Practical Pituitary Pathology

Arch Pathol Lab Med—Vol 132, August 2008 Pituitary Pathology—Asa 1231

Practical Pituitary PathologyWhat Does the Pathologist Need to Know?

Sylvia L. Asa, MD, PhD

● Context.—The sellar region is the site of frequent pa-thology. The pituitary is affected by a large number of path-ologic entities arising from the gland itself and from adja-cent anatomical structures including brain, blood vessels,nerves, and meninges. The surgical pathology of this arearequires the accurate characterization of primary adeno-hypophysial tumors, craniopharyngiomas, neurologic neo-plasms, germ cell tumors, hematologic malignancies, andmetastases as well as nonneoplastic lesions such as cysts,hyperplasias, and inflammatory disorders.

Objective.—To provide a practical approach to the di-agnosis of pituitary specimens.

Data Sources.—Literature review and primary materialfrom the University of Toronto.

Conclusions.—The initial examination requires routinehematoxylin-eosin to establish whether the lesion is a pri-mary adenohypophysial proliferation or one of the manyother types of pathology that occur in this area. The mostcommon lesions resected surgically are pituitary adeno-mas. These are evaluated with a number of special stainsand immunohistochemical markers that are now availableto accurately classify these tumors. The complex subclas-sification of pituitary adenomas is now recognized to re-flect specific clinical features and genetic alterations thatpredict targeted therapies for patients with pituitary dis-orders.

(Arch Pathol Lab Med. 2008;132:1231–1240)

Anumber of pathologic processes occur in the region ofthe pituitary gland. They include primary pituitary

lesions that are unique to this site, as well as disordersarising in adjacent anatomical structures such as brain,blood vessels, nerves, and meninges. The surgical pathol-ogy of this area requires the accurate characterization ofneoplastic lesions, including pituitary adenoma and car-cinoma, craniopharyngioma, neurologic neoplasms, germcell tumors, and hematologic malignancies, and their dis-tinction from nonneoplastic disorders such as cysts, hy-perplasias, and inflammatory lesions.1,2 The spectrum ofpituitary pathologies that represent the surgical pathologyof the pituitary is outlined in Table 1.

The commonest disorder is the pituitary adenoma, a le-sion that is increasingly recognized as a highly prevalentfinding. A recent meta-analysis has shown that the post-mortem prevalence of pituitary adenoma is 14.4% and thatradiologic studies identify a lesion consistent with pitui-tary adenoma in 22.2% of the population, providing anoverall estimated prevalence of 16.9%.3 Although many ofthese lesions are considered to be incidental findings,many have unrecognized impact on fertility, longevity,and quality of life, and their clinical significance is increas-ingly gaining attention. Moreover, the management ofthese lesions has seen major changes with the develop-

Accepted for publication January 30, 2008.From the Department of Pathology, University Health Network, To-

ronto, Ontario.The author has no relevant financial interest in the products or com-

panies described in this article.Reprints: Sylvia L. Asa, MD, PhD, Department of Pathology, Univer-

sity Health Network, 200 Elizabeth St, 11th Floor, Toronto, Ontario,Canada M5G 2C4 (e-mail: [email protected]).

ment of new pharmacotherapeutic agents, improved min-imally invasive surgical approaches, and targeted radio-therapeutic techniques. The surgical pathologist musttherefore recognize the important role of morphologicanalysis in classifying sellar pathology for the diagnosisand management of the pituitary patient.

THE ROLE OF CLINICAL INFORMATIONThe importance of clinical information cannot be over-

emphasized in this field. Patients with pituitary diseasemay present with symptoms and signs of hormone excess,or they may manifest features of a mass lesion, includingheadache, visual impairment, and hypopituitarism. Theformer usually indicates a primary adenohypophysial dis-order, but it should be recognized that hyperprolactinemiamay be a nonspecific finding because of a mass lesion thatobstructs the pituitary stalk, interrupting blood flow thatmaintains prolactin (PRL) under tonic inhibition. The lat-ter can be the result of any mass lesion in the region ofthe sella. The finding of diabetes insipidus or cranial nervedysfunction make the diagnosis of a primary adenohy-pophysial cell proliferation unlikely and instead suggestother tumor types or inflammatory disorders.

Despite the importance of clinicopathologic correlation,the reality is that many pathologists are faced with diag-nosing a lesion without clinical information. In most in-stances it is possible to determine a remarkable amountof information with careful morphologic evaluation usinga targeted approach.

