• Validation of the method

    First, the method used for analysis of portal images of oblique beams was validated. For this purpose, a metal sphere was irradiated with the Tetrahedron technique and portal images were collected from both the oblique beams and the AP and lateral verification beams. The sphere was moved by introducing a table displacement. Portal images were collected for eight different sphere positions with an accurately known displacement with respect to the reference position (determined from the table position readout with a precision of 0.1 mm). The average difference between the displacements determined from the orthogonal portal images and the actual displacement was 0.4 mm (1SD = 0.5 mm) in the lateral , –0.3 mm (1SD = 0.7 mm) in the ventrodorsal and 0.1 mm (1SD = 0.4 mm) in the craniocaudal direction. For the portal images from oblique beams de average difference was 0.1 mm (1SD = 0.2 mm) in the lateral, 0.0 mm (1SD = 0.7 mm) in the ventrodorsal and 0.2 mm (1SD = 0.3 mm) in the craniocaudal direction.

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