Cytopathology of pulmonary adenocarcinoma with a single histological pattern using the proposed...

Cytopathology of Pulmonary Adenocarcinoma With a

Single Histological Pattern Using the Proposed

International Association for the Study of Lung Cancer/

American Thoracic Society/European Respiratory Society

(IASLC/ATS/ERS) Classification

Erika F. Rodriguez, MD, PhD; Sanja Dacic, MD, PhD; Liron Pantanowitz, MD; Walid E. Khalbuss, MD, PhD;

and Sara E. Monaco, MD

BACKGROUND: Guidelines for histological subtyping in patients with surgically resected lung adenocarcinoma (ADC)

were recently proposed by the International Association for the Study of Lung Cancer/American Thoracic Society/Euro-

pean Respiratory Society. The objective of the current study was to investigate the cytomorphology of these subtypes of

ADC in cases with matched histology specimens demonstrating a single pure subtype. METHODS: The authors reviewed

their database for patients with histological diagnoses of primary lung ADC with a single histological pattern observed on

surgical resection and investigated the cytological findings in 18 matched cytology specimens to eliminate sampling

issues in cases of mixed ADC. RESULTS: Resections were classified as acinar (7 specimens), solid (6 specimens), lepidic

(2 specimens), mucinous (2 specimens), and papillary (1 specimen). Cytology specimens demonstrating a solid pattern

had a predominance of 3-dimensional clusters (5 of 6 vs 0 of 12 specimens) (P 5.0007, Fisher exact test), necrotic back-

ground (3 of 6 vs 0 of 12 specimens) (P 5.02), pleomorphic nuclei (6 of 6 vs 1 of 12 specimens) (P 5.0004), irregular

nuclear contours (6 of 6 vs 3 of 12 specimens) (P 5.009), and nuclear enlargement (5 of 6 vs 2 of 12 specimens) (P 5.01)

compared with the nonsolid patterns. Nuclear pseudoinclusions were present only in nonsolid patterns (5 of 12

specimens), although this finding was not statistically significant (P 5.05) CONCLUSIONS: Cytological features of lung

ADC subtypes proposed by the Study of Lung Cancer/American Thoracic Society/European Respiratory Society

classification overlap. However, architectural and nuclear features may be helpful, particularly in distinguishing the prog-

nostically adverse solid pattern from other patterns. Cancer (Cancer Cytopathol) 2015;123:306-17. VC 2015 American

Cancer Society.

KEY WORDS: lung; adenocarcinoma; International Association for the Study of Lung Cancer/American Thoracic Society/

European Respiratory Society (IASLC/ATS/ERS) classification; adenocarcinoma subtype; cytology.

INTRODUCTION

Lung cancer is one of the most common causes of cancer-related death worldwide.1 Although it has several dif-

ferent histological subtypes, lung cancer traditionally has been divided into 2 major histologic groups: non-

small cell carcinoma (NSCLC) and small cell carcinoma.2 In this dichotomous classification, NSCLC is more

common, representing approximately 80% of lung cancer cases, and patients with NSCLC have a survival rate

Presented at the 61st Annual Scientific Meeting of the American Society of Cytopathology; November 8-12, 2013; Orlando, FL.

Corresponding author: Sara E. Monaco, MD, Department of Pathology, University of Pittsburgh Medical Center, 5150 Centre Ave, POB 2, Ste 201,

Pittsburgh, PA 15232; Fax: (412) 623-4779; [email protected]

Department of Pathology, University of Pittsburgh Medical Center, Pittsburgh, Pennsylvania

Received: November 4, 2014; Revised: January 12, 2015; Accepted: January 20, 2015

Published online March 18, 2015 in Wiley Online Library (wileyonlinelibrary.com)

DOI: 10.1002/cncy.21532, wileyonlinelibrary.com

306 Cancer Cytopathology May 2015

Original Article

of approximately 15% at 5 years.3 However, NSCLC

encompasses a heterogeneous group of malignancies with

different morphologic features, etiology, pathogenesis,

and molecular changes,2 with the 2 major subtypes of

NSCLC being adenocarcinoma (ADC) and squamous

cell carcinoma.

