Summarize the radiology report findings into an impression within 3 sentences.
    INPUT: There is a subtle left retrocardiac opacity which is concerning for infectious process . The right lung is clear . The cardiomediastinal silhouette , hilar contours , and pleural surfaces are normal . There is no pleural effusion or pneumothorax .
    OUTPUT:
    
Subtle opacity in the left retrocardiac region possibly reflecting acute pneumonia . Repeat radiograph following treatment is advised .