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Figure 6.1 — Atypical Adenomatous Hyperplasia, Biopsy [H&E Stain; High Power]. This preinvasive neoplastic glandular proliferation is made up of cuboidal to low columnar cells with cytoplasmic snouts, which proliferate along alveolar septal surfaces. There is only mild nuclear atypia, with rare to no mitoses, and without associated septal thickening. Double nuclei, slight stratification, and cytoplasmic vacuolation may be present, and some examples have intranuclear pseudoinclusions. An important differential diagnostic consideration is reactive pneumocyte hyperplasia, in which the cells are more uniform, with associated inflammation and fibrosis in a clinical context of recent injury. Atypical adenomatous hyperplasia closely resembles nonmucinous adenocarcinoma in situ and lepidic-pattern invasive adenocarcinoma, and cannot be confidently distinguished from these on a small biopsy.

Figure 6.2a — Adenocarcinoma, Well-Differentiated, Sputum [Pap Stain; Medium Power]. This specimen is representative of what used to be known as “bronchoalveolar carcinoma,” a well-differentiated adenocarcinoma of the lung that is associated with a lepidic growth pattern on histology; it is no longer classified as a distinct entity according to International Association for the Study of Lung Cancer (IASLC)/the American Thoracic Society (ATS). Furthermore, World Health Organization classification of bronchoalveolar carcinoma requires histology in order to exclude invasion. Cytologic specimens demonstrate cytomorphologic features distinct from other adenocarcinomas of the lung. As seen here, the neoplastic cells are cuboidal, small, and have round uniform nuclei. The cells form a three-dimensional papillary structure, but other patterns include flat sheets and acinar arrays.

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Figure 6.2b — Adenocarcinoma, Well-Differentiated, Sputum [Pap Stain; Medium Power]. Well-differentiated adenocarcinoma with a lepidic growth pattern may be mucinous or, more commonly, nonmucinous. This field demonstrates a mucinous adenocarcinoma in which some neoplastic cells contain large mucinous vacuoles. This reduces the nuclear-to-cytoplasmic ratio and makes the cells appear bland. Due to growth within airways, the mucin-producing cells have created a mucin plug in the shape of the airway that contains neoplastic cells. Note the muciphages in the background, with bubbly cytoplasm, which may be difficult to distinguish from discohesive individual neoplastic cells at first glance.

Figure 6.2c — Adenocarcinoma, Well-Differentiated, Sputum [Pap Stain; High Power]. Some cells contain large mucin vacuoles while other cells have high nuclear-to-cytoplasmic ratios; however, the nuclei share similar features: moderately hyperchromatic, with moderate to marked nuclear border irregularities and a distinct nucleolus. The cells form loose clusters.

Figure 6.2d — Lepidic-Predominant Adenocarcinoma, Wedge Resection [H&E Stain; High Power]. The neoplastic cells are nonmucinous, with a “hobnail” appearance reminiscent of type II pneumocytes or Clara cells, and moderate to marked nuclear atypia; other examples are quite bland cytologically. Lepidic-pattern adenocarcinoma grows along the surfaces of alveolar septa. TTF-1 and napsin A are routinely positive. This appearance may be seen as a pure pattern in adenocarcinoma in situ and in minimally invasive adenocarcinoma, or as one of several patterns in invasive adenocarcinoma (“adenocarcinoma with a lepidic component”). The World Health Organization recommended terminology in small biopsies (including cytology specimens) with identifiable lepidic growth pattern is “adenocarcinoma (lepidic pattern).”

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Figure 6.3a — Adenocarcinoma, Papillary Growth Pattern, Fine Needle Aspiration [Diff-Quik Stain; Medium Power]. The malignant cells have elongated nuclei with nuclear grooves and are attached to a true papillary stalk, as defined by the presence of a fibrovascular core. Lung adenocarcinomas can have a papillary growth pattern, though the differential includes metastases of a papillary lesion, such as papillary thyroid carcinoma, urothelial carcinoma, and papillary renal cell carcinoma.