In recent years, the field of thoracic oncology has

changed. This change was driven primarily by progress in

our understanding of the molecular genetic features of

NSCLC and, as a result, many new targeted therapies

have become available for different subtypes of lung can-

cer. Thus, the prior dichotomous classification is no lon-

ger sufficient, and there is a need to further subclassify

NSCLC to help select a patient’s optimal treatment plan.4

The International Association for the Study of Lung

Cancer/American Thoracic Society/European Respiratory

Society (IASLC/ATS/ERS) classification of pulmonary

ADC proposed in 2011 introduced new entities such as

the concept of adenocarcinoma in situ and minimally

invasive ADC, and specific variants such as mucinous,

colloid, fetal, and enteric. Furthermore, it abandoned the

use of the terms “bronchioloalveolar carcinoma” and

“mixed ADC.”5 It also recognized 5 major patterns that

should be used in the classification of lung ADC: lepidic,

acinar, solid, papillary, and micropapillary, as well as sub-

type variants such as mucinous, colloid, fetal, and

enteric.5 These guidelines recommend subclassifying

ADC on surgical resection specimens according to the

predominant histopathologic pattern (primary pattern),

followed by an estimated percentage of the other ADC

patterns that may be present on the resection specimen in

increments of 5% (secondary pattern).5 The importance

of this approach is supported by studies demonstrating

the prognostic impact of the different patterns.6-8 Lepidic,

papillary, and acinar patterns are considered to have a

good/intermediate prognosis, whereas solid and micropa-

pillary patterns have an unfavorable prognosis.6-8 In up to

90% of cases, pulmonary ADC will have a combination

of patterns, and the prognosis will be driven by the pri-

mary pattern or by the presence of a solid or micropapil-

lary component in any percentage.6-8

The current recommendation for small biopsies and

cytology specimens is to precisely classify NSCLC into

specific categories, such as ADC and squamous cell carci-

noma.9,10 However, to the best of our knowledge, further

subtyping in cytology samples is not standard practice at

this time.9,10 In a recent study using cytology samples,

Sigel et al suggested that several cytomorphologic features

such as nuclear size, chromatin pattern, and nuclear con-

tours could be used in a scoring system that correlated

well with histologic grade and prognosis.10,11 In addition,

our group has recently published our experience subtyp-

ing ADC using the proposed IASLC/ATS/ERS classifica-

tion on cytology samples.12 In our prior study,

concordant subclassification of ADC between the domi-

nant/single pattern on surgical resection specimens and

matched cytology specimens was present in 40% of the

cases, and the majority of concordant cases had an acinar

pattern.

Currently, there is an increasing focus on subtyping

advanced lung ADC due to the prognostic implications

and the need to know whether cytology can reproducibly

answer this challenge.13 Thus, in the current study, we

described the cytomorphologic features of 18 cases with a

single histological pattern to better define cytomorpho-

logic criteria without the issue of sampling that can be

noted in ADCs with >1 histological pattern (so-called

mixed ADC).

MATERIALS AND METHODS

Patient Selection

We reviewed the database in the pathology department

for patients with diagnoses of primary ADC of the lung

who underwent surgical resections at the University of

Pittsburgh Medical Center from 2003 to 2007 and had a

positive cytologic specimen available for review.

Classification of Surgical PathologySpecimens

The surgical pathology specimens were reviewed by an

expert in pulmonary pathology (S.D.) using the 2004

World Health Classification criteria2 and were subclassi-

fied according to the current 2011 IASLC/ATS/ERS clas-

sification,5 which has been, in part, published

separately.14 Tumors with >1 pattern were given a domi-

nant and a secondary pattern classification. Specimens

with >1 pattern of ADC were excluded from the current

study and only cases with both cytology and surgical

material available for review were included. Patient demo-

graphics and clinical data (eg, smoking status) were also

recorded, in addition to the type of surgical specimen (eg,

lobectomy, wedge resection, and other) and primary

tumor size.