AT THE TIME OF SURGERYThe initial handling of tissue obtained at pituitary sur-

gery should ensure adequate fixation in formalin for his-tology and immunohistochemistry. In rare cases, there

1232 Arch Pathol Lab Med—Vol 132, August 2008 Pituitary Pathology—Asa

Table 1. Classification of Pituitary Pathology*

NeoplasticBenign†

Pituitary adenomaCraniopharyngiomaGangliocytoma/gangliogliomaGranular cell tumorMeningiomaSchwannomaChordomaVascular and mesenchymal tumors

MalignantPituitary carcinomaGliomasGerm cell tumorLymphoma/leukemia/Langerhans cell histiocytosisVascular and mesenchymal tumorsMetastasesMiscellaneous (salivary gland lesions, melanoma, etc)

NonneoplasticHyperplasia

Inflammatory lesionsInfectiousImmune

CystsRathke cleft cystsArachnoidDermoid/epidermoid

AneurysmsMeningoencephaloceleHamartomaBrown tumor of bone

* Surgical pathology only, not including developmental and meta-bolic lesions that are not biopsied.

† Although classified as benign, many of these lesions are locallyinvasive and cause significant morbidity and mortality.

may be a need for ultrastructural analysis; because thissituation is not often predicted clinically, it is recommend-ed that a small piece of tissue be routinely fixed for elec-tron microscopy and retained in the event that it is need-ed. Currently there is no need for special handling of tis-sue for other diagnostic techniques.

Most pituitary specimens are very small and the tissuemay be compromised by freezing artifact when surgeonsrequest intraoperative consultation and frozen sections areperformed. In some centers, pathologists use smear tech-nology for intraoperative consultation to prevent this ar-tifact; this method uses less tissue but requires experiencefor interpretation. Sometimes, the only diagnostic tissue isin the material used for the intraoperative procedure andit is fraught with artefact that precludes accurate evalua-tion. Because valid indication for intraoperative consulta-tion is rare, this procedure should be restricted for useonly when the clinical situation is unusual or when thesurgeon encounters unexpected findings and the intra-operative diagnosis might change the surgical approach.

THE AUTOPSY PITUITARYThe high prevalence of pituitary adenomas means that

pathologists will often identify these lesions as incidentalfindings in autopsies. The main concern is handling of thematerial for analysis. It is recommended that the sella tur-cica be examined after the brain is removed; the hypo-physial stalk should be cut as high as possible to leave the

gland intact. In the case of a large lesion, the sella may beeroded to the point where it must be resected en bloc.However, in most patients the sellar diaphragm can beopened and the dorsum sellae fractured to push it pos-teriorly, allowing the gland to be removed intact. Thegland then can be evaluated grossly and sectioned forcomplete histologic evaluation.

There are 2 approaches to the sectioning and embed-ding of the pituitary (Figure 1). Many investigators usesagittal sections through the gland; others prefer trans-verse sections. The former permit examination of the stalk;the latter provide a more thorough examination of thegland and more accurate determination of the geographicdistribution of the various cell types, at the expense ofexamining the stalk carefully.

HISTOLOGYThe initial evaluation of a pituitary specimen involves

review of material stained with hematoxylin-eosin. Thisroutine stain allows the distinction of primary adenohy-pophysial pathologies from other entities. Rathke cleftcysts, arachnoid cysts, and dermoid cysts (Figure 2) arerecognized based on preoperative clinical and radiologicfindings and confirmed with the identification of the ap-propriate cyst lining.4 Hypophysitis of any type5 can bereadily recognized with this conventional stain (Figure 3).The various tumors that arise in the sella—gliomas, me-ningiomas, schwannomas, and chordomas—are consid-ered based on this analysis and their workup is differentthan that of a pituitary adenoma. Unusual hypothalamicneuronal gangliocytomas and gangliogliomas can giverise to clinical features of hormone excess that can mimicpituitary adenoma,6 but must be recognized, either dis-tinct from an adenoma or associated with one.

Craniopharyngioma is a unique tumor of the sellar re-gion that is derived from the oropharyngeal remnants ofRathke pouch. These lesions have a characteristic mor-phology that requires only routine hematoxylin-eosinstaining for identification and classification (Figure 4).They are composed of cords or islands of squamoid epi-thelial cells in a loose fibrous stroma with varying degreesof desquamation and intervening cysts that often containa thick oily fluid.1 They can be subclassified as adaman-tinomatous and papillary types; the former are known toharbor mutations of the �-catenin gene as a specific mo-lecular pathogenetic mechanism.7,8 These lesions have abimodal distribution with peaks in childhood and in thesixth decade. Although adamantinomatous lesions pre-dominate in childhood, most craniopharyngiomas inadults have a mixed pattern and the clinical significanceof subclassification remains uncertain.