Figure 6.3b — Adenocarcinoma, Papillary Growth Pattern, Fine Needle Aspiration [Diff-Quik Stain; High Power]. Subclassifying the growth pattern of lung adenocarcinoma on cytology is not typically possible or necessary. While this field contains malignant cells from a papillary adenocarcinoma, the absence of fibrovascular cores in this field makes this distinction impossible. However, the features of adenocarcinoma are present: enlarged nuclei with irregular nuclear borders, anisonucleosis, and nuclear overlap. The cytoplasmic boundaries are difficult to discern, but most nuclei are eccentrically placed, a feature of glandular differentiation.

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Figure 6.3c — Adenocarcinoma, Papillary Growth Pattern, Fine Needle Aspiration [Diff-Quik Stain; Low Power]. The papillary fragment with a fibrovascular core indicates the papillary nature of this lesion. By contrast, the left-hand side of the field contains predominantly singly dispersed tumor cells with similar features as those seen on the papillary fragment. While it is likely that these cells have become dissociated from the papillary fragment, a papillary growth pattern is often seen as a mixed pattern in lung adenocarcinoma. Other growth patterns may not be sampled or, if sampled, may not be discernible by cytology.

Figure 6.3d — Adenocarcinoma, Papillary Growth Pattern, Fine Needle Aspiration [Diff-Quik Stain; High Power]. The malignant cells attached to the papillary stalk are monotonous appearing and show little nuclear size variation. However, the cellular nature of this lesion—as well as the presence of a fibrovascular stalk—leaves little doubt that it is neoplastic. The cells have a flask-shaped appearance, perhaps due to forces exerted during the aspiration procedure or creation of the smear. Some cells have become detached from the fragment.

Figure 6.4a — Adenocarcinoma, Papillary Growth Pattern, Lobectomy [H&E Stain; Low Power]. This adenocarcinoma is composed of papillae with variably sclerotic fibrovascular cores. In inflated specimens, the papillary features can be readily identified; if inflation is poor, compressed papillae can mimic lepidic growth.

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Figure 6.4b — Adenocarcinoma, Papillary Growth Pattern, Lobectomy [H&E Stain; High Power]. The neoplastic cells are arrayed in an orderly fashion along the papillae. There is only mild atypia, although crowding and overlapping are occasionally present. Some examples have intranuclear pseudoinclusions.

Figure 6.4c — Adenocarcinoma, Papillary Growth Pattern, Lobectomy [H&E Stain; High Power]. Exophytic finger-like fronds with fibrovascular cores are characteristic of the papillary pattern of pulmonary adenocarcinoma. The nuclei show moderate to marked pleomorphism and significant atypia, with frequent nucleoli. Most adenocarcinomas of lung origin have a mix of patterns, and papillary architecture is a common component. Expression of napsin A and TTF-1 is characteristic, and allows distinction from papillary adenocarcinoma originating in the gastrointestinal (CK20 and CDX2+) or GYN (PAX8 and ER+) tracts.

Figure 6.4d — Adenocarcinoma, Papillary Growth Pattern, Lobectomy [TTF-1 Immunohistochemical Stain; Low Power]. Expression of TTF-1 is strong and uniform in tumor cell nuclei. The major histologic differential consideration is metastatic papillary thyroid cancer, which can be excluded with history, imaging, and a thyroglobulin stain.

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Figure 6.4e — Adenocarcinoma, Papillary Growth Pattern, Lobectomy [Napsin A Immunohistochemical Stain; Low Power]. There is strong and uniform staining for napsin A in the characteristic granular cytoplasmic pattern. Napsin A is relatively specific, but does stain some tumors of renal origin, including some papillary tumors. A panel of stains, including TTF-1 and renal-associated antigens (CA-IX, PAX8) is therefore optimal, depending on the specific patient context.

Figure 6.5 (a, b) — Polypoid Endobronchial Adenocarcinoma, Excision [H&E Stain; Medium Power and Gross Examination]. Polypoid endobronchial adenocarcinoma is a descriptive term for adenocarcinomas that demonstrate an exophytic and polypoid growth within a large bronchus. This is an uncommon event. The histology is usually that of a well-differentiated papillary adenocarcinoma. Gross examination reveals the polypoid shape of this lesion.

