Breast spindle cell tumors are a relatively rare type of tumor and can be either benign or malignant. The literature I read generally recommends excision biopsy to determine whether the tumor is benign or malignant, as it is less conclusive with FNA or core biopsy.
As madge said, it has to do with the type of cell (spindle vs squamous I think). The main thing for you to find out is whether it falls into the benign or malignant category, and if it is benign, whether it is related to (arising from) other breast conditions that may raise your risk of malignancy.
I believe the spindle cell gets its name from the shape of the cells and it is a tumor that can be malignant or benign. Maybe if I bump this up one of the doctors can shed some light on the subject. God Bless, Madge.