Background: Current guidelines define primary and secondary endocrine resistance according to the periods of adjuvant endocrine therapy (adj-ET); however, the relationship between adj-ET period and endocrine resistance remains unclear.
Objective: We examined progression-free survival (PFS) after primary ET for recurrent hormone receptor-positive/HER2-negative breast cancer, and evaluated the relationship between endocrine resistance and the periods of adj-ET.
Methods: We assessed PFS among 183 patients who received ET as primary treatment for the first recurrence, according to the period of adj-ET (adj-ET < 1 year, 1-2 years, ≥2 years, and completion).
Results: Patients who relapsed during the first year of adj-ET had the significantly shortest PFS. PFS did not significantly differ between patients who relapsed at 1-2 years of adj-ET and patients who relapsed while on adj-ET but after the first 2 years.
Conclusions: Relapse at 1-2 years after adj-ET initiation might be better classified as secondary endocrine resistance rather than primary endocrine resistance.
Keywords: HER2-negative; Recurrent breast cancer; adjuvant endocrine therapy; endocrine resistance; hormone receptor-positive.