While high toxicity and/or radiation can likely cause tumors to develop as a defense mechanism, the most common cause of any cancer is your psyche (Psyche -> Brain -> Organ). That is, stress, fear, worrying, conflicts and similar triggers that causes a biological program to develop.
These videos are excellent sources of information on breast cancer.
Breast cancer and healing explained:
Breast cancer update interview – Patty talks about her experience of developing and healing from a breast tumor using German New Medicine:
For a deeper understanding, the five biological laws: