Residency · Residency · Pathology

Non-Neoplastic Lung Disease on Biopsy

Overview

Interstitial lung diseases (ILDs) represent a heterogeneous group of disorders affecting the pulmonary parenchyma. Accurate histologic pattern identification is essential but must be integrated with clinical and radiologic findings through multidisciplinary discussion (MDD). The pathologist's role is to identify and communicate the dominant histologic pattern and its differential diagnosis.

Usual Interstitial Pneumonia (UIP)

Clinical Context

The UIP pattern is the histologic correlate of idiopathic pulmonary fibrosis (IPF) when no identifiable cause is found. However, UIP pattern can also be seen in connective tissue disease-associated ILD, chronic hypersensitivity pneumonitis, drug toxicity, and asbestosis. IPF is a progressive, irreversible fibrotic disease with a median survival of 3 to 5 years without treatment. Antifibrotic therapy (pirfenidone or nintedanib) slows progression but does not reverse fibrosis.

Histologic Features

The defining characteristic of UIP is temporal heterogeneity: areas of dense established fibrosis (old injury) alternate with fibroblast foci (active/new injury) and normal lung tissue within the same biopsy. Spatial heterogeneity produces a patchy, subpleural, and paraseptal distribution with relative upper lobe sparing. Fibroblast foci appear as convex mounds of pale myofibroblasts and loose connective tissue situated at the interface between fibrotic and normal lung. Honeycombing consists of cystically dilated airspaces lined by bronchiolar-type epithelium, surrounded by dense fibrosis, with subpleural predominance. Additional features include architectural distortion with collapse and remodeling of alveolar walls, mild chronic inflammation (dense inflammation should prompt reconsideration of the diagnosis), and smooth muscle metaplasia within fibrotic areas. UIP pattern should lack granulomas, organizing pneumonia as the dominant pattern, or marked cellularity.

Differential Diagnosis of UIP Pattern

Beyond IPF (idiopathic), the UIP pattern may represent connective tissue disease-associated ILD (rheumatoid arthritis, systemic sclerosis), chronic hypersensitivity pneumonitis (which may have UIP-like pattern plus granulomas), drug-induced fibrosis, or asbestosis (UIP pattern plus asbestos bodies).

Nonspecific Interstitial Pneumonia (NSIP)

Histologic Features

The distinguishing feature of NSIP compared to UIP is temporal homogeneity: the changes appear uniform throughout the biopsy. Cellular NSIP shows diffuse, mild to moderate chronic interstitial inflammation (lymphocytes, plasma cells) with uniform thickening of alveolar septa and minimal fibrosis, carrying a better prognosis. Fibrotic NSIP shows diffuse interstitial fibrosis with uniform temporal appearance, potentially overlapping with UIP but lacking fibroblast foci and honeycombing. No honeycombing should be present (if found, UIP should be reconsidered), and fibroblast foci are absent or extremely rare. Subpleural sparing in some cases contrasts with the subpleural predominance of UIP. NSIP has a strong association with connective tissue diseases, especially systemic sclerosis.

Organizing Pneumonia (OP)

Clinical Context

Cryptogenic organizing pneumonia (COP) is diagnosed when no identifiable cause is found. Secondary causes include infection, drug reaction, connective tissue disease, radiation, and aspiration.

Histologic Features

The hallmark finding is Masson bodies: plugs of granulation tissue composed of fibroblasts and myofibroblasts in a myxoid matrix filling alveolar ducts and alveoli. The distribution is patchy with preservation of the underlying lung architecture. Mild chronic interstitial inflammation is present without significant interstitial fibrosis or honeycombing. Type II pneumocyte hyperplasia overlies the Masson bodies, and foamy macrophages may accumulate in adjacent alveoli (endogenous lipoid pneumonia). The clinical significance is that organizing pneumonia shows an excellent response to corticosteroids, unlike UIP.

Hypersensitivity Pneumonitis (HP)

Clinical Context

Hypersensitivity pneumonitis is an immune-mediated response to inhaled organic antigens (birds, mold, chemicals), presenting in acute, subacute, and chronic forms. A detailed exposure history is essential for diagnosis.

