[Histiocytosis X discovered after chemotherapy treatment for mesothelioma].

Langerhans cell histiocytosis is a rare disorder characterized by clonal proliferation of Langerhans cells. We report the case of a 40-year-old active smoker who was initially treated for a right pleural mesothelioma with chemo-immunotherapy, bullectomy with talc pleurodesis, and subsequent pleuropneumonectomy due to extensive pleuro-pulmonary involvement. During an oncological reassessment, multiple nodular lesions were detected in the contralateral lung parenchyma and were initially suspected to be metastatic. However, histological analysis of bronchoalveolar lavage and transbronchial biopsies revealed pulmonary Langerhans cell histiocytosis (formerly known as eosinophilic granuloma or histiocytosis X). This case highlights the importance of differential diagnosis in post-treatment oncologic follow-up, particularly in smokers.