Summarize the radiology report findings into an impression within 3 sentences.
    INPUT: Portable frontal radiograph of the chest demonstrates low lung volumes . Heart size is normal with normal mediastinal and hilar contours . Focal opacity at the left lung base . No pleural effusion or pneumothorax .
    OUTPUT:
    
Focal opacity at the left lung base could reflect atelectasis , aspiration or pre-existing mass . Recommendation further evaluation with conventional PA and lateral radiographs .