Squamous and Adenocarcinoma of the Respiratory Tract

Squamous cell carcinoma and adenocarcinoma are the two major non-small cell carcinomas of the lung, and distinguishing them on cytologic specimens has become clinically important because subtype guides molecular testing and therapy. Each has characteristic cytomorphology, and immunocytochemistry resolves cases that are poorly differentiated or ambiguous on morphology alone.

Definition

Squamous cell carcinoma and adenocarcinoma of the respiratory tract are the principal non-small cell lung carcinomas, defined cytologically by squamous differentiation (keratinisation, dense cytoplasm, sharp cell borders) and glandular differentiation (acinar groupings, vacuolated cytoplasm, mucin), respectively, and supported by immunocytochemical markers.

Scope

This topic covers the cytomorphologic features that separate squamous cell carcinoma from adenocarcinoma in respiratory specimens, the role of immunocytochemical markers in subtyping, the accuracy and limits of cytologic discrimination, and how subtyping connects to current classification and molecular testing. It is a reference-educational overview and does not prescribe diagnostic algorithms or treatment for individual patients.

Core questions

• Which cytomorphologic features distinguish squamous cell carcinoma from adenocarcinoma?
• When are immunocytochemical markers needed to assign subtype on cytology?
• How accurate is cytologic subtyping, and why does it matter for management?

Key concepts

• Squamous differentiation: keratinisation, dense cytoplasm, sharp borders
• Glandular differentiation: acini, cytoplasmic vacuoles, mucin
• Immunocytochemistry (e.g., p40/p63 for squamous, TTF-1/napsin A for adenocarcinoma)
• Poorly differentiated and not-otherwise-specified carcinoma
• Cytology-based molecular testing in adenocarcinoma
• Diagnostic accuracy and limits of cytologic subtyping

Mechanisms

Squamous cell carcinoma shows squamous differentiation, with dense or keratinised cytoplasm, sharp cell borders, and dense hyperchromatic nuclei, whereas adenocarcinoma shows glandular differentiation, with acinar or three-dimensional groupings, cytoplasmic vacuoles, and mucin. When cells are too poorly differentiated to subtype on morphology, immunocytochemistry on cytologic material discriminates the two lineages, supporting subtype assignment. Accurate subtyping matters because adenocarcinoma in particular requires preservation of material for molecular and biomarker testing that guides therapy, a principle integrated into the 2021 WHO classification (Nicholson 2022; Travis 2011).

Clinical relevance

Subtyping non-small cell lung carcinoma on cytology informs which molecular and biomarker tests are pursued and how scarce specimen material is allocated, because squamous and adenocarcinoma differ in their testing and therapeutic pathways. This entry describes the cytologic distinction and its rationale for reference; it does not provide treatment recommendations for individual patients.

Epidemiology

Squamous cell carcinoma and adenocarcinoma together account for the majority of lung cancers, with adenocarcinoma now the most common subtype in many populations; their classification is set out in the 2021 WHO scheme (Nicholson 2022).

Evidence & guidelines

A systematic review and meta-analysis quantifies the diagnostic accuracy of fine-needle aspiration cytology for discriminating squamous cell carcinoma from adenocarcinoma in non-small cell lung cancer (Layfield 2018). The IASLC/ATS/ERS classification of lung adenocarcinoma established cytology-relevant criteria and the importance of preserving tissue for molecular testing (Travis 2011), principles carried into the 2021 WHO classification (Nicholson 2022). Diagnostic guidance frames how cytologic specimens establish and subtype lung cancer (Rivera 2013).

History

Cytologic subtyping of non-small cell lung carcinoma gained prominence after the recognition that adenocarcinoma and squamous cell carcinoma diverge in molecular profile and therapy; the 2011 IASLC/ATS/ERS adenocarcinoma classification formalised subtype-driven, biomarker-aware reporting on small and cytologic specimens (Travis 2011), and subsequent WHO classifications consolidated this approach (Nicholson 2022).

Debates

How much can cytomorphology alone subtype non-small cell carcinoma?

Well-differentiated tumours are often classifiable on morphology, but a substantial fraction are poorly differentiated, and immunocytochemistry is needed to assign squamous versus glandular lineage reliably; the balance between morphology and ancillary testing is a recurring practical question.

Seminal works

• travis-2011
• nicholson-2022
• layfield-2018

Frequently asked questions

Why is it important to distinguish squamous cell carcinoma from adenocarcinoma on cytology?

The two subtypes differ in their molecular and biomarker testing and treatment pathways, so subtype assignment guides which tests are performed and how limited specimen material is used.

What is used when cytomorphology cannot subtype the tumour?

Immunocytochemical markers applied to the cytologic material, such as squamous-associated and adenocarcinoma-associated markers, help assign lineage in poorly differentiated cases.