This article will review the presentation and imaging appearance of tumors affecting the area around the pituitary gland. For convenience, the lesions will be discussed in order of the regions they usually affect. For example, lesions affecting the suprasellar region include craniopharyngiomas, optic or hypothalamic gliomas, germ cell tumors, epidermoids, dermoids, hamartomas, lipomas, and choristomas. Tumors that commonly affect the parasellar cavernous sinus include schwannomas and meningiomas. Tumors like chordomas may arise in the sphenoid bone or clivus and affect the sella. Metastasis, meningiomas and aneurysms may involve any area around the sella. For a detailed discussion of normal anatomy and imaging protocols, see the first article of this monograph.
The suprasellar cistern is the CSF filled space between the diaphragma sella and the floor of the third ventricle. It normally contains the optic chiasm and pituitary infundibulum.
Craniopharyngiomas are benign, slow growing neoplasms of epithelial origin. They are the most common suprasellar lesion, and are most commonly entirely suprasellar in location. 1-4 About 1/3, however, may extend into the sella, and occasionally they may be entirely intrasellar. They have a bimodal age distribution, with the first peak in incidence between 5-14 years old, and the second peak in older adults aged 65-74 years. There are two histological subtypes. The adamantinomatous subtype is seen mostly in children, while the squamous subtype occurs principally in older adults. This histologic variability in part accounts for the variability of imaging findings.(Figure 1)
The imaging hallmarks of craniopharyngioma are calcifications and cysts. 3 Calcifications occur in about 80%, and may be nodular or curvilinear. Calcifications may be difficult to see on MR images, but are well depicted by CT. Cysts are present in almost all craniopharyngioma. In children, the cystic parts tend to predominate, while in adults solid parts are larger, and the cysts may be small or multiloculated. In children the cysts frequently contain highly proteinaceous fluid, which may be very bright on pre-contrast T1 weighted MR images. 4 The solid portions and cyst walls of all craniopharyngiomas enhance, usually heterogeneously. It has been suggested that edema spreading along the optic tract may be a fairly specific sign of craniopharyngioma. 5
Hypothalamic/Optic Chiasm Glioma
Optic chiasm or hypothalamic gliomas are often considered a single disease entity, since the site of origin cannot usually be determined. 1,6-8 They are usually considered as a subset of optic pathway gliomas. The majority are pilocytic astrocytomas, with most of the rest being low-grade diffuse astrocytoma. They are tumors of childhood, with most occurring within the first decade of life. About half of patients with optic pathway gliomas have neurofibromatosis type 1 (NF-1), while about one third of patients with NF-1 will develop optic pathway gliomas. 9 Optic pathway gliomas generally behave very benignly, with very slow growth, however, tumors around the chiasm/hypothalamus are more aggressive, with only about a 50% 5 year survival. Those that occur in patients with NF-1 are slower growing, however, and have a much better prognosis, even in this region. 9 Pilomyxoid astrocytoma is a recently described subtype of hypothalamic glioma that typically occurs in young children and has unique histopathologic and clinical characteristics. Pilomyxoid astrocytomas seem to have a higher rate of recurrence and CSF dissemination than typical pilocytic astrocytomas. These tumors have been previously diagnosed as pilocytic astrocytoma (PA). 10 (Figure 2)
On imaging studies, hypothalamic/chiasmatic gliomas are usually large masses of the suprasellar region, infiltrating brain and third ventricle. They enhance homogenously. There is rarely necrosis, hemorrhage or calcification. They may encase the vessels of the circle of Willis, and hydrocephalus is common.
Germ Cell Tumors
Germ cell tumors primarily affect children and young adults. They are usually midline lesions. Germinomas are by far the most common of the germ cell tumors. About 80% of germinomas arise in the pineal area. Suprasellar or intrasellar lesions make up most of the remaining 20%. Suprasellar germinomas may arise alone, synchronously or metachronously with pineal lesions. Pineal region germinomas have a strong male predominance, but suprasellar germinomas have no sex preference. 8,11,12 (Figs. 3 & 4)
Germinomas (Figure 3) are seen on imaging studies as an infiltrative, homogenous mass in the midline. Signal intensity is similar to grey matter on T1 and T2 weighted MR images. There is homogenous, usually intense, contrast enhancement. It is rare to see any cystic change, hemorrhage, or calcification.
