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Cytopathology
Lung
Malignant

Authors: Jerome Givi, M.D., M.P.H., Reima El Naili, M.D.

Editorial Board Member: Bonnie Choy, M.D.

Last author update: 13 June 2024

Last staff update: 13 June 2024

Copyright: 2022-2024, PathologyOutlines.com, Inc.

PubMed Search: Cytopathology lung malignant

Table of Contents

Cite this page: Givi J, El Naili R. Malignant. PathologyOutlines.com website. https://www.pathologyoutlines.com/topic/cytopathologylungmalignant.html. Accessed December 26th, 2024.

Definition / general

This category is to be used only when a specimen has sufficient quality and quantity of cellular material that demonstrates unequivocal cytopathological features of malignancy (Acta Cytol 2023;67:80)
Risk of malignancy with this category varies slightly based on the sampling method but generally is close to 100% (Acta Cytol 2023;67:80)

Essential features

This category includes both primary and metastatic tumors of the lung
Precise cytologic and molecular characterization of tumors is essential due to advancements in personalized treatment of advanced non-small cell lung cancer (NSCLC)
- All lung adenocarcinomas should be tested for mutations or rearrangements in EGFR, ALK, ROS1 and BRAF to guide treatment
- Additionally, IHC for PDL1 is standard of care in NSCLC to evaluate potential treatment with immune checkpoint inhibitors
If there is insufficient material for ancillary testing or no correlation to clinical or imaging findings, then repeat fine needle aspiration biopsy (FNAB) with or without core needle biopsy should be considered (Acta Cytol 2023;67:80)

CPT coding

88104: cytopathology, fluids, washings or brushings, except cervical or vaginal
88108: sputum cytology smears
88112: concentrated cytology preparations (e.g., ThinPrep)
88173: diagnosis for a fine needle aspiration sample
88305: cell block, H&E
88342: immunohistochemical stain (qualitative), first stain
88341: immunohistochemical stain (qualitative), second and subsequent stains

Sites

Lungs
Pleura

Clinical features

This category includes both primary and metastatic tumors of the lung
- Whenever possible, the neoplasm should be subclassified based on cytopathologic features, immunohistochemistry (IHC) molecular testing and flow cytometry
- However, it is recommended to limit use of IHC for diagnosis and subtyping to preserve cells for molecular analysis (J Thorac Oncol 2011;6:244)
  - Small cell carcinoma and non-small cell lung cancers (NSCLC) can be differentiated with accuracy > 70% by cytomorphology alone (Acta Cytol 2023;67:80)
  - For cases of non-small cell carcinoma (NSCC) that require immunocytochemistry (ICC) to make a definitive diagnosis, they are to be reported as NSCC, favor adenocarcinoma / squamous cell carcinoma (Acta Cytol 2023;67:80)
  - In poorly differentiated carcinomas where NSCC cannot be further classified by cytomorphology or ICC, a final report of NSCC, NOS can be used (Acta Cytol 2023;67:80)
Precise cytologic and molecular characterization of tumors is essential due to advancements in personalized treatment of advanced non-small cell lung cancer
- All lung adenocarcinomas should be tested for mutations or rearrangements in EGFR, ALK, ROS1 and BRAF to guide treatment
- Additionally, IHC for PDL1 is standard of care in NSCLC to evaluate potential treatment with immune checkpoint inhibitors (J Thorac Oncol 2018;13:323, Drugs Context 2023;12:2022, Transl Lung Cancer Res 2020;9:898, Front Med (Lausanne) 2021;8:668612)
Overall cellularity of specimens and the proportion of tumor cells varies by specimen type as well as type and location of neoplasm
- If there is insufficient material for ancillary testing or no correlation to clinical or imaging findings, then repeat FNAB with or without core needle biopsy should be considered (Acta Cytol 2023;67:80)

