Cellular basaloid neoplasms of the salivary gland represent a diverse group of benign and malignant neoplasms with significant cytomorphologic overlap on fine-needle aspiration cytology. All are marked by the presence of monotonous and usually bland basaloid epithelium. Distinction between basaloid neoplasms on fine-needle aspiration cytology is based on the presence or absence of additional features, including a second cell population (eg, myoepithelial cells), an acellular stromal component, and/or cytologic atypia within the basaloid epithelium. This review highlights the cytomorphologic features of the most common cellular basaloid neoplasms of the salivary gland, with an emphasis on classification and subclassification within the Milan System.
To provide a comprehensive review of the cytologic features of basaloid epithelial neoplasms of the salivary gland, with an emphasis on classification within the Milan System for Reporting Salivary Gland Cytopathology.
Peer-reviewed literature, recent textbooks, and personal experiences of the author.
Some basaloid neoplasms, in particular pleomorphic adenomas and adenoid cystic carcinomas, may have characteristic findings on fine-needle aspiration that allow for definitive diagnosis. In other cases, however, fine-needle aspiration can confirm a neoplastic basaloid process, but specific classification of a benign or malignant neoplasm cannot be rendered. The Milan System for Reporting Salivary Gland Cytopathology acknowledges this difficulty, and recommends benign or malignant classification only when definitive diagnostic features of a specific neoplasm are present. For indeterminate cases, the subcategorization of salivary neoplasm of uncertain malignant potential is recommended.
Cellular basaloid neoplasms of the salivary gland are a diverse group of epithelial and biphasic epithelial/myoepithelial neoplasms characterized by a prominence of basaloid epithelium on fine-needle aspiration (FNA). They include a spectrum of benign neoplasms, such as pleomorphic adenoma (PA) and basal cell adenoma (BCA), and low- to intermediate-grade carcinomas, such as basal cell adenocarcinoma (BCAC) and adenoid cystic carcinoma (ACC).1–4 On FNA cytology, they are composed of relatively monotonous, crowded groups of ovoid epithelial cells with scant cytoplasm and round nuclei. Distinction between the different basaloid neoplasms is dependent on the presence or absence of a number of features in addition to monotonous basaloid epithelial cells, including cytologic atypia within the epithelial cells, a second cell population (eg, myoepithelial cells), and acellular matrix material.2,3
The differentiation of basaloid neoplasms on FNA is difficult. Specific diagnosis can be made for some neoplasms, such as PA, when classic cytomorphologic features are present in abundance. However, in many cases, even after careful inspection, specific classification is not possible on FNA. Recent publication of the Milan System for Reporting Salivary Gland Cytopathology (TMS) takes this into account, with basaloid neoplasms potentially falling under a number of classifications of varying expected malignant potential on surgical resection, including benign neoplasm, salivary gland neoplasm of uncertain malignant potential (SUMP), suspicious for malignancy, and malignant.4,5
This review highlights the features of cellular basaloid neoplasms of the salivary glands on FNA that may allow for specific diagnosis, including ancillary studies. It also emphasizes the present limitations of FNA for specific classification of cellular basaloid neoplasms in many cases, with a focus on classification and subclassification of basaloid neoplasms within TMS.
A number of benign tumors can present on FNA with a predominantly basaloid epithelial cellular pattern. Pleomorphic adenomas, particularly the cellular or stroma-poor variants, are most common. Although a biphasic tumor, PA not infrequently shows a predominance of basaloid epithelial cells on FNA. Basal cell adenoma accounts for most of the remainder of benign basaloid neoplasms, with a few adnexal and minor salivary gland tumors also diagnostic considerations. Benign basaloid tumors have a number of overlapping cytomorphologic features. The basaloid epithelial cells are tightly cohesive, with bland, regular nuclei, inconspicuous to absent nucleoli, and absence of mitoses or necrosis. The differential diagnosis is primarily dependent on the presence or absence of additional features, including extracellular matrix material and single, plasmacytoid myoepithelial cells.
Pleomorphic adenoma is the most common salivary gland neoplasm, with most arising in the parotid gland. In its textbook appearance, FNA of PA reveals a biphasic population of bland, monotonous basaloid epithelial cells and single, plasmacytoid or spindle-shaped myoepithelial cells, with chondromyxoid and fibrillary matrix material in the background that appears deep magenta on Romanowsky stain and pale to nearly translucent on Papanicolaou stain.2,5,6 Tumor cells, especially the myoepithelial component, are present embedded within this matrix material (Figures 1 through 3).
