A method for reducing ovarian doses in whole neuro-axis irradiation for medulloblastoma

Background and purpose Cranio-spinal irradiation for medulloblastoma can impair fertility in girls. The literature indicates that an ovarian dose of 4Gy causes permanent infertility in 30% of young females and that doses of <1. 5Gy over the whole treatment are desirable. We report a modified radiotherapy technique using a non-divergent beam edge inferiorly to reduce the ovarian dose. Patients and methods Eight female patients with medulloblastoma had magnetic resonance imaging (MRI) studies in the treatment position to identify the position of their ovaries relative to the radiation field. The information was transferred to the radiotherapy planning system and plans were generated using conventional spinal fields and modified fields with a half beam block at the inferior border. Results Identifying the position of the ovaries by MRI enabled the dose to be estimated for the two techniques. Using a non-divergent beam inferiorly, the mean ovarian dose was reduced in all cases by a median value of 2.45Gy (range 0.6-19.5Gy) and the median percentage reduction was 66.8% (range 2.6-84.6%). The position of the ovary relative to the beam edge was critical in determining the dose reduction for each case. The modified technique doubled the number of patients receiving <4Gy to a single ovary from three to six. With this alteration, three patients also had an ovary receiving <1.5Gy whereas all exceeded this dose with conventional treatment. Conclusion We recommend using asymmetry at the inferior spinal border to achieve a non-divergent edge to the treatment field to reduce the dose to the ovary. Using MRI to localise the ovaries is important in estimating their dose and in assisting the counselling of patients and their families about future fertility.