Sellar and Juxtasellar Tumors

CHAPTER 33 Sellar and Juxtasellar Tumors



The sellar region is defined by the sella turcica and its contents. The parasellar region has no precise boundaries but is considered to be the area surrounding the sella turcica, including the cavernous sinus. Many diverse lesions involve the sellar and juxtasellar regions, including primary benign and malignant neoplasms, metastases, perineurial spread of tumor, and direct invasion from adjacent tumors. The most common lesions are pituitary macroadenomas, craniopharyngiomas, meningiomas, metastasis, and hypothalamic-optic gliomas, which together account for about 75% of lesions in this area.1 Aneurysms clearly must be considered in the differential diagnosis of this region but are discussed elsewhere.


The size and exact location of the lesion determine patient symptoms. Differentiating lesions in this region may be difficult, especially for larger lesions where the point of origin is no longer certain. Placing the origin as either sellar or parasellar at least assists in narrowing the differential diagnosis.



PITUITARY ADENOMA


The most common intrasellar lesion is an adenoma of the adenohypophysis (anterior lobe of the pituitary gland). Rarely, an adenoma may arise from the neurohypophysis (posterior lobe of the pituitary gland). Pituitary adenomas are defined by their size as microadenomas (<10 mm) or macroadenomas (>10 mm). Macroadenomas are further described as invasive when they break through the sellar floor or extend into the cavernous sinus.




Clinical Presentation


Microadenomas frequently come to clinical attention because of hormonal secretion. Prolactin is the hormone most frequently elevated (25%) and can result in infertility, amenorrhea, and galactorrhea in women and decreased libido and impotence in men. Growth hormone–secreting adenomas result in acromegaly in adults and gigantism in children, whereas adrenocorticotropic hormone (ACTH)–secreting adenomas arising from the posterior pituitary or the neurohypophysial region cause Cushing’s syndrome. Thyroid-stimulating hormone (TSH) and folliclestimulating hormone/luteinizing hormone (FSH/LH)–secreting adenomas are rare. Twenty-five to 30 percent of pituitary microadenomas are nonfunctional.1


Macroadenomas frequently do not elaborate hormones. Instead, they present at larger size with mass effect on the chiasm causing visual impairment, compression of the pituitary gland and/or stalk causing pituitary insufficiency, compression of the third ventricle and foramina of Monro causing hydrocephalus, or invasion of the cavernous sinus causing cranial nerve palsies. In males, prolactinomas may reach large size, because the symptoms are less readily identifiable than in females. Occasionally, mass effect on the infundibulum causes elevated prolactin levels, but in these cases the prolactin levels typically do not exceed 150 ng/mL. Overall, about 40% of macroadenomas invade the cavernous sinus to some extent but rarely cause cranial nerve palsy.2


Intratumoral hemorrhage can occur in up to 15% of adenomas (Fig. 33-1). Both microadenomas and macroadenomas can undergo a rapid increase in size in the event of pituitary apoplexy, which is an acute hemorrhage or infarction of a pituitary adenoma. Apoplexy results in acute symptoms, such as headache, vomiting, ocular motility disturbance, and possibly a sudden decrease in vision.3 Seizures or decreased consciousness may also occur.




Pathophysiology


Pituitary adenomas arise from the adenohypophysis, or anterior pituitary gland. Traditional embryology describes the anterior pituitary as arising by invagination of the rostral stomadeum.4 The pars distalis makes up the majority of the anterior lobe. A portion of the pars distalis also extends superiorly and contributes to the anterior aspect of the infundibulum. Therefore, the infundibulum is composed of components from both the anterior and posterior pituitary lobes, explaining the occurrence of occasional adenomas along the stalk.4




Imaging


The physiology central to all pituitary imaging is that the normal pituitary gland lies outside the blood-brain barrier. For that reason, administered contrast agents wash in and then wash out very rapidly. On contrast-enhanced CT and MRI, slowly enhancing lesions may first appear as negative (filling) defects within the normally enhancing residual gland and then as enhancing lesions within the now nonenhancing (washed out) normal gland. Detection of a pituitary lesion and localization of the lesion within the gland require careful synchronization of the imaging study to precise times after administration of a contrast agent.




