We have two adrenal glands in our endocrine system and they sit just above our kidneys. The outside layer of the adrenal glands is called the adrenal cortex. The inside layer of the adrenal gland is called the adrenal medulla. A cancer tumour that starts in the chromaffin cells in the adrenal medulla is called pheochromocytoma. Although Pheochromocytoma is usually found in the adrenal glands as this is where the most chromaffin cells are found, this type of tumour may also start in chromaffin cells found in other parts of the body such as the area around the heart or bladder.