Raphex 10-13 Flashcards
What’s the ratio of neutrons to protons in heavy nuclei?
1.4 to 1.6
More neutrons help the nucleus overcome the repulsive forces b/w protons.
A ratio < 1 would make the nucleus highly unstable.
What do the energies of characteristic X Rays of a particular material depend on?
They equal the difference in the electron binding energies of the material.
In diagnostic XR machines, how is the contrast of the image related to beam energy?
Contrast ∝ 1/energy
In diagnostic XR machines, how is the contrast of the image related to beam energy?
Quantum noise ∝ 1/energy
Which radiation-matter interaction contributes to beam filtering?
PE effect
How does the electron applicator interlocking affect the LINAC?
The beam cannot be turned on until it is interlocked.
How are the waveguides in high-energy vs. low-energy LINACs mounted?
Low energy → Vertical, perpendicular to the gantry axis rotation
High energy → Parallel to the floor
How does neutron contamination vary b/w photons and electrons?
Photons (≥ 10 MV) > electrons
How do fast neutrons dissipate energy in tissues?
- Main Mech → Elastic collisions with hydrogen nuclei (protons) present in the tissue
- Inelastic collisions with heavier nuclei, resulting in disintegration, of which the reaction with nitrogen giving rise to a proton of 0.66 M e is the most important
- Elastic collisions with heavier nuclei present in tissue
- Neutron capture by hydrogen giving rise to 2.2 MeV γ-rays by the (n, γ) reaction
Is EM radiation deflected by the magnetic field?
No, only charged particles are affected by the magnetic field.
What does CT number depend on?
linear attenuation coefficient: μ
CT number = 1000 x [{μ_mat - μ_water)/μ_water]
Why is the mass attenuation coefficient similar for most low Z materials?
Because they have 2 nucleons for every electron
However, hydrogenous materials are notable exceptions, because they have 1 nucleon for every electron.
At what angle relative to the angle of the incident photon is the Compton electron ejected?
0o
At what angle relative to the angle of the scattered photon is the Compton electron ejected?
180o
If you collect all the charge produced by a photon beam in a small volume of air, what are you measuring?
Exposure
How do you convert exposure to absorbed dose?
Absorbed Dose = Exposure x f
When we say a beam has 10 MV energy, what are we referring to?
This is the max electron energy, which is also the max bremsstrahlung energy.
Avg beam energy is 1/3 rd the max energy.
How does distance impact the dose rate?
The rate decreased with the distance
What’s the highest beam energy produced by a cyclotron (proton therapy)?
250 MeV
What’s the highest energy produced by gamma knife (Cobalt-60 therapy)?
1.25 MeV
What’s the highest beam energy produced by Linacs?
25 MeV
What’s the highest energy produced by cyberknife?
6 MV
Dose measurements in an ion chamber need to be corrected to readings obtained at what temperature and pressure?
22oC, 760 mmHg
What is the homogeneity index?
Degree of dose uniformity in the target volume
What’s the density of bone as compared to soft tissue?
Bone Den = 1.6 x Soft Tissue Den
What is the attenuation per cm of a 6 MV beam?
3.5 %
Why are wide tangents (often used to treat breast IMN) not ideal?
Inadequate coverage of the breast tissue
How is dose rate and distance related?
Dose Rate ∝ 1/distance
How does the photon beam PDD curve vary from surface to deeper tissues?
- Increases from surface to dmax
- Exponentially decreases after dmax
What’s used for isocentric vs. SSD ΜU calculations?
Isocentric: TMR
SSD: PDD
How does the homogeneity of the dose profile for parallel opposed fields vary below dmax?
The higher the dose, the more homogenous the dose profile
How does the beam profile vary for flattened beams at shallow and deep depths?
- The filter “over-flattens” (profile at the center is dipped, profile on the outside has horns) the beam at shallow depts. This is usually the case at dmax for most beams
- The filter “under-flattens” (profile in the center is higher than that on the outside) the dose at deep depths
How does ISF vary with an extended SSD?
It decreases, since
ISF = [SSDref + dmax/SSD + dmax]2
How does the absolute dose vary with an extended SSD?
