Use of immunohistochemistry in the diagnosis of problematic breast lesions

Nonneoplastic and neoplastic sclerosing lesions of the breast

Sclerosing lesions of the breast encompass a spectrum of benign and malignant entities and often pose a diagnostic challenge. Awareness of key morphologic features and pitfalls in the assessment of morphology and immunophenotype is essential to avoid over- or underdiagnosis and ensure optimal clinical management. This review summarizes nonneoplastic sclerosing lesions such as radial scar/complex sclerosing lesion, sclerosing adenosis, sclerosing intraductal papilloma, sclerosing variants of ductal adenoma and nipple adenoma, and fibroadenoma with extensive sclerosis, including their clinical presentation, characteristic morphology, differential diagnostic considerations, appropriate immunohistochemical work-up, when needed, and the clinical significance. In addition, atypical or neoplastic entities (such as atypical ductal hyperplasia, ductal carcinoma in situ, low-grade adenosquamous carcinoma, and fibromatosis-like metaplastic carcinoma) that can involve these sclerosing lesions are also briefly discussed.

Immunohistochemistry in the Diagnosis and Classification of Breast Tumors

CONTEXT.—

In the clinical practice of breast pathology, immunohistochemistry (IHC) of different markers is widely used for the diagnosis and classification of breast lesions.

OBJECTIVE.—

To provide an overview of currently used and recently identified IHC stains that have been implemented in the field of diagnostic breast pathology.

DATA SOURCES.—

Data were obtained from literature review and clinical experience of the authors as breast pathologists.

CONCLUSIONS.—

In the current review, we summarize the common uses of IHC stains for diagnosing different types of breast lesions, especially invasive and noninvasive breast lesions, and benign and malignant spindle cell lesions. In addition, the cutting-edge knowledge of diagnostic carcinoma markers will lead us to further understand the different types of breast carcinoma and differentiate breast carcinomas from other carcinomas of similar morphology. Knowing the strengths and limitations of these markers is essential to the clinical practice of breast pathology.

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.

Avoiding "False Positive" and "False Negative" Immunohistochemical Results in Breast Pathology

Immunohistochemistry (IHC) plays an important role in the evaluation of breast pathology specimens to provide both diagnostic and prognostic/therapeutic information. Although most IHCs used in breast pathology can be easily interpreted, pitfalls do exist, especially in some uncommon scenarios. This review intends to focus on the challenging areas such as the interpretation of myoepithelial cell markers in differentiating benign proliferation and in situ carcinoma from invasive carcinoma, lobular cell markers in differentiating lobular from ductal carcinoma, cytokeratin and other markers in diagnosing metaplastic carcinoma, and breast tissue origin markers in diagnosing breast primary carcinoma. The challenges in interpreting prognostic and predictive markers will be also discussed.

Metastatic uveal melanoma: The final frontier

Treatment of primary intraocular uveal melanoma has developed considerably, its driver genes are largely unraveled, and the ways to assess its risk for metastases are very precise, being based on an international staging system and genetic data. Unfortunately, the risk of distant metastases, which emerge in approximately one half of all patients, is unaltered. Metastases are the leading single cause of death after uveal melanoma is diagnosed, yet no consensus exists regarding surveillance, staging, and treatment of disseminated disease, and survival has not improved until recently. The final frontier in conquering uveal melanoma lies in solving these issues to cure metastatic disease. Most studies on metastatic uveal melanoma are small, uncontrolled, retrospective, and do not report staging. Meta-analyses confirm a median overall survival of 10-13 months, and a cure rate that approaches nil, although survival exceeding 5 years is possible, estimated 2% either with first-line treatment or with best supportive care. Hepatic ultrasonography and magnetic resonance imaging as surveillance methods have a sensitivity of 95-100% and 83-100%, respectively, to detect metastases without radiation hazard according to prevailing evidence, but computed tomography is necessary for staging. No blood-based tests additional to liver function tests are generally accepted. Three validated staging systems predict, each in defined situations, overall survival after metastasis. Their essential components include measures of tumor burden, liver function, and performance status or metastasis free interval. Age and gender may additionally influence survival. Exceptional mutational events in metastases may make them susceptible to checkpoint inhibitors. In a large meta-analysis, surgical treatment was associated with 6 months longer median overall survival as compared to conventional chemotherapy and, recently, tebentafusp as first-line treatment at the first interim analysis of a randomized phase III trial likewise provided a 6 months longer median overall survival compared to investigator's choice, mostly pembrolizumab; these treatments currently apply to selected patients. Promoting dormancy of micrometastases, harmonizing surveillance protocols, promoting staging, identifying predictive factors, initiating controlled clinical trials, and standardizing reporting will be critical steppingstones in reaching the final frontier of curing metastatic uveal melanoma.

