Variably amount of extracellular basophilic mucin.
Focal papillae may be present.
Morphologic and immunohistochemical overlap with sarcomatoid papillary type 1 RCC (main DDX).
Tubular cells may look similar + spindle cells in sarcomatoid change.
Both tumors are CK7+ and AMACR+.
However, papillary RCC lacks extracellular mucin and shows trisomy 7 and 17. (Distinction is often morphologic)
Mostly have good prognosis. (Distinction from the more aggressive sarcomatoid papillary RCC is thus important)