Myxofibrosarcoma is a malignant fibroblastic neoplasm that commonly arises in the extremities, with mediastinum being a very rare location. The development of sarcomas is uncommon in patients with Lynch syndrome. We present a Lynch syndrome patient with synchronous cecal adenocarcinoma and mediastinal myxofibrosarcoma with both harboring the same loss-of-function MSH2 alteration (c.2634 + 1G > A splice region variant). Metastatic myxofibrosarcoma in the left chest wall developed 6 months after the initial diagnosis. The clinical presentation, imaging findings, histopathology, and molecular studies along with differential diagnoses are presented and discussed.
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