An 82-year-old man was referred for an abnormal shadow detected 4 months previously on computed tomography (CT) in a regular follow-up after bilateral clear cell renal cell carcinoma. He underwent left nephrectomy 14 years ago and right partial nephrectomy 5 years ago. At that time, he stated that he had a prolonged cough for several months. He had a smoking history of a half pack-year for 20 years. His father had gastric cancer, his mother had thyroid cancer, and his son had a brain tumor. He was a merchant and had no exposure history to asbestos. He had a bird mamelliha.
Chest CT showed a 14-mm nodular shadow on the apex of the right upper lobe of the lung with no lymphadenopathy or pleural effusion. PET/CT (positron emission tomography-computed tomography) revealed abnormal accumulation in 2 nodules of the upper lobe of the right lung and ipsilateral hilar- and mediastinal lymph nodes (Fig. 1). Abdominal CT and brain MRI (magnetic resonance imaging) were negative for metastasis. Laboratory examinations showed elevated levels of serum progastrin-releasing peptide [ProGRP; 115 pg/mL (normal range: 0-70.9 pg/mL)] and soluble IL-2 receptor [S.IL-2R; 992 U/mL (normal range: 122–496 U/mL)], and a slightly elevated level of squamous cell carcinoma antigen [SCC; 2.7 ng/mL (normal range: 0-2.5 ng/mL)]. Serum levels of carcinoembryonic antigen (CEA) and cytokerain-19 fragments (CYFRA) were within normal limits. Lung metastasis from renal cancer or primary lung cancer was clinically suspected and the patient underwent partial resection of the upper lobe of the right lung via video-assisted thoracotomy.
A gross examination revealed a solid tumor with an irregular border, measuring 25 × 12 × 10 mm with a white cut surface (Fig. 2). Paraffin sections showed that the tumor was composed of sheets of monomorphic small round cells with necrosis, which was not consistent with typical clear cell renal cell carcinoma (Fig. 2). There was no obvious or abrupt squamous differentiation or tube formation. Tumor cells had round- to oval-shaped nuclei with distinct nucleoli and a scanty cytoplasm. Mitotic figures (27/20 HPF) were clearly observed. There was lymphatic invasion and two intrapulmonary metastases of 2 and 1 mm in size. These intrapulmonary metastases were detected in the sub-pleura 8 mm from the main tumor. Primary and intrapulmonary metastatic lesions both extended into the soft tissue of bronchial vascular bundles and the structure of the pulmonary artery was maintained inside the tumor (Fig. 2). There was no pleural invasion, pleural dissemination, or venous infiltration. The surgical margin was negative. We initially suspected small cell carcinoma; however, immunohistochemistry was less typical because tumor cells were focally positive for neuron-specific enolase (NSE) and chromogranin A and negative for CD56 and synaptophysin. Although we searched for lymphoma, tumor cells were only focally positive for B-cell lymphoma-2 (BCL-2) and negative for other lymphocytic markers (leukocytic common antigen (LCA), CD3, CD5, terminal deoxynucleotidyl transferase (TdT), CD10, CD20, CD79a, and cyclin D1). Due to the focal positivity of BCL-2, we suspected synovial sarcoma as a differential diagnosis and attempted to exclude other diseases, but were unable to reach a final diagnosis based on general immunohistochemistry alone. Tumor cells were positive for epithelial membrane antigen (EMA) and vimentin and focally positive for CD99, HHF-35, desmin, calponin, and Sal-like protein 4 (SALL4). They were negative for AE1/AE3, cytokeratin 7 (CK7), cytokeratin 20 (CK20), cytokeratin 5/6 (CK5/6), CAM5.2, thyroid transcription factor 1 (TTF-1), tumor protein p63 (p63), p40, myoblast determination protein 1 (MyoD1), caldesmon, glypican 3 (GPC3), placental alkaline phosphatase (PLAP), calretinin, S-100, CD117 [c-kit], Melan A, HMB-45, Wilm’s tumor suppressor gene 1 (WT-1), paired box gene 8 (PAX8), and D2-40. EBER in situ hybridization was also negative. Our differential diagnoses for these small round cell tumors were small cell carcinoma, germ cell tumor, soft tissue sarcoma (synovial sarcoma, extra-skeletal Ewing’s sarcoma, Ewing-like sarcoma [CIC rearrangement sarcoma, and sarcoma with BCOR genetic alterations], and malignant glomus tumor), and NUT carcinoma.
We subsequently performed NUT immunohistochemistry (rabbit monoclonal, C52B1, x100, TRSpH9, Cell Signaling) using AutostainerLink48, which showed diffusely positive tumor nuclei with a speckled pattern. FISH and RNA sequencing (RNA-Seq) revealed the presence of the BRD4-NUT fusion gene (Fig. 2). These results led to the final diagnosis of primary pulmonary NUT midline carcinoma with the BRD4-NUT fusion (pT3NxM0).