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  1. the inverse square law states that:
    intensity is inversely proportional to the square of the distance

    • I1=(D2)2
    • I2 (D1)2
  2. if the distance from a point source is tripled, the intensity will be?
    one ninth
  3. 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)
  4. 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)
  5. eqation to solve for D2?
    • I1 = X2
    • I2 (D2)2
  6. decay constant (lamda) formula
    • .693
    • T 1/2
  7. T 1/2 formula:
    • = .693
    • lamda (decay constant)
  8. Activity formula
    A = Aoe (-lamda)(T)
  9. Ao =. A
  10. 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
  11. high half life typically means implants will be________?
  12. short half life typically means implants will be __________?
  13. most popular cancer treated with brachytherapy?
    prostate (treated interstitially with permanent seeds)
  14. 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!)
  15. tandems and ovoids are used for what treatment
    • cervical, endometrium
    • (give good distribution to the cervix,
  16. GYN most common sources used
    • radium 226
    • cesium 137
    • irridium 192
  17. 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
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