General principle:
Begin endocrine therapy only when its biologic trigger appears — biochemical recurrence, measurable receptor-positive disease, or symptom-producing tumor burden. Overtreatment wastes QoL; late treatment costs control.

Breast cancer

  • Adjuvant (after surgery): start within 12 weeks post-operation.

  • Neoadjuvant: start after biopsy confirmation ER/PR +, for 3–6 months to shrink the tumor.

  • Metastatic: start promptly at diagnosis unless there’s visceral crisis (then use chemotherapy first).

  • Switch or combine (AI → SERD ± CDK4/6) on progression or ESR1 mutation.

Prostate cancer

Setting Trigger Start rule
Localized high-risk + RT Before RT 2–3 months prior → continue during and after
Locally advanced/metastatic At diagnosis Immediate ADT ± AR antagonist
Biochemical recurrence PSA > 0.4 after prostatectomy or doubling time < 6 mo Early ADT improves MFS; may delay for slow PSA rise & asymptomatic

Clinical caution: confirm true biochemical failure (exclude lab noise or infection).
Quality of life: weigh bone, libido, and metabolic impact before early initiation.

(NCCN Breast 2025; ESMO Prostate 2024)

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