The radiological approach differentiates between:
- microadenomas(i.e., the greatest diameter is <10 mm),
- macroadenomas (i.e., the greatest diameter is ≥10 mm).
Using functional criteria, pituitary adenomas can be categorized into:
- prolactin (PRL)-producing,
- growth hormone (GH)-producing,
- thyrotropin (TSH)-producing, also known as thyrotroph, tumors.
Most pituitary adenomas are microadenomas. Macroadenomas, which account for 70–80% of adenomas, are seen in all ages, most commonly in between the ages of 25 and 60 years, with a clear preference in females.
Imaging and Location
In imaging studies, the tumor presents as a mass lesion in projection of the pituitary gland, in some cases with a typical figure eight appearance and a large suprasellar component (Cappaibanca et al.1999; Schwartzberg 1992). Macroadenomas may extend into the neighboring areas such as the suprasellar cistern, cavernous sinus, sphenoid sinus, and nasopharynx in up to 70% of the cases and are best shown on MRI studies. They can present with cysts, areas of necrosis and hemorrhage. An involvement of the carotid sinus is described and may encase the carotid artery as well. On MRI studies, depending on the size and different components of the tumor, macroadenomas are usually isointense with cortical gray matter, and enhance intensely with Gd- containing contrast materials. Depending on the amount of cysts, bleeding, and hemorrhage the signal patterns can be quite inhomogeneous.
Microadenomas less than 1 cm in diameter are best seen on thin-section coronal images and appear hypo- to isointense to the normal pituitary tissue. After contrast material application the microadenomas present with a less intense and later enhancement than the normal gland tissue, which can be nicely appreciated on the dynamic scans (Bartynski and Lin 1997; Rand et al. 2002).
Macroadenomas are usually endocrinologically inactive and present with clinical symptoms of mass effect or resulting hypofunction of the gland. Clinical important is its relation to the optical pathways, especially the deviation of the chiasm. The involvement of the chiasm, represented by a bitemporal hemianopia is an emergency that requires immediate treatment. The same is true for pituitary apoplexy, hydrocephalus, or cranial nerve involvement.
The prolactin-producing adenomas are the largest group, accounting for about 30% of all adenomas. The second largest group is that of clinically nonfunctioning (i.e., endocrine-inactive) adenomas. This group (20–25% of adenomas) is composed predominantly of gonadotroph adenomas. They synthesize follicle-stimulating hormone (FSH) and/or luteinizing hormone (LH), or the alpha or beta subunits of these heterodimers. They are usually detected incidentally, or because of the presence of neurologic symptoms. Gonadotroph adenomas are inefficient secretors of the hormones they produce, so they rarely result in a clinically recognizable hormonal hypersecretion syndrome.
On imaging studies and for treatment planning, e.g., surgical resection, the exact display of the size, the relation of the tumor to the optical pathways, and the infiltration of neighboring structures is essential and predicts the ability to completely remove the tumor via a transsphenoidal approach.
- M.F. Reiser, W. Semmler, H. Hricak (Eds.) "Magnetic Resonance Tomography", Springer 2008