Salivary Duct Carcinoma: Updates in Histology, Cytology, Molecular Biology, and Treatment

Abstract

Salivary duct carcinoma (SDC) is an aggressive subtype of primary salivary gland carcinoma, often with an advanced stage at presentation and high rates of metastasis and recurrence. It most commonly arises in the parotid gland of older men and microscopically resembles high-grade breast ductal carcinoma. While 50 years have lapsed since the first report of this entity, recent intensive studies have shed light on its biologic, genetic, and clinical characteristics. The diagnosis of SDC is aided by the immunohistochemical expression of androgen receptor (AR) coupled with its characteristic histomorphology. Fine needle aspiration typically reveals cytologic features of high-grade carcinoma, and ancillary studies using cell block material can facilitate the specific diagnosis of SDC. In surgical specimens, certain histologic features are important prognostic factors, including nuclear pleomorphism, mitotic counts, vascular invasion, and the morphology at the invasion front. Several clinical studies have shown promising results using targeted therapy for AR and human epidermal growth factor receptor 2 (HER2), and the latest version of the National Comprehensive Cancer Network guidelines recommends the evaluation of AR and HER2 status before treatment. Recent molecular analyses have revealed multiple heterogeneous alterations in well-known oncogenes and tumor suppressor genes, including TP53, HRAS, PIK3CA, PTEN, and BRAF. Clinical trials of drugs targeting these genes may broaden the treatment options for SDC in the near future.

INTRODUCTION

Salivary duct carcinoma (SDC) is the most aggressive tumor of the 21 subtypes of primary salivary gland carcinoma in the latest World Health Organization classification (Table 1),¹ showing high rates of local recurrence and distant metastases.^2,3 SDC mainly affects men over 50 years of age and accounts for approximately 5% to 10% of all salivary gland malignancies.^1,2 Most SDCs arise from the major salivary glands, especially the parotid gland, although tumors in the minor salivary glands, sinonasal tract, and lacrimal gland have also been reported.^2,4,5

TABLE 1.

Subtypes of Primary Salivary Gland Carcinoma

Mucoepidermoid carcinoma

Adenoid cystic carcinoma

Acinic cell carcinoma

Polymorphous adenocarcinoma

Clear cell carcinoma

Basal cell adenocarcinoma

Intraductal carcinoma

Adenocarcinoma, not otherwise specified

Salivary duct carcinoma

Myoepithelial carcinoma

Epithelial-myoepithelial carcinoma

Carcinoma ex pleomorphic adenoma

Secretory carcinoma

Sebaceous adenocarcinoma

Carcinosarcoma

Poorly differentiated carcinoma

Undifferentiated carcinoma

Large cell neuroendocrine carcinoma

Small cell neuroendocrine carcinoma

Lymphoepithelial carcinoma

Squamous cell carcinoma

Oncocytic carcinoma

Adapted from El-Naggar et al.¹

Radiologically, most SDCs show features of high-grade malignant tumors. By contrast-enhanced computed tomography, an ill-defined border, invasion into adjacent tissue, and necrosis are more frequently observed in SDC than in other salivary gland tumor subtypes (Fig. 1).^6,7 Calcification is also sometimes detected, but this may suggest the presence of a preexisting pleomorphic adenoma rather than SDC itself.⁶ Magnetic resonance imaging is another useful imaging modality with a high sensitivity and specificity.^8,9

FIGURE 1.

Contrast-enhanced computed tomography images of salivary duct carcinoma (SDC) in (A) the parotid gland and (B) the submandibular gland. SDCs (arrowheads) exhibit a heterogeneously enhanced mass with an ill-defined border. Occasional calcification and necrosis are noted.

Studies of SDC were previously limited to small cohorts or mixed populations with other subtypes of salivary gland carcinomas exhibiting different biologic behaviors. However, recent multi-institutional studies and the advent of new immunohistochemical and molecular techniques have better defined the biology of SDC and will likely affect future diagnostic and therapeutic options.

