Diagnostic Immunohistology of the Breast

Introduction

For diagnosing a majority of breast lesions, a high-quality hematoxylin-eosin (H&E) stained section is all that is needed. However, in our quest for improved diagnostic accuracy, immunohistochemistry (IHC) is frequently used in diagnostic breast pathology. The sheer volume and genuine difficulty of some cases, even for breast experts, result in frequent use of IHC in day-to-day practice.

This chapter addresses diagnostic issues involving stromal invasion, papillary lesions, atypical proliferative lesions, discrimination of ductal and lobular neoplasia, and identification of other breast tumor types, Paget’s disease of the breast, metastatic breast carcinomas, and fibroepithelial lesions. The prognostic and predictive markers in breast carcinomas are discussed in a separate chapter.

Assessment of Stromal Invasion

The distinction between invasive and in situ lesions may sound trivial, but it is not uncommon to deal with this situation in the daily practice of breast pathology. The most common scenario is the presence of extensive, high-grade ductal carcinoma in situ (DCIS) with foci suggestive of microinvasion. The other lesion categories that typically need to be differentiated include non-neoplastic proliferative lesions or pseudoinvasive lesions (sclerosing adenosis, radial scar, sclerosing papillary lesions), and invasive carcinomas.

In all of these diagnostic situations, it is the presence of the myoepithelial cell (MEC) in intimate relationship with the epithelial cells of the lesion that determines the difference between in situ and invasive disease and between benign pseudoinvasive lesions and invasive carcinomas. Microglandular adenosis, a distinct nonorganoid benign form of adenosis, is a well-known exception to this statement (see below). An extremely rare lesion called infiltrating epitheliosis also lacks a peripheral layer of myoepithelial cells. Some other sclerosing lesions (such as radial scar) can also show patchy loss of myoepithelium. Adenomyoepitheliomas show loss of some of the myoepithelial markers in the periphery of the lesion. Barring some exceptions, the presence of MECs that envelop ductal-lobular epithelium, situated on the epithelial basal lamina, has always been considered the important criterion that separates invasive from noninvasive neoplasms. Myoepithelial cells can be visualized rather easily in normal breast ductules and acini, but when these structures dilate and fill with proliferating cells, or are compressed, it is virtually impossible to visualize them on H&E stain. Several antibodies used in the past several years (S100, high molecular weight keratin, CD10, maspin, smooth muscle actin) have been gradually replaced by more sensitive and specific antibodies (calponin, myosin heavy chain, p63) to the myoepithelium ( Table 11.1 ).

Antibodies to S100 protein are not sensitive or specific for MEC nor stain MEC in an erratic manner. In addition, the recent use of antibodies to maspin and CD10 has been tempered by the fact that they stain a variety of cell types, including luminal cells of the terminal duct lobular unit and tumor cells. Cytokeratin cocktail antibodies (34 beta E12) in addition to CK5, CK14, and CK17 identify MEC, but the staining is often inconsistent; in addition, they immunostain acinar cells, which makes it difficult to differentiate MECs because of their proximity to these cells. Moreover, high molecular weight keratins are inconsistent in their staining for MECs. Anti-smooth muscle actins react with stromal myofibroblasts in addition to MECs and thus are not specific for MECs. The cross-reaction with myofibroblasts makes it difficult to identify MECs specifically, especially in DCIS, in which there may be periductal stromal desmoplasia. Anti-smooth muscle actin (DAKO, Carpinteria, Calif.) and muscle-specific actin HHF-35 (Enzo, Farmingdale, N.Y.) stain MECs in the majority of benign breast lesions, but there is substantial cross-reaction with stromal myofibroblasts, especially with smooth muscle actin (SMA).

