This article introduces pulmonary pathology and discusses an approach to lung specimens.

Medical lung disease is dealt with in the medical lung disease article. Tumours of the lung are dealt with in lung tumours article. Lung cytopathology is dealt with in the pulmonary cytopathology article.

Lung specimens

CT-guided or ultrasound-guided needle core biopsy - for peripheral lesions.
Transbronchial biopsy - for central lesions.
"Open lung biopsy" - typically a video-assisted thoracic surgery (VATS) - done for diffuse lung diseases.
- These specimens should be sectioned to create pieces with a large surface area; ideally, it should be one large piece per block, so one can appreciate the architecture.
Lobectomy - usually for cancer.
Pneumonectomy - usually for cancer.
Explantation - in the context of lung transplantation - done for cystic fibrosis Idiopathic pulmonary fibrosis and other causes.

Lung core biopsies

Inadequacy rate for percutaneous biopsies ~5% in one series.^[1]
Length 0.5-1.5 cm enough for EGFR testing.^[2]

Basic approach

All lung pathology can be grouped into one of six categories (as per Leslie). The radiology directly correlates to the pathologic grouping, except that air space disease encompasses three pathologic categories (ALI, CCI, AFD).

Pathologic groups:

Lung pathology

Acute lung
injury

Fibrosis

Chronic cellular
infiltrates (ALI)

Alveolar filling
defect (AFD)

Nodules

Near normal
histology

Identification of the groups:

Acute lung injury: hyaline membranes (very pink on H&E).
Fibrosis = thick walls - pink on H&E.
Chronic cellular infiltrates = inflammation (blue on H&E).
Nodules = look at the history/radiology - should say mass or nodule.
Alveolar filling defect = crap in the alveoli.
Near normal = looks almost normal.

Radiologic groups:

Lung radiology

Air space
disease

Interstitial
disease

Mass/nodules

Near normal

Radiologic-pathologic correlation:

Air space disease (radiologic).
- Acute lung injury.
- Chronic cellular infiltrates.
- Alveolar filling defects.
Interstitial disease (radiologic).
- Fibrosis.
Mass/nodules (radiologic).
- Nodules.
Near normal (radiologic).
- Near normal histology.

Most of the things that come to pathology are in the mass/nodules category and lung tumours (discussed below). The other categories are dealt with in the medical lung disease article.

Anatomical approach:

Pleura - thickening?
Airspace - filling?
Alveolar walls - thickening?
Airways - inflammation?
Vessels - thickening?

Normal lung

Benign lung redirects here.

Lung anatomy

Airway

Bronchus = has cartilage.
Bronchiole = non-cartilaginous airway.

Small airways

The trip to the alveolus:^[3]

Membranous bronchiole.
Terminal bronchiole - dilation distal to this = emphysema.
Respiratory bronchiole.
Alveolar duct - dilated in ARDS.
Alveolus.

Pleura

Visceral pleura = covers the lung.^[4]
Parietal pleura = covers the chest wall.

Note:

Pleural invasion is an important prognosticator in lung cancer and should be considered if the tumour is close to the pleura.

Lung lobule

Lung lobule:^[5]

Arterial vessels travels with the bronchus.
Venules travel in the septae.

Note:

Arterial vessels in the lung should be approximately the same size as its accompanying airway.^[6]

Memory device:

Arteries (which were once thought to contain air) are with the airway.

Lung histology

Cells

Common:

Type I pneumocyte - cover most of the alveolar surface.
Type II pneumocyte - stem cell, produce surfactant.
- Typical location: "angle of alveolus".
Macrophages.

Rare:

Pulmonary neuroendocrine cells:^[7]
- Histomorphology:
  - Single cells.
  - Small clusters ~ 6 cells ("neuroepithelial bodies").
- Identified with immunostains:
  - Serotonin.
  - Bombesin.
  - Chromogranin A.
  - Synaptic vesicle 2.

Images

BBE - low mag. (WC)
BBE - intermed. mag. (WC)
BBE - high mag. (WC)
BBE - very high mag. (WC)
Lung & BBE - intermed. mag. (WC)
Lung & BBE - high mag. (WC)

www:

Pneumocytes (proteinatlas.org).

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Missed endobronchial biopsy

RIGHT UPPER LOBE, ENDOBRONCHIAL BIOPSY:
- SMALL FRAGMENT OF BENIGN BRONCHIAL MUCOSA WITH INFLAMMATION.

COMMENT:
The clinical history of a mass is noted. 

This biopsy does not show neoplastic tissue; however, the biopsy may not be representative
of the lesion seen.

Alternate

Lung, Left Lower Lobe, Endobronchial Biopsy:
- Respiratory bronchiolitis.
- Benign bronchial epithelium.
- NEGATIVE for granulomatous inflammation.
- NEGATIVE for evidence of mass lesion.

Comment:
Immunostains were done and compatible with bronchial epithelium (napsin negative, 
TTF-1 negative, CK7 positive, CK20 negative, CDX2 negative, beta-catenin membranous 
staining) and lung parenchyma (napsin positive, TTF-1 positive, CK7 positive, 
CK20 negative, CDX2 negative, beta-catenin membranous staining).

Pathology terminology

Siderophages = mononuclear phagocyte with hemosiderin.^[8]

Hyaline membrane = glassy layering of an alveolus/small airways with material that is eosinophilic on H&E.^[9]
- Entomology of Hyaline: "... of glass" (Greek).
- Image: Hyaline membrane (path.upmc.edu).

Bronchiolization = ciliated (respiratory) epithelium or goblet cells in (distal) air space.^[10]
- Image: Bronchiolization (ucsf.edu).

Smoker's macrophages = brown-pigmented macrophages - assoc. with smoking.^[11]

Detail articles

Malignancy - lung cancer

Main article: Lung cancer

This pretty much always comes to the pathologist.

Medical lung disease

Main article: Medical lung disease

There are separate articles for:

Pneumonia.
Diffuse lung diseases, e.g. usual interstitial pneumonia.
Pulmonary hypertension.

Lung transplant pathology

Main article: Lung transplant pathology

Specific diagnoses

Bronchogenic cyst

General

Benign.
Usually in mediastinum or hilum.^[12]
Fit into the bigger category of foregut cyst.