harka

Well, it depends on what type of solid mass. The most common form of solid mass is a "neoplasm" or a "new growth of cells". Another name for a neoplasm is a tumour, which as most people know can be benign or malignant.

When a solid mass forms in the kidney, it is usually because one or more cells has grown in excess of what it should and forms a nodule. If the growth happens uncontrollably, it invades the tissue around it and it breaks off and goes to distant sites of the body, that is when the growth is considered malignant.

You can have other masses in the kidney which are NOT from cells overmultiplying including: fluid filled cysts (either from cystic renal disease or a cyst filled with urine called a urinoma), large stones (usually which are contained within the draining system of the kidney, metastases (cancer which has migrated from a distant site) and...and...I can't think of anything else.