Genetic analysis of microglandular adenosis and acinic cell carcinomas of the breast provides evidence for the existence of a low-grade triple-negative breast neoplasia family

Breast Carcinomas Resembling Acinic Cell Carcinoma: Comprehensive Analysis of 14 Cases and Review of the Literature

Acinic cell carcinoma (AciCC) of the breast is an exceptionally rare subtype of invasive breast carcinoma, often exhibiting a triple-negative phenotype and relatively indolent behavior. Since the first case reported by Roncaroli and colleagues in 1996, no more than 60 additional cases have been described in English medical journals, usually as case reports or small case series. In this study, we presented an in-depth analysis of 14 cases of AciCC of the breast, including 4 pure AciCCs and 10 AciCCs mixed with other histologic types. We reported the clinicopathologic characteristics, histologic components, treatment modalities including response to neoadjuvant treatment in 3 patients, and outcomes. In addition, we assessed the expression of nuclear transcription factor nuclear receptor subfamily 4 group A member 3 by immunohistochemistry and gene rearrangements by fluorescence in situ hybridization, which has been implicated in AciCC of the salivary gland. All 14 cases were negative for nuclear receptor subfamily 4 group A member 3 expression, and no gene rearrangements were detected. We also conducted a thorough review of the literature to highlight advancements in understanding this rare breast cancer subtype. This study aims to enhance clinical knowledge of AciCC of the breast and contributed to growing evidence that AciCC of the breast and AciCC of the salivary glands appear to be unrelated entities, despite sharing a similar histologic appearance.

Carcinoma arising in microglandular adenosis of the breast: clinicopathological and genetic analysis

AIMS

To study the clinicopathological, immunohistochemical and molecular features of carcinoma arising in microglandular adenosis (MGACA) of the breast.

METHODS

Clinicopathological features of 13 cases of MGACA were analysed. All tumours were molecular subtype by immunohistochemistry (IHC) of AR, CD8, FOXC1 and DCLK1 expression. Next-generation sequencing including 511 genes was analysed.

RESULTS

All tumours showed a histological spectrum ranging from microglandular adenosis (MGA) to atypical MGA (AMGA), ductal carcinoma in situ (DCIS) and MGACA. Invasive components in 10 of 13 tumours were invasive carcinoma of no special type (NST), 3 were metaplastic carcinoma with mesenchymal differentiation (including two cases of matrix-producing carcinoma) mixed with NST. All lesion-associated epithelial cells were triple negative (TNBC) and positive for S-100. Reticulin staining showed the presence of basement membrane in MGA, AMGA and DCIS, and its absence in invasive carcinoma. According to IHC-based TNBC molecular subtyping, 10 tumours were basal-like immune-suppressed (BLIS), 2 were luminal androgen receptor and 1 was immunomodulatory. 10 patients had gene mutations. Pathogenic germline mutations of the BRCA1 and BRCA2 genes were detected in four tumours (30.7%) and one tumour (7.7%). Somatic mutation rate of the TP53 gene was 69.2%. Amplification rates of MYC, FGFR2, JAK2 and MCL1 genes in our cohort were 46.2%, 15.4%, 15.4% and 7.7%, respectively.

CONCLUSION

MGACA is a rare breast carcinoma, with distinct morphological, immunohistochemical and molecular features. Most MGACA were BLIS molecular subtype of TNBC. TP53 and BRCA1 gene mutation and MYC gene amplification were the most common genetic changes in MGACA.

Breast acinic cell carcinoma with weak progesterone receptor expression: a case report and literature review

Rationale

Acinic cell carcinoma (AcCC) of the breast is an extremely rare malignant epithelial tumor characterized by acini cell differentiation, clinical low-grade malignancy, and a molecular triple-negative subtype.

Patient concern

A 47-year-old female presented with a 1-month history of a painless mass in her right breast.

Diagnosis

Ultrasound imaging, mammography and magnetic resonance imaging revealed a lesion, approximately 3.0cm×1.5cm in size, in the right breast, which was considered to be a malignancy. After the surgery, the AcCC of the breast was confirmed histologically.

Interventions

Right breast mastectomy and sentinel lymph node biopsy were performed. Adjuvant chemotherapy included 4 cycles of doxorubicin hydrochloride (Adriamycin) and cyclophosphamide followed by 4 cycles of docetaxel (Taxotere).

Outcomes

The patient was discharged from the hospital after surgery. There was no sign of recurrence during a 9-month follow-up period.

Lessons

Acinic cell carcinoma (AcCC) of the breast is an extremely rare malignant epithelial tumor that can be accurately diagnosed based on histopathologic morphology and immunohistochemistry. The weak positive progesterone receptor (PR) expressed in this case is extremely rare, which may provide a new research direction for the endocrine therapy of AcCC. Both AcCC and microglandular adenosis(MGA) exhibit microglandular growth, and the relationship between them remains unclear. Differentiation between them not only relies on histomorphology and pathological immunohistochemistry but also depends on clinical manifestations and other presentations. Optimal treatment of AcCC is the same as that for invasive breast cancer. The prognosis is generally good, with adjuvant therapy after surgery.

Unraveling complexity and leveraging opportunities in uncommon breast cancer subtypes

Special histologic subtypes of breast cancer (BC) exhibit unique phenotypes and molecular profiles with diagnostic and therapeutic implications, often differing in behavior and clinical trajectory from common BC forms. Novel methodologies, such as artificial intelligence may improve classification. Genetic predisposition plays roles in a subset of cases. Uncommon BC presentations like male, inflammatory and pregnancy-related BC pose challenges. Emerging therapeutic strategies targeting genetic alterations or immune microenvironment are being explored.

