Orthogonal fields: variations in dose vs. gap size for treatment of the central nervous system

Variations in dose at the junction at the lower end of two lateral skull portals and the upper end of a posterior spinal portal used for irradiation of the central nervous system (CNS) were studied using 60Co and 6 MV linear accelerator radiation. The relationship between dose and gap size was measured via film dosimetry. Small variations in gap size resulted in large variations in dose (+/- 40%) at the junction. These variations in dose were more pronounced for the 6 MV accelerator than for the 60Co, but the volume of high and low dose tended to be smaller with the 6 MV linear accelerator.