Absolute decrease is the dose w/ increasing SSD
Absolute dose ∝ 1/r2
The dose decreases according to the inverse square law (an absolute decrease)
How does PDD vary with an extended SSD?
Increase in PDD w/ increasing SSD
There is an absolute decrease in the dose, but the dose will no longer fall off as rapidly with depth (a relative increase) with increasing SSD
What is the difference between PDD & TMR?
- PDD is measured by moving the detector to different depths in a stationary phantom. The dose falls off due to both attenuation and distance (inverse square).
- TMR is measured by moving the phantom to different depths with a stationary detector. The dose falls off due to attenuation only
Therefore, TMR values can be higher than PDD at certain doses.
How does the contralateral breast dose vary b/w an open and a wedged field?
The contralateral breast receives more dose with a wedged field because of scatter from the wedge itself.
The wedge does block some of the head scatter, but its own scatter is considerably more.
How does beam hardening differ b/w dynamic and conventional wedges?
Dynamic wedges do NOT harden the beam
How does the scattered dose vary between dynamic and conventional wedges?
The scattered dose is far less for dynamic than for conventional wedges.
How is a dynamic wedge created?
By closing one collimator jaw during irradiation
What is the wedge angle?
The angle through which the isodose line at 10 cm is rotated from its position in an open beam.
What’s the beam energy for a tomotherapy unit?
6 MV only
Can a tomotherapy unit deliver e-?
No!
For an e- treatment, what contributes to the dose beyond the max range of the e-?
Bremsstrahlung
What e- field size is required to treat a x cm target?
You need at least 1 cm field on either side for adequate coverage, so field size → x + 2 cm
How much energy does an e- beam lose per cm in tissue?
2 MeV / cm
That’s why the e- range in tissues is MeV/2
Does the surface dose of an e- beam change with collimator(s) and foil(s)?
Yes
What’s the surface dose of orthovoltage photon beams?
100%
How does the electron dose decrease with increasing air gap?
2%/cm
The air gap is the gap between the applicator and the treatment surface.
For every cm of change, the airgap changes by 2%
What’s the tolerance for Linac daily, monthly, and yearly output checks per TG-40?
Daily → 3%
Monthly → 2%
Yearly → 2%
What are the steps that must be taken while decommissioning and trashing a Linac?
Check for any lead, depleted uranium, or activated metal parts.
What’s the air kerma rate at the pubic symphysis for patients receiving I-125 seed implantation in the prostate?
25 μGy/h
What’s the shielding design goal for an uncontrolled area?
- 1 mSv/year
- 0.02 mSv/week
- 0.02 mSv/hr
What are the shielding recommendations for controlled areas?
- 5 mSv/yr
- 0.1 mSv/wk
What’s a controlled vs. uncontrolled area?
Controlled: Limited access areas where the occupational exposure of personnel to radiation is under the supervision of a radiation protection program.
Uncontrolled: All areas not considered controlled areas are considered uncontrolled areas.
What is the formula for permissible dose equivalent for an area?
W = workload; total weekly radiation delivered
U = use factor; fx of operating time during which a Linac is directed towards a particular barrier
T = occupancy factor; fx of operating time during which the area is occupied
d = distance from the radiation source
B = transmission factor of a barrier
How much higher are the linac-leakage workloads for IMRT vs. conventional radiation?
2-10 x higher
What thickness of concrete is enough to shield Linacs outputting up to 18 MV of photons?
260 cm, or 8.6 ft
During treatment of breast cancer w/ tangents, what dose do the ovaries receive?
< 25 cGy (0.5%)
This comes from internal scatter and cannot be shielded against.
Can a lead apron shield against Linac’s head scatter?
No, a typical lead apron is only 0.5 mm.
What does the probability of Compton scatter depend on?
e- density
What are the steps in the conversion of photons into an electronic output in amorphous silicon electron portal imaging devices (EPIDs)?
- Metal plate (1 mm Cu): XR → e-
- Phosphor screen: e- ionization → visible light
- Photosensitive diode array: visible light → e--ion pairs
- Charges collected by a bias voltage onto a storage capacitor
What is the XR energy used to acquire portal images on a Linac?
6 MV
What are the primary XR-matter interactions for 6MV photon beams used for portal imaging?
