Ross, Dara S. MD; Hoda, Syed A. MD
In the 2010 edition of the American Joint Committee on Cancer staging system,8 the diagnosis of microinvasive carcinoma of the breast (T1mic) is applied to those invasive carcinomas with no focus measuring >1 mm. The current staging manual states that microinvasive carcinoma is nearly always encountered in a setting of ductal carcinoma in situ (DCIS) [or, less often, lobular carcinoma in situ (LCIS)] in which small foci of tumor cells have invaded through the basement membrane into the surrounding stroma. The term “less often” is not further quantified therein.
Several publications in the last 15 years have reported on the clinical and pathologic characteristics of microinvasive ductal carcinoma (MIDC).2,19,24,25,28 The latter entity is now well established and is regarded as a low morbidity disease, which is typically associated with high-grade DCIS and which only rarely involves regional nodal lymph nodes. Although invasive lobular carcinoma is the second most common form of invasive carcinoma, its incidence is apparently increasing,14 and its microinvasive form has not yet been fully and reliably characterized in clinicopathologic terms.
It is important to distinguish invasive lobular carcinoma from invasive ductal carcinoma because of their differences in clinical and radiologic presentations, histopathologic appearances, patterns of nodal and systemic metastases, and, at least in some cases, dissimilar management practices.15 It has, thus far, not been established whether or not it is clinically important to distinguish microinvasive forms of lobular and ductal carcinoma.
The paucity of reliable data on microinvasive lobular carcinoma (MILC) is most likely due to the following 3 reasons: First, the definition of MILC has been rather loosely applied even until quite recently; for instance, in a series of 6 MILC cases published in 1998, the size of the invasive carcinoma was reported as 4 mm in 1 case, stated to be <2 mm in another case, and measured to be 1 to 2 mm in yet another case, and not recorded at all in 2 cases.16 Second, MILC is an uncommon, perhaps underrecognized, disease; very few reports on MILC have appeared in the literature, some in the form of individual case reports and others in small series.10,12,13,16
Characterization of the clinicopathologic features of MILC, as we have attempted to do herein, could elucidate the clinical and pathologic value of MILC—possibly the most histologically subtle breast lesion that can be encountered in routine practice.
MATERIALS AND METHODS
Records of our Surgical Pathology Service and Breast Pathology Consultation Service from 1991 to 2009 were electronically searched for the diagnosis of MILC. Pathologic material of each case in which a diagnosis of MILC was either suspected or confirmed was studied by the authors. Only cases in which the final diagnosis of MILC could be indisputably established on routine Hematoxylin and Eosin stains, or supported by appropriate immunostains in equivocal cases, were included. The diagnostic criteria used for MILC were as follows: (a) cytologically malignant epithelial cells in the stroma, evidenced by cytokeratin (CK) AE1/AE3 immunostain in equivocal cases; (b) less than or equal to 1 mm extent of the largest focus of invasive carcinoma; (c) absence of myoepithelial cells surrounding the invasive cells, as supported by at least 1 immunostain in equivocal cases; (d) invasive carcinoma cells, supported to be of lobular differentiation by lack of E-cadherin staining in equivocal cases; and (e) no focus of a larger invasive carcinoma identified in any earlier biopsy sampling or subsequent excision specimen.
Details of antibodies used in immunohistochemistry appear in Table 1. Invasion was assessed using immunostains for either myoepithelial markers (CD10 and calponin, myosin, smooth muscle actin, or p63) or epithelial marker (CK AE1/3 or CK7).18 E-cadherin was used to determine ductal or lobular differentiation of in situ and invasive tumor cells in equivocal cases. Lack of cytoplasmic membrane immunoreactivity with E-cadherin, in addition to presence of appropriate cytologic features (smaller cells with high nuclear-to-cytoplasmic ratio and presence of intracytoplasmic vacuoles), was used to classify the tumor as lobular. Immunostains for hormone receptors and HER2/neu were performed on formalin-fixed, paraffin-embedded tissue sections. Estrogen receptor (ER) and progesterone receptor (PR) positivity in nuclei of invasive tumor cells was determined using quantitative and qualitative criteria (>1% immunoreactivity constituting positivity), and HER2/neu immunoreactivity in cytoplasmic membrane of invasive tumor cells was determined quantitively and qualitatively as well.
