Adenoid cystic carcinoma of breast: Recent advances

Abstract

Adenoid cystic carcinoma (ACC) of the breast is a rare special subtype of breast cancer characterized by the presence of a dual cell population of luminal and basaloid cells arranged in specific growth patterns. Most breast cancers with triple-negative, basal-like breast features (i.e., tumors that are devoid of estrogen receptor, progesterone receptor, and human epidermal growth factor receptor 2 expression, and express basal cell markers) are generally high-grade tumors with an aggressive clinical course. Conversely, while ACCs also display a triple-negative, basal-like phenotype, they are usually low-grade and exhibit an indolent clinical behavior. Many discoveries regarding the molecular and genetic features of the ACC, including a specific chromosomal translocation t(6;9) that results in a MYB-NFIB fusion gene, have been made in recent years. This comprehensive review provides our experience with the ACC of the breast, as well as an overview of clinical, histopathological, and molecular genetic features.

Key Words: Adenoid cystic carcinoma, Breast, Triple-negative and basal-like phenotype, Histology, Molecular genetic features

Core tip: Adenoid cystic carcinoma (ACC) of the breast is a rare, special subtype of breast cancer characterized by the presence of luminal and basaloid cells arranged in specific growth patterns. Although ACCs display a triple-negative, basal-like phenotype, these tumors are usually low-grade and exhibit an indolent clinical behavior. Many discoveries regarding the molecular genetic features of the ACC, including a specific chromosomal translocation t(6;9) that results in a MYB-NFIB fusion gene, have been made in recent years. This review provides our experience with ACCs, as well as an overview of its clinical, histopathological, and molecular genetic features.

INTRODUCTION

Invasive breast carcinoma comprises a heterogeneous group of tumors with various clinical, morphologic, and molecular genetic features[1,2]. According to the 2012 World Health Organization classification, invasive ductal carcinoma of no special type (NST) is the most common histologic type, accounting for up to 75% of all invasive breast carcinomas[3]. The remainder of the invasive cancers represent at least 18 different special and rare histomorphologic subtypes, including adenoid cystic carcinoma (ACC), a salivary gland-type of breast carcinoma[3].

A characteristic histologic pattern of ACC of the breast includes both epithelial and myoepithelial components and resembles a well-known tumor of the salivary gland origin known by the same name. However, patients diagnosed with ACC of the breast have a better prognosis than those who are diagnosed with ACC of the salivary gland[4-6]. ACC of the breast belongs to the basal-like subgroup of breast cancers[7-9]. Based on extensive molecular and genetic profiling studies, basal-like tumors are most often hormone receptor [estrogen receptor (ER) and progesterone receptor (PR)] negative, do not express human epidermal growth factor receptor 2 (Her2), but express one or more basal/myoepithelial cell markers [e.g., cytokeratins (CKs) 5, 5/6, 14 and 17][10]. Unlike other triple-negative breast cancers that are associated with poor prognosis, ACC has an overall excellent prognosis[11]. Because of these distinct clinicopathologic features that set it apart from the other triple-negative breast cancers, an understanding of ACC of the breast is essential for surgical pathologists, breast surgeons, and oncologists. This review will focus on ACC of the breast and will outline important updates in its epidemiology, clinical features, histomorphologic/immunohistochemical characteristics, molecular genetic features, and prognosis/treatment. In addition, we will address our team’s experience with this clinical entity.

EPIDEMIOLOGY

ACC is an uncommon subtype of invasive breast carcinoma and accounts for less than 0.1% of all primary carcinomas of the breast[3,12,13]. Recently, several independent studies based on large patient cohorts have provided more insight into its epidemiology and clinical characteristics[11,14-20]. This information, in the recent studies published in 2010 and after, is summarized in Table 1. The reported age distribution for patients diagnosed with ACC of the breast ranges from 38 to 81 years (with a median age of 60 years; Table 1) and is similar to that seen in other invasive breast cancer cases[3]. Moreover, a previous case series of 338 patients with ACC of the breast conducted over a 30-year period identified its age-adjusted incidence ratio (AAIR) to be 0.92 per 1 million person-years. The AAIR remained constant during the 30-year period and was 39%, lower in African-Americans than in Caucasian-Americans[11]. Most cases are in females, but occasional cases have been reported in male patients[21,22].

