Case 3 -

Low-Grade Adenosquamous Carcinoma (LGASC) Sandra J. Shin Weill Cornell Medical Center New York, NY

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Clinical History: 31 year old female presents with a right breast mass. Excisional biopsy is performed. Case 3 - Figure 1 Needle core biopsy: A small glandular proliferation with angled contours and an infiltrative growth pattern. Case 3 - Figure 2 Needle core biopsy: Tissue core containing a small glandular proliferation in a background of moderately heavy lymphocytic infiltrates. Case 3 - Figure 3 Needle core biopsy: Higher magnification of a small glandular proliferation with associated cellular stroma. Case 3 - Figure 4 Needle core biopsy: Extremely infiltrative growth pattern of glandular structures between benign mammary glands and in fat. Case 3 - Figure 5 Needle core biopsy: Extremely infiltrative growth pattern of glandular structures between benign mammary glands and in fat. Case 3 - Figure 6 Needle core biopsy: Glandular structures are cytologically atypical. A rare mitosis is identified. Case 3 - Figure 7 Excisional biopsy: Traits of squamous differentiation in participating glandular structures, in this case, by an increase of glassy eosinophilic cytoplasm in some foci. Case 3 - Figure 8 Excisional biopsy: Associated stromal cellularity in a lamellar pattern and imperceptible blending with adjacent stroma also known as "spindle-cell metaplasia". Case 3 - Figure 9 Immunohistochemical stain Smooth muscle actin: Strong staining of stromal myofibroblasts give the illusion of a myoepithelial layer in this invasive tubular carcinoma. Case 3 - Figure 10 Immunohistochemical stain p63: Variable circumferential staining around glandular structures. Some areas show diffuse or sporadic positivity while other areas are completely negative. Case 3 - Figure 11 Immunohistochemical stain p63: Some glandular structures show more diffuse staining of basal and luminal epithelium, likely foci with squamous differentiation. Case 3 - Figure 12 Immunohistochemical stain smooth muscle myosin heavy chain: Variable staining like p63 but in addition, closely apposed to glandular structures are stromal spindle cells demonstrating a lamellar and "wispy" staining pattern. Case 3 - Figure 13 Immunohistochemical stain K903: Shows diffuse but variably intense staining of glandular structures. Stromal cells are negative. Case 3 - Figure 14 Immunohistochemical stain K903: Some glandular structures demonstrate a "core" staining pattern where the luminal epithelium shows notably more intense staining than adjacent basal cells in the same gland. Introduction: Small glandular proliferations in the breast can represent benign, atypical and malignant entities. In recent times, the adjunctive role of immunohistochemistry in breast pathology has greatly increased. "...It is not because the new genre of breast pathologists are less well-trained than their 'experienced' counterparts; it is mainly because of the demands of more accurate and precise diagnoses, identification of new entities and availability of novel antibodies" [1]. This case presentation demonstrates the valuable uses as well as the interpretative pitfalls of immunohistochemistry in distinguishing mammary entities comprised of small glandular proliferations. Pathological/Microscopic Findings and any Immunohistochemical or Other Studies: This entity manifests as a mass-forming lesion. Grossly, these tumors are firm and poorly-circumscribed. Some tumors also have long, slender extensions radiating into the surrounding breast tissue. The histologic features of this entity consist of proliferating glandular structures with an ill-defined tumor border. The glands are often compressed and have elongated lumens. Some assume the shape of a "comma" or "tear-drop". The glands demonstrate an insidious growth pattern infiltrating between ducts and into lobules. Cytologic atypia is characteristically mild. Epithelial-associated mitoses can be identified but are typically few in number (Fig. 6). Necrosis is absent. This glandular proliferation is often associated with a mild to moderately heavy chronic inflammatory cell infiltrate which can be seen in patches or loose aggregates either throughout the tumor or concentrated at the tumor's periphery. Blending in the glandular structures are elements of squamous differentiation. The degree of squamous differentiation ranges from more abundant glassy eosinophilic or apocrine cytoplasm of individual cells to the formation of keratinizing squamous pearls or morules (Fig. 7). The tumor-associated stroma is collagenous or fibrotic but in some cases, more cellular and "fibromatosis-like". The presence of cellular stroma arranged in a lamellar fashion closely apposed to glandular structures is a diagnostic clue. These stromal "extensions" known in the