Should a patient be given extended endocrine therapy if she was on a lower dose than recommended for part of her treatment?

The patient is a pre-menopausal woman with low-grade, stage I IDC who initially could only tolerate 10 mg of tamoxifen for her first year of treatment. She then was able to tolerate 20 mg for the subsequent four years. Should she receive an extra year of 20 mg of tamoxifen?