Field matching considerations in craniospinal irradiation.

Radiotherapy of the craniospinal axis in leukemia and medulloblastoma patients usually involves parallel-opposed lateral cranial fields adjacent and orthogonal to a posterior spinal field. Most current treatment protocols require rotation of the cranial fields to compensate for the divergence of the spinal field such that the adjacent field edges abut along the match line in the mid-saggital plane. Some departments introduce gaps up to 1-2 cm wide between the matched fields out of concern for overdosing the spinal cord. The behavior of the dose distribution was studied in the junction region of divergence-compensated fields as the separation of their edges varied from 0.5 cm overlap to a gap 1 cm wide. Composite dose profiles and isodose maps were calculated for 60Co and 4 MV photon beams using static and moving junction (feathering) techniques. When the fields are appropriately matched there is no gap between them in the mid-saggital plane and the dose varies smoothly across the junction. Gapping of divergence-compensated fields is detrimental to dose uniformity, producing underdosage in a volume that extends to all depths in the body. With proper localization, there is no way that the commonly accepted critical dose for myelopathy can be exceeded in typical treatments of leukemia patients. It can occur in the treatment of medulloblastoma patients only in the unlikely situation where there is gross overlapping of the adjacent fields. Feathering may be considered as a safety margin against spinal cord damage in medulloblastoma but it is superfluous in leukemia. The importance of treatment machine quality assurance and verification of patient set up geometry are emphasized.