Precision pathology as applied to breast core needle biopsy evaluation: implications for management

With the shift to de-escalation of therapy for some breast cancers and fewer surgical excisions for high-risk lesions identified on breast imaging studies at one end of the spectrum, and the greater use of neoadjuvant systemic therapy at the other end, pathologists are ever more critical in guiding management decisions for women with breast disease following core needle biopsy. One important consequence of this shift in management paradigms is the elimination of the opportunity for a "second-look" with the excision specimen to confirm or refine the diagnosis rendered on core needle biopsy. Thus, not only is there the imperative for accuracy and precision of core needle biopsy diagnoses, increasingly it is the only opportunity for that diagnosis.

TAp63 and ΔNp63 (p40) in prostate adenocarcinomas: ΔNp63 associates with a basal-like cancer stem cell population but not with metastasis

Like other malignancies, prostate tumors are thought to contain cancer stem-like cells (CSCs) that are responsible for growth, metastasis, and therapy resistance. ΔNp63 (also called p40) is a regulator of normal prostate stem/progenitor cell activities and a marker of normal basal epithelial cells. The levels of ΔNp63 are reduced in prostate adenocarcinomas, although there is also evidence that ΔNp63 is involved in CSC regulation and drives metastasis to the bone. We studied metastatic deposits of prostate cancers with isoform-specific ΔNp63 and TAp63 antibodies. We identified p63-positive cells in only 3 of 42 metastatic prostate tumors (7%), including 2/38 (5.3%) "usual-type" adenocarcinomas. ΔNp63 and TAp63 isoforms were present in the nuclei of a small subpopulation (< 1%) of tumor cells in these metastases. ΔNp63-positive cells showed a basal-like cell phenotype (cytokeratin 8- and androgen receptor-negative, high molecular weight cytokeratin- and cytokeratin 19-positive), distinct from the tumor bulk. TAp63-positive cells were similar but were sometimes cytokeratin 8-positive. A subset of ΔNp63-positive tumor cells were CD44-positive, a marker of "basal" CSCs but were not positive for the "epithelial" CSC marker ALDH1. TAp63 was not associated with either of these CSC markers. None of the tumors containing p63-positive cells showed evidence of bone metastasis, compared with 28% of the p63-negative tumors. These data show that both ΔNp63 and TAp63 are present in only a small proportion of prostate adenocarcinomas and do not associate with metastasis. The data suggest heterogeneity of CSCs in prostate cancer, similar to other cancer types.

Malignant melanoma of the breast: controversies in the diagnosis and therapeutic management of a rare nosologic entity

