Pulmonary sarcomatoid carcinoma in the anterior mediastinum mimicking thymic carcinoma with rapid widespread metastases: A case report.

rationalePulmonary sarcomatoid carcinoma (PSC) is a rare and aggressive subtype of non-small-cell lung cancer. Marked histologic heterogeneity, nonspecific imaging findings, and limited representation in small biopsy specimens can cause diagnostic error, particularly when a pulmonary lesion abuts the anterior mediastinum and mimics a thymic tumor. PATIENT CONCERNS: A 67-year-old woman presented to a local hospital in Hangzhou with chest pain, low-grade fever, and intermittent hemoptysis. Imaging showed a mass abutting the anterior mediastinum and adjacent lung parenchyma. Core needle biopsy showed predominantly spindle cells with few atypical epithelioid cells in a lymphocyte-rich background and was interpreted as thymic carcinoma. She was subsequently transferred to Zhejiang Hospital for further evaluation. DIAGNOSES: Continued progression during docetaxel-cisplatin chemotherapy and persistent diagnostic uncertainty prompted video-assisted thoracoscopic wedge resection. The resected tumor showed markedly atypical spindle cells, pan-cytokeratin and vimentin positivity, thyroid transcription factor 1 negativity, and an elevated Ki-67 proliferation index. Programmed death-ligand 1 testing showed a tumor proportion score of approximately 10% and a combined positive score of approximately 20%. KRAS p.A146T and NRAS p.Q61K mutations were identified. Integrated histomorphologic, immunohistochemical, and molecular assessment established the diagnosis of PSC.

interventionsThe patient initially received docetaxel plus cisplatin. Because of continued disease progression, declining performance status, and myelosuppression and gastrointestinal adverse effects during platinum-based chemotherapy, cisplatin was discontinued. The patient subsequently received dose-reduced docetaxel plus pembrolizumab as an individualized treatment strategy intended to balance continued systemic therapy with tolerability. OUTCOMES: The disease progressed rapidly with metastases to the lungs, bone, soft tissue, liver, pancreas, and adrenal gland. The patient died of respiratory failure secondary to widespread tumor dissemination. LESSONS: PSC should be considered when an apparent anterior mediastinal mass shows spindle cell morphology or discordant clinicoradiologic and pathologic findings. Adequate representative tissue and integrated radiologic, morphologic, immunohistochemical, and molecular assessment are essential. Rapid progression despite docetaxel plus pembrolizumab indicates that programmed death-ligand 1 positivity alone may not reliably predict immunotherapy benefit, while the clinical significance of rare KRAS/NRAS co-mutations remains uncertain.