Term
| CT number (Hounsfield unit) is defined as: |
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Definition
H= μtissue - μwater x 1000
μwater
(μ= linear attenuation)
Ranges from -1000 for air to 0 for water to +1000 for bone . Has a linear relationship with attenuation coefficient, thus electron density. |
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Term
| MRI uses______ and ______ to image the body. |
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Definition
Electromagnetic field, radio frequency waves.
Adv: soft tissue contrast,no radiation, reconstruct in multiple planes
Disadv: lower resolution than CT, no metal objects. |
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Term
| Ultrasound uses______ at a frequency of ________. |
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Definition
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Term
| Ultrasound produces image by? |
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Definition
Reflection of waves from different tissue interfaces caused by variations in acoustic impedence of materials.
Acoustic impedence= product of density of material and velocity of ultrasound. |
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Term
| What radiomuclide does PET administer? |
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Definition
| Positron-emitting radionuclides 18F, 15O, 11C |
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Term
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Definition
| Positron is annihilated and the two annihilation photons (511keV) are detected by a pair of detectors. Suffers from low resolution |
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Term
| What is the film comprised of for port-localization? |
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Definition
| Single emulsion film with a single lead screen to maximize electron production. |
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Term
| Types of Electronic portal imaging (EPID)? |
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Definition
Fluorescent screen/video-based
Ion chamber or diode array
aSi panal
(all these are old models)
offers real-time imaging |
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Term
| How does Varian portal vision work? |
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Definition
| Array of amorphous silicon (aSi) detectors→ Scintillator converts radiation into visible photons→ photo diodes create charge captures. |
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Term
| What are the outdated corrections for contour irregularities methods (3)? |
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Definition
1Effective SSD (shift iso curve and apply Inv² to PDD)
2Tissue-Air Ratio (not SSD dependent) most accurate of the three
3Isodose shift -similar to effective SSD
1-2 for single point dose calculations
3 correct for the entire isodose chart |
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Term
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Definition
| Modern tx planning systems account for contour irreg by ray tracing, trackinig the primary photons as they enter the body and apply the maynard f factor(Eclipse)to adjust PDD or cconvolution/superposition to generate dose distribution maps (pinnacle) |
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Term
| Corrections for Tissue Inhomogeneities (traditional ways) |
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Definition
Tissue-Air Ratio
Batho power law TAR
Equivalent TAR
Isodose Shift |
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Term
Traditional inhomogeneity correction
TAR method |
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Definition
| Accounts for primary beam only. Overestimates dose. applies correction using equivalent depths(includes e- densities) in the TAR ratios to non-equivalent depth TAR. |
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Term
| What is the best Tissue correction for inhomogeneities to use for under 6MV? |
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Definition
Equivalent TAR method
Uses TAR corrections, but also accounts for change in scatter. |
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Term
| What is the best Tissue correction for inhomogeneities to use for energies greater that 10MV? |
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Definition
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Term
| What method for tissue correction for inhomogeneities do modern tx planning systems use? |
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Definition
| The most accurate(other than Monte Carlo) is density scaling ( Superposition) |
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Term
| What is density scaling based on? |
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Definition
| the theorm that dose in two different media with different densities, but = atomic numbers, will be the same if all dimensions in the media are scaled inversely with their density |
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Term
| What happens on the entrance side of tissue-bone interface? |
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Definition
| There is an increase in dose (8% in MV beams) within 2mm due to backscatter. |
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Term
| What happens on the transmission side of bone-tissue interface, for lower energies? |
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Definition
| For lower energies: lower dose at interface, then increase, then decrease. (due to forward scatter from e- in bone that buildup in soft tissue interface |
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Term
| What happens on the transmission side of bone-tissue interface, for higher energies? |
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Definition
| For higher energies: there is a dose increse at the interface (due to pair production) and then a drop-off. |
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Term
| What does the lower lung densities do to dose? |
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Definition
| Higher doses within and beyond the lung.Immediately after the lung, there is loss of secondary e-'s (due to longer distance lateral scattering) and decrease in dose. |
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Term
| What does lung densities do to dose profiles? |
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Definition
| Become less sharp; an increasing # of e-'s travel outisde the geometrical beam limits and a greater loss due to laterally scattered e-'s cause the CA dose to be lower. |
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Term
| What could happen if you treat with high energy (>6MV) and small fields(<6x6cm) in the lung? |
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Definition
| under-dose due to lack of lateral scattering equilibrium |
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Term
Approximate dose increase to tissues beyond healthy lung?
ortho
co-60
4MV
10MV
20MV |
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Definition
ortho +10% co-60 +4% 4MV +3% 10MV +2% 20MV +1%
%/cm of lung |
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Term
| Does presence of air cavity affect both sides of cavity? |
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Definition
Yes. due to partial loss of electronic equilibrium at the cavity surface.
Tissue beyond and in front is under-dosed. More significant for sm fields and large cavities, and higher energy. |
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Term
| Tissue compensators are designed to _____ at shallow depths and _____ at deep depths. |
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Definition
| Overcompensates, undercompensates |
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Term
| What are three devises that were used to make compensators? |
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Definition
Moire camera
Magnetic Digitizer
CT-based systems |
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Term
| Compensator thickness equation? |
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Definition
tc=TD(thickness ratio/density of material)
TD=tissue deficit at the point
A value of 0.7 for thickness ratio can be used for all compensators |
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Term
| Changes in PDD with depth (why we keep an eye on SSD's) |
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Definition
5%/cm for 6MV
4%/cm for 10MV
3-4% /cm for 18-20MV |
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