Laboratory

IHC can be used to confirm or subtype a primary lung tumor and differentiate from metastases; however, IHC use for subtyping should be minimized whenever possible to preserve samples for molecular analyses
Use of TTF1 and p40 is considered the gold standard for subtyping NSCLC; napsin A and CK5/6 can be used as second markers for adenocarcinoma and squamous cell carcinoma (SCC), respectively (J Thorac Oncol 2019;14:377)
- p40 must stain > 50% of tumor cell nuclei to be called positive (J Thorac Oncol 2019;14:377)
- Focal TTF1 staining is enough to be called positive (J Thorac Oncol 2019;14:377)
IHC for PDL1 is standard of care in NSCLC to evaluate potential treatment with immune checkpoint inhibitors (Transl Lung Cancer Res 2020;9:898, Front Med (Lausanne) 2021;8:668612)
In tumors with an adenocarcinoma component, IHC may be used as an alternative to FISH for ALK testing
- IHC may be used for screening prior to molecular testing for ROS1 but is not recommended for EGFR (J Thorac Oncol 2018;13:323)
Panel of synaptophysin, chromogranin and CD56 is the best combination to differentiate suspected neuroendocrine tumors (NETs) from other NSCLC (J Thorac Oncol 2019;14:377)
- TTF1 is also frequently positive in primary lung neuroendocrine tumors (Am J Clin Pathol 2014;142:320)
- Ki67 is used primarily to distinguish low grade NETs (Ki67: 1 - 25%) from small cell lung carcinoma (Ki67: 70 - 100%) and large cell neuroendocrine carcinomas (Ki67: 25 - 80%) (Virchows Arch 2021;478:5)

Radiology description

Radiologic features of pulmonary nodules concerning for malignancy include (Curr Radiopharm 2020;13:238)
- Irregular, spiculated margins
- Size > 3 cm; growth over time
- Multiple nodules
- Lymphadenopathy in mediastinum or hilum
- Thickening or nodularity of the pleura adjacent to mass
- Air bronchograms suggesting an obstructing lesion
- Cavitation, seen as air - fluid level, which suggests SCC
- Small cell carcinoma and SCC tend to be located centrally, while adenocarcinoma and large cell carcinoma tend to be peripheral

Radiology images

Images hosted on other servers:

Left lower lobe mass

Case reports

45 year old man with acute myeloid leukemia diagnosed by thoracentesis of a pleural effusion (Diagn Cytopathol 2021;49:E316)
46 year old man with primary Ewing sarcoma of the lung diagnosed on cytology (Diagn Cytopathol 2020;48:1098)
50 year old woman with combined small cell carcinoma with giant cell carcinoma component (Oncol Lett 2018;15:1907)
70 year old woman with synchronous diffuse large B cell lymphoma and lung adenocarcinoma diagnosed by effusion cytology (Respir Med Case Rep 2023;43:101852)
74 year old woman with primary malignant melanoma of the lung (Diagn Pathol 2020;15:11)