- Compton scatter (predominant)
- Pair production (secondary)
Which portal imaging system has the best resolution?
Radiographic films»_space;»» any digital system
What’s the advantage of the traditional MV electronic portal imaging system over the newer kV imaging systems?
In addition to bony anatomy, the position of the beam-shaping devices such as a multileaf collimator or a block can also be seen.
This is because the imaging beam literally originates from the head of the Linac for MV imaging, as opposed to KV imaging systems, which are independent entities.
How do the hotspots within the PTV compare between IMRT and 3D plans?
Usually, more hotspots with the IMRT plan
Does IMRT require more or fewer ΜUs than traditional RT delivery models?
3 to 5 times more
Which beam energy is chosen for prostate IMRT plans?
10 MV
More sparing of normal tissue
Less neutron contamination than 18 MV
What’s the formula for uncertainty for a photon counter for a given number of counts?
uncertainty =
How do you calculate the total uncertainty of a treatment delivery setup?
Total Uncertainty = √sum(error)2
What’s one of the health risks associated w/ LDR brachytherapy?
High risk of DVTs!
What’s another name for the Syed applicator?
Neblett applicator
How much does the activity of a radioisotope decay per day?
~ 1%
Which radioisotope is used to eye implants?
125I
There are 2 eyes!
What is the requirement for releasing a pt after a PET scan?
The dose rate at 1 m from the patient must be less than 5mR/h.
How does the integral whole-body dose vary between protons and XRs?
Integral whole-body dose is lower for protons because they have no exit peak!
What are the products of a β- decay?
- proton
- neutrino
How does the dose rate vary with half-life?
Just like source activity, the dose rate halves every 1 half-life.
How is contrast related to kVp and mA of an XR tube?
Contrast decreases with kVp
Contrast in unaffected by mA
In a standing waveguide, where is the residual microwave power absorbed?
Since the wave is reflected on both ends, it can be absorbed anywhere in the accelerating waveguide.
A TomoTherapy® unit incorporates which modalities into a single machine?
- MV CT scanner
- MV linear accelerator
At which energies is coherent scattering the dominant interaction?
< 10 keV
What happens during coherent scattering?
- photons bounce off e-
- there is no loss of energy
- there is no ionization
What is the probability of coherent scattering proportional to?
probability ∝ Z/E2
How do the lead attenuation coefficient and HVL of 1 MV to 20 MV photon beam vary?
- < 10 MV, Compton interaction is predominant. With increasing energy, the attenuation coefficient decreases, HVL increases
- > 10 MV, pair production takes over. The probability of this interaction increases dramatically with energy. Thus, the linear attenuation coefficient increases and HVL decreases
When measuring HVL, why must we use a narrow beam?
A broad beam could introduce scattered X-rays, giving a false reading
How does kerma compare to the absorbed dose in the build-up region?
kerma > absorbed dose up till dmax
How does kerma compare to the absorbed dose after dmax?
Dose is slightly greater than kerma
What is KERMA?
Kinetic Energy Released in Media
It is the energy transferred by the photon to all charged particles.
Which kerma contributes to the absorbed dose?
Collisional kerma only!
What is the quality factor?
It is the same as the weighting factor (WR)
- photons, e- → WR = 1
- protons, neutrons → WR = 2
What device does the AAPM recommend for the calibration of a 6MV photon beam?
Parallel plate chamber
NB: Diodes are never used to calibrate anything!
What’s the protocol for calibrating all MV therapy beams?
TG-51
Why do ionization chambers for machine calibration require temperature and pressure correction?
To account for variations in the air mass in the collection volume.
The charge collected is ∝ to mass of air in the chamber
What calibration specification is used to define ΜU?
This definition is department specific, as long as it is consistent with the data used for treatment planning within that department.
According to AAPM’s current calibration protocol, what are the ion chamber specifications used for calibration?
- Farmer-type OR parallel plate chambers can be used.
- Photon beam quality is defined by the percent depth dose at 10 cm depth in water
- The ion chamber must be calibrated at an accredited lab, in water, and in a Co-60 beam
- The chamber and electrometer must be calibrated every 2 years
Which data do you need to calculate PDD from published data?
