[Comparison between measured and computed absorbed dose values in the tangential treatment of the breast]

The calculated absorbed dose values to the reference point of the breast tangential irradiation, obtained by 2-dimensional (2D) Treatment Planning Systems (TPS), were compared with the measured absorbed dose values obtained with a ionization chamber in an anthropomorphic phantom treated with two opposed photon beams of 60Co, 6 MV and 10 MV. This comparison was aimed at assessing the 2D algorithm dose overestimation due to the effect of missing tissue in the irradiated field. The ratio between the computed dose, Dc, and the measured dose, Dm, reached 1.08 for 60Co beams and 1.04 for X-ray beams of linacs. The Clarkson method gives an adequate correction factor for the calculated absorbed dose value in the reference point. The portal films of some patients were studied to estimate the correction factor for the treatment time or monitor units evaluated by TPS using tomographic breast central plane. The values of the correction factors reached 1.10 for 60Co photon beams and 1.04 for X-ray beams. The measurements were carried out with TLDs positioned in the central plane of the breast, covered by a special bolus, of a second anthropomorphic phantom. The dose homogeneity as well as the accuracy of the relative computed dose values in the breast phantom irradiated with 10 MV X-ray beams were assessed.