Table 1 Clinical characteristics of adenoid cystic carcinoma of the breast in recently reported cohorts.

Ref.	No. of patients’		PathologicT1 or T2	Lymph nodeinvolvement	Distantmetastasis	Survival
	Cases	Age (yr)
Kulkarni et al[14]	933	60 (median)	Not reported	5.1%	Not reported	88% (5 yr)
Coates et al[15]	376	62 (mean)	90%	6.1%	1.1% (site not specified)	90% (10 yr)
Ghabach et al[11]	338	63 (mean)	95%	1.7%	< 1% (site not specified)	94.9% (10 yr)
Thompson et al[16]	244	62 (median)	92%	4.9%	2.9% (site not specified)	94.9% (10 yr)
Khanfir et al[17]	61	59 (median)	88%	0%	6.5% (bone, liver, lung)	94% (5 yr)
Defaud-Hénon et al[18]	30	61 (median)	95%	0%	10% (bone, liver, lung)	Not calculated
Vranic et al[19]	21	60.8 (mean)	85%	0%	20% (bone, kidney, lung)	90% (5 yr)

CLINICAL FEATURES

ACC of the breast affect the left and right breasts equally and tumors arise irrespective of the breast quadrants. However, in about 50 percent of patients, lesions are found in a subareolar region[23]. Pain or tenderness described in the minority of cases has not been correlated with histologically-confirmed perineural invasion[24]. Mammographically, these tumors may appear as asymmetric densities or irregular masses. Sonographically, they appear as well-defined, irregular, heterogeneous, or hypoechoic masses. Nonetheless, the radiographic findings are non-specific and can be misdiagnosed as benign lesions[13,25]. Subsequently, it could be challenging for a radiologist to make the correct diagnosis of carcinoma without histologic confirmation[25]. Lastly, although most patients present with a solitary tumor, a few cases of multifocal ACC of the breast have also been reported[26,27].

HISTOMORPHOLOGIC/IMMUNOHISTOCHEMICAL CHARACTERISTICS

The mean size of ACC is 3.0 cm (range, 0.7 to 12.0 cm)[28]. Most cases are macroscopically well-circumscribed. Occasionally, pink, tan, or gray microcysts are evident[28]. ACC usually presents as a localized disease of pathologic T1 or T2 (Table 1).

The histology of ACC of the breast is similar to that of their salivary gland counterparts. A variety of microscopic patterns detected in the ACC of the salivary glands may also be present in the ACC of the breast. A tumor typically consists of a dual-cell population of luminal and myoepithelial-basal cells which may be arranged in one or more of three architectural patterns: tubular-trabecular, cribriform, and solid-basaloid (Figure 1)[3]. There are two types of structures lined by these two different types of cells: true glandular spaces and pseudolumina. Luminal cells, characterized by round nuclei and eosinophilic cytoplasm, surround true gland lumina containing periodic acid-Schiff-positive neutral mucin. Immunohistochemically, the luminal cells are positive for CK7, CK8/18, epithelial membrane antigen, and CD117 (c-Kit)[2,29-31]. On the other hand, the myoepithelial-basal cells exhibit central oval nuclei and scant cytoplasm, and form pseudolumina, which result from intraluminal invaginations of the stroma. The myoepithelial-basal cells are immunoreactive for basal cytokeratins (CK5, CK5/6, CK14, CK17) (Figure 2), myoepithelial markers (p63, actin, calponin, S-100 protein), vimentin, and epidermal growth factor receptor (EGFR)[2,29-32]. Kasami et al[33] reported that the polarity of the different types of cells could be demonstrated by immunohistochemistry: myoepithelial-basal cells usually express laminin, fibronectin, basal lamina related proteins, and type IV collagen, whereas the luminal cells express proteins related to cell polarization and epithelial differentiation, including fodrin, E-cadherin, and β-catenin. The authors suggest that this preserved cell polarity and segregated cell differentiation could explain the lack of metastatic capacity observed in this tumor type. Other reports describe areas of squamous differentiation and even rare sebaceous differentiation in ACC of the breast[34,35].