Salivary gland-like tumors of the breast: Histopathologic and genetic features with clinical implications

Salivary gland-like tumors of the breast are rare neoplasms that share morphologic, immunophenotypic, and/or genetic features with their salivary gland counterparts, highlighting a shared underlying histopathogenesis in most cases. Salivary gland-like carcinomas included in the World Health Organization classification of breast tumors are adenoid cystic carcinoma, secretory carcinoma, mucoepidermoid carcinoma, acinic cell carcinoma, and the exceedingly rare polymorphous adenocarcinoma. These carcinomas are usually triple negative for estrogen receptor and progesterone receptor expression and HER2 overexpression, yet generally have favorable prognosis, in contrast to high-grade triple negative carcinomas of no special type. On the other hand, a small subset, such as solid-basaloid adenoid cystic carcinoma, rare high-grade carcinomas, and those associated with transformation to other types of high-grade invasive carcinoma can behave more aggressively. Other salivary gland-like tumors of the breast, such as pleomorphic adenoma and adenomyoepithelioma, are usually benign but can rarely undergo malignant transformation. Although clinical experience with salivary gland-like breast tumors is overall limited, their recognition and accurate classification has important implications for prognosis and clinical management, especially to avoid overtreatment of salivary gland-like carcinomas. The identification of characteristic genetic alterations and/or immunohistochemical surrogates in many of these tumors has practical applications to establishing an accurate diagnosis and directing clinical management. This review highlights the histopathologic and genetic characteristics of salivary gland-like breast tumors and the implications of the diagnosis for current clinical management.

New insights into acinic cell carcinoma of the breast: clinicopathology, origin of histology, molecular features, prognosis, and treatment

Acinic cell carcinoma (AciCC) of the breast is a rare malignant epithelial neoplasm, with approximately 60 cases reported in the literature. It predominantly affects women and exhibits significant histological heterogeneity. The diagnosis of breast AciCC is primarily based on the presence of eosinophilic and/or basophilic granular cytoplasm and markers of serous acinar differentiation. Despite being considered a low-grade variant of conventional triple-negative breast cancer (TNBC), over 25% of patients with breast AciCC have adverse clinical outcomes. Additionally, in early research, microglandular adenosis (MGA) and atypical MGA were considered potential precursors for various breast cancers, including intraductal carcinoma, invasive ductal carcinoma, adenoid cystic carcinoma, metaplastic carcinoma, and AciCC. Similarly, some studies have proposed that breast AciCC should be considered a type of carcinoma developing in MGA with acinic cell differentiation rather than a distinct entity. Therefore, the pathogenesis of breast AciCC has not yet been clarified. Moreover, to the best of our knowledge, the literature has not summarized the latest prognosis and treatment of breast AciCC. In this review, we synthesized the current literature and the latest developments, aiming at exploring the clinicopathology, histological origin, molecular features, prognosis, and treatment of breast AciCC from a novel perspective.

Elucidating the nature of acinic cell carcinoma of the breast with high-grade morphology: evidence from case report

BACKGROUND

Acinic cell carcinoma (AciCC) of the breast is a rare subtype of breast cancer. It was considered a low-grade triple-negative breast cancer (TNBC) with the potential to progress or transform into a high-grade lesion because of the molecular similarities with conventional aggressive TNBC in several genetic studies. Microscopically, the coexistence of classical low-grade and high-grade triple-negative components in breast AciCC is not uncommon. However, there is a scarcity of research on the comparative histopathological and genetic aspects of both components.

CASE PRESENTATION

A 34-year-old woman with a nontender mass in the upper outer quadrant of the left breast was initially diagnosed with a malignant small round cell tumor (undifferentiated or poorly differentiated carcinoma) based on a preoperative biopsy, which was later identified as breast AciCC with a high-grade solid component. Left breast-conserving surgery with sentinel lymph node biopsy was performed. Microscopically, the breast AciCC consisted of a classical acinic component and a high-grade component. The latter demonstrated a solid sheet-like pattern characterized by large, round, pleomorphic or vesicular nuclei, prominent nucleoli, and frequent mitotic activities. Classical acinic architectures focally merged together to form solid nests and transited into high-grade areas. Remarkably, in the high-grade lesion, conventional immunochemical markers for breast AciCC, such as α1-antitrypsin (AAT), Lysozyme (LYS), Epithelial membrane antigen (EMA), S100 protein (S100), and cytokeratin (CK) were negative, whereas cell cycle protein D1 (cyclin D1) and vimentin showed diffuse expression. Next‑generation sequencing (NGS) revealed that 43.5% of variants were identical in both components. Furthermore, PAK5 mutation; copy number (CN) loss of CDH1, CHEK1, and MLH1; and CN gains of CDK6, HGF, and FOXP1 were identified in the high-grade lesion. The patient was treated with eight cycles of adjuvant chemotherapy (epirubicin combined with cyclophosphamide) and radiotherapy after surgery, and she is currently alive for 43 months with no metastases or recurrences.

CONCLUSIONS

This case demonstrates a comparative analysis of the histopathological and genetic characteristics of classical low-grade and high-grade components of AciCC within the same breast. This information may serve as a morphological and molecular basis for further investigation into the molecular mechanisms underlying high-grade lesions in breast AciCC.

Clinicopathological, immunohistochemical and molecular features of acinic cell carcinoma of the breast

Breast acinic cell carcinoma (ACC) is a rare subtype of breast cancer. Accurate diagnosis of ACC using core needle biopsy (CNB) is pivotal for the use of effective treatments and patient prognosis. In the present study, a detailed analysis of the morphological, immunohistochemical and gene mutation features of 2 cases of ACC was performed. CNB was performed prior to surgical excision. The breast ACC in the present cases exhibited overt burrowing labyrinthine networks or 'hand-holding-hand' features. The tumor cells in both of the present cases expressed cytokeratin (CK)7, S100 and CK5/6, but were negative for p63, estrogen receptor and progesterone receptor. GATA binding protein 3 was positive in case 1 but negative in case 2. Fluorescence in situ hybridization indicated no ETS variant transcription factor 6 break-apart probe detection. Next-generation sequencing results revealed the same mutation and a similar abundance in exon 27 (NM_005120.2; c.3817G>T; p.A1273S) of the mediator of RNA polymerase II transcription, subunit 12 homolog (MED12) gene in both patients. To conclude, the findings of the present study suggested that recognition of this rare 'hand-holding-hand' structure could potentially be beneficial for avoiding patient misdiagnosis. In addition, it could be suggested that a mutation in the MED12 exon 27 was associated with the formation of a burrowing labyrinthine network or 'hand-holding-hand' feature.

