Practical Pituitary Pathology What Does the Pathologist Need to Know?

Sylvia L. Asa, MD, PhD

● Context.—The sellar region is the site of frequent pa- Conclusions.—The initial examination requires routine thology. The pituitary is affected by a large number of path- hematoxylin-eosin to establish whether the lesion is a pri- ologic entities arising from the gland itself and from adja- mary adenohypophysial proliferation or one of the many cent anatomical structures including brain, blood vessels, other types of pathology that occur in this area. The most nerves, and meninges. The surgical pathology of this area common lesions resected surgically are pituitary adeno- requires the accurate characterization of primary adeno- mas. These are evaluated with a number of special stains hypophysial tumors, craniopharyngiomas, neurologic neo- and immunohistochemical markers that are now available plasms, germ cell tumors, hematologic malignancies, and metastases as well as nonneoplastic lesions such as cysts, to accurately classify these tumors. The complex subclas- hyperplasias, and inflammatory disorders. sification of pituitary adenomas is now recognized to re- Objective.—To provide a practical approach to the di- flect specific clinical features and genetic alterations that agnosis of pituitary specimens. predict targeted therapies for patients with pituitary dis- Data Sources.—Literature review and primary material orders. from the University of Toronto. (Arch Pathol Lab Med. 2008;132:1231–1240)

number of pathologic processes occur in the region of ment of new pharmacotherapeutic agents, improved min- A the pituitary gland. They include primary pituitary imally invasive surgical approaches, and targeted radio- lesions that are unique to this site, as well as disorders therapeutic techniques. The surgical pathologist must arising in adjacent anatomical structures such as brain, therefore recognize the important role of morphologic blood vessels, nerves, and meninges. The surgical pathol- analysis in classifying sellar pathology for the diagnosis ogy of this area requires the accurate characterization of and management of the pituitary patient. neoplastic lesions, including pituitary adenoma and car- cinoma, craniopharyngioma, neurologic neoplasms, germ THE ROLE OF CLINICAL INFORMATION cell tumors, and hematologic malignancies, and their dis- The importance of clinical information cannot be over- tinction from nonneoplastic disorders such as cysts, hy- emphasized in this field. Patients with pituitary disease perplasias, and inflammatory lesions.1,2 The spectrum of may present with symptoms and signs of hormone excess, pituitary pathologies that represent the surgical pathology or they may manifest features of a mass lesion, including of the pituitary is outlined in Table 1. headache, visual impairment, and hypopituitarism. The The commonest disorder is the pituitary adenoma, a le- former usually indicates a primary adenohypophysial dis- sion that is increasingly recognized as a highly prevalent order, but it should be recognized that hyperprolactinemia finding. A recent meta-analysis has shown that the post- may be a nonspecific finding because of a mass lesion that mortem prevalence of pituitary adenoma is 14.4% and that obstructs the pituitary stalk, interrupting blood flow that radiologic studies identify a lesion consistent with pitui- maintains prolactin (PRL) under tonic inhibition. The lat- tary adenoma in 22.2% of the population, providing an ter can be the result of any mass lesion in the region of overall estimated prevalence of 16.9%.3 Although many of the sella. The finding of diabetes insipidus or cranial nerve these lesions are considered to be incidental findings, dysfunction make the diagnosis of a primary adenohy- many have unrecognized impact on fertility, longevity, pophysial cell proliferation unlikely and instead suggest and quality of life, and their clinical significance is increas- other tumor types or inflammatory disorders. ingly gaining attention. Moreover, the management of Despite the importance of clinicopathologic correlation, these lesions has seen major changes with the develop- 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 amount Accepted for publication January 30, 2008. of information with careful morphologic evaluation using From the Department of Pathology, University Health Network, To- a targeted approach. ronto, Ontario. The author has no relevant financial interest in the products or com- AT THE TIME OF SURGERY panies described in this article. Reprints: Sylvia L. Asa, MD, PhD, Department of Pathology, Univer- The initial handling of tissue obtained at pituitary sur- sity Health Network, 200 Elizabeth St, 11th Floor, Toronto, Ontario, gery should ensure adequate fixation in formalin for his- Canada M5G 2C4 (e-mail: [email protected]). tology and immunohistochemistry. In rare cases, there Arch Pathol Lab Med—Vol 132, August 2008 Pituitary Pathology—Asa 1231 Table 1. Classification of Pituitary Pathology* gland intact. In the case of a large lesion, the sella may be eroded to the point where it must be resected en bloc. Neoplastic However, in most patients the