Small Intestine Metastasis Leads to the Diagnosis of Thoracic SMARCA4-Deficient Undifferentiated Tumor: A Case Report

SMARCA4-deficient undifferentiated tumor (SMARCA4-UT) is a rare and aggressive malignancy characterized by the loss of SMARCA4 protein expression. It typically affects middle-aged male smokers and has a poor prognosis due to its rapid progression and metastatic potential. This case report presents a 73-year-old male diagnosed with a thoracic SMARCA4-UT. Initially diagnosed with stage IVA non-small cell lung cancer, the patient underwent brain tumor resection, radiation, and chemo-immunotherapy. Treatment was halted due to immune-related adverse events. During treatment, a progressing small intestine tumor was discovered, resected, and identified as SMARCA4-UT metastasis through immunohistochemistry, leading to a revised diagnosis of SMARCA4-UT with brain and small intestine metastases. The patient received multimodal treatment, including surgery, radiation, and chemo-immunotherapy. The small intestine metastasis showed resistance to systemic therapy, necessitating surgical intervention. This case highlights the diagnostic challenges and treatment complexities of SMARCA4-UT, emphasizing the importance of comprehensive diagnostic workup and personalized treatment strategies. It demonstrates the potential efficacy of combining systemic therapy with targeted interventions for oligoprogressive disease. The patient's progression-free survival at approximately two years post-diagnosis underscores the need for further research into optimal management strategies for this rare tumor.