Early stage of pulmonary Langerhans cell histiocytosis is characteristically by small ground glass lung nodules with upper lobe predominance. Based on imaging appearance it is not possible to differentiate Langerhans cell histiocytosis from respiratory bronchiolitis (RB) or RB-ILB, the clinical entity. 

As the disease progresses irregular shaped lung cysts appears. The mixture of lung nodules and irregular shaped cysts is suggestive of Langerhans cell histiocytosis in a patient with smoking history.

In minority of patients the disease progresses. The late stage is characterized by severe emphysemas and is no different to other causes of emphysema. 

In summary the cystic/ nodular phase of Langerhan cell histiocytosis is pathognomonic  whereas early nodular disease overlap with RB and RBILD and late disease cannot be differentiated from various causes of severe emphysema.