Breast Cysts . A breast cyst is a benign fluid-filled sac in the breast tissue. They are very common and typically affect women in their 40s who are in perimenopause (the period of time before menopause when a woman stops having periods), but they can occur at any age.
https://www.verywellhealth.com/understanding-breast-lumps-both-benign-and-cancerous-430415

