In the Tetrahedron technique, four radiation beams are used with equidistant angles of 109°. No opposing beams are used, which results in a steeper dose gradient between high and low dose regions. All beams have a common isocenter. Normally, the beams are oriented as described in Table 1. The gantry angle of beam 1 is chosen such that the beam does not hit the eyes. The angles of all other beams follow from the first beam. The patients were treated in supine position with the head in slight hyperextension and immobilized with a three-point thermoplastic head mask (Efficast, Orfit Industries, Belgium). A SAL correction protocol was used with Nmax = 4 and a = 4.7 mm. The deviation of patient position on the Linac with respect to the planning CT was determined by comparison of portal images of an AP and a lateral verification beam of 4 MU and 10×10 cm2 with the corresponding DRRs1. Furthermore, portal images were made of the radiation beams 2 – 4 shown in Table 1. The craniocaudal from ventral beam (nr. 1 in Table 1) cannot be used for verification because the imager has to be removed from the gantry to be able to setup this irradiation geometry. The deviation in the patient coordinates determined from the oblique beams was compared with that determined from the AP and lateral images.
The 6 MV photon beam of an Elekta Sli Linac (Elekta AB, Stockholm, Sweden) was used for all irradiations. The portal images were collected with the Elekta camera based iView system. The deviation of the portal images and DRRs with respect to simulator films was determined with a software package called myView, which was developed at the radiotherapy department of University Medical Center Leiden.

Table 1 (click to view)