Germ cell tumors of the sella resemble germ cell tumorsin other sites of the body.9 Hematologic malignancies ofthe sella are usually systemic disorders but occasionallyarise as primary plasmacytomas or lymphomas.10–12

Metastatic malignancies are common in the pituitary,13,14

usually at a late stage of disseminated malignancy whenthe clinical diagnosis is known. However, occasionallythese lesions are detected early, and particularly in pa-tients with no known history of a primary malignancy,they can be clinically challenging. The lesions that mostfrequently give rise to pituitary metastasis are breast,lung, and prostate carcinomas. The metastatic tumor usu-ally involves the neurohypophysis and extrasellar struc-tures, creating a clinical presentation of diabetes insipidus


Figure 1. a, There are 2 approaches to the sectioning and embedding of the autopsy pituitary, shown from a superior view on the left with theanterior lobe (AL) down, the posterior lobe (PL) up, and the stalk pointing out of the plane of the picture. Sagittal sections (S) allow examinationof the stalk but require multiple sections through each lobe to identify the various cell types. b, A transverse section (T) provides a full section ofthe gland that allows more accurate determination of the geographic distribution of the various cell types in the anterior lobe (AL) and identificationof basophil invasion of the intermediate lobe corticotrophs (arrow) into the posterior lobe (PL). The somatotrophs are mainly located in the lateralwings (GH), lactotrophs in the posterolateral wings (PRL), and corticotrophs in the median wedge (ACTH). Thyrotrophs are scattered with someconcentration in the anterior median wedge and gonadotrophs are found throughout the parenchyma (periodic acid–Schiff, original magnification�10).

and nerve palsies that is not consistent with pituitary ad-enoma.15,16 The common lesions are readily diagnosed onroutine histopathology, but a metastatic well-differentiatedneuroendocrine carcinoma (Figure 5) can be a diagnosticdilemma.15,17 The expression of specific markers, such asthyroid transcription factor 1, CDX-2, or peptides of gutor lung origin, may be required to distinguish these le-sions from a primary pituitary neoplasm.

PRIMARY ADENOHYPOPHYSIAL CELLPROLIFERATIONS

Once a pituitary lesion is determined to be composedof epithelial cells with neuroendocrine differentiation andthought to be of primary pituitary origin, the classificationof the lesion will require several steps. First, the lesionmust be identified as hyperplasia or neoplasia. Second, thecell population responsible for the proliferation must beestablished. Finally, in the case of a neoplasm, the behav-ior, prognosis, and potential therapy of choice must bedetermined.

ADENOMA OR HYPERPLASIA?

Hyperplasia is controlled cell proliferation that is in-duced by a stimulus and stops when the stimulus is re-moved. Pituitary hyperplasia can be physiologic, as whenlactotrophs proliferate during pregnancy, or pathologic

when induced by excess hypophysiotropic hormones. Ex-amples of the latter include cases of primary target organfailure, such as primary hypothyroidism,18,19 or hormoneexcess produced by neoplasms, such as hypothalamic gan-gliocytomas or ectopic sources of growth hormone–releas-ing hormone or corticotrophin-releasing hormone, such asbronchial, gastroenteropancreatic, adrenal, or prostatic en-docrine tumors.20 Hyperplasia may be clinically indistin-guishable from adenoma, usually in patients with acro-megaly or Cushing disease. Radiologic imaging some-times identifies differences; in hyperplasia, the prolifera-tion is diffuse and there is no normal rim that enhanceswith gadolinium.21 However, this subtle difference is oftenoverlooked and it falls to the pathologist to make the di-agnosis. In this regard, the reticulin stain is a very usefultool. Normal adenohypophysis is composed of small aciniof pituitary cells surrounded by an intact reticulin net-work (Figure 6, a). In hyperplasia, the acinar architectureis maintained and the reticulin network is preserved, butthe acini are increased in size (Figure 6, b). In contrast,pituitary adenomas are characterized by complete disrup-tion of the reticulin fiber network (Figure 6, c). Immuno-histochemical stains are required to determine the hyper-plastic cell population, and these stains will identify theadmixed normal cells that contain all of the normal ade-nohypophysial hormones.1


Figure 2. Pituitary Rathke cleft cyst. These cystic lesions are lined by ciliated columnar to cuboidal epithelium with occasional goblet cells. Thecyst is usually collapsed at the time of surgery and the lining is found adherent to adenohypophysial parenchyma (hematoxylin-eosin, originalmagnification �400).

