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the inverse square law states that:
intensity is inversely proportional to the square of the distance
if the distance from a point source is tripled, the intensity will be?
to apply the inverse sq. law, one must know...
- 2 distances and 1 intensity
- (you only need 3 pieces to the equation, then solve for the 4th/missing)
an instrument positioned 1m from a source is moved 50 cm closer, what will the radiation intensity do? (increase/decrease, and by what fraction)
increase by 4 times (increases by a square of the change in distance: .5m)
decay constant (lamda) formula
T 1/2 formula:
- = .693 lamda (decay constant)
A = Aoe (-lamda)(T)
advantages of brachy (3):
- -enables us to give a high tumoricidal dose
- -sparing normal tissues surrounging tumor (high dose fall off)
*ideally these 2 advantages enable us to help the therapeutic ratio of Tx
high half life typically means implants will be________?
short half life typically means implants will be __________?
most popular cancer treated with brachytherapy?
prostate (treated interstitially with permanent seeds)
What "R" of radiobiology is important to brachytherapy?
redistribution - high dose getting to the tumor, cells tend to redistribute into G2 (next phase is M = radiosensitive --> good!)
tandems and ovoids are used for what treatment
- cervical, endometrium
- (give good distribution to the cervix,
GYN most common sources used
- radium 226
- cesium 137
- irridium 192
side effects of GYN cancers with brachytherapy often have side effects that are... acute or chronic?
- chronic effects (due to high dose)
- ex: vaginal stenosis, atrophy, fibrosis
What would you like to do?
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