IASLC/ATS/ERS Classification of Lung Adenoca/Rodriguez et al

Cancer Cytopathology May 2015 307

Classification of Cytology Specimens

Cytology cases were carefully studied by 2 of the authors

(E.R. and S.M.) who were blinded to the histopathologic

diagnosis. Diff-Quik-stained and Papanicolaou-stained

aspirate slides and hematoxylin and eosin-stained cell

block slides for aspiration cytology specimens were used.

The cytomorphological features recorded were similar to

those studied previously by Sigel et al,10 including back-

ground features (eg, necrosis, mucin, clean, and inflam-

matory) and architectural morphology (eg, flat sheets [2-

dimensional (2D)], complex clusters [3-dimensional

(3D)] groups, or papillary structures with true fibrovascu-

lar cores). The presence of single cells was also noted

(abundant or focal), as well as the presence of giant cells

(tumor cell nuclei>the aggregate of 15 lymphocytes) and

signet ring cells. Nuclear size was categorized as small/

medium or large based on large nuclear size estimated as

being�the aggregate of 5 lymphocytes. In addition, other

features such as the presence of pleomorphism, nuclear

contours (smooth vs irregular), chromatin pattern (granu-

lar/fine vs coarse), prominent nucleolus (conspicuous at

low-power, 103 objective), and the presence of nuclear

pseudoinclusions and grooves were also evaluated.

Statistical Analysis

The Fisher exact test was used to test for associations

between the different categorical values. All tests were 2-

sided with P values <.05 considered to be statistically sig-

nificant. The online Fisher exact test calculator was used

(graphpad.com/quickcalcs/contingency1/).

RESULTS

Patient Demographics

Eighteen patients were identified, 14 of whom were

women and 4 of whom were men. The age range at diag-

nosis was 44 to 83 years (average, 68 years). Nine patients

were smokers, 8 were former smokers, and 1 patient was a

never-smoker.

Surgical Specimens

Nine patients underwent lobectomy, 3 underwent seg-

mentectomy, 3 underwent wedge resection, 1 patient

underwent pneumectomy, 1 underwent video-assisted

thoracoscopy, and 1 patient underwent bilobectomy.

Tumors ranged in size from 0.3 cm to 5.0 cm (median

size, 3.2 cm). Surgically resected tumors that measured

�3.0 cm in greatest diameter macroscopically were sub-

mitted in their entirety. One section per cm was submit-

ted on tumors measuring >3.0 cm. All specimens had a

final histological diagnosis of ADC of lung origin. Speci-

mens were classified as acinar pattern (7 specimens), solid

pattern (6 specimens), lepidic pattern (2 specimens), pap-

illary pattern (1 specimen), and those containing muci-

nous features (2 specimens). There were no surgical

specimens identified with a pure micropapillary pattern

and therefore this pattern was excluded from the current

analysis. Any ADCs that were mixed (ie, contained >1

pattern) were excluded. One case had only an exfoliative

cytology specimen and was excluded, resulting in 18 cases

with aspiration cytology and matched histology.

Cytology Specimens

The cytology specimens consisted of 18 fine-needle aspi-

ration specimens. Cytologic features by pattern are sum-

marized in Table 1 and in detail below.

Acinar Pattern

Of the 7 cases classified as acinar pattern (39%), the back-

ground was predominantly clean (6 cases; 86%), but was

inflammatory in 1 case (14%). The cells were distributed

predominantly in flat 2D sheets (7 specimens; 100%).

None of the cases demonstrated predominant 3D archi-

tecture, but 2 cases had focal 3D groups and 1 case had a

papillary architecture focally. The nuclei were small to

medium in 5 of the 7 cases (71%), and were large in the 2

other cases (29%). Significant nuclear pleomorphism was

not found to be present in any case and nuclear contours

were predominantly smooth (5 specimens; 71%),

although irregularities were observed in 2 cases (29%).

The chromatin was fine in all cases, nuclear grooves were

present in 2 cases (29%), and intranuclear pseudoinclu-

sions were noted in 2 other cases (29%). Nucleoli were

conspicuous in 5 cases (71%). No giant or signet ring cells

were present (Fig. 1).