Melanoma of the breast is an infrequent entity, presenting as either primary or metastatic from extramammary solid neoplasms. Depending on the involvement of the skin, primary malignant melanoma of the breast (PMMB) can be classified as cutaneous or noncutaneous. Cutaneous PMMB accounts for <5% of all melanomas and only 0.5% of all breast cancers. Furthermore, despite the rarity of metastatic breast neoplasms, melanoma comprises a frequent source of metastases. Metastatic potential of melanoma is associated with poor prognosis, and the majority of patients present more than one metastatic foci at the time of diagnosis. Diagnostic approach for both primary and metastatic melanomas of the breast is substantiated by fine needle aspiration (FNA) cytology along with immunohistochemistry. Nevertheless, verification of a metastatic mammary melanoma requires the discovery of a primary lesion. The mainstay of treatment for primary tumors is surgery, with radical local excision and axillary lymph node dissection or, on occasion, axillary sentinel node resection, while for metastatic tumors it depends on the specificities of the melanoma. Adjuvant therapy is always implemented. The aim of this survey is to meticulously review the literature of primary and metastatic malignant melanomas of the breast and report epidemiologic and clinicopathologic data for this rare entity. Clinical features, histogenesis, morphological, and immunochemical findings are discussed, while the role of current diagnostic and therapeutic management of this uncommon entity is emphasized.

∆Np63/p40 correlates with the location and phenotype of basal/mesenchymal cancer stem-like cells in human ER+ and HER2+ breast cancers

ΔNp63, also known as p40, regulates stemness of normal mammary gland epithelium and provides stem cell characteristics in basal and HER2‐driven murine breast cancer models. Whilst ΔNp63/p40 is a characteristic feature of normal basal cells and basal‐type triple‐negative breast cancer, some receptor‐positive breast cancers express ΔNp63/p40 and its overexpression imparts cancer stem cell‐like properties in ER⁺ cell lines. However, the incidence of ER⁺ and HER2⁺ tumours that express ΔNp63/p40 is unclear and the phenotype of ΔNp63/p40⁺ cells in these tumours remains uncertain. Using immunohistochemistry with p63 isoform‐specific antibodies, we identified a ΔNp63/p40⁺ tumour cell subpopulation in 100 of 173 (58%) non‐triple negative breast cancers and the presence of this population associated with improved survival in patients with ER⁻/HER2⁺ tumours (p = 0.006). Furthermore, 41% of ER⁺/PR⁺ and/or HER2⁺ locally metastatic breast cancers expressed ΔNp63/p40, and these cells commonly accounted for <1% of the metastatic tumour cell population that localised to the tumour/stroma interface, exhibited an undifferentiated phenotype and were CD44⁺/ALDH⁻. In vitro studies revealed that MCF7 and T47D (ER⁺) and BT‐474 (HER2⁺) breast cancer cell lines similarly contained a small subpopulation of ΔNp63/p40⁺ cells that increased in mammospheres. In vivo, MCF7 xenografts contained ΔNp63/p40⁺ cells with a similar phenotype to primary ER⁺ cancers. Consistent with tumour samples, these cells also showed a distinct location at the tumour/stroma interface, suggesting a role for paracrine factors in the induction or maintenance of ΔNp63/p40. Thus, ΔNp63/p40 is commonly present in a small population of tumour cells with a distinct phenotype and location in ER⁺ and/or HER2⁺ human breast cancers.

ALK or ROS1-rearranged breast metastasis from lung adenocarcinoma: a report of 2 cases

BACKGROUND

Breast metastases from extramammary sites are extremely rare, with an incidence of 0.4%-1.3% reported in the literature. Insufficient knowledge about its pathology and genetic mutation often leads to misdiagnosis and delayed treatment.

CASE PRESENTATION

We present 2 patients with synchronous (case 1) or metachronous (case 2) breast nodules, revealed as atypical breast metastases from pulmonary adenocarcinomas, in which ALK or ROS1 rearrangement were finally detected in breast mass. After they failed sequential treatments with standard chemotherapies for lung and breast cancer, we reexamined breast lump biopsy and found they were all lung adenocarcinoma metastasis to the breast with the presence of an ALK rearrangement in one case and of ROS1 rearrangement in the other. Eventually these 2 patients were treated with crizotinib. Both the primary tumor and the metastasis of these 2 patients were significantly regressed.

CONCLUSION

Whenever a diagnosis of a suspected tumor with sites at more than one organ is made, pathologic as well molecular pathologic examinations designated for organ-specific cancers should be done.

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.

