Quality assurance (QA) in radiotherapy is of particular importance if a new irradiation technique is introduced. The dosimetric aspects of such a QA program concern the check of the dose calculation procedure, i.e. the prediction of the relative dose distribution, as well as the verification of the absolute value of the target absorbed dose specified at a particular point. In our institution a QA program has been developed for a new conformal irradiation technique of prostatic cancer: the simultaneous boost technique. With this technique the dose of the boost field and the large field are given simultaneously, using customized 10 mm thick Roses-metal plates in which the boost field has been cut out. The computation of the dose distribution, using a procedure adapted from a commercially available 2D treatment planning system, has been compared with isodose distributions measured in a water phantom. Good agreement, better than 3% or 3 mm, was observed for both open and wedged 8 MV X-ray beams. In vivo dose measurements have been performed on individual patients to check the dose delivery at the specification point. An agreement better than +/- 2% with the calculated dose value was required. The average ratio for 18 patients of the actual and expected dose value amounted to 1.005 +/- 0.017 (1 SD) after a correction of the number of monitor units for 2 patients during the treatment. Quality control of the dose transmission factor of the Roses-metal plates has been performed.(ABSTRACT TRUNCATED AT 250 WORDS)