MRI


MRI can accurately demonstrate microadenomas as small as 3 mm and frequently succeeds in delineating even smaller lesions. Coronal and sagittal imaging are the most valuable for assessing microadenomas and macroadenomas. They are especially helpful for assessing cavernous sinus invasion and involvement of the optic nerves by macroadenomas. On T1-weighted (T1W) imaging, microadenomas are hypointense to isointense in comparison with the normal gland. They may deviate the infundibulum and bow the superior aspect of the gland superiorly. However, caution should be used when evaluating the tilt of the infundibulum, because some normal patients have infundibular deviation due to ectopic insertion of the infundibulum or eccentric gland position. Hemorrhage into the adenoma may cause T1 shortening. Suspected microadenomas may be evaluated by dynamic imaging, because they will enhance more slowly than the surrounding gland and therefore can be differentiated from the gland more easily (Fig. 33-2). On delayed contrast-enhanced MRI, the microadenomas will enhance like the gland itself and become difficult to distinguish. Still later, continued enhancement of the adenomas and washout of contrast from the gland may reverse the expected appearance, so the normal (now nonenhancing) gland is mistaken for the adenoma.



Macroadenomas are variable in their MRI appearance depending on the degree of necrosis and hemorrhage. They usually appear isointense to gray matter on T1W and T2-weighted (T2W) imaging, are hyperintense on fluid-attenuated inversion recovery (FLAIR) images, and show nearly constant heterogeneous contrast enhancement. They often show areas of cyst and hemorrhage, especially in the larger tumors. Coronal T2W images typically display the optic chiasm most clearly as a crescent of low signal crossing above the sella. The paired A1 segments of the anterior cerebral arteries pass immediately above the prechiasmal optic nerves or the chiasm itself. Compression of the chiasm by macroadenoma typically appears as elevation and thinning of the low signal “chiasmal crescent” with elevation of the flow voids of the A1 segments above it. Accurate detection of extension to the cavernous sinus is difficult. MRI is only 55% sensitive for invasion of the cavernous sinus. Invasion can be suspected when at least 50% of the cavernous segment of the internal carotid artery is surrounded by the macroadenoma. Unilateral carotid artery encasement is the most specific imaging sign of cavernous sinus involvement (Fig. 33-3A).6 Characteristically, pituitary macroadenomas do not narrow the carotid artery, even when they encase it, whereas meningiomas of the cavernous sinus do narrow the carotid artery when they surround it. Therefore, preservation of normal caliber of an encased internal carotid artery suggests that the cavernous sinus has been invaded by a macroadenoma, rather than meningioma. This sign, however, is imperfect. Macroadenomas can also erode through the floor of the sella turcica. Large lesions may extend into the sphenoidal sinus and then through the sphenoidal sinus into the nasopharynx, mimicking carcinoma of the sphenoidal sinus or nasopharynx (see Fig. 33-3B). Because nasopharyngeal carcinomas also tend to narrow the internal carotid arteries, preservation of the caliber of these vessels again suggests pituitary macroadenoma. MR spectroscopy may show a choline peak or no metabolites at all.7



Other lesions that may mimic pituitary macroadenoma include lesions of the posterior pituitary gland, such as granular cell tumors and pituicytomas. In such cases, the best approach may be to demonstrate that the lesion arises from the anterior lobe of the pituitary, eliminating posterior pituitary lesions from consideration. Metastases to the pituitary gland are also included in the imaging differential for pituitary lesions, but, like posterior pituitary lesions, are much less common than macroadenomas.


Pituitary adenomas can be graded by their imaging appearance. Grade I adenomas are the microadenomas, which by definition are less than 1 cm. These do not enlarge the sella turcica, although they may cause focal erosions in the sellar floor. Grade II lesions are greater than 1 cm but remain within the sella or have suprasellar extension with no invasion of adjacent structures. Grade III lesions are locally invasive. Grade IV lesions are large tumors that invade extrasellar structures including bone, hypothalamus, and cavernous sinus.




PITUITARY CARCINOMA


Pituitary carcinomas are very rare lesions that arise from the adenohypophysis. They are distinguished from adenomas by the presence of systemic, subarachnoid, or brain metastasis.







GRANULAR CELL TUMORS


Granular cell tumors are benign tumors of the neurohypophysis that arise from the granular cell–type pituicytes. They are also known as choristomas, myoblastomas, and infundibulomas.