sellar diaphragm can be Benign† opened and the dorsum sellae fractured to push it pos- Pituitary adenoma teriorly, allowing the gland to be removed intact. The Craniopharyngioma Gangliocytoma/ganglioglioma gland then can be evaluated grossly and sectioned for Granular cell tumor complete histologic evaluation. Meningioma There are 2 approaches to the sectioning and embed- Schwannoma ding of the pituitary (Figure 1). Many investigators use Chordoma sagittal sections through the gland; others prefer trans- Vascular and mesenchymal tumors verse sections. The former permit examination of the stalk; Malignant the latter provide a more thorough examination of the Pituitary carcinoma gland and more accurate determination of the geographic Gliomas distribution of the various cell types, at the expense of Germ cell tumor examining the stalk carefully. Lymphoma/leukemia/Langerhans cell histiocytosis Vascular and mesenchymal tumors HISTOLOGY Metastases Miscellaneous (salivary gland lesions, melanoma, etc) The initial evaluation of a pituitary specimen involves Nonneoplastic review of material stained with hematoxylin-eosin. This Hyperplasia routine stain allows the distinction of primary adenohy- pophysial pathologies from other entities. Rathke cleft Inflammatory lesions cysts, arachnoid cysts, and dermoid cysts (Figure 2) are Infectious recognized based on preoperative clinical and radiologic Immune findings and confirmed with the identification of the ap- Cysts propriate cyst lining.4 Hypophysitis of any type5 can be Rathke cleft cysts readily recognized with this conventional stain (Figure 3). Arachnoid The various tumors that arise in the sella—gliomas, me- Dermoid/epidermoid ningiomas, schwannomas, and chordomas—are consid- Aneurysms ered based on this analysis and their workup is different Meningoencephalocele than that of a pituitary adenoma. Unusual hypothalamic Hamartoma Brown tumor of bone neuronal gangliocytomas and gangliogliomas can give rise to clinical features of hormone excess that can mimic * Surgical pathology only, not including developmental and meta- 6 bolic lesions that are not biopsied. pituitary adenoma, but must be recognized, either dis- † Although classified as benign, many of these lesions are locally tinct from an adenoma or associated with one. invasive and cause significant morbidity and mortality. Craniopharyngioma is a unique tumor of the sellar re- gion that is derived from the oropharyngeal remnants of Rathke pouch. These lesions have a characteristic mor- may be a need for ultrastructural analysis; because this phology that requires only routine hematoxylin-eosin situation is not often predicted clinically, it is recommend- staining for identification and classification (Figure 4). ed that a small piece of tissue be routinely fixed for elec- They are composed of cords or islands of squamoid epi- tron microscopy and retained in the event that it is need- thelial cells in a loose fibrous stroma with varying degrees ed. Currently there is no need for special handling of tis- of desquamation and intervening cysts that often contain sue for other diagnostic techniques. a thick oily fluid.1 They can be subclassified as adaman- Most pituitary specimens are very small and the tissue tinomatous and papillary types; the former are known to may be compromised by freezing artifact when surgeons harbor mutations of the ␤-catenin gene as a specific mo- request intraoperative consultation and frozen sections are lecular pathogenetic mechanism.7,8 These lesions have a performed. In some centers, pathologists use smear tech- bimodal distribution with peaks in childhood and in the nology for intraoperative consultation to prevent this ar- sixth decade. Although adamantinomatous lesions pre- tifact; this method uses less tissue but requires experience dominate in childhood, most craniopharyngiomas in for interpretation. Sometimes, the only diagnostic tissue is adults have a mixed pattern and the clinical significance in the material used for the intraoperative procedure and of subclassification remains uncertain. it is fraught with artefact that precludes accurate evalua- Germ cell tumors of the sella resemble germ cell tumors tion. Because valid indication for intraoperative consulta- in other sites of the body.9 Hematologic malignancies of tion is rare, this procedure should be restricted for use the sella are usually systemic disorders but occasionally only when the clinical situation is unusual or when the arise as primary plasmacytomas or lymphomas.10–12 surgeon encounters unexpected findings and the intra- Metastatic malignancies are common in the pituitary,13,14 operative diagnosis might change the surgical approach. usually at a late stage of disseminated malignancy when the clinical diagnosis is known. However, occasionally THE AUTOPSY PITUITARY these lesions are detected early, and particularly in pa- The high prevalence of pituitary adenomas means that tients with no known history of a primary malignancy, pathologists will often identify these lesions as incidental they can be clinically challenging. The lesions that most findings in autopsies. The main concern is handling of the frequently