Figure 1 Adenoid cystic carcinoma of the breast. Adenoid cystic carcinomas predominantly showing tubular-trabecular (A), cribriform (B), and solid-basaloid patterns (C). Original magnification × 100.

Figure 2 Immunoreactivity of cytokeratin 5/6 in solid pattern of adenoid cystic carcinoma of the breast. The tumor cells are immunoreactive for cytokeratin 5/6, indicating myoepithelial-basal cell origin of tumor cells. Original magnification × 200.

In a way akin to the ACC of the salivary gland, ACCs of the breast are graded according to the proportion of solid growth: tumors with either cribriform or tubular-trabecular pattern and without solid elements are considered grade I, tumors with ≤ 30% of solid growth are classified as grade II, and tumors having more than 30% solid growth are designated grade III[4,36]. Ro et al[4] reported that tumors with a solid pattern (grade II and III) had a tendency to be larger than those without a solid pattern (grade I), and that grade II and III tumors were more likely to develop recurrences. In their series, three patients who developed metastatic ACC had grade II or III lesions. Furthermore, Shin et al[37] reported 9 cases of the solid (basaloid) variant of breast ACC in which the tumor cells tended to be larger, with hyperchromatic nuclei showing moderate to marked atypia, pleomorphism, and increased mitotic activity. This solid variant of ACC was associated with an aggressive clinical course. However, it is important to note that the histological grade defined by this system did not correlate with disease outcomes observed in two other studies[34,38]. The most recent American Joint Committee on Cancer staging manual (7^th edition) recommends that Nottingham histologic grading be provided uniformly for all breast carcinomas[39]. Based on this grading scheme, most ACCs would belong to the histologic grade 1 (3 - 1 + 1) or histologic grade 2 (3 + 2 + 1).

Phenotypically, both luminal and myoepithelial-basaloid cells in ACC of the breast are generally negative for ER, PR, and Her2 proteins (Table 2 and Figure 3)[11,14,40-43]. The immunohistochemical profile of ACC of the breast fits well within that of triple-negative breast cancers with basal-like features. In one study, ER and PR expression was detected in 46% and 36% of ACC cases, respectively[5]. Although this cohort was one of the larger series of ACCs reported to date (n = 28), the cases were collected from different institutions and did not undergo a central review of the diagnosis. Consequently, it cannot be ruled out that a substantial number of these cases were actually invasive cribriform carcinomas with ER and PR immunoreactivity. In addition, it should be noted that in the latter study, dextran-coated charcoal assay was used to assess expression for ER and PR instead of the now more widely used immunohistochemistry. Since normal breast lobules and ducts are often entrapped within the bulk of tumor tissues, it may lead to false positive results of the dextran-coated charcoal assay.

Table 2 Review of data reported on the expression of prognostic and predictive markers of breast adenoid cystic carcinoma (%).

Ref.	No. of cases	Percentage of cases showing positivity
		ER	PR	Her2
Kulkarni et al[14]	933	15	13	NA
Ghabach et al[11]	338	12	2	NA
Arpino et al[5]	28	46	36	NA
Mastropasqua et al[36]	20	15	10	0
Azouley et al[41]	18	0	0	0
Crisi et al[42]	6	0	0	0
Weigelt et al[43]	4	0	0	0

ER: Estrogen receptor; Her2: Human epidermal growth factor receptor 2; NA: Not available; PR: Progesterone receptor.

Figure 3 Immunohistochemical findings in adenoid cystic carcinoma of the breast. A: Estrogen receptor; B: Progesterone receptor; C: Human epidermal growth factor receptor 2. All these markers are negative in a case of adenoid cystic carcinoma of the breast. Original magnification × 100.