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Figure 6.6a — Well-Differentiated Adenocarcinoma, Fine Needle Aspiration [Diff-Quik Stain; High Power]. This is a corresponding Diff-Quik preparation from the same patient as the previous image. Note the arrangement of the nuclei in rosette and/or acinar formations. Because the chromatin pattern is not as readily identifiable on this preparation as compared to a Pap stain, it is more challenging to exclude a carcinoid tumor. The creation of cell block material for immunohistochemical studies could prove helpful, since low-grade neuroendocrine neoplasms typically express synaptophysin and/or chromogranin.

Figure 6.6b — Well-Differentiated Adenocarcinoma, Fine Needle Aspiration [Pap Stain; High Power]. The nuclei form small rosettes or acinar-like structures, an indication of gland formation. The presence of small rosettes in a well-differentiated neoplasm should always cause at least brief consideration of a neuroendocrine neoplasm, such as a carcinoid tumor. In this case, the nuclei have markedly atypical features, such as enlargement, irregular borders, and significant overlap, which favors an adenocarcinoma. However, the chromatin pattern is predominantly coarse and many cells have prominent nucleoli—a feature not typically seen in neuroendocrine neoplasms.

Figure 6.7a — Adenocarcinoma, Fine Needle Aspiration [Pap Stain; High Power]. This adenocarcinoma has a peculiar morphology, with optically clear chromatin and very little cytoplasm. The nuclei appear to mold against each other and may cause consideration of a small cell carcinoma. However, the cells are arranged in a three-dimensional shape with the nuclei lining the edges, suggesting a glandular differentiation. The cells also have nuclear size variation, ranging up to a 3:1 ratio, and are found predominantly in a tissue fragment rather than individually. These favor an adenocarcinoma over a small cell carcinoma.

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Figure 6.7b — Adenocarcinoma, Fine Needle Aspiration [Pap Stain; High Power]. The malignant cells form a papillary fragment, but the fragment lacks a fibrovascular core and thus is not a true papillae. The nuclei are enlarged, dark, and have distinct nucleoli. The nuclei are disorganized within the structure and are overlapping, vary greatly in size, and have irregular borders.

Figure 6.8a — Mucinous Acinar/Colloid Adenocarcinoma, Fine Needle Aspiration [Diff-Quik Stain; Low Power]. The current World Health Organization classification of adenocarcinomas having a plentiful amount of mucin is subdivided into invasive mucinous and colloid adenocarcinoma. The former term was newly introduced to replace mucinous bronchioloalveolar carcinoma. It differs from the colloid variant by preserving air spaces and growing primarily in a lepidic pattern along alveolar septa. With the colloid variant, mucin pools fill the alveolar space and replace the parenchymal architecture. Since spatial relationships between cells, stroma, and pulmonary architecture are lost in aspirate smears, it is doubtful that this subtle distinction between both invasive mucinous and colloid variants can be appreciated cytologically. Typical of all forms of colloid adenocarcinoma is the copious amount of mucin discharged onto glass slides. This stains metachromatically in air-dried smears and varies in thickness depending on the amount expelled. Mixed with this opaque stroma are variable-sized cell clusters.

Figure 6.8b — Mucinous Acinar/Colloid Adenocarcinoma, Fine Needle Aspiration [Pap Stain; High Power]. Colloid adenocarcinoma is rare as a pure subtype. Typical of lung adenocarcinomas, it is mostly a peripheral-based mass with no special imaging characteristics. Because there is so much mucinous stroma, smears are typically paucicellular. In this example, background mucin has a wispy diaphanous quality in contrast to the thick, opaque character in the prior image. Polygonal cells show variable degrees of disorder when seen in tight clusters, and some cells contain cytoplasmic mucin. The amount of intracellular mucin varies from being absent to having enormous goblet cell shapes.

