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Pathology of lung tumours — types, markers and carcinoid
Lung cancer is classified by a histological type that determines treatment — adenocarcinoma (TTF-1), squamous carcinoma (p40) and small cell carcinoma (neuroendocrine markers and very high Ki-67) — and in advanced adenocarcinoma molecular markers and PD-L1 must be tested before treatment.
Why it matters in the exam
The bank asks about location, association with smoking and carcinoid. Adenocarcinoma is the most common, usually peripheral, and occurs also in non-smokers and women; its patterns are lepidic (along alveolar walls), acinar, papillary, micropapillary and solid, and grade is set by the predominant pattern and the presence of 20% or more of a high-grade pattern. Adenocarcinoma in situ is a purely lepidic tumour up to 3 cm, and minimal invasion is up to 5 mm. Squamous carcinoma is central, strongly linked to smoking, with keratin pearls and intercellular bridges, and sometimes hypercalcaemia from PTHrP.
Small cell carcinoma is central, almost always linked to smoking, with small cells with scant cytoplasm, "salt-and-pepper" chromatin, nuclear moulding and crush artifact, very high Ki-67 and paraneoplastic syndromes — ACTH, SIADH and Lambert-Eaton. Carcinoid is a low-grade neuroendocrine tumour, usually central in a bronchus, at a younger age and without a clear link to smoking; it causes bleeding or obstruction, and carcinoid syndrome is rare. Typical carcinoid — fewer than 2 mitoses per 2 square millimetres and no necrosis; atypical carcinoid — 2–10 mitoses or necrosis.
In advanced adenocarcinoma at least EGFR, ALK, ROS1, BRAF, KRAS G12C, MET, RET, NTRK and HER2 are tested, preferably in an NGS panel, and PD-L1 by immunohistochemistry; the pathologist therefore preserves tissue and avoids "wasting" it on unnecessary stains. Pleural mesothelioma is linked to asbestos, stains for calretinin, WT1 and D2-40, and sometimes loses BAP1 — a sign that helps distinguish it from a benign reaction.
Key points
- Adenocarcinoma: peripheral, TTF-1, the most common.
- Squamous: central, p40, PTHrP.
- Small cell: central, high Ki-67, ACTH and SIADH.
- Carcinoid: typical fewer than 2 mitoses; atypical 2–10 or necrosis.
- Advanced adenocarcinoma: molecular panel and PD-L1.
Summary table
| Type | Location and smoking link | Markers | Exam point |
|---|---|---|---|
| Adenocarcinoma | Peripheral; also in non-smokers | TTF-1, Napsin A | Molecular markers mandatory in advanced stage |
| Squamous carcinoma | Central; smoking | p40 | PTHrP and hypercalcaemia |
| Small cell carcinoma | Central; almost always smoking | Neuroendocrine markers, high Ki-67 | ACTH, SIADH, Lambert-Eaton |
| Typical and atypical carcinoid | Central; not smoking-related | Neuroendocrine markers | Fewer than 2 versus 2–10 mitoses |
| Mesothelioma | Pleura; asbestos | Calretinin, WT1, D2-40; BAP1 loss | Distinguish from benign reaction |
Free exam questions
מה נכון לגבי Lung carcinoid ? (the question as examined, in Hebrew)
- א. יש לספור מיטוזות ולחפש אחר המצאות נמק בשאלה של atypical carcinoid
- ב. בניגוד ל- small cell carcinoma , ב- carcinoid אין dense core granules
- ג. ב typical carcinoid אופייני יחס גרעין/ציטופלסמה גבוה (גרעין גדול וציטופלסמה כמעט שאינה )נראית
- ד. ב typical carcinoid התאים אירגולרים והגרעינים לרוב אינם אחידים בגודלם
איזה מהבאים הכי פחות קשור לעישון במדינות מערביות? (the question as examined, in Hebrew)
- א. Squamous cell carcinoma
- ב. Adenocarcinoma
- ג. Carcinoid tumor
- ד. Small cell carcinoma
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Sources
- WHO Classification of Tumours: Thoracic Tumours, 5th edition (2021) — Lung epithelial tumours and neuroendocrine neoplasms
- IASLC grading system for invasive non-mucinous lung adenocarcinoma, 2020 — Predominant and high-grade patterns
- CAP/IASLC/AMP Guideline: Molecular testing for selection of patients with lung cancer for targeted therapy, 2018 — Biomarkers to test
Sources are given for further reading; the text is written afresh and not copied from them.
Reviewed by the Examqo AI Professor · 2026-11-06. The text may contain errors; when in doubt, check the source.
See also: Lung cancer and the pulmonary nodule