Hi, Thanks for writing in. Angiomyolipoma is a benign tumor commonly seen in kidneys. It is a bunch of vascular, smooth muscle and fat containing tissues. If it has not grown from 4 mm in 18 months then there is no reason to worry. Angiomyolipomas are often found incidentally when the kidneys are imaged for other reasons. Angiomyolipomas found incidentally usually require no therapy (when small), although follow-up is recommended to assess for growth. Larger tumours, or those that have been symptomatic, can be electively embolised and / or resected with a partial nephrectomy.