- HVL
- SSD
How is the secondary e- fluence (set in motion by the photon beam) affected by tissue inhomogeneities?
- Inhomogeneities cause the e- fluence change at the boundary of the inhomogeneity → affects dose at the boundaries and within the inhomogeneity
- Effectively, at the boundary of each inhomogeneity, e- equilibrium must be re-established (new build-up region)
- Tissue next to bone/metal may be overdosed (greater e- fluence due to high density of bone/metal)
- Tissue next to lung may be underdosed (low e- fluence due to low density of air/lung tissue)
For tissues after the inhomogeneity, what factor affects the dose?
Attenuation (NOT secondary e- fluence.
What are D5 and D95 used to evaluate in a DVH?
- D95 → target coverage (high for targets, low for OARs).
- D5 → hotspots; should be no more that 110% of the prescribed dose.
For dose calculations, should you use the field size before or after collimation?
After, since that is what’s gonna hit the patient!
For what breast separation are 6MV photons appropriate to deliver a uniform dose and keep the hotspot < 110%?
- 25 cm
- For > 25 cm, higher energies are required to keep the hot spot < 110%. However, this must be balanced against the fact that there will be a. lack of dose in the buildup region!
What beam energies are recommended for the treatment of lung cancer and why?
- AAPM recommends < 12 MV
- There is a loss of dose a the lung-tumor interface 2/2 low secondary e- fluence from the lung. This is worse for high-energy than low-energy beams. Thus, <12 MV should be used.
For dose calculations, 1 cm of lung tissue is equivalent to how many cms of regular tissue?
0.3
An e- field can be blocked down to what dimensions w/o affecting the central axis dose?
to the practical range of e-
At which depth does 90% of the dose occur for e-?
E/3.2 cm
At which depth does 80% of the dose occur for e-?
E/2.8 cm
How does the voltage of an ionization chamber relate to the charge collected?
If voltage is too low, it increased the chances of ion recombination, leading to a lower amount of charge collection.
What’s an advantage of using superficial XRs over e-?
Easy to shape the field w/ a think sheet of lead
What’re the advantages of using e- over superficial XRs?
- No increased dose to bone
- Small amount of skin-sparing
- Greater sparing of the underlying tissue
- Greater output means faster treatment
The amount of radiation that delivers 1 Gy to water will deliver how much to muscle?
0.99 Gy
What’s the pixel size of a standard CT scan?
512 x 512
How do you shield for neutrons?
- Using Borated polyethylene
- Lead/Steel is used on the outside of PE to capture γ rays produced by neutrons
What molecule is used in PET imaging?
18F
For images obtained using EPID, scatter photons increase which image properties?
- Signal
- Noise
- SNR
They decrease contrast-to-noise ratio (CNR)
How do the resolution and the patient dose of CBCT compare to diagnostic CTs?
- Resolution higher in the cephalocaudal direction
- Resolution is lower in the axial plane
- Dose is similar
NB: CBCT has equal resolution in all planes..
Over what length can a CBCT scan?
15 cm
How many rotations of the XR tube are required to obtain a CBCT?
Generally one full or partial rotation.
Are 2D or 3D images used to determine rotational errors?
3D
When is a CBCT used in a half-fan vs. a full-fan mode?
Half-fan:
- requires 360o rotation
- When the desired FoV is larger than the imaging panel, the imager is shifted laterally and only half the field is imaged at one time.
Full-fan:
- 180o rotation
- If the FoV is smaller than the imaging panel, the imager is kept centered, and the whole FoV is imaged at any time
Which imaging system is used to provide real-time imaging and monitoring intrafractional motion of bony targets in 3 dimensions?
Dual-mounted kV imaging systems.
You need to combine two 2D systems to get 3D data.
What imaging modality does Cyberknife use for target localization>
Real-time orthogonal XR imaging
Which imaging modality is used for γ knife treatment planning?
MRI
How many MUs per MV portal image?
2-3 MUs
How does the resolution of the DRRs compare to conventional radiographs?
Poorer than conventional radiographs in all directions, especially CC direction 2/2 CT slice thickness.