When all 3 elements are present and demonstrate typical cytologic features, the specific diagnosis of PA can usually be made on FNA.5 In particular, the presence of fibrillary acellular matrix material is distinctive among the basaloid salivary tumors.2,6–9 However, PA can show a wide spectrum of findings on FNA that may complicate the diagnosis. Cellular variant of PA often shows a predominance of either myoepithelial cells or basaloid epithelial cells on FNA, with less prominent extracellular matrix material. Matrix material can be scant to absent, particularly when sampling is limited.2,6 Such cases are difficult to impossible to definitively identify as PA on FNA, as the cytomorphologic overlap with BCA and ACC is significant. Squamous metaplasia is sometimes prominent in PA, and may raise the differential diagnosis of squamous cell carcinoma. However, cytologic features are bland and stromal and myoepithelial elements are typically still present.
Although classically fibrillary, the matrix material in PA can also take on a circumscribed or hyaline appearance, which can mimic the basement membrane–type material of ACC.10 Careful inspection typically reveals myoepithelial cells embedded within matrix material and/or other areas with more typical fibrillary stroma, important distinguishing factors that favor the diagnosis of PA over ACC.2 Among cellular basaloid neoplasms, the presence of fibrillary stroma on FNA is highly associated with benign diagnosis at resection.7,8
Ancillary studies are of limited benefit in the FNA diagnosis of PA. Myoepithelial expression of p40 and/or p63 is maintained in PA, whereas these markers are typically negative in ACC on immunohistochemical stain. S-100 immunohistochemistry can be helpful in separating PA (positive in epithelial component) from BCA (negative).11 Nuclear expression of PLAG1, a zinc finger protein, is a helpful marker, as translocation of the PLAG1 gene is frequently present in PA. Overexpression of PLAG1 is common in PA but rare in other basaloid salivary gland tumors, including malignancies.11
Basal Cell Adenoma
Basal cell adenoma, also referred to in past literature as monomorphic adenoma, is the second most common benign neoplasm of the salivary gland, and like PA typically occurs in the parotid gland. Histologically and cytologically, it is marked by the presence of crowded groups of basaloid epithelial cells associated with variable amounts of dense, acellular stromal material lacking the chondromyxoid appearance often seen in PA. The basal epithelial cells display a range of architecture, including solid, tubular, trabecular, and membranous subtypes.
On FNA, BCA is characterized by a predominance of tightly cohesive, 3-dimensional clusters and cords of basaloid epithelial cells with scant cytoplasm and uniform, bland nuclei (Figures 4 and 5). Myoepithelial cells are scant or absent. Extracellular matrix material can be present or absent, but when present it is typically denser and less fibrillary than PA matrix.2,3,5 Hyalinized stromal material, including hyalinized globules surrounded by tumor cells, can result in cytomorphologic overlap with ACC.9 However, although this matrix material is paucicellular, careful examination will often reveal the presence of tumor cells embedded within, a feature typically not seen in ACC. Additionally, BCA matrix has a wider spectrum of appearances than ACC. Palisading ribbons of matrix material can be prominent, and the stromal material is often present at the periphery of basaloid cell groups and interdigitating individual tumor cells (Figure 6). Nuclear palisading around matrix material can also be seen, particularly in the membranous subtype (Figure 7). In addition, loose aggregates of bland-appearing spindle cell stroma are often seen associated with the basaloid epithelial groups of BCA, a feature not typically seen in PA or ACC.12,13
As with PA, ancillary studies are of limited utility in the diagnosis of BCA. Lymphocyte enhancer binding factor 1 (LEF-1), a downstream mediator of the WNT/β-catenin signaling pathway, is overexpressed in the nuclei of BCA but is typically negative in ACC on immunohistochemical stain. However, LEF-1 expression can also be seen in other benign (PA) and malignant (BCAC) neoplasms.11,14
Other Benign Neoplasms
Pleomorphic adenoma and BCA account for the vast majority of benign salivary gland tumors with basaloid cytology, but a few other neoplasms may be diagnostic considerations depending on the location of the lesion and the background elements present on FNA. Canalicular adenoma is a tumor of minor salivary glands, and is sometimes regarded as a variant of BCA. It occurs most commonly on the upper lip and palate of older female patients. Fine-needle aspiration cytology is composed almost entirely of bland basaloid epithelial cells in sheets and trabecular arrangements. Histologically, the trabeculae of tumor cells are associated with hyalinized stromal material, but on FNA stroma is usually scanty to absent.2,15 Sclerosing polycystic adenosis is a rare neoplastic process of salivary gland epithelium, which often shows both acinar and ductal-type epithelial cells on FNA. Its histology is analogous to fibrocystic changes of the breast, including epithelial changes that mimic sclerosing adenosis and apocrine metaplasia. When bland, nonreactive-appearing ductal epithelium predominates, FNA may show bland epithelial sheets and clusters of cells with fairly high nuclear to cytoplasmic ratios. However, cytoplasm is typically somewhat more abundant and granular than other basaloid salivary tumors, even when acinar and/or apocrine differentiation is not present.2,16 Occasionally, basaloid adnexal tumors of the deep dermis of the skin of the face or neck may be mistaken for salivary gland origin on physical examination or radiographically. Squamous differentiation or the presence of anucleate squamous cells is often seen in basaloid adnexal tumors on FNA (eg, pilomatrixoma, trichilemmal cyst), which can provide a hint in such cases.17
Many cellular basaloid neoplasms prove to be malignant at surgical resection, and there is significant cytomorphologic overlap between the benign and malignant salivary gland neoplasms. However, a number of cytologic clues can point to a malignant diagnosis on FNA. Malignant basaloid neoplasms typically lack the fibrillary stroma classically seen in PA, and there is often significant nuclear atypia and loss of cellular cohesion. In particular, ACC can often be reliably diagnosed on FNA when classic features are present.
