IRRADIATION OF THE WHOLE NEURAXIS - A METHOD FOR FIELD POSITIONING

When the whole neuraxis is irradiated, as in medulloblastoma, the larg e target volume makes field arrangement complicated and 3-4 adjacent f ields have to be used. A calculation method, based on geometrical matc hing of field edges, is presented for determining optimal separation b etween the opposed cranial fields and the posterior spinal field as or thogonal movements of the treatment couch top. The dosimetric verifica tion of the method with a 4 MV photon beam indicates a dose inhomogene ity below +/-7%.