Solid Pattern

Of the 6 cases classified as solid pattern (33%), the back-

ground was necrotic in 3 cases (50%) or inflammatory in

3 cases (50%). The architecture was predominantly 3D in

5 of these 6 cases (83%), 3 of which also had single cells

and another had focal areas suggestive of papillary archi-

tecture. In 1 case, the cells were predominantly distributed

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http://graphpad.com/quickcalcs/contingency1/

in flat sheets. The nuclei were large in 5 cases (83%). All 6

cases demonstrated marked nuclear pleomorphism and

nuclear contour irregularities. Nucleoli were conspicuous

in 5 cases (83%). The chromatin was fine in 4 cases (67%)

and coarse in 2 other cases (33%). No nuclear grooves or

inclusions were noted and no giant or signet ring cells

were present (Fig. 2)

Papillary Pattern

One case was classified as papillary (6%). The background

was inflammatory with a predominant papillary architec-

ture. The nuclei were small/medium and uniform with

smooth nuclear contours and had fine chromatin. Nucle-

oli were inconspicuous at low power, but frequently seen

at higher power. Nuclear grooves were present as well as

intranuclear inclusions. No giant or signet ring cells were

present. Fibrovascular cores and empty cores mimicking

lumens were also observed (Fig. 3).

Lepidic Pattern

Of the 2 cases classified as lepidic (11%), the background

was clean in both. The cells were distributed in flat sheets

or collapsed back-to-back strips on the aspirate smears. In

the cell block sections, these cells were typically observed

in hobnailed strips similar to a string of pearls, analogous

to the appearance of mesothelial cells in washing speci-

mens, which appear as flat sheets on aspirates and strips

on cell block evaluation. No papillary structures or 3D

clusters were noted. In both cases, nuclei were small and

uniform with smooth nuclear contours and had fine/gran-

ular chromatin. Nuclear grooves and inclusions were con-

spicuous in 1 case (50%). The nucleoli were

TABLE 1. . Summary of the Cytomorphologic Features in Primary Lung ADC Categorized by the IASLC/ATS/ERS Classification

Cytologic Features Acinar n=7 Solid n=6 Papillary n=1 Lepidic n=2 Mucinous n=2

Background

Necrosis 0 3 0 0 0

Clean 6 0 0 2 1

Inflammatory 1 3 1 0 0

Predominant architecture of groups

Flat sheets 7 1 0 2 2

3D clusters 0 5 0 0 0

Papillary structures 0 0 1 0 0

Architecture: lumens or acinar pattern

Present 7 2 1 1 1

Absent 0 4 0 1 1

Nuclear size

Small/medium 5 1 1 2 2

Large 2 5 0 0 0

Pleomorphism

Present 0 6 0 0 1

Absent 7 0 1 2 1

Nuclear contours

Smooth 5 0 1 2 1

Irregular 2 6 0 0 1

Chromatin pattern

Fine/granular 7 4 1 2 2

Coarse 0 2 0 0 0

Nucleoli

Conspicuous 5 5 0 0 1

Inconspicuous 2 1 1 2 1

Intranuclear inclusions

Present 2 0 1 1 1

Absent 5 6 0 1 1

Nuclear grooves

Present 2 0 1 1 0

Absent 5 6 0 1 2

Presence of signet ring cells

Present 0 0 0 0 0

Absent 7 6 1 2 2

Presence of giant cells

Present 0 0 0 0 0

Absent 7 6 1 2 2

Abbreviations: 3D, 3-dimensional; ADC, adenocarcinoma; IASLC/ATS/ERS, International Association for the Study of Lung Cancer/American Thoracic Society/

European Respiratory Society.



inconspicuous at low power in both cases. No giant or sig-

net ring cells were present (Fig. 4).

Mucinous Features

Of the 2 cases classified as mucinous ADC (11%), 1 had a

mucinous background (50%) and the other had a clean/

bloody background (50%). In both cases, the cells were

predominantly distributed in flat sheets. One of the cases

had focal 3D clusters and one also had conspicuous single

cells. Papillary formation was not observed. The nuclei

were small to medium sized in both cases. The nuclei were

pleomorphic in one case and one of the cases had nuclear

contour irregularities. The chromatin was granular/fine in

both cases, and in one of the cases nuclear inclusions were

noted. Nucleoli were conspicuous in one of the cases. No

giant or signet ring cells were present (Fig. 5).