Clinical Presentation


Granular cell tumors are hormonally silent, so they usually present as masses. Most patients have visual complaints from compression of the optic chiasm.10 Approximately 50% of patients show secondary signs of pituitary dysfunction, including hyperprolactinemia, which is presumed to result from compression of the pituitary stalk by the primary mass. Compression of the stalk impedes transport of pituitary-releasing hormones from the hypothalamus to the pituitary gland, leading to endocrine dysfunction (so-called stalk effect).





Imaging




MRI


On MRI, granular cell tumors have a nonspecific appearance. They are hypointense to isointense to gray matter on both T1W and T2W imaging.9 Granular cell tumors are vascular lesions, so MRI typically shows an enhancement within the sellar/suprasellar mass. Granular cell lesions do not cause significant edema in the surrounding brain. Their appearance is nonspecific, so imaging does not differentiate these tumors from other sellar/suprasellar lesions, including craniopharyngioma, pituitary adenoma, and meningioma. Absence of the posterior pituitary bright spot may provide a clue that the lesion originates from the posterior pituitary gland, but absence of the posterior pituitary bright spot on incidental MRI is another nonspecific sign present in up to 20% of normal subjects.9



PITUICYTOMA


Pituicytomas are low-grade astrocytomas that involve the neurohypophysis, or posterior pituitary, and are histologically benign. They are also referred to as astrocytomas of the posterior pituitary.








CRANIOPHARYNGIOMA


Craniopharyngiomas (Rathke’s pouch tumor, craniopharyngeal duct tumor) are benign epithelial neoplasms of the suprasellar and sellar regions.






Pathology


Craniopharyngiomas consist of cystic and solid components and frequently show calcification. Multiple different cysts may be present, each appearing different because each contains a different proportion of cholesterol, methemoglobin, triglycerides, protein, and desquamated epithelium. The lesions are usually suprasellar, depress the diaphragma sellae, elevate the chiasm, and, when large, may obstruct the foramina of Monro to cause hydrocephalus. Large craniopharyngiomas typically extend far posteriorly into the depths of the interpeduncular fossa. They typically grow upward through the circle of Willis, displacing the vessels of the circle outward circumferentially. The adamantinomatous tumors have a tendency to encase adjacent arteries. Craniopharyngiomas have a variable relation to the pituitary stalk but usually displace it anteriorly.


Pathologically, craniopharyngiomas are divided into adamantinomatous and squamous papillary variants. The adamantinomatous type resemble the enamel-forming neoplasms of the oropharynx. They tend to be more cystic, calcify more frequently, and are most common in children. The squamous papillary form is often more solid, less frequently calcified, and more frequent in adults. However, most tumors have mixed features. Attempts to correlate pathology with imaging and recurrence rate have not proved fruitful.15 Although craniopharyngiomas are histologically benign, slowly growing tumors, they are difficult to resect completely. They are often adherent to the pituitary stalk, the hypothalamus, the thalamoperforate vessels, and other small penetrating arteries and veins, so complete resection is difficult or impossible. For that reason they recur repeatedly, are locally aggressive, and may extend into the adjacent brain parenchyma. If the tumors are completely resected, recurrence is rare. With subtotal resection, however, only 47% of patients are disease free at 5 years.9 Craniopharyngiomas can recur locally, along the surgical tract, and even distant from the original site, suggesting cerebrospinal fluid (CSF) seeding.15



Imaging






METASTASIS, DIRECT TUMORAL SPREAD, AND PERINEURIAL SPREAD


Metastases can spread to the sellar-parasellar region by hematogenous spread, perineurial spread, or direct invasion. In hematogenous spread the tumor invades local blood vessels and rafts downstream to lodge in end-arteries of the sellar region. In perineurial spread the tumor invades the regional nerves and grows along their perineurium or endoneurium to reach a noncontiguous area.



Epidemiology


Hematogenous metastases to the pituitary and sellar region are uncommon, representing 0.14% to 28.1% of all brain metastases.18 They most frequently originate from lung and breast carcinomas. Direct local invasion most commonly results from squamous cell carcinoma, nasopharyngeal carcinoma, and rhabdomyosarcoma. Perineurial spread is most frequent with squamous cell carcinoma and adenoid cystic carcinoma of the head and neck.



