The World Health Organization classification of tumors of the breast recognizes several special type carcinomas and benign lesions with features comparable to those of salivary gland tumors.
To discuss the histologic, immunophenotypic, molecular, and clinical features of salivary gland–like carcinomas of the breast. These breast tumors are often negative for hormone receptors and human epidermal growth factor receptor 2 (HER2), that is, triple-negative, but they generally have a much better prognosis than triple-negative breast carcinomas of no special type. We compare the immunophenotypic, molecular, and clinical features of these breast tumors with their salivary gland counterparts, highlighting similarities and differences. We also discuss benign salivary gland–like breast tumors. Finally, we highlight recent developments in understanding the molecular pathogenesis of these breast tumors and novel ancillary studies that can be used to support their diagnosis.
A literature review was conducted, and papers were selected for further analysis and discussion by the authors of this review based on their novelty, applicability, and impact in the field.
Breast tumors that exhibit morphologic overlap with salivary gland tumors have been recognized by pathologists for decades, but the similarities and differences in their molecular pathogenesis have not been understood until more recently. These developments have led to novel diagnostic tools and further knowledge of these rare breast lesions.
The World Health Organization (WHO) recognizes several special type breast carcinomas that share morphologic, immunophenotypic, and molecular genetic features with salivary gland carcinomas. These breast carcinomas are typically negative for estrogen receptor (ER), progesterone receptor (PR), and human epidermal growth factor receptor 2 (HER2; “triple-negative”), yet generally have a much better prognosis than triple-negative breast carcinomas of no special type. These special type salivary gland–like breast carcinomas include adenoid cystic carcinoma (AdCC), secretory carcinoma (SC), mucoepidermoid carcinoma (MEC), and polymorphous adenocarcinoma (PAC).1 This classification also includes acinic cell carcinoma (AciCC), which deserves separate mention as a breast carcinoma that shares some morphologic similarities with AciCC of the salivary gland, but, based on recent evidence, appears to have a different molecular pathogenesis.2,3 The breast WHO classification also includes 2 other salivary gland–like tumors, pleomorphic adenoma (PA) and adenomyoepithelioma (AME), under combined epithelial-myoepithelial tumors; the salivary gland counterparts are PA and epithelial-myoepithelial carcinoma (EMC), respectively; notably, a large proportion of the latter arise ex PA in the salivary gland. We highlight the clinical, pathologic, and molecular genetic features of these breast tumors, with a special emphasis on novel diagnostic tools.
ADENOID CYSTIC CARCINOMA
Histologic and Immunohistochemical Features
AdCC is a special type of breast carcinoma and a well-recognized type of salivary gland carcinoma characterized by its biphasic morphology, meaning that the invasive carcinoma consists of both a luminal epithelial cell component and a basaloid/myoepithelial cell component. The luminal component forms true ductal structures with epithelial cell polarization around extracellular lumina and occasional intraluminal secretions (Figure 1, A through C). The basaloid/myoepithelial cell component forms pseudolumina, with the cells surrounding basement membrane material and/or myxoid extracellular matrix (Figure 1, A through C). The cells of the luminal component show immunohistochemical staining for low–molecular weight (luminal-type) cytokeratins (eg, CK7; Figure 1, D) and KIT (Figure 1, E), and the cells of the basaloid/myoepithelial component show staining for basal epithelial and myoepithelial markers (eg, CK 5/6, p63, smooth muscle actin; Figure 1, F).4,5 In reality, AdCCs are composed of cells with a broad spectrum of differentiation and a range of expression of immunohistochemical markers. At one end of the spectrum are cells that exhibit the histologic and immunohistochemical features of fully differentiated luminal epithelial cells; at the other end of the spectrum are cells that show all the features of basaloid epithelial/myoepithelial cells. In addition, many AdCCs show cells with hybrid features. This spectrum of differentiation results in variable staining of lineage-related markers in histologically similar cells (eg, morphologically basaloid/myoepithelial cells that show little or no staining for some or all myoepithelial or basal epithelial markers, and luminal cells that show variable staining for high–molecular weight/basal cytokeratins).