Molecular pathology in breast disease: diagnostic, prognostic, and therapeutic tools

Molecular testing in breast cancer gained increasing attention and importance as specific molecular results can tailor not only oncological decisions on systemic adjuvant or neoadjuvant or in metastatic setting, but increasingly serve in diagnostic routine histopathological services to differentiate between morphologically overlapping or ambiguous histological pictures. Diagnostic tools involve in most cases a broad spectrum of immunohistochemical panels, followed by entity-specific in situ hybridization probes and in given cases NGS-based sequencing. Workflow of which methodology is applied and in which order depends on the specific entity resp. on the given differential diagnosis in question. Regarding prognostic/predictive molecular testing, the choice of assay and the workflow are based on clinical algorithms and on the evidence of targeted therapies following the molecular alterations. In this review paper, we aim to address the use of molecular technics in [1] the histological diagnostic setting (such as subtyping of invasive carcinomas/malignant spindle cell tumors and sarcomas and some B3 lesions) and [2] in the context of adjuvant or neoadjuvant or other clinical settings with special focus of targeted therapies.

Salivary Gland-like Tumors of the Breast

CONTEXT

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.

OBJECTIVE

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.

DATA SOURCES

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.

CONCLUSIONS

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.

NR4A3 Expression Is Consistently Absent in Acinic Cell Carcinomas of the Breast: A Potential Nosologic Shift

Acinic cell carcinoma (AciCC) is a tumor that is recognized in both the breast and salivary glands. Recently, the recurrent genomic rearrangement, t(4;9)(q13;q31) was identified in salivary AciCC that results in constitutive upregulation of the nuclear transcription factor NR4A3, which can be detected by immunohistochemistry. In this study, we sought to evaluate NR4A3 expression in breast AciCC using immunohistochemistry. Strong and diffuse nuclear staining was considered a positive result. Sixteen AciCCs were studied, including 8 pure AciCCs and 8 AciCCs admixed with other types (invasive carcinoma of no special type in 5 cases and metaplastic carcinoma in 3 cases). All 16 AciCCs showed negative results for NR4A3 expression. Four cases with available material were evaluated for rearrangements of the NR4A3 gene by fluorescence in situ hybridization and no rearrangements were observed. Whole-genome sequencing of 1 AciCC revealed a TP53 splice-site mutation, high levels of genomic instability, and genomic features of homologous recombination DNA repair defects; a structural variant analysis of this case did not reveal the presence of a t(4;9) rearrangement. We conclude that breast AciCCs consistently lack NR4A3 rearrangement or overexpression, unlike most of the salivary AciCCs, and that consistent with previous results, breast AciCCs are associated with genomic alterations more similar to those seen in triple-negative breast carcinomas than salivary gland AciCCs. These results suggest that unlike other salivary gland-like tumors that occur in the breast, the molecular underpinnings of the salivary gland and breast AciCCs are different and that the salivary gland and breast AciCCs likely represent distinct entities.

Luminal-Type Invasive Carcinoma in Association With Microglandular Adenosis/Atypical Microglandular Adenosis: A Case Report and Molecular Comparison

Microglandular adenosis (MGA) is a proliferative breast lesion composed of small, uniform glands lacking a myoepithelial cell layer while still invested by the basement membrane. The glands percolate haphazardly through the breast parenchyma rather than maintaining a lobular architecture, typical of other forms of adenosis.MGA is a benign lesion though atypical forms have been well described, often in close association with carcinoma. MGA, atypical MGA (AMGA), and the vast majority of MGA-associated carcinomas (MGACA) are negative for estrogen receptor (ER), progesterone receptor (PR), and human epidermal growth factor 2 (HER2) by immunohistochemistry. In light of these findings and early molecular studies, MGA is hypothesized to represent a clonal process and nonobligate precursor of basal-type breast carcinomas. We present the case of a 58-year-old woman and the first published molecular comparison of a luminal-type invasive ductal carcinoma with its associated MGA/AMGA. Analysis of small nucleotide variants (SNVs) revealed that 63% of the SNVs identified in the MGA were present in the AMGA while only 10% of them were present in the MGACA, suggesting a direct relationship between MGA and AMGA but not MGA and MGACA.

A case of extensively spreading acinic cell carcinoma of the breast with microglandular features

Acinic cell carcinoma (ACC) is an exceptionally rare type of breast carcinoma with a low-grade morphology and a favorable prognosis. It is postulated to be a type of invasive carcinoma arising in microglandular adenosis (MGA). We report a case of extensively spreading ACC of the breast with MGA-like features. Macroscopically, yellowish nodules were widely distributed throughout the right breast, up to the axilla, without mass formation. Microscopically, the tumor consisted of two distinct carcinoma components: one was multiple nodular lesions showing invasive carcinoma with fused solid nests, and the other was a widely spreading lesion exhibiting MGA-like features with uniform small single glands. Immunohistochemically, both components were negative for ER, PR, and HER2, and expressed EMA, S100 and lysozyme. The distinct morphology and molecular expression indicated ACC. The single glands in the MGA-like area lacked myoepithelial cells but were linearly surrounded by type IV collagen, a basement membrane component. This case supports the hypothesis that ACC and MGA have the same lineage and indicates that ACC is not necessarily a low-grade malignancy and can be aggressive.