Figure 3. Lymphocytic hypophysitis. The adenohypophysis is infiltrated by chronic inflammatory cells including lymphocytes and plasma cells.The few residual parenchymal cells are oncocytic (hematoxylin-eosin, original magnification �200).

Figure 4. Craniopharyngioma. This papillary craniopharyngioma is composed of abundant squamous epithelium in a loose fibrous stroma withfocal inflammation (hematoxylin-eosin, original magnification �100).

Figure 5. Metastatic neuroendocrine carcinoma. This metastasis from a primary endocrine carcinoma of lung mimics a pituitary adenoma inarchitecture and cytology. Both lesions can be positive for synaptophysin and chromogranin; therefore, other markers are required to establish thediagnosis. Strong nuclear positivity for thyroid transcription factor 1 (not shown) confirmed the diagnosis (hematoxylin-eosin, original magnification�200).

PITUITARY ADENOMA CLASSIFICATIONClinically pituitary adenomas are classified as hormon-

ally active functioning adenomas and nonfunctioning ad-enomas that often present with visual impairment and hy-popituitarism. Approximately two thirds of clinically di-agnosed lesions are functioning adenomas.

Pituitary adenomas are also classified based on size andinvasiveness. Microadenomas are defined as less than 1cm; macroadenomas are larger than 1 cm. Large tumorsgrowing upward are defined as showing suprasellar ex-tension. Tumors are also classified radiologically and bythe neurosurgeon as invasive or not, based on their infil-tration into surrounding structures (dura, bone, sinuses,etc).

The pituitary is composed of at least 6 distinct celltypes. Each cell is responsible for the production and se-cretion of at least 1 hormone. Recent advances in molec-ular biology have clarified 3 major pathways of cytodif-ferentiation of adenohypophyseal cells (Figure 7) that aredetermined by a complex pattern of transcription factorexpression.22,23 These transcription factors can help in clas-sifying adenomas.24–28

Corticotrophs differentiate first in the human fetal pi-tuitary and the expression of the proopiomelanocortin(POMC) gene is regulated by the Tpit transcription factor29

that mediates its action in concert with Ptx1 andneuroD1,30,31 which were previously identified as cortico-troph upstream transcription element-binding proteins.


Figure 6. Reticulin staining in normal, hyperplastic, and neoplastic pituitary. The normal gland (a) has an intact reticulin pattern identifying acini.Hyperplasia (b) has intact but expanded acini. In contrast, an adenoma (c) has total breakdown of acinar architecture (Gordon-Sweet silver stain,original magnifications �100).

Figure 7. Pathways of cell differentiation inthe adenohypophysis. The 3 main pathwaysof cell differentiation are determined by tran-scription factors that can serve as diagnosticmarkers. Pit-1 indicates pituitary transcriptionfactor 1; SF-1, steroidogenic factor 1; ER, es-trogen receptor; TEF, thyrotroph embryonicfactor; and GH, growth hormone.

The second line of differentiation temporally in the humangland is determined by Pit-1, a protein that activates thegrowth hormone (GH), PRL, and �-thyrotropin (�-thy-roid–stimulating hormone [TSH]) genes.32–37 Pit-1 initiatesGH expression and somatotroph differentiation. Expres-sion of estrogen receptor allows the expression of PRL andGH a bihormonal population of mammosomatotrophs.38

The development of mature lactotrophs is dependent onthe presence of a putative GH repressor that has yet to beidentified. Some of the Pit-1–expressing cells further ex-press thyrotroph embryonic factor39 and develop into thy-rotrophs in the presence of a GH repressor and GATA-2.40

In physiologic states, somatotrophs, mammosomato-trophs, and lactotrophs transdifferentiate in what isthought to be a reversible fashion.41 It has been shown inanimal models that somatotrophs can also transdifferen-tiate into thyrotrophs in severe hypothyroidism and thistoo is thought to be reversible.42 These changes indicatefluidity of 4 cell types that are all dependent on Pit-1. Thethird line of cytodifferentiation is that of the gonadotrophs

whose hormone production is dependent on steroidogenicfactor 1 and GATA-2 in the presence of estrogen recep-tor.28,40

Each cell type can give rise to tumors that are clinicallyfunctioning or silent. Some tumor types have morphologicvariants based on patterns of immunoreactivity for hor-mones and subcellular structures and, in occasional cases,ultrastructural features1,2; the variants are thought to re-flect differing pathogenetic mechanisms and may predictdiffering responses to therapy. The current clinicopatho-logic classification of pituitary adenomas is shown in Table2 and the detailed morphologic subclassification is out-lined in Table 3.

