Rosettes are characteristically seen in -

Robbins basic pathology 9th edition page no 261, Differentiated structures are found within many retinoblastomas, the most common of these being Flexner-Wintersreiner rosettes. These structures consist of clusters of cuboidal or sho columnar cells arranged around a central lumen. The nuclei are displaced away from the nucleus. In well-differentiated tumors, there are Flexner-Wintersteiner rosettes and fleurettes reflecting photoreceptor differentiation