# Shared Flashcard Set

## Details

Chapter 13
Radiation Field Shaping, Skin Dose, Field Seperation
18
Physics
Graduate
05/06/2010

## Cards Return to Set Details

Term
 Equation for 5% transmission using a number of HVL's,and solve.
Definition
 1/2n = 0.05 where n is the number of HVL's. 2n= 1/0.05 2n= 20 nlog2=log20, n=4.32HVL
Term
 Ballpark idea of required thickneses to provide 5% transmissionCo606MV10MV25MV(in lead)
Definition
 Co60-5cm 6MV-6.5cm 10MV-7cm 25MV-7cm
Term
 Cerrobend melting point vs lead?
Definition
 70 vs 327 degrees C.
Term
 Density for Cerrobend vs lead?
Definition
 9.4g/cc 11.4g/cc
Term
 When you half-beam block what happens to the isodose lines near the surface?
Definition
 Throws a hotspot to the outer edge of the field.
Term
 How many pairs do MLC come in and what are their sizes?
Definition
 26, 40, or 60 leaf pairs. Come in size of 0.5m or 1.0cm
Term
 What is the transmission of MLC leaves?In between leaves?
Definition
 <1% transmission through leaves <1-2% between leaves
Term
 What is the transmission through the jaws of the machine?
Definition
 <2%
Term
 Is Penumbra larger or smaller with MLC vs blocks?
Definition
 Penumbra with MLC is larger.
Term
 By what photon interaction contributes to skin dose?
Definition
 Anything put into the beam will produce Compton electrons.
Term
 What instruments do we use to measure Dose Distributions in the buildup region?
Definition
 Extrapolation chambers are best but not widely available. Many use fixed seperation plane-parallel ionization chambers. TLDs are another choice (LiF)
Term
 Skin Dose equation
Definition
 ½(100% + entrance%)
Term
 What are electron filters?
Definition
 When placed in photon beam, they are designed to attenuate secondary e-'s . Made of medium z material (copper, tin, lead)
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.
Term
 [image]
Definition
 Gap seperation for adjacent fields S1=½*L1*[d/SSD1] S2=½*L2*[d/SSD2]
Term
 Total seperation for adjacent fields
Definition
 Total separation = (½L1 . 1/SSD1) + (½L2 . 1/SSD2)
Term
 How do we eliminate the "three-field overlap" (when L1 and L2 are different lengths) using AP/PA fields?
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'
Term
 Craniospinal collimator and couch kick equation.
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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