Cytology description

Morphologic features based on the WHO Reporting System for Lung Cytopathology (IAC-IARC-WHO Joint Editorial Board: WHO Reporting System for Lung Cytopathology, 1st Edition, 2023)
Adenocarcinoma
- Cells may be arranged in flat sheets, papillae with fibrovascular cores, glands, acini and micropapillae
- Tumor cells are cuboidal to columnar, with large, round to oval, eccentric nuclei with fine to coarse chromatin
  - Poorly differentiated adenocarcinomas show more marked nuclear pleomorphism
- Nuclear grooves, intranuclear pseudoinclusions and one or more prominent nucleoli are commonly seen
- Cells typically have moderate to abundant amount of granular to vacuolated cytoplasm that is located toward the center of glandular structures
  - Vacuoles are typically fine but may be large and contain mucin droplets
- Other common findings
  - Intra and extracellular mucin
  - Psammoma bodies
  - Necrotic background, especially in high grade carcinomas
- Features consistent with invasive mucinous carcinoma
  - Flat sheets of cells filled with mucin containing vacuoles (may be single vacuole or many small vacuoles)
  - Irregular arrangement of bland nuclei with fine chromatin within sheets of cells
  - Mucinous carcinoma may be negative for TTF1 but a mucin stain may aid in identifying intracellular mucin (J Thorac Oncol 2022;17:362)
Squamous cell carcinoma (SCC)
- Keratin is eosinophilic on Pap and robin's egg blue on Giemsa; it may be present intracellularly or as keratin pearls in cohesive tissue fragments
  - Keratinized cells often have pyknotic nuclei
- Cells may be arranged both singly and in multilayered sheets
- Well defined cell borders with dense cytoplasm and intercellular bridges
- Polygonal, elongated or bizarrely shaped cells; tadpole or spindle shaped cells may be present
- Central, large, hyperchromatic nuclei with irregular nuclear membranes and clumped chromatin
- Prominent nucleoli are typically absent
- Extensive background inflammation and necrosis is common
- Basaloid SCC
  - Predominantly larger cell groups with variable amount of palisading at their periphery; pseudoglandular spaces may be seen
  - Cells often have high N:C ratio, dense, granular chromatin and a scant rim of cytoplasm
  - Focal nuclear molding may be present
Low grade neuroendocrine tumor (NET) / carcinoid tumor
- Highly cellular specimens
- Cohesive groups of cells that may branch around thin fibrovascular strands or form rosette-like structures
- Cells are small and can be round / oval, plasmacytoid or spindled
- Nuclei are monotonous, round / oval shaped with salt and pepper chromatin and smooth nuclear outlines
- Cytoplasm: scant to moderate amount of finely granular cytoplasm
  - Granules are eosinophilic on H&E, grayish blue on Pap or red to magenta on Giemsa
- Nuclear grooves, binucleation, nuclear pseudoinclusions and naked nuclei may be present
- Necrosis is typically absent
Small cell carcinoma
- Highly cellular aspirates of loosely arranged and discohesive cells
- Small, blue cells with scant cytoplasm
- Cell size is usually < 3 times the size of a small lymphocyte
- Angulated nuclei that display nuclear molding in larger sheets of cells (best appreciated on Giemsa stain)
- Chromatin smearing is very frequent and caused by crush artifact but preserved cells have salt and pepper chromatin (best appreciated on Pap)
- Very scant cytoplasm
- Paranuclear blue bodies (Giemsa) are a distinctive feature of SCLC (Cancers (Basel) 2021;13:820)
- Mitotic rate is usually very high (10 mitoses per 10 high power fields)
- Necrosis is frequently extensive and in large zones
- Can be combined with a NSCLC component
Large cell neuroendocrine carcinoma
- Moderate to highly cellular specimens
- Small, loosely cohesive tissue fragments in background of scattered single cells
  - Pseudorosette architecture may be present
- Cells are intermediate to large in size (> 3 times the size of a small lymphocyte)
- Nuclei are enlarged with irregular nuclear membrane; molding may be present to a lesser degree than small cell carcinoma
- Moderate to abundant cytoplasm
- Chromatin is typically granular; chromatin smearing is common
  - May see perinucleolar clearing on Pap
- Prominent background of necrosis
- Frequent mitoses
- Difficult to diagnose on cytology without cell blocks that demonstrate neuroendocrine morphology and architecture (i.e., nesting, palisading, rosettes)
Other tumor types in this category
- Salivary gland type carcinomas
- Adenosquamous carcinoma
- Pleomorphic carcinoma
- Pulmonary blastoma
- Carcinosarcoma
- NUT carcinoma
- Thoracic SMARCA4 deficient undifferentiated tumor
- Lymphoproliferative malignancies
- Mesenchymal tumors
- Mesothelioma
- Angiosarcoma
- Metastatic malignancies

Cytology images

Contributed by Reima El Naili, M.D.