Clinical Presentation


About 40% of patients with hematogenous metastases develop diabetes insipidus. Visual deficits and hypopituitarism are also common.18 Direct invasion by regional neoplasms usually causes symptoms from local mass effect. Perineurial spread may cause hypoesthesia and burning or stinging pain. Perineurial spread most frequently involves the fifth (trigeminal) and seventh (facial) cranial nerves, so it may be misdiagnosed as trigeminal neuralgia or Bell’s palsy. It may be asymptomatic in up to 45% of patients. There may be a long latency period between development of perineurial spread and tumor treatment, with reports of perineurial spread manifesting as late as 45 years after resection of the original tumor.20 Because perineurial spread changes the status of a lesion from resectable to nonresectable,19 and because many patients retain normal nerve function on clinical examination, imaging studies must try to identify perineurial spread that escapes clinical detection.



Pathophysiology


Hematogenous metastases to this region most frequently go to the pituitary gland, infundibulum, and tuber cinereum, because these structures lie outside the blood-brain barrier. Metastases to the infundibulum cause visual disturbance by compression of the chiasm or endocrine disturbance by disruption of the hypothalamic-pituitary axis. Hematogenous metastases to the pituitary gland itself most commonly pass to the infundibulum and posterior lobe, rather than the anterior lobe of the pituitary, because the posterior lobe is supplied directly through the inferior hypophysial branches of the internal carotid artery, whereas the anterior lobe is supplied indirectly via hypothalamic-portal vessels.18 The cavernous sinus may be involved by direct extension from regional neoplasms such as nasopharyngeal carcinoma, by hematogenous metastases, or by perineurial spread. Tumors may spread along any nerve, but perineurial spread most commonly involves the fifth and seventh cranial nerves. Squamous cell carcinoma and adenoid cystic carcinoma are the two most common tumors to spread by perineurial extension, but melanoma, lymphoma, leukemia, basal cell carcinoma, and mucoepidermoid carcinoma also reach the sellar-parasellar region by this route. Perineurial spread usually indicates a poor prognosis and may change the lesion’s status from resectable to nonresectable.




Imaging




MRI


Most pituitary metastases are difficult to distinguish from adenomas, especially if no other metastasis is identified. They typically appear as dumbbell-shaped intrasellar-suprasellar masses, indented at the level of the diaphragma sellae (Fig. 33-8). These lesions do not widen and remodel the sella because they grow too rapidly.21 Instead, they may destroy the sphenoid bone and invade into the cavernous sinus, partially occluding it. The flow void from the cavernous segment of the internal carotid artery may be displaced. Flow through it may be reduced or obstructed.



Metastatic lesions are usually hypointense to isointense to gray matter on T1W imaging and hyperintense on T2W imaging. Metastases can enhance homogeneously or heterogeneously after contrast agent administration and occasionally show rim enhancement.


Perineurial spread is identified by isointense thickening of the nerve, loss of the normal intraforaminal fat plane, and concentric enlargement of the foramen on T1W images. Contrast-enhanced studies show increased nerve size, nerve enhancement, and widening of the cavernous sinus (Fig. 33-9). Fat suppression can help to identify the abnormal enhancement. Atrophy of the muscles supplied by the affected nerves is an important secondary sign of perineurial invasion. Because nasopharyngeal carcinoma is a common source of perineurial spread, imaging of the parasellar region should always include the nasopharynx.




LANGERHANS CELL HISTIOCYTOSIS


Langerhans cell histiocytosis is a reactive clonal disease of the macrophage-monocyte system. It can involve almost any organ.



Epidemiology


Langerhans cell histiocytosis is a rare disorder, seen in up to 2 per 100,000 people.22 It most frequently affects children and adolescents but can occur in adults. There is no gender predilection.




Pathophysiology


Langerhans cells are a dendritic cell line derived from bone marrow cells. The cells have antigen-presenting and antigen-processing properties22 and are normally found in the pituitary and brain parenchyma. The pathophysiologic mechanism of Langerhans cell histiocytosis has not been elucidated.



Jan 22, 2016 | Posted by in NEUROLOGICAL IMAGING | Comments Off on Sellar and Juxtasellar Tumors
Premium Wordpress Themes by UFO Themes