The Pathology of Hormone Excess SyndromesMost patients with Cushing disease have small lesions

that are difficult to localize by magnetic resonance imag-ing. The differential diagnosis is pituitary adenoma versuscorticotroph hyperplasia. The former is far more common,but the distinction is important and requires the use of


Table 2. Clinicopathologic Classification of Pituitary Adenomas*

Clinically Functioning Adenomas Clinically Silent Adenomas

Adenomas causing GH excessSomatotroph adenomas Silent somatotroph adenomasMammosomatotroph adenomas

Adenomas causing hyperprolactinemiaLactotroph adenomas Silent lactotroph adenomasLactotroph adenomas with GH reactivity

Adenomas causing TSH excessThyrotroph adenomas Silent thyrotroph adenomas

Adenomas causing ACTH excessCorticotroph adenomas Silent corticotroph adenomas

Adenomas causing gonadotropin excessGonadotroph adenomas Silent gonadotroph adenomas

Plurihormonal adenomas Hormone negative adenomas

* GH indicates growth hormone; TSH, thyroid-stimulating hormone (thyrotropin); and ACTH, adrenocorticotropic hormone.

Table 3. Immunohistochemical Classification of Pituitary Adenomas*

Tumor Transcription Factor Hormone(s) CAM 5.2

Adenomas containing GH Pit-1Somatotroph adenomas GH

Densely granulated somatotroph adenomas �-Subunit PerinuclearSparsely granulated somatotroph adenomas Fibrous bodies

Mammosomatotroph adenomas GH, PRL, �-subunitMixed somatotroph-lactotroph adenomas

Plurihormonal GH-producing adenomas GH, PRL, �-subunit, �-TSH

Adenomas containing PRL Pit-1, ERLactotroph adenomas PRL

Sparsely granulated lactotroph adenomas PRL (Golgi pattern)Densely granulated lactotroph adenomas PRL (diffuse)

Acidophil stem cell adenomas PRL, GH Fibrous bodies

Adenomas containing TSH Pit-1, TEF, GATA-2Thyrotroph adenomas �-Subunit, �-TSH

Adenomas containing ACTH TpitDensely granulated corticotroph adenomas ACTHSparsely granulated corticotroph adenomas ACTHCrooke cell adenoma ACTH Dense bands

Adenomas containing gonadotropins SF-1, ER, GATA-2Gonadotroph adenomas �-Subunit, �-FSH, �-LH

Plurihormonal adenomas ? MultipleSilent subtype 3 adenomas MultipleUnusual plurihormonal adenomas Multiple

Hormone-negative adenomas NoneNull cell adenomas None

* GH indicates growth hormone; Pit-1, pituitary transcription factor 1; PRL, prolactin; TSH, thyroid-stimulating hormone (thyrotropin); TEF,thyrotroph embryonic factor; ACTH, adrenocorticotropic hormone; SF-1, steroidogenic factor 1; ER, estrogen receptor; FSH, follicle-stimulatinghormone; and LH, luteinizing hormone.

reticulin staining and adrenocorticotropic hormone(ACTH) immunohistochemistry. The classical microade-noma is a densely granulated adenoma composed ofstrongly basophilic cells. These adenomas exhibit strongpositivity with the periodic acid–Schiff stain. Immunohis-tochemically, they demonstrate expression of ACTH andgenerally have very strong reactivity with the CAM 5.2antibody to keratins 7 and 8.

The pathologist should also examine the nontumorousgland to determine if there is Crooke hyaline change, amorphologic marker of feedback suppression that is usu-ally found in nontumorous corticotrophs and confirmsthat the patient has elevated circulating glucocorticoids

(Figure 8). This change is seen in patients with pituitarycorticotroph adenoma, ectopic ACTH secretion, primaryadrenal pathology, or iatrogenic administration of gluco-corticoids. When this change is present but no tumor isseen, the pathologist must search for a microadenoma thatmay be only 1 to 2 mm and multiple sections through thespecimen may be required to find the lesion. In the ab-sence of an identified adenoma, the pathologist can onlyissue a report that indicates the presence of Crooke hya-line, consistent with Cushing syndrome, and the outcomeof surgery alone will indicate the true nature of this dis-order. Because very small microadenomas can be lost dur-ing surgery, perhaps suctioned during aspiration of blood


