Irradiation of the whole neuraxis. A method for field positioning.

When the whole neuraxis is irradiated, as in medulloblastoma, the large target volume makes field arrangement complicated and 3-4 adjacent fields have to be used. A calculation method, based on geometrical matching of field edges, is presented for determining optimal separation between the opposed cranial fields and the posterior spinal field as orthogonal movements of the treatment couch top. The dosimetric verification of the method with a 4 MV photon beam indicates a dose inhomogeneity below +/- 7%.