Adenoid Cystic Carcinoma
Adenoid cystic carcinoma is the second most common salivary gland malignancy, and the most common malignancy that may present as a cellular basaloid neoplasm on FNA. It is characterized by clusters, sheets, and trabeculae of basaloid epithelial cells associated with globules of hyaline basement membrane–like matrix material that have an acellular, “punched-out” appearance within groups of tumor cells (Figure 8).2,3,9 In contrast to PA and BCA, tumor cells are not embedded within the matrix material of ACC, an important distinguishing feature on FNA. Fibrillary-type matrix is typically absent.
The solid subtype of ACC, in which basaloid epithelium predominates and matrix is scant to absent, is a difficult diagnostic challenge. A predominance of relatively bland, basaloid epithelial cells without matrix material is a feature that can be seen not only in solid ACC, but also cellular PA and BCA. Cytologically, the nuclei may be deceptively bland, although careful examination often reveals irregular, angulated nuclear contours, coarse chromatin, and irregular nucleoli in the solid subtype (Figure 9). Tumor cells are often less cohesive than those of cellular PA and BCA, and cytoplasm may be so scant as to impart a “naked nuclei” appearance to individual tumor cells.18 Mitoses and necrosis are not always observed but are suggestive of malignancy when present.
Immunohistochemistry is of limited benefit in the differential diagnosis of ACC. Strong expression of CD117 and/or DOG1 favors ACC over PA, although expression of these markers can be seen occasionally in PA. In the differential diagnosis between ACC and BCA, S-100 expression favors ACC whereas LEF-1 expression favors BCA.11,13,19
Basal Cell Adenocarcinoma
Basal cell adenocarcinoma is a rare salivary gland malignancy, most commonly occurring in the parotid gland. It is considered the malignant counterpart of BCA. Its cytology is very similar to that of BCA, and the diagnosis is often dependent on histologic confirmation of invasion, similar to the distinction between PA and carcinoma ex PA.20,21 Basal cell adenocarcinoma shows packed groups of ovoid to spindle-shaped, basaloid epithelial cells in solid, trabecular, and membranous arrangements. Matrix material is typically scanty, and when present it resembles the patterns seen in BCA. Loose spindle cell stroma can be associated with basaloid groups, similar to BCA.20
In contrast to BCA, cytologic atypia can often be observed in BCAC, including coarse chromatin, anisonucleosis, and nuclear molding.3,20,21 The nuclei of BCAC are often elongated and spindled in appearance (Figure 10). Groups of tumor cells are typically more crowded than those of BCA, and loss of cohesion can be seen at the edges of groups.20 When present, mitoses and necrosis also suggest a diagnosis of BCAC over BCA. Immunohistochemistry is not typically helpful, as BCA and BCAC share similar expression profiles, including LEF-1 and β-catenin expression and negativity for PLAG1 and S-100.11,22
Other Malignant Neoplasms
Adenoid cystic carcinoma and BCAC are the malignancies that classically present on salivary gland FNA as cellular basaloid neoplasms. However, other carcinomas can also present with a predominance of basaloid epithelial cells depending on sampling and individual tumor biology.
