Pathological approach to idiopathic interstitial pneumonias: useful points for clinicians

Key points

• IPF has been narrowed down to a more homogeneous group of patients with a very poor prognosis (median survival <3 yrs) and (by definition) the histological pattern of UIP.

• Histological evaluation of IIPs in general (and cases formerly interpreted clinically as IPF in particular) has resulted in subclassification into prognostically significant histopathological groupings.

• Histopathological classification of interstitial pneumonias generally requires a surgical lung biopsy (exceptions: some cases of COP and AIP); clinical diagnosis may not always require a biopsy (see below).

• Histological classification of interstitial pneumonias requires (by definition) histopathology. HRCT is a surrogate for surgical lung biopsy in some settings, especially UIP. The other histological patterns have less distinctive HRCT features. In all the categories, there remain cases that are hard to classify.

Educational aims

1. To highlight the pathological features of the IIPs.

2. To show the correlation of histology and radiology in the IIPs.

3. To improve understanding of when to get a biopsy and what type of biopsy to get in the setting of an IIP.

Summary Most of the IIPs are diagnosed histologically on the basis of patterns of inflammation and fibrosis. Most require open lung biopsy for definitive histological diagnosis. There is good histological/radiological (HRCT) correlation among the IIPs, but both have their limitations. Radiology gives the overview of gross pathology, but the resolution is limited, whereas biopsy shows the histological features but is limited by sampling; the two modalities are complementary. Histology is not always the gold standard. Histology is a relative gold standard that is modified by other findings (particularly those of HRCT). The new ATS/ERS classification of IIPs is based on clinical-radiological-pathological correlations. Coordinating clinicalradiological-pathological data in the diagnosis of IIP is significantly better than the sum of the individual data. NSIP is still evolving.