| |
Disorders
We Treat
 Pituitary Tumor
Incidence: The true incidence of pituitary adenomas is difficult
to know with certainty because they are often asymptomatic; autopsy estimates
range from 2.7 to 27%.
Gender: There is not a predominance
in either men or women. However, these tumors are most common in adults,
and the incidence peaks in the third and fourth decades; children and
adolescents account for about 10% of the total. These tumors are not hereditary
except for rare families with multiple endocrine adenomatosis, an autosomal
dominant trait, shown by a high incidence of pituitary adenomas in addition
to tumors of other endocrine glands.
Pathology: Historically,
pituitary adenomas were classified by a now-obsolete system that identified
them as acidophilic, basophilic, or chromophobic as determined by staining
characteristics with hematoxylin and eosin. Growth hormone-secreting tumors
were acidophilic adenomas, adrenocorticotropic hormone (ACTH)-secreting
tumors were basophilic adenomas, and chromophobic adenomas were thought
to be nonsecretory. With new methods of immunohistochemistry, pituitary
hormone secretion could be detected within an adenoma, and it soon became
clear that methods based on the traditional histologic dyes were unreliable.
Most pituitary tumors that secrete prolactin, ACTH, or growth hormone
are actually chromophobe adenomas and not hormonally inactive as originally
thought. Current strategies involve a functional classification based
on endocrinologic activity, dividing tumors into secreting and nonsecreting
types. Secreting tumors are less common and produce one or more anterior
pituitary hormones, including prolactin (the most common endocrinologically
active tumor), growth hormone, ACTH, follicle-stimulating hormone, or
luteinizing hormone. Mixed secretory tumors account for 10% of adenomas,
and their ability to secrete more than one hormone has implications for
medical therapies. Some tumors secrete the alpha subunit of the large
precursor molecules that form one of the polypeptide chains of glycoprotein
hormones, even if the tumor is nonfunctional. Null cell adenomas or nonsecreting
adenomas demonstrate no clinical or immunohistochemical evidence of hormone
secretion.
Size: For clinical purposes,
pituitary adenomas are arbitrarily divided by size into microadenomas
(<1.0 cm in diameter) or macroadenomas (>1.0 cm in diameter). When
tumors erode the dura or bone, they are considered invasive and may infiltrate
surrounding structures, such as the cavernous sinus, cranial nerves, blood
vessels, sphenoid bone, and sinus or brain. Locally invasive pituitary
adenomas are nearly always histologically benign. Different staging systems
have been based on invasive or extension characteristics, which are useful
for prognosis and treatment.
Clinical Features: Clinical
manifestations of pituitary adenomas stem from endocrine dysfunction or
from mass effect with invasion or compression of surrounding neural and
vascular structures. Mass effects include headache, hypopituitarism, and
visual loss. Headaches result from stretching of the diaphragma sellae
and adjacent dural structures that transmit sensation through the first
branch of the trigeminal nerve. Visual loss may be accompanied by optic
disc pallor, loss of central visual acuity, and visual field defects,
but papilledema is rare. Visual field abnormalities are caused by compression
of the crossing fibers in the optic chiasm, first affecting the superior
temporal quadrants and then the inferior temporal quadrants. Further expansion
compromises the noncrossing fibers and affects the lower nasal quadrants
and finally the upper nasal quadrants. Patients usually note blurring
or dimming of vision. Formal visual field testing is important because
some tumors affect only the macular fibers to cause central hemianopic
scotomas that may be missed on routine screening. Although bitemporal
hemianopia is most common, any pattern of visual loss is possible, including
unilateral or homonymous hemianopia.
Lateral extension of the tumor with compression
or invasion of the cavernous sinus can compromise third, fourth, or sixth
cranial nerve functions, manifest as diplopia in 5 to 15% of pituitary
tumor patients. The third cranial nerve is most commonly affected. There
may be numbness in the cranial nerve V1 or V2 distribution.
Complete endocrine evaluation is necessary
for all patients with pituitary tumors, not only to make the diagnosis
of a secreting adenoma, but also to determine the presence of hypopituitarism.
Hypopituitarism may result from compression of the normal pituitary gland
or blood supply; adequate replacement and long-term follow-up are then
needed. Hormonal replacement most commonly includes thyroid and adrenal
hormones. Nonsecreting tumors may be associated with slight elevations
of serum prolactin levels to 100 mg/ml, which is attributed to compression
of the pituitary stalk, interrupting dopaminergic fibers that inhibit
prolactin release. Mild elevations are common and must be distinguished
from prolactin-secreting tumors because bromocriptine has little or no
effect on nonsecretory tumors.
Pituitary adenomas may enlarge during pregnancy.
This may be problematic when pregnancy is induced in a woman with infertility
problems and an unrecognized pituitary adenoma.
 Radiographic
Features: MRI is the best procedure for evaluation of pituitary
pathology, imaging soft tissue without interference from the bony surroundings
of the sella and producing images in any plane. Normally the anterior
lobe of the pituitary gland has the same signal as white matter on T1
-weighted imaging. With gadolinium, the normal gland enhances homogeneously.
