Introduction/Background Malignant transformation can rarely occur within ovarian teratomas. Due to its scarcity and limited evidence in the literature, such cases prove to be a diagnostic challenge. DNA methylation technology is a developing research tool which has not been clinically validated in gynaecological tumours. We present two cases of malignant transformation relating to immature teratoma where DNA-methylation profiling has raised question to the current diagnostic classification criteria.

Case reports Patient 1 (20yo) presented with disease recurrence, 2-years after resection of a grade 1 immature teratoma with peritoneal gliomatosis. Multi-specialist histology review showed a malignant tumour with dual neuroepithelial and rhabdomyoblastic differentiation.

Patient 2 (19yo) presented with a rapidly growing ovarian mass; histology showed a malignant primitive neuro-ectodermal tumour and a minor yolk-sac tumour component arising within a high grade immature teratoma.

Results DNA-methylation profiling interestingly classified both tumours as sarcoma, with a variable level of diagnostic confidence, respectively as embryonal and MYOD1-mutant rhabdomyosarcoma (patient 1) and sarcoma, rhabdomyosarcoma-like (patient 2).

Conclusion Both cases demonstrate how DNA-methylation profiling might aid in the diagnosis for tumours exhibiting divergent differentiation. This technique will improve our understanding of the pathogenesis and molecular profile of rare ovarian germ cell tumours and might therefore influence future classification.

Disclosures None.