give rise to pituitary metastasis are breast, material for analysis. It is recommended that the sella tur- lung, and prostate carcinomas. The metastatic tumor usu- cica be examined after the brain is removed; the hypo- ally involves the neurohypophysis and extrasellar struc- physial stalk should be cut as high as possible to leave the tures, creating a clinical presentation of diabetes insipidus 1232 Arch Pathol Lab Med—Vol 132, August 2008 Pituitary Pathology—Asa 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 the anterior lobe (AL) down, the posterior lobe (PL) up, and the stalk pointing out of the plane of the picture. Sagittal sections (S) allow examination of the stalk but require multiple sections through each lobe to identify the various cell types. b, A transverse section (T) provides a full section of the gland that allows more accurate determination of the geographic distribution of the various cell types in the anterior lobe (AL) and identification of basophil invasion of the intermediate lobe corticotrophs (arrow) into the posterior lobe (PL). The somatotrophs are mainly located in the lateral wings (GH), lactotrophs in the posterolateral wings (PRL), and corticotrophs in the median wedge (ACTH). Thyrotrophs are scattered with some concentration in the anterior median wedge and gonadotrophs are found throughout the parenchyma (periodic acid–Schiff, original magnification ϫ10). and nerve palsies that is not consistent with pituitary ad- when induced by excess hypophysiotropic hormones. Ex- enoma.15,16 The common lesions are readily diagnosed on amples of the latter include cases of primary target organ routine histopathology, but a metastatic well-differentiated failure, such as primary hypothyroidism,18,19 or hormone neuroendocrine carcinoma (Figure 5) can be a diagnostic excess produced by neoplasms, such as hypothalamic gan- dilemma.15,17 The expression of specific markers, such as gliocytomas or ectopic sources of growth hormone–releas- thyroid transcription factor 1, CDX-2, or peptides of gut ing hormone or corticotrophin-releasing hormone, such as or lung origin, may be required to distinguish these le- bronchial, gastroenteropancreatic, adrenal, or prostatic en- sions from a primary pituitary neoplasm. docrine tumors.20 Hyperplasia may be clinically indistin- guishable from adenoma, usually in patients with acro- PRIMARY ADENOHYPOPHYSIAL CELL megaly or Cushing disease. Radiologic imaging some- PROLIFERATIONS times identifies differences; in hyperplasia, the prolifera- Once a pituitary lesion is determined to be composed tion is diffuse and there is no normal rim that enhances of epithelial cells with neuroendocrine differentiation and with gadolinium.21 However, this subtle difference is often thought to be of primary pituitary origin, the classification overlooked and it falls to the pathologist to make the di- of the lesion will require several steps. First, the lesion agnosis. In this regard, the reticulin stain is a very useful must be identified as hyperplasia or neoplasia. Second, the tool. Normal adenohypophysis is composed of small acini cell population responsible for the proliferation must be of pituitary cells surrounded by an intact reticulin net- established. Finally, in the case of a neoplasm, the behav- work (Figure 6, a). In hyperplasia, the acinar architecture ior, prognosis, and potential therapy of choice must be is maintained and the reticulin network is preserved, but determined. the acini are increased in size (Figure 6, b). In contrast, pituitary adenomas are characterized by complete disrup- ADENOMA OR HYPERPLASIA? tion of the reticulin fiber network (Figure 6, c). Immuno- Hyperplasia is controlled cell proliferation that is in- histochemical stains are required to determine the hyper- duced by a stimulus and stops when the stimulus is re- plastic cell population, and these stains will identify the moved. Pituitary hyperplasia can be physiologic, as when admixed normal cells that contain all of the normal ade- lactotrophs proliferate during pregnancy, or pathologic nohypophysial hormones.1 Arch Pathol Lab Med—Vol 132, August 2008 Pituitary Pathology—Asa 1233 Figure 2. Pituitary Rathke cleft cyst. These cystic lesions are lined by ciliated columnar to cuboidal epithelium with occasional goblet cells. The cyst is usually collapsed at the time of surgery and the lining is found adherent to adenohypophysial parenchyma (hematoxylin-eosin, original magnification ϫ400). 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 ϫ200). Figure 4. Craniopharyngioma. This papillary craniopharyngioma is composed of abundant squamous epithelium in a loose fibrous stroma with focal inflammation (hematoxylin-eosin, original magnification ϫ100). Figure 5. Metastatic neuroendocrine carcinoma. This metastasis from a primary endocrine carcinoma of lung mimics a pituitary adenoma in architecture and cytology. Both lesions can be positive for synaptophysin and chromogranin; therefore, other markers are required to establish the diagnosis. Strong nuclear positivity for thyroid transcription factor 1 (not shown) confirmed the diagnosis (hematoxylin-eosin, original magnification ϫ200).

