Brachytherapy Flashcards
What is a mgRaEq?
a 1mgRaEq source is a source with the same air exposure rate as 1mg of Radium
1 mgRaEq = gamma*A/(8.25R/cm2/hr)
What are the dose rate cutoffs for LDR, MDR, HDR?
LDR: 0.4-2 Gy/hr
MDR: 2-12 Gy/hr
HDR: >12 Gy/hr
What is the average photon energy of a Ra source?
0.83 MeV. While the decay of Ra releases alpha, beta, and gamma rays, the encapsulation is designed to absorb everything except the gamma rays
How do you define gamma for brachytherapy?
This is the exposure rate constant: the exposure rate per mCi of isotope at 1 cm distance
Why are I-125 and Pd-103 particularly well suited for permanent implants?
They have a very low dose rate (allows for sublethal repair)
They have a low energy (22-28 keV; allows for steep dose falloff)
What is the effective half life equation for an unsealed source?
teff = (tbiol*tphys)/(tbiol + tphys)
What is the equation for activity?
A=Aoe^(-lambda*t)
What is the equation for Dose rate?
D=Doe^(-lambda*t)
How do you calculate total dose for an implant?
Dtot=Do(1.44*t1/2)
How do you calculate exposure rate in air (X)?
X=gamma*A/r^2
What does the TG-43 geometry factor define? G(r,x)
this is the inverse square dose falloff
What does the TG-43 anisotropy factor define? F(r,x)
this accounts for differential attenuation from the source capsule
Which historical brachytherapy loading system uses crossed ends?
Paterson-Parker and Quimby (NOT Paris: PARis is PARallel-only)
How is pt A calculated?
What does it represent?
2cm cephalad to flange, 2cm lateral to tandem
Represents the paracervical triangle, where vessels cross the ureter
How is pt B calculated?
What does it represent?
3cm lateral from pt A
Represents the obturator nodes