Clinical (including follow-up) and radiologic information for all cases of MILC were compiled. All relevant histopathologic material from earlier or subsequent needle core biopsy, excisional biopsy, or mastectomy was studied.
Sixteen cases of MILC were detected among 75,250 (0.02%) breast cases examined in our institution from 1991 to 2009. The 16 MILC cases included 7 inhouse cases and 9 consultation cases. Distribution of MILC amid total number of breast, in situ, and invasive lobular carcinoma cases are listed in Table 2.
Table 3 includes relevant clinical data on the MILC series. The mean age of the 16 patients with MILC was 52 years. Presentation included a radiographic abnormality in the majority of cases. MILC was unilateral in all 16 cases (right, 8; left, 8).
Table 4 includes relevant pathologic information. A gross lesion was identified in only 4 cases, and the lesion was ill defined in all 4 cases. The mean size of the gross lesion in these 4 cases was 1.9 cm (range, 0.5 to 2.6 cm, 3 of these were palpable). The mean number of microinvasive foci was 1.5 (range 1 to 5); however, 1 focus of MILC was identified in 11 cases. All foci of MILC were intimately associated with LCIS. The associated LCIS, at least in part, was of the classical type in all 16 cases (Fig. 1). The nuclear grade of MILC cells and adjacent LCIS in 15 cases was low to intermediate. Florid LCIS (LCIS with diffuse involvement of lobules, which are overly distended with incipient central necrosis and calcific deposits) existed in 4 of 16 (25%) cases (Fig. 2). Synchronous pleomorphic LCIS (with apocrine cytology, high-grade nuclei, and central necrosis, Fig. 3) existed in 1 of 16 (6%) cases (nuclear grade of MILC cells in this case was high). Neither perilesional stromal reaction nor inflammatory infiltrate was present in any case, and the only histologic hint of MILC was enhanced stromal cellularity. The diagnosis of MILC was supported by immunostaining (ie, CK AE1/AE3 immunoreactivity in the microinvasive cells and absence of myoepithelial cells around these cells, as evident by at least 1 myoepithelial immunostain) in 8 cases, which were considered equivocal for microinvasion on routine H&E stain (Figs. 1C–E).
Negative immunoreactivity for E-cadherin in the constituent neoplastic cells supported lobular differentiation in all the 4 cases of florid LCIS and in the 1 case of pleomorphic variants. E-cadherin was negative in the 4 additional cases in which the differential diagnosis included solid type of intraductal carcinoma. Three cases had synchronous DCIS of solid, cribriform, or micropapillary types; however, the DCIS lesion was not in the immediate vicinity of MILC in any case (DCIS did not appear in the same histologic slide as MILC in all 3 cases). Six cases showed calcifications within the LCIS, including 5 of the classical type and 1 of the pleomorphic type. One case showed calcifications only in inactive ducts.
Each of 6 MILC cases, in which results could be obtained, was found to be positive for ER and PR (Fig. 1F). MILC was no longer present in the subsequently prepared ER-stained and PR-stained sections in 5 additional cases. HER2/neu was not overexpressed in each of 5 cases in which results could be obtained (MILC was not present in HER2/neu-stained sections of 5 cases), and 1 case showed equivocal 2+ (on a scale of 0 to 3+) immunostaining—with no fluorescence in situ hybridization confirmation.
The final surgical procedure was excisional biopsy in 8 cases, ipsilateral mastectomy in 6 cases, and bilateral mastectomies in 2 cases. Four cases showed additional significant disease in a different region of the ipsilateral breast: 1 case showed MIDC associated with DCIS, 2 cases showed DCIS, and 1 case showed atypical duct hyperplasia. One case showed invasive ductal carcinoma (9 mm) associated with DCIS in the contralateral breast. Eight of 16 cases had residual LCIS or atypical lobular hyperplasia (ALH) in a subsequent specimen. No lymphovascular involvement by tumor cells was evident in any case of MILC.