Classic AdCCs exhibit a variety of architectural patterns, including tubular, cribriform, trabecular, and solid patterns, often in combination (Figure 1, A through C). These tumors in the breast are associated with a good prognosis, with a low risk of lymph node and distant metastasis and low risk of recurrence after excision.6 Despite it being a triple-negative carcinoma, there is no clear benefit to the use of chemotherapy.7 In contrast, AdCC in the salivary gland is a locally aggressive malignancy, shows frequent perineural invasion, and can have a protracted clinical course with late recurrences and distant metastases.8
The solid variant of AdCC with basaloid features has been recognized in the breast as a morphologic variant where the invasive component consists predominantly of basaloid/myoepithelial cells but with a haphazard, solid, and infiltrative growth pattern and a lack of cribriform architecture (Figure 1, G and H). Nevertheless, at least focal lumen formation or focal expression of CK7 reveals such lesions as AdCC.9 The solid-basaloid variant is considered to have more aggressive clinical behavior than classic AdCC, with higher rates of lymph node involvement and distant recurrence, and shorter interval to recurrence.6,10 The WHO also recognizes AdCC with high-grade transformation, in which case a high-grade triple-negative breast carcinoma develops in association with an underlying AdCC.1
In the salivary gland, AdCC is graded based on histologic growth pattern,11 with the presence of solid growth associated with a poor prognosis12,13 ; AdCC with high-grade transformation (AdCC associated with other histologic patterns of pleomorphic, high-grade carcinoma) can also occur in the salivary gland.11 It has been proposed that breast AdCC also be graded according to the presence of solid growth or additional patterns of high-grade carcinoma, because of the associations with clinical behavior in this entity: classic AdCC would be grade 1, the solid-basaloid variant would be grade 2, and AdCC with high-grade transformation would be grade 3.14 However, this grading system has not been universally accepted. Some authors distinguish breast AdCC with a basaloid component of greater than 50% from those in which the basaloid component is less prominent because those with greater than 50% basaloid component have a significantly higher frequency of lymph node and distant metastases and other adverse features compared with conventional AdCC.6 How best to grade AdCC of the breast is an unresolved issue (ie, whether to grade these tumors using the salivary gland AdCC grading system or whether to use the Nottingham combined grading system used for other types of invasive breast carcinomas). However, when the Nottingham system is used to grade breast AdCC,7 classic AdCC tends to be of lower grade than the solid-basaloid variant.10
AdCCs are associated with recurrent genetic alterations, most frequently t(6;9) MYB::NFIB fusions15 and more rarely MYBL1 rearrangements or MYB gene amplification,16 leading to MYB overexpression. The MYB oncogene signaling program promotes breast cancer development.17 In addition, the solid variant with basaloid features has been associated with recurrent NOTCH1 and CREBBP mutations18 in addition to MYB alterations.19
MYB protein overexpression by immunohistochemistry (IHC) can be a useful surrogate for MYB rearrangement or amplification and can be used to support the diagnosis of AdCC in concert with characteristic morphologic and immunohistochemical staining features (Figure 1, I).20 Although KIT can be positive in AdCC (Figure 1, E), sensitivity and specificity are limited. Because MYB immunohistochemistry has some limited sensitivity and staining may be seen in several other carcinomas with basaloid morphology, it may be helpful to perform confirmatory molecular testing, such as fluorescence in situ hybridization (FISH) studies or sequencing-based techniques targeting the fusion.21 RNA in situ hybridization (ISH) may also be employed to detect MYB overexpression.22
Histologic and Immunohistochemical Features