For IMRT treatment planning, what parameter is not set by the planner (automatically optimized by the TPSS)?
Beam weights
How does the size of an appropriate dosimeter compare to the size of the field?
Dosimeter size < field size
What is the length (size) of a 0.6cc farmer chamber?
> 1 cm
For γ knife, which isodose line is used to prescribe the dose?
50%
Does the conformity index (CI) provide any information about the spatial distribution of the dose with respect to the target?
No, just like the DVH
What is the magnification factor (MF)?
Image size/object size
If you incorrectly use a higher magnification factor, you calculate the object size to be smaller than it is.
If you incorrectly use a lower magnification factor, you calculate the object size to be larger than it is.
Do we need to do inhomogeneity corrections for tandem and ovoid brachy treatments?
No! so you do not need an additional CT scan
How do the dose distributions of an HDR cylinder plan vary with distance?
At shorter distances, the ends will be hotter than the center, whereas at longer distances, the center will be hotter.
How does Pd-103 decay?
- Pd-103 is proton rich
- Decays via e- capture
How do Isotopes with more neutrons than the stable isotope decay?
- If Z ≤ 80 → decay via β-
- If Z > 80 → decay via α emission
What happens to an e- beam if it does not pass through a scattering foil?
It remains a pencil beam of 2-3 mm diameter
How does the e- current differ b/w 15 MV and 6 MV photon beams for the same dose rate?
- Bremsstrahlung production efficiency increases w/ increasing e- energy
- Higher current required at lower energies to give the same dose rate
What’s the energy threshold for neutron production?
8 MV
For a Compton interaction, an e- emitted at 90o to the direction of the incident photon will have what energy?
0.511 MV
For which range of photon energies is Compton scatter the most dominant interaction?
25 keV - 25 MV
Do hard wedges and flattening filters increase or decrease PDD?
- Increase
- Wedge and flattening filter selectively attenuate lower energy photons, leaving a beam with higher average energy → higher PDD
How does the dose rate compare between FFF and flattened beams?
- FFF → higher dose rate in the center, but comparable dose rate around the edges
- Deliver higher dose rates only for small field sizes
Which tissue (fat, muscle, bone) has the highest e- density?
- Fat: ↑ H content → ↑ e- density
- Muscle: intermediate H content
- Bone: Lowest H content
What’s the average leakage dose through MLCs?
1-2%
What’s the average leakage dose through x-y jaws?
0.5%
Which detectors have been specifically designed to measure neutron doses?
- Bonner sphere
- Bubble detector
- Lithium fluoride TLD-600 plus TLD-700
Can a parallel plate ionization chamber differentiate b/w neutron and proton dose?
No!
Therefore, it cannot be used to detect neutron contamination in proton beam therapy.
What tissue thickness of a tissue-equivalent phantom is required to sufficiently establish a backscatter dose when doing X-Ray beam calibration?
≥ 5 cm
How does the dose required to obtain suitable images vary between radiographic and radiochromic films?
Radiochromic films require a higher dose for suitable image quality
What are standard temp and pressure?
- Temp: 295.15oK (22oC)
- Pressure: 760 mmHg
What is the picket fence test?
- It exposes a radiographic film or EPID to radiation through a narrow slit formed by MLCs
- MLCs are moved to preset positions. If they are out of alignment, it will show on the film.
CT numbers scale linearly w/ e- densities of different tissues, except for this tissue.
Bone: with its high calcium content, bones do not follow this pattern.
Which algorithm is the least accurate for treatment planning for a lung lesion?
Pencil beam scanning, as it does not account for changes in scatter dose from inhomogeneities.
Which wedge, dynamic, universal, or physical, produces the largest scatter dose?
Physical
How does the ease of conforming isodose lines vary with energy?
6 MV photons produce e- that have a shorter range than those produced by higher beam energies. Thus, it is easy to conform those isodose lines.
How do the isodose lines shift when the field size is decreased?
They shift closer to the surface as the field size is decreases, due to a decrease in scatter dose!
What’s the radiation dose per CBCT?
10-50 mGy!
or 1-5 cGy
What’s the radiation dose from a single KV image?
0.1-0.5 mGy
In which situation is MV CBCT better than kV CBCT?