literature as "spindle-cell metaplasia", blend imperceptibly with the surrounding stroma (Fig. 8). Osteocartilaginous metaplasia has been described in some examples of this entity [2]. This entity has been found to arise in association with papillary, adenomyoepitheliomatous, and complex sclerosing lesions [3, 4, 5]. Differential Diagnoses: Radial sclerosing lesion, sclerosing adenosis, syringomatous adenoma, invasive well-differentiated duct carcinoma (especially of the tubular type), florid papillomatosis of the nipple (adenosis predominant pattern), skin carcinoma of adnexal origin (i.e. microcystic adnexal carcinoma) Final Diagnosis: Low-grade adenosquamous carcinoma (LGASC) Case Discussion: As with other small glandular proliferations in the breast, the differential diagnosis spans the gamut of benign to malignant entities. Among benign lesions, sclerosing adenosis, radial sclerosing lesion, and syringomatous adenoma are histologic mimics due to their common feature of cytologically bland small and angulated glands in an infiltrative-looking growth pattern. Due to its infiltrative growth pattern, microglandular adenosis is another consideration however these glands are typically more round and less angulated or compressed. Especially in limited samples such as needle core biopsies, the lobular origin from which sclerosing adenosis arises may not be represented or fully appreciated. Radial sclerosing lesions can be additionally problematic due to additional similar traits of fibrotic/elastotic stroma and variable immunoreactivity of participating glands with myoepithelial markers (i.e. focal negativity in a particularly sclerotic nidus of some radial sclerosing lesions). On the other hand, squamous differentiation is not commonly seen in this lesion. Syringomatous adenoma and LGASC both show areas of squamous differentiation however, the former typically arises in the nipple/areolar complex (central and superficial region of the breast) where as LGASC predominantly originates in the breast's periphery and is situated deeper in the mammary parenchyma. Moreover, the tumor- associated stromal cellularity (i.e. spindle cell metaplasia) and chronic inflammatory infiltrate seen in LGASC are not features of syringomatous adenoma. In rare cases, LGASC can arise from pre-existing papillary or adenomyoepitheliomatous lesions. This phenomenon has not been described in cases of syringomatous adenoma. Immunohistochemical stains are generally assessed in two ways: the number of lesional cells staining and the intensity of the staining. However, more information can be gleaned by scrutinizing the pattern of staining even if there is no scientific explanation for such a distribution. The same immunostains can be "positive" in two morphologically similar entities but the difference in staining patterns can prove to be a distinguishing factor that can ultimately lead to the correct diagnosis. Currently, there are sparse descriptions of the immunoprofile of low-grade adenosquamous carcinoma most of which do not describe in detail any characteristic staining patterns that would suggest the diagnosis [6, 7, 8]. In cases of LGASC, immunohistochemical stains show variable but in some instances, characteristic staining patterns which are not seen in morphologic mimics. Myoepithelial Markers (p63, Smooth muscle myosin heavy chain) Immunohistochemical stains specific for myoepithelium such as smooth muscle myosin heavy chain or p63 show strong continuous circumferential staining in participating glands of sclerosing adenosis and radial sclerosing lesions. In the latter, attenuated or focally absent myoepithelial immunoreactivity can occur due to a heavily sclerotic nidus. This staining pattern can also be misinterpreted as an invasive duct carcinoma arising within a pre-existing radial sclerosing lesion. Among malignant considerations, invasive well-differentiated carcinoma - in particular the tubular type - should be excluded. Myoepithelial immunostains should assist in ruling out invasive duct carcinoma in the majority of cases. p63 and SMM-HC are typically very "clean" stains that show crisp and specific staining of myoepithelium. Often the myoepithelial layer of a normal mammary duct will serve as a reliable internal positive control. It is recommended that such specific markers for myoepithelial cells be utilized and less specific stains such as smooth muscle actin be avoided due to concurrent staining of adjacent myofibroblasts in the surrounding stroma. Positive staining myofibroblasts can give the illusion of myoepithelium around the glands in question and be diagnostic problematic. Moreover, the myofibroblasts immediately around lesional glands typically show notably increased staining than that of neighboring more distant myofibroblasts (Fig. 9). This enhances the deceptive appearance of