Dosimetric precision requirements in radiation therapy.

Based on simple radiobiologic models the effect of the true distribution of absorbed dose in therapy beams on the response of uniform tumor volumes are investigated. Under assumption that the dose variation in the beam is small it is shown that the response of the tumor to radiation is determined by the mean dose to the tumor volume. Quantitative expressions are also given for the loss in tumor control probability as a function of the degree of dose variations around the mean dose level. When the dose variations are large the minimum tumor dose is best related to tumor control. It is finally shown that high tumor control rates can only be achieved with a very high accuracy in dose delivery. If the normalized dose response gradient is higher than 3, as is frequently the case, the relative standard deviation of mean dose in the target volume should be less than 3 per cent to achieve an absolute standard deviation in tumor control probability of less than 10 per cent.