Background: The cumulative incidence of subsequent solid malignant neoplasm (SMN) after treatment of TGCT is about 10% over a 25-year period. i(12p) has been used to define tumor of germ cell origin. We evaluated the prevalence of i(12p) on various subsequent SMN in patients with a history of TGCT. Methods: From the Tumor Registry database at MD Anderson, we identified 655 TGCT patients who developed a second (third or fourth) malignancy between January 1990 and September 2018. Of those 114 SMN from the GI tract, GU system, chest, brain, sarcomas or melanomas between 2011 and 2017, 43 had tumor tissue available at our institution and of sufficient quality for fluorescence in situ hybridization (FISH) of i(12p). Results: Our cohort comprised 43 patients, excluding 2 cases (#6 and 7) whose SMN tested positive and their non-tumor controls negative for i(12p). Diagnosis of primary TGCT and SMN: age, mean 41 yrs (range 20-68 yrs) and 59 yrs (range 34-86 yrs), respectively, and interval of 18 yrs (range 0-57 yrs). Of those 43 cases (5 did not hybridize), 5 (11%) harbored i(12p), while 16 (37%) had additional copies of chromosome 12 without apparent gain of 12p. Conclusions: Prevalence of i(12p) on subsequent SMN in our patient cohort with a history of TGCT was 11%. Detection of i(12p) in those patients with TGCT in general and with nonseminomas in particular suggests somatic transformation, emphasizes importance of a complete surgical resection of residual teratomatous tumor after chemotherapy, and implicates a stem-cell origin and nature (e.g., plasticity, heterogeneity, dormancy) of cancer. *deceased; Rx – initial treatments; S – seminoma; NS – nonseminoma; BEP – bleomycin, etoposide, cisplatin; CEB – carboplatin, etoposide, bleomycin; XRT – radiation therapy. Groot HJ, Lubberts S, de Wit R, et al. Risk of solid cancer after treatment of testicular germ cell cancer in the platinum era. J Clin Oncol 2018;36:2504-13.