J Breast Cancer.  2021 Apr;24(2):164-174. 10.4048/jbc.2021.24.e17.

Clinical Outcomes Following Letrozole Treatment according to Estrogen Receptor Expression in Postmenopausal Women: LETTER Study (KBCSG-006)

  • 1Department of Surgery, Gangnam Severance Hospital, Seoul, Korea
  • 2Department of Surgery, Samsung Medical Center, Seoul, Korea
  • 3Department of Surgery, Asan Medical Center, Seoul, Korea
  • 4Department of Surgery, Ajou University School of Medicine, Suwon, Korea
  • 5Department of Surgery, Gachon University Gil Hospital, Incheon, Korea
  • 6Department of Surgery, Yeungnam University College of Medicine, Daegu, Korea
  • 7Department of Surgery, CHA University Ilsan Medical Center, Goyang, Korea
  • 8Department of Surgery, Seoul National University College of Medicine, Seoul, Korea
  • 9Division of Oncology/Hematology, Department of Internal Medicine, Korea University College of Medicine, Seoul, Korea
  • 10Department of Surgery, Kangbuk Samsung Hospital, Seoul, Korea
  • 11Department of Surgery, Soonchunhyang University Seoul Hospital, Seoul, Korea
  • 12Department of Surgery, Jeonbuk National University Medical School, Jeonju, Korea
  • 13Department of Surgery, Busan St. Mary's Hospital, Busan, Korea
  • 14Department of Surgery, Konkuk University Medical Center, Choongju, Korea
  • 15Department of Surgery, Kyung Hee University Hospital, Seoul, Korea
  • 16Department of Surgery, Ulsan University Hospital, Ulsan, Korea
  • 17Department of Surgery, Inje University Sanggye Paik Hospital, Seoul, Korea
  • 18Department of Surgery, Hanyang University Hospital, Seoul, Korea
  • 19Department of Surgery, Soonchunhyang University Cheonan Hospital, Cheonan, Korea
  • 20Department of Surgery, Dong-A University Hospital, Busan, Korea
  • 21Department of Surgery, Kangseo MizMedi Hospital, Seoul, Korea
  • 22Department of Surgery, Dankook University Hospital, Cheonan, Korea
  • 23Department of Surgery, Ewha Woman's University Mokdong Hospital, Seoul, Korea
  • 24Department of Surgery, Hallym University Sacred Heart Hospital, Anyang, Korea
  • 25Department of Surgery, Gangseo Mizmedi Hospital, Seoul, Korea
  • 26Department of Surgery, Chonnam National University Medical School, Gwangju, Korea
  • 27Department of Surgery, Inje University Busan Paik Hospital, Busan, Korea
  • 28Department of Surgery, Keimyung University School of Medicine, Daegu, Korea
  • 29Department of Surgery, Soonchunhyang University Bucheon Hospital, Bucheon, Korea
  • 30Department of Surgery, Pusan National University Hospital, Busan, Korea
  • 31Department of Pathology, Asan Medical Center, Seoul, Korea
  • 32Department of Pathology, Yeungnam University College of Medicine, Daegu, Korea
  • 33Department of Pathology, Seoul St. Mary's Hospital, The Catholic University of Korea, Seoul, Korea


In this trial, we investigated the efficacy and safety of adjuvant letrozole for hormone receptor (HR)-positive breast cancer. Here, we report the clinical outcome in postmenopausal women with HR-positive breast cancer treated with adjuvant letrozole according to estrogen receptor (ER) expression levels.
In this multi-institutional, open-label, observational study, postmenopausal patients with HR-positive breast cancer received adjuvant letrozole (2.5 mg/daily) for 5 years unless they experienced disease progression or unacceptable toxicity or withdrew their consent. The patients were stratified into the following 3 groups according to ER expression levels using a modified Allred score (AS): low, intermediate, and high (AS 3–4, 5–6, and 7–8, respectively). ER expression was centrally reviewed. The primary objective was the 5-year disease-free survival (DFS) rate.
Between April 25, 2010, and February 5, 2014, 440 patients were enrolled. With a median follow-up of 62.0 months, the 5-year DFS rate in all patients was 94.2% (95% confidence interval [CI], 91.8–96.6). The 5-year DFS and recurrence-free survival (RFS) rates did not differ according to ER expression; the 5-year DFS rates were 94.3% and 94.1%in the low-to-intermediate and high expression groups, respectively (p = 0.6), and the corresponding 5-year RFS rates were 95.7% and 95.4%, respectively (p = 0.7). Furthermore, 25 patients discontinued letrozole because of drug toxicity.
Treatment with adjuvant letrozole showed very favorable treatment outcomes and good tolerability among Korean postmenopausal women with ER-positive breast cancer, independent of ER expression.


Breast neoplasms; Letrozole; Postmenopause; Receptors; estrogen
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