Prolactin-secreting adenomas

Prolactin-secreting adenomas, also called prolactinomas, comprise nearly 80% of functioning pituitary tumors and about 40% to 50% of all pituitary adenomas. However, due to the trend for medical control of these adenomas with dopamine agonist therapy, the frequency of prolactinomas in a surgical series tends to be smaller. In our institution, prolactinomas represent 15% of the tumor specimens (see Table 9-3 ).

Most prolactinomas are microadenomas occurring in reproductive-age women who usually have amenorrhea, galactorrhea, and infertility. Macroadenomas (tumors >1 cm) occur in only 30% of hyperprolactinemic women. Conversely, in men and elderly women, prolactinomas are usually macroadenomas and are most commonly associated with headaches, neurological defects, and visual loss. Impotence and decreased libido are also common symptoms of hyperprolactinemia in males. The diagnosis of a prolactinoma is confirmed by sustained hyperprolactinemia and neuroradiological evidence of a pituitary tumor.

Prolactinomas contain cells that are medium sized with chromophobic or slightly acidophilic cytoplasm and a central, oval nucleus ( Figure 9-3 ). Approximately 10% to 20% of cases show variably prominent microcalcifications that occasionally are so abundant as to form a “pituitary stone.” Prolactinomas may also produce an amyloid-like substance, forming small hyaline bodies. Calcifications and amyloid bodies are not pathognomonic but occur more frequently in prolactinomas than in other adenoma types. Immunohistochemistry shows significant positivity for PRL. The pattern of staining is very characteristic in that the staining is localized near the nucleus in a “dotlike” pattern ( Figure 9-3, B ). This pattern of immunoreaction is designated by some authors as a Golgi pattern, reflecting hormone localization in the Golgi complex.

PRL-secreting adenoma. A, PRL-cell adenoma showing chromophobic cytoplasm and central nuclei with delicate chromatin. B, PRL immunoreaction is typically seen in a paranuclear location (Golgi pattern). C, The ultrastructure of a PRL-secreting adenoma shows sparse granulated cells with prominent RER and small-sized secretory granules. D, Granular extrusion ( circle ) is characteristic of PRL cells.

The ultrastructural features of prolactinomas are well characterized. The majority of the adenomas are sparsely granulated adenomas and their cells resemble actively secreting lactotrophs of the normal pituitary gland. The cells have a prominent rough endoplasmic reticulum (RER) network, conspicuous Golgi complex, and a sparse number of small (150 to 300 nm) secretory granules ( Figure 9-3, C and D ). Misplaced exocytosis, characterized by secretory extrusions on the lateral cell surfaces, are typical of these tumors. The utility of electron microscopy in PRL-cell adenomas is questionable since it may not have clinical significance.

Currently, the majority of patients with prolactinomas that undergo surgical resection have been previously treated to some degree with dopamine agonists. The morphological effects of these drugs are consequently present in the great majority of surgical specimens. Dopamine agonists produce atrophy of lactotroph cells, with resultant tumor shrinkage. The cells become smaller with shrinkage of the cytoplasm and hyperchromasia of the nuclei ( Figure 9-4 ). Additionally, various degrees of perivascular and interstitial tumoral fibrosis may result from long-term drug administration.

Medically treated PRL-secreting adenoma. A and B, The adenoma shows extensive shrinkage of the tumor cells and interstitial fibrosis after medical treatment. C, PRL immunoreaction is focally present in tumor cells. D, Ultrastructure shows striking decrease of tumor cell size and paucity of organelles (compare with Figure 9-3 ).

Growth hormone–secreting adenomas

About 20% of pituitary adenomas are associated with clinical or immunohistochemical evidence of GH secretion. GH-secreting adenomas are accompanied by high serum GH and insulin-like growth factor I (IGF-I) levels and signs and symptoms of acromegaly or gigantism. Most acromegalic patients have macroadenomas when first diagnosed, many of them with suprasellar expansion and parasellar invasion. Consequently, symptoms secondary to an expanding tumor mass, including headaches and visual field defects, may also be present in patients with large tumors. In about 30% to 50% of the patients, co-secretion of PRL with GH by the tumor results in signs and symptoms of hyperprolactinemia. The mixed GH/PRL-secreting tumors will be discussed in the following subsection.

GH-secreting adenomas are classified in either densely granulated or sparsely granulated adenomas reflecting the variable amount of secretory granules present in the cellular cytoplasm. In densely granulated adenomas, the adenoma cells are composed of eosinophilic cytoplasm showing considerable granularity, reflecting the great numbers of secretory granules seen at the ultrastructural level ( Figure 9-5 ). The nucleus tends to be central and oval with prominent nucleoli. Immunohistochemical stains for GH shows diffuse stain occupying the entire cytoplasm of the tumor cells, and the GH positivity is dispersed diffusely within the entire tumor.