SC is a rare special type breast carcinoma with characteristic morphologic features. The invasive carcinoma consists of tubules, cords, and nests of infiltrative epithelial cells with abundant eosinophilic and vacuolated cytoplasm and associated brightly eosinophilic extracellular secretions,23 often in microcystic and macrocystic spaces in the tumor (Figure 2, A and B). SC was originally described in children (so-called juvenile carcinoma)24 ; however, this tumor can occur at any age. SC is typically triple-negative but can express low levels of hormone receptors. The tumor cells can express SOX10, mammaglobin, and gross cystic disease fluid protein-15.25 SC of the salivary gland has morphologic and immunophenotypic features similar to those of SC of the breast.26,27 In the salivary gland, MUC4 has been shown to be helpful in distinguishing SC from its morphologic mimics.28 In the breast, SC is considered an indolent tumor with low rates of distant metastasis and excellent long-term survival, despite the triple-negative phenotype.1 Similarly, SC of the salivary gland is considered a low-grade malignancy with an excellent prognosis in most cases.29,30
SC in both breast and salivary gland is associated with a recurrent translocation t(12;15), resulting in an ETV6::NTRK3 fusion.31 This fusion gene is thought to contribute to pathogenesis through activation of growth signaling pathways.32,33 Diagnostic assays interrogating this fusion have been developed. FISH testing for ETV6 rearrangement is highly sensitive and specific for the diagnosis of SC in the breast.34 More recently, IHC using an antibody that recognizes an epitope common to TRKA, TRKB, and TRKC (pan-TRK) has been employed as a surrogate marker for TRKC overexpression driven by the fusion. Strong, diffuse, nuclear positivity (with or without cytoplasmic positivity) is a surrogate for the ETV6::TRKC fusion protein and supports the diagnosis of SC along with other morphologic features (Figure 2, C and D).35 IHC for pan-TRK can be used as a screening tool to identify cancers that may harbor NTRK fusion.36 Identification of a TRK fusion suggests the carcinoma may be sensitive to TRK inhibitors, a class of targeted therapy that has been approved in the advanced or metastatic setting by the US Food and Drug Administration regardless of histologic type of cancer or primary site.37–39 Finally, as with AdCC, next-generation sequencing for the fusion RNA transcript or DNA fusion may be of value in selected cases or clinical situations.40
Histologic and Immunohistochemical Features
MEC is a rare breast carcinoma, representing an estimated 0.2% to 0.3% of breast carcinomas.41 In contrast, MEC is the most common histologic type of malignancy of the salivary gland.42,43 Characteristically, MEC shows multiple different cell types within the tumor, in various proportions, including epidermoid/squamoid cells, intermediate cells, basaloid cells, and mucous cells (Figure 3, A).44 The mucous cells show a prominent mucin-rich cytoplasm that can be highlighted by a mucicarmine stain (Figure 3, B). True keratinization is not seen41 ; the epidermoid/squamoid cells of MEC should be contrasted with squamous cell metaplastic breast carcinomas and low-grade adenosquamous carcinomas, which have a different appearance and can show true keratinization.1 MEC is usually negative for ER, PR, and HER2 and positive for both high–molecular weight cytokeratins (eg, CK 5/6, CK14, CK17) in the basaloid and intermediate cells, and low–molecular weight cytokeratins (eg, CK7) in the intermediate, epidermoid/squamoid, and mucous cells.44 The epidermoid/squamoid, intermediate, and basaloid cells are positive for squamous/basal-like markers p63 and p40 (Figure 3, C). Salivary MEC has several proposed grading systems that incorporate the proportion of cyst formation, growth pattern, nuclear pleomorphism, mitotic count, necrosis, and bone or neural invasion, etc45–47 ; these grading systems, as well as the conventional Nottingham system for breast carcinoma grading, have been used for breast MEC.44 Compared with triple-negative breast carcinoma of no special type, the prognosis is generally good: there were no deaths from disease among all 19 previously published low-grade breast MECs, whereas 5 of 16 patients with high-grade breast MECs developed distant metastasis and died of disease,41 suggesting that low-grade breast MEC may have an indolent course and high-grade breast MEC may behave more aggressively. MEC in the salivary gland shows similar stratification of behavior based on grade.
