If breast cancer reccurs after having had a masectomy, will the new growth be benign?

Will be malignant.: By definition, a recurrence of breast cancer is malignant and potentially serious. Recurrence can be localized to the breast or chest wall and could potentially be treated by surgical resection or radiation @/or chemotherapy or hormonal therapy. Spread to other organs is more serious and is usually beyond surgical control so chemo or hormonal therapy is used.

Breast cancer: If breast cancer recurs someplace else in the body, the recurrence is still a cancer (malignant, not benign).

Not necessarily: Medically, a "recurrence" after any cancer removal is, by definition, a cancer- therefore not benign. A new growth must be biopsied or excised and tested to prove if it is benign or malignant.