Which of the following statements about Xanthogranulomatous is not true -

Xanthogranulomatous pyelonephritis is relatively rare form of chronic pyelonephritis charecterized by accumulation of foamy macrophages intermingled with plasma cells ,lymphocytes,polymorphonuclear leukocytes, and occational giant cells.Often associated with Proteus infections and obstruction,the lesions sometimes produce large , yellow orange nodules that may be grossly confused with renal cell carcinoma. From Pathologic basis of disease Robbins,volum 2 pg no:934