Any breast-conserving procedure necessarily includes local radiation of the residual mammary gland and possibly the lymph drainage system, depending on how far the tumour has spread. In rare cases and at an advanced stage of cancer, radiation may even be prescribed after a mastectomy. Radiation therapy takes a period of five weeks. Over this period, clearly demarcated areas of skin are subjected to intensive radiation for a few minutes at a time. This radiotherapy may cause some skin irritation.