When a pt has metallic objects. MV imaging leads to less scatter!
How does the contrast to noise ratio compare b/w MV and kV CBCT for soft vs. bone?
kV CBCTs have better contrast-to-noise ratios for both soft tissues and bone.
Which beam-modifying device is used for kV cone-beam CT generation?
- Bow-tie filter
– it reduces the intensity of the beam near the edges, where patient thickness is less than the thickness at the center - W/o the bow-tie filter, the edges would be over-exposed!
How does XR contamination to the pt differ between head-on vs. angled beams?
X-ray contamination occurs mostly along the central axis. So, angled beams have less xray patient contamination.
What’s the minimum recommended total arc length for VMAT?
- 360o
- Usually one rotation (360o) is enough, but occasionally two rotations (720o) may be required.
For uniform dose coverage during HDR cylinder treatment, how will the source dwell times differ b/w the center and the ends?
- Longer dwell times at the ends.
- The center is always closer to the dwell positions than the edge, so more dwell time is needed at the edges.
Why is the dose distribution for an I-125 brachytherapy seed asymmetric?
- The thickness of the metal casing is variable.
- Attenuation by the seed’s metal casing depends on the angle of XR incidence, which can be variable
Why is the dose within 5 cm of the Ir-192 source very close to the inverse square law despite attenuation by tissue?
- < 5 cm from the source: Scatter dose compensates for XR attenuation, therefore, the dose is very similar to what the inverse square law predicts
- > 5 cm from the source: Attenuation dominates, and dose falls of more than the inverse square law predict
Which EM energy is used to deliver hyperthermia?
- Microwave
- RF radiations
What’s the RBE of clinically-used protons?
1.1 (slightly higher thaWSX#όn e-!)
Which brachytherapy isotope decays via β- decay but does not subsequently emit x-rays or γ rays?
Sr-90
How does the relative e- density of bone compare to its mass density?
E- density is lower than mass density
Which nuclei do not have a net magnetic moment?
Nuclei w/ unpaired protons OR neutrons
How does the dose rate of FFF beams compare b/w different beam energies?
The higher the beam energy, the more the dose rate in the center
Do higher magnetic field strengths have higher or lower distortion from metallic objects?
More!
Is radiochromic film good for measuring radiation doses?
No!
Are EPIDs used for dose output measurements?
NO
Which ionization chamber represents a safety hazard for in vivo dosimetry?
Thimble: Requires high voltage
If a question stem mentions a very large field, what are they trying to suggest?
That the PDD at 100 SSD will not vary significantly with Δ field size
How does the PDD for a dynamic wedge compare to that of an open field?
They’re the same
Why do we use effective SSD or virtual source position for e- beams?
To correct the inverse square law relationship for the output change with distance
Why do e- beams fan out?
E- are negatively charged and they repel each other.
Through which interaction do e- deposit dose within the patient?
Coulomb scattering
How does any directly ionizing radiation exert its effect in a medium?
It exerts coulombic forces to directly kick out e- from atoms.
What are secondary barriers?
- They are barriers meant to protect against scatter and leakage.
- Usually half the primary barrier size
What is the cumulative dose limit for the lifetime of a radiation worker?
10 mSv x Age
How does the light field compare to the XR field size for LINACs with rounded MLCs?
- Light field is slightly smaller than the XR field
- Rounded MLCs block the light completely but some XRays penetrate through the edges.
What is another name for inverse planning?
Simulated annealing
Does VMAT as compared to IMRT greatly improve dose conformality?
No! generally, VMAT and IMRT plans are comparable. IMRT plans require fewer MUs
What is the purpose of a beam spoiler during TBI?
Some of the bone marrow lies close to the skin and needs to be treated. The spoiler is used to increase dose near the skin.
Which portion of an e- beam is usually contaminated by X-rays?
The central portion
How do you minimize the penumbra during gamma knife treatments?
We do it using collimators that are very close to the target, which reduces the geometric penumbra.
Why does the γ knife have an inferior penumbra as compared to MV beams?
2/2 large source size and more scattering
What does the WInston-Lutz test measure?
It measures the coincidence between mechanical and radiation isocenters.