PITUITARY ADENOMA CLASSIFICATION The pituitary is composed of at least 6 distinct cell Clinically pituitary adenomas are classified as hormon- types. Each cell is responsible for the production and se- ally active functioning adenomas and nonfunctioning ad- cretion of at least 1 hormone. Recent advances in molec- enomas that often present with visual impairment and hy- ular biology have clarified 3 major pathways of cytodif- popituitarism. Approximately two thirds of clinically di- ferentiation of adenohypophyseal cells (Figure 7) that are agnosed lesions are functioning adenomas. determined by a complex pattern of transcription factor Pituitary adenomas are also classified based on size and expression.22,23 These transcription factors can help in clas- invasiveness. Microadenomas are defined as less than 1 sifying adenomas.24–28 cm; macroadenomas are larger than 1 cm. Large tumors Corticotrophs differentiate first in the human fetal pi- growing upward are defined as showing suprasellar ex- tuitary and the expression of the proopiomelanocortin tension. Tumors are also classified radiologically and by (POMC) gene is regulated by the Tpit transcription factor29 the neurosurgeon as invasive or not, based on their infil- that mediates its action in concert with Ptx1 and tration into surrounding structures (dura, bone, sinuses, neuroD1,30,31 which were previously identified as cortico- etc). troph upstream transcription element-binding proteins. 1234 Arch Pathol Lab Med—Vol 132, August 2008 Pituitary Pathology—Asa 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 ϫ100).

The second line of differentiation temporally in the human whose hormone production is dependent on steroidogenic gland is determined by Pit-1, a protein that activates the factor 1 and GATA-2 in the presence of estrogen recep- growth hormone (GH), PRL, and ␤-thyrotropin (␤-thy- tor.28,40 roid–stimulating hormone [TSH]) genes.32–37 Pit-1 initiates Each cell type can give rise to tumors that are clinically GH expression and somatotroph differentiation. Expres- functioning or silent. Some tumor types have morphologic sion of estrogen receptor allows the expression of PRL and variants based on patterns of immunoreactivity for hor- GH a bihormonal population of mammosomatotrophs.38 mones and subcellular structures and, in occasional cases, The development of mature lactotrophs is dependent on ultrastructural features1,2; the variants are thought to re- the presence of a putative GH repressor that has yet to be flect differing pathogenetic mechanisms and may predict identified. Some of the Pit-1–expressing cells further ex- differing responses to therapy. The current clinicopatho- press thyrotroph embryonic factor39 and develop into thy- logic classification of pituitary adenomas is shown in Table rotrophs in the presence of a GH repressor and GATA-2.40 2 and the detailed morphologic subclassification is out- In physiologic states, somatotrophs, mammosomato- lined in Table 3. trophs, and lactotrophs transdifferentiate in what is thought to be a reversible fashion.41 It has been shown in The Pathology of Hormone Excess Syndromes animal models that somatotrophs can also transdifferen- Most patients with Cushing disease have small lesions tiate into thyrotrophs in severe hypothyroidism and this that are difficult to localize by magnetic resonance imag- too is thought to be reversible.42 These changes indicate ing. The differential diagnosis is pituitary adenoma versus fluidity of 4 cell types that are all dependent on Pit-1. The corticotroph hyperplasia. The former is far more common, third line of cytodifferentiation is that of the gonadotrophs but the distinction is important and requires the use of

Figure 7. Pathways of cell differentiation in the adenohypophysis. The 3 main pathways of cell differentiation are determined by tran- scription factors that can serve as diagnostic markers. Pit-1 indicates pituitary transcription factor 1; SF-1, steroidogenic factor 1; ER, es- trogen receptor; TEF, thyrotroph embryonic factor; and GH, growth hormone.

