Term
| Horns are created by what? |
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Definition
| Flattening filter which is designed to overcompensate near surface in order to obtain flat isodose curves at 10cm. |
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Term
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Definition
| Dose rate decreases rapidly as a function of lat distance from the CA. |
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Term
| What is the falloff near the edge of the beam caused by? |
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Definition
| Geometric penumbra and reduced side scatter |
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Term
| Physical Penumbra consists of...? |
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Definition
Physical penumbra consists of:
-Geometric penumbra (due to finite source size)
-Transmission penumbra ( due to transmission through jaws)
-Scatter penumbra (due to in-phantom scatter) |
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Term
| What does Physical Penumbra depend on? |
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Definition
Energy
Source size
SSD
Source to collimator distance
Depth in phantom |
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Term
| Definition on Physical Penumbra? |
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Definition
| the lateral distance between two specific isodose lines at a specified depth (eg lateral distance between 80 and 20% isodose lines at the depth of Dmax.) |
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Term
| What do you measure Isodose curves with? |
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Definition
Most reliable- Ionization chambers because of flat energy response and precision
Solid state detectors (diodes), ideal for e- measurements
Film, not the easiest |
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Term
| What is "Golden Data" and if using it what do you need to do? |
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Definition
It is preloaded isodose info from manufacturer. Must
-verify PDD data in a water phantom
-a deviation of 2% or less is acceptable up to a d=20cm. |
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Term
| At higher energy, what is the attributes to the side scatter? |
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Definition
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Term
What has greatest effect on Iso curves,
-collimators
-flattening filter
-other absorbers? |
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Definition
Flattening filter. if no filter, conical field distribution.
Filter causes change in beam quality |
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Term
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Definition
The variation of dose relative to the CA over the ventral 80% of the f.s. at 10cm depth.
A variation of +/- 3% is acceptable. |
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Term
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Definition
The variation of dose at any pair of points symmetrically situated with the CA ray.
It shouldn't differ by more that 2%. |
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Term
| What distance should be kept between wedge and skin? |
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Definition
| 15cm which will preserve the skin-sparing effect |
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Term
Wedge systems:
Individualized
Universal
Physical |
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Definition
| Wedge systems: Individualized- a seperate wedge for each beam width to minimize the loss of beam output Universal-a single wedge for all beam widths, fixed centrally in the beam, always used in linacs Physical-Individual 15,30,45,60 (varian) and Universal, one 60degree mix open and wedge fields(elekta) |
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Term
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Definition
Wedge factor= Dose with wedge/Dose without wedge
measured at a suitable depth beyond dmax (5 to 10cm)
Presence of wedge decreases the output and hardens the beam |
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Term
| Physical wedges affect the beam how? |
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Definition
-Harden the beam
-Compton scattering which results in energy degradation (beam softening)
-Hardening also affects the DD distribution, wedged PDD should be measured |
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Term
| POP advantages/disadvantages? |
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Definition
-Simple
-Less chance of a geometric miss
-Fairly homogeneous through the tumor
Disadvantages
-Excessive dose to normal tissue, often more than the tumor dose |
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Term
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Definition
| As the pt thinckness increases or the beam NRG decreases: -The central axis maximum dose near the surface increases relative to the midpoint dose |
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Term
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Definition
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Term
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Definition
ICRU 50
gross palpable or visible extent of tumor.
may consist of primary tumor, metastatic lymphadenopathy or other metastases. |
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Term
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Definition
ICRU 50
Clinical Target Volume
It contains the GTV and sub-clinical microscopic disease. Must be tx'd adequately for cure. |
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Term
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Definition
ICRU 62
added to the CTV to compensate for physiological movements and size, shape, and position of the CTV during therapy.
CTV+IM=ITV (internal target volume) commonly asymmetric around the CTV, accounts for movement, respiration, filling of bladder/rectum. |
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Term
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Definition
ICRU 62
accounts for uncertainties in pt positioning and alignment during tx planning and delivery. |
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Term
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Definition
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Term
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Definition
ICRU 50
The volume enclosed by an isodose surface deemed appropriate to achieve tx goal. |
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Term
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Definition
ICRU 50
The tissue volume which receives a dose that is considered significant in relation to normal tissue tolerance. Depends on treatment technique used |
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Term
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Definition
-Always at the center of the PTV
-and when possible, at the intersection of the beam axes
The dose at the ICRU reference point is the ICRU reference dose
-For single field: on CA at center of target
-for POP equal weight: on CA, midplane.
-for POP unequal weight: at center of target
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Term
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Definition
Depends on Source size, distance from source, SDD.
exists both inside and outside of the geometric boundaries of the beam.
edge sharpness... also depends on reduced side scatter, so geometric penumbra isn't best to describe beam edge sharpness- use physical penumbra |
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Term
| Rotational therapy dose rate |
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Definition
Diso= Dref x T
Dref= reference dose rate related to quantity T.
T is the avg TAR or TMR.
If using TMR, Diso=D0 x Sc x Sp x TMR
D0 is the Dmax for a 10x10 field at SAD |
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Term
Example of Rotational calc:
250 rad at iso
SAD=100
avg TMR=0.746, Sc=0.98, Sp=0.99
Machine output is 200MU/min |
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Definition
Diso=Do x Sc x Sp x avgTMR
=200 x 0.98 x 0.99 x 0.746= 144.8 rad/min
Tx time= 250 rad/144.8rad/min= 1.73min
Total MU to be set= 200MU/min x 1.73 min= 345MU |
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Term
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Definition
| An area outside the target volume that is hotter than the prescribed dose. Must be at least 2cm² to be clinically significant |
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Term
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Definition
| the mean of the absorbed dose at a large number of points uniformly distributed in the target area |
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Term
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Definition
| value between the max and min absorbed dose values within the target. |
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Term
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Definition
| The absorbed dose that occurs most frequently within the target area. |
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Term
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Definition
| a measure of the total energy absorbed in the treated volume. If the volume receives a uniform dose, then it equals mass x dose. It is one way to compare dose distributions of different-quality beams. Its units are kg·Gy or joule (since 1Gy=1J/kg). Try to keep intregral dose to a minimum. |
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