Calponin and smooth muscle myosin heavy chain (SMMHC) are two antibodies that are more specific for MECs. SMMHC is a structural component (200 kD) unique to smooth muscle cells that functions within the hexagonal array of the thick-thin filament contractile apparatus. Calponin, a 34 kD polypeptide, modulates actomyosin adenosine triphosphatase (ATPase) activity in the smooth muscle contractile apparatus and is unique to smooth muscle. In their analysis of 85 breast lesions, Werling and colleagues found that calponin and SMMHC always detected MEC in benign lesions and that SMMHC stained myofibroblasts in 8% of cases compared to calponin, which stained myofibroblasts in 76% of cases. It is also our experience that SMMHC and calponin are excellent antibodies, but calponin does stain stromal myofibroblasts to a greater extent than SMMHC.

p63, a homologue of the tumor suppressor protein p53, has gained use as a multitasker in multiple organs for the detection of MEC, basal cells (prostate), and myoepithelial differentiation (breast metaplastic carcinomas and salivary gland tumors), and as a marker for squamous differentiation. The advantages of p63 in the diagnosis of stromal invasion are that it is present only in the nucleus, which renders it most specific for MECs in the breast, and that it does not stain myofibroblasts. Some have used a cocktail of dual-staining for SMMHC and p63 together. In our experience, using SMMHC and p63 is optimal for discerning MEC on difficult breast biopsies, especially diagnostic core biopsies ( Figs. 11.1 to 11.5 ). Another antibody, p40 (also known as ΔNp63, an isoform of p63), used more frequently in lung pathology to identify squamous differentiation, has a staining pattern similar to p63 in the breast. Distinguishing DCIS from invasive carcinomas on core biopsies can be crucial, as almost all patients with invasive carcinomas will have a sentinel lymph node biopsy.

An important pitfall to note is that around 1% to 5% of DCIS cases (especially DCIS in the background of a papillary lesion) completely lack MEC using any antibody ( Fig. 11.6 ). In these situations, critical appraisal of the histological section is crucial to arrive at the correct diagnosis. It is also important to remember that p63 nuclear immunostaining results in apparent gaps of immunostaining because staining of cytoplasm of the MEC does not occur ( Fig. 11.7 ). Any nuclear staining around nests of tumor cells can be construed as evidence of the presence of MECs. Special care must be taken to exclude nuclear staining of tumor cells around the periphery of neoplastic ducts, as p63 stains tumoral cells in approximately 10% to 15% of cases ( Fig. 11.7 ).

Lesions that are especially difficult on core biopsies include the distinction of carcinoma in situ from invasive carcinoma in the presence of prominent periductal stromal desmoplasia (regressive changes) or heavy lymphoid infiltrates; infiltrating cribriform carcinoma; sclerosing adenosis (with or without DCIS involvement); cancerization of lobules; radial scars with stromal elastosis-desmoplasia; tubular carcinoma; and sclerosing papillary lesions. The optimal MEC antibodies needed to resolve these difficult cases include both SMMHC and p63 ( Figs. 11.8 to 11.12 ). A significant pitfall for misinterpretation of MEC antibodies such as calponin and even SMMHC is that these antibodies may immunostain the microvasculature around tumor nests.

IHC for MECs is also useful to help discriminate the three dominant benign lesions of the breast—sclerosing adenosis, microglandular adenosis (MGA), and tubular carcinoma ( Table 11.2 )—but a detailed morphological study of the lesion is essential. The MECs are seen by IHC in all forms of adenosis except the microglandular form, the only benign lesion that is known not to contain MECs. However, basement membrane is always present around MGA as highlighted by collagen IV stain ( Fig. 11.13 ). In addition to the distinct nonorganoid morphology of MGA, tubular adenosis, described by Lee and colleagues, may mimic both MGA and carcinoma but differs from MGA in that it contains MECs. Microglandular adenosis is positive with S100 protein, whereas sclerosing adenosis and tubular carcinomas are S100 negative (S100–).

Although the presence of the peripheral layer of myoepithelium indicates a benign or noninvasive lesion and the absence of this layer indicates an invasive and potentially malignant lesion, there are some exceptions to the rule. The benign lesions that show complete loss of the peripheral layer of myoepithelium include microglandular adenosis and infiltrating epitheliosis. Patchy loss is not uncommonly seen in several sclerosing lesions, such as radial scar and sclerosing papillomas. A peculiar phenomenon is seen in benign adenomyoepitheliomas where expression of some myoepithelial markers is significantly reduced while other markers are still expressed. The noninvasive or other indolent lesions that show absence of myoepithelium in the periphery include encapsulated and solid papillary carcinomas, and tall cell carcinoma with reverse polarity (see section on immunohistochemistry of papillary lesions). In contrast, some malignant tumors express staining for myoepithelial markers, such as expression of p63 around tubules in low-grade adenosquamous carcinoma and frequent p63 staining in adenoid cystic carcinomas. The presence or absence of the peripheral layer of myoepithelial cells is an important characteristic to consider in the diagnosis of breast lesions; however, a diagnosis of invasion or potential for malignancy is defined by the overall features of a particular lesion rather than by a single stain.

