Calculating a dosage figure that balances these two goals can be complex, since the size and stage of the tumor have to be taken into account and since different tissues tolerate different levels of radiation. The liver, for example, will tolerate 3,000 cGy, the lungs 2,000 and the kidneys 1,800. Higher doses can be delivered to small parts of one of these organs, but if the entire organ is given higher doses than these, normal tissues will be harmed.
The radiation oncologist prescribes the total dose necessary to destroy the tumor, then calculates the daily dose over a specific period. This is called the fractionation schedule. Throughout, the radiation oncologist works with a figure called the therapeutic ratio, defined as a comparison of the damage to the tumor cells compared with normal cells. The therapeutic ratio can be enhanced in a number of ways—by using altered time fractionation schedules, careful treatment planning, selection of the optimum radiation energy for the specific problem and by the use of experimental techniques such as high LET radiation or chemical modifiers that either make the tumor cells more sensitive to radiation or better protect normal tissues.
The Number of Treatments Radiation is usually given daily five days a week. That schedule can continue for two to eight weeks depending on the tumor, the kind of treatment being used and the dosage required. The point of using multiple treatments instead of a single treatment is to give normal cells a chance to recover and repair themselves.