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Figure 6.8c — Mucinous Acinar/Colloid Adenocarcinoma, Fine Needle Aspiration [Pap Stain; High Power]. This cluster exemplifies the cytologic uniformity and nuclear blandness of malignant cells. Only slight anisonucleosis is present, allowing one to confuse these cells as either reactive pneumocytes or bronchial cells that have lost their cilia. An almost transparent globule of mucin is seen at the lower left of this image. Colloid adenocarcinoma routinely expresses intestinal-type immunohistochemical markers such as cytokeratin 20, cytokeratin 7, and CDX2. TTF-1 staining is weak at best. Because of this, one must clinically exclude a metastatic lesion from the gastrointestinal tract, pancreas, and possibly ovary.

Figure 6.9a — Adenocarcinoma, Colloid Type, Lobectomy [H&E Stain; High Power]. This special subtype of adenocarcinoma is defined histologically by the filling of air spaces with pools of mucin, often acellular, or with interrupted strips of neoplastic glandular epithelium along septal surfaces or floating in the mucin. The neoplastic cells in this image are mildly to moderately atypical, with only slightly increased nuclear:cytoplasmic ratios and focal crowding. Colloid adenocarcinomas tend to be weakly and heterogeneously PET-avid, probably due to the large zones of acellular mucin. The major differential consideration is metastasis from the gastrointestinal tract, breast, ovary, or pancreas.

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Figure 6.9b — Adenocarcinoma, Colloid Type, Gastrointestinal-Like Area, Lobectomy [H&E Stain; High Power]. The neoplastic epithelium in this field strongly resembles tumors of gastrointestinal origin (especially the colon, appendix, and pancreas). Further confounding interpretation is the fact that colloid adenocarcinoma of the lung expresses gastrointestinal markers (CK20, CDX2, MUC2), and is often negative or weak/focal+ for CK7, TTF-1, and napsin A. History and imaging findings are therefore important in establishing the correct diagnosis.

Figure 6.9c — Invasive Mucinous Adenocarcinoma, Lobectomy [H&E Stain; Low Power]. This entity, newly defined in the 2015 World Health Organization classification of lung tumors, is composed of a dominant (greater than 90%) population of cells with abundant cytoplasmic mucin. Surrounding alveoli may be mucin-filled, causing these tumors to appear larger than they are, grossly and by imaging studies; and fine needle aspiration may yield paucicellular mucin. Growth patterns include varying proportions of any or all of those observed in nonmucinous tumors, except solid—papillary, micropapillary, acinar, and lepidic; adenocarcinoma in situ and minimally invasive adenocarcinoma also occur. This image shows acinar and papillary patterns.

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Figure 6.9 (d, e) — Invasive Mucinous Adenocarcinoma, Wedge Resection [H&E Stain; High Power]. This neoplasm is composed of a uniform population of goblet cells and/or columnar cells with abundant intracytoplasmic mucin and small, basally oriented nuclei with minimal to mild atypia. Adjacent alveolar spaces may be filled with mucin. Often predominantly lepidic, these lesions may also have acinar, papillary, and micropapillary growth patterns. Differential diagnostic considerations include mucinous adenocarcinoma in situ and minimally invasive mucinous adenocarcinoma, both of which require tissue sections for diagnosis; and colloid carcinoma (more mucus than cells; occasional strips of variably mucinous columnar cells). Mucinous lung tumors of all types typically co-express CK7 and CK20, with variable expression of CDX2, and are often negative for TTF-1 and napsin A. Thus, metastasis from the gastrointestinal and pancreatobiliary tracts should be carefully excluded.

Figure 6.10a — Adenocarcinoma, Micropapillary, Lobectomy [H&E Stain; High Power]. This neoplastic gland contains multiple “micropapillae,” small detached tufts of neoplastic glandular cells that lack a fibrovascular core. This pattern occurs most often as a component of a mixed-pattern adenocarcinoma, but rarely does it occur in pure form. Micropapillae may be associated with psammoma bodies and intranuclear pseudoinclusions, features that can be striking on fine needle aspiration biopsies. Micropapillary adenocarcinoma can arise as a primary lesion in numerous organs (e.g., breast, urinary bladder); distinguishing primary from metastatic disease may thus require a panel of immunohistochemical stains.

