Brainstem

The midbrain, pons and medulla oblongata, running from the base of the brain down to the foramen magnum, where it continues as the spinal cord. It sits directly behind the clivus and nasopharynx.

The brainstem connects the brain to the spinal cord. It controls automatic functions such as breathing, heart rate and swallowing, and most cranial nerves (for eye movement, facial movement and feeling, hearing and swallowing) start here.

Like the spinal cord, the brainstem is a serial organ: injury to a small part can cause serious neurological damage, so its limits are written as maximum doses, often with a separate limit for a planning risk volume (PRV). It matters most for nasopharynx cancer and for tumors that spread toward the skull base, where the high-dose target can sit only millimeters away. For brain treatment it is evaluated in nearly every plan: glioma targets and radiosurgery targets near the posterior fossa can sit right against it.

Brouwer 2015 defines it as follows:

• Top: the bottom section of the lateral ventricles, which is clearly visible on both CT and MRI.
• Bottom: the tip of the dens of C2, which is also where the spinal cord contour starts.

MRI is recommended. Atlases disagree most on the upper limit: the EPTN 2018 atlas, written for brain tumors, contours on MRI and splits the brainstem into midbrain, pons and medulla, starting the midbrain at the nigral substance of the cerebral peduncle. It uses the same lower limit (the tip of the dens). Check which definition the source behind your limit used.