Indeterminate dendritic cell neoplasm accompanied by eosinophilic pneumonia successfully treated by systemic steroid therapy: Report of the first case with muscular and parotid involvement and review of published work

A 34‐year‐old Japanese man presented with an indolent nodule on the right flank. Computed tomography of the chest and abdomen demonstrated a large nodule measuring 55 mm × 50 mm in the abdominal oblique muscle layer of the right flank, and several small nodules were seen in the muscle layer throughout the body and subcutaneous tissue of the lower abdomen. 18F‐fluorodeoxyglucose‐positron emission tomography/computed tomography revealed nodular lesions in the bilateral parotid glands, bilateral cervical lymph nodes and lower lobe of the right lung. Intermittently, ground‐glass shadows developed in the bilateral lungs. Histologically, sheet‐like nodules in the abdominal oblique muscle layer and parotid gland were composed of large polygonal cells with convoluted nuclei and ample eosinophilic cytoplasm. Several lymphocytes and considerable eosinophils were intermingled. Lung biopsy demonstrated an inflammatory infiltrate of lymphocytes and considerable eosinophils in the alveoli. Immunohistochemically, polygonal cells were positive for S100 protein and CD1a, but negative for langerin and BRAFV600E. Some cells were positive for CD68. Electron microscopy demonstrated histiocytic cells with phagosomes and interdigitating processes. However, no Birbeck granules were observed. Eosinophilia was seen in the peripheral blood. Multifocal nodules and ground‐glass shadows gradually diminished following systemic administration of oral prednisolone. We describe a case of indeterminate dendritic cell neoplasm with multifocal involvement of the muscle, subcutis, lymph node and parotid gland accompanied by chronic eosinophilic pneumonia that was successfully treated by systemic steroid therapy. Neither muscular nor parotid indeterminate dendritic cell neoplasms accompanied by eosinophilic pneumonia have been previously reported.