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Figure 6.10b — Adenocarcinoma, Micropapillary, Lobectomy [H&E Stain; High Power]. These micropapillae show marked nuclear grooving, pleomorphism, and prominent nucleoli, with moderately abundant eosinophilic cytoplasm. The micropapillary pattern of glandular neoplasia tends to be associated with a strong propensity for stromal and vascular invasion; pure or predominantly micropapillary tumors are thus often regionally and distantly metastatic at presentation.

Figure 6.11a—Adenocarcinoma With Mixed Acinar and Solid Patterns, Wedge Resection [H&E Stain; Low Power]. Adenocarcinoma of the lung most often has a mix of growth patterns. This image shows a solid pattern at the top, with an acinar pattern at the bottom. Note the mucin vacuole at the top left, in the solid portion. Thorough sampling of adenocarcinoma will allow accurate determination of the dominant subtype and the percent of all other patterns present; the World Health Organization recommends that final reporting include all patterns, in 5% increments.

Figure 6.11b—Adenocarcinoma With Solid and Acinar Patterns, Wedge Resection [Gross Examination]. This wedge resection shows a peripheral yellow-gray lesion with indistinct infiltrative borders. The tumor broadly abuts the pleura; elastin staining of multiple sections across the tumor-pleura interface may be required to fully evaluate for foci of invasion of the visceral pleural elastic lamina (PL1 or PL2 disease).

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Figure 6.12—Adenocarcinoma, Mixed Mucinous and Nonmucinous Type, Lobectomy [H&E Stain; High Power]. Invasive mucinous adenocarcinoma, acinar pattern, is present at the bottom, while the top half of the image shows nonmucinous adenocarcinoma with cribriform pattern, considered a variant of the acinar pattern. If both mucinous and nonmucinous cytologies exceed 10% of the tumor as a whole, the lesion is designated “mixed mucinous and nonmucinous adenocarcinoma,” with all growth patterns reported, in standard fashion.

Figure 6.13a — Adenocarcinoma, Solid Pattern, Lobectomy [H&E Stain; Low Power]. This tumor had a uniformly solid growth pattern; studies suggest such tumors tend to be more aggressive. Note the nested pattern and the absence of specific cytoplasmic differentiation. Squamous cell carcinoma is a differential consideration.

Figure 6.13b — Adenocarcinoma, Solid Pattern, Lobectomy [H&E Stain; High Power]. Higher magnification shows disorganized nests of pleomorphic cells with nuclear anisocytosis, nuclear creases and irregularities, and frequent nucleoli.

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Figure 6.13c — Adenocarcinoma, Solid Pattern, Lobectomy [TTF-1 Immunohistochemical Stain; Low Power]. TTF-1 staining is strong and uniform. In the 2015 World Health Organization classification, TTF-1 or napsin A expression is sufficient to classify a solid-pattern tumor as adenocarcinoma, even in the absence of stainable mucin. TTF-1 expression is identifiable in about 75% of adenocarcinomas; napsin A is slightly more sensitive. Note that TTF-1 expression is not specific to pulmonary adenocarcinoma, but is also expressed in large cell neuroendocrine carcinoma and some carcinoid tumors, as well as thyroid carcinomas.

Figure 6.13d — Adenocarcinoma, Solid Pattern, Lobectomy [Napsin A Immunohistochemical Stain; Low Power]. There is diffuse granular cytoplasmic staining for napsin A. This protein is secreted, and often taken up by resident alveolar macrophages; at low power, solid aggregates of alveolar macrophages may therefore mimic solid adenocarcinoma with napsin A staining. Some non–small cell carcinomas lack specific growth patterns and are negative for napsin A, TTF-1, p40/p63, and CK5/6; these are designated “large cell carcinoma—null” on resection specimens, and “non–small cell carcinoma, NOS” on small biopsy specimens.