High grade acinic cell carcinoma of the breast with clear cytoplasm mimics clear cell carcinoma in a BRCA1 mutation carrier: a case report and review of the literature on the molecular analysis

Acinic cell carcinoma of the breast is an extremely rare tumor. To the best of our knowledge, only one case is reported to have bilateral tumors and had both BRCA1 and TP53 mutation. Herein, we report another case of acinic cell carcinoma of the breast in a 29-years-old female carrying germline BRCA1 and TP53 mutation, and the tumor showed a complex combination of histological features which had not only the reported common features such as diffuse infiltrative small acinar or glandular structures mixed with solid nests, but also the uncommon widespread clear cells, high grade tumor cells. The immunohistochemical profile of the tumor cells was strongly positive for lysozyme and triple negative for ER, PR, HER2. Although she had bilateral high grade breast cancers, this patient refused postoperative adjuvant therapy this time and has been doing well in the past 12 months. As a rare form of triple-negative breast cancer with a relatively not so bad prognosis, more reports are needed to understand its biological characteristics.

Pitfalls in breast pathology

Accurate pathological diagnosis is the cornerstone of optimal clinical management for patients with breast disease. As non-operative diagnosis has now become the standard of care, histopathologists encounter the daily challenge of making definitive diagnoses on limited breast core needle biopsy (CNB) material. CNB samples are carefully evaluated using microscopic examination of haematoxylin and eosin (H&E)-stained slides and supportive immunohistochemistry (IHC), providing the necessary information to inform the next steps in the patient care pathway. Some entities may be difficult to distinguish on small tissue samples, and if there is uncertainty a diagnostic excision biopsy should be recommended. This review discusses (1) benign breast lesions that may mimic malignancy, (2) malignant conditions that may be misinterpreted as benign, (3) malignant conditions that may be incorrectly diagnosed as primary breast carcinoma, and (4) some IHC pitfalls. The aim of the review is to raise awareness of potential pitfalls in the interpretation of breast lesions that may lead to underdiagnosis, overdiagnosis, or incorrect classification of malignancy with potential adverse outcomes for individual patients.

Update on triple-negative breast cancers - highlighting subtyping update and treatment implication

Triple-negative breast cancer (TNBC) remains a major challenge in breast cancer management. Continuing research in the past years aimed at understanding the biology of this tumour and developing more effective therapeutic options. It is now clear that TNBC is vastly heterogeneous with diverse histological, molecular, immunological profiles and clinical differences. Current evidence suggested the existence of at least four predominant subtypes based on expression profiling across studies. These subtypes exhibited specific genomic alterations and tumour microenvironment. Subtype-specific therapeutic strategies were identified. Recognising these subtypes allows not only an improved prognostication but also a better treatment decision. Herein, we provide an overview of the recent findings on TNBC heterogeneity at different levels and corresponding subtyping. The characteristic of subtypes and the implication of these subtypings in therapeutic approaches are also discussed.

Acinic cell carcinoma of the breast: A comprehensive review

Acinic cell carcinoma of the breast is a rare special subtype of breast cancer in the category of salivary gland-type tumors. It is morphologically similar to acinic cell carcinomas of salivary glands and pancreas and has a triple-negative phenotype (estrogen receptor-negative, progesterone receptor-negative, and Her-2/neu negative). Its molecular genomic features are more similar to triple-negative breast cancer of no special type than to its salivary gland counterpart. However, the clinical course of the mammary acinic cell carcinoma appears to be less aggressive than the usual triple-negative breast carcinomas. This review comprehensively summarizes the current literature on the clinicopathologic, immunohistochemical, and molecular features of this rare and distinct subtype of breast cancer.

Special Histologic Type and Rare Breast Tumors - Diagnostic Review and Clinico-Pathological Implications

Breast cancer is the most common malignant tumor in females. While most carcinomas are categorized as invasive carcinoma, no special type (NST), a diverse group of tumors with distinct pathologic and clinical features is also recognized, ranging in incidence from relatively more common to rare. So-called "special histologic type" tumors display more than 90% of a specific, distinctive histologic pattern, while a spectrum of tumors more often encountered in the salivary gland may also arise in the breast. Metaplastic carcinomas can present diagnostic challenges. Some uncommon tumors harbor pathognomonic genetic alterations. This article provides an overview of the key diagnostic points and differential diagnoses for this group of disparate lesions, as well as the salient clinical characteristics of each entity.

Low-risk triple-negative breast cancers: Clinico-pathological and molecular features

Triple-negative breast cancers (TNBC) comprise biologically and clinically heterogeneous diseases characterized by the lack of hormone receptors (HR) and HER2 expression. This subset of tumors accounts for 15-20% of all breast cancers and pursues an ominous clinical course. However, there is a spectrum of low-risk TNBCs with no/minimal metastatic potential, including the salivary gland-type tumors, those with extensive apocrine differentiation and/or high tumor-infiltrating lymphocytes, and small-sized, early-stage (pT1a/bN0M0) TNBCs. De-escalating the treatment in low-risk TNBC, however, is not trivial because of the substantial lack of dedicated randomized clinical trials and cancer registries. The development of new diagnostic and/or prognostic biomarkers based on clinical and molecular aspects of low-risk TNBCs would lead to improved clinical treatment. Here, we sought to provide a portrait of the clinicopathological and molecular features of low-risk TNBC, with a focus on the diagnostic challenges along with the most important biological characteristics underpinning their favorable clinical course.

Pathogenesis of Triple-Negative Breast Cancer

Triple-negative breast cancer (TNBC) encompasses a heterogeneous group of fundamentally different diseases with different histologic, genomic, and immunologic profiles, which are aggregated under this term because of their lack of estrogen receptor, progesterone receptor, and human epidermal growth factor receptor 2 expression. Massively parallel sequencing and other omics technologies have demonstrated the level of heterogeneity in TNBCs and shed light into the pathogenesis of this therapeutically challenging entity in breast cancer. In this review, we discuss the histologic and molecular classifications of TNBC, the genomic alterations these different tumor types harbor, and the potential impact of these alterations on the pathogenesis of these tumors. We also explore the role of the tumor microenvironment in the biology of TNBCs and its potential impact on therapeutic response. Dissecting the biology and understanding the therapeutic dependencies of each TNBC subtype will be essential to delivering on the promise of precision medicine for patients with triple-negative disease.

