Hyalinizing clear cell carcinoma of the left main bronchus diagnosed with Ewing sarcoma breakpoint region 1::activating transcription factor 1 fusion: A case report

Hyalinizing clear cell carcinoma (HCCC) of the bronchus is extremely rare. In addition, its rarity makes an accurate histological diagnosis difficult. Herein, we report a case of HCCC in the left main bronchus diagnosed by the detection of Ewing sarcoma breakpoint region 1::activating transcription factor 1 (EWSR1::ATF1) fusion transcript using reverse transcription-polymerase chain reaction (RT-PCR). A 56-year-old woman presented with a tumor obstructing the left main bronchus on physical examination computed tomography. A bronchoscopic biopsy confirmed a malignant tumor but did not provide a definitive diagnosis. Sleeve resection of the left main bronchus was performed. The intraoperative frozen section was negative for cancer in both the proximal and distal bronchial stumps. General immunohistochemical staining was performed, and mucoepidermoid carcinoma and HCCC were suspected. RT-PCR revealed positive results for EWSR1::ATF1 fusion transcript, and the tumor was finally diagnosed as HCCC. The patient was discharged without any postoperative complications. Sixty-five months have passed since surgery, and no cancer recurrence has been observed. Although hematoxylin and eosin staining and immunostaining alone were not sufficient to distinguish HCCC from mucoepidermoid carcinoma and other malignant tumors, adding genetic testing for EWSR1 rearrangement led to a definitive pathological diagnosis.