The tumors they produce are yellow to orange in color. Few tumors contain only clear cells, however. Darker granular cells usually are present to some degree. These have a larger, darker nucleus and are full of tiny pink granules called mitochondria. The tumors they produce tend to be gray to white in color.
Mitochondria are small, oval bodies that provide energy for cell growth.
Their presence indicates a more aggressive form of cancer. Tumors that contain both clear and granular cells are considered mixed . This is the most common form of RCC and indicates the most aggressive form of kidney cancer. Mixed tumors that contain spindle shaped, sarcomatoid cells have the least favorable prognosis. Although tumors composed exclusively of spindle cells are uncommon, the presence of sarcomatoid cells indicates a form of cancer that grows and spreads quickly.