Immunohistochemistry of papillary lesions

Papillary lesions range from benign papillomas to atypical papillomas to papillary carcinomas (in situ and invasive). There are several reports on the use of myoepithelial cell markers to distinguish between the different categories. A papillary lesion could be classified as a papilloma if there is a uniform layer of myoepithelial cells in the proliferating intraluminal component of the lesion, whereas the absence of myoepithelial cells would be suggestive of a papillary carcinoma. Some papillomas show the features of an atypical papilloma—that is, areas in which there is atypical ductal epithelial hyperplasia that overgrows the papilloma. These atypical areas lack MECs by immunoperoxidase examination. Atypical papillomas and papillary carcinoma in situ also lose high molecular weight keratin immunostaining with 34βE12 and CK5/6. The distinction between a papilloma, atypical papilloma, and papillary DCIS (either de novo or DCIS involving papilloma) is quite straightforward in the majority of cases and can be made using morphology and IHC staining ( Fig. 11.14 ). The more difficult and confusing area is the distinction between a well-circumscribed papillary tumor and an invasive carcinoma. Well-circumscribed papillary tumors have been referred to by different names in the literature. The term intracystic papillary carcinoma has been used for a single mass-forming cystic lesion with malignant papillary proliferation while papillary ductal carcinoma in situ is a term that has been used for more diffuse lesions.

The use of myoepithelial cell markers to assess invasion in these lesions has yielded variable results. In an IHC study of papillary breast lesions, Hill and Yeh found consistent staining patterns in cases originally diagnosed as papilloma or invasive papillary carcinoma, but found variable staining in cases diagnosed as intraductal papillary carcinoma. Of the nine intraductal papillary carcinomas in their series, four cases showed unequivocal basal myoepithelial cells by IHC, one case showed partial discontinuous staining, and four cases were predominantly negative for basal myoepithelial cells. The authors found that lesions originally classified as intraductal papillary carcinoma but lacking basal myoepithelial cells by IHC were uniformly large, expansile papillary lesions with pushing borders and a fibrotic rim. The authors hypothesized that such lesions form a part of the spectrum of progression between in situ and invasive disease and suggested that these lesions should be termed encapsulated papillary carcinoma . Collins et al. have also favored such a designation. Subsequent reviews of papillary lesions have made an attempt to classify the lesions in a uniform fashion using morphology and IHC. Papillary lesions are now classified as papilloma, papilloma with atypical ductal hyperplasia (atypical papilloma), papilloma with DCIS, papillary DCIS, intracystic papillary carcinoma (encysted or encapsulated papillary carcinoma or EPC), solid papillary carcinoma (SPC), and invasive papillary carcinoma. Some have included both intracystic and solid papillary carcinomas under the umbrella term encapsulated papillary carcinoma .

The problem in diagnosis arises from the fact that encapsulated and solid papillary carcinomas have the morphology of an in situ lesion but lack the presence of MECs around the periphery ( Fig. 11.15 ). Since type IV collagen is an integral component of the basal lamina (BL) that envelops normal and proliferative benign lesions, we studied its expression in intracystic papillary carcinoma and compared it to a variety of papillary lesions and invasive carcinomas. We have found that a continuous strong collagen type IV staining is often seen around the periphery of encapsulated papillary carcinomas similar to benign lesions and DCIS, but generally a weak and discontinuous type of staining was seen around invasive carcinomas. Our results were very similar to a previous study regarding the usefulness of collagen type IV which was published several years ago. This pattern of staining supports the in situ nature of most encapsulated papillary carcinomas. Moreover, the clinical behavior of these lesions is more akin to in situ disease. A subsequent study, however, did not find the collagen IV stain helpful. Due to inconsistent staining with collagen IV, interpretation issues, and periodic collagen IV staining around carcinomas at metastatic sites, we do not recommend using it in routine clinical practice. Encapsulated papillary carcinomas and solid papillary carcinomas are circumscribed papillary tumors that often lack basal myoepithelial cells around the periphery, and in the absence of frank invasion (invasion into fat or within/beyond the fibrotic rim), their behavior is similar to in situ carcinomas. However, it is extremely important to analyze the resection specimen on these lesions in their entirety through histological evaluation due to the not-so-infrequent presence of frank invasion in the periphery of these lesions. We believe it is these foci of invasive carcinomas that are mostly responsible for occasional metastatic disease reported with encapsulated papillary carcinomas. Given that sentinel lymph node mapping is not a highly morbid procedure, it should be offered to patients diagnosed with encapsulated or solid papillary carcinoma on core needle biopsy.