Low-grade triple-negative breast carcinomas. A report of 2 cases and an update of current concepts

Triple negative breast cancer is defined by the lack of expression of estrogen, progesterone and HER2 receptors. Significant molecular, morphological and clinical heterogeneity is present in this group of neoplasms. Although the majority are high-grade tumors, low-grade triple negative breast cancers can occur and their evolution, molecular characteristics and therapeutic management vary from the former. In the current review, we focus on the histological and immunohistochemical phenotypes of two new low-grade cases: an acinic cell carcinoma and an adenoid cystic carcinoma. Data originated from the pathology department of a third-level hospital over an 18-month period, within a breast cancer screening program. Low-grade triple negative cancers should be suspected in triple negative breast cancers with low proliferative rates as, unlike high-grade tumors, they require a multidisciplinary approach. They can be diagnosed at an early stage by immunohistochemistry using core needle biopsy.

Triple-Negative Breast Cancer Histological Subtypes with a Favourable Prognosis

Triple-negative breast cancers (TNBC), as a group of tumours, have a worse prognosis than stage-matched non-TNBC and lack the benefits of routinely available targeted therapy. However, TNBC is a heterogeneous group of neoplasms, which includes some special type carcinomas with a relatively indolent course. This review on behalf of the European Working Group for Breast Screening Pathology reviews the literature on the special histological types of BC that are reported to have a triple negative phenotype and indolent behaviour. These include adenoid cystic carcinoma of classical type, low-grade adenosquamous carcinoma, fibromatosis-like metaplastic carcinoma, low-grade mucoepidermoid carcinoma, secretory carcinoma, acinic cell carcinoma, and tall cell carcinoma with reversed polarity. The pathological and known molecular features as well as clinical data including treatment and prognosis of these special TNBC subtypes are summarised and it is concluded that many patients with these rare TNBC pure subtypes are unlikely to benefit from systemic chemotherapy. A consensus statement of the working group relating to the multidisciplinary approach and treatment of these rare tumour types concludes the review.

Morphologic and immunohistochemical features of carcinoma involving microglandular adenosis of the breast following neoadjuvant chemotherapy

Microglandular adenosis (MGA)-related lesions, including atypical MGA (AMGA) and carcinoma involving MGA (C-MGA), are characterized by epithelial atypia, negative hormone receptors, and HER2 status, and can mimic invasive triple negative breast cancer (TNBC) in core needle biopsies (CNB) resulting in selection for treatment with neoadjuvant chemotherapy (NAC). We identified 12 cases of AMGA and/or C-MGA in post-NAC excision specimens (EXC) and analyzed their morphologic and immunohistochemical (IHC) features. All CNBs were initially diagnosed as containing TNBC. Upon re-review, TNBC was confirmed in nine cases. In three CNBs AMGA and/or C-MGA had been interpreted as TNBC. AMGA was initially recognized in only one case but AMGA and/or C-MGA were present in an additional nine CNBs. At EXC, no residual TNBC was present in 5 of 9 EXCs and all 12 cases showed residual AMGA and/or C-MGA. Similar to conventional MGA, AMGA, and C-MGA were positive for S-100, laminin and collagen IV and negative for calponin and p63. Following NAC, these lesions retained their typical staining pattern despite acquiring treatment-related morphologic alterations, most notably of which were areas of single cell growth pattern seen in eight EXCs. This study is the first to report the effects of NAC on AMGA and C-MGA. Our data showed no response of the AMGA and/or C-MGA following NAC in contrast to the high response rate of conventional TNBC. In particular, the infiltrative single cell pattern of post-NAC MGA-related lesions closely mimicked residual TNBC. The persistence of AMGA and C-MGA following NAC supports the notion that these lesions are distinct from conventional TNBC. Our findings also highlight the challenges in recognizing AMGA and C-MGA in CNBs which may lead to unwarranted treatment with NAC in the absence of conventional TNBC.

Molecular Classification and Future Therapeutic Challenges of Triple-negative Breast Cancer

Triple-negative breast cancer (TNBC) is an extremely diverse group of breast tumors, with aggressive clinical behavior, higher rates of distant recurrence and worse overall survival compared to other types of breast cancers. The genetic, transcriptional histological and clinical heterogeneity of this disease has been an obstacle in the progression of targeted therapeutic approaches, as a ubiquitous TNBC marker has not yet been discerned. In terms of that, current studies focus on the classification of TNBC tumors in subgroups with similar characteristics in order to develop a treatment specialized for each group of patients. To date, a series of gene expression profiles analysis in order to identify the different molecular subtypes have been used. Complementary DNA microarrays, PAM50 assays, DNA and RNA sequencing as well as immunohistochemical analysis are some of the methods utilized to classify TNBC tumors. In 2012, the Cancer Genome Atlas (TCGA) Research Network conducted a major analysis of breast cancers using six different platforms, the genomic DNA copy number arrays, DNA methylation, exome sequencing, messenger RNA arrays, microRNA sequencing and reverse-phase protein arrays, in order to assort the tumors in homogenous subgroups. Since then, an increasing number of breast cancer data sets are being examined in an attempt to distinguish the classification with biological interpretation and clinical implementation. In this review, the progress in molecular subtyping of TNBC is discussed, providing a brief insight in novel TNBC biomarkers and therapeutic strategies.

