*3.6. Rhabdoid Cells*

Rhabdoid tumors have been documented in many different topographies across the body [62], always linked to biological aggressiveness and bad prognosis. Its histogenesis is still unclear. A handful of rhabdoid tumors of the bladder have been published, particularly in children and young adults [63–65]. There are, however, isolated cases reported in adulthood [66–68].

Aside from genuine rhabdoid tumors, a focal *rhabdoid* phenotype can be observed sometimes in UC [68], where large and ovoid cells with large atypical nuclei and lateralized eosinophilic cytoplasm may appear growing without any specific pattern usually in high-grade neoplasms. A possible rhabdomyoblastic dedifferentiation in the context of a sarcomatoid UC should be ruled out, at least theoretically, in these cases.

By immunohistochemistry, rhabdoid cells are positive for CK7, CK20, vimentin, E-cadherin, and β-catenin, p63, and INI-1 [68].

#### *3.7. Lipid*/*Lipoblast-Like Cells*

These two terms refer to a rare variant of UC composed of lipidic appearing tumor cells intermingled with transitional cells in variable proportions. It was first recognized by Mostofi et al. in 1999 [69]. Since then only single case reports and two short series [70,71] have been published. The longest series so far analyzes 27 cases collected from different international institutions [71]. Lipidic-appearing cells may resemble either adipocytes or adipoblasts (Figure 2e) and usually take part

in a high-grade UC, not otherwise specified. Immunohistochemistry confirms the epithelial nature in all cases, including the co-expression of CK7 and CK20 [70,71].
