| Summary: | Extragonadal germ cell tumors (GCTs) are rare neoplasms, accounting for 1–5 % of all germ cell tumors. Mixed germ cell tumors are neoplasms with more than one GCT component. The majority of somatic-type malignancies arising in testicular GCTs originate from the teratomatous component of a mixed GCT.We present a 43-year old man with a history of recurring retroperitoneal teratoma. Initially the patient was diagnosed with a retroperitoneal mixed germ cell tumor with an embryonal carcinoma component and a cystic mature teratoma component in 2001. The patient received neoadjuvant cisplatin-based chemotherapy, followed by surgical resection. He had several recurrences of the teratoma component in 2004, 2014 and the emergence of a somatic-type malignancy in 2023. Microscopic examination in 2023 revealed a biphasic tumor with malignant stromal overgrowth and a characteristic phyllodes-like pattern, leading to the diagnosis of Müllerian adenosarcoma-like tumor. To the best of our knowledge, this is the first reported case of Müllerian adenosarcoma-like tumor arising in an extragonadal teratoma.This case expands the spectrum of somatic-type malignancies arising in extragonadal GCTs and emphasizes the importance of long-term surveillance. Extragonadal GCTs are rare and gonadal origin should always be excluded by imaging. Our case also brings awareness to the growing teratoma syndrome where residual teratomas persist or enlarge post-chemotherapy. Because of its extremely low incidence, Müllerian adenosarcoma in males should be studied further to determine the prognosis and best treatment options.
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