Figure 8. Crooke hyaline change. Nontumorous corticotrophs in thepituitary of a patient with elevated glucocorticoids exhibit accumula-tion of hyaline material in the cytoplasm; the hyaline traps large lyso-somes known as ‘‘enigmatic bodies’’ that are found in corticotrophs.The periodic acid-Schiff–positive hormonal content is sequestered atthe periphery and in the juxtanuclear region of the cell (original mag-nification �400).

in the operative field, an operative success can be assumedif biochemical normality is achieved along with regressionof clinical signs and symptoms. In contrast, a surgical fail-ure will require more careful clinical evaluation of the pa-tient to exclude an ectopic source of ACTH or ACTH-likepeptide, primary adrenal disease, or a missed pituitarylesion elsewhere in the gland.

In a patient in whom no tumor is found and there is noCrooke hyaline change of nontumorous corticotrophs, thediagnosis becomes complex. One possibility is that the pa-tient has corticotroph hyperplasia; this diagnosis requiresa careful evaluation of reticulin and corticotroph distri-bution that can be very difficult.43,44 Another possibility ispseudo-Cushing, a significant medical pitfall that can oc-casionally result in unnecessary pituitary surgery.

Generally, larger lesions tend to be obvious adenomasbut not obviously basophilic adenomas, because they areusually sparsely granulated, chromophobic adenomas.The presence of periodic acid–Schiff positivity and weakACTH reactivity makes the diagnosis evident, but thesestains can also be equivocal in this setting. The additionof Tpit is very helpful.

Crooke cell adenoma is a rare variant of corticotrophadenoma. In this unusual lesion, the adenomatous cellsexhibit the features of suppressed corticotrophs. These tu-mors are often associated with atypical clinical histories,and the diagnosis may be unclear, or there may be a his-tory of cyclical Cushing syndrome. The morphology ofthese lesions is quite atypical, with prominent nuclearpleomorphism and large cells that can resemble ganglio-cytoma or metastatic carcinoma. Periodic acid–Schiff pos-itivity and immunoreactivity for ACTH as well as thedense ring of keratin that fills the tumor cell cytoplasmand is identified with CAM 5.2 define this rare entity.

When the patient is known to have acromegaly or gi-gantism, the diagnosis of a GH-secreting adenoma is al-most certain. However, rarely these patients can have so-matotroph hyperplasia resulting from ectopic production

of growth hormone–releasing hormone by endocrine tu-mors as described previously. Reticulin is a critical stainto exclude this possibility and ensure appropriate man-agement for these patients.

Growth hormone–secreting adenomas may be mono-hormonal somatotroph adenomas, bihormonal mammo-somatotroph adenomas, or plurihormonal adenomas ofthe Pit-1 family that also make TSH. Monohormonal so-matotroph adenomas can be densely granulated or sparse-ly granulated. The distinctions can impact medical thera-py in the event of surgical therapy45,46; therefore, accurateclassification is important. Indeed, it appears that the mostimportant distinction is between densely and sparselygranulated types, because the pathophysiology of theselesions will determine their response to the current ther-apies that are available: somatostatin analogues versusGH-antagonists and possibly dopamine agonists.47

All of these adenomas exhibit nuclear Pit-1 reactivityand variable cytoplasmic GH positivity. Mammosomato-trophs stain for PRL as well, and the unusual plurihor-monal adenomas contain �-TSH. All of the densely gran-ulated variants are acidophilic and also contain �-subunit.The sparsely granulated somatotroph adenoma is the mostdifficult to diagnose, because it is often either negative oronly weakly positive for GH. Indeed the most critical im-munostain in this setting is the CAM 5.2 keratin stain. Itidentifies perinuclear keratin in densely granulated ade-nomas, including mammosomatotrophs and plurihormon-al lesions (Figure 9, a). In contrast, it clearly decorates fi-brous bodies in the sparsely granulated adenomas (Figure9, b). These fibrous bodies can often be recognized with-out the keratin stain. The tumor cells often have bilobedor concave, pleomorphic nuclei that are distorted by palehomogenous eosinophilic globules.

The patient presenting with hyperprolactinemia usuallyis treated with medical therapy. Patients who come to sur-gery either have failed medical therapy or suffer signifi-cant adverse effects induced by all of the dopaminergicagonists now available. Because most lactotroph adenomasrespond well to these drugs with hormone normalizationand tumor shrinkage, it is important for the pathologistto exclude the many other causes of hyperprolactinemia,including hypophysitis and all of the various nonadeno-hypophysial neoplasms identified previously.