Figure 6.14a — Moderately Differentiated Adenocarcinoma, Fine Needle Aspiration [Diff-Quik Stain; High Power]. At high magnification, notice the eccentric placement of the nuclei. At the top left-hand side of the fragment, the nuclei are arranged outward from a central point, forming an acinar structure. This can also be appreciated in other areas of the fragment, such as at the lower right-hand corner. These features help identify this as an adenocarcinoma.

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Figure 6.14b — Moderately Differentiated Adenocarcinoma, Fine Needle Aspiration [Pap Stain; High Power]. The malignant cells are present in a complex epithelial sheet and have prominent nucleoli, wispy cytoplasm, mildly irregular nuclear borders, and moderate anisonucleosis. The cells are disorganized within the epithelial fragment, resulting in a large amount of nuclear overlap. While there is no question that this is an adenocarcinoma, the determination of whether this is a primary lung lesion or metastatic lesion would require immunohistochemical studies.

Figure 6.14c — Moderately Differentiated Adenocarcinoma, Fine Needle Aspiration [Diff-Quik Stain; High Power]. The malignant cells have round nuclei, mild nuclear border irregularities, and prominent nuclei. Most cells have an eccentrically placed nucleus and are hyperchromatic. The cells are arranged in a three-dimensional structure. Can you identify the benign ciliated epithelial cells in the background? The ciliated cells are smaller and have smaller nuclei and a different chromatin appearance as compared to the malignant cells. Comparing atypical cells to benign cells—especially their nuclear features—can help determine whether the atypia is secondary to reactive atypia or malignancy.

Figure 6.14d — Moderately Differentiated Adenocarcinoma, Fine Needle Aspiration [Pap Stain; High Power]. There is little variation in nuclear size among the malignant cells. Many cells have prominent nucleoli and greatly enlarged nuclei with minimal cytoplasm. In some areas, the tumor cells form three-dimensional structures and have significant overlap. While any adenocarcinoma can have prominent nucleoli, they are most often seen in renal cell carcinoma and prostate carcinoma. In this case, the patient had a lung primary.

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Figure 6.15a — Moderately Differentiated Adenocarcinoma, Fine Needle Aspiration [Pap Stain; Medium Power]. At lower magnification, the arrangement of cells within these tissue fragments can be better appreciated. The cells form three-dimensional sheets with smooth edges. Even though the nuclei overlap significantly, there are “empty” areas within the sheets in which the nuclei are absent, a suggestion that a glandular or acinar structure is present.

Figure 6.15b — Moderately Differentiated Adenocarcinoma, Fine Needle Aspiration [Pap Stain; High Power]. Compare the malignant cells in the center of the field to the numerous ciliated benign bronchial respiratory epithelial cells in the background. Some malignant nuclei are easily more than five times the size of the bystander cell nuclei. The ciliated cells show little variation in nuclear size, whereas there is anisonucleosis in the malignant cells. The malignant cells have prominent nucleoli. While the chromatin of the malignant cells is not significantly darker than that of the bystander benign cells, the malignant cells must be aneuploid to maintain the same amount of chromasia over such enlarged nuclei.

Figure 6.16a — Poorly Differentiated Adenocarcinoma, Fine Needle Aspiration [Diff-Quik Stain; Medium Power]. The field is packed with discohesive cells. The cells have abundant cytoplasm and dramatic nuclear size variations. The cellularity of the field suggests a neoplasm and the greatly enlarged nuclei possessed by some cells indicate a malignant process. However, the neoplasm does not have features to suggest a particular origin. Because the cells are discohesive and do not form tissue fragments, this could be a poorly differentiated carcinoma, melanoma, or sarcoma. Because poorly differentiated carcinomas may be negative for keratin, immunohistochemical studies are not always useful.

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Figure 6.16b — Poorly Differentiated Adenocarcinoma, Fine Needle Aspiration [Pap Stain; High Power]. The malignant cells are discohesive; while most only contain one nucleus, some are binucleate. While the chromatin pattern is not overtly malignant, some cells have highly complex nuclear borders and elevated nuclear-to-cytoplasmic ratios. This could safely be diagnosed as a malignant neoplasm, and if cell block material is available, immunohistochemical studies may help confirm this as a carcinoma.

