H&E Diagnosis & Classification of Lung Cancer: An Overview

From specimen to diagnosis

Lung cancer rarely announces its precise identity at first glance. The pathologist begins with whatever tissue the clinical situation allows — a small bronchoscopic biopsy, a transthoracic needle core, a cytology specimen, or, less often, a fully resected lobe. That starting material matters enormously: a generous resection lets the pathologist survey the full architecture of a tumor, whereas a tiny biopsy may capture only a fragment of a heterogeneous mass. The first and most consequential task is deceptively simple to state — decide whether the tumor is small cell or non–small cell, and within the latter, whether it is adenocarcinoma or squamous cell carcinoma. This division is not academic. As the current WHO framework makes explicit, histology gates which molecular tests are run downstream, so the morphologic call directly shapes the entire diagnostic and therapeutic pathway [1].

The workhorse of this process is the hematoxylin and eosin (H&E) stain, which renders nuclei blue-purple and cytoplasm/stroma pink, exposing the tissue architecture and cellular detail. When H&E alone cannot resolve the subtype — common in small or poorly differentiated biopsies — a focused panel of immunohistochemistry (IHC) is added, and tissue is conserved for the molecular work that may follow.

The H&E picture (synthesized morphology)

Across lung carcinomas, the H&E appearance maps onto a few recurring themes, and learning to read them is the foundation of classification.

Glandular versus squamous differentiation anchors the non–small cell side. Adenocarcinoma shows gland formation, mucin, or growth of tumor cells along intact alveolar walls (the lepidic pattern), and it displays a spectrum of architectures — lepidic, acinar, papillary, micropapillary, and solid [1,2]. Squamous cell carcinoma instead shows keratinization, intercellular bridges, or a basaloid arrangement, and is classically central in location [1].

Neuroendocrine morphology is a distinct visual language: organoid nests, palisading at the edges of cell groups, and rosette-like structures. This pattern spans a biological range from indolent to highly aggressive, and the H&E reader grades it largely by counting mitoses and assessing necrosis [1].

Cell size and chromatin separate the high-grade neuroendocrine tumors. Small cell carcinoma is defined by small cells with scant cytoplasm, finely granular ("salt-and-pepper") chromatin, nuclear molding, brisk mitotic activity, and necrosis [1].

When morphology is ambiguous, IHC clarifies. The foundational split uses p40 (and p63, CK5/6) to mark squamous differentiation versus TTF-1 and napsin A (with CK7) for adenocarcinoma [1]. Neuroendocrine tumors are confirmed with synaptophysin, chromogranin, and INSM1, with Ki-67 helping gauge proliferation [1].

The major categories

The current classification organizes lung carcinoma along a few axes; each named entity below links to its own detail article.

Non–small cell carcinomas

• Lung adenocarcinoma — the glandular/lepidic tumor that carries nearly all of the actionable molecular drivers (EGFR, ALK, ROS1, and others). TTF-1, napsin A, and CK7 positivity are typical [1,2].
• Lung squamous cell carcinoma — keratinizing, p40-positive, TTF-1-negative. Targetable drivers are comparatively scarce here, so PD-L1 testing carries proportionally more weight in the biomarker workup [1].

Preinvasive and minimally invasive lesions

• Adenocarcinoma in situ (AIS) / minimally invasive adenocarcinoma (MIA) — the indolent end of the adenocarcinoma spectrum. AIS shows pure lepidic growth with no invasion; MIA shows lepidic growth with an invasive focus of ≤5 mm. When fully resected, prognosis is excellent [1].

Neuroendocrine neoplasms

• Carcinoid tumors (well-differentiated neuroendocrine tumors) — separated into typical (<2 mitoses/2 mm² and no necrosis) and atypical (2–10 mitoses/2 mm² and/or focal necrosis). These mitosis/necrosis thresholds are the spine of the classification [1].
• Large cell neuroendocrine carcinoma (LCNEC) — large cells with neuroendocrine architecture, a high mitotic rate, and necrosis, requiring neuroendocrine marker confirmation. Biologically, it straddles small cell and non–small cell carcinoma [1].
• Small cell lung carcinoma (SCLC) — high-grade by definition, with near-universal TP53 and RB1 loss, and a distinct treatment paradigm [1].

How grading and staging work

Grading and staging answer different questions. Grading describes how aggressive the tumor looks under the microscope; staging describes how far it has spread (and is established through clinical, radiologic, and surgical assessment beyond the scope of this H&E page).

Grading is now entity-specific. For invasive adenocarcinoma, the IASLC system grades by combining the predominant architectural pattern with the presence of any high-grade pattern, producing well, moderately, or poorly differentiated categories [2]. This is a meaningful change from grading by predominant pattern alone, because even a small amount of high-grade (solid or micropapillary) architecture can reclassify an otherwise low-grade tumor. (Readers and writers should verify the precise IASLC cut-points at the time of use.) Squamous cell carcinoma still uses conventional differentiation grading [1]. Neuroendocrine tumors are graded through their own logic: carcinoids by mitoses and necrosis, while SCLC and LCNEC are high-grade by definition [1].

Why classification matters

The reason this morphologic taxonomy carries such weight is that histology determines molecular-test triage. Actionable drivers concentrate in adenocarcinoma, so comprehensive molecular profiling has become standard in advanced non-squamous non–small cell disease — the histologic diagnosis is what makes a patient eligible to be tested, and therefore potentially eligible for a targeted-therapy drug class [1,2]. The p40-versus-TTF-1/napsin split is not a formality; it routes the specimen toward, or away from, that profiling. Squamous tumors yield few targetable alterations, so their workup leans toward PD-L1 and immunotherapy considerations [1]. Distinguishing SCLC and LCNEC from non–small cell carcinoma changes the treatment paradigm entirely [1]. In short, the A↔B linkage between histology and molecular testing is unusually tight in the lung, and the H&E slide sits at the hinge.

What's current (recent reclassification)

The 5th edition of the WHO Classification of Thoracic Tumours (2021) is the present reference standard, and several shifts are worth flagging [1]. First, the formal IASLC architectural grading of invasive adenocarcinoma integrates high-grade patterns rather than relying on the predominant pattern alone — a change clinicians and trainees should be aware of when reading reports [2]. Second, the neuroendocrine neoplasms are increasingly framed as a unified family with well-differentiated tumors (carcinoids) at one end and high-grade carcinomas (SCLC, LCNEC) at the other. Third, the evidence suggests that transcription-factor–based molecular subtypes of SCLC (ASCL1, NEUROD1, POU2F3, and others) may eventually refine classification, but this remains a research/emerging area not yet part of routine H&E-based diagnosis [1]. As always with a fast-moving field, specific grading cut-points and emerging subtypes should be verified against the current primary sources at the time of reading.

References

1. WHO Classification of Tumours Editorial Board. Thoracic Tumours, 5th ed. IARC, Lyon; 2021. ISBN 978-92-832-4506-3.
2. Moreira AL, Ocampo PSS, Xia Y, et al. A Grading System for Invasive Pulmonary Adenocarcinoma: IASLC Pathology Committee. J Thorac Oncol. 2020. doi:10.1016/j.jtho.2020.06.001.