Langerhans cell histiocytosis (LCH) describes a group of syndromes that share the common pathologic feature of infiltration of involved tissues by Langerhans cells. Typically, the skeletal system is involved, with a characteristic lytic bone lesion form that occurs in young children or a more acute disseminated form that occurs in infants.
Pulmonary involvement is not unusual in systemic forms of LCH, but symptoms are rarely a prominent feature. [1, 2] Pulmonary Langerhans cell histiocytosis is considered a reactive proliferation of dendritic Langerhans cells to chronic tobacco-derived plant proteins resulting from incomplete combustion, but it can also occur as a tumor-like systemic disease in children.  CT scans of eosinophilic granuloma are depicted in the images below.
Localized pulmonary LCH (also termed pulmonary eosinophilic granuloma) is a rare pulmonary disease that occurs predominantly in young adults. The precise incidence and prevalence of pulmonary LCH are unknown, although studies of lung biopsy specimens from patients with interstitial lung disease identified pulmonary LCH in only 5%.
A confident diagnosis of pulmonary LCH often can be made based on the patient's age, smoking history, and characteristic high-resolution computed tomography (HRCT) scan findings, especially if patients are followed without treatment.
Definitive diagnosis, if necessary, can be made by identification of Langerhans cell granulomas in lung biopsy samples acquired by video-assisted thoracoscopy. Biopsy sites are selected on the basis of HRCT scan findings.  Transbronchial biopsy has a low diagnostic yield (10-40%) because of the patchy nature of the disease and the small amounts of tissue obtained.
Treatment consists of smoking cessation, which stabilizes symptoms in most patients. Corticosteroids are used in progressive or systemic disease. Cytotoxic agents (eg, cyclophosphamide) can be employed for patients who do not respond to smoking cessation and steroids. No treatment has been confirmed to be useful, and no double-blind therapeutic trials have been reported. Lung transplantation also has been performed for treatment of LCH.
The typical HRCT pattern of pulmonary Langerhans cell histiocytosis includes small poorly limited nodules, cavitated nodules, and thick- and thin-walled cysts. In rare cases, HRCT allows the disease to be diagnosed before the development of cysts. [2, 6, 7, 8] In HRCT scanning, the small star-like scars can still be detected even after complete cessation of tobacco smoking. 
Chest radiographs are normal in fewer than 10% of patients. Usually, diffuse reticulonodularity (3- to 10-mm nodules that may cavitate) is observed in a symmetrical pattern, predominantly in the upper and middle lobes (bases tend to be spared). 
Lung volumes are generally preserved, or even increased, in contrast to most other pulmonary infiltrative diseases.  Associated pneumothorax is found in 15-25% of patients, and pleural effusion is rare.
In the late stage, diffuse cysts may be found that spare only the costophrenic angles.
Small nodules (1-5 mm, may be up to 1.5 cm)
Cystic cavitation of small nodules; this is not readily visible on plain radiographs 
Cysts (initially thick walled, typically progressing to thin walled over time)Chest CT in a patient with pulmonary eosinophilic granuloma demonstrates scattered cavitary and noncavitary nodules.High-resolution chest CT scan in a patient with pulmonary eosinophilic granuloma shows the typical combination of nodules, cavitated nodules, and thick- and thin-walled cysts. Image courtesy of European Respiratory Society Journals LTD.High-resolution chest CT scan in a patient with advanced pulmonary eosinophilic granuloma shows numerous pulmonary cysts of various sizes, which are confluent in some places. Image courtesy of European Respiratory Society Journals LTD.
Some authors believe that nodules do not cavitate and cysts represent a paracicatricial emphysematous change adjacent to nodules. Cysts usually are smaller than 10 mm, although cysts larger than 10 mm are found in more than 50% of patients. Cyst walls usually are thin (less than 1 mm) but can vary, and the cysts are not necessarily round; they may be bilobed or branching. The intervening lung parenchyma appears normal. The extent of cystic involvement seen on HRCT scans has been correlated with the degree of lung function impairment. 
In the early stages, only nodules may be seen. In the late stages, diffuse cysts may be seen, with no nodules evident (approximately 20% of patients). Late-stage disease may be indistinguishable from lymphangiomyomatosis; however, sparing of the costophrenic angles suggests the diagnosis of eosinophilic granuloma. Mediastinal adenopathy has been described in some series but usually is uncommon. Other authors report mediastinal adenopathy in as many as 30% of patients. [13, 14]
On CT scans of 40 patients with Langerhans cell histiocytosis, 25 patients were found to have cysts involving upper lung zones with costophrenic sparing, 9 had a micronodular pattern in the middle-upper zone, and 6 had a combination of the 2 radiologic patterns. Pulmonary hypertension was seen in 4 patients. 
Gallium-67 scans usually are negative.