Arch Pathol Lab Med—Vol 132, August 2008 Pituitary Pathology—Asa 1235 Table 2. Clinicopathologic Classification of Pituitary Adenomas* Clinically Functioning Adenomas Clinically Silent Adenomas Adenomas causing GH excess Somatotroph adenomas Silent somatotroph adenomas Mammosomatotroph adenomas Adenomas causing hyperprolactinemia Lactotroph adenomas Silent lactotroph adenomas Lactotroph adenomas with GH reactivity Adenomas causing TSH excess Thyrotroph adenomas Silent thyrotroph adenomas Adenomas causing ACTH excess Corticotroph adenomas Silent corticotroph adenomas Adenomas causing gonadotropin excess Gonadotroph adenomas Silent gonadotroph adenomas Plurihormonal adenomas Hormone negative adenomas * GH indicates growth hormone; TSH, thyroid-stimulating hormone (thyrotropin); and ACTH, adrenocorticotropic hormone. reticulin staining and adrenocorticotropic hormone (Figure 8). This change is seen in patients with pituitary (ACTH) immunohistochemistry. The classical microade- corticotroph adenoma, ectopic ACTH secretion, primary noma is a densely granulated adenoma composed of adrenal pathology, or iatrogenic administration of gluco- strongly basophilic cells. These adenomas exhibit strong corticoids. When this change is present but no tumor is positivity with the periodic acid–Schiff stain. Immunohis- seen, the pathologist must search for a microadenoma that tochemically, they demonstrate expression of ACTH and may be only 1 to 2 mm and multiple sections through the generally have very strong reactivity with the CAM 5.2 specimen may be required to find the lesion. In the ab- antibody to keratins 7 and 8. sence of an identified adenoma, the pathologist can only The pathologist should also examine the nontumorous issue a report that indicates the presence of Crooke hya- gland to determine if there is Crooke hyaline change, a line, consistent with Cushing syndrome, and the outcome morphologic marker of feedback suppression that is usu- of surgery alone will indicate the true nature of this dis- ally found in nontumorous corticotrophs and confirms order. Because very small microadenomas can be lost dur- that the patient has elevated circulating glucocorticoids ing surgery, perhaps suctioned during aspiration of blood

Table 3. Immunohistochemical Classification of Pituitary Adenomas* Tumor Transcription Factor Hormone(s) CAM 5.2 Adenomas containing GH Pit-1 Somatotroph adenomas GH Densely granulated somatotroph adenomas ␣-Subunit Perinuclear Sparsely granulated somatotroph adenomas Fibrous bodies Mammosomatotroph adenomas GH, PRL, ␣-subunit Mixed somatotroph-lactotroph adenomas Plurihormonal GH-producing adenomas GH, PRL, ␣-subunit, ␤-TSH Adenomas containing PRL Pit-1, ER Lactotroph 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-2 Thyrotroph adenomas ␣-Subunit, ␤-TSH Adenomas containing ACTH Tpit Densely granulated corticotroph adenomas ACTH Sparsely granulated corticotroph adenomas ACTH Crooke cell adenoma ACTH Dense bands Adenomas containing gonadotropins SF-1, ER, GATA-2 Gonadotroph adenomas ␣-Subunit, ␤-FSH, ␤-LH Plurihormonal adenomas ? Multiple Silent subtype 3 adenomas Multiple Unusual plurihormonal adenomas Multiple Hormone-negative adenomas None Null 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-stimulating hormone; and LH, luteinizing hormone. 1236 Arch Pathol Lab Med—Vol 132, August 2008 Pituitary Pathology—Asa of growth hormone–releasing hormone by endocrine tu- mors as described previously. Reticulin is a critical stain to 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 of the 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, accurate classification is important. Indeed, it appears that the most important distinction is between densely and sparsely granulated types, because the pathophysiology of these lesions will determine their response to the current ther- apies that are available: somatostatin analogues versus GH-antagonists and possibly dopamine agonists.47 All of these adenomas exhibit nuclear Pit-1 reactivity and variable cytoplasmic GH positivity. Mammosomato- Figure 8. Crooke hyaline change. Nontumorous corticotrophs in the trophs stain for PRL as well, and the unusual plurihor- pituitary of a patient with elevated glucocorticoids exhibit accumula- monal adenomas contain ␤-TSH. All of the densely gran- tion of hyaline material in the cytoplasm; the