Quality assurance of the simultaneous boost technique for prostatic cancer: dosimetric aspects

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. On average, this factor was 63.4 ± 0.6% (I SD), which is in good agreement with the value of 64% accepted in the simultaneous boost technique. The uniformity of the transmission factor over the whole plate inside the irradiation field was required to be better than 2%, and has been checked using megavoltage imaging. In about 10% of the cases a replacement of the transmission plate was required due to enclosed air cavities. The results of our QA program demonstrated that with regard to the dosimetric aspects the simultaneous boost technique can be considered to be a reliable treatment technique.