In summary, encapsulated and solid papillary carcinomas are circumscribed papillary tumors that lack peripheral myoepithelial cells, have a low risk of local recurrence, and have a very low risk of distant recurrence. These lesions (without associated frank invasion) should be managed like DCIS in most instances. On the flip side, one should also be mindful of not overdiagnosing EPC and SPC. Tumors with solid papillary growth and large confluent/coalescing nodules are invasive carcinomas and should not be called SPC. Tumors with an SPC and EPC growth pattern but pleomorphic grade 3 nuclei and estrogen receptor (ER)-negative (ER–) status likely represent triple-negative breast cancers, which have significant potential to metastasize and should be called invasive carcinomas. EPC and SPC terminology should be limited to only grade 1 and grade 2 ER-positive (ER+) tumors. Lastly, metastatic tumors (particularly gynecological serous cancers) may occasionally present with an EPC/SPC-like growth pattern, and when suspicion is high they should be evaluated using an IHC panel.

There are two other lesions which have papillary architecture but are usually not considered in the differential diagnosis of papillary lesions due to their uncommon occurrence. The first lesion is an adenomyoepithelioma, which, as the name suggests, shows an admixture of glandular and myoepithelial cells. Adenomyoepitheliomas often arise in the background of adenosis or a papillary lesion. A pancytokeratin stain highlights glandular elements, and the myoepithelial cells (which are often in abundance) are highlighted by myoepithelial markers. However, the reactivity for myoepithelial markers varies from case to case, and it is not uncommon to see a patchy loss of some myoepithelial markers in the periphery of the lesion. This latter finding should not be misconstrued as evidence of invasion. Most adenomyoepitheliomas are clinically benign, with only occasional cases reported to undergo malignant transformation. Malignancy in adenomyoepithelioma is diagnosed based on morphological features and infiltration into the surrounding tissue rather than on IHC stains.

The second uncommon lesion is the so-called “breast tumor resembling tall cell variant of papillary thyroid carcinoma” (BTRPTC). This lesion has characteristic cytomorphological features but often comes across as a solid papillary lesion. Clinically, it is identified as an incidental mass lesion measuring 1 to 3 cm and showing solid papillary nests of bland-appearing proliferating epithelium. The epithelium can show grooves and occasional nuclear inclusions. The lesional nuclei are arranged away from the stromal aspect (known as reverse polarization), and hence it is also referred to as a solid papillary neoplasm with reverse nuclear polarization (SPNRP).

Today both BTRPTC and SPNRP are called tall cell carcinomas with reverse polarity (TCCRP). The proliferating epithelium of TCCRP stains strongly for CK5 (similar to usual ductal hyperplasia), but is only weak and patchy positive for ER ( Fig. 11.16 ). The cells often express S100 and calretinin and, occasionally, smooth muscle myosin heavy chain, indicating myoepithelial cell differentiation. Since TCCRP often demonstrates mutation of the IDH2 gene, a mutant antibody for IDH2 protein has been shown to stain TCCRP cells, while other papillary lesions have been reported to be negative. The exact clinical course of this lesion is uncertain, but limited clinical experience suggests an indolent clinical course. The lesion may locally recur if incompletely excised. The potential for distant recurrence is questionable. TCCRP is a distinct lesion with a characteristic staining pattern that needs to be distinguished from other established papillary lesions for proper patient management. The IHC staining pattern for each papillary lesion is summarized in Table 11.3 .