Chromosome 2q gain and epigenetic silencing of GATA3 in microglandular adenosis of the breast

Microglandular adenosis (MGA) represents a rare neoplasm of the mammary gland, which in a subset of cases may be associated with triple-negative breast cancer (BC). The biology of MGA is poorly understood. In this study, eight MGA cases (n = 4 with and n = 4 without associated BC) were subjected to a comprehensive characterization using immunohistochemistry, genome-wide DNA copy number (CN) profiling, fluorescence in situ hybridization (FISH), next-generation sequencing (NGS), and DNA methylation profiling using 850 K arrays and bisulfite pyrosequencing. Median patient age was 61 years (range 57-76 years). MGA lesions were estrogen receptor (ER)-negative, progesterone receptor-negative, HER2-negative, and S100-positive. DNA CN alterations (CNAs) were complex or limited to few gains and losses. CN gain on chromosome 2q was the most common CNA and was validated by FISH in five of eight cases. NGS demonstrated an average of two mutations per case (range 0-5) affecting 10 different genes (ARID1A, ATM, CTNNB1, FBXW7, FGFR2, MET, PIK3CA, PMS2, PTEN, and TP53). CNAs and mutations were similar in MGA and adjacent BC, indicating clonal relatedness. DNA methylation profiling identified aberrant hypermethylation of CpG sites within GATA3, a key transcription factor required for luminal differentiation. Immunohistochemistry showed regular GATA3 protein expression in the normal mammary epithelium and in ER-positive BC. Conversely, GATA3 was reduced or lost in all MGA cases tested (8/8). In conclusion, MGA is characterized by common CN gain on chromosome 2q and loss of GATA3. Epigenetic inactivation of GATA3 may provide a new clue to the peculiar biology of this rare neoplasia.

Acinic Cell Carcinoma of the Breast: Report of a Case With Immunohistochemical and Next-Generation Sequencing Studies

Acinic cell carcinoma of the breast is a rare subtype of triple-negative breast cancer that recapitulates the appearance of tumors seen in salivary glands. We present the case of a 42-year-old woman with an irregular, nontender mass above the left nipple during routine obstetric appointment at 24 weeks gestation. She was subsequently diagnosed with triple-negative invasive ductal carcinoma of the left breast, Nottingham grade 3, via core needle biopsy. She was treated with neoadjuvant therapy (doxorubucin and cyclophosphamide) antenatally and paclitaxel in the postpartum period followed by left mastectomy with sentinel node biopsy. The carcinoma in the mastectomy specimen showed a spectrum of morphologic patterns with immunohistochemistry revealing strong positivity for alpha-1-antichymotrypsin, epithelial membrane antigen (EMA), lysozyme, and S100. The histomorphology paired with the immunoprofile led us to the diagnosis of acinic cell carcinoma. We retrospectively performed immunostains in the core biopsy specimen, which demonstrated GATA-3 and DOG-1 positivity. Next-generation sequencing of the postneoadjuvant specimen using a 70-gene panel revealed 2 single-nucleotide variant (SNV) mutations: tumor protein 53 (TP53) (c.747G>T) SNV mutation and rearranged during transfection (RET) (c.2899G>A) SNV mutation.

Problematic breast tumors reassessed in light of novel molecular data

Breast cancer is a vastly heterogeneous disease encompassing a panoply of special histological subtypes. Although rare breast tumors have largely not been investigated systematically in large scale genomics series, recent studies have shed light on the genetic underpinnings of special histologic subtypes of breast cancer. Genomic analyses of estrogen receptor-positive special histologic types of breast cancer have not resulted in the identification of novel pathognomonic genetic alterations in addition to the confirmation of the presence of CDH1 loss-of-function mutations in invasive lobular carcinomas. By contrast, the analyses of triple-negative breast cancers have demonstrated that low-grade triple-negative breast cancers categorically differ from the common forms of high-grade triple-negative disease biologically and phenotypically and are underpinned by specific fusion genes or hotspot mutations. A subset of low-grade triple-negative disease has been shown to harbor highly recurrent if not pathognomonic genetic alterations, such as ETV6-NTRK3 fusion gene in secretory carcinomas, the MYB-NFIB fusion gene, MYBL1 rearrangements or MYB gene amplification in adenoid cystic carcinomas, and HRAS Q61 hotspot mutations coupled with mutations in PI3K pathway genes in estrogen receptor-negative adenomyoepitheliomas. A subset of these pathognomonic genetic alterations (e.g., NTRK1/2/3 fusion genes) now constitute an FDA approved indication for the use of TRK inhibitors in the advanced/metastatic setting. These studies have also corroborated that salivary gland-like tumors of the breast, other than acinic cell carcinomas, harbor the repertoire of somatic genetic alterations detected in their salivary gland counterparts. Reassuringly, the systematic study of special histologic types of breast cancer utilizing state-of-the-art sequencing approaches, rather than rendering pathology obsolete, has actually strengthened the importance of breast cancer histologic typing and is providing additional ancillary markers for the diagnosis of these rare but fascinating entities.

[Microglandular adenosis of the breast in a patient with a history of mediastinal radiotherapy]

The risk of secondary tumors in patients who have received mediastinal radiation therapy is well-known. Microglandular adenosis of the breast is a rare lesion that is considered benign, although its possible role as a precursor of invasive breast carcinoma has been considered. We present a case of microglandular adenosis in a patient who received mediastinal radiation therapy in childhood for Hodgkin's lymphoma. To our knowledge, this is the first reported case of microglandular adenosis in a patient with mediastinal radiotherapy which may shed light on its pathogenesis.

[Histological lesions of risk of breast carcinoma. Survival guide for the general pathologist]

Proliferative epithelial lesions are risk factors for breast cancer. They are a heterogeneous group of lesions in which the presence of atypia is related to varying degrees of risk. They should be considered in the differential diagnosis with benign lesions, in situ ductal carcinoma and infiltrating carcinoma. An accurate histopathological diagnosis is important in choosing the best therapeutic option, including vacuum assisted biopsy and surgery. We revise diagnostic criteria and the differential diagnosis of usual ductal hyperplasia, radial scar and complex sclerosing lesions, distinct types of adenosis, papillary lesions, atypical ductal hyperplasia, flat epithelial atypia and lobular neoplasia in situ. Furthermore, we summarize the degree of risk associated with the different conditions and management possibilities.

