CNS / Brain contouring

The brain controls thought, memory, speech, movement, sensation and vision.

The EPTN 2018 atlas contours the brain including the cerebellum, the cerebrospinal fluid and small brain vessels, and excluding the brainstem and the large venous sinuses (sigmoid, transverse and superior sagittal). It also leaves out the carotid canals and cavernous sinuses in the middle cranial fossa, which are easiest to see on contrast-enhanced T1 MRI. It recommends a CT brain window (WW/WL 350/40), or a planning-system auto-contour that is checked afterwards. A "supratentorial brain" structure is the brain minus the cerebellum.

Brouwer 2015 also excludes the brainstem but suggests CT bone settings, so the two atlases differ slightly in detail.

"Brain" is defined differently between sources, and the definition changes the number. HyTEC's V12 Gy, V20 Gy and V24 Gy include the target volumes; other sources subtract the GTV or PTV. Check the definition used by the source behind your limit, and name the structure so the difference is obvious (AAPM TG-263 gives standard names).

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.

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.

The hearing organ of the inner ear. It turns sound vibrations into nerve signals that travel to the brain.

Brouwer 2015 places it in the temporal bone, lateral to the internal auditory meatus, and recommends a CT bone window to find it. The EPTN 2018 atlas adds:

• It sits in a small bony cavity in the petrous temporal bone, below the semicircular canals and lateral to the internal auditory canal.
• On a bone window (WW/WL 120/1500 in the atlas) it shows up as a small cavity. It is also well seen on T2 MRI.
• The semicircular canals are not included.

Because it is so small, Brouwer 2015 suggests expanding it by 5 mm when calculating DVH data, to make the numbers more consistent (while avoiding overlap with the PTV), and recommends a contrast CT with slices of 2 mm or less.

The eyes focus light onto the retina at the back of the eye, which turns it into nerve signals for the brain.

Brouwer 2015 splits each eyeball into two structures rather than one whole globe:

• Anterior segment: everything in front of the vitreous — the cornea, iris, ciliary body and lens.
• Posterior segment: everything behind the lens — the vitreous, retina and choroid. It includes the whole retina but excludes the optic nerve.

The EPTN 2018 atlas instead contours the cornea and the retina as thin separate structures (drawn with a 2–3 mm brush). Many protocols evaluate the whole globe, so check which structure your limit refers to. The lens is contoured as its own structure.

The hippocampus, on the inner side of each temporal lobe, is central to forming new memories and to learning.

The EPTN 2018 atlas defines the hippocampus as the gray matter medial to the temporal horn of the lateral ventricle, bordered medially by the quadrigeminal cistern, following Gondi et al. Its size varies with age and disease (2.8–4.0 cm³ in the literature it cites). For research it can be split into an anterior part (head) and a posterior part (body), using the lateral ventricle as a landmark for the anterior part. Delineation on T1-weighted MRI is essential.

The NRG CC001 protocol requires the hippocampi to be contoured according to the RTOG hippocampal contouring atlas, and names three structures: both hippocampi together, each side separately, and the hippocampal avoidance region grown from them. Gondi 2010 walks through the contouring and planning.

The clear lens behind the iris focuses light onto the retina. Clouding of the lens is a cataract.

The EPTN 2018 atlas describes the lens as a clearly visible, biconvex structure up to 10 mm across, between the vitreous and the iris, which is easy to contour on CT. It notes that the lens can move during treatment unless the patient is told where to look.

Brouwer 2015 does not contour the lens on its own — it is included in the anterior segment of the eyeball.

The optic nerves carry what each eye sees to the brain. At the chiasm the signals are sorted, so each side of the brain receives the opposite half of what both eyes see.

Optic nerves — from Brouwer 2015 and the EPTN 2018 atlas:

• Usually 2–5 mm thick and generally clear on CT.
• Contour all the way from the back of the eyeball, through the bony optic canal, to the chiasm.
• MRI is recommended, at least close to the chiasm. EPTN stresses contouring the nerves in continuity with the chiasm, because dose gradients there can be very steep.

Optic chiasm

• Brouwer places it in the suprasellar cistern, typically about 1 cm above the pituitary gland, bordered at the sides by the internal carotid arteries. MRI is recommended.
• EPTN adds that it sits just in front of the pituitary stalk, in the front-lower part of the third ventricle, with the optic nerves in front, the optic tracts behind, and the anterior cerebral and anterior communicating arteries in front of it. It recommends T1 MRI.

Because it is so small, Brouwer 2015 suggests expanding it by 5 mm when calculating DVH data, to make the numbers more consistent (while avoiding overlap with the PTV), and recommends a contrast CT with slices of 2 mm or less.

A small gland that makes hormones controlling growth, the thyroid, the adrenal glands and the reproductive glands, often called the body's master gland.

Brouwer 2015 notes the gland is very small and generally hard to see on CT; the inner part of the sella turcica can be used as a bony surrogate, and its borders are easiest to set on the sagittal view. The EPTN 2018 atlas adds:

• Oval, up to 12 mm from top to bottom, sitting in the sella turcica.
• The cavernous sinuses border it at the sides (well seen with IV contrast); the brain lies just above it, connected by the pituitary stalk to the hypothalamus.

Because it is so small, Brouwer 2015 suggests expanding it by 5 mm when calculating DVH data, to make the numbers more consistent (while avoiding overlap with the PTV), and recommends a contrast CT with slices of 2 mm or less.

The temporal lobes handle hearing and understanding language, recognizing faces and objects, and memory — the hippocampus, important for forming new memories, sits on the inner side of each lobe.

Brouwer 2015 lists the temporal lobe as beyond its scope. Sun 2014, an open-access atlas for nasopharynx cancer, gives these boundaries (Table 3):

• Top: the top edge of the sylvian fissure.
• Bottom: the base of the middle cranial fossa.
• Front: the temporal bone, the sylvian fissure and the greater wing of the sphenoid.
• Back: the petrous part of the temporal bone, the tentorium of the cerebellum and the preoccipital notch. (The table prints "petrous part of temporal lobe", which reads as a typo for bone.)
• Outside: the temporal bone.
• Inside: the cavernous sinus, the sphenoid sinus, the sella turcica and the sylvian fissure.

Sun 2014 includes the hippocampus, parahippocampal gyrus and uncus, and excludes the basal ganglia and insula. Some other methods leave the hippocampus out, so check which your protocol uses. Contour each side separately.