Merkel cell carcinoma, melanoma, metastatic mimics of breast cancer

Merkel cell carcinoma and melanoma can each occur primarily in breast skin, or metastasize to the breast. The breast is a rare site of metastasis of essentially any and every type of tumor, including carcinomas, sarcomas, and hematolymphoid neoplasms, and 10-30% of breast metastases may represent the initial presentation of disease. Although metastases generally recapitulate histologic features of the primary tumor, they are diagnostically challenging given their rarity and morphologic overlap with breast carcinoma, including special types of breast cancer. Histologic clues may include lack of carcinoma in situ, lack of central elastosis, pattern of infiltration around normal breast structures, yet none of these are specific. Careful correlation with clinical history and judicious use of immunostain panels is essential in approaching these cases.

Multiplexed imaging reveals heterogeneity of PI3K/MAPK network signaling in breast lesions of known PIK3CA genotype

PURPOSE

Activating genetic changes in the phosphatidylinositol-3-kinase (PI3K) signaling pathway are found in over half of invasive breast cancers (IBCs). Previously, we discovered numerous hotspot PIK3CA mutations in proliferative breast lesions. Here, we investigate the spatial nature of PI3K pathway signaling and its relationship with PI3K genotype in breast lesions.

METHODS

We identified PI3K phosphosignaling network signatures in columnar cell change (CCL), usual ductal hyperplasia (UDH), ductal carcinoma in situ (DCIS), and IBC in 26 lesions of known PIK3CA genotype from 10 human breast specimens using a hyperspectral-based multiplexed tissue imaging platform (MTIP) to simultaneously quantitate PI3K/MAPK pathway targets (pAKT473, pAKT308, pPRAS40, pS6, and pERK) in FFPE tissue, with single-cell resolution.

RESULTS

We found that breast lesional epithelia contained spatially heterogeneous patterns of PI3K pathway phosphoprotein signatures, even within microscopic areas of CCL, UDH, DCIS, and IBC. Most lesions contained 3-12 unique phosphoprotein signatures within the same microscopic field. The dominant phosphoprotein signature for each lesion was not well correlated with lesion genotype or lesion histology, yet samples from the same patient tended to group together. Further, 5 UDH/CCL lesions across different patients had a common phosphosignature at the epithelial-stromal interface (possible myoepithelial cells) that was distinct from both the adjacent lesional epithelium and distinct from adjacent stroma.

CONCLUSION

We present the first spatial mapping of PI3K phosphoprotein networks in proliferative breast lesions and demonstrate complex PI3K signaling heterogeneity that defies simple correlation between PIK3CA genotype and phosphosignal pattern.

Estrogen Receptor and Cytokeratin 5 Are Reliable Markers to Separate Usual Ductal Hyperplasia From Atypical Ductal Hyperplasia and Low-Grade Ductal Carcinoma In Situ

Context.—High–molecular weight cytokeratins, such as cytokeratin 5 (CK5), are helpful to distinguish usual ductal hyperplasia (UDH) from atypical ductal hyperplasia (ADH) or low-grade ductal carcinoma in situ (DCIS). Few studies have looked at combining CK5 with estrogen receptor (ER) to differentiate UDH from ADH.

Objective.—To evaluate the expression pattern of CK5 and ER as single or combined markers to separate UDH from ADH and low-grade DCIS.

Design.—A total of 23 ADH, 10 low-grade DCIS, and 32 UDH whole-tissue slides were stained for ER, CK5, progesterone receptor (PR), and Bcl-2. Nuclear staining of ER and PR was scored as diffuse (&gt;80%), focal (10%–80%), or negative (&lt;10%). Cytoplasmic staining of CK5 and Bcl-2 was scored as diffuse (&gt;60%), focal (10%–60%), or negative (&lt;10%). Differences in staining patterns were evaluated.

Results.—For ER staining: 94% of ADH/DCIS cases showed a diffuse staining pattern, whereas none of the 32 UDH cases showed diffuse staining. For CK5 staining: 96% of ADH/DCIS cases were negative or focally positive, whereas all 32 UDH cases had diffuse staining. The combination of ER and CK5 increased the sensitivity (94% to 97%). For PR staining: 11 of 23 ADH cases (48%), 6 of 10 DCIS cases (60%), and 4 of 32 UDH cases (13%) showed diffuse staining. Bcl-2 staining showed no statistical significance (P = .73).

