In the Bellinzona technique two oblique radiation beams are used covering the spinal cord: a right anterior oblique (RAO; gantry angle » 290°) and a left anterior oblique beam (LAO; gantry angle » 70°). In the other main beams the spinal cord is blocked out. In addition, several small beams are used to obtain a homogeneous dose distribution. All beams have a common isocenter. Only the right and left anterior oblique beams can be used for verification, because the other beams hardly contain any bony structures.
The patients were treated in supine position and immobilized with a five-point thermoplastic head mask (Efficast, Orfit Industries, Belgium). A SAL correction protocol was used with Nmax = 4 and a = 5 mm. The deviation of patient position on the Linac with respect to the planning CT was determined by comparison of portal images of an anterior posterior (AP) and a lateral verification beam of 4 MU and 10 × 15 cm2 (15 cm in the craniocaudal direction) with the corresponding Digitally Reconstructed Radiographs (DRRs).
Furthermore, portal images of the right and left anterior oblique radiation beams that are used for treatment were made and compared with the corresponding DRRs. The deviation in the patient coordinates determined from the oblique radiation beams was compared with that determined from the orthogonal verification images of the verification beams.