Craniospinal axis irradiation: an improved electron technique for irradiation of the spinal axis

In this work we review the dosimetric features of craniospinal axis irradiation in the areas of matching cranial and spinal fields, with reference to the normal structures within the spinal field. The implications of the use of photon or electron modalities for the spinal port were evaluated. A novel method of matching the cranial photon and the spinal electron fields involving a computer-aided junction design is presented. The technique involves moving the photon beam in three steps to degrade its penumbra to match that of the electron field. Thermoluminescent dosimetry in a Rando phantom and computed tomography-based dose-volume histogram study for an illustrative paediatric case were used to compare the dose to normal structures within the spinal field. Our results show that the use of electrons for the spinal field leads to better sparing of deep seated normal structures. In the case of bone marrow, the use of a customized bolus for the spinal field results in an improved dose distribution, making electrons potentially superior to photons for radiobiological reasons.