Term
Equation for 5% transmission using a number of HVL's,
and solve. |
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Definition
1/2n = 0.05
where n is the number of HVL's.
2n= 1/0.05
2n= 20
nlog2=log20, n=4.32HVL |
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Term
Ballpark idea of required thickneses to provide 5% transmission
Co60
6MV
10MV
25MV
(in lead) |
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Definition
| Co60-5cm 6MV-6.5cm 10MV-7cm 25MV-7cm |
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Term
| Cerrobend melting point vs lead? |
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Definition
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Term
| Density for Cerrobend vs lead? |
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Definition
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Term
| When you half-beam block what happens to the isodose lines near the surface? |
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Definition
| Throws a hotspot to the outer edge of the field. |
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Term
| How many pairs do MLC come in and what are their sizes? |
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Definition
26, 40, or 60 leaf pairs.
Come in size of 0.5m or 1.0cm |
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Term
What is the transmission of MLC leaves?
In between leaves? |
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Definition
<1% transmission through leaves
<1-2% between leaves
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Term
| What is the transmission through the jaws of the machine? |
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Definition
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Term
| Is Penumbra larger or smaller with MLC vs blocks? |
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Definition
| Penumbra with MLC is larger. |
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Term
| By what photon interaction contributes to skin dose? |
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Definition
| Anything put into the beam will produce Compton electrons. |
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Term
| What instruments do we use to measure Dose Distributions in the buildup region? |
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Definition
Extrapolation chambers are best but not widely available.
Many use fixed seperation plane-parallel ionization chambers.
TLDs are another choice (LiF) |
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Term
|
Definition
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Term
| What are electron filters? |
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Definition
| When placed in photon beam, they are designed to attenuate secondary e-'s . Made of medium z material (copper, tin, lead) |
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Term
| What is the purpose of electron filters and for what energy? |
|
Definition
| They help preserve skin sparing and are useful in large fields on 60Co machines. |
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Term
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Definition
Gap seperation for adjacent fields
S1=½*L1*[d/SSD1]
S2=½*L2*[d/SSD2] |
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Term
| Total seperation for adjacent fields |
|
Definition
Total separation =
(½L1 . 1/SSD1) + (½L2 . 1/SSD2) |
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Term
How do we eliminate the "three-field overlap"
(when L1 and L2 are different lengths) using AP/PA fields? |
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Definition
ΔS' = ΔS x d'-d/d
where d'=depth of cord from surface.
d= depth of Midline
and ΔS= S1-S2.
New gap required = S1+S2+ΔS' |
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Term
Craniospinal collimator and couch kick equation.
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Definition
1. Careful not to exit through mouth!
Collimator = arctan(½(Lspinefld)*1/SSD)
Couch kick= arctan(½(LWBlength)*1/SAD) |
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