Adenocarcinoma

Cell block, adenocarcinoma

TTF1 in adenocarcinoma

p40 in adenocarcinoma

Squamous cell carcinoma

p40 in SCC

CK5/6 in SCC

TTF1 in SCC

Small cell carcinoma

Prominent necrosis in small cell carcinoma

Synaptophysin in small cell carcinoma

CD56 in small cell carcinoma

TTF1 in small cell carcinoma

CD45 in small cell carcinoma

Ki67 in small cell carcinoma

Molecular / cytogenetics description

All lung adenocarcinomas should be tested for mutations or rearrangements in EGFR, ALK, ROS1 and BRAF to guide treatment (J Thorac Oncol 2018;13:323, Drugs Context 2023;12:2022)
- MET, RET, ERBB2 (HER2) and KRAS should also be included in any expanded molecular panel for lung adenocarcinomas (J Thorac Oncol 2018;13:323)
- Invasive mucinous carcinomas frequently have KRAS mutations or alterations in NRG1 or ERBB2 (J Thorac Oncol 2022;17:362)
For primary lung cancers without an adenocarcinoma component, molecular testing may be conducted if the clinical presentation suggests a heightened likelihood of an oncogenic driver (J Thorac Oncol 2018;13:323)
Small cell carcinoma
- 95% of tumors have loss of RB1, 91% have 3p deletions and 65% have TP53 mutations (Cancers (Basel) 2021;13:820)
- It has recently been suggested to subtype small cell lung carcinoma (SCLC) by mRNA expression of ASCL1, NEUROD1, POU2F3 and YAP1 (J Thorac Oncol 2020;15:1823)

Molecular / cytogenetics images

Images hosted on other servers:

FISH showing ALK rearrangement

Sample pathology report

Lung, fine needle aspiration (FNA):
- Specimen adequacy: adequate for evaluation
- Interpretation: malignant
- Non-small cell carcinoma, favor adenocarcinoma (see comment)
- Comment: The cytology specimen demonstrates malignant cells consistent with non-small cell lung carcinoma (NSCLC). The cells exhibit features indicative of poorly differentiated adenocarcinoma, including enlarged nuclei, prominent nucleoli, high N:C ratio and intracellular mucin. Immunohistochemical staining for TTF1 is positive and staining for p40 is negative, supporting a diagnosis of adenocarcinoma. Additional studies for molecular profiling will be performed for targeted therapy selection. Clinical correlation and further evaluation with imaging studies are advised for staging and treatment planning.
Lung, fine needle aspiration (FNA):
- Specimen adequacy: adequate for evaluation
- Interpretation: malignant
- Small cell carcinoma (see comment)
- Comment: The cellular features seen are consistent with small cell carcinoma and include small to intermediate sized cells with scant cytoplasm, nuclear molding, finely granular chromatin and frequent mitotic figures. By immunohistochemistry, the tumor cells are positive for TTF1, synaptophysin, chromogranin and CD56, while negative for CD45 and p40. Ki67 stain shows a proliferation index of > 90% within the tumor cell nuclei. The morphology and immunoprofile supports the diagnosis.

