Brachytherapy I Flashcards
Define low dose rate brachytherapy?
< 2 Gy / hour
Define high dose rate brachytherapy?
> 12 Gy /hour
What are the two examples of isotopes used in LDR treatments?
125 Iodine, 103 Pd
What is the mode of energy emission in LDR for 125 Iodine, 103 Pd?
Electron capture
What is the commonly used isotope in HDR?
192 Ir
What is the mode decay of 192 Ir often used in HDR?
beta-minus decay
What does 192 Ir decay into?
Excited state of 192 Ro via beta-minus and decays down into the ground state, emitting photons of various energies.
What is the definition of exposure?
The charge per unit mass of air created in the chamber,
Exposure, X = Q / m(air)
What is the unit of exposure?
Roentgens, R (defined as 2.58 x 10^-4 C / kg)
For brachytherapy, which one do we usually consider, exposure or exposure rate?
exposure rate since brachytherapy source are continuously emitting and are characterized by their activity which is a rate (number of decays per second)
Exposure rate is proportional to the activity divided by
the square of the distance r
What is the definition of exposure?
the ionization equivalent of collisional kerma (J/kg) in air.
It is defined as the quantity of charge produced in a unit mass of air for a low energy photon beam
What does KERMA stand for?
What is the unit for KERMA?
Kinetic Energy Released to Mass of Air
Gy = J/Kg
1 rad = 1 cGy = 0.01 Gy
Compare the LDR and HDR brachy interms of dose per hour, half life, energy, usage for permenant implant vs temporary (seed of wire)
For brachytherapy applications, we typically consider not exposure but exposure rate, and not dose but dose rate.
What is the exposure rate equation for brachy?
exposure rate constant X activity / (radius)^2
