Cancer cells are more radiosensitive than their cells of origin !!!

1. normal cells repair sublethal damage better
2. cancer cells have high mitotic activity (cells are more radiosensitive during mitosis)

FRACTIONATION = dose / time

  - treating patients over a period of time with smaller doses

• Can catch cells in cell division by fractionation (breaking up treatment over several weeks)
• Hypoxix (low oxygen) cancer cells (cells that are not close to a blood supply) are resistant to radiation
• Need to be treated with more radiation than actively dividing and well oxygenated cancer cells

Can’t raise the levels of radiation to where normal cells are damaged so fractionalism with several smaller doses is needed

Conventional fractionation :

5x per week

200cGy per day     =    1000cGy week

hyperfractionation (B.I.D.) – patient is treated twice a day

• 5 hour gap inbetween treatments
• a lot of head and neck treatments are performed this way

protracted fractionation – stretching out the treatment for a longer period of time with smaller doses

**the bigger the area you treat the lower the dose will be**

Reasons for fractionation :

1. allows normal cells to recover
2. catch cancer cells in mitotic phase (mitosis)
3. to shrink the tumor to allow for better oxygenation (the smaller a tumor gets the larger the blood supply will be therefore each successive treatment will be mopre and more effective!!!)

erythema dose – irradiate until the skin turns red (was used as a way to measure the amount of radiation to be given)

Roentgen (R) – amount of electron exposed to a given amount of air

• tells how much radiation arrives
• tells exposure
• used to measure x-rays and gamma rays