Nine of 16 cases had a sentinel lymph node biopsy procedure, and 4 of 16 cases had axillary lymph node dissection, with negative results obtained in all 13 cases. All patients remained alive without evidence of recurrence and/or metastases in a mean follow-up of 24 months (range, 1 to 72 mo).
Five cases had tissue sampled from the contralateral breast (2 excisional biopsies, 2 mastectomies, and 1 reduction mammoplasty), with significant diseases in 3 cases (including 1 case each of ALH, LCIS, and invasive and in situ ductal carcinoma).
At a practical level, LCIS can be divided into 3 main types: classical, florid (ie, necrotic type with massive distension of the acini, as per World Health Organization classification), and pleomorphic.26 The cells of the classical type of LCIS are uniformly small, noncohesive, bear round nuclei and inconspicuous nucleoli, and may contain intracytoplasmic mucoid globules (lending a signet ring cell appearance).20 The appellation florid is applied to LCIS when the noninvasive neoplastic process diffusely involves numerous lobules, considerably distends and distorts the individual lobules, and exhibits central necrosis.1 Pleomorphic LCIS shares the architectural features of florid LCIS; however, in terms of cytology, the cells are larger and variable (ie, pleomorphic).4 All types of LCIS can be associated with invasive carcinoma, either of the ductal or of the lobular type. In this series, 11 cases of microinvasive carcinoma showed classical lobular type of architectural and cytologic features and in 5 others, in which such features were nonclassical, absence of E-cadherin immunostaining supported lobular differentiation. Currently, pleomorphic and florid LCIS are regarded as more aggressive forms of LCIS,1,4 and as such more attention ought to be paid to the perilesional stroma in these variants to detect early invasive carcinoma.
The classical type of invasive lobular carcinoma is characterized by a linear (so-called “Indian filing”) or circumferential arrangement around lobules and ducts (in a “targetoid” or “bulls-eye” manner). The reported frequency of finding LCIS in association with the classical type of invasive lobular carcinoma varies widely from 65% to 98%.7,17 The pleomorphic type of invasive lobular carcinoma is its most distinctive (and potentially aggressive) variant.21 The architectural pattern of invasive pleomorphic lobular carcinoma is similar to that of the classical type; however, the cytologic appearance of this variant lives up to its name—the individual tumor cells are larger with variable nuclei.
The histologic and cytologic features of most forms of lobular and ductal carcinoma (in situ and invasive) are different enough to enable distinction between these. However, immunohistochemistry can be helpful for this purpose in equivocal cases. With rare exceptions in which it is a manifestation of nonfunctional cadherin-catenin complex, cytoplasmic membrane immunoreactivity of in situ carcinoma cells for E-cadherin supports the diagnosis of ductal differentiation.6 p120 (intracytoplasmic reactivity) and 34 βE12 (also intracytoplasmic reactivity) can be used to further support lobular differentiation.3,5
Mammary ducts and lobules afflicted by high-grade intraductal carcinoma are usually surrounded by a cuff of myxoid-appearing specialized connective tissue. It has been suggested that the presence of invasive carcinoma into the aforementioned cuff does not constitute true invasion, and that only invasive carcinoma into nonspecialized interlobular or interductal fibrous or adipose tissue ought to be considered as truly invasive.12 This suggestion may be of value in cases of intraductal carcinoma involving radial scars or sclerosing papillary lesions in which there may be considerable stromal activation and sclerosing entrapment of neoplastic cells. However, it may not apply to MILC cases in which such a cuff is generally not in evidence.
Minute foci of invasive carcinoma may be difficult to detect on histologic sections stained with routine H&E stain. The detection of such obscure invasive foci may be facilitated by paying attention to changes in the stroma immediately in the vicinity of LCIS—because such invasive foci are usually present close to a focus of LCIS in lobules or its extension into ducts. In some cases of LCIS, there are subtle irregularities in the borders of the afflicted glands, which in combination with perilesional enhanced cellularity lend an appearance that is suspicious of microinvasion. This increased cellularity is due to the presence of either lymphocytes, plasma cells, histiocytes, or epithelioid myofibroblasts. CK immunostains (including those for CK AE1/3 and CK 7) can highlight any occult invasive carcinoma. Minimal trimming of the tissue block should be ensured to avoid losing diagnostic tissue. Neither granulomatous inflammation nor lymphovascular involvement is typically associated with invasive lobular carcinoma (and these findings were absent in all MILC cases in this series).
