Isolated Mediastinal Langerhans Cell Histiocytosis.

Langerhans cell histiocytosis (LCH) is a rare clonal proliferative disorder originating from dendritic cells and primarily occurring in children. Although pulmonary LCH is relatively common among adult smokers, isolated mediastinal involvement is very rare. This study reports a 33-year-old female patient who presented with a 2-month history of right upper back pain. Positron emission tomography/computed tomography revealed an isolated 6.2 × 3.5 cm mixed-density mass in the right anterior mediastinum, raising suspicion for invasion of the pericardium, major vessels, and diaphragmatic pleura. However, during surgical resection, the lesion appeared well-encapsulated, with no evidence of invasion. This discrepancy was explained by postoperative histopathology, which revealed prominent thymic hyperplasia containing a small 3 mm focus of LCH. This rare presentation of isolated anterior mediastinal LCH in an adult highlights potential for radiologic overestimation when a small LCH focus is accompanied by prominent thymic hyperplasia, underscoring the need for careful individualized evaluation.