There have been several case reports suggesting an association between ACC of the breast and various benign lesions including microglandular adenosis, tubular adenosis, adenomyoepithelioma, and fibroadenoma[44-48]. Acs et al[44] suggested that ACC of the breast may develop in a background of and in continuity with microglandular adenosis. Following this hypothesis, their group described a morphological spectrum of lesions with a trend of progression, encompassing microglandular adenosis, “atypical microglandular adenosis” (also described as “ACC in situ”), and invasive ACC[44]. Da Silva et al[45] reported a morphological characterization of tubular adenosis arising concurrently with ACC in the breast, although the comparative genomic hybridization (CGH) analysis performed on these two lesions failed to provide evidence of molecular evolution from tubular adenosis to ACC. Importantly, breast that harbors an ACC can rarely also contain other types of carcinoma, as was shown in a case where the ACC of the breast coexisted with an invasive ductal carcinoma of NST[49,50].

ACC of the breast that exhibits a cribriform/tubular pattern should be distinguished from invasive cribriform/tubular carcinoma or a benign condition termed collagenous spherulosis[51,52]. This is especially important when a pathologist is provided with tiny tissue specimens obtained by core needle biopsies[36]. Invasive cribriform/tubular carcinomas are characterized by the hyper-proliferation of a single type of neoplastic cells (i.e., luminal cell) only, in contrast to the dual cell types observed in ACC. Moreover, cribriform/tubular carcinomas are generally immunoreactive for ER and PR, whereas ACCs are negative for both[53]. In addition, limited evidence exists of c-Kit and/or p63 immunoreactivity in ACCs of the breast (positive for both), compared to the invasive cribriform/tubular carcinomas which are negative for both markers[40]. In collagenous spherulosis, collagenous spherules are irregular, mostly observed at the periphery of the lesions, and no mucosubstance is detected within lumina. Immunohistochemically, ACCs are c-Kit (+), calponin (-), and smooth muscle myosin (-), whereas collagenous spherulosis lesions are c-Kit (-), calponin (+), and smooth muscle myosin (+), which may help to differentiate between these two types of lesions[54]. The differential diagnosis of the solid (basaloid) variant of ACC includes small cell carcinoma (neuroendocrine carcinoma), solid papillary carcinoma, metaplastic carcinoma, and malignant lymphoma[37]. Although an extensive and careful search for a more typical cribriform pattern of ACC should be performed, immunohistochemistry can also be helpful to distinguish these tumors from ACC.

MOLECULAR GENETIC FEATURES

Microarray-based gene expression profiling studies have been performed in common types of breast cancer, such as the invasive ductal and lobular carcinomas[7-9]. However, most of these studies did not focus on special types of breast cancer, and consequently, there is only limited transcriptomic data on the ACC features. A recent molecular subtype analysis using a single sample predictor (i.e., centroid) performed on 4 ACCs revealed that two of the samples were classified as basal-like, while the other two were shown to exhibit the normal breast-like phenotype. Based on this divergence in the results, they could be an artifact of sample representation, perhaps caused by the contamination with normal tissues[55]. In fact, molecular subtype assignment following hierarchical clustering showed that all four ACCs consistently displayed a basal-like phenotype, and all of them clustered with one of the five subgroups of the triple-negative breast cancers. In another study that utilized the immunohistochemical staining analysis and microarray-based gene expression profiling for a series of 113 tumors that belonged to 11 special histologic types of breast cancer (including 4 ACCs), Weigelt et al[43] reported that the ACC, medullary carcinoma, and metaplastic carcinoma were highly similar in their immunohistochemical and gene expression profile. However, ACCs did not intermingle with medullary and metaplastic carcinomas in the hierarchical clustering, but formed a separate group. Another study, an array-based CGH analysis of 59 breast cancers that belonged to 10 special histologic special types established that while medullary and metaplastic carcinomas displayed complex genomes, ACCs consistently exhibited simpler patterns of gene copy number aberrations[56]. In line with these results, a recent CGH analysis study revealed that ACC of the breast manifested significantly lower frequencies of genetic instability and lower copy number alterations than the histologic grade-matched basal-like and invasive ductal carcinomas of NST[29]. At the genomic level, ACC is substantially different from the other basal-like breast cancers. Studies show that it rarely harbors genomic aberrations associated with basal-like invasive ductal carcinomas of NST, such as gains of 1q, 6p, 8q, and 10p, and losses of 4p, 5q, and 10q[29,57,58]. Furthermore, aneuploidy is reported in fewer than 10% of cases with ACC of the breast[5]. Together, these findings illustrate the heterogeneity of triple-negative, basal-like breast cancers. Although the majority of these tumors are high grade cancers with high levels of genetic instability and an aggressive clinical course (e.g., grade 3 invasive ductal carcinoma of NST, medullary carcinoma, and metaplastic carcinoma), there is also a subgroup of low grade tumors with low frequencies of genetic instability and an indolent clinical behavior (e.g., ACC and secretory carcinoma)[10,41,43,59-61]. Thus, we emphasize that based solely on molecular subtyping and without proper histologic classification, ACCs, which have an indolent clinical behavior, would be classified as triple-negative, clinically aggressive tumors. Therefore, information regarding the histologic type of triple-negative breast cancers should be included in histopathology reports and taken into account for clinical decision-making.

