In tumors with a high probability of cerebrospinal seeding both cranial and spinal axis irradiation is indicated. Although cranium is irradiated through two lateral fields that are vertical to the sagittal plane, spinal canal is irradiated through a posterior portal that is vertical to the coronal plane. Several techniques that are developed to find a solution for hot and cold spots that may exist at the matchline of two vertical fields also carry some problems. In this study two of the techniques, "Divergence matching technique (DMT)" and "Half-beam irradiation technique (MBIT)" are studied to calculate the dose distribution at the junction and to compare them in terms of simplicity of application. A special fantom similar to random fantom which has two sagittal symmetric slices is build from 50 % wax and 50 % paraffin and is used for dose distribution calculations at the junction using x-ray films and TLD rods at several points including thyroid. In conclusion, no gross difference between DMT and MBIT is found at the junction at sagittal plane in terms of dose distribution. Both techniques had a homogenity of 5 %, and, thyroid received 88 cGy by DMT and 100 cGy by MBIT.

Keywords: Cranio-spinal irradiation technique, dosimetry