The need for the moving junction in craniospinal irradiation.

The moving junction is used in craniospinal irradiation (CSI) to smooth out any dose inhomogeneity across the head/spine junction. The aim of this study was to demonstrate the extent of the head/spine junction inhomogeneity in treatment plans of actual patients and to compare stationary and moving junction data. The radiotherapy plans, prescriptions and case notes of 18 patients (12 medulloblastomas, 3 supratentorial primitive neuroectodermal tumours, 2 pineoblastomas and 1 pineal germinoma) treated with CSI (35 Gy in 21 fractions over 29 days) were examined. At 16 months median follow-up (range 1.5-35.5 months), no junctional recurrences or myelopathy were observed. Using the moving junction technique the mean maximum anterior cord dose, from 5 cm caudal to 3 cm cephalad of the Day 1 junction, was 36.3 Gy, and the mean minimum anterior cord dose was 32.9 Gy, with a mean within-patient variation of 3.4 Gy (9.7% of 35 Gy). In four patients, comparison of dose variation across the field junction was made between the original plans and a re-plan using a stationary junction. The effect of a matched junction, a 2 mm overlap and a 2 mm gap were studied both for moving junction and stationary junction techniques. Dose variations were similar in all cases for exactly-matched fields, but for a 2 mm overlap or gap the dose variation was smaller in all but one case for the moving junction technique. These data suggest that the moving junction is important to minimize the risk of overdose or underdose across the spine/head junction in CSI.