GH-secreting adenoma. A, Densely granulated (DG) GH-cell adenoma showing eosinophilic, granular cytoplasm, and central nucleus with prominent nucleoli. B, Immunostain for GH shows intense and diffuse stain. C and D, DG-GH-cell adenoma exhibits well-developed organelles and abundant large secretory granules ( D ) by EM analysis.

In the sparsely granulated adenomas, the cytoplasm is more chromophobic in appearance, and the nucleus tends to be eccentric ( Figure 9-6 ). In these tumors, paranuclear eosinophilic structures called “fibrous bodies” are seen. These structures represent accumulations of intermediate filaments and tubular formations at the ultrastructural level. Immunostains for GH reflect the sparse amount of secretory granule with focal stain within the tumor and localized in a paranuclear distribution, similar to the Golgi pattern seen in prolactinomas. In addition, “fibrous” bodies are strongly positive for cytokeratin ( Figure 9-6, C ).

GH-secreting adenoma. A, Sparsely-granulated (SG) GH-cell adenoma has a more chromophobic appearance showing paranuclear “fibrous bodies.” B, Immunoreaction for GH shows dotlike pattern within a less diffuse distribution. C, Cytokeratin immunostain highlights the paranuclear “fibrous body” seen in SG-GH adenomas. D, Ultrastructure: a SG-GH cell displays sparse secretory granules and the characteristic “fibrous bodies” ( arrowheads ).

A number of GH-secreting adenomas display secondary reactivity for other pituitary hormones. Immunopositivity for PRL can be seen focally, even in patients without clinical or biochemical evidence of hyperprolactinemia. In a similar way, the presence of immunoreactivity for the glycoprotein hormones β-FSH, β-LH, α-SU, and less frequently β-TSH, can be demonstrated in a number of GH-secreting adenomas. Apart from the well-characterized mixed GH/PRL-secreting adenomas (see later discussion), plurihormonal differentiation is not clinically symptomatic in the majority of cases.

The two subtypes of GH-cell adenomas, densely and sparsely granulated adenomas, are well-characterized by ultrastructural analysis. The densely granulated GH-adenoma is composed of cells that resemble the normal somatotrophs of the pituitary gland, and are characterized by a well-developed RER network, prominent Golgi complexes, and numerous large (300 to 600 nm) secretory granules ( Figures 9-5, C and 9-5, D ). The sparsely granulated GH-cell adenomas have moderately prominent RER and Golgi membranes, and few small (100 to 250 nm) secretory granules. The most characteristic feature of these adenomas is the presence of “fibrous bodies,” which consist of an accumulation of intermediate filaments and tubular smooth-surfaced endoplasmic reticulum ( Figure 9-6, D ).

The clinical importance for distinction of these two subtypes of GH-cell adenomas is controversial. The sparsely granulated GH-cell adenomas appear to exhibit more aggressive biological behavior than the densely granulated tumors. In a review of almost 90 acromegalic patients followed in our institution, although no significant difference in cure rate and survival was present between these two subtypes of GH-secreting adenomas, sparsely granulated adenomas were more likely to be locally invasive than densely granulated tumors.

Additionally, the discrimination between densely- and sparsely-granulated GH-cell adenomas appears to be of significance in terms of specific medical treatment of acromegalic patients. Tumor subtyping (DG-GH) was the strongest predictor of IGF-I normalization in patients with acromegaly receiving postoperative Somatostatin analogue therapy.

As in prolactinomas, medical therapy of acromegaly using long-acting somatostatin analogues, mainly octreotide, is common practice in endocrinology. These drugs may change the morphology of GH-secreting adenomas; most commonly these changes are characterized by varying degrees of perivascular and interstitial fibrosis. At the ultrastructural level, there is an increase in the size/number of secretory granules and lysosomal activation. Unlike bromocriptine effects, significant reduction of cell size due to involution of RER and Golgi membranes are uncommonly seen in these treated tumors.

Mixed GH/PRL-secreting adenomas

A large percentage of GH-secreting adenomas also secrete PRL. About half of the patients with surgically removed GH-secreting adenomas in our institution presented signs and symptoms of both acromegaly and hyperprolactinemia. Three morphological tumor types that secrete simultaneously GH and PRL can be identified: the mixed GH-cell/PRL-cell adenoma, the mammosomatotroph cell adenoma, and the acidophilic stem cell adenoma. These categories of tumors are the ones in which the full study of the tissues by immunohistochemistry and ultrastructure is of fundamental importance because their distinction has clinical and prognostic implications. Both mixed GH-cell/PRL-cell adenomas and mammosomatotroph adenomas tend to be more slowly growing tumors than the acidophilic stem cell adenomas. In our experience, these mixed tumors behave more aggressively than any pure GH-secreting adenomas with a lower surgical cure rate.

