The simultaneous boost technique: the concept of relative normalized total dose.

The simultaneous boost technique in radiotherapy consists of delivering the boost treatment (additional doses to reduced volumes) simultaneously with the basic (large-field) treatment for all treatment sessions. Both the dose per fraction delivered by the basic-treatment fields and by the boost-treatment fields have to be reduced to end up with the same total dose in the boost volume as in the original schedule, where the basic treatment preceded the boost treatment. These dose reductions and corresponding weighting factors have been calculated using the linear-quadratic (LQ) model and the concept of Normalized Total Dose (NTD). Relative Normalized Total Dose (RNTD) distributions were computed to evaluate the dose distributions resulting for the simultaneous boost technique with respect to acute and late normal tissue damage and tumor control. For the example of the treatment of prostatic cancer the weighting factors were calculated on the basis of the NTD for late normal tissue damage. For the treatment of oropharyngeal cancer the NTD for acute normal tissue damage was used to determine the weighting factors. In this last example a theoretical sparing of late normal tissue damage can be demonstrated. A second advantage of the simultaneous boost technique is that the megavoltage images of the large basic-treatment fields facilitates the determination of the position of the patient with respect to the small boost-treatment fields.