Why would a breast mri be recommended when you have breast cancer?

Several reasons: A breast MRI gives a more detailed picture of the breasts than mammogram or ultrasound. It is recommended in young women whose breasts are dense and hard to see on mammogram. It is also better for detecting certain types of breast cancer (lobular or inflammatory breast cancer.) the downside is that it finds many "spots" that aren't breast cancer.

Controversial: This is controversial. An MRI is an additional tool in breast imaging. The sensitivity of MRI is high and it may pick up smaller cancer that were not detected on mammogram. It may also lead to increase false positives and unnecessary procedures.

Preop planning: Breast MRI should be performed before surgery for breast cancer. Mri finds additional cancer in the breast with known cancer up to 23% of the time (sometimes changing a lumpectomy to mastectomy) and cancer in the opposite breast up to 8% of the time . Breast MRI can be used to screen women after lumpectomy for recurrence. It is extremely sensitive, finding nearly 100% of breast cancer.

Yes: I too agree with the use of MRI in preop planning for finding additional cancer (in the same breast or the other breast) and for post treatment (after chemotheraphy or surgery). There are other scenarios where breast MRI is useful as well. In my opinion, MRI of the breasts is underutilized.

Usually for: Preoperative treatment planning or to assess neo adjuvant chemotherapy response. MRI is the most accurate imaging modality for assessing the extent of the cancer. It is also useful for diagnosing multi centric/multi focal cancers, and cancers in the contraleyeral breast