Mixed GH-cell/PRL-cell adenoma

The predominant clinical feature of mixed GH-cell/PRL-cell adenomas is acromegaly. Signs and symptoms of hyperprolactinemia are not always apparent. Morphologically, the tumors may resemble GH-secreting adenomas but immunohistochemistry is demonstrated for both GH and PRL with varying degrees of staining and distribution ( Figure 9-7 ). The two cell types may form small groups or may be scattered. At the ultrastructural level, these adenomas are bimorphous tumors, consisting of two separate cell populations, densely or sparsely granulated GH cells and PRL cells.

Mixed GH-cell/PRL-cell adenoma. A, The appearance of a mixed GH/PRL adenoma on H&E stain may be indistinguishable from a GH adenoma, displaying eosinophilic/chromophobic cells. B and C, The tumor shows immunoreactivity for GH ( B ) and PRL ( C ) in distinct cell populations. D, Ultrastructure confirms the dual cell population of this tumor composed of lactotrophs (L) and somatotrophs (S).

ACTH-secreting adenomas

ACTH-secreting adenomas associated with Cushing’s disease represent approximately 10% to 15% of all adenomas. On rare occasions, corticotroph cell hyperplasia may be the source of Cushing’s disease. However, controversy exists both from the clinical and pathological viewpoints regarding this event (see later discussion). The great majority of ACTH-secreting adenomas are microadenomas and approximately 15% are invasive at the time of surgery.

ACTH-secreting adenomas are usually basophilic by H&E stain. The adenomatous cells have granular cytoplasm and the nucleus is large with coarse chromatin and a prominent nucleolus ( Figure 9-8 ). Some degree of nuclear pleomorphism can be present. The cells have very distinct cytoplasmic borders and tend to touch each other in a “tile-like” arrangement. Papillary formations are very common. Adenomas with a more chromophobic appearance and less granular cytoplasm are also seen. Occasionally, hyaline bundles encircling the cytoplasm, giving a “target-cell” appearance, are observed and represent Crooke hyaline change (see Figure 9-8, D ). These hyaline bundles correspond to the accumulation of cytokeratin intermediate filaments and appear to be a direct action of high serum levels of cortisol on the pituitary cells. Crooke’s hyalin change are also present in the normal pituitary gland of Cushing’s patients and in patients with other pathological or iatrogenic hypercortisolemic state.

ACTH-cell adenoma. A, Corticotroph cell adenomas are composed of large cells with angular, amphophilic cytoplasm and large nuclei. B, Intense ACTH immunoreactivity in a corticotroph adenoma. C, Ultrastructure is remarkable for well-differentiated cells with abundant large-sized secretory granules. D, Crooke hyaline changes are characterized by the accumulation of hyalin bundles in the cytoplasm.

ACTH adenomas in the setting of Nelson’s syndrome are histologically similar to adenomas associated with Cushing’s disease, with the exception that accumulation of intermediate filaments and Crooke hyaline changes are not present.

Immunohistochemical studies demonstrate the presence of ACTH with various degrees of immunoreactivity. In addition, other peptides related to the pro-opiomelanocortin (POMC) precursor molecule, including β-lipotropin, β-endorphin, and α-melanocyte-stimulating hormone, are also present. In practice, the demonstration of these other related peptides is less relevant than the presence of ACTH.

The ultrastructural features of ACTH-secreting adenomas are characterized by well-differentiated cells that resemble normal corticotrophs, with well-formed organelles including RER, SER, and conspicuous Golgi and numerous large (250 to 700 nm) secretory granules ( Figure 9-8, C ). The secretory granules are often of different shapes (teardrop-, spherical-, heart-shaped) and vary in electron density. In the adenomas from patients with Cushing’s disease, bundles of intermediate filaments adjacent to the nucleus and/or forming large circles are easily identified. In Nelson’s syndrome adenomas, there is lack of accumulation of intermediate filaments in the cells.