hyaline traps large lyso- ulated variants are acidophilic and also contain ␣-subunit. somes known as ‘‘enigmatic bodies’’ that are found in corticotrophs. The sparsely granulated somatotroph adenoma is the most The periodic acid-Schiff–positive hormonal content is sequestered at the periphery and in the juxtanuclear region of the cell (original mag- difficult to diagnose, because it is often either negative or nification ϫ400). only weakly positive for GH. Indeed the most critical im- munostain in this setting is the CAM 5.2 keratin stain. It identifies perinuclear keratin in densely granulated ade- in the operative field, an operative success can be assumed nomas, including mammosomatotrophs and plurihormon- if biochemical normality is achieved along with regression al lesions (Figure 9, a). In contrast, it clearly decorates fi- of clinical signs and symptoms. In contrast, a surgical fail- brous bodies in the sparsely granulated adenomas (Figure ure will require more careful clinical evaluation of the pa- 9, b). These fibrous bodies can often be recognized with- tient to exclude an ectopic source of ACTH or ACTH-like out the keratin stain. The tumor cells often have bilobed peptide, primary adrenal disease, or a missed pituitary or concave, pleomorphic nuclei that are distorted by pale lesion elsewhere in the gland. homogenous eosinophilic globules. In a patient in whom no tumor is found and there is no The patient presenting with hyperprolactinemia usually Crooke hyaline change of nontumorous corticotrophs, the is treated with medical therapy. Patients who come to sur- diagnosis becomes complex. One possibility is that the pa- gery either have failed medical therapy or suffer signifi- tient has corticotroph hyperplasia; this diagnosis requires cant adverse effects induced by all of the dopaminergic a careful evaluation of reticulin and corticotroph distri- agonists now available. Because most lactotroph adenomas bution that can be very difficult.43,44 Another possibility is respond well to these drugs with hormone normalization pseudo-Cushing, a significant medical pitfall that can oc- and tumor shrinkage, it is important for the pathologist casionally result in unnecessary pituitary surgery. to exclude the many other causes of hyperprolactinemia, Generally, larger lesions tend to be obvious adenomas including hypophysitis and all of the various nonadeno- but not obviously basophilic adenomas, because they are hypophysial neoplasms identified previously. usually sparsely granulated, chromophobic adenomas. Lactotroph adenomas are subclassified into sparsely The presence of periodic acid–Schiff positivity and weak granulated and densely granulated variants. Sparsely ACTH reactivity makes the diagnosis evident, but these granulated adenomas are usually highly responsive to do- stains can also be equivocal in this setting. The addition pamine agonists and therefore usually exhibit major of Tpit is very helpful. changes because of the previous therapy. Only untreated Crooke cell adenoma is a rare variant of corticotroph adenomas of this type exhibit the usually chromophobic adenoma. In this unusual lesion, the adenomatous cells morphology with abundant cytoplasm and characteristic exhibit the features of suppressed corticotrophs. These tu- juxtanuclear PRL immunoreactivity (Figure 10). More mors are often associated with atypical clinical histories, commonly, the lesions are composed of small cells in a and the diagnosis may be unclear, or there may be a his- fibrous stroma, resembling inflammation, plasmacytoma, tory of cyclical Cushing syndrome. The morphology of or lymphoma. The diagnosis is confirmed by the identi- these lesions is quite atypical, with prominent nuclear fication of strong nuclear positivity for Pit-1; usually they pleomorphism and large cells that can resemble ganglio- have at least focal PRL positivity. The rare densely gran- cytoma or metastatic carcinoma. Periodic acid–Schiff pos- ulated lactotroph adenomas are composed of acidophilic itivity and immunoreactivity for ACTH as well as the cells with strong and diffuse cytoplasmic positivity for dense ring of keratin that fills the tumor cell cytoplasm PRL. Another unusual pituitary adenoma causing hyper- and is identified with CAM 5.2 define this rare entity. prolactinemia is the so-called acidophil stem cell adeno- When the patient