Organizing pneumonia components in non-specific interstitial pneumonia (NSIP): a clinicopathological study of 33 NSIP cases

Aims: To review the clinical, radiological and pathological features of non-specific interstitial pneumonia (NSIP), mainly to characterize organizing pneumonia (OP) components in NSIP.

Methods and results: Lung biopsy samples from 33 NSIP patients were collected over a period of 10 years. Microscopic analysis revealed that 13 cases showed a cellular pattern and 20 showed a mixed/fibrosing pattern. OP components were detected in 26 cases (13 with a cellular pattern; 13 with a mixed/fibrosing pattern), and were found to constitute a median proportion of 9% (range, 1-40%) of the affected tissues. In nine cellular and four mixed/fibrosing NSIP cases, the OP components accounted for ≥10%. A proportion of ≥20% was found in only five cellular pattern cases. Twenty-nine patients were followed up: 17 showed improvements, five were stabilized, and seven showed progression.

Conclusions: OP components are common basic lesions in NSIP cases, although their proportion in cellular and mixed/fibrosing pattern cases varies substantially between patients. OP components do not impact on prognosis, even when they constitute ≥20% of the affected tissue. Thus, a high level of OP components does not exclude a diagnosis of NSIP in cases that otherwise show pathological and radiological findings characteristic of NSIP.