MNEMONIC: Winston, Lutz, two different guys agreeing!
Why do we prescribe to the 80% isodose line for SBRT?
To ensure rapid dose fall-off outside the targeted volume.
Is the dose around an HHDR source (such as Ir-192) isotropic?
No, because differential absorption of x and γ rays by the metal casing makes the dose anisotropic.
Patterson Parker rules cannot be used for which brachy source?
I-125 since the average energy is only 35 keV
How often do we need to do a radiation shielding survey of adjacent areas for an Ir-192 source?
Everytime the source is changed
Are inhomogeneity corrections used in HDR brachytherapy TPS?
Not currently
Do protons undergo exponential attenuation?
No, only X-rays and γ rays do.
Protons have a finite range.
Does SRS use IMRT?
No!
It uses cones, which are small, circular apertures.
What does the spectrum of unfiltered 150 kV X-rays from an X-Ray tube look like>?
What does the spectrum of filtered 150 kV X-rays from an X-Ray tube look like>?
- Filtration selectively removes lower energy photons through PE interactions
- It increases the effective energy of the photons
What other process does the production of characteristic X-rays compete with?
Production of Auger e-
Does the tube current have any effect on the energy of an x-ray beam?
- No!
- The energy depends on the potential difference (kVp) that accelerates the e- b/w cathode and the anode. It does not matter how many (#n; mAs) e- are being accelerated.
What determines the maximum energy of the X-rays from an X-ray tube?
Max Energy = kVp
Are the characteristic X-rays characteristic of the anode or the cathode?
Anode!
How is the Linac output (cGy/MU) change if the registered ΜU decrease (reading by the pressurized, sealed ion chamber)?
Output will increase because more dose will be required to register a monitor unit.
Are FFF beams harder or softer?
- It’ll be softer, since a FF removes lower energy photons.
- Without FF, these photons decrease the energy spectrum (soften) of the beam.
What’s the rationale behind the tongue-in-groove design of MLCs?
To minimize leakage to less than 3-4%
In an MV Linac, if the target is thicker than the range of incident e-, how would the dose rate and average energy of the resulting bremsstrahlung X-rays change?
- Average energy: Increase, as the thicker target will attenuate lower energy photons.
- Dose Rate: Decrease. Some of the bremsstrahlung X-rays, especially those produced near the surface, will be absorbed by the thicker target, maybe to produce auger e-, thus the dose rate will decrease.
What is photonuclear disintegration?
- At 8-16 MV energies, a photon can strike a nucleus and chip off parts of it. It mostly results in neutron ejection and is most probable at energies > 10 MV.
- It is the main source of neutron contamination in photon beams. It happens when the beam interacts with the metal components of the LINAC head.
Why is neutron contamination bad?
It causes significantly more late effects.
Between e- and photons, which requires higher beam fluence to deliver the same dose?
Photons since only a portion of their dose is deposited in tissue
What is the f factor?
It is the exposure-to-dose conversion factor.
Which detector is best for detecting radiation behind a barrier of a Linac?
An ion chamber should be used for any quantitative measure of radiation!
Are the ΜU readings from a Linac monitor chamber subject to variations in temperature and pressure?
No! The monitor chambers are sealed so they are not affected by temperature and pressure.
How does a TLD record dose?
- Radiation causes e- to jump from a crystal’s e- orbitals into a metastable state
- E- are trapped in a metastable state by impurities, such as Mg in the crystal lattice
- Later, by either heating or cooling, these e- are released from their metastable trap.
- They emit photons, which can be measured.
- The photon energy (“glow peaks”) corresponds to e- valence energies
What is the primary standard for dosimetry calibration?
- The free-air ionization chamber
- It does not require calibration against any other dosimeter
- Even radiochromic films require calibration against a free-air ionization chamber
What kind of OAR shielding is used for superficial orthovoltage treatment machines?
- Lead plates
If a photon and an e- field are matched at the skin, what happens to the hot and cold spots immediately beneath the skin?
E- field scatters out more than the photon field, resulting in:
- Cold spot on the e- side
- Hot spot on the photon side
Which method of radiation delivery does not require feathering?
Tomotherapy. It literally treats slice by slice using a fan beam.