Although studies using next-generation sequencing (NGS) for whole exome or microRNA expression profiling for ACC of the salivary gland have been recently reported[62-65], there have been few studies using NGS for ACC of the breast. In one study utilizing microRNA expression profiling for two cases each of ACC of the salivary gland and breast, Kiss et al[65] reported that the let-7b was overexpressed in ACC of the salivary gland, while decreased in ACC of the breast. In addition, the miR-24 was decreased in salivary gland-derived but overexpressed in breast-derived adenoid cystic carcinomas.

Similar to ACCs of the salivary gland, ACCs of the breast are characterized by the t(6;9) (q22-23; p23-24) chromosomal translocation, which generates fusion transcripts involving the oncogene MYB and the transcription factor gene NFIB. Several previous studies reported that this chromosomal translocation is present in over 90% of ACC cases and is a key ACC oncogenic mechanism[29,66,67]. The myeloblastoma (MYB)- nuclear factor I/B (NFIB) fusion protein retains the DNA-binding and transactivation domains of a wild-type MYB, and is therefore expected to activate MYB target genes[29,66]. MYB is a leucine zipper transcription factor that plays an important role in the control of cell proliferation, apoptosis, and differentiation[68,69], while its target genes include BCL2 and GRP78/BIP, which are essential for cell survival[70]. MYB is a direct target of EG signaling and is highly expressed not only in ACCs, but also in cell lines of ER-positive breast cancers[71,72]. Recently, one study reported that 67% (8/12 cases) of dermal cylindroma displayed the t(6;9) and MYB-NFIB fusion transcripts and that the composition of these chimeric transcripts was identical to that seen in ACC[73].

Approximately 7% of breast cancer cases are related to hereditary conditions and caused by mutations in the BRCA1 and BRCA2 genes[3]. Although medullary and metaplastic breast carcinomas, with which ACC shares immunohistochemical and molecular findings, show a frequent promotor methylation of BRCA1 gene, ACC of the breast usually retains normal BRCA1 gene function[2,29]. To our knowledge, BRCA2 gene status has not been investigated in ACCs of the breast.

