An interesting image popped up on Reddit recently, of a breast cancer that was out of control. NSFW pictures are below the fold
From all indications the woman had previous surgery (based on the scars under her arm it involved removal of the lymph nodes to prevent spread).
Tumors crop as masses, hard knots of cells. If left unchecked they create that bubbled, volcanic, reptile skin like surface area. In the case of gland tumors they may produce excess fluid. In the case of mammary glands, well, putrid lactation.