MEC in both the breast and salivary gland is characterized by a recurrent translocation, t(11;19)(q21;p13) forming a CRTC1::MAML2 fusion,48 which almost always is seen in low-grade MEC.49 MAML2 FISH49 or molecular diagnostic assays targeting the fusion RNA transcript50 can be performed to confirm a rearrangement involving this locus. This fusion gene is not typically found in other triple-negative breast carcinomas. In addition, MEC typically has a low mutational burden with few other alterations,50 supporting that the fusion gene is sufficient for pathogenesis. There is currently no specific immunohistochemical surrogate for this fusion gene.
ACINIC CELL CARCINOMA
AciCC deserves special mention as an entity in the breast WHO classification that has an identically named carcinoma in the salivary gland, but the 2 entities share only some morphologic features and harbor different molecular underpinnings, unlike the foregoing carcinomas, for which the breast and salivary gland analogs are convincingly related by morphology and shared recurrent genetic alterations.
Histologic and Immunohistochemical Features
AciCC of the breast has distinctive morphologic features, consisting of a haphazard infiltrative proliferation of small glands and solid nests of varying size, with the epithelial cells of the invasive carcinoma demonstrating eosinophilic cytoplasm and a variable degree of fine and coarse eosinophilic cytoplasmic granules and brightly eosinophilic luminal secretions (Figure 4, A and B). Ultrastructural studies demonstrated that these cytoplasmic granules are electron dense and have features consistent with zymogen granules.51 Some cases demonstrate larger, densely eosinophilic granules, resembling Paneth cells (Figure 4, B). AciCC of the breast is typically triple-negative, and it is characteristically positive for S100 protein and variably positive for markers such as α1 antitrypsin, α1 antichymotrypsin, and lysozyme (Figure 4, C and D). Many of the morphologic and immunophenotypic features overlap with microglandular adenosis and atypical microglandular adenosis.52 In contrast, salivary gland AciCC has a distinctive tumor cell population, with basophilic-to-amphophilic small, compact cytoplasmic granules that are PAS positive and diastase resistant, reminiscent of the zymogen granules seen in normal salivary acini (Figure 4, E). These characteristic basophilic tumor cells are admixed with a heterogeneous tumor cell population (including cuboidal, glandular, vacuolated, or clear cell morphology), which can vary from tumor to tumor; cells with Paneth cell–like morphology are not a feature. Salivary AciCC is typically positive for basal-like markers such as SOX10 and positive for DOG1, a calcium-activated chloride channel and marker of acinar differentiation.53
Salivary AciCC, when of low histologic grade, is generally considered an indolent tumor with slow growing behavior but the potential to be locally aggressive, depending on involvement of adjacent structures, or it can undergo high-grade transformation.54 Similarly, in its pure form, breast AciCC is generally indolent, but in some cases is associated with aggressive, high-grade breast carcinomas of no special type (ie, conventional triple-negative breast carcinoma) or metaplastic carcinoma, both of which have aggressive clinical behavior with a propensity for lymph node and distant metastasis.2
More recently, a recurrent t(4;9) translocation has been described in salivary AciCC, leading to translocation of the NR4A3 locus with the active enhancers of the secretory calcium-binding phosphoprotein gene cluster, and constitutive overexpression of NR4A3, which is oncogenic in salivary acini.3 NR4A3 overexpression (facilitated by enhancer hijacking) can be detected by immunohistochemistry in salivary AciCC (Figure 4, F)55 and is more sensitive and specific than FISH for NR4A3 rearrangement, and other translocations can also occur, such as those involving a related orphan nuclear receptor NR4A2.56 In contrast, breast AciCC shows recurrent mutations more similar to triple-negative breast carcinoma of no special type, such as frequent TP53 and PIK3CA mutations and high levels of chromosomal instability and copy number variants,2,57 alterations that are also seen in examples of microglandular adenosis.58–61 Moreover, breast AciCC is consistently negative for NR4A3 overexpression and largely negative for DOG1.62 Thus, the evidence suggests that breast AciCC is unique among this group of entities as a carcinoma that has key morphologic, immunohistochemical, and molecular genetic differences from salivary AciCC, suggesting that these are unrelated entities. The similarities between breast AciCC and microglandular adenosis suggest relatedness between these breast entities63 and morphogenesis distinct from that of salivary AciCC.
