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 s uch a QA program concern the check of the dose calculation procedure, i.e the prediction of the relative dose distribution, as well as the v erification of the absolute value of the target absorbed dose specifie d at a particular point. In our institution a QA program has been deve loped for a new conformal irradiation technique of prostatic cancer: t he simultaneous boost technique. With this technique the dose of the b oost field and the large field are given simultaneously, using customi zed 10 mm thick Roses-metal plates in which the boost field has been c ut out. The computation of the dose distribution, using a procedure ad apted 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 performe d on individual patients to check the dose delivery at the specificati on point. An agreement better than +/-2% with the calculated dose valu e was required. The average ratio for 18 patients of the actual and ex pected dose value amounted to 1.005 +/- 0.017 (1 SD) after a correctio n of the number of monitor units for 2 patients during the treatment. Quality control of the dose transmission factor of the Roses-metal pla tes has been performed. On average, this factor was 63.4 +/- 0.6%;, (1 SD), which is in good agreement with the value of 64% accepted in the simultaneous boost technique. The uniformity of the transmission fact or over the whole plate inside the irradiation field was required to b e better than 2% and has been checked using megavoltage imaging. In ab out 10% of the cases a replacement of the transmission plate was requi red due to enclosed air cavities. The results of our QA program demons trated that with regard to the dosimetric aspects the simultaneous boo st technique can be considered to be a reliable treatment technique.