Conclusions.—Although morphology remains the gold standard, ER and CK5 are useful makers to differentiate UDH from ADH. Progesterone receptor staining may have limited value, and Bcl-2 staining is not useful.

Breast carcinoma metastasis to the lacrimal gland: Two case reports

A 77-year-old female, with proptosis, reduced eye motility and diplopia which had developed over two to three months and a 69-year-old female with proptosis, oedema of the eyelid, reduced motility and ptosis, which had developed over three weeks, are presented in the present study. Computed tomography scans revealed irregular lacrimal gland tumours in the two patients. The two patients had history of breast cancer. The first breast cancer metastasis in the lacrimal gland demonstrated a cribriform growth pattern containing ductal elements. The epithelial tumour cells stained positive for cytokeratin (1-8, 10, 14-16, 18 and 19), oestrogen receptor, epithelial membrane antigen (EMA), carcinoembryonic antigen (CEA) and gross cystic disease fluid protein 15 (GCDFP-15). The second metastatic tumour was positive for EMA and estrogen receptor, but variably positive for CEA and GCDFP-15. The metastasis in the lacrimal gland was a pleomorphic tumour. The tumour cells were positive for EMA and variably positive for oestrogen and CEA. Metastases to the lacrimal gland are extremely rare, and metastases to the lacrimal gland should be considered in the diagnoses of lacrimal gland tumours. The present study aimed to describe two such cases and draw attention to breast carcinomas as a differential diagnosis and the most frequent cause of lacrimal gland metastasis.

Immunostaining of ∆Np63 (using the p40 antibody) is equal to that of p63 and CK5/6 in high-grade ductal carcinoma in situ of the breast

As a result of breast cancer screening programs, high-grade ductal carcinoma in situ (DCIS) of the breast is diagnosed more often. Frequently, a DCIS diagnosis can only be made using immunohistochemical stains to visualize the myoepithelial layer in order to assess microinvasion. Standard markers for myoepithelial cells are CK5/6 and p63. An isoform of the latter, ∆Np63, is recognized by a recently developed antibody, p40. Here, we compare the standard myoepithelial markers CK5/6 and p63 with p40. We immunostained full sections of tissue samples of 35 high-grade DCIS and compared the staining pattern of CK5/6, p63 and p40 in tumour tissue and in normal glands. Staining patterns of myoepithelial cells for p63 and p40 were similar in terms of the percentage of stained nuclei. In all cases, p63 was strongly expressed, while this was the case for p40 in 31 (89%) and moderately in 4 (11%) cases. All but one case (97%) showed a similar percentage of stained myoepithelial cells in comparing CK5/6 and p40 staining. CK5/6 expression was heterogeneous and strong/moderate/weak in 60, 34 and 6 % respectively. Compared to surrounding normal glands, staining of myoepithelial cells for all three markers in the neoplastic lesion was attenuated. In high-grade DCIS, p40 staining is highly specific for myoepithelial cells. Its staining pattern and intensity are equal to p63, which opens up its use for daily practice. Staining with p40 is less heterogeneous than that for CK5/6.

Lobular carcinoma of the breast with gastrointestinal metastasis

We present the case of a 74-year-old woman with metastatic lobular carcinoma with an occult breast primary presenting as a suspected ampullary tumor due to its ampullary metastasis. The patient's clinical presentation is of interest in two aspects. First, lobular carcinoma of the breast metastatic to the ampulla is extremely rare. Second, in the absence of a detectable primary lesion, prior history of malignancy, or distinguishing clinical, radiological, and endoscopic features, histopathological assessments are pivotal for arriving at the appropriate diagnosis and for optimizing treatment.

