What Could Cause A Large Tumour Behind The Ears?

Hello, I am a nursing student finishing my RN in May but have a question about my mom. My mother is a 50-yr-old woman, with a recent Dx of DCIS of the right breast, stage 1. She is being treated for this and will be undergoing a mastectomy in January. However, after her biopsy and lumpectomy both she had pretty major infections, and excess bleeding (large hematoma from the biopsy). Anyhow, she also has a very large tumor behind her ear (egg size), that her family Dr. says is nothing to worry about. She is neurologically intact, but I am very concerned about this tumor. Her dogs have also had to have tumors removed in the past couple years. I am just wondering if there is a correlation between the kerosene heaters and the tumors?