Figure 6.16c — Poorly Differentiated Adenocarcinoma, Fine Needle Aspiration [Pap Stain; High Power]. Careful examination of the specimen may reveal focal clues as to a tumor’s origin. In this case, while most of the cells are discohesive, the malignant cells form a small fragment at the top of the field as well as a glandular structure at the bottom left-hand side of the field. At the very least, adenocarcinoma can be favored here if immunohistochemical studies cannot be performed or are not useful.

Figure 6.16d — Poorly Differentiated Adenocarcinoma, Fine Needle Aspiration [Diff-Quik Stain; Medium Power]. The cells are large and the size of the nucleus varies greatly between cells. Most cells have an eccentrically placed nucleus and delicate cytoplasm, features of adenocarcinoma. Squamous cell carcinomas tend to have centrally placed nuclei and dense cytoplasm. Prominent nucleoli are also seen, a feature more often seen in adenocarcinomas but also sometimes seen in squamous cell carcinomas. Definite gland formation is not seen, though the cells form three-dimensional fragments that almost form acinar arrangements.

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Figure 6.17a — Poorly Differentiated Adenocarcinoma, Fine Needle Aspiration [Pap Stain; Medium Power]. The anisonucleosis seen here—with some nuclei being more than four times larger than adjacent nuclei—is a common finding in adenocarcinomas. The chromatin is dark but has a granular appearance, and the nuclear borders are moderately irregular. In squamous cell carcinomas, variation in nuclear size is also accompanied by pyknotic nuclei and/or markedly irregular nuclear borders.

Figure 6.17b — Poorly Differentiated Adenocarcinoma, Fine Needle Aspiration [Diff-Quik Stain; Medium Power]. Here the adenocarcinoma shows some signs of glandular differentiation, with more distinct gland-like formations seen at the top of the field. However, in other areas the adenocarcinoma cells are present singly and have marked anisonucleosis, features found in poorly differentiated adenocarcinomas. Many cells contain an appreciable amount of cytoplasm, possibly due to mucin production.

Figure 6.17c — Poorly Differentiated Adenocarcinoma, Fine Needle Aspiration [Pap Stain; Medium Power]. The cells are discohesive or form small fragments. There is granular debris in the background that suggests this is a necrotic tumor, and this may also explain how the individual cells have become detached. The cells have eccentric nuclei and elongated cytoplasm. Some cells are columnar but lack cilia and have the prominent nucleoli seen in other malignant cells. The discohesive nature of the cells, rare binucleate forms, and prominent nucleoli may cause one to consider the possibility of melanoma—however, the nuclei are too pleomorphic to make melanoma very likely.

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Figure 6.18 — Non-Small Cell Carcinoma, Sputum [Pap Stain; High Power]. The malignant cells form a sheet and have prominent nucleoli, marked nuclear size variation, high nuclear-to-cytoplasmic ratios, and irregular nuclear borders. The cells are poorly differentiated, and in the absence of keratinization, gland formation, and mucin, it is challenging to determine whether this represents a squamous cell carcinoma, an adenocarcinoma, or an adenosquamous carcinoma. The use of immunohistochemical studies on cell block material, if enough residual sputum remains, can help narrow the differential diagnosis.

Figure 6.19a — Adenocarcinoma, Fine Needle Aspiration [CK7 Immunohistochemical Stain; High Power]. A cell block was created from dedicated passes of a patient’s lung tumor, allowing for immunohistochemical studies to be performed. Most lung adenocarcinomas are positive for CK7, but the pattern is nonspecific, as adenocarcinomas from many other sites express CK7, as well as some lung squamous cell carcinomas. CK7 may be helpful to prove that a poorly differentiated neoplasm is a carcinoma.

Figure 6.19b — Adenocarcinoma, Fine Needle Aspiration [TTF-1 Immunohistochemical Stain; High Power]. The carcinoma cells present in this fragment stain positively for TTF-1. While not all cells are positive, typically lung adenocarcinomas stain diffusely and strongly for TTF-1 in a nuclear pattern. Lung squamous cell carcinomas and mesotheliomas are usually negative for TTF-1.

