Whether 6mm of endometrial lining is normal for a 47 year old woman who is not menopausal after breast cancer surgery is dependent on the menstrual cycle.
The thickness of the endometrium in an unmenopausal woman changes cyclically with the menstrual cycle. Typically, a woman’s thickness is around 3-5mm just after her period, and by the time she ovulates, the endometrial thickness should be around 8-10mm. Before menstruation the thickness of the endometrium should be about 10-12mm.
So a woman’s endometrial thickness of about 6mm is normal if it is detected close to the time of ovulation. If it is just after menstruation then it is not normal and is slightly thicker. If it is detected during ovulation or just before menstruation, it is also abnormal and suggests that the lining of the uterus is too thin.