Histologic and Immunohistochemical Features
PA of the breast is a rare benign tumor with epithelial and myoepithelial components, morphologically resembling the very common benign salivary gland counterpart. The term pleomorphic in the name refers to the tendency of this tumor to have a wide range of morphologic appearances of the tumor cells and growth patterns, both from one patient to another and within a given tumor. The 2 cellular components are organized in cytologically bland glandular and ductal structures enveloped by chondromyxoid stroma (Figure 5, A through C), which can occasionally exhibit overt cartilaginous or osseous metaplasia.64,65 The immunohistochemical staining pattern recapitulates the underlying cells of origin: myoepithelial cells are positive for markers such as smooth muscle actin and S100 protein, whereas the luminal cells are positive for low–molecular weight cytokeratins, EMA, and ER.64 Some PAs of the breast have been found in association with or seeming to arise from an underlying intraductal papilloma (Figure 5, A), suggesting these entities may be related or exist on a spectrum.66
PA in the salivary gland is characterized by rearrangements involving PLAG1 or HMGA2 genes, and both FISH for these rearrangements as well as IHC to detect overexpression of their protein products can be employed for the diagnosis.67,68 A subset of breast PAs has been shown to harbor PLAG1 or HMGA2 rearrangements,69,70 suggesting related molecular pathogenesis in certain cases.
PA in the salivary gland is a common benign tumor, although there is a well-recognized but rare tendency for some PAs to undergo malignant transformation to carcinoma ex-PA. Carcinoma ex-PA comprises a heterogeneous group of morphologic types, including EMC, myoepithelial carcinoma, and salivary duct carcinoma. Salivary duct carcinoma is a high-grade carcinoma with apocrine morphology, growth patterns resembling high-grade ductal carcinoma in situ, necrosis, frequent expression of androgen receptor, and occasional overexpression of HER2.71 Salivary gland carcinomas ex-PA of all types also demonstrate PLAG1 or HMGA2 rearrangements (in addition to other genomic alterations associated with malignant progression, such as TP53 mutations or copy gain of ERBB272) and/or foci of precursor PA. The WHO classification of breast tumors considers PA to be a benign entity.1 However, breast PA may recur locally if not completely excised,73 and rare cases of carcinoma ex-PA have been reported in the breast.74 Some authors consider breast PA to be of uncertain malignant potential given these possible outcomes and the rarity and lack of data on this lesion.75
AME AND EPITHELIAL-MYOEPITHELIAL CARCINOMA
Histologic and Immunohistochemical Features
AME in the breast is a biphasic neoplastic proliferation of epithelial and myoepithelial cells, with the myoepithelial component exhibiting prominent proliferation (Figure 6, A through D). Malignant transformation can occur in breast AME, so-called malignant AME, and can involve only the epithelial component, in which case it is characterized by the resultant histologic type of epithelial-derived breast carcinoma—that is, only the myoepithelial component (ie, myoepithelial carcinoma), or both (ie, EMC; Figure 6, E and F).1 In the salivary gland, EMC is composed of populations of eosinophilic ductal cells and myoepithelial cells (which frequently show abundant cleared cytoplasm) arranged in characteristic bilayered ductal formation, but it can show varied architectural features, including pseudopapillary growth, cyst formation, and myoepithelial cell overgrowth. Salivary EMC is generally considered a low-grade malignancy with a favorable prognosis, but there is no recognized equivalent of a benign AME in the salivary gland; some salivary EMCs may arise ex-PA.71 Most cases show expansile and infiltrative growth, with appreciable nuclear enlargement of epithelial cells (distinguishing them from normal ductal cells) and mitotic activity in all cell components; notably, some cases presenting as a single nodule may be difficult to distinguish from PA.