Salivary gland-type mammary carcinoma arising in microglandular adenosis: A case report and clinicopathological review of the literature

INTRODUCTION

Microglandular adenosis (MGA) is a rare benign proliferative lesion lacking a myoepithelial cell layer; 27% of all reported cases have progressed to invasive carcinoma. Salivary gland-type carcinomas of the breast are also uncommon, representing 2% of all breast cancers. This wide spectrum of neoplasms tends to be triple negative and generally has an excellent prognosis. Given the rarity of salivary gland-type carcinomas of the breast arising from MGA, there are few reports of these cases in literature. As such, there is uncertainty regarding their diagnosis and treatment strategies.

PRESENTATION OF CASE

We report the rare case of a 66-year-old woman who presented with a triple negative, invasive carcinoma with salivary gland-type features, arising from MGA. The patient underwent mastectomy with sentinel lymph node biopsy, followed by Taxotere and Cyclophosphamide (TC) chemotherapy and 50 Gy in 25 fractions of radiation to her chest wall. We reviewed the available literature on salivary gland-type breast carcinomas arising from MGA.

DISCUSSION

Despite the generally unfavourable characteristics associated with carcinoma arising in microglandular adenosis (MGACA), most patients with MGACA have favourable outcomes.

CONCLUSIONS

The findings of the present case and reviewed cases are consistent with the literature on MGA, atypical MGA (AMGA), and MGACA. Future study of this rare entity is warranted to establish a consensus surrounding its clinical significance and treatment methods.

Whole-exome sequencing and RNA sequencing analyses of acinic cell carcinomas of the breast

AIMS

Acinic cell carcinoma (ACC) of the breast is a rare histological form of triple-negative breast cancer (TNBC). Despite its unique histology, targeted sequencing analysis has failed to identify recurrent genetic alterations other than those found in common forms of TNBC. Here we subjected three breast ACCs to whole-exome and RNA sequencing to determine whether they would harbour a pathognomonic genetic alteration.

METHODS AND RESULTS

DNA and RNA samples from three breast ACCs were subjected to whole-exome sequencing and RNA-sequencing, respectively. Somatic mutations, copy number alterations, mutational signatures and fusion genes were determined with state-of-the-art bioinformatics methods. Our analyses revealed TP53 hotspot mutations associated with loss of heterozygosity of the wild-type allele in two cases. Mutations affecting homologous recombination DNA repair-related genes were found in two cases, and an MLH1 pathogenic germline variant was found in one case. In addition, copy number analysis revealed the presence of a somatic BRCA1 homozygous deletion and focal amplification of 12q14.3-12q21.1, encompassing MDM2, HMGA2, FRS2, and PTPRB. No oncogenic in-frame fusion transcript was identified in the three breast ACCs analysed.

CONCLUSIONS

No pathognomonic genetic alterations were detected in the breast ACCs analysed. These tumours have somatic genetic alterations similar to those of common forms of TNBC, and may show homologous recombination deficiency or microsatellite instability. These findings provide further insights into why breast ACCs, which are usually clinically indolent, may evolve into or in parallel with high-grade TNBC.

Core needle biopsy of benign, borderline and in-situ problematic lesions of the breast: Diagnosis, differential diagnosis and immunohistochemistry

Core needle biopsy (CNB) is the most common sampling technique for the histologic evaluation of breast abnormalities. Diagnosing benign proliferative, borderline and some in-situ lesions in CNB is challenging and subject to a significant degree of interobserver variability. In addition, due to the inherent limitations of CNB, "upgrading" to a more significant pathology at excision is an important consideration for some lesions. Pathologists carry a major responsibility in patient diagnosis, risk stratification and management. Familiarity with the histologic features and the clinical significance of these common and problematic lesions encountered in CNB is necessary for adequate treatment and patient follow-up. This review will focus on benign, atypical and in-situ epithelial proliferations, papillary lesions, radial sclerosing lesions, adenosis and cellular fibroepithelial lesions. Highlights of histologic features, useful strategies for accurate diagnosis, basic immunohistochemistry and management will be presented.

Microglandular Adenosis: A Possible Non-Obligate Precursor to Breast Carcinoma With Potential to Either Luminal-Type or Basal-Type Differentiation

Microglandular adenosis (MGA) of the breast is exceedingly rare, with only a few case reports and series published to date. Previous studies have elegantly demonstrated the progression of benign MGA to atypical MGA to MGA-in situ carcinoma to invasive carcinoma and therefore suggest MGA as a possible non-obligate precursor lesion to a subset of breast carcinomas. Immunohistochemically, MGA is negative for estrogen receptor (ER), progesterone receptor (PR), and HER2-neu oncoprotein expression, and carcinomas arising in the setting of MGA are often reported to be triple negative. In this article, we present a unique case of an ER+/PR+/HER2- invasive carcinoma associated with MGA and atypical MGA. Our case highlights the diagnostic pitfall of MGA and suggests that MGA is a heterogeneous group of lesions with potential for either luminal-type or basal-type differentiation during progression to breast carcinoma.

Recent advances in breast cancer research impacting clinical diagnostic practice

During the last decade, the genomics revolution has driven critical advances in molecular oncology and pathology, and a deeper appreciation of heterogeneity that is beginning to reshape our thinking around diagnostic classification. Recent developments have seen existing classification systems modified and improved where possible, gene-based diagnostics implemented and tumour-immune interactions modulated. We present a detailed discussion of this progress, including advances in the understanding of breast tumour classification, e.g. mixed ductal-lobular tumours and the spectrum of triple-negative breast cancer. The latest information on clinical trials and the implementation of gene-based diagnostics, including MammaPrint and Oncotype Dx and others, is synthesised, and emerging targeted therapies, as well as the burgeoning immuno-oncology field, and their relevance in breast cancer, are discussed. Copyright © 2018 Pathological Society of Great Britain and Ireland. Published by John Wiley & Sons, Ltd.