Lactotroph adenomas are subclassified into sparselygranulated and densely granulated variants. Sparselygranulated adenomas are usually highly responsive to do-pamine agonists and therefore usually exhibit majorchanges because of the previous therapy. Only untreatedadenomas of this type exhibit the usually chromophobicmorphology with abundant cytoplasm and characteristicjuxtanuclear PRL immunoreactivity (Figure 10). Morecommonly, the lesions are composed of small cells in afibrous stroma, resembling inflammation, plasmacytoma,or lymphoma. The diagnosis is confirmed by the identi-fication of strong nuclear positivity for Pit-1; usually theyhave at least focal PRL positivity. The rare densely gran-ulated lactotroph adenomas are composed of acidophiliccells with strong and diffuse cytoplasmic positivity forPRL. Another unusual pituitary adenoma causing hyper-prolactinemia is the so-called acidophil stem cell adeno-ma, an oncocytic lesion characterized by Pit-1 nuclearstaining, variable PRL and GH reactivity, and fibrous bod-ies identified with the CAM 5.2 immunostain.

The presentation of a patient with TSH excess requires


Figure 9. Keratin patterns in somatotroph adenomas. Densely granulated somatotroph and mammosomatotroph adenomas exhibit a perinuclearpattern of keratin (a) identified with the CAM 5.2 antibody. In contrast, sparsely granulated somatotroph adenomas have a unique pattern of stainingthat identifies globular ‘‘fibrous bodies’’ (b) with this same antibody (original magnifications �200).

Figure 10. Prolactin staining in sparsely granulated lactotroph adenoma. This tumor is usually treated medically, but when untreated it exhibitsa highly specific staining pattern for prolactin that is localized to the Golgi complex (prolactin immunohistochemistry, original magnification�1000).

Figure 11. Gonadotroph adenoma. Most clinically nonfunctioning adenomas are of gonadotroph differentiation. They often have areas of solidarchitecture, but their characteristic feature is the formation of trabeculae of elongated cells with distinct polarity that form pseudorosettes aroundvascular channels (hematoxylin-eosin, original magnification �400).

the exclusion of thyrotroph hyperplasia (see ‘‘Adenoma orHyperplasia?’’). The rare thyrotroph adenomas are usu-ally highly infiltrative macroadenomas with stromal fibro-sis and marked nuclear atypia. Immunohistochemically,thyrotroph adenomas express �-subunit and �-TSH.

Clinically Nonfunctioning AdenomasThe diagnosis of a clinically nonfunctioning adenoma

requires appropriate classification for prognostication. Themajority of these lesions are gonadotroph adenomas; thesevery rarely present with clinical or biochemical evidenceof hormone excess. Nevertheless, they produce follicle-stimulating hormone and/or luteinizing hormone andthey express the transcription factors that prove gonado-

troph differentiation. They have a highly characteristic his-tologic pattern, in which solid sheets, nests, and even si-nusoidal patterns are interrupted by pseudopapillae andstriking pseudorosettes around vascular channels (Figure11). They usually are composed of admixtures of 2 celltypes: tall columnar cells line pseudopapillae and rosettes,and polygonal cells comprise the bulk of the lesion. On-cocytic change can be observed in all patterns. Gonado-troph adenomas express �-subunit, �-follicle–stimulatinghormone, and �-luteinizing hormone in scattered patternsand to variable degrees; they also express steroidogenicfactor 1 with strong nuclear reactivity.

Occasional clinically silent adenomas are positive forPit-1 and GH, PRL, or �-TSH; these lesions should be clas-


sified as adenomas of the appropriate type as indicatedpreviously, with the additional qualification of ‘‘silent’’ ad-enoma. Silent corticotroph adenomas are thought to arisefrom cells that fail to process the ACTH precursor, pro-opiomelanocortin, into the biologically active 1-39 ACTH.These lesions manifest Tpit and ACTH immunoreactivityand they are strongly positive for keratins 7 and 8. Indeedthey resemble functioning corticotroph adenomas of the 2types, sparsely and densely granulated variants; however,they are invariably macroadenomas and there is no asso-ciated Crooke hyaline in nontumorous corticotrophs.These lesions are generally much more aggressive thanother silent adenomas, and recurrence is extremely com-mon. The pathophysiology of the lack of clinical symptom-atology of other silent adenomas is not known.

As immunohistochemical markers become more sophis-ticated, the number of truly unclassified adenomas is fall-ing. The rare tumor that is completely negative for all hor-mones and transcription factors is classified as a ‘‘null celladenoma.’’ These usually behave like gonadotroph ade-nomas.

