For adult intracranial (posterior fossa) ependymoma, how do you approach spinal cord constraints with planning to 54-59.4 Gy?

As you have realized, there is no way to keep SC to 54 with this prescription, and the more important point is: where does the radio-sensitivity of the brainstem transition to the spinal cord? After all, the distal brainstem is really a series organ as well. So, given that the control of disease is ...