Differential diagnosis

All from WHO Reporting System for Lung Cytopathology (IAC-IARC-WHO Joint Editorial Board: WHO Reporting System for Lung Cytopathology, 1st Edition, 2023)
For adenocarcinoma:
- Adenocarcinoma cannot be distinguished from precursor lesions such as carcinoma in situ or atypical adenomatous hyperplasia by cytology alone
- Metastatic adenocarcinoma:
  - If metastasis is suspected, then ancillary tests, such as IHC, should be considered
- Adenocarcinoma variants, including fetal and colloid adenocarcinoma:
  - Fetal adenocarcinoma: small glycogen rich inclusions and occasional squamoid morules
  - Colloid adenocarcinoma: single cells in thick extracellular mucin
- Sclerosing pneumocytoma:
  - Features favoring pneumocytoma: cuboidal tumor cells with single bland nuclei and single nucleoli
- Inflammatory processes: acute or granulomatous inflammation, diffuse alveolar damage, pulmonary infarcts:
  - Features favoring benign inflammation: low cellularity with few atypical fragments, preservation of normal tissue architecture and cell polarity, absence of hyperchromasia, dense cytoplasm
- Poorly differentiated adenocarcinoma may be difficult to distinguish from poorly differentiated SCC:
  - If unable to distinguish with ancillary testing, the diagnosis NSCLC, NOS may be used
For SCC:
- Metastatic SCC:
  - Well differentiated metastatic SCC is indistinguishable from primary lung SCC
  - Correlation with clinical and imaging findings, as well as HPV testing may help differentiate
- Reactive, metaplastic and dysplastic squamous cells:
  - Correlation with clinical and imaging findings is important to avoid overcalling cases as malignant
- Poorly differentiated SCC may be difficult to distinguish from poorly differentiated adenocarcinoma:
  - If unable to distinguish with ancillary testing, the diagnosis NSCLC, NOS may be used
For basaloid SCC:
- Small cell carcinoma:
  - Features favoring small cell carcinoma: discohesive cells, nuclear molding, granular chromatin, apoptotic debris
- NUT carcinoma:
  - Features favoring NUT carcinoma: discohesive, monotonous cells with high N:C and prominent nucleoli
- Adenoid cystic carcinoma:
  - Features favoring adenoid cystic carcinoma: cribriform architecture, sharply defined matrix; in some cases where these features are lacking, IHC may be helpful
For low grade NET:
- Distinction between typical and atypical carcinoid is not possible in most cases and should be deferred to surgical resection (Diagn Cytopathol 2013;41:689, Acta Cytol 1995;39:1152)
  - In these cases, carcinoid tumor, NOS should be reported
- Small cell neuroendocrine carcinoma / large cell neuroendocrine carcinoma:
  - Features favoring low grade NET: absent / minimal necrosis, minimal cellular pleomorphism, spindle cells, small cells with minimal atypia and small nucleoli, absent / minimal crush artifact and few mitoses
- Adenocarcinoma:
  - Features favoring adenocarcinoma: mucin, irregular nuclear contours, prominent nucleoli and glandular, acini or papillary architecture
- Lymphoma:
  - Features favoring lymphoma: small, discohesive cells with scant cytoplasm; IHC positivity for lymphoid markers
- Melanoma:
  - Features favoring melanoma: plasmacytoid discohesive cells with moderate cytoplasm, large nuclei with prominent nuclei, melanin pigment may be present; IHC positivity for melanocytic markers
- Adenoid cystic carcinoma:
  - Features favoring adenoid cystic carcinoma: monotonous basaloid cells with high N:C ratio and bland nuclei and indistinct nucleoli, sharply defined eosinophilic tumor matrix
- Paraganglioma:
  - Features favoring paraganglioma: small tissue fragments or single cells with fine chromatin and moderate pale cytoplasm
For small cell lung carcinoma:
- Other neuroendocrine neoplasms:
  - Features favoring low grade NET: absent / minimal necrosis, minimal cellular pleomorphism, spindle cells, small cells with minimal atypia and small nucleoli, absent / minimal crush artifact and few mitoses
  - Features favoring large cell neuroendocrine carcinoma: cell > 3x size of small lymphocytes, prominent nucleoli, only focal nuclear molding
- Basaloid SCC:
  - Features favoring basaloid SCC: medium sized cells with high N:C ratios in nests with peripheral palisading and frequent mitoses
- Lymphomas:
  - Features favoring lymphoma: small, discohesive cells with scant cytoplasm; IHC positivity for lymphoid markers
- NUT carcinoma and SMARCA4 deficient thoracic tumors:
  - Features favoring NUT carcinoma and SMARCA4 deficient thoracic tumors: high N:C ratio, focal abrupt keratinization
- Merkel cell carcinoma:
  - Indistinguishable from small cell lung carcinoma on cytopathology; clinical correlation and ICC is required
For large cell neuroendocrine carcinoma:
- Small cell lung carcinoma:
  - Features favoring small cell lung carcinoma: cell size < 3x size of small lymphocyte, prominent nuclear molding, marked crush artifact, scant cytoplasm
- Lymphoma:
  - Features favoring lymphoma: small, discohesive cells with scant cytoplasm; IHC positivity for lymphoid markers
- Low grade NET:
  - Features favoring low grade NET: absent / minimal necrosis, minimal cellular pleomorphism, spindle cells, small cells with minimal atypia and small nucleoli, absent / minimal crush artifact and few mitoses
- Basaloid SCC:
  - Features favoring basaloid SCC: medium sized cells with high N:C ratios in nests with peripheral palisading and frequent mitoses