The Question of PlurihormonalityReports of various combinations of hormones in unusu-

al plurihormonal pituitary adenomas were extremely com-mon in the past. However, the application of highly spe-cific monoclonal antibodies and the understanding of celldifferentiation have clarified many of the controversies.Reports of adenomas expressing GH or PRL with gonad-otropins are now recognized to reflect nonspecific cross-reactivity.48 The fact that cells of the Pit-1 lineage express�-subunit and that many antisera raised against follicle-stimulating hormone or luteinizing hormone recognized�-subunit explains many of these anomalies. The use ofhigh-quality monoclonal antisera has made the occurrenceof unusual plurihormonal profiles exceptionally rare. In-deed, some of these lesions represent double adenomas or‘‘collision’’ tumors.49 Most lesions respect the lines of dif-ferentiation attributable to the 3 transcription factor line-ages. Even the rare silent subtype 3 adenoma is usuallypositive for Pit-1, PRL, GH, and �-TSH with other reac-tivities likely reflecting �-subunit cross-reactivity. This le-sion is characterized by intense stromal fibrosis and highvascularity. Other distinct features are identified by elec-tron microscopy.

The Role of Electron MicroscopyThe classification of pituitary adenomas is based on

careful studies that used immunohistochemistry, electronmicroscopy, and immunoelectron microscopy to identifystructure-function correlations.50 Many of the ultrastruc-tural features that were recognized as characterizing spe-cific tumor types are now identified by immunohisto-chemistry. For example, fibrous bodies were consideredthe hallmark of the sparsely granulated somatotroph ad-enoma, and these are now readily identified with theCAM 5.2 immunostain.

There remain situations in which the histology and im-munohistochemical profile are atypical and these cases re-quire electron microscopy for accurate classification. Thebest approach to the use of this diagnostic tool is to fixand embed a small fragment of all pituitary tumors at thetime of receipt in the event that electron microscopy maybe required, recognizing that only a small number of spec-imens will ever require sectioning and ultrastructural ex-

amination. If this is impractical, certainly specimens frompatients with atypical histories deserve this type of han-dling, because they will be the cases most likely to requirethis ancillary study.

PrognosisPrognostication remains a major challenge in pituitary

pathology. Proliferative activity51–53 using markers such asproliferating cell nuclear antigen, Ki-67/MIB-1, and anti-apoptotic Bcl-2 have unfortunately demonstrated no con-sistent correlation with tumor invasiveness or recurrence.52

Although invasive pituitary adenomas and carcinomas ex-hibit a high DNA topoisomerase II� index, this indicatorhas no significant advantage over MIB-1 as a prognosticmarker.54 Cyclooxygenase 2 expression correlates with pa-tient age, but not with tumor size or invasiveness.55 De-tection of telomerase expression may predict recurrence inpituitary adenomas.56 Galectin-3, a �-galactoside–bindingprotein implicated in cellular differentiation and prolifer-ation as well as angiogenesis, tumor progression, and me-tastasis, may play a role in pituitary tumor progression.57

Unfortunately, none of these is a true marker of biologicbehavior. The best predictive marker remains the tumorclassification based on hormone content and cell structure.For example, among acromegalics who fail surgical resec-tion, response to long-acting somatostatin analogues isbest predicted by the subtype of somatotroph adenoma asdensely or sparsely granulated.45,46 This finding rendersthe value of a CAM 5.2 keratin stain more important thanalmost any other immunostain in this setting. A silent cor-ticotroph adenoma will recur more often and more ag-gressively than a silent gonadotroph adenoma. A silentsubtype 3 adenoma will almost certainly behave invasi-vely, infiltrating the base of the skull, whereas a silent ad-enoma of the gonadotroph lineage will usually grow byexpansion upward.

Pituitary carcinoma, by definition a lesion that exhibitsdistant cerebrospinal and/or systemic metastasis, is an ex-ceptionally rare lesion that cannot be defined by morpho-logic parameters of the primary tumor.

CONCLUSIONThe approach to pituitary pathology is complex and re-

quires recognition of many pathologic entities. Familiaritywith inflammatory and neurologic diseases must be cou-pled with a detailed understanding of pituitary hyperpla-sia and adenoma classification. In the past, a diagnosis of‘‘adenoma’’ was considered sufficient for many patients,but the advances in pituitary medicine demand a morethorough clinicopathologic diagnosis that will guide pa-tient management.

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