Silent “corticotroph” adenomas

These variants of ACTH adenomas are characterized by immunoreactivity for ACTH despite the fact that the patients have neither clinical signs of Cushing’s disease nor serum levels reflecting excess ACTH secretion. Consequently, a complete morphological, immunohistochemical, and ultrastructural analysis of the tumors is essential for a correct evaluation of these silent adenomas. Because silent adenomas are clinically nonfunctioning, the majority of them are macroadenomas, and the patients have signs and symptoms of a mass lesion. Characteristically, these adenomas show a high tendency for hemorrhage and apoplexy, which may be the presenting symptom in about a third of the patients.

Two types of silent corticotroph adenomas have been described. The silent “corticotroph” adenoma subtype 1 is morphologically indistinguishable from functioning corticotroph adenomas associated with Cushing’s disease. However, these adenomas tend to arise in older patients than in those with Cushing’s disease. Both at light microscopy and ultrastructurally, these adenomas are identical to corticotroph-cell adenomas associated with Cushing’s disease as described previously ( Figure 9-9 ).

Silent corticotroph adenoma. A, An adenoma showing extensive necrosis accompanied by macrophage infiltration in a patient with a macroadenoma. B, Tumor cells with granular cytoplasm were seen among the areas of necrosis. C, Intense immunopositivity for ACTH was demonstrated in the adenoma.

The silent corticotroph adenoma subtype 2 shows different morphology from corticotroph-cell adenomas. Histologically, the tumors resemble the more typical nonfunctioning null-cell adenomas (see later discussion). Immunohistochemistry demonstrates ACTH and POMC-related peptide reactivity. The ultrastructure is less characteristic of a typical corticotroph-cell adenoma and silent corticotroph subtype 1 adenomas. However, the morphology of the secretory granules has corticotroph characteristics.

The normal cell phenotypes giving rise to the silent corticotroph adenomas have yet to be established. It has been speculated that the posterior lobe basophilic cell, morphologically similar to the anterior lobe corticotrophs, could be a potential progenitor cell of the silent type 1 tumors.

TSH-secreting adenomas

TSH-secreting or thyrotroph-cell adenomas are the least frequent pituitary adenomas (see Table 9-3 ). Although functioning adenomas in many patients present clinically with inappropriately elevated TSH levels and hyperthyroidism, some tumors may arise in the setting of hypothyroidism or in clinically euthyroid patients. Most tumors are invasive macroadenomas.

Thyrotroph cell adenomas are frequently chromophobic at light microscopy. The adenomas are composed of elongated, angulated cells possessing long cytoplasmic processes ( Figure 9-10 ). Immunohistochemistry usually reveals variable TSH immunoreactivity, and α-SU is commonly positive as well.

TSH-secreting adenoma. A and B, Thyrotroph-cell adenomas are mostly composed of angulated cells with a central nucleus and a prominent nucleolus. C, TSH immunoreactivity is variable in these tumors. D, Ultrastructure is diagnostic by well-differentiated cells with moderate numbers of small secretory granules located along the cytoplasmic border.

By electron microscopy, the cells are moderately differentiated, with scant RER network and Golgi complexes. Secretory granules are small (100 to 200 nm), spherical, and evenly electron dense, and often are lined up along the cytoplasmic membrane ( Figure 9-10, D ).

The diagnosis of TSH-secreting adenomas can be problematic if the clinical presentation and TSH immunoreactivity are not convincing. In this instance, electron microscopy is mandatory for appropriate diagnosis.

Gonadotropin-secreting (gonadotroph) adenomas

Pituitary adenomas that secrete the gonadotropins—follicle-stimulating hormone (FSH) and luteinizing hormone (LH)—have traditionally been regarded as uncommon tumors. Unlike other secreting tumors, gonadotroph adenomas do not usually cause a clinical syndrome related to hormone overproduction. The hormonal production from these tumors is inefficient and the detection of excess hormone levels is challenging. With the advent of modern laboratory techniques, a large number of the pituitary adenomas previously classified as nonfunctioning adenomas have been found to produce gonadotropins or their subunits. Based on these studies, gonadotroph adenomas may account for a large proportion of clinically nonfunctioning adenomas and about 20% of all adenomas (see Table 9-3 ).

Gonadotroph adenomas most frequently occur in patients in the sixth decade and older, and have a slight male predominance. Typically these adenomas present as clinically nonfunctioning tumors with most symptoms related to local mass effects. Mass effects can include visual deficits, headache, hypopituitarism, loss of libido, and cranial nerve palsies. Visual field loss due to suprasellar extension and compression of the optic chiasm is found as a presenting symptom in more than 70% of patients with presumed gonadotroph adenomas.

