Clear cell stromal tumor of the lung: Report of 3 cases with emphasis on multifocal tumors

We describe 3 patients with clear cell stromal tumor (CCST) of the lung, all of whom presented with multifocal disease. The patients were 2 men and 1 woman aged 47-58 years (mean, 54 years). Two patients had evidence of autoimmune disease and their pulmonary disease was an incidental finding; one patient presented with non-specific respiratory symptoms. Radiologic imaging revealed multiple pulmonary nodules in all patients. Histologically, the tumors were solid-cystic and composed of cytologically bland, medium-sized ovoid to spindle cells with eosinophilic to clear cytoplasm arranged in a subtle nested pattern. These tumor cells were set in a highly vascularized stroma. Occasional cytologic atypia with multinucleated tumor cells was noted but mitotic activity was low. An infiltrate of mixed inflammatory cells was apparent in all tumors. Immunohistochemical analysis demonstrated diffuse expression of vimentin and TFE3 in all cases. Next generation sequencing revealed the presence of YAP1::TFE3 fusion in 1/1 case. All patients have remained alive albeit with stable or progressive disease, 24-66 months after diagnosis. These cases highlight the existence of multifocal pulmonary CCST and seem to support the notion that multifocality in CCST may be associated with more protracted clinical course. Awareness of the existence of multifocal pattern is important for patient management and prognosis.