Histologic Features

The classic triad (often incomplete) consists of bronchiolocentric chronic inflammation (cellular interstitial pneumonia centered on bronchioles), poorly formed non-caseating granulomas (small, often peribronchiolar, sometimes reduced to isolated giant cells), and organizing pneumonia with Masson bodies in airspaces. Chronic HP may develop fibrosis with a UIP-like pattern, but the presence of granulomas and centrilobular distribution helps distinguish it from IPF. Peribronchiolar metaplasia (lambertosis) is common. Giant cells may contain birefringent particles or cholesterol clefts.

Granulomatous Lung Disease

Sarcoidosis

Sarcoidosis produces non-caseating granulomas distributed along lymphatics: bronchovascular bundles, interlobular septa, and pleura. The granulomas are well-formed and compact ("naked") with minimal surrounding inflammation. Schaumann bodies (laminated calcified concretions) and asteroid bodies (stellate inclusions) may be found within giant cells. Necrosis is absent or limited to minimal fibrinoid necrosis. Infections must be excluded with AFB and GMS stains, and foreign material must be ruled out.

Infections

Mycobacterial infections produce caseating granulomas with central necrosis, diagnosed by AFB stain. Fungal infections (Histoplasma, Coccidioides, Blastomyces, Cryptococcus) produce granulomatous inflammation diagnosed by GMS and PAS stains. Granulomatosis with polyangiitis (GPA) shows necrotizing granulomatous inflammation with vasculitis, geographic "dirty" necrosis, and multinucleated giant cells.

Pulmonary Vasculitis

GPA (formerly Wegener granulomatosis) produces necrotizing granulomatous inflammation, geographic necrosis, and vasculitis of small and medium vessels, and is associated with c-ANCA/PR3 positivity. Eosinophilic granulomatosis with polyangiitis (formerly Churg-Strauss) shows eosinophilic infiltration, necrotizing vasculitis, and extravascular granulomas with p-ANCA/MPO positivity. Microscopic polyangiitis produces capillaritis with diffuse alveolar hemorrhage without granulomas, also with p-ANCA/MPO positivity.

Diffuse Alveolar Damage (DAD)

DAD is the histologic correlate of acute respiratory distress syndrome (ARDS). The exudative phase (first week) shows hyaline membranes lining alveolar ducts and alveoli, interstitial edema, and type I pneumocyte necrosis. The organizing/proliferative phase (1 to 3 weeks) shows type II pneumocyte hyperplasia, interstitial fibroblast proliferation, and organization of hyaline membranes. The fibrotic phase produces dense interstitial fibrosis if the patient survives. Causes include infections, drugs, toxins, connective tissue disease, radiation, and idiopathic presentation (acute interstitial pneumonia/Hamman-Rich syndrome).

Other Patterns

Lymphoid Interstitial Pneumonia (LIP)

LIP shows dense, diffuse lymphocytic infiltration expanding alveolar septa with germinal center formation within the interstitium. It is associated with Sjogren syndrome, HIV, and CVID. Lymphoma must be excluded through clonality studies.

Desquamative Interstitial Pneumonia (DIP)

DIP features diffuse filling of alveolar spaces by pigmented macrophages with uniform involvement and mild interstitial thickening. It is strongly associated with smoking, carries a better prognosis than UIP, and responds to smoking cessation and steroids.

Respiratory Bronchiolitis-ILD (RB-ILD)

RB-ILD shows pigmented macrophages confined to respiratory bronchioles (bronchiolocentric distribution) with peribronchiolar fibrosis. It occurs in smokers and represents a milder form on the same spectrum as DIP. It resolves with smoking cessation.

Pulmonary Alveolar Proteinosis (PAP)

PAP shows alveoli filled with granular, eosinophilic, PAS-positive lipoproteinaceous material with preserved alveolar architecture. Forms include autoimmune (anti-GM-CSF antibodies), secondary (hematologic malignancies, dust exposure), and congenital. The autoimmune form is treated with whole lung lavage.