Board review style question #1

A 65 year old woman with an extensive smoking history is found to have a 4.2 cm right lower lobe lung mass. Fine needle aspiration of the mass is shown above. What set of genes should always be interrogated for mutations or rearrangements in this type of tumor in order to guide treatment?

EGFR, ALK, ROS1, BRAF
EGFR, MET, ALK, ASCL1
ERBB2 (HER2), KRAS, NEUROD1, BRAF
MET, RET, ERBB2 (HER2), YAP1

Board review style answer #1

A. EGFR, ALK, ROS1, BRAF. The cytology specimen demonstrates adenocarcinoma and all adenocarcinomas should be tested for mutations or rearrangements in EGFR, ALK, ROS1 and BRAF to predict response to treatment with tyrosine kinase and BRAF inhibitors (although it may be reasonable to perform sequential testing on EGFR and ALK, followed by ROS1 testing). Additionally, IHC for PDL1 is standard of care in NSCLC to evaluate potential treatment with immune checkpoint inhibitors. Answers B - D are incorrect because it is appropriate to include MET, RET, ERBB2 (HER2) and KRAS as part of larger panels performed initially or when routine testing is negative but they are not currently required. ASCL1, NEUROD1, POU2F3 and YAP1 are genes involved in SCLC subtyping (J Thorac Oncol 2018;13:323, Drugs Context 2023;12:2022, J Thorac Oncol 2020;15:1823).

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Reference: Cytopathology - Malignant (lung)

Board review style question #2

A 78 year old man is found to have a 5.2 cm left upper lobe mass with central cavitation. Fine needle aspiration of the mass is shown above. What is the expected staining pattern of this mass?

TTF1+, napsin A+, p40-
TTF1+, napsin A-, p40+
TTF1-, napsin A-, p40+
TTF1-, napsin A+, p40+

Board review style answer #2

C. TTF1-, napsin A-, p40+. The radiographic description and cytology specimen is consistent with squamous cell carcinoma, which typically stains positive for p40. Answers A, B and D are incorrect because TTF1 and napsin A are typically positive in adenocarcinomas. Of note, TTF1 can also positive in tumors of neuroendocrine lineage and may also stain background benign bronchial cells.

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Reference: Cytopathology - Malignant (lung)

Board review style question #3

Fine needle aspiration of a central lung mass in a patient with an extensive smoking history is diagnosed as small cell lung carcinoma. What feature likely aided this diagnosis?

Absence of necrosis
Ki67 of 35%
Light blue cytoplasmic inclusions
Round nuclei with prominent nucleoli

Board review style answer #3

C. Light blue cytoplasmic inclusions describe paranuclear blue bodies, a distinctive finding of small cell carcinoma. Answers A and D are incorrect because small cell lung carcinomas typically have angulated nuclei that demonstrate nuclear molding and typically have a background of prominent necrosis. Answer B is incorrect because while not a diagnostic criteria, small cell carcinomas typically have a Ki67 of 70 - 100%.

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Reference: Cytopathology - Malignant (lung)

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