Small punctate areas of heterogeneity may be due to local variations in
vascularity, microcyst formation, or granularity within the gland. The
cavernous sinuses also enhance.
Microadenomas are sometimes difficult to
see directly on MRI but may be inferred by glandular asymmetry, focal
sellar erosion, asymmetric convexity of the upper margin of the gland,
or displacement of the infundibulum (Fig. 54-3) (Figure Not Available)
. The normal gland usually shows more enhancement than the microadenoma
(Fig. 54-4) (Figure Not Available) . In the presence of a macroadenoma,
the normal gland may not be visualized, and the bright signal of the posterior
lobe may be absent. Areas of increased signal on T1 -weighted image may
be due to hemorrhage; areas of low signal may represent cystic degeneration.
Although the cavernous sinus often appears expanded in the presence of
a macroadenoma, it is difficult to distinguish invasion or compression
from stretching of the cavernous sinus.
MRI alone is usually sufficient for adequate
imaging of a pituitary adenoma. CT, however, may show the bony anatomy
in better detail. MRI can usually exclude an aneurysm, but an angiogram
is indicated if uncertainty exists.
Differential Diagnosis: Most
lesions in the differential diagnosis have characteristic radiographic
or clinical syndromes that distinguish them from pituitary adenoma. Craniopharyngiomas
have a predilection for children, are calcified, and usually contain cystic
areas with highly proteinaceous fluid with cholesterol crystals. Rathke
cleft cysts are similar to cranio pharyngiomas but have a cystic appearance
without any solid component. Meningiomas are commonly found in the diaphragma
sellae, planum sphenoidale, and tuberculum sellae and may be difficult
to distinguish from a macroadenoma. Distinguishing characteristics of
meningiomas include enhancement, visualization of a cleavage plane between
the mass and the sellar contents, normal-sized sella, and the presence
of a dural ``tail'' of enhancement. Optic glioma, hypothalamic glioma,
germinoma, dermoid tumor, metastasis, and nasopharyngeal carcinoma are
less common entities to be considered. Chordomas characteristically show
extensive clival bony destruction. Mucoceles of the sphenoid sinus may
simulate pituitary adenoma. Visual symptoms and sellar enlargement may
also result from chronic increased intracranial pressure of any origin.
Characteristic signal voids on MRI usually distinguish an aneurysm.
Treatment: Treatment of pituitary
adenomas begins with the correction of electrolyte dysfunction and replacement
of pituitary hormones, if necessary, immediately after diagnostic blood
specimens have been sent. Replacement of thyroid or adrenal hormones is
of particular importance. Steroid replacement must be adequate for stress
situations, including the perioperative period.
The goals of treatment differ according
to the functional activity of the tumor. For endocrinologically active
tumors, an aggressive approach toward normalization of hypersecretion
is essential while preserving normal pituitary function. This can usually
be achieved by surgical excision, but some prolactinomas are better controlled
medically.
For nonsecreting tumors, treatment is directed
toward surgical reduction of the mass effect responsible for symptoms,
while maintaining pituitary function. Although complete surgical resection
is desired, the radiosensitivity of these tumors invites subtotal debulking
followed by radiation therapy to reduce the risk of recurrence or progression.
Incidental asymptomatic adenomas require
no intervention but should be followed with periodic visual field examination
and MRI. Onset of symptoms or MRI documentation of growth are indications
for treatment.
Surgery: The efficacy and
safety of the transsphenoidal approach make it the procedure of choice
for the removal of adenomas. Most tumors are soft and friable, and transsphenoidal
access, although limited, allows for complete removal even if there is
significant suprasellar extension or the sella is not enlarged. Transsphenoidal
surgery was originally developed by Cushing and popularized by others,
especially Hardy. Refinements in microsurgery and the availability of
steroid replacement and antibiotics have dramatically improved the results
of transsphenoidal surgery. Mortality rates are less than 1%. Major morbidity,
including stroke, visual loss, meningitis, CSF leak, or cranial palsy,
is less than 3.5%. Permanent diabetes insipidus appears after surgery
in 2 to 5% of patients and is treated by replacement.
Radiation Therapy: Radiation
therapy is complementary to surgery in preventing progression or recurrence.
Standard radiation techniques involve the use of three fields (parallel
opposed fields with a coronal field) or rotational techniques to avoid
unnecessary dosage of the temporal lobes. Dosages of 4500 to 5000 cGy
delivered in 180-cGy fractions are recommended. In general, patients with
subtotally resected tumors are given radiation therapy. Although radiation
reduces the risk of recurrence or delays recurrence after gross total
resection, we follow these patients with serial MRI scans and visual field
examination and withhold radiation unless there is documented tumor regrowth.
For pituitary gland tumors
including pituitary adenoma, prolactinoma and Cushings disease, the decisions
related to treatment for the pituitary gland tumors depend upon the complete
understanding of the competing risks vs. benefits for the different treatments.
Options for pituitary gland tumor treatments may include surgery, radiosurgery
and gamma knife.
Top
|