ACCs of the breast typically do not express the full-length ER-α (ER-α66) and PR[11,14,39-42]. However, several studies have shown that the ACC, apocrine carcinoma, and triple-negative breast cancer of NST exhibited a frequent membranous/cytoplasmic immunoreactivity for ER-α36, a novel ER-α66 splice variant implicated in membrane-initiated estrogen signaling[74-76]. In the experimental cell models of breast cancer, ER-α36 was shown to transduce the membrane-initiated steroid signaling cascade, and served as a dominant-negative modulator of ER-α66 mediated transcription activity[75]. In addition, ER-α36 was reported to be related to non-genomic ER activities, in which activation of the mitogen-activated protein kinase (MAPK/ERK) signaling pathway plays a major role[75]. The MAPK/ERK signaling pathway is activated in response to antiestrogens (e.g., tamoxifen), indicating a subset of ER-α66 (-)/ER-α36 (+) breast carcinomas might still respond to antiestrogen based therapy[74,75]. Finally, ER-α36 protein closely interacts with EGFR protein, which is commonly expressed in ACC and triple-negative breast cancers[75]. Some investigators have reported that ACCs of the breast frequently overexpress EGFR protein in the absence of underlying EGFR gene alterations[19,29].

Cancer stem cells have been reported to be associated with tumor initiation, progression, survival, and resistance to therapy[77]. However, the cancer stem cell field is still fairly controversial and stem cell markers have not been fully elucidated. In the majority of studies, breast cancer cells with a CD44 (+)/CD24 (-) phenotype have been proposed to have tumor-initiating properties with stem cell-like features[78], and Defaud-Hénon et al[18] recently reported that a characteristic CD44 (+)/CD24 (-) phenotype is commonly observed in the ACC of the breast. On the other hand, frequent overexpression of c-Kit and EGFR proteins was observed in undifferentiated carcinomas with stem cell-like features[79]. Although several studies illustrated that a consistent c-Kit protein expression was detected in most ACCs[29,40-43], underlying KIT gene alterations, such as gene mutations, have not been previously detected[80]. Finally, SOX10 transcription factor appears to support stem-like properties in normal tissues and cancer cells[81]. Recently, Ivanov et al[82] described SOX10 as a novel diagnostic marker for ACCs of the salivary gland and breast basal-like carcinomas, indicating that SOX10 expression might be worth examining in ACCs of the breast.

Although triple-negative NST breast cancers usually have high proliferative activity, ACC of the breast exhibits a low proliferation rate using standard Ki-67 labeling index[29,83]. Interestingly, their typical proliferation rate is even lower than that of low-grade conventional breast carcinomas[84]. Mastropasqua et al[40] suggested that proliferative indices showed grater values in high-grade ACCs when compared to low-grade lesions. However, another study reported that the proliferative activity is not associated with the outcome of ACC patients with ACC[38]. In addition to low Ki-67 labeling index, ACCs of the breast, including high-grade solid-basaloid lesions, also show low p53 protein expression[29,39,83]. Trendell-Smith et al[53] described a slightly higher p53 protein expression in ACC than that in invasive cribriform carcinoma.

Finally there are several recent studies that identified potential breast ACC biomarkers. Insulin-like growth factor-II mRNA-binding protein 3 (IMP3) is an oncofetal protein and a component of the insulin-like growth factor-II pathway. Studies indicate that it could serve as a biomarker for basal-like breast carcinomas[84-87], and a recent report showed that the IMP3 is commonly overexpressed in ACCs of the breast[88]. In another report, the molecular genetic analysis of a primary ACC of the breast and its renal metastasis revealed PTEN and PIK3CA gene mutations[89].

PROGNOSIS AND TREATMENT

A striking feature of ACC of the breast, which is in stark contrast with other triple-negative, basal-like breast cancers and the ACC of the salivary gland, is its excellent prognosis. As shown in Table 1, the 10-year survival rate is 90%-100%, and lymph node metastasis is rare, as well as distant metastases, which affect mainly visceral organs[11,14-19,90]. Based on its indolent clinical course and favorable outcome, the ACC of the breast is generally cured by breast-conserving surgery, such as wide excision or quadrantectomy with or without radiotherapy[11,17,91]. Mastectomy is recommended for invasive lesions when a cosmetically satisfactory excision is not possible, especially when the tumor has a high-grade pattern[4,36,92]. A recent study of a large patient cohort reported a considerable benefit of adjuvant radiotherapy on overall and disease-specific survival in patients with ACC[15]. Moreover, because a high rate of positive surgical margins has been detected following breast conserving surgery, adjuvant radiotherapy may be beneficial[93]. Furthermore, while some clinicians recommend systemic adjuvant chemotherapy for patients with high-grade lesions or axillary lymph node/distant metastasis[36], its role in breast ACC patients remains controversial.