This review provides a comprehensive overview of these updates to raise awareness of SDC when making a cytological diagnosis.

HISTOLOGY

SDC is defined as an aggressive epithelial malignancy resembling high-grade mammary ductal carcinoma characterized by tumor cell proliferation with comedonecrosis and a cribriform growth pattern (Fig. 2A).² The tumor cells have large pleomorphic nuclei with conspicuous nucleoli and abundant eosinophilic cytoplasm (Fig. 2B). Although SDC histologically resembles breast carcinoma, it generally shows more marked cellular atypia and more mitoses than the latter.¹⁰ Several histologic variants of SDC have been reported, including mucin-rich, sarcomatoid, invasive micropapillary, oncocytic, and rhabdoid, but these histomorphological features essentially appear in combination with more typical histologic features of SDC (Fig. 2C).^11–15 Sarcomatoid, invasive micropapillary, and rhabdoid SDC are suggested to show a poorer prognosis than conventional SDC.^12,13,15 Approximately 20%−59% of SDC cases arise from preexisting pleomorphic adenoma (ex PA), and as such, SDC is the most common subtype of carcinoma ex PA (Fig. 2D). Contrary to a common misunderstanding, previous data have shown that there is no marked difference in the prognosis between SDC ex PA and de novo SDC.^3,16–18 Encapsulated and minimally invasive SDC ex PA were shown to exhibit a better prognosis than widely invasive SDC ex PA.^19,20 There is no consensus concerning the cutoff value for minimal invasion, and reported values have ranged from 1.5 to 6 mm beyond the fibrous capsule of PA.^19,20

FIGURE 2.

Histology and immunohistochemistry of salivary duct carcinoma (SDC). (A) Low-power view of an SDC tumor shows tumor cell proliferation with comedo-type central necrosis resembling breast ductal carcinoma. (B) Tumor cells of SDC have large nuclei with conspicuous nucleoli and abundant eosinophilic cytoplasm. (C) Sarcomatoid SDC. Abrupt transition from conventional SDC (left portion) to fascicular proliferation of highly atypical spindle cells is observed. (D) SDC ex pleomorphic adenoma. Ducts composed of atypical SDC cells (left portion) abut myxochondroid stroma of preexisting pleomorphic adenoma (right portion). (E) Invasive morphology of the tumor front. Many tumor buds (nests comprising <4 cells, indicated with arrows) and poorly differentiated clusters (nests comprising >5 cells, indicated with arrowheads) are observed. (F) Diffuse and strong immunohistochemical staining for androgen receptor (left panel) and human epidermal growth factor receptor 2 (right panel).

Although SDC is basically a highly malignant tumor, some histologic features are prognosticators independent of TNM stage.^3,10,21–24 Because no screening system for its early detection is available, most cases of SDC present at an advanced stage, and reported cases of noninvasive in situ SDC are limited.^25,26 The histomorphology of the invasive front is an important feature predicting its malignant potential; cases with a high number of tumor buds (tumor cell clusters composed of up to 4 cells) and poorly differentiated clusters (tumor cell clusters composed of more than 5 cells without gland formation) show a poorer prognosis than others (Fig. 2E).¹⁰ In a recent study, Nakaguro et al¹⁰ proposed a risk stratification model based on 4 histological features: prominent nuclear pleomorphism, mitoses, vascular invasion, and poorly differentiated clusters (Table 2).

TABLE 2.

