Breast cyst. Magnetic resonance imaging (MRI) scans of sections through the right breast of a 40-year-old female patient with a history of breast cancer, showing a cyst in the retro-mammary space - a section of tissue that separates the breast from the pectoralis major muscle of the chest. Cysts are fluid-filled sacs. Breast cysts are generally benign (non-cancerous) but may be pre-cancerous or cancerous (malignant).