Summary of Results

The cytomorphologic features of the different patterns

overlapped. However, some distinguishing cytologic

Figure 1. Lung adenocarcinoma with an acinar pattern. Orderly, 2-dimensional flat sheets of neoplastic cells are shown with cen-

tral lumen formation and mild to moderate pleomorphism with nucleoli within a clean background (A: Diff-Quik stain, 3400; B:

Papanicolaou stain, 3400). The cell block shows tumor cells with vague 2-dimensional glandular arrangements, which resemble a

broken or incomplete string of pearls in some areas, and a lack of large complex clusters (C: cell block; H & E, 3200). The histol-

ogy of this subtype shows an adenocarcinoma with an acinar infiltrative pattern of growth (D: H & E stain, 3400).

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features and trends could be discerned. In particular, cases

classified as solid pattern on histology had a predomi-

nance of 3D clusters (5 of 6 vs 0 of 12 specimens)

(P 5 .0007, Fisher exact test), necrotic background (3 of

6 vs 0 of 12 specimens) (P 5 .02), pleomorphic nuclei (6

of 6 vs 1 of 12 specimens) (P 5 .0004), irregular nuclear

contours (6 of 6 vs 3 of 12 specimens) (P 5 .009), and

nuclear enlargement (5 of 6 vs 2 of 12 specimens)

(P 5 .01) in comparison with the other nonsolid patterns.

Conversely, nuclear inclusions were present only in non-

solid patterns (6 of 12 specimens), although this finding

was not statistically significant (P 5 .05). There was no

statistically significant difference noted between the

groups with regard to the presence of nuclear grooves (0

of 6 vs 4 of 12 specimens) (P 5 .25), conspicuous nucleoli

(5 of 6 vs 6 of 12 specimens) (P 5 .32), or coarse chroma-

tin (2 of 6 vs 0 of 12 specimens) (P 5 .098) in the solid

versus nonsolid pattern groups, respectively (Table 2).

Figure 2. Lung adenocarcinoma with a solid pattern. Complex 3-dimensional clusters of tumor cells are seen with architectural

disorganization and nuclear pleomorphism in a necroinflammatory background (A: Diff-Quik stain, 3400; B: Papanicolaou stain,

3400). The cell block shows large solid clusters of neoplastic cells without discrete lumen formation and demonstrates an

absence of the tumor cells in a string-of-pearl arrangement (C: cell block; H & E stain, 3400). The histology of this solid type of

adenocarcinoma demonstrates markedly pleomorphic tumor cells in a sheet without a definitive glandular pattern of growth (D:

H & E stain, 3400).



DISCUSSION

The current recommendation for the histological classifi-

cation of pulmonary ADC was published by the IASLC/

ATS/ERS in 2011,5 based on prior studies demonstrating

prognostic differences among the different subtypes of

ADC.6-8 This includes the finding that low-grade ADCs

with a lepidic growth pattern have been shown to have a

favorable prognosis (a 5-year recurrence-free survival rate

of approximately 92%), ADCs with papillary or acinar

patterns are considered to have an intermediate prognosis

(a 5-year recurrence-free survival rate of approximately

84%), and ADCs with a solid or micropapillary pattern

have a poor prognosis (a 5-year recurrence-free survival

rate of approximately 73%).7,8,15 Due to these prognostic

implications, the IASLC/ATS/ERS classification has

emphasized the significance of histological subtyping of

surgical specimens of resected lung ADC in routine

pathology practice. However, there currently is no

Figure 3. Lung adenocarcinoma with a papillary pattern. Fine-needle aspiration cytology demonstrating elongated clusters of

neoplastic cells with some palisading along the edges (A: Diff-Quik stain, 3200). Some areas demonstrate true fibrovascular

cores traversing the papillary fronds (B: Papanicolaou stain, 3200). The cell block shows a few papillary tufts with central clear-

ing, which can be difficult to distinguish from an acinar or lepidic pattern of growth (C: cell block; H & E stain, 3400). The histol-

ogy shows an adenocarcinoma with a papillary growth pattern (D: H & E stain, 3400).

