Subtypes of NF-PitNETs
Non-functioning/silent gonadotroph tumors are SF-1 cell lineage derived tumors that typically demonstrate at least focal immunolabeling for β-FSH, β-LH, and α-subunit [
2]. Despite the methodological difficulties related to suboptimal quality of the available anti SF-1 antibodies, the nuclear labeling can usually be detected in a significant proportion of tumor cells, enabling the diagnosis in cases with sparse or no gonadotroph hormone expression [
22]. Gonadotroph tumors comprise almost 80% of NF-PitNETs, when both antibodies toward gonadotroph hormones and SF-1 are used in classification [
18,
35]. However, they have been underestimated and usually classified as null cell adenomas in previous studies, in which transcription factors were not available [
36,
37]. It is the only type of PitNET where the non-functioning form dominates [
38,
39].
Non-functioning/silent corticotroph tumors are T-Pit lineage related tumors, usually with sparse ACTH expression without clinical evidence of Cushing disease [
2,
40,
41]. They constitute about 15% of NF-PitNETs, thus, representing the second largest group of these tumors [
18,
39]. The proportion of silent corticotroph tumors among NF-PitNETs is expected to increase with greater use of immunohistochemistry with anti-T-Pit antibody allowing for identification among the tumors with sparse or no ACTH expression [
18]. Similar to their functioning counterparts, silent corticotroph tumors can be morphologically and ultrastructurally subdivided into densely and sparsely granulated [
2,
42]. Rarely, Crooke-cell adenoma with a typical perinuclear ring-like accumulation of cytokeratin and relocation of ACTH positivity to the sub-membranous zone can manifest as clinically silent [
41,
43]. Transformation of silent corticotroph tumors into functioning PitNET (or, exceptionally, vice versa) has been comprehensively analyzed in corticotroph tumors [
40,
44‐
46]. However, the mechanisms remain unclear despite several potential explanations [
40,
47‐
49]. Independently of their morphological variants, silent corticotroph tumors are recognized as tumors with the more aggressive clinical behavior due to their tendency for invasive growth, apoplexy, and recurrences [
2,
40,
50,
51].
Non-functioning/silent somatotroph tumors are Pit-1 and GH immunoreactive tumors without clinical signs of acromegaly [
2,
52,
53]. They represent 2–3% of all pituitary tumors [
53]. Similar to their much more frequent functioning counterparts, they can be divided into sparsely and densely granulated somatotroph tumors based on low molecular weight cytokeratin (LMWCK) [CK7/8 (Cam5.2) and CK18] staining, demonstrating either fibrous bodies or diffuse cytoplasmic pattern (Table
1) [
2,
53]. NF-somatotroph PitNETs are predominantly sparsely granulated, in contrast to functioning somatotroph PitNETs, where the frequency of sparsely and densely granulated tumors is equal [
52,
53] or in favor of densely granulated subtype [
54,
55]. They have a lower proportion of GH-immunoreactive cells, which suggests lower differentiation. Majority of silent somatotroph tumors co-express prolactin [
53]. Sparsely granulated somatotroph tumors are, according to the WHO classification [
2], designated as more aggressive tumors based on several studies demonstrating that they are usually larger and more invasive compared to the densely granulated subtype [
54‐
56]. In patients with acromegaly, the sparsely granulated somatotroph tumors are less responsive to somatostatin analogues, probably due to the lower expression of somatostatin receptors (SSTRs) [
55].
Non-functioning/silent lactotroph PitNET are PRL-immunoreactive tumors with no clinical signs of hyperprolactinemia, apart from elevated prolactin levels due to the stalk effect [
57]. In addition to transcription factor Pit-1, they also express estrogen receptor alpha [
2]. Pure silent lactotroph tumors are exceptional. More often, prolactin is co-expressed with GH in Pit-1 positive tumors. Silent lactotroph tumors are very rare in surgical, but not in autopsy series [
37,
58]. Both silent and functioning lactotroph tumors are sub-classified into sparsely granulated, demonstrating Golgi-like prolactin immunoreactivity and densely granulated with diffuse cytoplasmic expression of prolactin in the tumor cells [
2]. Acidophil stem-cell adenoma has only been exceptionally described in silent form [
37]. More aggressive behavior of the functioning lactotroph tumors in men seems to correlate with the lower expression of ERα compared with the lactotroph tumors in women [
33]. However, the expression of ERα and its correlation with the clinical characteristics of silent lactotroph tumors is still obscure. Rare silent lactotroph tumors is important to identify, as addition of dopamine agonists may be considered in the treatment, although the effects are less well documented than in the clinically-functioning prolactinomas [
59].
Non-functioning/silent thyrotroph tumors belong to Pit-1 lineage tumors and express β-TSH without clinical and biochemical signs of central hyperthyroidism [
2,
60,
61]. They seem to occur with slightly higher frequency than their rare functioning counterparts [
62,
63]. Silent and functioning thyrotroph tumors behave in a similar manner regarding the recurrence rate and time from surgery to the recurrence [
62]. Non-functioning thyrotroph tumors rarely change its endocrine activity, transforming into functioning thyrotroph PitNET [
35,
64].
Null-cell PitNETs are defined by the lack of immunohistochemical evidence of differentiation toward any anterior pituitary cell, using antibodies to adenohypophysial hormones and transcription factors [
2]. They comprise only a few percent of all pituitary tumors [
18]. However, their frequency has frequently been overestimated due to suboptimal immunohistochemical protocols and lack of reliable antibodies to pituitary-specific transcription factors. Further investigations are needed to elucidate whether null-cell adenomas really exist, or whether this category just reflects methodological limitations in the current diagnostic procedure. Since null-cell adenomas are the diagnosis of exclusion, they may sometimes pose the challenge in differential diagnosis with other, non-adenohypophysial neuroendocrine tumors of the sellar region [
65].
Plurihormonal poorly differentiated Pit-1 positive tumors (previously designated as “silent subtype 3 adenoma”) are rare tumors composed of large polygonal or spindle shaped cells, with atypical nuclei, sometimes containing inclusions, so called nuclear spheridia that can be observed on routine HE stains or, more readily, on electron microscopy [
29,
66‐
68]. They may demonstrate variable and patchy immunoreactivity for GH, PRL, and TSH in different combinations. Although traditionally categorized as silent, plurihormonal Pit-1 positive tumors are associated with clinical signs of hypersecretion of Pit-1 lineage hormones in about 30% of the cases [
29]. The correct diagnosis of this uncommon type of NF-PitNETs is important since it belongs to the group of potentially aggressive tumors, usually macroadenomas with propensity to invade cavernous sinus and clivus and to recur [
2,
29,
66,
68].
Double/triple pituitary tumors are very rare tumors composed of demarcated components originating from different cell lineages, which can be confirmed by expression of more than one pituitary specific transcription factors. In the same tumor, silent and hormone-secreting components may be present [
2]. Based on sporadic reports and small series, double/triple pituitary tumors seem to be clinically active in the majority of cases [
69]. Combination of two or three NF PitNETs has been more frequently reported in autopsy material, suggesting that some tumors have been clinically silent [
58,
70].