<image>A medical illustration comparing the major interstitial lung disease patterns. Panel A (UIP): Low-power view showing patchy subpleural fibrosis with honeycombing (cystic spaces lined by bronchiolar epithelium), fibroblast foci (pale myofibroblastic plugs at the interface of fibrotic and normal lung), and intervening normal alveolar tissue demonstrating spatial and temporal heterogeneity. Panel B (NSIP): Diffuse, uniform thickening of alveolar septa by chronic inflammatory cells (cellular NSIP) with temporal homogeneity, no honeycombing, and no fibroblast foci. Panel C (Organizing pneumonia): Masson bodies (plugs of loose granulation tissue) filling alveolar spaces and alveolar ducts, with preserved underlying architecture. Panel D (Diffuse alveolar damage, exudative phase): Pink hyaline membranes lining alveolar ducts and alveolar walls, interstitial edema, and type I pneumocyte necrosis.</image>

<image>A medical illustration of granulomatous lung diseases. Panel A (Sarcoidosis): Well-formed, compact non-caseating granulomas with minimal surrounding inflammation distributed along a bronchovascular bundle, Schaumann body visible within a giant cell (inset). Panel B (Mycobacterial infection): Caseating granuloma with central amorphous necrosis surrounded by epithelioid histiocytes and Langhans-type giant cells, AFB stain inset showing red acid-fast bacilli. Panel C (Hypersensitivity pneumonitis): Bronchiolocentric chronic inflammation with poorly formed loose granulomas and isolated multinucleated giant cells in the peribronchiolar interstitium, with background organizing pneumonia. Panel D (Granulomatosis with polyangiitis): Geographic basophilic necrosis (dirty necrosis) with palisading histiocytes and multinucleated giant cells, vasculitis of a small pulmonary artery with fibrinoid necrosis of the vessel wall (inset).</image>

<image>A medical illustration of smoking-related lung diseases and PAP. Panel A (DIP): Alveolar spaces uniformly filled with pigmented macrophages containing brown cytoplasmic pigment, mild interstitial thickening. Panel B (RB-ILD): Pigmented macrophages confined to respiratory bronchioles (bronchiolocentric distribution) with peribronchiolar fibrosis. Panel C (Pulmonary alveolar proteinosis): Alveoli filled with dense granular eosinophilic material, PAS stain inset showing strongly positive lipoproteinaceous material. Panel D (Langerhans cell histiocytosis): Stellate nodular lesion centered on a bronchiole with Langerhans cells (grooved/reniform nuclei), eosinophils, and CD1a immunostain inset showing membrane positivity.</image>

Clinical Pearls

The single most important distinction in ILD pathology is between UIP and NSIP because they have fundamentally different prognoses and treatment responses. Temporal heterogeneity (fibroblast foci adjacent to dense fibrosis adjacent to normal lung) is the hallmark of UIP, while NSIP shows temporally uniform changes. UIP pattern should never be diagnosed in isolation; multidisciplinary discussion should always be recommended because UIP pattern can be seen in IPF, connective tissue disease-ILD, chronic HP, and asbestosis, each requiring different management.

Small, poorly formed granulomas on a lung biopsy should always prompt AFB and GMS stains before attributing them to sarcoidosis or HP, since infectious granulomatous disease must be excluded. Organizing pneumonia (Masson bodies) is a nonspecific reaction pattern rather than a specific disease, occurring as a component of many conditions including HP, connective tissue disease-ILD, drug reactions, and infections. Transbronchial biopsies are insufficient for reliably distinguishing UIP from NSIP due to sampling limitations; surgical (VATS) biopsy or transbronchial cryobiopsy provides adequate tissue for pattern recognition. When a biopsy shows UIP pattern plus granulomas, chronic hypersensitivity pneumonitis should be strongly considered and a detailed exposure history obtained before diagnosing IPF.

References

• Travis WD, et al. An official American Thoracic Society/European Respiratory Society statement: Update of the international multidisciplinary classification of the idiopathic interstitial pneumonias. Am J Respir Crit Care Med. 2013;188(6):733-748.
• Raghu G, et al. Diagnosis of idiopathic pulmonary fibrosis: An official ATS/ERS/JRS/ALAT clinical practice guideline. Am J Respir Crit Care Med. 2018;198(5):e44-e68.
• Leslie KO, Wick MR. Practical Pulmonary Pathology: A Diagnostic Approach. 3rd ed. Elsevier; 2018.
• Mukhopadhyay S, et al. Lung biopsy in interstitial lung disease: A practical approach for the pulmonary pathologist. Arch Pathol Lab Med. 2012;136(3):223-231.

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