Conformation therapy to improve the irradiation of the spinal axis.

Abstract

Techniques for the irradiation of the spine have remained largely unchanged since the introduction of megavoltage therapy. The main disadvantages of these techniques are the inhomogeneous dose distribution and the problems associated with field joins. The tracking method of conformation therapy employs a short field length and translates the patient through the beam on the couch of a computer controlled treatment unit. Dose uniformity along the spinal axis is achieved by controlling the speed at which the target travels through the radiation field. Treatment is given at standard source-axis distance (SAD) and the need for external shielding is eliminated by shaping the width of the field. The join with the cranial fields is independent of any skin marks, removing the risk of over- or under-dosage. Treatments are planned automatically from computed tomography (CT) information by an in-house planning system. The calculated doses have consistently shown good agreement with phantom measurements. Three cobalt dose distributions are presented and compared: a tracking treatment; a single posterior field, and two matched fields. The superiority of conformation therapy over the other commonly used treatment methods is clearly demonstrated.