is known to have acromegaly or gi- ma, an oncocytic lesion characterized by Pit-1 nuclear gantism, the diagnosis of a GH-secreting adenoma is al- staining, variable PRL and GH reactivity, and fibrous bod- most certain. However, rarely these patients can have so- ies identified with the CAM 5.2 immunostain. matotroph hyperplasia resulting from ectopic production The presentation of a patient with TSH excess requires Arch Pathol Lab Med—Vol 132, August 2008 Pituitary Pathology—Asa 1237 Figure 9. Keratin patterns in somatotroph adenomas. Densely granulated somatotroph and mammosomatotroph adenomas exhibit a perinuclear pattern of keratin (a) identified with the CAM 5.2 antibody. In contrast, sparsely granulated somatotroph adenomas have a unique pattern of staining that identifies globular ‘‘fibrous bodies’’ (b) with this same antibody (original magnifications ϫ200).

Figure 10. Prolactin staining in sparsely granulated lactotroph adenoma. This tumor is usually treated medically, but when untreated it exhibits a highly specific staining pattern for prolactin that is localized to the Golgi complex (prolactin immunohistochemistry, original magnification ϫ1000). Figure 11. Gonadotroph adenoma. Most clinically nonfunctioning adenomas are of gonadotroph differentiation. They often have areas of solid architecture, but their characteristic feature is the formation of trabeculae of elongated cells with distinct polarity that form pseudorosettes around vascular channels (hematoxylin-eosin, original magnification ϫ400). the exclusion of thyrotroph hyperplasia (see ‘‘Adenoma or troph differentiation. They have a highly characteristic his- Hyperplasia?’’). The rare thyrotroph adenomas are usu- tologic pattern, in which solid sheets, nests, and even si- ally highly infiltrative macroadenomas with stromal fibro- nusoidal patterns are interrupted by pseudopapillae and sis and marked nuclear atypia. Immunohistochemically, striking pseudorosettes around vascular channels (Figure thyrotroph adenomas express ␣-subunit and ␤-TSH. 11). They usually are composed of admixtures of 2 cell types: tall columnar cells line pseudopapillae and rosettes, Clinically Nonfunctioning Adenomas and polygonal cells comprise the bulk of the lesion. On- The diagnosis of a clinically nonfunctioning adenoma cocytic change can be observed in all patterns. Gonado- requires appropriate classification for prognostication. The troph adenomas express ␣-subunit, ␤-follicle–stimulating majority of these lesions are gonadotroph adenomas; these hormone, and ␤-luteinizing hormone in scattered patterns very rarely present with clinical or biochemical evidence and to variable degrees; they also express steroidogenic of hormone excess. Nevertheless, they produce follicle- factor 1 with strong nuclear reactivity. stimulating hormone and/or luteinizing hormone and Occasional clinically silent adenomas are positive for they express the transcription factors that prove gonado- Pit-1 and GH, PRL, or ␤-TSH; these lesions should be clas- 1238 Arch Pathol Lab Med—Vol 132, August 2008 Pituitary Pathology—Asa sified as adenomas of the appropriate type as indicated amination. If this is impractical, certainly specimens from previously, with the additional qualification of ‘‘silent’’ ad- patients with atypical histories deserve this type of han- enoma. Silent corticotroph adenomas are thought to arise dling, because they will be the cases most likely to require from cells that fail to process the ACTH precursor, pro- this ancillary study. opiomelanocortin, into the biologically active 1-39 ACTH. Prognosis These lesions manifest Tpit and ACTH immunoreactivity and they are strongly positive for keratins 7 and 8. Indeed Prognostication remains a major challenge in pituitary they resemble functioning corticotroph adenomas of the 2 pathology. Proliferative activity51–53 using markers such as types, sparsely and densely granulated variants; however, proliferating cell nuclear antigen, Ki-67/MIB-1, and anti- they are invariably macroadenomas and there is no asso- apoptotic Bcl-2 have unfortunately demonstrated no con- ciated Crooke hyaline in nontumorous corticotrophs. sistent correlation with tumor invasiveness or recurrence.52 These lesions are generally much more aggressive than Although invasive pituitary adenomas and carcinomas ex- other silent adenomas, and