A Rare Case of Acute Fibrinous Organizing Pneumonia in a Patient With Post-obstructive Pneumonia and Malignancy.

Acute fibrinous organizing pneumonia (AFOP) is a rare form of interstitial lung disease characterized histopathologically by intra-alveolar fibrin "balls" and organizing pneumonia. AFOP is associated with infections, malignancies, and immune dysregulation, posing diagnostic and therapeutic challenges. We present the case of a 66-year-old female with rectal adenocarcinoma, chronic obstructive pulmonary disease (COPD), and a posterior mediastinal mass who was admitted with sepsis and post-obstructive pneumonia. Imaging revealed consolidations in the right middle and lower lobes with adjacent abscess formation. Lung biopsy confirmed AFOP, demonstrating intra-alveolar fibrin balls and organizing pneumonia. Cultures identified Streptococcus anginosus and Prevotella melaninogenica, indicating a polymicrobial infection. The patient was treated with antibiotics; corticosteroids were withheld due to active infection and immunosuppression from malignancy. Surgical intervention addressed the mediastinal mass and abscess. The patient recovered without AFOP recurrence. This case highlights the diagnostic complexity of AFOP, especially in patients with multiple risk factors such as malignancy, infection, and immunosuppression. The decision to withhold corticosteroids underscores the importance of individualized therapy. Clinicians should consider AFOP in the differential diagnosis of acute lung infiltrates, particularly in patients with malignancies or infections. Prompt recognition and tailored management can improve outcomes in this rare condition.