Histologic Risk Stratification Model for Salivary Duct Carcinoma

Adverse prognostic factors

Prominent nuclear pleomorphism

≥30 Mitoses/10 high-power fields

Vascular invasion (H&E stain)

≥5 Poorly differentiated clusters

Total number of positive factors

0, 1: Low risk

2, 3: Intermediate risk

4: High risk

Adapted from Nakaguro et al.¹⁰

CYTOLOGY

Aspirates of SDC show overt nuclear features of high-grade carcinoma, making them readily identifiable by fine needle aspiration (FNA) as “malignant” in the Milan System for Reporting Salivary Gland Cytopathology (MSRSGC).^27,28 FNA specimens of SDC are composed of large tumor cells with abundant eosinophilic cytoplasm and large, round to oval atypical nuclei with prominent nucleoli. Tumor cells are either isolated or occur in cohesive 3-dimensional clusters that sometimes exhibit a papillary cytoarchitecture (Fig. 3A–D).^29,30 The background of the FNA may contain necrotic cellular debris reflecting the surgical pathology correlate of comedonecrosis (Fig. 3C). The cytological diagnosis of malignancy is straightforward when the sample is adequate; however, a specific diagnosis of SDC based solely on cytomorphologic features is not always possible. The differential diagnosis of SDC in FNA samples includes high-grade mucoepidermoid carcinoma; high-grade transformation of various primary salivary gland carcinomas; adenocarcinoma, not otherwise specified; and metastatic cancers from other anatomic sites.³⁰ The features of the histological variants of SDC can be observed in FNA (Fig. 3E).

FIGURE 3.

Fine needle aspiration of salivary duct carcinoma (SDC). (A) A low-power view shows 3-dimensional pseudopapillary clusters of tumor cells. (B) Tightly cohesive cluster with nuclear crowding. Tumor cells show nuclear pleomorphism suggestive of high-grade malignant tumor. (C) Loose cell clusters are observed in a necrotic background. Tumor cells have large pleomorphic nuclei with coarse chromatin and conspicuous nucleoli and abundant eosinophilic cytoplasm. (D) Isolated tumor cells in the aspirates have round to oval nuclei and abundant cytoplasm. Markedly enlarged nucleus, mutinucleation, and cannibalism are noted. (E) Aspirate from a sarcomatoid SDC tumor shows isolated atypical spindle cells on a hemorrhagic background. (F) Immunohistochemical staining of a formalin-fixed cell block specimen. Tumor cells express androgen receptor (left panel) and human epidermal growth factor receptor 2 (right panel).

One pitfall in the FNA diagnosis of SDC is SDC ex PA. As described in the Histology section, SDC is among the most common types of carcinoma ex PA. A cytologic diagnosis of malignancy is sometimes difficult in cases of carcinoma ex PA and the sensitivity is variable depending upon sampling of the malignant component (29%−80%).^31–33 In many cases of SDC ex PA, the SDC component will predominate to the near exclusion of any residual classical PA component, thus limiting the ability to recognize the aspirate as SDC ex PA versus a de novo SDC. However, this distinction is usually of less clinical importance, except perhaps for noninvasive examples. It is sufficient to recognize the aspirate as a high-grade carcinoma to guide the proper clinical management.

Most FNA cases of SDC should be readily diagnosed as high-grade carcinoma and classified appropriately using the MSRSGC; however, ancillary immunohistochemical studies on formalin-fixed paraffin-embedded cell block material can be used to more specifically classify the FNA as SDC. Characteristic immunohistochemical findings in the context of the corresponding high-grade cytomorphologic features are sufficient to render a diagnosis of SDC (Fig. 3F).^34–36

IMMUNOHISTOCHEMISTRY

Immunohistochemically, a strong and diffuse AR expression is a characteristic feature and a practical tool in the diagnosis of SDC (Fig. 2F), especially when paired with positive staining for GATA3 or GCDFP-15.^{16,22,37–39} Recent cohorts with a large number of patients showed AR expression in over 90% of cases, and the diagnosis of “AR-negative SDC” requires the careful exclusion of other entities, such as high-grade mucoepidermoid carcinoma and high-grade transformation of other subtypes of salivary carcinomas.^{18,22,37,40–47} The p63 expression is helpful for differentiating mucoepidermoid carcinoma from SDC, which is usually negative for p63.^48,49