Epithelial-myoepithelial carcinoma is a biphasic tumor, most commonly occurring in the parotid glands of elderly patients. Fine-needle aspiration cytology typically reveals a cellular neoplasm composed of branching pseudopapillary groups and tubules, with central laminated, acellular cores surrounded by an inner layer of basaloid epithelium, which is in turn surrounded by an outer layer of larger myoepithelial cells with moderately abundant clear cytoplasm.23 Hyaline globules may also be present. In most cases, the myoepithelial component predominates over the basaloid epithelial component, distinguishing epithelial-myoepithelial carcinoma from basaloid malignancies such as ACC. However, the diagnosis is more difficult in cases where the basaloid component predominates in the sampled material. Careful examination of cytologic material and immunohistochemical stains for myoepithelial markers (eg, p63, smooth muscle actin) typically reveal the biphasic nature even in cases where there is an abundance of basaloid cellular elements.21,24
Polymorphous adenocarcinoma, previously referred to as polymorphous low-grade adenocarcinoma, is a rare, indolent malignancy occurring almost exclusively in the minor salivary glands, particularly of the hard palate. Histologically, its polymorphous growth includes tubular, cribriform, papillary, and solid patterns.25 On FNA, polymorphous adenocarcinomas are cellular neoplasms composed of sheets and groups of basaloid cells with scant, eosinophilic cytoplasm.26 Hyaline globules are sometimes identified, but overall matrix material is scant. The presence of solid and cribriforming basaloid epithelial cells with absent or scant hyaline stroma raises the differential diagnosis of ACC and BCA/BCAC. Strong S-100 expression and absence of expression of myoepithelial markers and CD117/DOG1 are characteristic of polymorphous adenocarcinoma.
Carcinomas ex PA are rare malignancies that typically require histologic evaluation for invasion to differentiate from PA. Cytologic clues to this diagnosis include typical findings for PA in addition to frankly malignant cytologic features, such as loss of epithelial cohesion, significant cytologic atypia, and necrosis (Figure 11).2
Primary neuroendocrine carcinomas of the salivary gland are rare malignancies that show a predominance of loosely cohesive cells with high nuclear to cytoplasmic ratios, and they may present as small cell or large cell variants. The small cell variant is similar to that seen in the lung and other locations, with extremely scant cytoplasm, fine chromatin, absent to inconspicuous nucleoli, and nuclear molding. The large cell variant shows larger cells with more cytoplasm, coarse chromatin, and prominent nucleoli. In both cases, expression of neuroendocrine markers such as synaptophysin and chromogranin aid in the diagnosis.2,3,27
Finally, metastasis or direct extension of basaloid carcinomas into the salivary gland may be a diagnostic consideration depending on the patient's clinical history. Basal cell carcinoma of the skin can resemble cellular basaloid salivary gland neoplasms. It is typically very tightly cohesive, but cytologic atypia and mitoses are usually readily identifiable (Figure 12).3 Metastasis of squamous cell carcinoma, especially those that are high-risk human papillomavirus associated, can also present with a predominant basaloid cytologic appearance. Identification of squamous differentiation and/or ancillary studies for high-risk human papillomavirus involvement (eg, high-risk human papillomavirus in situ hybridization) are helpful in establishing this diagnosis.2,28
TMS AND CLASSIFICATION OF CELLULAR BASALOID NEOPLASMS
Although specific features of a particular neoplasm can be identified for some cellular basaloid neoplasms, in practice the degree of cytomorphologic overlap among stroma-poor PA, BCA, ACC, BCAC, and other basaloid tumors is significant and prevents specific classification in many cases (Figure 13, A through D). The recent publication of TMS highlights this reality. Categorization of salivary gland lesions in general is similar to that for thyroid lesions under the Bethesda System for Reporting Thyroid Cytopathology, with FNA categories of nondiagnostic, nonneoplastic, atypia of undetermined significance, neoplasm, suspicious for malignancy, or malignant. Cellular basaloid neoplasms can fall under multiple categories of varying risk of malignancy at resection, including neoplasm, suspicious for malignancy, and malignant.4
The neoplastic category in TMS includes 2 subcategories: benign neoplasm and SUMP. The benign neoplasm category is reserved for those cases where a specific benign neoplasm can be diagnosed.5 In practice, most benign basaloid salivary gland neoplasms that can be specifically diagnosed on FNA are PAs, as the presence of fibrillary stroma and embedded myoepithelial cells in addition to basaloid epithelial cells is specific for this diagnosis.
The SUMP subclassification is used for cases where a diagnosis of neoplasm is established, but definitive features of a specific benign or malignant neoplasm are not present. Most cellular basaloid tumors that fall under the SUMP classification will show a predominance of basaloid epithelial cells without significant cytologic atypia (Figure 13, A through D). They also generally have scant or no myoepithelial cells and fibrillary stromal material.5 Fine-needle aspiration in such cases is diagnostic for a basaloid neoplasm, but neither a PA nor a carcinoma diagnosis can be definitely established. On resection, these cases include PAs, BCAs, and ACCs, among other basaloid neoplasms.
