My mother is 80 years old. in 2008 she was diagnosed with breast cancer. she received treatment but it has reappeared again. what caused it to reappear again?

Recurrence: Depends on the type & stage of the first breast cancer & the treatments received (chemotherapy & radiation therapy). Depends on where it has reappeared: skin, chest wall, bone, lung, lymph node, etc. Breast cancers can recur and close exams & follow up after the first treatments are important. An oncologist will help direct the correct care & treatment options.

Recurrence Defined: Breast cancer may recur locally (in breast after lumpectomy) or systemically (distant organs=metastasis). If the former, mastectomy is recommended; if the latter, chemotherapy. The "why" part is a lot harder to answer, relating to the unique type of cancer she had and its original rx. I suggest you accompany your mother to her oncology appointments for support & a better understanding of the "why".