Other germ cell tumors that may rarely arise in the suprasellar region include yolk sac tumors, embryonal cell carcinoma, choriocarcinomas, and teratomas. Teratoma (Figure 4) may be suspected when the mass is very heterogeneous, often with areas of fat or calcification. 13,14
Epidermoid tumors are benign, congenital, epithelial inclusion cysts. The wall of the cyst is lined with squamous epithelium. They do not have any of the deeper dermal structures, such as hair follicles or sweat glands. If these elements are present, the lesion is called a dermoid (see below). Epidermoids tend to arise away from the midline, and are most common in the cerebellar-pontine angle, but may also be seen in the suprasellar region. Although they are thought to be formed during fetal development, epidermoids usually present clinically in the 4th and 5th decades, when the cyst has grown large enough to exert mass effect on adjacent structures. Epidermoids grow by the accumulation of desquamated epithelial cells into the center of the cyst. 1,8,15,16 (Figure 5)
It is a remarkable coincidence of nature that this desquamated debris, which contains dead cells, keratin, and cholesterol crystals, appears almost identical to CSF on CT scans, and on T1 and T2 weighted MR images. Fortunately, epidermoids can be distinguished from CSF containing arachnoid cyst by the fact that epidermoids are usually bright on fluid attenuated inversion recovery (FLAIR) and diffusion weighted MR images, while the arachnoid cyst is dark on both sequences. Another distinguishing feature is that epidermoids tend to insinuate between and surround vessels and other adjacent structures, while arachnoid cysts displace them. Neither epidermoids nor arachnoid cysts enhance.
Like epidermoids, dermoids are benign, congenital epithelial inclusion cysts. They are also lined with squamous epithelium, but, unlike epidermoids, they also contain deeper dermal elements, including hair follicles and sweat and sebaceous glands. Unlike epidermoids, they most commonly arise in the midline. They are usually found in the posterior fossa or suprasellar area. They tend to present in the 20-30 year age range, earlier than epidermoids, and there is a male predominance. 1,2,8,15-17 (Figure 6)
On imaging studies, dermoids are well circumscribed heterogeneous masses. They may contain areas of fat and or calcification. They do not enhance. They may rupture, spilling high signal debris into the CSF, and causing a chemical meningitis. 17,18
Hypothalamic hamartoma may present as a mass in the suprasellar area. They are composed of disorganized, ectopic foci of grey matter. They are not true neoplasms, but may increase in size slowly over time. They most frequently arise from the region of the tuber cinereum and mammillary bodies, and are thus frequently called tuber cinereum hamartomas. Most commonly they present in children with precocious puberty. Less commonly patients may have partial seizures. Rarely, but characteristically, the seizures may take the form of spasmodic laughter, so called "gelastic seizures". 19,20 (Figure 7)
Hamartoma have imaging characteristics similar to grey matter on CT and all MR imaging pulse sequences. They may be slightly higher signal than grey matter on T2 or FLAIR MR images. They do not enhance. The appearance of a pedunculated mass arising between the pituitary stalk and the mammillary bodies is characteristic. 21,22
Lipomas are benign fatty tumors, thought to derive from remnants or maldevelopment of the primitive meninx. In the sellar region they occur as lesions adherent to the surface of the infundibulum, floor of the third ventricle, or adjacent cranial nerves. 23,24 They are usually discovered incidentally, but rarely may enlarge or produce symptoms. (Figure 8)
Lipomas appear identical to fat on CT and all MR imaging sequences. They are well circumscribed and do not enhance. They may have rim calcifications. Lipomas can be distinguished from fat containing dermoids and teratomas by the fact that they are very homogenous. Fat suppressed MR images may be useful in distinguishing lipomas (which suppress) from hemorrhagic or proteinaceous lesions, which are also bright on pre contrast, T1 weighted images. 1,4,8
Choristomas are rare tumors that arise from the infundibulum or posterior lobe. They are also known as myoblastomas or granular cell tumors. On MR imaging studies they typically are isointense to brain, and enhance homogenously. 25-27 (Figure 9)
The cavernous sinus is a blood filled space created by reflections in the dura along the side of the sella. The internal carotid artery and cranial nerves 3-6 pass through the sinus or within its walls. Detailed discussion of the cavernous sinus anatomy is contained in the first article of this monograph. Lesions that may arise in this area include meningiomas, Schwannomas and metastatic neoplasm. Meningiomas and metastatic lesions are discussed later, in the section on lesions that may arise in any region around the sella.
