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[Influence of position of ionization chamber and leaf-end on verification value in IMRT verification for prostate cancer].

In IMRT for prostate cancer, MU verification is performed by the actual measurement. We have experienced a remarkable improvement in results, once succeeding in finding out the more suitable and optimal evaluation dose point in some cases even though the deviation between a designed MU score and our actual record gained at the iso-center was more than 3%. In this study, we tried to demonstrate how much influence would be given to the point dose verification by the 3D arrangement between an ion chamber and tips of the MLC. The five cases in which the bias between each actual datum and planed MU score showed that about 3% were picked up and through these MLC configurations, 8 leaf-ends around the chamber were highlighted as the influential ones. After each distance from 4 pairs, a total of 8 leaves to the axis (the mid-line) of our ion chamber were mapped. The indexes (PlanLeafScores) were computed through these distances and segmental MU scores. The ratio of these scores and results obtained at the 12 sites within 1 cm from the iso-center were carried out by single regression analysis. In all cases the ratios of planed MU values to the actual ones tended to go down in inverse proportion to the increase in PlanLeafScores (r<-0.77, p<0.002). As the dimensional arrangement between the ion chamber and the edges of the MLC were thought to determine the result of the verification. PlanLeafScores will enable us to determine the optimal evaluation of the dose point.