Conversely, AR expression has been reported in approximately 10% of cases of salivary gland tumors other than SDC, including pleomorphic adenoma, adenoid cystic carcinoma, and acinic cell carcinoma; however, even if AR expression is present, it is weak to intermediate and focal. The reported sensitivities and specificities of AR expression for diagnosing SDC were 73% to 100% and 76% to 100%, respectively.^37,40,48 The diagnosis of SDC should be based primarily on the morphologic features with the support of AR immunohistochemistry.⁴⁰

By definition, SDC morphologically resembles invasive ductal carcinoma of the breast, and AR is also expressed in 85% to 100% of cases of invasive ductal breast carcinoma.^46,50 The differential diagnosis between SDC and metastatic carcinoma of breast origin can thus be challenging from histopathological aspects alone. The lack of estrogen receptor or progesterone receptor expression in SDC may be helpful in such cases.⁴⁶

In SDC, 16% to 83% of cases show human epidermal growth factor receptor 2 (HER2) expression by immunohistochemistry or HER2 amplification using FISH or next-generation sequencing (Fig. 2F).^{3,18,23,39–44,46,47,51–55} Although most benign salivary gland tumors are negative for HER2, some malignant tumors—including mucoepidermoid carcinoma and adenoid cystic carcinoma, acinic cell carcinoma, and squamous cell carcinoma—may show expression or amplification of HER2.^40,51 AR and HER2 expression is not specific for SDC; however, evaluation of AR or HER2 status is recommended to help guide therapeutic strategies.⁵⁶ The biomarker classification of SDC based on the AR and HER2 status and Ki-67 labeling index is reminiscent of that of breast carcinoma.^41,42 The system suggested by Takase et al⁴² is useful for predicting the patient survival or selecting appropriate therapy (Table 3). Other immunohistochemical markers of SDCs are listed in Table 4.^{16,23,38,39,41,42,46–48,52,57}

TABLE 3.

Revised Salivary Duct Carcinoma Classification Based on Biomarker Immunoprofiling

	HER2
	(+)	(−)
AR
(+)	Apocrine HER2	Apocrine A (Ki-67-low)
		Apocrine B (Ki-67-high)
(−)	HER2-enriched	Double negative

Abbreviations: AR, androgen receptor; HER2, human epidermal growth factor receptor 2.

A case was considered positive for AR when ≥20% of the tumor cell nuclei showed strong staining. A Ki-67 labeling index <40% was considered Ki-67–low, while ≥40% was Ki-67–high.

Adapted from Takase et al.⁴²

TABLE 4.

Immunohistochemical Findings in Salivary Duct Carcinoma

Markers	Positive Cases
AR	73%–100%
HER2 (IHC or FISH)	16%–83%
GATA3	73%–100%
GCDFP-15	76%–100%
CEA	71%–72%
CK5/6	14%–76%
EGFR	12%–33%
p53 (extreme negative or positive)	42%
p63	Usually negative
Average Ki-67 labeling index	35%–46%

Abbreviations: AR, androgen receptor; CEA, carcinoembryonic antigen; EGFR, epidermal growth factor receptor; FISH, fluorescence in situ hybridization; HER2, human epidermal growth factor receptor 2; IHC, immunohistochemistry.

Regarding the relationship between immunohistochemical markers and prognosis, an improved prognosis in patients with AR and its coregulatory protein forkhead box protein A1 (FOXA1) expression has been reported,^42,58 but this finding was not observed in another large cohort.¹⁸ Although HER2 status does not affect patient survival, p53 extreme negative/positive staining (null or diffuse strong expression), which suggests the presence of a TP53 point mutation, and CK5/6 staining are significantly associated with a worse prognosis.^18,42 “Apocrine A” type SDC (AR+, HER2-, Ki-67 low) on biomarker classification (Table 3) showed a better prognosis than other SDC tumors.⁴²

The expression of PD-L1 in SDC has been evaluated in some studies; however, the clones of PD-L1 antibodies and the evaluation criteria varied from study to study, and the rate of positive cases ranged from 6% to 50%.^59–64 The association between PD-L1 expression and prognosis were inconsistent between studies,^60,64 and further investigations are therefore warranted.