Proliferative Ductal Epithelial Lesions and in Situ Carcinomas

Differences in cytokeratin expression have been described between hyperplasia and ductal carcinoma in situ (DCIS). The antibody 34βE12 recognizes CK1, CK5, CK10, and CK14, and these keratins are typically found in duct-derived epithelium and squamous epithelium. Normal breast MECs and luminal cells express 34βE12, as does proliferative duct epithelium of the usual type ( Fig. 11.17 ). The expression is generally lost in atypical ductal hyperplasia (ADH). DCIS is largely negative in 81% to 100% of cases for 34βE12 ( Fig. 11.17 ), but may show some positive cells. Most cases of DCIS are uniformly positive for CAM5.2, reflecting a shift away from high molecular weight keratins to the simpler keratins 8 and 18. The 34βE12 immunostaining profile for DCIS and ADH is very similar and cannot be used to help distinguish DCIS from ADH, but it can be an aid to histomorphology in separating DCIS from florid usual ductal hyperplasia (UDH) in difficult cases. The clone D5/16B4 antibody CK5/6 is largely negative in DCIS. This expression of high molecular weight keratins (or basal keratins) in usual hyperplasia with loss in ADH and DCIS suggests that atypical lesions try to acquire a more “luminal” phenotype. Adding to the same theme, usual hyperplasia is generally negative or weak/patchy positive for ER, but atypical hyperplasia and low/intermediate-grade DCIS are often strongly ER+. So, in a lesion with ambiguous morphology for ADH, a combination of CK5 and ER may be helpful in rendering a more definitive diagnosis. A CK5+ and ER low/negative immunophenotype of the proliferative component would favor usual hyperplasia, while the opposite (a CK5-negative [CK5–], ER+) profile would favor ADH/DCIS. There are a few pitfalls for using these IHC stains for making a diagnosis of ADH/DCIS. First, this panel is not valid for columnar cell lesions as even benign columnar cell changes strongly express ER. Second, apocrine DCIS or atypical apocrine lesions (atypical apocrine adenosis) are generally negative for ER and variably express CK5. Finally, basal-like DCIS is almost always CK5+ and ER–. Therefore, CK5 and ER should be used in conjunction with defined morphological criteria for diagnosing ADH/DCIS.

The diagnosis of atypia in papillary lesions is also very challenging. Fortunately, the same cytokeratin patterns of immunostaining hold up for the differential of ADH/DCIS in a papilloma versus florid hyperplasia in a papilloma.

Tumor Type Identification by Immunohistochemistry

Cell Adhesion: Ductal Versus Lobular Carcinoma

Based on cell cohesiveness, the two broad categories of breast carcinoma (invasive and in situ) are ductal and lobular. Ductal carcinoma in situ (DCIS) increases the risk of invasive malignancy at the local site whereas lobular carcinoma in situ (LCIS) is considered a marker of generalized increased risk of invasive malignancy, although data also suggest precursor properties for LCIS. Invasive ductal carcinomas are often unifocal lesions compared to invasive lobular carcinomas, which are not uncommonly multifocal and/or more extensive than what is estimated on clinical and mammographic examination. Distant metastases from ductal carcinomas preferentially involve the lungs and brain, whereas metastases from lobular carcinomas more often involve the peritoneum, bone, bone marrow, and visceral organs of the gastrointestinal (GI) and gynecological tracts.

In spite of these differences, with today’s combined multimodality therapy there appears to be no significant difference in disease-free or overall survival between ductal and lobular carcinomas. However, there are enough significant differences in patient preoperative evaluation and subsequent treatment that an accurate diagnosis is warranted at the time of core biopsy. At some breast cancer centers, a preoperative (before lumpectomy or mastectomy) magnetic resonance imaging (MRI) scan of the breast is performed to evaluate the extent of disease with a core biopsy diagnosis of invasive lobular carcinoma. A core biopsy diagnosis of ductal versus lobular carcinoma is also important if the patient will be treated with neoadjuvant chemotherapy as only a subset of ductal cancers show response, with no or minimal effect on lobular cancers. Some preliminary data suggest better response to aromatase inhibitors in invasive lobular carcinomas than in invasive ductal carcinomas. Therefore, pathologists have to strive hard to give the best diagnosis possible for current management, and for the future as specific therapies become available. Moreover, a correct and consistent morphoimmunohistological diagnosis will avoid confusion in clinical charts and upon specimen review among institutions.