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Figure 6.19c — Adenocarcinoma, Fine Needle Aspiration [TTF-1 Immunohistochemical Stain; High Power]. In this case, the dispersed cell population is seen in a poorly differentiated neoplasm. TTF-1 should only be interpreted as positive if there is nuclear staining—cytoplasmic staining can occasionally be seen nonspecifically, such as in liver tissue. While TTF-1 is a sensitive stain for lung primary adenocarcinomas (75%) and small cell carcinomas (90%) it is nonspecific and is usually positive in some other neoplasms, such as those derived from thyroid follicular cells, and is sometimes positive in carcinomas from other sites.

Figure 6.20 — Adenocarcinoma, Fine Needle Aspiration [Napsin-A Immunohistochemical Stain; High Power]. Napsin A is a less sensitive but more specific marker than TTF-1 for lung adenocarcinomas. The staining pattern is typically cytoplasmic and granular. Napsin A is typically not expressed in small cell carcinomas or squamous cell carcinomas of the lung. It is typically used to prove that an adenocarcinoma has a lung origin, but can also be expressed in renal cell carcinomas, especially papillary renal cell carcinoma (79% positive in one study).

Figure 6.21a — Well-Differentiated Squamous Cell Carcinoma, Sputum [Pap Stain; Medium Power]. This image is rather “busy” with numerous cells encompassing nearly the entire field. Approximately half of the cells have distinct orangeophilic, keratinized cytoplasm, identifying these as squamous cells. These cells have enough atypical features to warrant a diagnosis of well-differentiated squamous cell carcinoma. The nuclei are hyperchromatic, with many nuclei additionally being quite large (inflammatory cells may be used for internal size comparison). These large, hyperchromatic nuclei additionally exhibit high nuclear to cytoplasmic ratios. The individual, dyshesive keratinized cells also identify this specimen as a well-differentiated squamous cell carcinoma.

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Figure 6.21b — Well-Differentiated Squamous Cell Carcinoma, Sputum [Pap Stain; High Power]. A single cluster of atypical cells exhibits nuclear molding in addition to a high nuclear:cytoplasmic ratio and nuclear hyperchromasia, all features of malignancy. The few neutrophils present in an otherwise clean background provide a helpful size comparison. The cytoplasm, particularly prominent in the cell in the upper right of the cluster, has the deep turquoise hue and dense, “hard” character of keratin production. Given that this cluster came from a sputum sample, and all malignant cells had similar features, the ultimate diagnosis was well-differentiated squamous cell carcinoma.

Figure 6.21c — Well-Differentiated Squamous Cell Carcinoma, Sputum [Pap Stain; High Power]. The single cluster of atypical cells hovers over a background of rosy streaked mucous strands. Within the cluster, numerous hyperchromatic, angulated, molded atypical nuclei with a faint rim of blueish cytoplasm aggregate into a tight ball-like structure in the center, flanked on the left by dense, turquoise cytoplasm consistent with keratinized cytoplasm that identifies the structure as having squamous differentiation and keratinization, which suggests a well-differentiated process. The nuclear features identify the process as malignant.

Figure 6.21d — Well-Differentiated Squamous Cell Carcinoma, Sputum [Pap Stain; Medium Power]. First to catch the eye is the central, almost arachnoid-appearing, isolated orangeophilic keratinized cell with pronounced curvilinear dendrites. This atypical and irregular keratinized cell provides strong evidence of a malignant squamous process. Further evidence of malignancy is the atypical cells with hyperchromatic nuclei and increased nuclear:cytoplasmic ratios. These cells, in combination with the irregularly shaped anucleate swaths of cytoplasm with that “hard” blue-turquoise squamous appearance of keratin production seen on Pap stain and the background of necrotic debris and inflammatory cells all support a diagnosis of squamous cell carcinoma. Squamous cell carcinoma of the lung is often associated with a history of smoking.