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differential treatment recommended based on the histo-

logical pattern or grade of the ADC, and this subtyping is

not routinely applied in cytological specimens.

Because many patients with advanced lung cancer

will not undergo surgical resection, cytology specimens

may represent the only material available for diagnostic

interpretation in a substantial number of cases. However,

to the best of our knowledge there are limited studies

describing the cytomorphological criteria of the 5 main

patterns of lung ADC proposed by the IASLC/ATS/ERS

classification (ie, acinar, solid, lepidic, papillary, and

micropapillary) on cytology samples. Thus, the current

study focused strictly on the examination of cytology cases

with matched histology demonstrating a single pattern of

ADC to further characterize the cytological features and

to identify cytologic criteria that would help cytopatho-

logists in differentiating the solid pattern of ADC with a

poor prognosis from the nonsolid patterns with a good/

intermediate prognosis. Although the case numbers in the

current study are limited, the majority of lung ADCs are

Figure 4. Lung adenocarcinoma with a lepidic pattern. Flat 2-dimensional sheets of bland-appearing neoplastic cells are shown

with occasional intranuclear inclusions and uniform nuclei (A: Diff-Quik stain, 3400; B: Papanicolaou stain, 3400). The cell block

shows tumor cells in strands, imparting a string-of-pearls appearance, with intranuclear inclusions (C: cell block; H & E stain,

3400). The resection specimen shows tumor cells with a lepidic or alveolar pattern of growth (D: H & E stain, 3200).



of mixed histological types when meticulously evaluated,

which limit the number of surgical cases with a pure pat-

tern available for analysis.14 In fact, in a prior study from

the study institution, approximately 74% of the lung

adenocarcinomas were found to have a mixed subtype,

leaving only 26% of cases with a pure histological pat-

tern.14 Although this current study contains a fewer num-

ber of cases, the percentage of ADCs with a single

histological pattern is similar, with only 18 of 66 cases

having matched FNA cytology and histology (27%).12

Given the limited sample size, the cytomorphologic fea-

tures that were found to be statistically significant in dif-

ferentiating solid from nonsolid ADC were compared,

and included the presence of a necrotic background, pre-

dominant 3D architecture, pleomorphic nuclei, irregular

nuclear contours, and nuclear enlargement. It is known

that the majority of ADCs of the lung have mixed pat-

terns, which makes this difficult to apply in practice. In

addition, some cases lacked matched specimens, which

makes our sample size necessarily small. However, the

Figure 5. Lung adenocarcinoma with mucinous features. The aspirate smears and cell block show clusters of cells with basally ori-

ented nuclei and abundant vacuolated cytoplasm (A: Diff-Quik stain, 3400; B: Papanicolaou stain, 3400; C: cell block; H & E

stain, 3400). The histological features of adenocarcinoma with mucinous features show tumor cells with abundant vacuolated

cytoplasm and extracellular mucin (D: H & E stain, 3400).

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results of the current study may be useful in planning

future studies with a larger number of cases that can more

adequately address the issue of cases containing prognosti-

cally unfavorable patterns (eg, solid, micropapillary) that

are clinically significant when present in any amount.

Prior studies have examined the subtyping of differ-

ent patterns of ADC on cytology specimens,11,12 and the

majority of these articles have shown that cytology cannot

reliably and accurately subtype ADC.12,16,17 However,

this may in part reflect sampling issues given that many

prior studies did not restrict their cases to ADC cases with

a single pattern, which may have resulted in cytology

specimens that had a variety of patterns in a different per-

centage from that observed on the histology. In a study by

Rudomina et al, in which only 10 of the cases had a single

histological pattern, the presence of acinar architecture in

cytology specimens was found to have a predictive value

of 94% for the diagnosis of ADC with an acinar pattern,

the presence of papillary clusters with fibrovascular cores

was found to have a predictive value of 75% for the diag-

nosis of papillary ADC, and the presence of large clusters

with smooth borders was found to have a predictive value

TABLE 2. Comparison of Solid Versus NonsolidPatterns Based on Cytomorphological Features