Strong E-Cadherin (ECAD) membranous staining has long been used to define ductal carcinomas. Ductal carcinomas (in situ or invasive) retain membranous ECAD because they do not show homozygous mutation/silencing of the ECAD gene. Mutation of the ECAD gene either leads to a mutant protein that loses its adhesive properties or there is not enough protein to function as an adhesive molecule.

The ECAD gene, CDH1, is a large gene located on 16q22.1. The ECAD protein has an intracytoplasmic portion, an intramembranous portion, and an extracellular domain. Cell-to-cell adhesion through ECAD is also critically dependent on the subplasmalemmal cytoplasmic catenin complexes (alpha, beta, gamma, and p120 isoforms) that link ECAD to the actin cytoskeleton of the cell. Abnormalities of the catenins or ECAD gene expression can result in a variety of immunohistochemical ECAD pathologies. Lobular carcinomas studied at the genetic level have often shown ECAD mutation that accounts for loss of cohesiveness of the tumor cells. The majority of these mutations have been found in combination with loss of heterozygosity (LOH) of the wild type ECAD locus (16q22.1), a hallmark of classical tumor suppressor genes.

Immunohistochemically, this correlates with either a complete absence of the ECAD protein or abnormal localization (apical, perinuclear, cytoplasmic, beaded, or partial membrane). This abnormal localization may be dependent on the type of mutation. Truncation mutations produce an ECAD product that is inept at binding to neighboring cells, resulting in a histological pattern of widely dyshesive cells that are completely negative for ECAD through IHC (e.g., classic infiltrating lobular carcinoma) ( Fig. 11.18 ). Loss of membrane staining may be associated with granular cytoplasmic immunostaining ( Fig. 11.19A and B ) that represents cytoplasmic solubilization of a portion of the truncated protein. Proximal truncation mutations may result in the inability of ECAD to bind to the catenin complex, resulting in a short ECAD represented by focal or dot-like membrane immunostaining ( Fig. 11.19C ). Patients with focal staining of LCIS cells with ECAD may have an ipsilateral risk of carcinoma akin to low-grade DCIS. Mutations in the catenin complex can also lead to dysfunctional ECAD and loss of membrane staining. While deletions of the CDH1 gene as a result of LOH are seen in ductal carcinomas, they are not early events and are not usually associated with the point mutations seen in lobular neoplasia. A comprehensive molecular analysis of invasive lobular carcinoma has confirmed the presence of ECAD mutations in these tumors; however, the study failed to identify promoter hypermethylation of the promoter region as the cause of ECAD protein loss.

In the majority of cases, ECAD staining is unequivocal (positive or negative), and can be solely used in distinguishing ductal from lobular carcinomas. In a minority (approximately 15%) of cases, the stain may be difficult to interpret. Another stain that could be used in such situations is p120. This stain represents p120 catenin, which binds with ECAD on the internal aspect of the cell membrane to form the cadherin-catenin complex ( Fig. 11.20 ). This complex is essential for the formation of intercellular tight junctions, and is composed of an external domain of calcium-dependent ECAD and an internal domain of ECAD to which are bound the alpha, beta, and p120 catenins. The alpha and beta catenins are complexed with the carboxy-terminal cytoplasmic tail of ECAD, whereas the p120 catenin is anchored to ECAD in a juxtamembranous site. P120 is actively involved in the status of cell motility, ECAD trafficking, ECAD turnover, promotion of cell junction formation, and regulation of the actin cytoskeleton. The binding of p120 to ECAD stabilizes the complex and increases the half-life of membranous ECAD by slowing the normal turnover of ECAD that usually occurs by cellular endocytosis. P120 that is bound to ECAD exists in equilibrium with a small cytoplasmic pool of p120. When ECAD is absent, the cytoplasmic pool of p120 increases. Therefore, in normal ducts and ductal carcinomas, p120 shows a membranous pattern of staining ( Fig. 11.21A and B ). In contrast, lobular carcinomas with absent or nonfunctional ECAD show strong cytoplasmic p120 immunoreactivity ( Fig. 11.21C and D ). This positive cytoplasmic staining for lobular carcinoma is much easier to interpret than ECAD negative staining. A combination of ECAD and p120 drastically reduces the number of ambiguous diagnoses and better delineates (or diagnoses with increased confidence) the category of mixed ductal and lobular carcinoma ( Fig. 11.21E ). These mixed carcinomas comprise no more than 10% of all breast carcinomas and probably arise due to late ECAD inactivation within a ductal carcinoma. In contrast, loss of ECAD protein occurs very early in lobular carcinogenesis. Lack of ECAD staining and strong p120 cytoplasmic staining is observed in all morphologically characterized lobular carcinomas in situ and atypical lobular hyperplasias ( Fig. 11.21E to J ). Additionally, lack of ECAD within minimal epithelial proliferation in the breast terminal duct lobular unit defines atypical lobular hyperplasia and distinguishes it from mild ductal hyperplasia. This distinction is important because patients with ALH are typically referred to a high-risk clinic, and in some cases diagnostic excision procedure is performed. The IHC stains support the notion that the term lobular hyperplasia has no significance in breast pathology.

