Embryonal tumor with multilayered rosettes, DICER1-mutated, showing histologically unique neuronal differentiation after chemoradiotherapy.

Embryonal tumors with multilayered rosettes (ETMRs) are rare and highly aggressive embryonal central nervous system tumors that predominantly affect infants younger than 3 years old. These tumors typically have a C19MC alteration (ETMR, C19MC-altered) or, more rarely, a DICER1 mutation (ETMR, DICER1-mutated). Post-chemotherapeutic or post-chemoradiotherapeutic histological changes of C19MC-altered ETMRs, such as maturation or loss of histological characteristics of ETMR have been described in several reports. However, histological changes of recurrent DICER1-mutated ETMRs have not been reported to date. Herein, we report a case of DICER1-mutated ETMR with unique post-treatment morphological changes, including both maturation and loss of histological characteristics. Although pathological examination of tissue from the first resection revealed typical ETMR histology, the recurrent tumor after chemoradiotherapy was composed predominantly of a primitive embryonal component without multilayered rosettes or neuropil-like areas. Furthermore, the recurrent tumor contained a component composed of unique tumor cells with oval eccentric nuclei and eosinophilic cytoplasm demonstrating a neuronal immunohistohemical phenotype. No mitotic figures were found in the component. Molecular analysis identified a mutation in the DICER1 RNase IIIb domain in the primary tumor and the primitive embryonal component of the recurrent tumor, but not in the unique neuronal area.