Conventional Craniospinal Irradiation with Patient Supine and Source-Skin Distance (SSD) 100 cm for Spinal Field

Abstract

We describe a method of craniospinal irradiation (CSI) in the supine position and at a source-skin distance (SSD) of 100 cm for the spinal fields. The procedure is carried out with a 100-cm isocenter linear accelerator and conventional simulator, and the treatment is delivered with 2 opposed lateral cranial fields at source-axis distance (SAD) of 100 cm and 1 or 2 direct posterior spinal fields at SSD, 100 cm. The half beam–blocked cranial fields with a collimator rotation is used to match the superior border of the spinal field at the level of C2 vertebral body. The length of the spinal field is fixed, and is the same if 2 spinal fields are used. The position of the isocenter of the spine field is defined by longitudinally moving the couch a distance from the isocenter of the cranial fields and adjusting the SSD = 100 cm to the surface of the couch with the gantry rotated to the angle of 180° (posteroanterior position), and the distance can be calculated easily according to a few parameters. It only needs a simple calculation without couch rotation, extended SSD, or markers. The inferior and superior borders of the spinal field do not require visualization under fluoroscopy when it is beyond the visual field of the simulator. The entire simulation takes no more than 20 minutes. Supine craniospinal treatment using this technique may substitute the traditional prone position as a potentially beneficial alternative to CSI.