Breast AMEs are characterized by recurrent activating mutations in genes involved in phosphatidylinositol-3-kinase/AKT pathway signaling,76–78 and some also show HRAS mutations. It has been suggested that PIK3CA or AKT1 activating mutations are characteristic of ER-positive AME, whereas HRAS p.Q61 hotspot mutations are found in ER-negative AME.79 HRAS mutations are exceedingly rare in other types of breast carcinoma, and it has been suggested that this driver mutation is specific to ER-negative AME and malignant AME.79 Salivary gland EMC is characterized by recurrent HRAS mutations, suggesting a relationship to ER-negative breast AME80 ; a RAS p.Q61R mutation-specific antibody has been investigated as a surrogate marker for this recurrent alteration and adjunctive diagnostic tool in salivary EMC81 and in breast AME.82
PAC AND OTHER RARE TUMOR TYPES
PAC is an extremely rare type of breast carcinoma, with only 4 known published cases. It is described as having morphologic features similar to its salivary gland counterpart: a monotonous population of tumor cells with a variety of infiltrative growth patterns, including solid, trabecular, tubular, cribriform, and corded growth. The stroma is collagenous, with production of basement membrane material by the neoplastic cells.83 Tumor cells are typically negative for CK7 and positive for p63, and they may be variably positive for Bcl-2; tumor cells are negative for ER, PR,83 and HER2.84
In the salivary gland, a subset of PAC is recognized to have distinct morphologic features, with tumor cells having optically clear nuclei and more cribriform and solid growth with glomeruloid structures, termed cribriform adenocarcinoma of salivary gland (CASG) by some authors. Alterations involving PRKD genes are common in both subtypes.85,86 The p.E710D hotspot activating mutation in PRKD1 is most common in conventional PAC, whereas gene rearrangements involving PRKD1, PRKD2, or PRKD3 are seen in CASG; however, there can be overlap, and 7% of conventional PACs were reported to have a gene fusion, whereas 13% of CASGs have a point mutation.87 Furthermore, 1 large multi-institutional study showed only fair-to-moderate interobserver agreement between experienced head and neck pathologists in classifying conventional PAC and CASG.87 Only one breast PAC has been analyzed for genomic alterations by next-generation sequencing in the literature; no PRKD alteration was reported, although alterations were seen in KDR, ESR1, FGFR3, KEAP1, MAP3K1, MET, PDGFRA, SF3B1, SMO, and TP53.84
As with PAC, there are other tumor types that are well described in the salivary gland, such as Warthin tumor and basal cell adenoma/basal cell adenocarcinoma,71 but are exceedingly rare in the breast (there are few case reports of basal cell adenoma88 or basal cell adenocarcinoma89 occurring in the breast, but no evident publications relating to Warthin tumor in the breast). The reasons for these differences in distribution of these tumor types are unclear.
There is a rare entity recognized in the salivary gland, intraductal carcinoma, which demonstrates papillary, cribriform, and solid patterns of epithelial cell proliferation confined to the ducts, with myoepithelial cells surrounding the involved spaces; concurrent invasive carcinomas may occur alongside an intraductal component.71 The epithelial cells may have amphophilic, apocrine, or oncocytic cytomorphology; although some cases may resemble salivary duct carcinoma, these are considered distinct entities.71 Rearrangements involving RET are frequent.90 In some cases with an invasive component, the same RET rearrangement was found throughout the lesion.91 Apart from these rare examples, salivary intraductal carcinoma is not seen with any regularity in association with salivary carcinomas. In contrast, breast ductal carcinoma in situ is common and often screen detected, and it is a well-known precursor of invasive breast carcinoma.1
Several tumors can occur in both the breast and salivary gland and exhibit overlapping morphologic features, protein expression profiles, and often have shared molecular pathogenesis. These entities can sometimes be in the differential diagnosis of each other or have overlapping features with other types of breast carcinoma, especially triple-negative breast carcinomas. Therefore, recognition of their unique features and use of immunohistochemical surrogates for gene rearrangements, or probing directly for known recurrent genetic alterations, can facilitate the diagnosis. Accurate classification of these tumors can have important prognostic and treatment implications.
The authors have no relevant financial interest in the products or companies described in this article.