Schwannomas are benign neoplasms thought to arise from the Schwann cells of sensory nerve sheaths. The trigeminal nerve is the second most common site of intracranial origin after the vestibular nerve (CN VIII), but trigeminal Schwannomas are still quite rare. 28 (Figure 10)
On imaging studies the Schwannoma is similar to grey matter on CT and T1 weighted images, usually hyperintense to brain on T2 weighted images, with homogenous, bright enhancement after contrast. Schwannomas may be distinguished from meningiomas if they enlarge and follow the expected course of a cranial nerve or its branches. They may enlarge the cranial nerve outlet foramen in the skull base, such as the foramen ovale or rotundum. 2,6,28
Chordomas are benign neoplasms thought to arise from remnants of the primitive notochord. Most commonly they arise around the ends of the notochord, within the sacrum and clivus. Rarely they may arise within the sella, in which case they are difficult to distinguish from pituitary macroadenomas. Although histologically benign, they can be quite aggressive, causing extensive bony destruction in the skull base. 29,30 (Figure 11)
CT scan of patients with clival chordoma shows a bone destroying mass, centered on the spheno-occipital synchondrosis. There are frequently intra-tumoral calcifications. MR images show a heterogeneous mass with internal septations, and heterogeneous enhancement. 7,8,29,30
Lesions that may arise in any area around the sella include meningiomas, metastases, and anurysms.
Meningioma are the second most common tumor in the sellar region in adults, second only to pituitary macroadenomas. Five to 10% of meningiomas occur in this region. They may be suprasellar, when arising from the planum sphenoidale, tuberculum sella, or clinoid processes. They may also arise in the cavernous sinus, presenting as parasellar masses, or arise from the diaphragm sella, in which case they may be intrasellar or suprasellar. 31-33 (Figure 12)
Imaging features of meningioma frequently allow pre-operative diagnosis. Lesions arise from the dura and have a broad attachment to the dura or fill and expand the cavernous sinus. There may be a linear, enhancing dural tail extending along the dura away from the lesion. Bone reaction, with bone thickening and sclerosis, or expansion of sphenoid sinus air space (pneumo sinus dilitans) may be seen. Meningiomas typically are similar to grey matter in CT density and T1 and T2 weighted image signal intensity. They enhance homogenously and brightly. Some diffuse calcification is not uncommon. Less commonly, they may have cystic areas, or even areas of fat. 1,7,33 When meningiomas encase blood vessels, they tend to narrow the lumen more than other tumors. 34
Metastatic neoplasm may affect the pituitary gland by direct local invasion, spread through the CSF, or hematogenously. Pituitary metastases are common at autopsy of patients with widespread metastatic disease, but it is distinctly unusual for metastatic disease to the pituitary to be the initial clinical presentation of a patient with cancer. For this reason, the diagnosis may be suspected based on clinical history of known neoplasm. Breast and lung cancer are the most common primary tumors to hematogenously metastasize to the pituitary. The posterior lobe of the pituitary gland is said to be more commonly involved than the anterior, perhaps because the anterior lobe receives most of its blood supply through the hypothalamic portal system, while the posterior lobe has a systemic arterial supply. 35,36 (Figure 13)
The imaging appearance of metastatic neoplasm to the pituitary is nonspecific, and frequently cannot be distinguished from pituitary macroadenomas. Rapid progression, older age, the presence of diabetes insipidus, or the history of known neoplasm all favor the diagnosis of metastatic disease. 1,2,6 Of course, if the pituitary is involved by direct local extension of skull base or nasopharyngeal tumor, then there will be evidence of a destructive mass in the sphenoid or clivus. CSF spread of CNS neoplasm or lymphoma may be evidenced by enhancing leptomeninges or cranial nerves in other areas.
Obviously, preoperative diagnosis of aneurysm causing a parasellar mass is of critical importance. These aneurysms may arise from the cavernous or supraclinoid internal carotid artery, the anterior or posterior communicating arteries, or the ophthalmic artery. 37,38 (Figure 14)
Imaging characteristics of aneurysms are varied. Usually there is some residual lumen with blood flow. This area will be dark on pre contrast MR images and enhance brightly and homogenously after contrast. There may be variable amount of thrombosed lumen, which may contain crescent or ring shaped layers of different aged blood products or fibrosis. There may be rings or arcs of calcification, especially at the periphery. If there is high velocity flow or significant pulsation, there may be phase encoding error related artifacts on MR images. If within the sella, aneurysms are usually eccentrically located. 1,2,4,8,38 There have been many case reports of aneurysms associated with pituitary adenomas, so don't stop looking when you see the aneurysm. 38,39
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