distance before annihilating. This travel distance adds uncertainty to the PET scan and contributes to poorer spatial resolution.
- Using positrons with lower energy w/ decrease the travel distance and improve uncertainty and resolution.
Are annihilation photons emitted exactly anti-parallel in a PET scan?
No. This contributes to the poorer spatial resolution of PET scans.
What is the dynamic range of a CT scanner?
It is the ratio of the largest signal value to the smallest signal value
What’s the normal shape of the dose-response curve?
Sigmoidal!
Note the difference between the dose-response curve and the survival curve!
What is the mechanical iso-center?
Point of intersection of the couch, collimator, and gantry axes of rotation.
How do you identify the dosimetric isocenter of a linac?
Star shot
Can intraop radiation be delivered using a linac?
Yes!
As it related to its energy, which brachytherapy source would require the most anisotropy correction?
Source w/ the lowest energy, eg Pd-103, since it will be most affected by absorption by the metallic seed casing.
How is the backscatter factor related to dose and time of treatment?
- BSF ∝ Dose
- BSF ∝ 1/ tx_time
Which dosimeter has the least energy dependence in the kV range?
Radiochromic film
How is the total scatter factor measured?
It’s a combination of collimator and phantom scattered, and is measured in water.
Can EPIDs measure e- outputs?
No! They need photons, which are sequentially converted into e- and visible light, etv.
Can ion chambers be used in vivo?
No! They have a high bias voltage (300 V), which would endanger the patient.
Which dosimeter is unsuitable for SRS dosimetric measurements?
A farmer chamber, as its size is too large compared to SRS field sizes.
How often do radiation survey meters need to be calibrated at an accredited laboratory?
Annually
How often do radiation therapy dosimeters need to be calibrated at an accredited laboratory?
Bi-annually
How does the penumbra 𝛥 with ↑ beam energy?
↑ beam energy → ↑ penumbra 2/2 higher lateral range of secondary e-
What’s the % attenuation for 6 MV and 15 MV photon beams?
- 6 MV: 3%/cm
- 15 MV: 2%/cm
How does the degree of loss of charged particle eq at tissue boundaries related to field size?
↓ field size → ↑ loss of equilibrium
Why do protons deposit almost all of their energy at the end of their range while e- do it in a more random manner?
Because proton are 2000 times heavier than e-, they travel in a nearly straight line.
What does narrow beam geometry mean?
Means that the beam geometry is such that only the primary radiation can reach the detector.
How does the penumbra of a Co-60 compare with a 6 MV Linac beam?
Co-60 will have a larger penumbra 2/2 small source size!
What particles contribute to the skin dose of an MV photon beam?
- e- contamination
- incident beam
- backscattered beam
What happens to dose calculations in a patient with a metal prosthesis?
- Metal prosthesis saturates CT value, resulting in an underestimation of the CT number
- This underestimation leads to a falsely low attenuation value compared to the true value
Can metal implants affect RF fiducial tracking?
Yes!
Why do we use effective SSD for e- treatments?
To produce correlation with the inverse square law!
Which DICOM format contains dose matrices?
- DICOM-RT dose file
What’s the occupancy factor of a room set to when calculating the maximum allowable dose rate?
1
When designing shielding for a Linac room, should lead or steel be placed inside with concrete on the outside or the converse?
- Lead or steel should be placed inside as they can produce photoneutrons
- Concrete should be placed outside to block all neutrons
What device is used for portal dosimetry for a Linac?
MV imager (NOT kV imager)
What is a piezoelectric material and which imaging modality uses it?
- Material that produces acoustic waver from radiofrequency
- Used by ultrasounds
What part of the dynamic jaw corresponds to the heel of a physical wedge?
The part that’s covered by the jaw for most of the treatment.
How do you calculate ITV from CTV
- ITV = CTV + Internal Margin
- ITV; Internal Treatment Volume
- IM; Internal Margin
What is the most consistent and reproducible part of the breathing cycle?
- End exhalation!
Why is collimator rotation used during IMRT?
It decreases the dose banding effect from the tongue in groove design of the MLCs
What’s the main advantage of using EPID vs. diode array for in vivo dosimetric measurements?
↑ resolution
What’s the main advantage of VMAT vs. IMRT?
Shorter treatment time, less opportunity for intrafraction movement
What’s the purpose of using a compensator for TBI treatments?
- To compensate for differences in tissue
- Improved dose homogeneity
What’s the primary purpose of a beam spoiler, and how should it be positioned relative to the patient?
- ↑ skin dose
- Place as close to patient as possible
What agreement between calculated and measured doses and dose uniformity is considered acceptable for TBI protocols?
- Dose agreement: 5%
- Dose uniformity: 10%
What are fundamental particles?
- Particles that are not made of smaller component particles
- Include e-, and quarks (which make up protons, neutrons, etc)
What does insufficient vacuum inside an XR tube cause?
- Short-circuit, or tube arcing
- air/impurities present in the tube cause current to flow between the cathode and usually the tube envelope
- No useful particles are produced by the XR tube when this happens
What’s the reasoning behind making the effective smaller than the actual focal spot for an XR tube?
- ↑ actual focal spot: Heat distributed over a larger area → greater heat capacity
- ↓ effective focal spot → improved image quality
What’s the primary goal when using an FFF beam?
↑ dose rate and ↓ tx time
How does the dmax of an FFF beam compare to the dmax of a filtered beam?
- FFF beam dmax < Filtered beam dmax
– FFF beams have lower average energy than filtered beams
What XR energies are available for a tomotherapy unit?
- 6 MV only
- Used for tx & acquiring images
What’s the SAD for a tomotherapy unit?
85 cm
What kind of MLCs do tomotherapy units have?
Binary MLCs
What’s the unique feature of tomotherapy XR shielding?
Tomotherapy units have internal shielding, which can attenuate the primary beam by 99.9%!
In which kind of treatment is using a bolus over OSLDs inappropriate?
When doing TSET, since we want to measure skin dose itself!
What’s the integral dose and how does it 𝛥 with photon energy, field size, incidence dose, & patient thickness?
Dose x Massirradiated
- ↑ Photon energy → ↓ Integral dose
- ↑ Field size, patient thickness, incident dose → ↑ Integral dose
What’s the difference between a mask and a filter?
- A mask blocks the beam, affecting intensity and field size but NOT energy.
- A filter attenuates the beam, affecting its energy and intensity but NOT the field size.
If a person has irregular breathing, how would it affect his 4D CT scan?
Some phases will image the same phases, leading to discontinuities b/w slices.
How many gray levels in a x bit image?
2x
What’s the formula for the dose rate?
It’s basically the denominator in the ΜU formula.
Dose Rate = O x PDD (or TMR) Scp x WF x TF
