In normal circumstances, cells grow and divide as our body requires new cells.
When cells get damaged or grow old they are replaced by new cells. Sometimes this natural process goes wrong and new cells appear without our body needing them, while old or damaged cells may build up rather than die. A gradual build up of these cells can form a lump, tumour or mass. Lumps in the breast can be benign (non-cancerous) or malignant (cancerous).