Can breast cancer cause breast cysts to develop? i know it's rare for cancer in a cyst, but can a tumor be the cause of a simple cyst?

Generally: A simple breast cyst is a benign condition not associated with breast cancer. It is rare for breast cancer to present as a cystic structure, and would be viewed as a complicated cyst, not a simple cyst, by ultrasound.

It's possible: A breast cancer may exist in a cyst if there is a mass in the cyst or in the wall of the cyst. A simple cyst is unlikely to be cancerous. Usually cancerous cysts are complex and may have separations within it. In any event either removal, aspiration with follow up, or repeat imaging my be needed to ensure the cyst is not cancerous.