The use of imunohistochemical stains in diagnostic breast pathology has become ubiquitous. Unfortunately, it is rare that a contemporaneous H&E-stained section is obtained at the time of requesting immunostains. Hoda and Rosen10 reported the case of a 63-year-old woman (not included in this series) who underwent an excisional biopsy for a palpable breast mass. The original H&E-stained sections had shown in situ carcinoma, which could either have been of the ductal or lobular type. Recut slides prepared for immunostains showed in situ carcinoma, and a focus of microinvasive carcinoma was negative for E-cadherin; a result supportive of LCIS and MILC. No contemporaneous H&E-stained section was available, and a diagnosis of MILC was made solely on the basis of the E-cadherin-immunostained slide.
We have encountered additional cases in which sections obtained for assessing hormonal receptor stains on in situ carcinoma have shown microinvasive carcinoma. Conversely, immunostains for biomarkers on microinvasive carcinoma have occasionally shown either no residual microinvasive carcinoma or invasive carcinoma larger than 1 mm.
In our opinion, barring artifactual dislodgement thereof, the presence of neoplastic epithelial cells (beyond the limits of myoepithelial cells and basement membrane) constitutes evidence of invasive carcinoma. Needling procedures of the breast (including needle core biopsies and fine needle aspirations) can disrupt, and dislodge, breast tissue. Such dislodged cells usually take the form of single cells or minute cell clusters, or minuscule glands that lie along the tract of biopsy.30 These cells, which are almost invariably derived from the targeted hyperplastic or neoplastic lesions, cytologically (if not architecturally) resemble the parent lesion. We have not encountered such dislodgement in the setting of classical type of LCIS; however, it may potentially be observed in the setting of either florid or pleomorphic types of LCIS (ie, more cellular types of LCIS). These displaced cells of LCIS can be distinguished from MILC by virtue of their distribution exclusively along the healing biopsy tract. Immunostains for myoepithelial cells are of limited help, but study of at least 3 deeper sections taken from the corresponding block could be helpful in this regard.
No metastasis in any lymph node was identified in any of the 16 MILC cases in this series. The reported incidence of axillary metastases in MIDC has ranged from 0% to 28%,11 and in a review of the literature of axillary lymph node involvement in MIDC by Guth et al,9 the cumulative total extent of nodal involvement was 6% (43 of 709 cases in which axillary staging was performed). Cumulative data have helped ordain the role of sentinel lymph node biopsy in the management of MIDC.23,27,29 Until such information is amassed for MILC, it would be prudent to perform sentinel lymph biopsy in these cases as well.
Patients with invasive lobular carcinoma are generally considered to have a relatively high frequency of bilateral carcinoma when compared with women who have other types of carcinoma. In this series, significant breast disease (1 case each of ALH, LCIS, and invasive ductal carcinoma) was found in 3 of the 5 cases in which tissue was sampled from the contralateral (clinically asymptomatic) breast. Clinical and radiologic evaluation of the contralateral breast is recommended in all cases of MILC.
Finally, clinicians and pathologists should be wary of the term microinvasive carcinoma in needle core biopsies, even if only <1 mm of invasive carcinoma is identified in such samples.22 In such cases, it is prudent to state the maximal extent and number of foci of invasive carcinoma therein with the caveat that additional residual invasive carcinoma may be identified in subsequent samples.
MILC is a rare, histologically subtle, lesion associated with classical LCIS. MILC comprised 0.02% of all breast specimens examined in our Department from 1991 through 2009. The disease constituted approximately 0.4% of all invasive lobular carcinomas and was seen in association with approximately 0.4% of all LCISs in this period. MILC seems to be a low morbidity disease with neither recurrences nor metastases observed (at least in the short term).
The assistance of Ms Pat Kuharic with preparation of photomicrographs is gratefully acknowledged.
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