Background: The role of infection as a cause of focal organizing pneumonia (OP) is not fully understood. Objectives: This study aimed to determine the clinical, radiological and pathological characteristics of patients with OP presenting a solitary nodular shadow on a chest radiograph. Methods: Fourteen patients who presented with a solitary nodular shadow on a chest radiograph, pathologically diagnosed as OP after surgical resection, were allocated into two groups according to the histological findings. The first had OP with aggregates of neutrophils in airways and/or pulmonary parenchyma with or without necrosis or destruction of lung architecture (group 1: OP with neutrophilic infiltration; n = 10). The second consisted of patients with OP presenting neither neutrophilic infiltration, necrosis nor destruction of lung architecture (group 2: OP without neutrophilic infiltration; n = 4). Results: Cough, sputum, and chest pain were the common symptoms in both groups of patients. Computed tomography (CT) revealed that all nodules in both groups were located in the peripheral lung parenchyma and had irregular margins, and their shapes varied from round to wedge-shaped. Haemophilus influenzae was isolated from sputum or transbronchial aspirates from 3 patients in group 1. Conclusions: The specimens resected from patients with focal OP mostly show small aggregates of neutrophils. However, the dominant histological feature is OP and neutrophilic infiltration, suggesting infection was a minor histological component in all cases. Clinical symptoms and chest CT findings did not clearly distinguish these two groups of patients. It is thus reasonable to place these OPs in the same category and to treat them in the same way. Surgical excision appears to be the only method for a precise diagnosis.
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