MOLECULAR BIOLOGY

An increasing number of studies have reported the molecular characteristics of SDC. In addition to HER2 amplification, various hotspot mutations and fusion genes have been detected (Table 5).^{43,59,65–73} In comparison with other subtypes of salivary gland carcinomas, the results are heterogeneous, and most of the recurrent mutations are well-known oncogenes or tumor suppressor genes whose mutations are also observed in malignancies of other organs. Regarding the results of next-generation sequencing, because the panels, threshold, and analysis and validation methods have differed among studies, the assessment and comparison of the true mutation frequency is difficult, but TP53, PIK3CA, HRAS, and PTEN mutations are commonly and described frequently (Table 5). TP53 mutations, including missense and truncating mutations, and PI3K/AKT pathway activation evaluated by the p-Akt expression are associated with poor prognosis.^67,74

TABLE 5.

Genetic Alterations Reported in Salivary Duct Carcinoma

Pathway	Genes
Gene mutation
DNA damage	TP53 (38%–68%), ATM
MAPK	HRAS (13%–27%), BRAF (3%–13%), KRAS, NRAS
RTK	HER2 (7%–20%), EGFR, HER4, RET, PDGFRA, KDR, FGFR1, FLT3
PI3K/AKT	PIK3CA (18%–53%), PTEN (10%–53%), AKT1 (1.5%–19%)
Cell cycle	CDKN5A, FBXW7, RB1
Wnt	APC, CTNNB1
Other	ALK (2.7%–7%), KMT2C, ABL1, CDH1, SMO, VHL, STK11, PTPN11, SMAD4, NOTCH1, FOXA1
Amplification	HER2 (30%−40%), NOTCH1, AKT1, SMARCA4
Fusion gene	ETV6-NTRK, BCL6-TRADD, ABL1-PPP2R2C, CTNNB1-PLAG1, LIFR-PLAG1, NCOA4-RET, HNRNPH3-ALK

Abbreviations: MAPK, mitogen-activated protein kinases; PI3K, phosphoinositide 3-kinase; RTK, receptor tyrosine kinases.

Values within parentheses indicate the incidence of genetic alterations.

Interestingly, the genetic changes observed specifically in certain subtypes of salivary gland carcinoma, such as the HRAS mutations in epithelial-myoepithelial carcinoma,⁷⁵ CTNNB1 mutation in basal cell adenoma/adenocarcinoma,^76–78 BRAF mutation in sialadenoma papilliferum,⁷⁹ ETV6-NTRK3 fusion gene in secretory carcinoma,⁸⁰ and NCOA4-RET in intraductal carcinoma,⁸¹ have also been detected in SDC without any histologic evidence of these tumor subtypes.^65,70 The presence of subtype-specific genetic changes in SDC may suggest the origin of SDC, but evidence supporting this hypothesis remains sparse.

SDCs are reported to be microsatellite-stable.^59,63 The overall tumor mutational burden (TMB) is 1.7 to 2.5 mutations/Mb, and a high TMB has been reported in only a small number of cases.^59,70,82 The overall TMB value is higher than that in adenoid cystic carcinoma but close to that of breast, pancreas, prostate, and kidney cancers.^70,73,82