Other stains have also been evaluated in the distinction between ductal and lobular neoplasia, but their diagnostic ability is limited. In addition to p120, other catenins (alpha-catenin, beta-catenin, plakoglobin) play a role in anchoring ECAD to the actin cytoskeleton. As per the reported literature, beta-catenin either is negative or shows cytoplasmic reactivity in lobular carcinomas, while membranous staining with or without cytoplasmic staining is seen in ductal carcinomas. A reduction in membranous beta-catenin expression has been shown with transition from LCIS to infiltrating lobular carcinoma (ILC). In a study assessing cadherin-catenin complex dissociation in lobular neoplasias (11 pure LCIS, 18 LCIS with ILC, and 7 ILC), Morrogh et al. identified membranous beta-catenin expression in 82% of pure LCIS, 28% of LCIS associated with ILC, and 0% of ILC. Although a lack of beta-catenin expression could be used in identifying ILC, it is less reliable than ECAD. The high molecular cytokeratin cocktail CK903 (which recognizes CK1, CK5, CK10, and CK14) has been reported to be expressed in lobular neoplasia and absent in ductal cancers (excluding basal-like cancers). We personally do not have much experience with CK903 in lobular cancers, but it is to be noted that lobular neoplastic lesions are negative for the CK5 antibody (clone XM26), and CK5 cannot be used to distinguish atypical ductal hyperplasia from atypical lobular hyperplasia.

IHC and molecular methods not only aid in diagnostic issues, but to some extent they demand that one looks at the morphology that has been learned in a new light. All invasive breast carcinomas that infiltrate in a single-file pattern with a low nuclear grade are not lobular carcinomas, as invasive ductal carcinomas also have this pattern.

The morphological assessment of a ductal or lobular phenotype is not without controversy, and has limitations. The classic ILC is composed of small cells with bland cytology and some plasmacytoid features. The growth pattern is completely dyshesive. Breast tissue can grossly appear normal (as can the mammogram) yet show widespread dyshesive carcinoma of the classic type. These tumors are uniformly ECAD negative (ECAD–) and are associated with specific patterns of systemic metastases. Invasive ductal carcinomas (IDCs) can also show patterns seen in ILC, such as single-filing of tumor cells, targetoid patterns, and regional dyshesiveness. Such patterns may be confusing but are readily resolved with ECAD immunostaining. There are subgroups of morphologically indeterminate lobular/ductal phenotypes. ECAD separates these groups distinctly in most cases, and demonstrates the existence of mixed lobular-ductal phenotypes in a minority of cases. ECAD stains myoepithelial cells (albeit in a weak fashion), a pitfall corresponding to misinterpretation of LCIS as DCIS (see Fig. 11.18A ).

The morphological reproducibility of distinguishing IDC from ILC and LCIS from DCIS is less than optimal. There can be substantial variation in the interpretation of ILC versus IDC and LCIS versus DCIS. For this reason alone, ECAD and p120 IHC could be justified to aid in correctly classifying these lesions.

Ductal Versus Lobular Carcinoma

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  ECAD staining is a useful diagnostic adjunct in cases with indeterminate morphology.

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  p120 further enhances diagnostic accuracy by being a positive stain for lobular carcinoma.

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  Lobular lesions are characteristically negative for ECAD and demonstrate intense cytoplasmic immunoreactivity for p120.

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  Normal ducts and ductal lesions demonstrate membranous staining for ECAD and p120.

Lobular Carcinoma Variants and Former Lobular Variants