myoepithelium and can lead to a misdiagnosis of invasive duct carcinoma as a benign glandular proliferation. In cases of LGASC, participating glandular structures show variable staining with p63 and SMM-HC. In some foci, there is unequivocal positivity of the basal epithelium as evidenced by strong, complete circumferential staining. However, other glands are completely devoid of immunoreactivity (Fig. 10). In addition, some glands will demonstrate p63 positivity in not only the peripheral, basal cells but also the inner luminal cells (Fig. 11). The latter pattern is most likely highlighting areas of squamous differentiation which is a feature that can be also appreciated histomorphologically. SMM-HC does not show this staining pattern but instead, can show lamellar and "wispy" immunoreactivity in spindle cell metaplasia immediately surrounding glandular structures (Fig. 12). Although it has been reported that syringomatous adenomas maintain a myoepithelial layer [9], variable staining with myoepithelial immunostains is commonly seen in this entity (personal observation). Some lesional glands can be entirely negative. Therefore, utilizing myoepithelial immunostains are not helpful in excluding syringomatous adenoma in the differential diagnosis. High-molecular weight Cytokeratins (K903, CK 5/6) High molecular weight cytokeratins like K903 and CK5/6 show variable staining in LGASC which is generally diffuse in distribution but of varying staining intensities (Fig. 13). Some cases demonstrate largely negative staining in lesional glandular structures. However, in other examples glandular structures can exhibit a "core" staining pattern where luminal cells are strongly positive in comparison to immediately adjacent basal cells in the same gland (Fig. 14). This core staining pattern is not observed in other entities in the differential diagnosis. Hormone receptors (ER, PR) Prognostic markers estrogen and progesterone immunostains are consistently negative in LGASC [3, 6, 7, 8]. This ER/PR negative immunoprofile can prove helpful in situations where a misdiagnosis of invasive, well-differentiated duct carcinoma has been made and only becomes apparent at the time of evaluating routine biomarkers. The unexpected immunonegativity for both markers in an apparent case of invasive duct carcinoma should raise suspicion of the diagnosis rendered. Conclusion(s): Low-grade adenosquamous carcinoma is an indolent malignancy which represents a subset of metaplastic carcinoma. Due to the cytologically bland, well-formed glandular structures and infiltrative growth pattern, this entity can be mistaken for a variety of morphologically similar benign and malignant proliferating glandular lesions. In addition to key morphologic features, immunohistochemical stains can greatly aid in classifying these entities. Low-grade adenosquamous carcinoma has been found to have characteristic staining patterns with p63, smooth muscle myosin heavy chain, and high molecular weight cytokeratins such as K903 and CK5/6. Although neither diagnostic nor appreciated in every example, these staining patterns appear unique to LGASC and different from the patterns seen in morphologic mimics using the same immunostains. When these staining patterns are observed, the possibility of a LGASC should be entertained. This tumor is also characteristically negative for estrogen and progesterone receptors which can help to distinguish it from invasive, well-differentiated duct carcinoma. References: Bhargava and Dabbs. "Use of immunohistochemistry in diagnosis of breast epithelial lesions" Adv Anat Pathol; March 2007:14(2)::93-107. Rosen PP, Ernsberger D. Low-grade adenosquamous carcinoma: a variant of metaplastic mammary carcinoma. Am J Surg Pathol 1987;11(5): 351-358. Van Hoeven KH, Drudis T, Cranor ML, et al. Low-grade adenosquamous carcinoma of the breast: a clinicopathological study of 32 cases with ultrastructural analysis. Am J Surg Pathol 1993;17(3): 248-58. Gobbi H, Simpson JF, Jensen RA, et al. Metaplastic spindle cell breast tumors arising within papillomas, complex sclerosing lesions, and nipple adenomas. Mod Pathol 2003;16(9):893-901. Denley H, Pinder SE, Tan PH, et al. Metaplastic carcinoma of the breast arising within complex sclerosing lesion: a report of five cases. Histopathol 2000;36:203-209. Drudis T, Arroyo C, Van Hoeven K, et al. The pathology of low-grade adenosquamous carcinoma of the breast. An immunohistochemical study. Pathol Annu 1994; 29(pt2):181-97. Ferrara G, Nappi O, Wick MR. Fine-needle aspiration cytology and immunohistology of low-grade adenosquamous carcinoma of the breast. Diagn Cytopathol 1999;20(1):13-18. Ho B C-S, Tan HW, Lee V K-M et al. Preoperative and intraoperative diagnosis of low-grade adenosquamous carcinoma of the breast: potential diagnostic pitfalls. Histopathol 2006;49:603-611.