Conclusion
In summary, primary mediastinal immature teratoma with somatic-type malignancy is extremely rare with extremely poor prognosis. When laboratory and imaging findings are conflicting for patients with immature teratoma after chemotherapy, somatic-type malignancy should be considered. Due to its rarity, comprehensive reporting of clinical, radiological and pathological features is needed for a diagnosis of an immature teratoma with SM. We reported a primary giant mediastinal immature teratoma with sarcoma and squamous malignant transformation which was resistant to chemotherapy and radiotherapy but resected completely. Rare as it is, additional cases of this pathology are needed to add to the literature for a better understanding of somatic-type malignancy and management of this disease.