Craniospinal irradiation phase dosimetry. The radiation target is the whole brain and spinal canal (dark green line) inclusive of the thecal sac. The doses received by these regions are shown by a series of isodose lines for the 5 Gy (dark purple), 20 Gy (light purple), 23 Gy (dark blue), and 23.4 Gy (light blue) dose levels. Note the lack of low-dose radiation received by structures anterior to the spinal canal when using a proton beam that enters from the posterior surface.