DISCUSSION: This case demonstrates the importance of monitoring for progression of germ cell tumors with serial imaging, in addition to tumor markers. Similar to other case reports, this patient experienced normalization of his tumor markers, but demonstrated increasing tumor size on imaging. Detecting the expanding tumor prompted a concern for the presence tumor cells that were unaffected by treatment, which, in this case, were revealed to be those of angiosarcoma. Because angiosarcomas are very unlikely to respond to chemotherapy, the prognosis is poor. Novel therapies are under investigation.