TREATMENT

General Aspects

The rarity of SDC has made it impossible to conduct large definitive randomized trials to establish standard approaches to treatment. The current treatment options and trends for patients with SDC have been reviewed by various researchers.^83–87 Because there is a considerable variety in the treatment of SDC between institutions, we herein mainly discuss the contents of the recent updates to the National Comprehensive Cancer Network (NCCN) guidelines.⁵⁶ The major therapeutic approach for SDC is adequate and appropriate surgical resection. Elective neck dissection is recommended for patients with neck metastases and T3–4 patients without neck metastasis. Postoperative radiation therapy is an effective and appropriate therapeutic option according to the NCCN guidelines, regardless of the T stage and margin status.^56,88–90 For recurrent unresectable and/or metastatic SDC, there are no randomized trials that have established the most effective cytotoxic chemotherapy, but smaller trials have shown. overall response rate of taxane (paclitaxel or docetaxel) plus platinum (carboplatin) chemotherapy ranging from 39% to 50% for SDC.^91,92 Other chemotherapeutic agents have also been studied, including taxanes, platinum, cyclophosphamide, doxorubicin, gemcitabine, vinorelbine and eribulin; however, these study cohorts included other carcinoma subtypes, so the results cannot be simply applied to SDC.^93–98

AR- and HER2-Targeted Therapy

Based on biomarker profile of SDC, treatments targeting AR and HER2 have been introduced. Small cohort studies have provided promising results for targeted therapy in SDC, including recent data from a larger cohorts comprising more than 35 patients with histological reviews from Japan and the Netherlands.^99–102 These data have confirmed the efficacy of targeted therapy, and the latest NCCN guidelines include AR therapy and anti-HER2 therapy as therapeutic options for AR-positive and HER2-overexpressing tumors, respectively.⁵⁶ The NCCN guidelines further recommend the evaluation of the AR and HER2 status for SDC and adenocarcinoma, not otherwise specified.⁵⁶

Drugs used for androgen deprivation therapy include AR blockers (eg, bicalutamide) and luteinizing hormone-releasing hormone analogues (eg, leuprolide). The response rate for advanced or recurrent patients was shown to be 17% to 65%, and median overall survival was 17 to 44 months.^{99,101,103,104} AR-V7, an AR therapy–resistant splicing variant of AR in prostate cancer, is also expressed in 15% to 50% of SDC patients, but the relationship between the expression of AR-V7 and AR therapy resistance has not been proven in SDC.^59,70

Protocols of HER2-targeted therapy for HER2-overexpressing SDC tumors are based on those for breast cancer treatment. Combination therapy of an anti-HER2 antibody (trastuzumab, ado-trastuzumab emtansine, or pertuzumab) with conventional taxane-based chemotherapy increased the response rates and time to disease progression. The response rates ranged from 67% to 100%, and the median progression-free survival was 3 to 18 months.^{54,55,100,105–108}

Immune Checkpoint Inhibitor Therapy and Other New Treatment Options

In SDC, the predictive biomarkers for immune checkpoint inhibitors indicate a small potential for achieving therapeutic benefit.⁵⁹ Most SDC cases are microsatellite-stable and have a low TMB.^{59,63,70,73,82} There are no consistent data on PD-L1 expression.^59–64 A study of immune checkpoint inhibitor (pembrolizumab) treatment for advanced salivary gland carcinoma reported a 12% response rate, and another study of pembrolizumab included 1 SDC case in the cohort.^109,110 As mentioned above, recent molecular analyses have revealed additional “druggable” genetic alterations, including PIK3CA, HRAS, BRAF, PTEN, NTRK, and RET. Basket trials targeting these molecules have shown therapeutic activity in some salivary gland tumors.^65,111–114 It may therefore be worthwhile for recurrent and/or metastatic SDC patients to undergo molecular testing to identify pertinent targeted therapies and consider their suitability for enrollment in basket trials, such as the NCI-MATCH Trial.¹¹⁵

CONCLUSIONS

The cytologic and histopathologic diagnosis of primary salivary gland tumors is often a challenging issue for general pathologists because of their histomorphologic diversity, the plethora of histologic subtypes, and their low incidence (<1% of malignancies from all body sites).¹¹⁶ Recent research on SDC has improved our understanding of its biology and clinicopathologic characteristics, which has led to more accurate diagnosis of SDC and has resulted in new treatment strategies, although some are not yet globally available. Considering the aggressiveness of SDC, future basic and clinical studies will be useful for the development of more effective therapeutic strategies.