A sellar mass is any tumor located in the sella turcica. The sella is the bony cup that holds the pituitary gland, and the gland fills it with little room to spare. So anything that grows there presses on what is next to it. The gland and the structures around it can be affected by a wide variety of disorders, and “sellar mass” names a place, not a diagnosis.
What can grow in the sellar region?
The sella holds the gland, bone, vessels and membranes, and above it lies the brain. A mass there can therefore start from any of these tissues. Which of them actually give rise to masses, and which is the most familiar?
Pituitary adenomas are the most familiar of these disorders. The sellar region can also be affected by:
- neoplasms of the adenohypophysis with endocrinological manifestation, such as Cushing disorder and acromegaly;
- cysts or neoplasms derived from remnants of Rathke’s pouch;
- tumors of the neurohypophysis and pituitary stalk;
- neoplasms of the parasellar bones.
The first group makes sense once you remember that a tumour of hormone-making cells keeps making their hormone. The excess of that hormone gives the endocrine picture. The second group has an embryological reason. A piece of the embryonic structure that the gland grows from can fail to disappear, and what remains can form a cyst. Finally, sellar masses can arise from brain, hypothalamus, or pituitary tissue.
Where did the mass start?
Because a mass can come from brain, hypothalamus or gland, the next question is how to tell which. Each of these tissues has a specific magnetic resonance imaging (MRI) pattern. That pattern can help in the differential diagnosis of the origin of the mass. The help has limits, because several common sellar lesions look alike on imaging, so the reading is hard.
What is a pituitary adenoma?
The gland is one possible origin, so what is a tumour of the gland? Pituitary adenomas are benign neoplasms. They are neoplasms of the neuroendocrine epithelial cells of the adenohypophysis. Each grows from one cell type, so it keeps that cell’s hormone. This explains why, for adenomas, the cell type they came from sets the hormonal picture.
The adenohypophysis can be affected by primary tumors or by secondary, metastatic tumors. Most of the time tumors in the adenohypophysis are adenomas. In very rare cases there can be carcinomas in the adenohypophysis as well.
Why does benign not mean harmless?
An adenoma is usually benign, yet it can grow into what lies beside the sella. A benign tumour can cause harm through its size and position. Although called benign, pituitary adenomas can be locally invasive and destructive. The sella is walled by bone and the cavernous sinus lies beside it, so an adenoma from the gland can reach them. A tumor that invades the surrounding tissues is called invasive.
How are adenomas sized and described?
Invasion is one measure of how far a benign growth reaches, and size is another. Pituitary adenomas are divided by size into microadenoma and macroadenoma. The cut-off between them is a size of more than 10 mm. A larger mass presses harder on what is next to it, and the closed sella leaves it less room. Whether the tumour invades the surrounding tissues adds a second description, invasive or not.
Where do adenomas come from?
Size and invasion describe the tumour. The question now is what started it. The origin of pituitary adenomas can be sporadic, which is most of the cases. It can also be familial.
Familial pituitary tumors can present as isolated tumors, found only in the hypophysis. They can also present as part of a syndrome in which several tumours appear across the body. An inherited change is present in every cell from birth, so the tumour can appear in one gland or in several places. That is why familial forms can be part of a wider syndrome.
How does a mass change the gland’s hormones?
A sellar or suprasellar mass can affect the gland, so what does the gland lose or gain? The effect of sellar and suprasellar masses on the pituitary gland can be variable, and it depends on what the mass is made of.
- A mass can leave the gland underactive (hypopituitarism), as in the case of craniopharyngiomas. Compression of normal gland cells, and of the stalk that carries releasing hormones down the portal vessels to the gland, lowers the output of the normal hormones.
- A mass can make the gland overactive (hyperpituitarism), as in the case of Cushing’s disease. An adenoma keeps the output of its cell type, so its excess hormone can raise blood levels.
What if the mass makes no hormone?
A mass can be non-functional, with no hormonal effect. It makes no excess hormone, yet it still presses on the normal gland and stalk, so it can still matter. If the sella can hold more than adenomas, what else grows there, and how do those masses differ?