Basaloid neoplasms with more worrisome cytologic features, such as marked nuclear atypia and necrosis, may fall under the suspicious for malignancy or malignant categories.23,29 Adenoid cystic carcinoma is the basaloid malignancy that can most commonly be specifically diagnosed on FNA. The presence of the typical acellular basement membrane–like hyaline material associated with basaloid epithelial cells with angulated, hyperchromatic nuclei and distinct nucleoli are hallmarks of ACC on FNA.23
Cellular basaloid neoplasms of the salivary gland represent a unique challenge on FNA, because of the morphologic overlap between benign and deceptively bland malignant neoplasms. Although PA and ACC often can be specifically diagnosed on FNA, reproducible diagnostic features for the other basaloid neoplasms have largely not been established. Even in cases of PA and ACC, there exists significant cytomorphologic overlap with other basaloid tumors that can at times prevent a specific diagnosis on FNA (Figure 13, A and B).
Given the cytomorphologic overlap of basaloid neoplasms, the primary goal of TMS classification is stratification based on risk of malignancy at resection. In its initial publication, estimation of rates of malignancy (ROMs) by neoplastic or greater categories were benign neoplasm, less than 5%; SUMP, 35%; suspicious for malignancy, 60%; and malignant, 90%. FNA diagnosis of PA (benign neoplasm category) is still associated with a small risk of malignancy at resection, as carcinoma ex PA typically requires histologic confirmation of invasion, and occasionally low-grade basaloid malignancies can be mistaken for PA on FNA. In cases with classic features of ACC, or when other cytologic features concerning for malignancy are present (eg, marked nuclear atypia, necrosis), suspicious for malignancy or malignant categorization is appropriate depending on the cytopathologist's degree of concern.
For many cellular basaloid neoplasms, though, the indeterminate FNA subcategorization of SUMP is necessary (Figure 13, A through D). As discussed, PA and ACC do not always display their classic appearances on FNA, and cannot be definitively diagnosed in all cases. FNA sampling of low-grade malignancies, such as BCAC, epithelial-myoepithelial carcinoma, and low-grade forms of polymorphous adenocarcinoma, can have such bland basaloid-type epithelium and scant or nonspecific stromal elements that malignant classification is impossible on FNA. Basal cell adenoma can have somewhat unique cytologic and ancillary findings that can suggest the diagnosis at FNA, such as polymorphous stromal elements, nuclear palisading around stroma, loose spindle cell aggregates, and LEF1 expression. However, in practice, the cytologic overlap of BCA with other benign and malignant neoplasms is significant, and reproducible diagnostic criteria for BCA at FNA have not been established.
The ROM for SUMP upon resection was estimated to be 35% at the time of publication of TMS, and early studies have found malignancy rates in this approximate range. A recent study30 with literature review found 40.3% (85 of 211) ROM for all subtypes of SUMP (eg, basaloid, oncocytic) across 10 studies between 2017 and 2019. Two retrospective studies31,32 looking specifically within the SUMP category found overall ROM for cellular basaloid neoplasms (basaloid SUMP) of 40% (10 of 25) and 28% (8 of 29), respectively, at surgical resection. Not surprisingly, follow-up surgical resection in these 2 studies showed a spectrum of benign and malignant neoplasms. Among the 36 cases with benign histology, there were 19 PAs, 15 BCAs, and 2 benign cystadenomas. Among the 18 cases with malignant histology, there were 7 ACCs, 5 epithelial-myoepithelial carcinomas, 2 myoepithelial carcinomas, 2 metastatic basaloid squamous cell carcinomas, 1 carcinoma ex PA, and 1 salivary duct carcinoma.31,32
In summation, cellular basaloid neoplasms are a challenging group of epithelial and biphasic salivary gland neoplasms, ranging from benign tumors to fairly aggressive malignancies. Whenever possible, a specific diagnosis of a benign (eg, PA) or malignant neoplasm (eg, ACC) should be made. However, in many cases a diagnosis of neoplasm is clear but the benign or malignant nature of the tumor is not. Such cases are best classified as SUMP. Early studies have found PA and BCA to be the most common diagnoses at surgical resection following a diagnosis of basaloid SUMP. However, basaloid SUMP is associated with a significant ROM at resection (28%–40%).
The author has no relevant financial interest in the products or companies described in this article.
Presented in part at the New Frontiers in Pathology meeting; September 27–29, 2018; Ann Arbor, Michigan.