Cytologic Features Solid n=6 Nonsolid n=12 Pa

3D clusters 5/6 0/12 .0007

Necrotic background 3/6 0/12 .02

Pleomorphic nuclei 6/6 1/12 .0004

Irregular nuclear contours 6/6 3/12 .009

Nuclear enlargement 5/6 2/12 .01

Nuclear inclusions 0/6 6/12 .05

Nuclear grooves 0/6 4/12 .25

Conspicuous nucleoli 5/6 6/12 .32

Coarse chromatin 2/6 0/12 .098

Abbreviation: 3D, 3-dimensional.a Determined using the Fisher exact test.

TABLE 3. Summary Comparing Histological and Cytological Findings in the 5 Main ADC Patterns [5]

ADC Pattern Histological Features Cytological Features

Acinar Majority of gland-forming clusters that are round-to-oval

in contour with peripheral nuclear polarization and

apical cytoplasm.

Tumor cells with central lumen formation that may demonstrate

polarization with more peripherally located nuclei. Usually

more 2-dimensional sheets, moderate pleomorphism, and

clean background.Note: Cribriform architecture is also considered as an

acinar pattern. Pitfall: Broken or incomplete acinar structures on the cell block

can mimic a “string of pearls,” as described with ADC with

a lepidic pattern.

Solid Majority of polygonal cells growing in sheets that lack

definitive acinar, papillary, micropapillary, or lepidic

growth.

Tumor cells with large complex, and rounded 3-dimensional clus-

ters without a cribriform or papillary architecture. Usually more

pleomorphism and background necrosis or inflammation.

Note: Tumors with pure solid morphology should have at

least 5 tumor cells with intracellular mucin in each of

2 high-power fields, confirmed with histochemical

stains.

Pitfall: Some clusters may have a vague acinar architecture

when falling apart or appearing in smaller groups with less

complexity. Special stains and immunostains are helpful to

exclude large cell neuroendocrine carcinomas and squa-

mous cell carcinoma.

Papillary Majority of glandular cells growing along central fibro-

vascular cores with polarization of the nuclei.

Elongated or finger-like clusters with palisading along the

edges and endothelial cells streaming through the central

core (eg, fibrovascular core). May see intranuclear inclusions

and/or grooves.

Pitfall: Some papillary tufts with central clearing and an

absence of a definitive fibrovascular core can be difficult to

distinguish from an acinar or lepidic pattern of growth. Intra-

nuclear inclusions and/or grooves may be observed in the

lepidic pattern also.

Lepidic Nonmucinous glandular cells with growth along preexist-

ing alveolar structures (eg, lepidic).

Strips of cells with mild pleomorphism, usually showing hob-

nailing or a “string-of-pearls” arrangement on the cell block.

Usually an absence of marked pleomorphism and necrosis.

May see intranuclear inclusions and/or grooves.

Pitfall: Clusters may demonstrate back-to-back collapsed

strips, which may impart a cribriform or acinar appearance.

Intranuclear inclusions and/or grooves may be observed in

the papillary pattern also. In cases of limited cellularity,

these cases may be difficult to distinguish from benign/reac-

tive epithelial cells.

Micropapillary Small, cuboidal cells growing in detached or loosely

connected papillary tufts, which typically lack the

fibrovascular cores observed in the papillary pattern.

Psammoma bodies may occur.

No case reported in the current study. Previously reported

cases describe small tight tufts of cells without discrete

fibrovascular cores.

Pitfall: Micropapillary tufts may be seen in acinar, lepidic, and

papillary ADC.

