These scars tend to come in symmetrically placed pairs, placed on lower chest, right below pectorals. They are sometimes slightly curved upwards.
They are associated with mastectomy (colloquially referred to as "top surgery" in LGBT circles) - a procedure that removes a person's breasts. Common for transgender men, for whom it is used to achieve a more masculine appearance, but it is also used for breast cancer or gynecomastia patients.
