Figure 2 (click to view)

Portal images of nine patients treated with the Tetrahedron technique were analyzed. The radiation beams 2, 3 and 4 from Table 1 were imaged if possible. For two patients it was not possible to collect the radiation beam nr. 2 because of the danger of collision of the imager with the patient. In addition, an AP and one lateral verification beam were imaged. This resulted in 71 sets of portal images from two oblique and the verification beams and 51 sets of portal images from three oblique and the verification beams. In Fig. 2 the scatter plots of the set-up errors determined from the oblique beams versus those determined from the verification beams are given for the lateral (a), the ventrodorsal (b) and the craniocaudal (c) direction. The results calculated from two oblique radiation beams are indicated with circles and those from three oblique radiation beams with plus signs. An overview of the Pearson correlation coefficients and the mean diffe-rences between the set-up errors determined from the oblique radiation beams and the verification beams is given in Table 3.

Table 3 (click to view)

The results show a statistically significant correlation between the set-up errors determined from the oblique and the AP and lateral verification beams. However, the correlation coefficient is only 0.4-0.5 for two oblique beams and 0.5-0.7 for three oblique beams.

Figure 2 Set-up error in the lateral, height and craniocaudal direction of patients treated with the Tetrahedron technique. Set-up errors determined from oblique radiation beams are plotted as a function of those determined from orthogonal verification beams.

Table 3 Comparison of set-up errors determined from the oblique fields and the verification fields of patients treated with the Tetrahedron technique.