recurrence is extremely com- hibit a high DNA topoisomerase II␣ index, this indicator mon. The pathophysiology of the lack of clinical symptom- has no significant advantage over MIB-1 as a prognostic atology of other silent adenomas is not known. marker.54 Cyclooxygenase 2 expression correlates with pa- As immunohistochemical markers become more sophis- tient age, but not with tumor size or invasiveness.55 De- ticated, the number of truly unclassified adenomas is fall- tection of telomerase expression may predict recurrence in ing. The rare tumor that is completely negative for all hor- pituitary adenomas.56 Galectin-3, a ␤-galactoside–binding mones and transcription factors is classified as a ‘‘null cell protein implicated in cellular differentiation and prolifer- adenoma.’’ These usually behave like gonadotroph ade- ation as well as angiogenesis, tumor progression, and me- nomas. tastasis, may play a role in pituitary tumor progression.57 Unfortunately, none of these is a true marker of biologic The Question of Plurihormonality behavior. The best predictive marker remains the tumor Reports of various combinations of hormones in unusu- classification based on hormone content and cell structure. al plurihormonal pituitary adenomas were extremely com- For example, among acromegalics who fail surgical resec- mon in the past. However, the application of highly spe- tion, response to long-acting somatostatin analogues is cific monoclonal antibodies and the understanding of cell best predicted by the subtype of somatotroph adenoma as differentiation have clarified many of the controversies. densely or sparsely granulated.45,46 This finding renders Reports of adenomas expressing GH or PRL with gonad- the value of a CAM 5.2 keratin stain more important than otropins are now recognized to reflect nonspecific cross- almost any other immunostain in this setting. A silent cor- reactivity.48 The fact that cells of the Pit-1 lineage express ticotroph adenoma will recur more often and more ag- ␣-subunit and that many antisera raised against follicle- gressively than a silent gonadotroph adenoma. A silent stimulating hormone or luteinizing hormone recognized subtype 3 adenoma will almost certainly behave invasi- ␣-subunit explains many of these anomalies. The use of vely, infiltrating the base of the skull, whereas a silent ad- high-quality monoclonal antisera has made the occurrence enoma of the gonadotroph lineage will usually grow by of unusual plurihormonal profiles exceptionally rare. In- expansion upward. deed, some of these lesions represent double adenomas or Pituitary carcinoma, by definition a lesion that exhibits ‘‘collision’’ tumors.49 Most lesions respect the lines of dif- distant cerebrospinal and/or systemic metastasis, is an ex- ferentiation attributable to the 3 transcription factor line- ceptionally rare lesion that cannot be defined by morpho- ages. Even the rare silent subtype 3 adenoma is usually logic parameters of the primary tumor. ␤ positive for Pit-1, PRL, GH, and -TSH with other reac- CONCLUSION tivities likely reflecting ␣-subunit cross-reactivity. This le- sion is characterized by intense stromal fibrosis and high The approach to pituitary pathology is complex and re- vascularity. Other distinct features are identified by elec- quires recognition of many pathologic entities. Familiarity tron microscopy. with inflammatory and neurologic diseases must be cou- pled with a detailed understanding of pituitary hyperpla- The Role of Electron Microscopy sia and adenoma classification. In the past, a diagnosis of The classification of pituitary adenomas is based on ‘‘adenoma’’ was considered sufficient for many patients, careful studies that used immunohistochemistry, electron but the advances in pituitary medicine demand a more microscopy, and immunoelectron microscopy to identify thorough clinicopathologic diagnosis that will guide pa- structure-function correlations.50 Many of the ultrastruc- tient management. tural features that were recognized as characterizing spe- References cific tumor types are now identified by immunohisto- 1. Asa SL. Tumors of the Pituitary Gland. Washington, DC: Armed Forces In- stitute of Pathology; 1998. Atlas of Tumor Pathology ; 3rd series, fascicle 22. chemistry. For example, fibrous bodies were considered 2. 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