By light microscopy, most gonadotroph adenomas have chromophobic cytoplasm and the nucleus displays a fine chromatin pattern ( Figure 9-11 ). The tumor cells may be arranged in a diffuse pattern, but a distinct papillary arrangement of tumor cells is commonly seen in these tumors. The papillary structures are typically formed by elongated cytoplasmic processes extending toward the vessels in a pattern resembling perivascular pseudorosette formation.

Gonadotropin-secreting adenoma. A and B, Gonadotropin-secreting adenomas characteristically show chromophobic cells with papillary arrangements. C, Strong FSH immunoreactivity in an adenoma with papillary arrangement. D, In the same adenoma, α-subunit of glycoproteins was focally present.

Immunohistochemistry demonstrates varying degrees of reactivity for β-FSH, β-LH, α-SU, or combinations of the three hormones ( Figure 9-11, C and D ). Immunoreactive cells can be scattered throughout the adenoma, but are often clustered. Immunoreactivity for β-FSH appears to be more common than the other glycoproteins.

Gonadotroph adenomas are ultrastructurally characterized by elongated, polar cells containing scant numbers of small (50 to 200 nm) secretory granules. The secretory granules are distributed unevenly within the cytoplasm or, more commonly, tend to be located along the cytoplasmic membrane. A typical vacuolar transformation of the Golgi complex, giving a honeycomb appearance to the Golgi apparatus, is also seen in the tumors.

The correlation between β-FSH and/or β-LH immunoreactivity, the degree of ultrastructural differentiation, and clinical symptomatology is relatively poor in gonadotroph adenomas. Although classifying a glycoprotein-immunopositive tumor as a gonadotroph adenoma is of academic interest, it does not alter clinical management of these patients. Most of the patients are at present treated as having a “clinically nonfunctioning adenoma” with therapeutic goals targeting restoration of visual deficits, preservation of pituitary function, and prevention of recurrence.

Null-cell adenomas and oncocytomas

Approximately 20% of adenomas show neither clinical nor pathological evidence of hormone production (see Table 9-3 ). These tumors are designated “null-cell adenomas” and they are characteristically composed of cells without specific pituitary cell type differentiation. Null-cell adenomas develop in a similar population and with similar clinical conditions as gonadotroph adenomas. Usually the tumors arise in postmenopausal females and elderly males, and likewise have signs and symptoms of a mass lesion. The great majority of null-cell adenomas are macroadenomas at presentation.

Null-cell adenomas are typically chromophobic by light microscopy, and tumor cells may be arranged in a diffuse pattern and/or in well-defined papillary arrangements. Oncocytic degeneration can be seen in a percentage of cases, and consequently the designation of oncocytic null-cell adenoma (oncocytoma) is applied to these adenomas. By immunohistochemistry, adenomas may lack immunoreactivity for any pituitary hormone (“immunonegative” adenomas) or often demonstrate focal and weak reaction for β-FSH, β-LH, and/or α-SU.

Null-cell adenomas demonstrate poorly developed organelles at the ultrastructural level, and only sparse small secretory granules. Abundance of mitochondria is seen in the tumors with oncocytic degeneration.

The cytogenesis of null-cell adenomas is still not completely understood. Since some null-cell adenomas may show focal immunoreactivity for glycoprotein hormones, a considerable overlap exists between these tumors and gonadotroph adenomas. Criteria for differentiation between null-cell and gonadotroph adenomas are still debatable. However, the relative lack of synthetic organelles in null-cell adenomas by ultrastructure differs from that of normal, mature gonadotrophs. These two tumors may be derived from a single progenitor cell that has the capacity to differentiate within a spectrum from the more differentiated gonadotroph cell to less differentiated cells.

Plurihormonal adenomas

Plurihormonal adenomas are rare adenomas that have an unusual mixture of hormonal immunoreaction that is mostly unrelated to the normal cytogenesis and development of the anterior pituitary. Due to the rarity of such tumors, the clinical presentation is not well characterized, although most reported cases have symptoms of mass effect due to the large size of the adenomas at the time of diagnosis. Plurihormonal adenomas can be either monomorphous or plurimorphous in nature. Monomorphous plurihormonal adenomas are composed of one distinct cell type that produces two or more hormones. Plurimorphous plurihormonal adenomas consist of two or more morphologically individual cell types.

Any combination of hormones may be found. However, they should not include the combinations of (1) GH, PRL, and TSH, or (2) FSH and LH since these are commonly seen combinations, respectively, in GH-secreting and gonadotroph adenomas (see previous discussion). Combinations of FSH and GH or PRL and TSH have been reported. Rarely, plurihormonal adenomas show immunoreactivity for ACTH. A rare variant of corticotroph adenoma that also expresses α-SU has shown recurrence and aggressive behavior.