Abbreviation: ADC, adenocarcinoma.



of 39% for a solid-type ADC.16 This study also demon-

strated that the presence of small micropapillary-like tufts

in cytology specimens was not an accurate indicator of a

micropapillary-type ADC.16 Given the small sample size

in the current study, we chose to combine the good/inter-

mediate subtypes to determine whether there were differ-

ences compared with the solid-type ADCs, given that the

cytomorphologic features of the good (lepidic) and inter-

mediate (papillary or acinar) prognostic patterns appeared

to overlap in the current study and in other studies.16-21

The majority of the nonsolid ADC subtypes have been

described as having flat sheets of cells in an orderly

arrangement and round uniform nuclei, similar to the

findings noted in the current study for the acinar, papil-

lary, and lepidic patterns.16-18 These patterns also had a

clean background and lacked pleomorphism. The papil-

lary pattern tended to have a more prominent papillary

architecture with fibrovascular cores traversing through

the clusters compared with the acinar and lepidic patterns,

and fewer prominent nucleoli at low power. Although

intranuclear inclusions and grooves have been reported in

papillary-type and lepidic-type ADC, they were also

observed in some acinar patterns.18 Recognition of a pre-

dominance of a papillary architecture appeared to be

important for the papillary pattern, given that a focal pap-

illary architecture may have less significance, as it was

noted in 1 acinar pattern and 1 solid pattern. However,

the current study did not identify any pure micropapillary

ADC cases, and therefore it is uncertain whether the fea-

tures of this prognostically poor pattern are distinctly rec-

ognizable. In general, the micropapillary pattern is

relatively rare (compared with acinar and solid), is usually

observed in combination with other patterns, and has

been described as lacking definitive fibrovascular

cores.16,19,20 A side-by-side comparison of the histological

and cytological findings reported for the 5 main patterns

of ADC is shown in Table 3, which also cited some of the

pitfalls and diagnostic challenges in these patterns.

The difficulties in subclassifying ADC in cytology

specimens may be related to the lack of available, estab-

lished, cytomorphologic criteria; the overlap in some fea-

tures; and the lack of experience in defining specific

patterns of ADC, as well as sampling issues in ADC of

mixed patterns. However, the problems in applying the

IASLC/ATS/ERS classification are not limited to cytol-

ogy, because it has been shown that there is poor reprodu-

cibility of the patterns among surgical pathologists

examining histological specimens.21,22 However, there is

hope in that training appears to improve the concordance

rate for surgical pathologists.22,23 Thus, with more publi-

cations and further characterization of these patterns, the

cytopathology community will likely improve their ability

to recognize these patterns, which may enable characteri-

zation of the prognostically significant patterns in small

specimens. Although the presence of ADC with lepidic,

papillary, or acinar histology, in the absence of a solid

growth pattern, has been shown to be a significant predic-

tor of a potential epidermal growth factor receptor muta-

tion, it is important to remember that the current

molecular testing guidelines specifically state that the his-

tological subtype of an ADC should not be used as a selec-

tion criteria for molecular testing.14,24 Thus, practical

application of subtyping for ADCs in cytological material

would currently only be of value in providing prognostic

information, particularly among patients who will not

undergo primary surgical resection. At the moment, for

practical purposes, the current recommendation is to pre-

cisely classify NSCLC into specific categories such as

ADC and squamous cell carcinoma on small biopsies and

cytology specimens, while limiting the number of immu-

nohistochemical stains to preserve tissue for molecular

studies as needed.

Although the subclassification of pulmonary ADC

in cytologic samples is challenging and criteria remain to

be developed, the results of the current study have demon-

strated that there are features that can be helpful in distin-

guishing some of these patterns of ADC in cytology. In

particular, specific cytologic features (background, archi-

tecture, and pleomorphism) may allow for better identifi-

cation of the prognostically adverse solid pattern.

Additional studies with greater sample sizes, as well as

studies examining the interobserver variability among

practicing cytopathologists who evaluate these challenging

specimens, will be helpful in defining these patterns in

cytology as we are asked to do more with smaller

specimens.

FUNDING SUPPORT

No specific funding was disclosed.

CONFLICT OF INTEREST DISCLOSURES

The authors made no disclosures.

Abbreviations: 3D, 3-dimensional; ADC, adenocar-

cinoma; IASLC/ATS/ERS, International Association for

Original Article


the Study of Lung Cancer/American Thoracic Society/

European Respiratory Society.

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