High rate of PIK3CA mutations but no TP53 mutations in low-grade adenosquamous carcinoma of the breast

AIMS

Low-grade adenosquamous carcinoma of the breast (LGASC) is a rare variant of metaplastic carcinoma characterised by a favourable outcome and histologically composed of glandular and squamous elements in a spindle cell background typically associated with a lymphocytic stromal reaction. Because of its rarity, the immunophenotypic and genetic profile of LGASC has not been sufficiently characterised. The aim of this study was to gain insights into the molecular and phenotypic characteristics of LGASC.

METHODS AND RESULTS

We reviewed the clinical and morphological features and detailed the immunohistochemical characteristics of a retrospective series of 13 LGASCs. Targeted sequencing of 50 genes was performed in 10 of 13 cases. Identified mutations were further assessed by Sanger sequencing in a validation series of 11 additional cases. All tumours showed a triple-negative immunophenotype, expressed 'basal' keratins, showed variable levels of epidermal growth factor receptor expression, and did not express androgen receptor. Sequencing analysis of the screening set of LGASCs revealed a high rate (seven of 10 cases) of PIK3CA mutations, whereas no TP53 mutations were found. All PIK3CA mutations were missense mutations located either in exon 20 (n = 6) or in exon 9 (n = 1). The global PIK3CA mutation rate, including the validation series, was 52% (11 of 21 cases). No disease recurrences were observed. [Correction added on 11 June 2018, after first online publication: The percentage of mutation rate was corrected to 52%] CONCLUSIONS: Our results indicate that LGASC of the breast is a low-grade triple-negative breast cancer that harbours a basal-like phenotype with no androgen receptor expression, and shows a high rate of PIK3CA mutations but no TP53 mutations.

The Spectrum of Triple-Negative Breast Disease: High- and Low-Grade Lesions

Triple-negative breast cancer is viewed clinically as an aggressive subgroup of breast cancer. In fact, most triple-negative breast cancers are poor-prognosis tumors with a complex genomic landscape. However, triple-negative disease is vastly heterogeneous, encompassing multiple entities with marked genetic, transcriptional, histologic, and clinical differences, with neoplasms in this group ranging from low to high grade. Among the less common low-grade triple-negative lesions, two large subgroups, both with a rather indolent behavior, can be distinguished: a low-grade triple-negative breast neoplasia family, which includes nonobligate precursors of triple-negative breast cancer, and, despite being low-grade, harbors the complex genomic landscape of usual triple-negative breast cancer, and the salivary gland-like tumors of the breast, lacking all the cardinal molecular features of conventional triple-negative breast cancer and underpinned by specific fusion genes or hotspot mutations, which may be of diagnostic and possibly therapeutic utility. Progression to high-grade triple-negative breast cancer likely occurs in both subgroups but at different rates. In this review, we describe the heterogeneity of triple-negative disease, focusing on the histologic and molecular features of the low-grade lesions. Recognition that triple-negative breast cancer is an operational term and that triple-negative disease is heterogeneous and includes low-grade forms driven by distinct sets of genetic alterations is germane to the successful implementation of precision medicine.

Genetic analysis of uterine adenosarcomas and phyllodes tumors of the breast

Uterine adenosarcomas and breast phyllodes tumors (PTs) are morphologically similar, being composed of stromal projections in a leaf-like fashion lined by epithelial cells. Here, we investigated whether their histologic similarities would be mirrored at the genetic level. The previously reported repertoires of somatic genetic alterations found in 19 adenosarcomas and 22 PTs (six benign, six borderline, and 10 malignant) were compared. PTs significantly more frequently displayed mutations affecting MED12, the TERT gene promoter and bona fide cancer genes, whereas adenosarcomas harbored a higher rate of MDM2/CDK4 and TERT gene amplifications. Pathway analyses based on the genes affected by somatic genetic alterations in these tumors indicated that Wnt signaling likely plays a role in the biology of adenosarcomas and benign/borderline PTs. In conclusion, despite the differences at the gene level, PTs and adenosarcomas share remarkable morphologic similarities and enrichment for somatic genetic alterations affecting Wnt pathway-related genes.

Triple-negative breast cancer: the importance of molecular and histologic subtyping, and recognition of low-grade variants

Triple-negative breast cancers (TNBCs), defined by lack of expression of estrogen receptor, progesterone receptor and HER2, account for 12–17% of breast cancers and are clinically perceived as a discrete breast cancer subgroup. Nonetheless, TNBC has been shown to constitute a vastly heterogeneous disease encompassing a wide spectrum of entities with marked genetic, transcriptional, histological and clinical differences. Although most TNBCs are high-grade tumors, there are well-characterized low-grade TNBCs that have an indolent clinical course, whose natural history, molecular features and optimal therapy vastly differ from those of high-grade TNBCs. Secretory and adenoid cystic carcinomas are two histologic types of TNBCs underpinned by specific fusion genes; these tumors have an indolent clinical behavior and lack all of the cardinal molecular features of high-grade triple-negative disease. Recent studies of rare entities, including lesions once believed to constitute mere benign breast disease (e.g., microglandular adenosis), have resulted in the identification of potential precursors of TNBC and suggested the existence of a family of low-grade triple-negative lesions that, despite having low-grade morphology and indolent clinical behavior, have been shown to harbor the complex genomic landscape of common forms of TNBC, and may progress to high-grade disease. In this review, we describe the heterogeneity of TNBC and focus on the histologic and molecular features of low-grade forms of TNBC. Germane to addressing the challenges posed by the so-called triple-negative disease is the realization that TNBC is merely a descriptive term, and that low-grade types of TNBC may be driven by distinct sets of genetic alterations.