# Clinicopathologic Features of Isolated AFOP Nodules Radiologically Mimicking Malignancy in Post COVID-19 Patients: A Case Series Study

## TL;DR

This case series study explores how rare AFOP nodules in post-COVID-19 patients can look like lung cancer on imaging, emphasizing the need for combined radiological and histopathological analysis.

## Contribution

The study identifies a novel clinicopathologic association between post-COVID-19 infection and isolated AFOP nodules mimicking malignancy.

## Key findings

- AFOP nodules were strongly associated with prior COVID-19 infection compared to organizing pneumonia.
- AFOP nodules showed distinct radiological features like upper-lobe location and mixed GGO-consolidation patterns.
- Histologically, AFOP was marked by intra-alveolar fibrin 'balls' correlating with radiological consolidation.

## Abstract

Background/Objectives: Acute Fibrinous and Organizing Pneumonia (AFOP) is a rare pulmonary condition histologically characterized by intra-alveolar fibrin deposition and organizing pneumonia without hyaline membranes. This study aims to describe the clinicopathologic and radiologic features of isolated AFOP nodules presenting as solitary pulmonary nodules (SPNs) mimicking malignancy in patients with recent COVID-19 infection. Methods: We retrospectively analyzed consecutive cases of histologically confirmed AFOP (n = 20) and organizing pneumonia (OP; n = 119) presenting radiologically as SPNs suspicious for malignancy from January 2021 to December 2023. Clinical data, COVID-19 status, radiologic features (including nodular characteristics, ground-glass opacity [GGO], and consolidation), and histopathological findings were collected and analyzed. Digital image analysis quantified the intra-alveolar fibrin content. Results: AFOP nodules showed a significant association with previous COVID-19 infection compared to OP (55% vs. 0.8%, p < 0.001). Radiologically, AFOP lesions were predominantly located in the upper lobes, frequently exhibiting a mixed pattern of GGO and consolidation within solitary nodules (8–28 mm diameter), distinctly differing from the predominantly lower-lobe homogeneous consolidations in OP. Histologically, AFOP was defined by prominent intra-alveolar fibrin “balls,” correlating significantly with radiological consolidation patterns (r = 0.991, p < 0.05). Regions of consolidation demonstrated higher fibrin contents compared to areas of predominant GGO. Conclusions: Isolated AFOP nodules presenting as SPNs post-COVID-19 infection strongly mimic malignancy radiologically, highlighting the necessity for multidisciplinary diagnostic approaches integrating radiological and histopathological data to avoid unnecessary interventions. Recognition of this rare but distinctive clinical entity is essential for appropriate patient management.