Elucidating encounters of atypical ductal hyperplasia arising in gynaecomastia

AIMS

Atypical ductal hyperplasia (ADH) rarely arises in gynaecomastia. We set out to understand more clearly the clinical, histological and immunohistochemical features of ADH in this setting.

METHODS AND RESULTS

Twenty-five cases of ADH arising in gynaecomastia, nine cases of ductal carcinoma in situ (DCIS) and 36 cases of gynaecomastia with usual ductal hyperplasia (UDH) were studied. Reviews of clinical, morphological and immunohistochemical findings were performed. The extent of cytokeratin 5/6 (CK5/6) luminal epithelial cell staining was assessed (0% = 0, < 10% = 1, 10-50% = 2 and > 50% = 3). Oestrogen receptor (ER) was evaluated using the H-scoring system. The average age of ADH patients was 35 years (range 14-78). ADH was bilateral in 20% and less frequent in active gynaecomastia (24%). ADH often showed a cribriform pattern (72%), with less nuclear variation/size and similar frequency of mitoses than UDH cells. CK5/6 luminal epithelial staining was decreased in ADH (68%) versus UDH (11%). ADH showed high ER expression compared to UDH (H score > 270 in 88% and 14%, respectively).

CONCLUSIONS

ADH in gynaecomastia can be distinguished from UDH by morphological and immunohistochemical features. We also identified a subset of young patients (< 25 years) with extensive bilateral ADH. More studies are needed to characterize this patient subset more clearly.

Primary malignant melanoma of the breast: A case report and review of the literature

Malignant melanoma predominantly occurs in the skin and mucous membranes, thus, malignant melanoma of the breast is particularly rare. In the current study, a case of a 26-year-old female with a malignant melanoma of the breast is presented. On diagnosis of the patient, extensive metastasis had occurred. The patient refused any treatment and succumbed two months after the initial diagnosis. The prognosis for patients with this rare tumour of the breast is somewhat poor. Early diagnosis, correct surgical resection and comprehensive adjuvant therapy are the key procedures that may improve the patient survival rate. The current case report aims to increase the awareness of uncommon tumours of the breast.

Clinicopathological significance of 'atypical ductal proliferation' in core needle biopsy of the breast

Atypical ductal proliferation or ADP has been used in histopathological diagnosis of core needle biopsy (CNB) but its details have not been well studied. Therefore, we examined the clinicopathological characteristics of the initial CNB cases diagnosed as 'ADP ' who subsequently turned out to be malignant, and compared the findings to those that did not. Among 101 cases initially diagnosed as ADP in CNB, the second biopsy revealed no carcinoma (38), ductal carcinoma in situ (DCIS) (45) and invasive carcinoma (18). Significant differences were detected between those which turned out to be carcinoma and those that did not, in the status of myoepithelial cells identified by p63 immunohistochemistry (P = 0.026) and ultrasound (US) categories (P < 0.001). We further compared the histopathological characteristics of those initially diagnosed as ADP and subsequently as DCIS or invasive ductal carcinoma (IDC) with those initially diagnosed as such. DCIS or IDC cases initially diagnosed as ADP had significantly lower Ki67 labeling index (P < 0.01, P < 0.01) and histological grade using Van nuys prognostic index (P < 0.01) or Nottingham histological grades (P < 0.01) respectively than those initially as DCIS or IDC. An assessment of myoepithelial components with US findings might contribute to determine the subsequent clinical algorithm of the patients diagnosed as ADP at initial CNB.

[Papillary lesions of the breast]

Papillary lesions of the breast encompass a spectrum of both benign and malignant lesions despite sharing a similar basic architecture. A reliable distinction between the different entities is possible even in biopsies with precise knowledge of the diagnostic criteria and using immunohistochemistry as a diagnostic adjunct. These include papilloma, papillary ductal carcinoma in situ, encapsulated papillary carcinoma and solid papillary carcinoma. Architectural features, cellular composition and distribution of myoepithelial cells as highlighted by immunohistochemistry are the major diagnostic criteria. In this review the most useful morphological and immunohistochemical criteria for routine diagnostic practice are presented.