When patients with ACC demonstrate local recurrence or distant metastases, a prolonged and indolent clinical course is still likely[94-97]. However, long-term follow-up is recommended, since their long clinical course carries a risk of secondary malignancies[98,99], and the risk of distant metastases increases with time[100].

As treatment of cancer enters a new stage with the development of targeted therapies, the common MYB-NFIB fusion gene may provide new therapeutic avenues for the management of advanced ACC of the breast. Consequently, further functional studies investigating the biological consequences of the MYB gene of function due to the MYB-NFIB fusion are needed. Gene silencing experiments are also necessary to demonstrate that MYB expression is required for the survival of cancer cells with genetically activated MYB. Finally, the functional role of the ER-α36 variant in ACC merits further research as experimental evidence in triple-negative breast cancer cell lines suggests that breast cancer cells with ER-α66 (-)/ER-α36 (+) phenotype might still be responsive to antiestrogens[72,73].

HOUSTON METHODIST EXPERIENCE OF ACC OF THE BREAST

A search of the electronic data base at Houston Methodist Hospital from 2004 to 2010 yielded five cases of ACC of the breast. The clinicopathologic and follow-up status of these five patients are summarized in Table 3. The five female patients ranged from 48 to 76 years in age, with a mean age of 60 years. All tumors had distinct morphologic features of classic ACC: histologic grade 1 with cribriform, trabecular or glandular architectural patterns, and basement membrane deposition. No cases of grade II and III tumors were identified. Perineural invasion was identified in one case. Lymphovascular invasion was not seen in any of the cases. An associated adenomyoepithelioma was observed in one case. All patients received lumpectomy and two of these patients had axillary lymph node dissections, with no nodal metastasis found. No patients received adjuvant chemotherapy or radiotherapy. Pulmonary metastasis developed in one case (case 2) seven years after the initial diagnosis. All of the tumors, including the pulmonary metastatic lesion in case 2, were ER/PR negative and did not express Her2. No synchronous/metachronous in-situ carcinoma, invasive ductal/lobular carcinoma, or microglandular adenosis was reported in any of the cases. Four patients without metastasis were alive and showed no evidence of disease for an average (follow-up) of 45.3 mo (range 12-90 mo). The last patient (case 2) who was diagnosed with pulmonary metastasis is alive with disease at 85 mo (one month after metastasis was detected).

Table 3 Houston Methodist experience of adenoid cystic carcinoma of the breast (2004 to 2010).

Case No.	Age (yr)	Laterality	Tumor size (cm)	Perineural invasion	Lymph node metastasis	Distant metastasis	TMN stage (AJCC)	Follow-up (mo)
1	61	Left	1.6	-	pN0	-	IA	14
2	83	Right	3.0	-	pN0	Lungs, multiple	IV	85
3	51	Right	2.2	-	cN0	-	IIA	12
4	57	Left	4.5	+	cN0	-	IIA	65
5	48	Left	2.0	-	cN0	-	IIA	90

AJCC: American Joint Committee on Cancer.

CONCLUSION

The correct classification of the histological special types of breast cancer is not just an academic exercise, as it has both prognostic and predictive implications. Although the majority of triple-negative, basal-like breast carcinomas are high-grade tumors, ACC is a subgroup of low-grade tumors with an indolent clinical behavior that also displays a triple-negative, basal-like phenotype. Because of its low incidence, there have been only few comprehensive studies of ACC of the breast, which is one of the major limitations of this review. However, this review of recent updates, including certain molecular genetic features in breast ACC, herein will hopefully serve as a prognostic and treatment guide for surgical pathologists, breast surgeons, and oncologists, and lead to the development of more specific, personalized therapies for this rare tumor subtype.