Portal dose measurements by a 2D array.

A 2D array (PTW, type 10024), equipped with 729 vented plane parallel ion-chambers, has been calibrated as a detector for the in vivo comparison between measured and predicted portal doses for head-neck tumors. The comparison of absolute portal doses measured to ones predicted by a commercial treatment planning system within the field of view of the CT scanner, can help the delivered dose verification during different treatment fractions, in particular when the patient's present weight loss. This paper reports the preliminary results of the comparison of the portal doses measured by a PTW 2D array during several radiotherapy fractions and the predicted portal doses for seven patients undergoing head-neck tumor radiotherapy. The gamma index analysis supplied an agreement of more than 95% of the dose-point P(gamma)>95% within acceptance criteria, in terms of dose difference, DeltaD(max), and distance-agreement, Deltad(max), equal to 5% and 4mm, respectively. After the third week, one patient showed a decrease of P(gamma) values due to the markedly reduced patient's thickness. Even if the spatial resolution of the 2D array was 1cm, there were two advantages in the use of this 2D array as a portal dose device for IMRT quality control. The first one was the use of a stable and efficient absolute dosimeter for in vivo verification, although its construction and behavior for other gantry angles need to be tested, and the second one was the time efficiency in verifying the correct dose delivery in several fractions of the therapy. This study presents acceptance criteria for the comparison of TPS-predicted portal dose images with in vivo 2D ion-chamber measurements for IMRT. In particular, portal dose measurements offer clues for additional studies as to which indicators can signal the need for replanning during treatment.